Friday, January 26, 2018

Seismic Structural Imaging

Ask a geophysicist for a simple definition of structural imaging and you might get an analogy like this, echoing the parallel between optics and acoustics. But in direct terms, structural imaging boils down to this:  the branch of seismology in which processed seismic data undergo additional passes to create a large-scale picture of the subsurface. Its goal is to provide a map to locate traps and plan a drilling strategy for optimal drainage. 

Structural imaging lays the foundation for other seismic techniques that investigate progressively smaller features. After structural imaging, seismic stratigraphy jumps to the next level of detail, characterizing the arrangement of layers within rock formations. Next, lithostratigraphic inversion attempts to describe lithology of individual rock layers and evaluate properties and distribution of pore fluids, through analysis of variation of seismic signal amplitude with spacing between source and receiver, called offset. The quality of these finer-scale techniques rest largely on the quality of structural imaging. 

Today, structural imaging is advancing on two fronts. One is improving image quality of conventional structures-their position and shape become known with greater accuracy. The other is the abilty to image areas of more complex structure associated with large, rapid changes in velocity. Examples are low-velocity layers, structure below salt or gas, multiply folded and faulted formations. Understanding of these difficult settings promises to better quantify reserves in established fields and help define new prospects that eluded more conventional approaches.  

In all but the simplest geologic settings, imaging with seismic energy has three fundamental problems: the image starts out blurry, has the wrong shape and is in the wrong place. These problems are  caused mainly by refraction -bending of rays as they pass through rocks in different velocity- and diffraction of seismic energy as it passes through rocks of different velocities, shapes and thicknesses. To make the image interpretable, seismic energy must be focused into a sharp, correctly shaped image, and the image moved to the correct lateral and vertical position. The sharper the image of a structure and truer its shape and position, the more accurately the structure can be evaluated and drilled. The method of sharpening, shaping and relocating images is loosely termed migration, often considered synonymous with structural imaging.

Mathematically, migration is performed by various solutions to the wave equation that describe the passage of sound through rock. Called migration types, these numerous solutions or algorithms often take the name of their authors (Gazdag,Stolt) or the type of solution (finite difference, integral).
Migration types may be thought of as a family of tools, each with shortcomings and advantages. Choice of the optimal type is not always obvious and relies on the experience of the seismic practitioner.

Types of migration are applied in a broader category of migration called class: poststack or prestack, two-dimensional or three-dimensional (2D or 3D), time or depth. These classes form eight possible combinations. The trend in imaging is from postack to prestack, 2D to 3D and time to depth migration. This trend is best understood by examining the strengths and weaknesses of these classes. 

Poststack migration, still the most common form, assumes the section built of stacked traces is equivalent to a zero-offset section, meaning each trace is made as if the source and receiver are coincident. Chief advantages of poststack migration derive from stacking: compression of data, removal of multiples and other noise, and fast, inexpensive processing.  Postack migration holds up even in fairly strong lateral velocity variation, stacking breaks down and prestack processing is required. A limitation of stacked data is its removal of true amplitude information.

Prestack imaging is done on unstacked traces, taking 60 to 120 times longer than postack imaging, but with potential to retain amplitude variation with offset (AVO) and phase changes useful for later analysis. Prestack time migration is preferred when two or more events occur at the same time but with different stacking velocities.  Prestack depth migration is advantageous when velocities in the overburden or the target are complex, but requires large computer resources and remains rare. However, as massively paralel computers become more widely available, migration technology will shift toward prestack depth migration.

In 2D migration, energy reflected only in the plane of the section is correctly imaged, whereas 3D migration uses energy from both in and out of the plane of the section. In general, 3D migration will have higher resolution because it can move energy from outside the plane back to its correct position. The cost of higher resolution, however, is  greater acquisiition costs with longer processing time - what takes days in 2D might take weeks in 3D.

The choice of 2D or 3D migration is first determined by acquisition geometry. Data acquired with a 2D scheme- a single acquisition line with shot and receivers in a line - can only be 2D migrated, but 3D data can undergo 2D or 3D migration. An inexpensive, fast approximation to 3D migration is 2D migration in orthogonal directions, called two-pass 3D migration. It is strictly correct only for a constant-veloctiy earth, but errors are small if the vertical velocity gradient and dip angle are small.

Time and depth migration differ on several levels. In simplest terms, time migration locates reflectors in two-way travel time - from the surface to the reflector and back as measured along the image ray- whereas depth migration locates reflectors in depth. A migrated seismic section with a time axis, however, is not necessarily a time migration. A depth migration may be converted to time. This is sometimes done to compare velocity modeling for depth migration with velocity assumptions used for time migration.

The significant difference between time and depth migration is the detail with which they view the behaviour of sound in the earth. To time migration, the earth is simple in both structure and velocity; to depth migration, it may be complex. Time migration assumes negligible lateral variation in velocity and therefore hyperbolic moveout. Using only stacking velocity , or some approximation, time migration can make sharp and correctly shaped and positioned images - if structure and velocity are generally simple. Time migration can handle some complexity of structure , but only limited variaton in velocity. 

When velocity and structure become obviously complex, rays are bent, producing nonhyperbolic arrival times and distorting the resulst of time migration. Reflectors become blurry, move to the wrong place or become too long. Accounting for this ray bending requires what is called a macro model - a model of velocities between reflectors. This is needed mainly to eliminate lateral positioning error caused by refraction, but also to sharpen the image. Construction, revision and verification of this macro model are the goals of depth migration, are the main contributors to its difficulty and cost. Depth migration is also more sensitive to errors in velocity than time migration.



Picture above: Visualizing a North Sea salt diapir structure with 2D time migration, depth migration and depth migration output in time. Colors denote the interval velocity field determined prior to depth migration. The velocity model permits a more certain migration by including lateral variations. In the posstack time-migrated section, reflectors are poorly defined, on both the flanks and base of the diapir. Definition improves in the prestack depth migration, performed with the velocity field shown. Finally, for comparison between the prestack depth and time migrations, the depth migration output is converted back to time, revealing a marked improvement in definition of the diapir flanks and base.

 Although time migration handles only simple problems exactly, it remains the dominant technique in exploration, and usually lays the foundation for the depth migration macro model. In the production setting, some operators prefer to strecth time migration to the limit before jumping to depth migration. Still, depth migration has become a valuable tool because it is the only one that can handle the most difficult problem in imaging - strong, rapidly varying lateral changes in velocity. Depth migration also remains the focus of most imaging research. 

When is depth migration needed? Or, to put it another way: When is the velocity field complex enough to mislead time migration? For many operators, a step before depth migration is a processing step to convert time-migrated section to depth using image-ray depth conversion. This procedure uses an image ray, which is shot downward perpendicular to the surface and is bent by an amount predicted by Snell's law applied to the velocity model. The ray passes through the correct lateral position of an event, which in the time migration would appeear vertically below the starting point of the ray. If the image ray strikes the reflector a considerable lateral distance from the starting point, velocity variation may be interpreted to be sufficient to warrant depth migration.



In many areas, the decision to use depth migration usually arises when imaging the near-vertical flanks of salt domes. In this setting, BP tests the macro model with ray tracing. if the depth or lateral position of the image is not displaced far enough to affect well placement, then BP does not bother with depth migration. To preserve steep-dip events, BP is careful not to finish processing with a low-cut filter (5 to 40 Hz). In removing noise, the filter may inadvertently erase low-frequency, steep-dip events. 

Selection of the approriate type and class of migration is only half the story of imaging, however, and probably the less important half. The main concern in imaging is the engine that drives the depth migration algorithm - the velocity macro model. 



The macro model is a numerical description of the subsurface on the scale of hundreds of meters. It contains either two-way travel time or depth to the main reflectors and the velocities and densities between them. It describes the acoustic propagation characterics of the subsurface and is used for depth migration to include ray bending. In other words, the macro model functions as the air traffic controller for the migration algortihm. It tells the algorithm how far to move the reflector - up a little, down, to the left or righ, or hold the line. Inadequate knowledge of velocity results in the image being under- or overmigrated, misplaced or blurred. Even the most advanced migration algorithm will fail to focus the image if directed by a flawed macro model.


Techniques of macro model building are controversial, proprietary and fast evolving. It remains the weak link in the depth migration, so attention today focuses on increasingly sophisticated ways to model, update and verify velocity for depth migration. The cost and computation time of these techniques increase with their capability to handle large, rapid changes in velocity.


A starting point is stacking velocity, obtained for conventional time processing. Stacking velocity is calculated from the difference in arrival time at different offsets, assuming the layers over the reflector have a constant velocity. Stacking velocity values are often inaccurate because they are an average over a large volume of rock, which often has nonuniform velocity. Stacking velocities may be constrained with data from sonic logs or previous seismic data. Still, stacking velocities may provide the best, first macro model. 


A first-guess model of the earth is usually developed by picking main reflectors from a poststack time migration. Times are assigned to each reflector and velocities to intervals between reflectors. A first order approximation is the computation of constant velocity values between reflectors. Now, workstations readily permit estimation of velocity gradients vertically between reflectors and sometimes horizontally along the event. The macro model may then be used to make a synthetic seismogram based on part of the model, which is iteratively modified until the synthetic matches the measured seismic data. 

















Thursday, January 11, 2018

Seismic While Drilling

Seismic while drilling promises elegant solutions to some limitations of conventional borehole seismics. But so far, it has failed to grab a significant slice of the market. Recent advances may create new opportunities for the technique.

 The aim is to turn conventional borehole seismics on its head. Instead of locating seismic sources on the surface and conveying receivers downhole, seismic while drilling uses bit vibrations as a downhole source and surface geophones to measure signals. This inversion promises timely seismic information without interrupting drilling and without deploying any downhole hardware. Further, the service may be employed in environmetally sensitve areas where surface seismic sources are disruptive - for example, in jungle roads built specially to accomodate sources. Despite these advantages, a widely-used seismic-while-drilling service has so far proved elusive.

This article outlines the princlpes of seismic while drilling and its potential uses, looks at developments to date, and examines how recent research initiatives are rekindling interest in the technique.






The principles

Well location is usually selected using surface seismic images. Once drilling is underway, it is useful to know the bit's position relative to the seismic section. However, this information is not easily available because the vertical axis of the seismic section is measured not in distance but in "two-way-time" - the time the seismic waves take to travel through the earth, bounce off a subsurface reflector and return to surface.

To relate the position of the bit to the seismic section, it is necessary to convert the vertical axis from time to depth. This conversion requires knowledge of the velocity of seismic waves through the formation. Velocity varies significantly with rock type and usually has to be measured, rather than modeled, using a combination of sonic logs and borehole seismics of a well after it has been drilled. 

The theory of borehole seismics has been known for many decades. At its simplest, a geophone deployed on wireline records the time that seismic waves take to travel from a surface source to a receiver at known depth in the well. These times are doubled to tie in with two-way time on the surface seismic section. This simple service is known as a "checkshot" survey.

But there are subtleties that add significantly to the usefulness of borehole seismics. Good quality data, sampled finely and in sufficient depth, enable a vertical reflection image or vertical seismic profile (VSP) to be created. In a basic VSP survey, the seismic source is static and the geophone  is moved to different levels in the well. The image may be displayed either in time, to match the surface seismic section, or in depth, to match wireline logs.

Alternatively, the geophone location may be fixed and the surface source moved along a line that "walks away" from the rig. Walkaway VSP produces an image of the subsurface with lateral coverage that is typically between half and a quarter the well depth. In deviated wells, various combinations of VSP and walkaway VSP may be employed to provide the required images.

Today, borehole seismics delivers a range of high-resolution images. However, like all wireline-delivered services, drilling must stop and the drillstring must be removed prior to running the survey. Therefore, borehole seismics is typically carried out during openhole logging, usually just before casing is run. The results certainly offer useful information, but this may be too late.The well may already be in the wrong place - for example, on the wrong side of a fault subsequently revealed by walkaway VSP - and a costly sidetrack may be needed. Furthermore, it may be expensive or impossible to locate sufficient surface sources to create a satifactory walkaway VSP image.

 In seismic while drilling, compressional waves emitted by the active bit radiate both directly to surface and downward from the bit, reflecting off formation boundaries. By using surface geophones to detect this sound, the inverse checkshot, VPS and walkaway VSP surveys may be obtained. 

  These techniques offer several advantages over conventional borehole seismics: drilling need not stop and, because the measurements are made continously, the information allows well trajectory decisions to be made before it is too late. Further, using the bit as a source may make it practical to perform large-scale borehole seismic jobs where surface sources are impractical - for example in towns or environmentally sensitive areas. However, seismic while drilling presents significant technical challenges. The signal emitted by conventional seismic sources is well controlled - either an impulsive explosion or a sweep from a vibrator of known signature - making the time between its emission and detection relatively easy to determine. On the other hand, the bit's signal is essentially continous and uncontrollable. A geophone on surface records continous seismic radiation as it is transmitted through the ground.


In addition, the environment around a drilling rig is very noisy. The comparatively low-level energy of the drillbit seismic signal is often completely submerged in noise. Onshsore, the geophone traces include several noise components. Some noise that correlates with seismic signal is caused by bit vibrations travelling up the drillstring and the fluid-filled annulus and then "rolling" along the air-ground interface to the geophones-this is called correlated ground roll. Uncorrelated ground roll comes from the vibrations of surface equipment like the mud pumps and engines. Random noise is caused by events like a passing truck or train. 

The challenge is to recognize the unknown and variable signature of the bit, to improve the signal-to-noise ratio and to convert a continous emission to one in which discrete seismic events may be recognized.








For seismic while drilling, the filter exploits differences in the moveout of components within the traces. Ground roll approaches the geophones from the side and exhibits moveout across the array. However, the wavefront of the seismic signal approaches the array from below, has zero moveout and is in phase. By using these differences in moveout to distinguish between different parts of the trace, the adaptive filter effectively attenuates the ground roll, while allowing the seismic signal to pass. 

Random noise may be removed by cross-correlating the individual geophone traces with the average of the two traces measured by the accelerometers on the drillstring. This crosscorrelation also gives the time shift between accelerometer and geophone signals- the difference in signal velocity through the drillstring and formation.

 Determining formation velocity also requires knowing the drillstring travel time. As already noted, the many components in the string complicate calculation of this travel time and a number of methods have been proposed - like Elf's double crosscorrelation process.

The continous checkshot system uses a new technique called drillstring imaging, also devised at SCR, to model changes in the acoustic impedance of the drillstring, giving a better understanding of the velocity of the signal through the drillstring. The time shift and the drillstring travel time are then used to compute the formation travel time.










The processing capitalizes on the relative abundance of geophone data and tracks the wavefront as it travels through the formation, potentially estimating the bit signal and the earth's response without using accelerometer data. However, by employing the accelerometer input, data may be compressed, making it feasible to store the massive volume of information collected over three or four days.

Each trace contains a common bit signature, and noise that varies from trace to trace. With time-delay curves, stacking and deconvolution filtering, new signals are created that represent what the traces would have looked like if the source had been a noiseless pulse - the earth impulse response. This converted form is then migrated to create an image.





Monday, January 8, 2018

Overcoming Limitations of Sequence Stratigraphy

Sequence stratigraphy has proven useful for petroleum exploration, but it is commonly misapplied. There is controversy over whether the technique can be applied to carbonate systems since it was designed to explain sand-shale systems. Some experts maintain that sequence stratigraphy is easier in carbonates because carbonates are extremely sensitive to sea level change. There is unanimous agreement, however, that low sedimentation rates often pose special problems. When sedimentation rate is moderate to high, layers within a sequence are tens to hundreds of meters thick, comfortably within the resolving power of a typical seismic wave. But when sedimentation rate is low, several sequences might fit within a seismic wavelength. Sequence stratigraphy cannot be confidently applied here, but it has been done countless times. A useful interpretation in thinly-bedded regions requires abandoning small-scale features and concentrating on larger scale, longer term processes that control the generation of sequences. 

 With this in mind, Vail and coworkers proposed a hierarchy of stratigraphic cycles based on duration and amount of sea level change. Duval and Cramez at TOTAL Exploration worked with Vail to provide subsurface examples and to expand the application to hydrocarbon exploration. The hierarchy assigns frequencies to the mechanisms of eustasy  enumerated by Fairbridge, viewed in light of plate tectonics. The first-order cycle, which is the longest, track creation of new shorelines resulting from the breakup of the continents. The second-order cycle is landward and basinward oscillation of the shoreline that lasts 3 to 50 million years. This oscillation is produced by changes in the rate of tectonic subsidence and uplift, caused by changes in rates of plate motion. 
Both first - and second order cycles may cause changes in the volume of the ocean basins resulting in long-term variations in global sea level. The third-order cycle is the sequence cycle, lasting 0.5 to 3 million years. Fourth- and higher order cycles may be correlated with periodic climatic changes.

The following example, with its low deposition rate, approaches the limit of interpretation in terms of third-order cycles. It comes from the Outer Moray Firth basin in the UK sector of the North Sea, where the initial basin shape, tectonic activity and variation in the rate of deposition add a twist to the interpretation.

Stratigraphic interpretation of the last 65 million years of sediments in the Outer Moray Firth is more difficult than in the Gulf of Mexico because slower deposition in the Central North Sea resulted in thinner units, many of which cannot be resolved by seismic waves. During this period, the Outer Moray Firth has 17 sequences totaling 5000 feet of sediments, compared to the Gulf Coast, with 10 sequences totaling 9000 ft. In the North Sea, however, depositional processes juxtaposed a variety of lithologies, provide reliable calibration points for accurate conversion of logs from depth to time using synthetic seismograms. 





In the Gulf of Mexico, this conversion is typically done with only nearby checkshots; sands and shales commonly show periodic alternation with depth at wavelength that make comparisons between seismic sections and synthetic seismograms nonunique. 

 Stratigraphy study is always preceded by structural interpretation. In addition, a paleogeographic interpretation of the Outer Moray Firth shows that late in the Cretaceous period - when the sequences under study began to deposited - a smooth basin floor sloped gently from northwest to southeast. During a relative fall in sea level, sediments were deposited as slope fans. Their seismic expressions indicate lobes with channels and some chaotic flows- large scale slumps with jumbled seismic character. As sea level rose, a wedge of out-building deltas was deposited. Sea level maximum is associated with a depositional hiatus, shown only as a thin line. Deposits synchronous with this surface may be found on what is now land in Europe, but in the basin, sediments that correspond to periods of high relative sea level are rare. 

 Why are elements of the classic Vail model missing from this sequences in this basin? One explanation is the the competing influences of tectonic uplift and sea level change. As global sea level rose and fell, continual regional uplift kept the sea from reaching levels high enough to allow formation of units typical of high relative sea level. Only once, at the top of the third sequence, does a thin layer of high relative sea level sediments appear. Another interpretation is that thin, high relative sea level sediments were deposited, but eroded and so are not preserved in the section.





 This section can also be interpreted in terms of second-order cycles. The entire set of 17 depositional sequences can be bracketed by five second-order cycles, based on physical stratigraphy and biostratigraphy. Major biostratigraphy gaps exist at the boundaries of each second-order cycle, and the boundaries can be seen to represent major changes in the depositional style of basin fill.

If the volume of earth in a study area is small enough, workstations can add a new dimension to sequence stratigraphy. In the Green canyon area of the Gulf of Mexico, interpreters concentrated on a fan deposited in a syncline on the continental slop 1 to 2 million years ago. Regional sequence stratigraphy was established using 2D seismic data and paleontologic control from six nearby wells. Zooming in on a subset of this data, interpreters assembled a series of 2D panels for 3D interpretation.

The top and base of the slope fan were interpreted over a six-block area (138 km2). The thickest part of the slope fan coincides with the stacked channels that carried shallow-water shelf and delta sands into deep water, greater than 200 m. A series of stacked channels, possibly filled with sand, is visible within the slope fan interval.

The goal of this interpretation is to identify exploration targets. Although lithology of the channel deposits is difficult to identify in the horizon slice, geology predicts that the channel will terminate in a sand-rich fan. The channel was tracked south, and a fan was discovered in the next block of seismic data.




Sequence stratigraphy continues to evolve. One area of investigation is high resolution sequence stratigraphy, which is performed at a higher resolution than seismic wavelengths, usually with log and outcrop studies. 







Thursday, January 4, 2018

Searching for Sand - A case Study

Sequence stratigraphy was applied in 1992 in the East Breaks area, offshore Texas. Data included a two-dimensional (2D) seismic line and logs from seven wells. The seismic line was processed for structural imaging and the structural interpretation used for other lines to view the basin as whole. In this case, the big picture shows a basin controlled by normal faulting to the north, which was the direction of the sediment source. Layers dip and thicken to the south. 

Initially ,seismic data and logs were interpreted indepedently to identify sequences and their bounding unconformities. Log-derived boundaries were compared with those from seismic data and the interpretation refined iteratively. Detailed seismic interpretation began with the most easily interpreted reflection patterns, and was pieced together - working upward, downward and back toward the wells - respecting the stratigraphy suggested by the sequence model.

Logs from wells on the seismic lines were converted from depth to time using nearest check shot - here, 3 miles (4.8 km) away. Sands interpreted on spontaneous potential, gamma ray and resistivity logs were associated with seismic reflections at the well and tracked along the seismic section. Shales indicated by logs were noted for correlation with fossil data from cuttings.

Next was integration of biostratigraphy. Fossils from cuttings help identify and date boundaries of each rock sequence. Fossil diversity and abundance are measured versus depth, which is converted to seismic travel time for easy comparison with seismic section. Fossils of planktonic (floating) organisms are more widespread than those of benthic (bottom-dwelling) organisms and are therefor more useful in establishing regional time correlations. However, in shallow-water environments, bethic fossils are used because nearshore conditions may be too variable for planktonic fossils.  

Fossils are also indicators of relative sea level. High fossil counts, or peaks, are associated with shales deposited during low sedimentation. Such conditions occur in the basin during time of high relative sea level, but also in deep water between fan deposits and outbuilding delta deposits. Two shale sections are expected within each sequence, one at the top of the slope fan and the other at the maximum flooding surface, associated with the furthest landward position of the shoreline. Biostratigraphy also holds the key to paleobathymetry - a measure of topography of the ancient ocean floor - needed to interpret the depositional environment.







Paleodepth is derived from benthic fossils with known depth habitats. Knowing water depth helps to interpret deep or shallow water rock types and expected layer thicknesses.

Once seismic, log and biostratigraphic data are combined, a final, color-coded interpreted section is made. Very high amplitude reflections may be highlighted with hatcing. These so-called bright spots are analyzed for anomalies in amplitude variation with offset associated with hydrocarbons. In the East Breaks example, the most promising prospect is a large, sandy basin floor fan. Shales interpreted above and below could provide seal and source rock, respectively. 
 

Wednesday, January 3, 2018

A Detailed View of Sequence Stratigraphy

The components of depositional sequences are called system tracts. Systems tracts are divided into three groups according to relative sea level at the time of deposition -lowstand at low relative sea level, transgressive as the shoreline moves landward, and highstand at high relative sea level. Systems tracts are depositional groups that have a predictable stratigraphic order and predictable shapes and contents. A close look at system tracts, their geometries and lithologies, shows how sequence stratigraphy can be used to foretell reservoir location and quality. 

Each systems tracts exhibits a characteristic log response, seismic signature and paleontologic fingerprint, and performs a predictable role  in the oil and gas play -reservoir rock, source rock or seal. Gamma ray (GR) and spontaneous potential (SP) logs are expected to read low in sands and high in shales. Resistivity logs show the reverse, reading high in hydrocarbon-filled sands and low in shales. 

 Apparent layering interpreted on seismic sections-called stratal patterns -is determined by tracing seismic reflections to their terminations. The termination is categorized by its geometry and associated with a depositional style. Fossils are described by their abundance, diversity and first or last occurence, allowing dates to be determined based on correlation with global conditions. 

Starting with the lower lowstand systems tract at the bottom of a sequence, basin floor fans are typically isolated massive mounds of well-sorted grain flows or turbidite sands derived from alluvial valleys or nearshore sands. Log responses are blocky, with a sharp top and bottom bracketing clean sand. Seismic reflections curve down and terminate on the underlying sequence boundary-a feature called downlad - while the top may form a mound. The lowstand facies makes an execellent reservoir, with porosity often over 30% and permeability of several darcies. It may be overlain by a thin clay-rich layer that can act as a seal , but more often it is overlain directly by the next depositional unit. In these cases, the basin floor fan acts as a hydrocarbon migration pathway. Fossil content is minimal, since deposition rates are often very high. Basin floor fans derive their hydrocarbon from previous sequences.

In areas of high deposition , the major component of the lower lowstand systems tract is the slope fan complex. Slope fans can be extensive and can exhibit several depositonal styles , depending on the vertical gradient of the slope face and on the sediment source. The complex may include submarine channels with levees, overbank deposits, slumps and chaotic flows. Log responses commonly are crescent shaped. A sharp base within the crescent commonly indicates sand in a channel , with a bell shape indicating fining upward as the channel is abandoned. On the other hand, channels may fill with mud. On seismic sections, leveed channels in the fan show a characteristic mound with a slight depression in the top. Sand-filled channels make excellent exploration targets, but may be difficult to track. Sands flowing over channel levees may be deposited as overbank sheets and alternate with shales, createing subparallel reflectors.  Such sands can provide stacked reservoirs with porosities of  10 to 30% , but are usually very thin. Slumps from shelf edge deltas create a chaotic or jumbled pattern "hummocky" in interpreter vernacular - easily identifiable on seismic data. Hydrocarbon sources for channel and overbank reservoirs are deeper sequences. Seals are provided by a widespread "condensed" section of shale, a thin layer representing prolonged deposition at very low rates that comes with the rise in sea level. The sealing shale also contains abundant marine fossils used for dating. 

Part of the upper lowstand, the prograding wedge complex derives its name from shallowing-upward deltas that build basinward from the shelf edge and pinch out landward at the preceding shoreline. Log response shows more sand higher in the section and less sand basinward, indicating a coarsening upward. The seismic signature shows moderate to high amplitude continuous reflector that downlap onto the basin floor. This depositional unit often contains ample sand, especially near the sediment source. Updip seals are typically ppor, however, and structural trapping is required for hydrocarbon accumulation.

The transgressive systems tracts represents sedimentation during a rapid rise in sea level. The shoreline retreats landward, depriving the basin of sediment. SP and gamma ray logs show a fining upward. Retreat of the shoreline gives rise to seismic patterns that appear to truncate basinward. In practice, this systems tract is commonly thin, and such patterns are usually impercetible on typical seismic sections. Basal transgressive sands derived from reworked lowstand sands can be excellent reservoirs, except where shell fragments may later cement the sands. Shoreface sands will follow strike-oriented trends. 

The top of the transgressive systems tract is the limit of marine invasion and is called the maximum flooding surface. Widespread shale deposition results in a condensed section. Abundant fossils provide ages and well ties across the seismic section. This clay-rich layer shows low resistivity and high gamma ray readings. The seismic pattern of this surface is downlap, which becomes conformal-parallels adjacent reflectors-basinward , and disappears above the shelf. This surface is usually a very continous reflector. At the shelf edge, it can commonly be identified by changes in reflection patterns above and below.


 Layers deposited during highest relative sea level are known as the highstand systems tractk (above, "E). Early highstand sediments are usually shaly. The late highstand complex, deposited as the rise in sea level slows, contains silts and sands. Some late highstand sediments are deposited in the open air as fluvial deposits. Gamma ray and SP responses show a gradual decrease in gamma ray, indicating coarsening upward associated with decreasing water depth. Seismic reflections are characterized by sigmoidal -S shaped -stratal patterns ,similiar to prograding wedge reflections. There may be deltaic and shoreface sands at the top of the section, but in general, this systems tract has poor reservoir sands, and updip seals are uncommon. Fossil abundances diminish as the marine environment becomes restricted to the deeper parts of the shelf.