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:
FD = the total footage of dipmeter blank or scattered dip zones
Fr = the total footage of dipmeter red patterns
Fb = 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.
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