Dipmeter Surveys (Computation)

Computation

Methods and Parameter Selection

Computation Methods

One method used to obtain dip information from the raw data involves correlating intervals of the dip curves. To a mathematician, a correlation coefficient is a measure of agreement between any two curves. Numerically, coefficients may run from zero (representing two completely dissimilar curves) to one (representing two identical curves).

The computer calculates the similarity between a section of one curve and an equal section of a second curve. The length of the interval on the first curve is the correlation length or interval. The computer then moves the first curve by some small, preset increment and recomputes the coefficient. This process is repeated many times.

When plotted with respect to depth, the resultant series of coefficients forms a function called the correlogram. This correlogram shows a peak value where the curves have the best fit with each other ( Figure 1 ). The position of this peak with respect to the center of the interval chosen on the first curve is the shift, or displacement, between curves.

 

The process is repeated for all curve pair combinations at that depth; the result is the relative position of correlated points around the borehole, which (when combined with the other measurements such as tool orientation, drift, and caliper data) are used to calculate the dip answer for that depth. A new interval is then chosen on the first curve at a distance equal to the step distance from the previous round of correlations just described, and the process is repeated to produce another dip answer displaced in depth from the previous one by an amount equal to the step distance. This step distance is normally chosen to be some fraction (usually 25 to 50%) of the correlation interval.

During the curve-to-curve comparison it is essential to prescribe for the computer the distance up and down the second curve to which the first curve is to be compared. This distance is fixed by the choice of the input parameter called search angle.

Search angle is chosen according to the dip environment. For low structural dip areas, a 45° search is common, as most stratigraphic dips fall within that range.

In tectonically disturbed areas, higher search angles may be required. The choice in such circumstances must be guided by both local knowledge and close inspection of the dip curves. Large displacements may be visually evident and an approximate dip range may be estimated.

The user of the computed data should be aware of a particular characteristic of the interval correlation system. In order to prevent some data from not being used in the computation, the step distance is normally (as mentioned above) less than the correlation interval. This may allow a dominant anomaly (a large sharp peak or trough) to influence the dip answer for each step in which it is included in the correlation interval. This can cause two or more adjacent dips to be essentially identical, giving the user the impression that several parallel beds exist when in fact there may be only one. For example, a 25% step may produce four similar dips from one anomaly, a 33% step may produce three similar dips, and a 50% step may produce two similar dips.

If the user is aware of the parameters used for the computation he will recognize the duplications and interpret the dips correctly. However, if the effect is not desirable, a method called pooling may be used to present the results. In pooled plots, adjacent dips within a very small solid angle (2° to 3°) are presented as one dip answer. Dips that do not pool are still presented, so that no data is discarded.

Figure 2 shows another interval with both the unpooled and pooled results side by side. Note the groups of four dips on the unpooled data set that appear as single dips on the pooled result. Also evident is the marked decrease in dip density in the pooled data for the upper half of the log. This can be a desirable presentation, particularly when plotting data on reduced scales, such as 1:600 or 1:1200, for structural dip analysis.

 

Computation Parameter Selection

There are three basic types of interpretation problems that users of dipmeter data may wish to solve:

structural interpretation

large-scale stratigraphic features

maximum detail, very fine stratigraphic features, as observed on detailed core inspection.

It is often desirable to interpret a combination of the above from a single dipmeter log. As a result, a variety of systems have evolved to handle widely different requirements.

The most commonly used and generally applicable approach is the correlation interval system described earlier. For analysis of structure and large-scale sedimentary features, a 4-ft correlation interval and a 1-ft or 2-ft step is usually the first approach to analysis. For special applications or difficult logging conditions, other values of these parameters may be more useful. In fact, if the user of the data is specifically interested in one of the three interpretations mentioned above, parameters must be chosen to optimize that result.

Therefore, it is important to understand how the tadpole plot is affected by the choice of these parameters. For each step, a single dip answer is produced, and all the data within that correlation interval are used to obtain that single dip. A 4-ft interval may contain from 0 to more than 100 correlations, due to bedding contrasts, but only a single dip is calculated, based on the best fit of the correlation curves. Large correlation intervals tend to smooth the dip results. Short correlation intervals allow the system to find more detailed results.

Figure 3 contains a 4-m section of dipmeter computed in a sand section using several correlation intervals. Note that although the dip direction trend is similar in each, the implied cross-sectional view of the formation is significantly different.

 

Plot A clearly shows detailed internal sedimentary structures with a much better suggestion of environment than do B and C. Plot B retains most of the characteristics of Plot A, but with some apparent averaging and smoothing at dip magnitude boundaries. Plot C suggests large-scale, almost parallel crossbedding. This plot fails to indicate the more complex internal sedimentary structures evident on the A plot.

It is apparent from comparing these three computations that the choice of the computation parameters should be influenced by the type of information required to support exploration and production programs. Although the basic principles described in the foregoing apply to all correlation interval techniques, algorithms differ significantly for different tool types, allowing the best adaptation to the data obtained by the tool.

Dip Computations with the 4-Curve Dipmeter Tool

For the 4-curve tool, two correlation techniques are available to determine the magnitude of the dip and the azimuth of its direction: interval correlation (CLUSTER* Program), as described above; and feature correlation (GEODIP* Program), where individual peaks and troughs are first classified by size, shape, and other characteristics, and these features are matched from curve to curve, taking into account certain constraints. The objective of the latter method is to adapt the program to variations in bedding frequency and thickness, with the result that dip computations are made at points on the dip curves rather than over preselected intervals. This system then frees the computation from a fixed interval constraint, and allows computation of dips of individual bed boundaries.

Note: Throughout this document an asterisk (*) indicates a mark of Schlumberger.

The overlapping correlation sequences of CLUSTER processing are an improvement over previous programs, but it still has the disadvantage of a fixed, rigid correlation "window," unresponsive to variations in the density of geologic data in the curves.

A close study of dipmeter curves shows that many curve features or elements are identifiable from curve to curve. As shown in Figure 1 , these features have various thicknesses (from 1 in. to several feet), amplitudes, and shapes. Each feature may be considered to be the signature of a geological event in the depositional sequence of the formation. Moreover, the dip of the bed boundaries is not necessarily constant, and sometimes varies rapidly.

 

In the GEODIP program, each of the four dip curves to be correlated is mathematically decomposed into a depth-ordered sequence of ranked elements.

In feature extraction, which is the first phase of the program, elements such as peaks, troughs, spikes, and steps are identified in the curves. Each feature has one or two boundaries and a set of parameters that describe its shape.

In the second phase, the GEODIP program attempts to match elements of one curve with similar elements of the others according to the following logic:

· By a built-in order of precedence (e.g., first large troughs, then large peaks, then medium troughs, and so on), the program first evaluates higher-order features, then when necessary also evaluates lower-order ones. This is done during multiple passes through the four sets of elements.

· Because geologic strata are deposited in succession, their boundaries do not cross. So, if event A appears above event B on one curve, it cannot appear below event B on another. This is the rule of noncrossing correlations.

If no correlation can be found within the specified search angle among all four curves, the program lowers its standards and looks for 3-curve correlations instead. Planarity is monitored continuously, and if it fails to meet preset standards, the program makes no attempt at 4-curve dips, but computes the four different 3-curve dips and displays them all.

Because the program works from identifiable features on the curve, each one corresponds to a geologic event and the density of the output data depends on the density of geologic information at that level. This makes GEODIP processing particularly successful in fine-structured sedimentary sections and for definition of lithological changes, such as scour surfaces.

The calculation of dip angle at each depth is from displacements measured on boundaries rather than on feature centers. These boundaries are shown on the correlation curves of a GEODIP log. They are themselves useful features for interpreting lithology, as Figure 2 suggests.

 

Determining Data Quality

The geologic validity of each dip determination may be tested in several ways.

Closure If displacements are determined between each adjacent pair of curves, taken cylindrically (1-2, 2-3, 3-4, 4-1) they should have an algebraic sum of zero. (Moving from one electrode to the next, you should return to where you began after making a traverse of all four electrodes.) This condition is called perfect closure. Small closure errors may be due to inaccuracies in the computed displacements; large closure errors indicate that one or more of the correlations are in error.

Planarity Another test is for planarity, the condition that the four points should define a plane. After four displacements have been calculated, the lines joining diametrically opposed electrodes may fail to intersect, if there is an anomaly in the calculation or in the bedding.

For the 4-curve tool, the geometry of the pad linkage ensures that distances between opposing adjacent pairs remain equal. Displacements computed from opposite pairs of curves (h 1-2 and h 3-4 for example) must therefore be equal but opposite if the bedding surface is planar. (The line segment connecting Pads 1 and 2 on the dipping plane parallels and equals in length-but is oppositely directed to-the line segment connecting Pads 3 and 4, for example.) For perfect planarity, h_1-2+h_3-4 = 0  and h_2-3 + h_4-1 = 0.

Likeness A third test is for likeness, a quality derived from the correlogram, to compare the similarity of the curves. The highest correlation coefficient computed over the search interval is the likeness of the two curves, and the trial displacement of that maximum is the displacement retained for that interval of the curves. Since more than one cross correlation is required to compute a dip, the credibility of the dip answer is roughly proportional to the lowest likeness of all the correlations used.

Despite these tests, the results sometimes show excessive scatter that is not of geologic origin, particularly when shorter correlation lengths are selected to improve resolution. The CLUSTER program reduces the scatter in the output by statistically reducing the data. It is assumed that random noise does not repeat itself through small changes of the correlation environment. Thus, at a given level the redundancy inherent in having four correlation curves allows the curves to be grouped in various combinations in a search for consistency. In addition, coherence between consecutive overlapping levels above and below each point in the hole is checked.

The program computes correlations between five of six possible pairings of the four curves, taken two at a time. To define a plane, any two of these pairs must have one curve in common. The CLUSTER program, working with this output, considers eight such solutions. Each of the eight yields a solution for the true dip plane, and generally each is slightly different. Calculations from an adjacent level yield another set of eight solutions. Since the correlation interval is greater than the step distance, neighboring correlation intervals overlap. Comparison of dips from several overlapping levels (eight solutions from each level) shows statistical scatter among the different solutions, but there should be a tendency for many of them to "cluster" near some numerical value. When several solutions (not all from one level) fall within an acceptable range of values, the program quotes the value for the group, rejecting those that scatter outside. As a result, legitimate dip trends can be sorted from noise.

Computing Dip with 8-Curve Data

This section discusses the methods developed specifically for processing 8-curve data using the principles of interval and feature correlation, the presentation of the results, and the presentations available at the wellsite and at the computing centers.

The determination of formation dip measurements using the 4-curve dipmeter tool depends on the bedding plane being detected by at least three of the four measure electrodes. This, in turn, implies that the formation is well-bedded or laminated. Unfortunately this is not always the case, and for many formations pad-to-pad correlations are impossible to establish, making sedimentary studies difficult or impossible. Also, pad-to-pad correlations may be difficult in highly dipping formations or in highly deviated holes.

The 8-curve tool was designed specifically to overcome this limitation by providing two microresistivity curves, 3 cm apart, on each of the four pads. The density of the results is an order of magnitude higher than with previous 4-pad hardware and processing. In addition, the improved sonde velocity correction, using accelerometer data to compute instantaneous sonde speed and length of travel along the borehole, greatly increases the coherence of the results and helps salvage data affected by severe hole conditions.

The processing methods discussed here have been developed to take advantage of the tool improvements. They provide three independent computations of formation dip and allow adaptation of the interpretation of the results to the specific problem of interest (e.g., structural, sedimentary, geometry of the sand body).

Programs for computing dip from 8-curve measurements include the basic interval correlation program, called mean square dip (MSD), which uses all 28 possible cross correlations to compute 28 displacements (if all are successful). Since only two adjacent displacements are needed to define a plane, considerable redundancy has been built into the measurement system. The program thus tries to find a "best fit" plane that satisfies most of the displacements.

A second interval correlation method called continuous side-by-side (CSB) is also used. It only considers displacements computed from the side-by-side buttons on the pad. These four computed displacements represent the apparent angle of the set of bedding planes that cut across the borehole.

Finally, feature correlation is provided by the LOCDIP* computation. These pad-to-pad correlations are made over short intervals centered on bed boundaries, as defined by the major inflection points on the microresistivity curves. This method is used to identify and then correlate major individual curve features. The correlation lines are displayed with the actual microresistivity curves in a way similar to the GEODIP computation and presentation.

Mean Square Dip (MSD) Processing

At any one depth level, there are 28 possible cross correlations for the 8-electrode measurements, as compared to six for the 4-curve recording. As in 4-curve processing, the correlation method for the eight curves requires defining an interval length, a step, and a search angle; however, there is a significant difference in the way the cross correlation is made. In the standard interval correlation program, a specific interval of a reference curve is defined and then slid along the interval of the matched curve. For the 8-curve dipmeter tool, the MSD method considers the same depth interval on each curve and uses only the data within that interval to make correlations. In the case of low apparent dip, nearly all the data points within the interval are considered when the correlation is made. As the apparent dip increases, fewer and fewer points enter into the correlation. A limit is imposed when the search angle is increased until only half the points in the intervals are being used. This corresponds to an apparent dip of about 72° in an 8-in. borehole with a 4-ft correlation interval.

In areas where high dips (or high apparent dips due to deviated hole conditions) are expected, this limitation can be overcome by displacing the curves by a known amount before cross-correlations are attempted. The amount of the curve displacement or shift would be that corresponding to the displacement one would expect if the actual dip plane were the same as the assumed or "focusing" plane. Hence, the net displacement used in the dip computation is the interval shift plus the displacement computed between the curves after the shift. The focusing plane can be chosen as either

• a fixed plane defined by the analyst (default is a horizontal plane), or
  

a plane defined by a previously computed dip

For moderate structural dip computations, experience has shown that the following input parameters are usually satisfactory:

• interval length, typically 4 ft.
  

  step distance, expressed as a percent (usually 50%) of interval length-(e.g., for a 4-ft interval, step distance would be 2 ft)
  

search angle; 45° usually find most dips relative to a horizontal plane

The MSD program, then, is primarily used to determine structural dip by finding strong planar events crossing the borehole. The button-button displacements are computed and the best-fit plane through these displacements is found.

This initial best-fit can then be refined by an iterative process in which points beyond k (which varies from 2.5 to 1.4) standard deviations from this initial best-fit plane are rejected, and a best-fit plane through the remaining points is calculated. An empirical quality factor is assigned to the final best-fit plane. This factor, ranging from 0 to 20, is a function of the number of iterations made and the final number of displacements retained.

There is no vertical continuity logic or clustering routine in the MSD computation; each level is autonomously processed. The redundancy available (28 possible displacements, when two are enough to define a dip) reduces the possibility of producing mathematical dips or noise correlations.

Continuous Side-by-Side (CSB) Processing

Continuous side-by-side (CSB) processing is a unique feature of the 8-curve measurement and takes advantage of the fact that there is great similarity between the two microresistivity curves recorded by each pad since the two measure electrodes are separated by a horizontal spacing of only 3 cm. With side-by-side correlations, CSB processing is able to define formation dip that may not be apparent on pad-to-pad correlation. Even more important, the CSB program is responsive to the fine bedding structure of the formation, making it particularly effective for defining stratigraphic features. This is illustrated in
Figure 1 , where the curves recorded by Pads 2 and 3 are shown for 12 ft of hole. Side-by-side correlations are shown as thin lines, and, for reference, the pad-to-pad correlations found for the same interval are shown as thick lines. From this example, we see that the number of side-by-side correlations is approximately an order of magnitude greater than the pad-to-pad correlations, and that the resolution is on the order of a few inches.

 

Another important feature, due to the proximity of the buttons on the pad, is that the displacements found by side-by-side correlations are much smaller than pad-to-pad displacements. This allows the measurement of very high dips that are not detected by pad-to-pad correlation. For such cases, once credible dips are found by CSB processing, they can be used as input to the focusing option for the MSD program.

Figure 2 shows a conventional pad-to-pad MSD correlation for a case of high apparent dip. The well is deviated about 35° to the southwest, in the same direction as the regional structural trend (30° to 40°). Thus, a given bedding surface would cut the borehole high on the northeast side and low on the southwest side. Obviously, getting a good correlation is difficult, although the quality of the dip curves and the borehole condition is excellent. Figure 3 shows the results obtained with side-by-side CSB processing. In this case, the 3-cm spacing of the buttons allows an unambiguous correlation to be made.

 

In the standard CSB computation, each pair of microresistivity curves (e.g., buttons 1-lA) is cross-correlated using short correlation intervals of 12 in. or less, and under favorable conditions even 4 in. or 3 in. The step distance can be taken equal to half or three-quarters of the correlation interval. This gives a vector parallel to the dip plane. Under ideal conditions (planar beds) another vector is found at the same depth by cross-correlating the microresistivity curves of an adjacent pad (e.g., buttons 2-2A). These two vectors are then used to define a dip plane.

With only four side-by-side correlations, a cross-check is needed to verify that the bed is indeed planar. If it is, then displacements obtained using microresistivity curves from opposite pads (e.g., buttons 1-lA, 3-3A) should be equal in value but opposite in sign, and the dip can be obtained from any two orthogonal pairs at that depth. If this is not the case, however, a window is opened around the level under examination, and the vertical continuity of the displacements a certain number of levels above and below it is checked. The pad showing the best vertical continuity is kept. A similar procedure is then followed for Pads 2 and 4 and, again, the pad showing the best vertical continuity is kept. The orthogonal pair showing the smoothest continuity within the window is used for dip computation.

In order to evaluate the credibility of the dip, a quality value ranging from 0 to 20 is assigned to each dip according to the vertical continuity and the quality of the correlograms at the various levels or depths.

If the environment of deposition produces little contrast between beds or the formation is highly crossbedded with sequences terminating over lateral distances of the same order as the borehole diameter, then pad-to-pad correlation may be difficult or impossible due to curve dissimilarity. CSB provides an excellent solution to this problem.

Correlation intervals as small as 2 in. have been matched with detailed core information, although 6-in. to 1-ft correlation intervals are most commonly used.

Figure 4 shows the detail available from the CSB as compared to visible core features. To make this comparison the CSB was processed with a 6-in. correlation interval and a 2-in. step and then plotted on a scale one-quarter of full size in order to match with the core photographs. Good dip agreement is apparent. Note the low contrast on the dip curves correlating to the fore-sets in the lower one-third of the photo. The truncation visible on the core is also evidenced on the dip plot. Such detail would not be possible with standard pad-to-pad correlation systems.

 

The good likeness of the side-by-side curves is useful in cases of high apparent dip. Under these conditions it becomes difficult to find an unambiguous curve match between the pads. Use of the side-by-side configuration allows reliable measurement of displacements between the curves from the same pad and computed dip values.

LOCDIP Computation

As discussed earlier, inflection points on the microresistivity curves describe geological events in the depositional sequence of the formation. The purpose of the LOCDIP program is to detect the geological events, or boundaries, and where applicable to associate a dip precisely at that boundary independent of dips at other depths. Instead of correlating intervals of curves, it detects features (inflection points) on each curve and attempts to link these around the borehole, in a manner somewhat similar to GEODIP processing. There are, however, some important differences:

· To be retained as a LOCDIP result, an event must be recognized on at least seven of the eight microresistivity curves; GEODIP logic requires only three out of the four curves. Thus, LOCDIP logic is more demanding than GEODIP logic.

· A measurement of the planarity is derived for each of the possible dip planes at any level. The retained value corresponds to the surface that best approximates the set of these planes. By convention, a perfectly planar surface has a planarity of 100.

· Some events are recognized on only a few of the dip curves. In this case, the available correlations are traced across the applicable curves, with an "options" notation of "F" (fracture) or "P/L" (pebble or lens) for single-pad events or two/three-pad events, respectively. These interpretations, however, are not to be considered as certain, but rather as possible.

The processing of the 8-curve data is designed to extract the maximum amount of dip information from the raw curves. A well may present several interpretation problems due to variations in lithology and bedding characteristics. A single computation system may not offer the total solution. It is useful, therefore, to be able to combine the results of several types of computation in one presentation.

 

 

DUALDIP* Presentation

The DUALDIP presentation for the 8-curve dipmeter tool allows results from more than one computation to be combined. Figure 5 is an example of multiple computations on a short section. In the figure, the dips on the left side are side-by-side (CSB) results with a correlation length of 8 in. and a step of 4 in. This produces three dips per foot, or about 10 dips per meter.

 

The tadpoles on the right are of two types. The round-headed tadpoles were computed from pad-to-pad correlations with a correlation interval of 4 ft and a step of 2 ft. This is the MSD computation.

The triangular-headed tadpoles are LOCDIP computations, also known as pad-to-pad feature correlations. These dips usually correspond to the more prominent bed boundaries, and are computed by the earlier mentioned pattern-recognition system. For each LOCDIP computation which used seven or eight of the dip curves, a solid correlation line is drawn on the plot showing exactly where the bed boundary was interpreted. For each of these correlations a local dip is shown. If fewer than seven curves are correlated, then the correlation is shown as a dotted line, but dip is not computed.

This presentation not only gives a visual impression of the frequency of stratification and its planarity and parallelism, but it also allows the user to judge the validity of the correlations. This is of particular value in detailed studies of sedimentary features.

All three systems may not, nor should they necessarily, give the same dip answer. This characteristic can be used to great advantage in interpreting sedimentary features, particularly thin, highly bedded clastics.

In Figure 5 , the two local dips at A and B correspond to the top and bottom of a distinct sedimentary unit. They suggest the boundaries both dip at 1° northerly. All the finer bedding within these boundaries produced CSB or round-headed dips consistently north-northeast between 4° and 10°.

The internal bedding indicates sediment transport direction from south-southwest to north-northeast, with topset and bottomset surfaces approximately 1° northerly. The CSB result is different from that obtained from LOCDIP and MSD processing, whose computation system is restricted to major events, which can be correlated from pad to pad. The CSB logic favors events with some continuity; individual single events are less likely to be computed, particularly where both types are visible within the correlation interval. This tendency for different systems to favor different types of bedding planes has been very useful, particularly in the interpretation of fluvial environments.

Note also that the 4-ft MSD correlation showed the dip at C to be southwest about 90 and consistent over 4 ft. This is easily explained, considering the previous discussion of overlap effects, and it is supported by the LOCDIP computation at that depth. This boundary presents a dominant anomaly to the 4-ft correlation system, and for fine stratigraphy would be misleading by itself. When all bedding features, large and small, are parallel, all systems should give the same answer as at D.

Formation-Imaging Tool

Successful dipmeter interpretation depends greatly upon the accurate evaluation of geological features. The application of the classic dip patterns is a relatively simple matter when geological events such as current bedding or lateral accretion are known. In many complex environments this is a severe problem. A whole core over the zone of importance solves these problems, but whole core availability is the exception rather than the rule. Formation imaging provides a continuous oriented borehole representation that can be used in conjunction with a whole core or, in most cases, by itself to evaluate geological events.

Interpretation The goal of formation-image interpretation is to characterize formation properties to assist sedimentological interpretation, determine the presence of permeability paths and permeability barriers, help calculate net pay, plan perforation and fracturing, and to help decide whipstocks and where to drill next.

Formation images must always be interpreted after lithology has been fairly well defined, so supplemental data are usually necessary to enhance the confidence of image interpretation. As with other dipmeter interpretations, the more supplemental data available, the better the interpretation.

Measurement The clustered microresistivity buttons on two or four of the microscanner pads provide a continuous electrical image of the borehole wall. The pads are oriented at right angles to achieve a three-dimensional perspective. These resistivity data are then mapped to a gray-scale or color "corelike" borehole wall image. This allows fine-scale features to be described through essentially the same interpretation procedure as that used in the examination of slabbed cores. The images characterize many types of structural and stratigraphic features. These oriented features, combined with a conventional dipmeter plot, are used to evaluate these events to extend the reservoir geometry beyond the wellbore.

Images of the rock formation exposed by the wellbore are processed from the microresistivity traces. Each image pad covers 2.8 in. of the borehole wall. Thus, 22% of an 8-in. borehole can be imaged with two pads and 44% with four pads on each logging pass. This coverage can be increased with multiple logging passes. The tool also contains a triaxial accelerometer and three magnetometers for orientation and to enable speed corrections to be made on the acquired data.

Presentation of Images

Several presentations are available for displaying the data. The vertical scale provides the most striking difference between the formation-imaging presentations and other logs. The normal detail scale for logs is 1:240, while the formation images are presented on a 1:5 scale. The standard presentations can be broadly classified into two types: straight-line images and azimuthal images.

Straight-Line Images A straight-line presentation shows the images in a stationary horizontal scale ( Figure 1 ). This presentation is divided into several sections. The left section contains the depth scale, the pad orientation, and the borehole deviation. The long arrow on the tadpole indicates the direction of borehole drift; the body of the tadpole indicates the magnitude of deviation by its position on the horizontal scale. The small arrow shows the azimuth of Pad 1. The next section contains the caliper and resistivity correlation curves. The calipers from Pads 1-3 and 2-4 are shown. The resistivity curve is used only for correlation and not for quantitative purposes. Pads 3 and 4 of the 2-pad tool provide the image. Both the raw microresistivity traces and the processed images are presented. The microresistivity traces are from the 27 image buttons. The image traces are computer enhanced using 16 gray levels; they range from white (resistive) to black (conductive).

 

 

Another popular presentation is shown in Figure 2 . In this example, the formation images are displayed on the same depth scale as the dipmeter log. This scale is not as effective for identifying individual sedimentary features but is better for displaying the overall features of a zone and showing how they relate to dip patterns.

 

Azimuthal Images A BORMAP presentation is shown in Figure 3 . The horizontal scale shifts according to the respective azimuths of each pad. Thus, multiple passes can be merged to portray a more complete picture of the wellbore. In this example, images from two logging passes (from a tool with two imaging pads) were merged to cover approximately 44% of the well-bore. There are vugs present at 4208.7 ft and at 4210.4 ft. This presentation is very effective for secondary porosity evaluation and for sedimentary structure identification.

 

Image-Examiner Workstation

Image interpretation can be enhanced by means of a computer workstation equipped with image-examiner processing programs. This allows such interactive processing features as

scale changes of both the vertical and horizontal, to enhance the interpretation

a display of other logs for correlation on the same scales

graphic enhancement of specific features, such as bedding, texture, vugs, and fractures

dip computation of bedding surfaces, fault planes, and fractures

correlation of images to whole core sections, extending the interpretation to noncored sections

orientation of cores from features present in both the core and the formation images

quantification of images (such as sand count and calibration to core porosity) to increase interpretation accuracy

Dip Computation/Thin Bed Definition Computation of the dip magnitude and azimuth of specific beds is essential to many interpretations and can be performed on an image-examiner workstation. An example is shown in Figure 4 . The magnitude is measured from horizontal (0°) to vertical (90°). The azimuth of the downdip direction is measured from true north. The thin sand shown at 6969 ft dips to the northwest. A sine wave is fit through both the upper and lower surface of the sand, indicating a 39° dip magnitude and an azimuth of 317°.
These dips are "true dip", since hole deviation is compensated. Apparent dips may be presented if a direct comparison with a whole core is required The actual thickness of the sand stringer, measured be-the sine waves, is 1.61 ft.

 

»»  read more

Dipmeter Surveys (Depositional Interpretation)

Depositional Interpretation

Eolian Environment

Introduction

A dune is a hill of sand, deposited by wind, that rises to a single summit and possesses a slip face. Dunes may be various sizes and shapes depending on wind conditions, sand type, and sand supply. Dunes may be oriented perpendicular to the prevailing wind (e.g., barchan and transverse dunes), parallel to the prevailing wind (e.g., seif or longitudinal dunes), or they may acquire complex formations (e.g., dome-shaped or star-shaped dunes).

Dunes are the most impressive and important feature of a desert environment. They are also important geologically. The Nugget formation of the western United States, the Norphlet formation of the U.S. Gulf Coast, and the European Rotliegendes formation form important hydrocarbon reservoirs. The eolian Botucatu of Brazil is a large freshwater aquifer.

Much of the following information is based on the work of Reineck and Singh. (See Depositional Sedimentary Environments, 1980, New York: Springer-Verlag.) Figure 1 illustrates some typical dip patterns in eolian environments.

 

Parabolic Dunes

Parabolic dunes are U-shaped sand ridges with their concave side toward the wind. Parabolic dunes are associated with blow-up features. The middle part of the parabolic dune moves forward ahead of the arms, which are believed to be hindered by vegetation.

The characteristic dip pattern of parabolic dunes is a red pattern at the center of the dune dipping in the direction of the prevailing wind ( Figure 1 ).

The dips found near the tips of the arms may be skewed more than 90° from the direction of the prevailing wind.

Foreset laminae of parabolic dunes are low-angled relative to other dune types. The foreset laminae are characteristically concave-downward as a result of slip-face shape and the presence of vegetation. The azimuth spread of the dip of foreset laminae is rather large-up to 200°.

Barchan Dunes

Barchan dunes are crescent-shaped sand mounds occurring as isolated bodies, in chains, or in colonies of individual dunes coalescing into complex forms. Barchan dunes are formed by a unidirectional wind, and they migrate by sand avalanching on the slip face. The extremities or horns of a barchan extend forward and downwind, as the horns migrate more rapidly than the main body.

Simple barchans may be made complex by the coalescence of many sand dunes. In regions where the wind blows periodically from directions other than that of the prevailing wind, small, oblique slip faces may be produced, but the general dune form and direction of movement are retained.

When interpreting eolian zones, note that the structural dip is at the left edge of the tadpole cloud unless the log is from an area that has undergone appreciable structural uplift. The general dip direction is in the direction of the prevailing wind. Near the horns, dips are less and may be almost 90° to the direction of the prevailing wind. The zones of crossbedding dips with constant magnitude near the center of barchan dunes reflect the angle of repose during deposition.

The angle-of-repose zones are underlain by fore set-generated blue dip patterns. The minimum dip found at the base of these blue patterns reflects the dip of interdunal layers and approximates structural dip. The foreset laminae of crossbedded units in barchan dunes are mainly planar (tabular types with a dip from 20 to 35° in the central part).

Figure 2 is a dipmeter log through the eolian Rotliegendes sand in Holland. The dip patterns are typical for barchan-type dunes.

 

 

Dome-Shaped Dunes

Dome-shaped dunes are low, circular sand ridges lacking a well-developed, downwind, steep slip face. Dome-shaped dunes develop when dune height is checked by a strong, unobstructed wind. The characteristic internal structure displays low-angled foreset laminae.

Dome-like dunes produce red dip patterns similar to parabolic dunes. The central portion of the dome contains dip in the direction of the prevailing wind. The dip left and right of the center may be skewed as much as 75° to the prevailing wind.

Transverse Dunes

Transverse dunes are elongate, almost straight sand ridges perpendicular to the predominant wind direction. These ridges are regularly spaced and are separated by broad interdune areas that may have developed as inland sabkhas.

Transverse dunes originate in areas of inland sabkhas, where the damp sabkha surface inhibits the growth of barchan horns. When the interdune sabkhas eventually disappear, a sand sea with transverse dunes may be produced. Dip patterns produced from transverse dunes are similar to the patterns found near the center of barchan dunes; the patterns consist of zones of angle-of-repose dips underlain by foreset-generated blue patterns.

Crossbedded units are mostly of the planar-tubular type. The foreset laminae are relatively long, even, and high-angled. The azimuth spread of foreset laminae dip is probably less than that of all other types of sand dunes, with one well-developed maxima in the direction of the prevailing wind.

Longitudinal Dunes

Seif or longitudinal dunes are elongate, continuous, serrated, straight sand ridges. Their long axes parallel the prevailing wind direction. Several seif dunes commonly occur as a series of long parallel ridges separated by broad interdune areas.

Sand is deposited alternately on opposite sides of the sand dunes. Crossbedding dips are normal to the elongation of the sand ridge; therefore, the two maxima of high-angle foresets are almost l80° apart. Locally, some low-angle bedding is present, especially in the lower part of a seif dune.

It has been suggested that the most important factor in generating seif dunes is the existence of a strong wind with a uniform direction. The higher the wind velocity, the larger the seif dune and the greater the interdune spacing. All other conditions being equal, barchan dunes develop at lower wind velocities than seif dunes.

Seif dunes may be modified to barchan dunes if wind velocities are not strong enough to maintain the seif dune form. The depositional pattern in seif dunes produces red and blue dip patterns with an azimuth normal to the prevailing wind.

Occasionally an azimuth reversal occurs within the blue patterns.

Whalebacks

Whalebacks are large-scale features associated with seif dunes. They are platforms of rather coarse-grained sediments left by the passage of a series of seif dunes along the same path.

The platform and the sides are composed of horizontally bedded sediments with crossbedded seif dune sediments below.

Wadis

Wadis are predominantly dry desert streambeds that are only active following sporadic, but often heavy, rains. Wadis are better developed near hills where the rainfall is slightly higher. Wadis are characterized by sporadic and abrupt fluvial activity and by a low water-to-sediment ratio. Deposition by flash floods is very rapid because of the sudden loss of velocity as the water is absorbed underground. Most wadis diverge downslope and deposit the bulk of their sediment in fan-shaped bodies at the downstream limit of the flow.

Wadi channels are not permanent, and they may be filled by their own detritus or by wind-blown sediments. During subsequent seasons, a new channel system is likely to cut into the older sequences. Wadi channels produce dip patterns similar to those found in braided streams.

Small ripples, megaripples, and plane beds are the bedforms developed in wadi channels by the variable flow conditions. Deposits within the wadi channels may be conglomeratic and fanglomeratic. During certain phases of flow, the sediment transported through the wadi may resemble mud flows. The nature of the sediment is strongly controlled by source rocks and the availability of various grain sizes. Wadi deposits may lack pebbles and may contain only well-sorted sand. The deposits produce ripple and horizontal beddings.

Desert Basins

Desert basins represent areas of inland drainage with water flowing towards the center. Basins are often low depressions resulting from deflation of tectonic origin. Water accumulates in these low-lying areas, producing shallow, ephemeral lakes. The larger examples may be semipermanent desert lakes.

Inland sabkhas are formed when sediments are subjected to wetting by inflowing wadis or ground seepage, subsequent drying, and deposition of damp, salt-encrusted sediments. In deflation hollows, where the water table is higher than the ground surface, a small lake may develop. Sand dunes may be drowned and preserved as a consequence of a rising water table caused by seepage and inflowing water.

Abundant detrital sediment is brought to desert lakes and inland sabkhas during floods. As current velocity is almost nil, the deposition of silt and clay occurs from suspension, and individual thin beds may contain graded bedding. Gypsum, halite, and other evaporite minerals are commonly associated with these deposits. The uppermost clay layers may crack and curl during dry seasons, and these features may be preserved if covered by blown sand.

Detrital sediment is rarely deposited in lakes resulting from groundwater seepage; instead, salt pans are built. Some windblown detrital sediment may be incorporated as thin layers or impurities within the chemical precipitates.

Sediments of inland sabkhas are usually parallel-bedded with silty and clay-rich layers alternating with thin, sandy, gypsum or gypsiferous clay layers. These sediments are deposited as inflowing wadi sediments settle from suspension, or as wind-blown sediments are captured by adhesion ripples on the sabkha surface. Bedding is better developed in desert lake sediments than in inland sabkha sediments. Sabkha sediments sometimes generate only blank zones on the dipmeter plot.

Deltaic Environment

Introduction

Only a small percentage of modern coastlines are delta-dominated at any given time. Most coastlines are located in interdistributary environments where sediments deposited by older deltas are undergoing reworking and redeposition.

Deltas are constructed where rivers enter the sea. Where long-shore currents are weak and abundant sediments are available, deltas prograde seaward, forming elongate or birdfoot deltas. The modern Mississippi Delta is a classic example of a birdfoot delta. Strong longshore currents prevent or retard seaward progradation, and the resulting deltas form cuspate-arcuate shapes.

Deltas discharge seaward through active distributaries. Fan, crescent, or elongate sand bodies called distributary mouth bars or distributary front sands are deposited seaward of the mouth of each distributary. These and the following features are illustrated in Figure 1 .

During periods of high water, breaks occur in the natural levees formed along the distributary channel margins. Discharge through breaks or crevasses in the natural levees forms crevasse splays. Crevasse splays have the same shapes as distributary mouth bars. As the distributary channel progrades, bodies of water between distributary channels are constrained by sedimentary deposition into interdistributary embayments.

 

Figure 2 is an example of dipmeter plots from a deltaic environment and a tide/wave-dominated environment. Distributary front deposition at rates of tens of feet of sediment per year exist. The associated rapid burial and subsidence appear important in sediment preservation because they prevent reworking of the sediments by waves, tides, and currents.

 

Identifying the Environment

It is possible to confuse a thin eolian sand section with a deltaic sand; therefore, recognizing a fossil delta depends in part on local knowledge that the sediments under investigation were deposited in a marine environment. If it is known that deltaic conditions existed during the deposition of a zone of interest, log character can be used to determine the probability of preserved deltaic sediments.

A strong family of mostly blue dip patterns is a good indicator. The blue patterns would be intermixed with a few red patterns with azimuths 90° from the blue patterns. Funnel-shaped SP and gamma ray curves are indicative of preserved deltaic sediments; however, a funnel shape alone does not identify a deltaic environment. Shale resistivities may provide clues on a strictly local basis to indicate that the zone of interest was deposited in a deltaic environment.

Identifying Deltaic Features

Once it has been determined that a zone of interest was deposited in a deltaic environment, the data should be compared to a generalized deltaic model. If the entire deltaic system was preserved, which is unlikely, the system would consist of the following:

• distributary channels
  

distributary mouth bars

crevasse splays

longshore current sand waves

marshes

These features become the pieces of the jigsaw puzzle you wish to solve. In the worst-case scenario, the entire delta would have been reworked, and all of the pieces would be missing. Usually, however, several of the pieces are present. They may be from adjacent parts of the puzzle, or they may fit randomly into the model with no adjacent pieces.

There are several facts to help solve the puzzle. If the zone of interest was deposited during a deltaic period of deposition, strong dip patterns can be assigned a deltaic, rather than a reworked, origin. Also, the location of land during the time of deposition is known, at least approximately. Logs are responding to only fragments of each deltaic feature, not the entire system.

Distributary Channels

If a complete depositional sequence were preserved, the dip patterns on the following illustrations would be seen. Figure 3 shows the expected dip patterns within a distributary channel. When the channel is penetrated at or near its axis, only blue dip patterns, indicating flow down the channel, are recorded. The south-dipping blue pattern on this figure indicates flow down a north-south striking channel.

When the channel is penetrated near the edge, only red dip patterns, dipping toward the channel axis, are found. Current velocities are lower near the channel edge; therefore, only laminar deposition occurs.

Between the two zones, a red-blue dip pattern combination is usually found. The basal layer of fill mimics the dip of the surface it is deposited on; therefore, the drape over the sloping surface of the channel cut creates a red pattern dipping toward the channel axis. This red pattern (or patterns) is overlain by blue patterns with a dip azimuth 90° to the underlying red pattern. These blue patterns result from flow down the channel. Foreset beds deposited by sediments transported down the channel are formed after the basal portion of the channel is filled and leveled.

 

Distributary Mouth Bars

Within the distributary mouth bar seaward of the distributary channel mouth, only blue dip patterns would be recorded ( Figure 4 ). These patterns indicate the direction of sediment transport.

 

The dip magnitude spread of the family of blue patterns is an indicator of the type of depositional environment and the probable sand geometry. If the magnitude spread of the family of dips is greater than 10°, the sand was probably deposited in an inertia-dominated environment, and the shape of the distributary mouth bar is probably elongate. If the family magnitude spread is 10° or less, the environment was friction-dominated, and the shape of the distributary mouth bar is probably fanlike or crescent.

The subsurface deltaic sediments usually consist of a stack of fossil delta remnants rather than sediments deposited by a single active delta. Dips belonging to patterns measured in the subsurface tend to be steeper than their original depositional angles. This steepening is probably the result of compaction.

Discharge Direction

The discharge directions of a delta are not always directly seaward. Some active distributaries of the modern Mississippi Delta discharge to the north-northeast-not to the south-southeast, which is the main direction of progradation.

Figure 5 is an example of a preserved distributary mouth bar from the East Cameron Block 270 field. The distributary prograded to the northeast, a direction similar to the Main Pass distributary of the modern Mississippi Delta. Deltas may prograde almost across the continental shelf, as has the Mississippi Delta.

 

Distributary Channels and Distributary Mouth Bars

Dip meters run in wells that penetrate both distributary channels and distributary mouth bars create the patterns shown in Figure 6 .

 

Location A: A red pattern resulting from drape over an east-west striking channel overlies a blue pattern generated by distributary mouth bar sands transported from west to east.

Location B: The deepest blue pattern indicates distributary mouth bar sands. The overlying red pattern indicates drape over the base of the distributary channel. The shallowest blue indicates flow down the channel.

Location C: There is no red pattern indicative of drape at this location. Relative to the information from the other wells, it is possible to identify this as the channel axis. The underlying blue pattern indicates a distributary mouth bar sand. The overlying blue pattern indicates flow down the channel.

This sequence is repeated on the opposite side of the channel, with red patterns dipping to the north (Locations D and E).

Cuspate-Arcuate Deltas

Rivers create cuspate-arcuate deltas by discharging their fresh water and sediments seaward, but the strong longshore currents transport the marine sediments in the direction of current flow, subparallel to the fossil coast-line. Figure 7 is an example of a cuspate-arcuate delta from the Bekapai field, Mahakam Delta, Kalimantan. Strong longshore currents transported sediments that were carried to the sea by the ancestral Mahakam to the southwest. The dominant dip is southwesterly dipping blue patterns.

 

 

Creation and Destruction of a Delta

The sediments deposited at a river mouth create increasing resistance to flow. Eventually, the river follows the path of least resistance and changes course. When the sediment supply to the delta is eliminated, deposition ceases, and destruction begins. Deltaic sediments exposed on land and the seafloor are attacked by rains, waves, currents, and tides. These destructive forces remove, re-sort, retransport, and redeposit the previously deposited sands and clays in new forms.

The amount of a fossil delta that is preserved depends on many variables: the depth of subsidence, the period of deltaic deposition, the thickness of the deltaic column, and the amount of protection from the open sea. One estimate by a knowledgeable geologist, Dr. John Kraft, estimates a worst-case preservation rate of less than one percent. The remaining 99% may be transported by waves, tides, and currents to be redeposited in an interdeltaic environment. Figure 8 illustrates the dipmeter response in zones where some of the original bedding planes were destroyed by reworking.

 

Interdeltaic Environment

Introduction

Many sediments deposited in an interdistributary environment by waves, tides, and currents were originally deposited within deltaic environments. Later reworking provided the raw materials for the interdeltaic deposition.

Dipmeter logs that were run through sediments deposited in interdeltaic environments tend to look rather sparse. They contain blank zones resulting from bioturbation and rooting, and open tadpoles from low-quality correlations.

As is true in other marine environments, the direction to land during the time of deposition is a key direction ( Figure 1 ). This information allows tentative identification of landwarddipping foresets deposited within tidal flood deltas, washover fans, and slipface deposits.

Other transport directions indicated by blue patterns are seaward-dipping ebb delta sands and sand waves deposited by longshore currents, paralleling the coast.

Beach sands dip seaward on their front portions and landward on their slip-face portions. Tidal flat sediments exhibit blue patterns dipping in opposing directions as a result of landward- and seaward-dipping foreset beds. Tidal channels in microtidal and mesotidal ranges generate red patterns dipping toward their axes.

 

In a microtidal range (less than 2 m), any ebb delta present would be small ( Figure 2 ). In microtidal environments, tidal inlets with pronounced flood deltas on their landward side exist. In a mesotidal range (between 2 and 4 m), a prominent ebb delta would be formed. In a macrotidal range (more than 4 m), a tidal estuary would be formed. Macrotidal estuaries contain sand bodies elongate in the directions of tidal flow.

 

 

Ebb Delta

In the ebb delta shown in Figure 3 , it is assumed that land is to the west and the coastline strike is north-south. A dipmeter log run at location A, in the southern portion of the ebb delta, would contain southeasterly dip, not directly seaward in an easterly direction. A dipmeter log run from a well drilled at location B would exhibit seaward or east dips. A well drilled at location C would penetrate both the marginal flood channel and the underlying ebb-deposited sediments. Foresets dipping back into the tidal channel were deposited on the flood tide; as a result, they dip to the southwest. Beds deposited during the ebb dip to the northeast.

Tidal Channel

Tidal channel dip patterns resemble patterns from other channel types. The basal layer of channel fill mimics the dip of the surface on which it is deposited. The channel base is a sloping surface except at the axis; therefore, red dip patterns are created with azimuths toward the channel axis and normal to the channel axis strike.

After the fill smoothes and levels the channel base, foreset beds with dip along the channel are deposited. The type of deposition preserved-flood or ebb-depends on the location within the channel.

Sand waves formed within tidal channels may contain a large amount of shell hash. If preserved and buried, these waves would generate seaward-dipping blue dip patterns.

In Figure 4 , the well location B is at or near the channel axis. Only sand-wave foresets would be deposited, because of the relatively flat underlying surface.

 

Flood Deltas

Deposition within flood deltas occurs as landward-dipping foresets. Similar landward-dipping foresets are found within washover fans and slipface sands.

 

Dipmeter logs run at locations A, B, and C on Figure 5 contain blue dip patterns dipping to the northwest, to the west, and to the southwest, respectively. Particular caution must be exercise in the interpretation of dipmeter logs run through sediments deposited in a tidal environment. The most significant dips are de rived from sediments deposited within the flood delta, the ebb delta, and the tidal inlet.

Swash Bar and Recurved Spit

Two groups of dips that may produce conflicting interpretations are swash bars and recurved spits. Swash bar dips create land-ward-dipping blue patterns, which can be mistaken for flood delta dips. This can lead to an offset seaward of the terminal lobe. The best approach to identifying swash bar deposits is to expect them to be preserved near the tops of tidal sands or carbonates; therefore, beware of blue patterns existing only in the top of a tidal sequence. Landward-dipping blue patterns from flood deltas should extend throughout most of the sand or carbonate under study.

Recurved spit dips are the other set of problem blue dip patterns. They tend to dip away from the inlet and can contribute to offsets in the wrong direction. These dips can be recognized by their dip in the direction of coastline strike.

Longshore Current Sand Waves

Longshore current sand waves are composed of fore set beds that generate blue dip patterns paralleling the fossil coastline. Deep water contains longshore currents strong enough to redistribute sands previously deposited by turbidity flows.

To identify sand waves, one must (1) know that the sequence being interpreted is from an interdeltaic depositional environment and

(2) determine the direction to land during the time of deposition.

Beach Sands

Shoreface Sands

Shoreface sands were deposited between the beach and a water depth of 20 m, the fair-weather wavebase. These sands were deposited in a high-energy environment, and few, if any, of the bedding planes were deposited flat. After deposition, bioturbation occurred, destroying or distorting the original bedding.

Dipmeter logs run through shore face sands record a few widely scattered dips and blank zones because of bioturbation. Bioturbation decreases in the shallowest portion of the shore face zone; therefore, more dips are recorded as the mean low water line is approached. Beginning at depths of about 5 m, some low-angle, seaward-dipping crossbeds were deposited and preserved. These beds initially dip seaward 1° or 2°. Flaser bedding is also present in the lower shoreface zone.

Beachface Sands

Beachface sands were deposited as parallel crossbeds dipping seaward plus or minus 5°.

Runnel

Deposition within a runnel may appear as megaripples dipping parallel to the beach, small ripples, or laminations. Preserved megaripples generate small blue dip patterns best identified by CSB computation. Small ripples usually create blank zones or false correlations; however, they can be identified on the multisensor dipmeter output of the 8-curve tool.

Berm Crest

Deposition on the berm crest is essentially horizontal. If preserved, the beds would indicate structural dip. Dunes, which also form on the berm crest, contain festoon cross-bedding, which generates a wide dip scatter.

Back Beach

If the back beach escaped bioturbation by fiddler crabs or their ancestors, it would contain landwarddipping foresets that generate blue dip patterns. These landwarddipping patterns are the best indicators for determining the strike of a fossil beach.

Washover Fans

Washover fans generate landward-dipping blue patterns similar to slip face foresets and flood delta foresets. The character of other log responses provides clues for distinguishing these features.

Washover fans were deposited over marsh deposits by a catastrophic event. This process did not allow for appreciable sorting. Flood deltas were deposited in a subaqueous environment with winnowing before final deposition.

Slipface sands were deposited on a land surface containing some plant material; this, in turn, created a rooted layer. The rooted layer generates blank dip zones and is electrically more homogeneous than undisturbed bedding.

Barlike or Convex-Upward Sands

Barlike or convex-upward sands may be formed at the wave break point or as beach ridges. There is one distinct difference between these two types of sand Break point-bar sands are winnowed until there is little internal electrical contrast; therefore, dipmeter logs exhibit mostly blank zones. In contrast, beach ridges exhibit many internal dips ( Figure 6 ).

 

When either type of sand is penetrated on the flanks, the drape the overlying beds creates a red dip pattern just above the sand A fault can create the same dip pattern; therefore, faulting must be ruled out before any stratigraphic interpretation is attempted The direction of the red pattern is toward the shaleout and normal to the strike of the bar or beach ridge. If the bar or ridge is penetrated at or near the crest, no drape would be present, and the sand would appear blanket-like.

These same guidelines can be applied to oolitic bars. The drape extends beyond the limits of barlike sands. A red pattern in the silty zone, where a bar should have been located, dips away from the bar, and it can be used to determine the direction of sidetrack.

In some cases only blue patterns dipping toward the shaleout are found above a bar ( Figure 7 ). These patterns tend to be components of a very subtle red dip pattern, and may be partially related to slump of the clays deposited above the bar.

 

Deep Water Environment

Introduction

This chapter addresses the processes of deposition and the resulting dip patterns encountered in deepwater environments. The processes of mass transportation are able to move, transport, and lay down sediments between their zone of origin and a topographically lower zone under the influence of gravity. Generally, these mechanisms provide intermittent and catastrophic transfers of large amounts of sediments, which are deposited at or near the base of a slope.

 

Mass transport consists of rockfalls, slides and slumps, and gravity flows ( Figure 1 ).

Rockfalls

Rockfalls are formed by free-falling bodies of sediments accumulating at the bases of fault scarps, canyon floors, and other steep slopes. The deposited sediments generally exhibit distinct limits, but no bedding.

The dimensions of clasts that form rock falls vary from sand-size to blocks measuring several tens of meters. The clasts are in contact and generally contain intergranular porosity.

The sequences resulting from submarine rock falls are often related to forereef escarpments or platform edges. On slopes in deep-sea environments, rockfalls may contain abyssal sediments. The accumulation of sediment blocks caused by rock slides can only occur at the foot of strongly inclined slopes, which are often characteristic of carbonate margins.

Slides and Slumps

Subaqueous slides occur when a mass of semiconsolidated sediments moves along a basal shear surface. These slides are able to transfer considerable masses (up to tens of cubic kilometers) of sedimentary materials from the inner or outer continental platform to the abyssal plain. Any internal bedding characteristics of the mass are preserved during movement. Slides can be divided into translational or glide and rotational or slump types. The basal shear surface of a glide is a plane of slightly undulating surface paralleling the stratification.

In a slump, the concave shear surface permits rotation of the slump block. As a slump block moves down a slope, compression occurs at the foot of the block, and tension occurs at its rear. Compression produces thrusts and folds, and tension produces normal faults and open cracks. The central part of the block is generally not deformed.

Slump blocks penetrated in the subsurface are not always easy to identify. They may appear on the dipmeter as an isolated trend. When this occurs, the most probable explanation is either a tilted fault block (most commonly found between two nearby faults) or a slump block. If faulting can be ruled out, then the slump block becomes the most probable explanation.

 

 

 

Gravity Flow

Sediment gravity flow is a general term for flows of mixed sediments and fluids in which the bedding coherence is destroyed and the individual grains move in a fluid medium. This includes mud flows or debris flows, grain flows, liquefied flows, and turbidity flows.

Mud flows exhibit essentially plastic behavior with the muddy carrier phase creating sediment coherence. The matrix containing the clasts is the main driving and lubricating force behind the flow.

The dynamics of grain flows are governed by the reciprocal interaction of clasts. This granular interaction causes sandy flows to exhibit plastic mechanical behavior rather than fluid behavior. In contrast, liquefied flows, fluidized flows, and turbidity currents exhibit a fluid behavior. Grain flows consist of cohesion-less sediment supported by dispersive pressure. This process requires steep slopes for initiation and sustained downs lope movement.

Liquefied flows consist of cohesionless sediment supported by upward displacement of fluid as loosely packed structures collapse. The sediments settle into tightly packed textures. Liquefied flows require slopes of greater than 3°.

Fluidized flows consist of cohesionless sediment supported by upward motion of escaping pore fluid. These flows are thin and short-lived.

Turbidity current flows contain clasts supported by fluid turbulence. These flows can move long distances on low-angle slopes.

 

Submarine Channel-Fan Complex

Figure 2 illustrates the features found during the growth of submarine fans. Of these features, the obvious deepwater features interpretable by dipmeter logs are debris flows, which result in blank zones; feeder channels, which produce typical red dip patterns at the base and blue patterns with a 90° azimuth difference above; and midfans, which generate blue dip patterns. Outer fan sediments generate structural dips.

 

A submarine channel-fan complex can exhibit the same features as a delta complex, including natural levees. Submarine feeder channels are cut by downs lope sediment flows and later filled ( Figure 3 ). As with other types of channels, the basal layers of fill mimic the dip of the underlying surface. Deposition on a sloping surface produces a red pattern dipping toward the channel axis and normal with the channel strike. After the bottom was filled and leveled, foresets dipping down the channel were deposited; these, in turn, generated blue patterns dipping 90° from the underlying red patterns.

In the midfan portion of the system, only blue patterns dipping in the direction of sediment transport are detected. Few obvious foreset beds are found within midfan outcrops, and this raises the question of what the dipmeter tools are measuring. It is possible the dipmeter sensor is detecting some type of permeability change associated with timelines or climbing ripples. Permeability changes do not always have a visual representation and may appear only on X-ray photographs.

In the outer fan portion of the system, only structural dips are detected because deposition was essentially horizontal. This is an environment in which the deposition of alternating laminations of sand and shale may become low-resistivity pay zones.

Transport Directions

A common feature of deepwater sands is that transport directions are not directly offshore. Some sediments were transported parallel to the continental shelf while others were transported back into land ( Figure 4 ).

Landward transport can be a function of seafloor topography or it can be initiated by the presence of a down-to-the-basin growth fault. Whatever the cause, inshore transport in deepwater depositional environments does occur.

 

Deepwater Longshore Currents

In some areas, considerable numbers of blue dip patterns indicate sediment transport parallel to the slope. This is a result of the reworking of previously deposited sediments by deepwater longshore currents. This is another environment conducive to the deposition of alternating sand-shale laminations.

Submarine Canyons

Submarine canyons exhibit alternating up and down canyon sets of blue dip patterns generated by deepwater tidal action ( Figure 5 ).

 

Submarine canyon fill sands closely resemble tidal sands on the dipmeter plot. The fill sands contain many blue patterns dipping both up and down the canyon. These patterns were probably generated by deepwater tidal action within the canyon. Canyon fill sands may be up to a thousand feet or more in thickness. They may also exhibit indicators of compaction underneath-e.g., downward-decreasing resistivity or increasing interval transit time gradients.

 

Turbidity Flows

Turbidity flows produce complex sand packages containing multiple depositional units. Separate reservoirs may be present, though sand-to-sand contact seems probable. Figure 6 illustrates dip patterns encountered in sand packages produced by turbidity flows. This package is made up of at least six submarine fans, two scour channels, and a sediment layer deposited in the upper portion of a submarine feeder channel. In other areas, this portion of the channel is filled with conglomerate. Shale layers are not required to separate one reservoir from another; an inch or so of silt suffices.

 

Debris Flows

Debris flows are best recognized by the dual-dip curves themselves, since few (if any) meaningful dips are produced. Some correlations not extending around the four pads are seen on the presentation, but no tadpoles are produced. Conglomerates can produce these features.

Feeder Channels

The depositional environment of the sand at 6400 ft in High Island Block 560-561 is a continental slope environment; therefore, feeder channels would be the most probable feature ( Figure 7 ). The expected dip model would be a red dip pattern at the base of the sand section, with blue patterns above. The red pattern azimuth is toward the channel axis and normal to its strike. The blue dip patterns indicate flow down the channel. The azimuth of the blue patterns is approximately 90° from the azimuth of the red patterns.

 

The dipmeter log on Well 4 of High Island Block 561 exhibits the expected dip patterns for a filled feeder channel. The basal red pattern dips to the northwest, which is the direction of the channel axis. The overlying blue patterns dip to the southwest, which indicates flow down the channel from northeast to southwest.

The lower portion of the example shows the same dip pattern combination from the sand at 8900 ft. In this example the red pattern dips to the north; therefore, the channel axis lies north of the well, and the channel strike is west to east. The east-dipping blue pattern indicates sediment transport down the channel from west to east.

The relative magnitudes of the dip patterns in these examples indicate their approximate positions within their respective channels. The sand example at 6400 ft contains several blue patterns, but only one red pattern; this indicates a position near the channel axis, where the blue patterns dominate. Had the location been nearer the channel axis, only blue dip patterns would have been present.

The thin 8900-ft sand contains a strong red pattern and a weak blue pattern. This indicates a position near the edge of the channel, where current velocities were lower and drape over the underlying surface was the dominant type of deposition. Transport in this channel was from west to east paralleling the fossil coastline. This orientation may result from flow parallel to a down-to-the-south growth fault system in the area.

Dip Scatter as an Environment Indicator

Dip scatter results from both the original attitude of deposition and postdepositional deformation. The products of both processes are diagnostic depositional indicators. The following comments on scatter are confined to a marine environment.

The marine ecological zones, their defining depths, and their location on the continental shelf and slope are illustrated in Figure 8 . The original concept of less scatter on the lower continental slope was due to a lack of paleo-calibrated dipmeter logs run through lower slope sediments. Later observations of dipmeter logs run in paleo-identified lower slope sediments confirmed that some of the greatest sediment jumbles exist at the base of the continental slope. Dip scatter is best used with shale resistivities, density-neutron responses, and other indicators.

 

When deltaic deposition is preserved in its original form, it can mask effects of the surrounding depositional environment. Indicators are more obvious in a tide/wave-dominated environment than in a delta-dominated environment ( Figure 9 , Figure 10 , Figure 11 ).

 

 

Inner neritic deposition in a tide/wave-dominated environment generates a 40° dip scatter and blank zones. Some scatter results from a high initial angle of deposition, but much of it is the result of bioturbation. Bioturbation produces zones of no correlation or zones where miscorrelations are probable. Near the 20-m boundary between the inner and middle neritic zones, the amount of bioturbation and the corresponding dip scatter decrease.

The scatter across the middle neritic zone ranges from 20° on the shoreward side to 3° on the seaward side. Local experience allows additional subdivision of the middle neritic zone into 50- to 100-ft, 100- to 200-ft, and 200- to 300-ft ranges.

Dip scatter in outer neritic sediments ranges from none, where parallel laminations exist, to 2°. Sediment spreading by long-shore currents in this zone can produce laminated, low-resistivity pay zones.

Continental Slope Sediments

Dipmeter logs run through continental slope sediments tend to be difficult to interpret because of postdepositional deformation by downslope creep, slump, and fracturing. Shales, in particular, may be so severely deformed that few meaningful dips can be computed. Sometimes the only intact bedding planes are found within sands; when this occurs, the sand dips are used for determining structure.

Shale resistivities can provide clues to the proximity of sand bodies associated with shorelines or deltaic depositions. Shale resistivities are partially a function of grain size; therefore, the presence of silt-sized particles increases the resistivity values. Assuming a model progressing from sand to silt to clay, the presence of increased silt creates higher shale resistivity values.

In the northern Gulf of Mexico, shale resistivities less than 0.8 ohm-m usually indicate deposition in a slope or abyssal depth range. There are, however, exceptions to this general rule.

Dip scatter of 60° on the continental slope results from postdepositional deformation. The scattered dips result from deformation; they are not related to structural or stratigraphic dips. The dip scatter again decreases to a maximum of 2° in the abyssal range. Some sediment transport by deepwater longshore currents also occurs in this environment.

Sea Level Fluctuations

A Pleistocene example illustrates changes in scatter resulting from sea level fluctuations during glacial and interglacial periods. During the interglacial periods, sea level is high and deposition occurs in low-energy environments-probably the outer shelf. This permits layer-cake deposition with dip variations less than 3°. During glacial periods, the sea level drops, and deposition occurs in inner and midshelf environments. The environmental changes increase dip scatter considerably.

Compaction Features

Many thick, channel-like sands were formed by compaction, not by the cut-and-fill process. Sands deposited on a mud bottom gradually sank downward, compressing and dewatering the underlying muds.

Shales formed from compressed muds exhibit downward-decreasing resistivity gradients and downward-increasing interval transit time gradients. Density-neutron log response gradients are also present ( Figure 12 ).

 

The dip pattern resulting from compaction is a mega-red pattern with interspersed blue groups dipping in the same direction. No right-angle relationship exists between the azimuth of the red and blue dip groups, as it does in features resulting from the cut-and-fill process.

Deepwater Chalks

Localized dipmeter interpretation rules are occasionally convenient. The following set of rules was developed for the deepwater chalks of the Norwegian Central Graben. Figure 13 illustrates an Ekofisk chalk example. In developing these rules, it was noted that chalk wells whose dipmeter logs exhibited many blank or scattered dip zones and dip patterns were better producers than wells containing zones exhibiting mainly structural dips.

To quantify the interpretation process, multipliers or weights of 4, 2, and 1 were assigned respectively to blank or scatter zones, red patterns, and blue patterns. These arbitrary weights are based on the permeability of each type of zone.

Blank or scatter zones result primarily from chalk debris flows or conglomerates that contain the highest permeability; therefore, they were assigned the highest weight.

 

Red dip patterns represent beds draped over a sloping surface. These draped layers permit laminar flow, which has a lower permeability; therefore, red patterns were assigned a weight of two.

Blue dip patterns indicate foreset-generated crossbeds cutting across the reservoir at some angle that interfered with flow into the well. Blue dip patterns have the lowest permeability and the lowest weight factor.

Reservoir Quality Factor

The following equation was developed to determine whether a chalk interval is capable of commercial production. The potential for commercial production was called the quality factor. The quality factor is the proportion of the total footage under study contributed by each type of zone multiplied by the weight factor for the zone. The equation is given below:

where:

F_D = the total footage of dipmeter blank or scattered dip zones

F_r = the total footage of dipmeter red patterns

F_b = the total footage of dipmeter blue patterns.

Using this approach on operator data, it was discovered that chalk intervals such as the Ekofisk, Tod, or Hor with quality factors of 2.6 or greater contained intervals capable of commercial production.

For the commercial threshold of 2.6 to be a reliable indicator, the interval must be sufficiently thick. Quality factors can be contoured on both regional and fieldwide bases for the Central Graben area. Other weight factors could have been chosen that would have worked as well. The value of the commercial threshold would have changed. Similar techniques may have applications in areas where sandstones have undergone some downs lope creep and slump or shallow-water working.

 

Fluvial Environment

UNDER CONSTRUCTION …!

Fluvial Channel Environment

Interpretation of Fluvial channels

The following are the five basic steps in interpreting a dipmeter log for a fluvial channel:

Determine the structural dip (and delete it if necessary).

Determine the stratigraphic encasement.

Define the depositional environment.

Orient the sand trend.

Locate the offset.

Structural Dip

Structural dip is determined from the shales above and below the channel. These dips may be different, since channels frequently occur as unconformities. The shale above the channel usually reflects the structural dip required for interpretation. In very low-angle situations, the stratigraphic gain may be more important than structural dip. The influence of coalescing channels repeatedly affects the structural position of fluvial channels.

Generally, if structural dip is greater than 4°, then the structural dip should be deleted. Sometimes, if the dip magnitudes are low (less than 10°), then even 2° dip should be removed.

Stratigraphic Encasement

The stratigraphic encasement is the interval in which the channel facies occur, including both the sand and shale components of a channel. Detailed correlations with offset logs are used for defining the stratigraphic encasement. Under special circumstances, the channel abandonment facies, or clay plug, can be identified from higher gamma ray or more resistive shale log responses. Red dip patterns, reflecting compaction features, may also be used to define channel facies. In all cases, identifying the interval is critical to the interpretation.

Depositional Environment

Success in defining the depositional environment depends on the geologist's input, core and sample data, log responses, formation images, and dipmeter arrow plots.

Local knowledge of the geology is very important in identifying the environment. Cores and samples are an integral component in new areas and are always useful in any area.

Electrical formation images are a valuable aid to the interpretation of the thin, fluvial sand zones.

Log responses are generally used to identify a fining-upward sequence, which infers a channel system. Distinguishing a braided stream from a meandering stream is only possible when very simple depositional sequences are penetrated. The braided stream contains several fining-upward sequences within the sand. A meandering stream contains one overall fining-upward sequence. This becomes very complex when the borehole penetrates several coalescing units. Coalesced point bars occurring in meandering streams may be interpreted as braided streams.

Dipmeter patterns are very similar in braided and meandering stream environments. Families of red and blue dip patterns with 90° azimuth differences typically occur in both environments. Braided streams can sometimes be recognized by the identification of transverse bars within the stratigraphic encasement.

 

Channel Orientation

Channel orientation from dipmeter patterns is usually determined by the following priorities:

1. strong blue

2. strong red

3. weak blue

4. weak red

5. erosional or drape

Channels are generally elongated parallel to the blue patterns and perpendicular to the red patterns.

Locating the Offset

Considerations for locating an offset include reservoir geometry, reservoir quality, structural position, surface restrictions, and secondary-recovery prospects.

Reservoir quality is determined primarily by permeability, bed thickness, and porosity. In a point bar, for example, the coarser, better-developed sand is generally near the thalweg, and fine-grained, poor-permeability sand is generally near the inside bank. Also, the leading edge of a point bar usually has better sand quality than the trailing edge.

Structural position is critical when a water or gas contact has been penetrated. The structural position often depends on compaction over coalescing channels and stratigraphic gains within the channel system.

Optimum location of an injection or a production well in a secondary-recovery project is dependent on the relative position of the well in the reservoir. A well near the leading edge of a point bar, for example, usually depletes quickly on primary production. The well can only produce from one direction, toward the middle of the point bar. This well can be used quite effectively as an injection well.

Required Accuracy Fluvial channels are usually narrow in width. This requires high accuracy in the measurement of the channel orientation. Typically, fluvial channels have a productive width of approximately 40 times their productive thickness. A 10-ft thick channel sand has an estimated productive width of 400 ft.

An azimuth diagram, as shown in Figure 1 , can help in defining the channel orientation. There are several segments to the diagram.

 

1. Depths are determined from the logs and the dipmeter plot.

2. The geologist and the dipmeter interpreter agree up on the type of deposition.

3. Structural dip is determined from the surrounding shale sections. If the dip above and below the sand is different, the sand is assumed to be deposited at an unconformity and the structural dip above the sand is recorded.

4. Confidence rating is a means to rank the quality of the interpretation. The rating is from A (highest) to D (lowest): A = strong blue and red, B = strong blue or red, C = weak blue or red, and D = erosion, drape, or intuition.

5. Orientation is shown as a line along the sand trend.

6. Current flow is the arrow on the end of the orientation line.

7. Channel thalweg is the arrow in the center of the azimuth diagram.

 

 

Determination of Well Position in a Point Bar

A relation between the blue and red dip patterns allows the determination of relative position in a point bar ( Figure 2 ). Blue pattern azimuths are usually parallel to the sand axis, since current flow is across the point bar. Red pattern azimuths are generally perpendicular to the sand axis and point toward the channel thalweg.

 

For wells located on the leading edge of a point bar, blue and red pattern azimuths are normally greater than 90° in angle difference. When a well is located midpoint, the blue and red pattern azimuths are approximately 90° different (perpendicular) to each other. For wells positioned on the trailing edge of a point bar, the blue and red pattern azimuths are usually less than 90° in angle difference.

Figure 3 shows a dipmeter plot through a Cretaceous sand interval in a fluvial meander channel. The strong NE red and SE blue dip patterns show the channel thalweg to be N67E, a current direction of S47E, and an orientation of N47W-S47E. The angle difference between the red and blue dip pattern azimuths is slightly less than 90°. This indicates the well position to be on the trailing edge of a point bar.

Exercise 1.

This exercise uses a classic deltaic example. The sand, shown in Figure 1 , from 6744 ft to 6900 ft was deposited in a deltaic environment.

In which part of the delta complex was this sand deposited? What is the strike of the sand?

In what direction is the thickest part of the sand body? What was the direction of current flow?

Was the entire sand deposited as one feature, or was there more than one feature deposited?

 

Solution 1:

This sand was deposited as fill within a distributary channel.

The strike of the channel is NE-SW.

The axis lies to the NW of the well.

Current flow was down the channel from NE to SW.

There is more than one channel present.

The main channel is below 6784 ft. This is the feature to consider when offsetting the well.

Above 6784 ft, the current flow diminished as the channel began to fill with sand, and channel switching occurred.

There is another minor channel between 6784 and 6761 ft. Its strike is also NE-SW, and its axis lies to the NW. Flow was from the NE to SW. The few scattered dips within this interval indicate some reworking.

Exercise 2.

In Figure 1 , the sand between 3810 and 4060 ft was deposited in an interdeltaic environment.

What type of sand is it?

What are its attributes?

 

Solution 2:

This sand is the product of previously deposited deltaic sediments reworked by waves, tides, and currents.

The top of the sand is now barlike, and it shales out to the NE. The strike of the sand is NW-SE.

The blank zone near the top results from shallow-water reworking and bioturbation.

 

Exercise 3.

Figure 1 represents a Pennsylvanian sand deposited in a fluvial meander channel.

What is the current flow direction?

What is the thalweg direction?

What is the channel orientation?

Construct the azimuth diagram.

What is the position of this well on the point bar?

 

Solution 3:

The current direction is south.

Channel thalweg is N41W.

The orientation is along the blue pattern azimuth (NS)

The angle difference between the red and blue pattern azimuths is greater than 90°. This indicates that this well is on the leading edge of the point bar.

»»  read more