Abstract Techniques for the study of outcrop geology have gone through a paradigm shift in the last decade. Traditional techniques such as analogue photography, logging and manual mapping using pens and pencils have been complemented and partly replaced by digital techniques. These digital techniques include LIDAR scanning, photorealistic mapping, remote sensing techniques and digital mapping tools. Recording methods have switched from paper to handheld computer tablets, GPS and digital data. The digital techniques allow for much higher precision and efficiency in acquisition of field geological data. The techniques complement but does not replace traditional scientific insight and empirical data when interpreting the outcrops. The combination of digital outcrop data with behind-the-outcrop borehole and geophysical data provides an unprecedented ability to move outcrop examples from analogues towards homologues. The ability to compare outcrop data, where much larger extents of geological systems can be viewed, with subsurface data where little primary data such as cores are available and geophysical data which may have unsatisfactory resolution, has revitalized outcrop geology in the last decade. Therefore, presently, and in the foreseeable future, outcrops will increase in their importance, both in terms of their independent value as the primary data source and training ground for geologists, but also for invaluable comparisons with subsurface data.

Abstract The ultimate goal of collecting outcrop data is to be able to understand the underlying physical controls on the formation of the rock record, and these techniques provide an essential step in the right direction towards this goal. In particular, development of new techniques for acquiring data from outcrops has led to an ongoing evolution in how outcrop data can be analyzed and utilized in the petroleum industry. Understanding which technique to use depends strongly on the problem being addressed, as the choice of techniques provides different cost-benefit outcomes to different problems. The key to adopting these techniques successfully is to understand their limitations and where the successes have been in applying them. This brief review defines the problems being addressed in collecting outcrop data, examines the role of the “digital” geologist and the collection, application, and deployment of three-dimensional data, and the future of outcrop studies. Reference is made to case studies described in other contributions in this volume.

Abstract An outcrop’s geometry will always have an effect on how data from that outcrop are interpreted; therefore those data should be used only when the implications of that geometry have been taken fully into account. Problems inevitably arise whenever the geologist attempts to do this, the greatest of which stems from the uncertainty that necessarily exists in the position and orientation of the outcrop relative to the underlying rock body. This problem is best handled by means of a hypothesis-based approach for outcrop interpretation, for instance by using one of the three techniques outlined in this paper. The first of these techniques—the use of geometric probability arguments—is illustrated here in the context of recognizing clinoforms in outcrop. It is shown that clinoforms will rarely be seen as sigmoid-shaped curves on randomly positioned and randomly oriented outcrop faces; they are more likely to be seen as simple convex-up or concave-up curves. The second technique—the use of Monte Carlo methods—is illustrated in the context of interpreting dog-legged outcrops in flat-lying sedimentary successions. It is shown that the probability of failing to recognize simple features on the face of dog-legged outcrops can be high, and that this failure probability is highest for relatively long and sinuous outcrops with relatively many segments. This result conflicts totally with conventional geological wisdom. The third technique—the use of standard statistical tests—is illustrated by showing how isolated outcrops can be used to test correlation hypotheses in areas of broken exposure. The paper warns of the danger of conflating rock body and outcrop, then finally offers guidance on hypothesis selection. Key words: stratigraphy, correlation, model, hypothesis, Monte Carlo

Abstract Digital survey methods, including terrestrial laser scanning (LIDAR) and differential GPS, allow geological and topographic data from outcrops to be recorded very rapidly, in 3D, at detailed resolutions and with high spatial precision. Geological interpretations of outcrop datasets (e.g., fault or bedding traces) can be extended into the subsurface using geometric, probabilistic, or deterministic methods. Geometric methods based on interpolation and extrapolation of observed surfaces and surface traces are generally associated with high uncertainty. This can be reduced in areas of highly irregular topography. Another approach is to use geological heuristics to constrain the subsurface interpretation. This approach can help to limit the number of possible interpretations when creating multiple realizations. Deterministic methods encompass both invasive and non-invasive approaches. Invasive methods include mining and quarrying, as well as small-scale excavation of unconsolidated sediments. Behind-the-outcrop boreholes are only slightly invasive, and can provide very useful constraint of the subsurface. In contrast, geophysical methods such as near-surface seismics and ground penetrating radar (GPR) allow indirect imaging of the subsurface and are non-invasive. Excellent coastal exposures of Namurian turbidites near the Bridge of Ross in County Clare, western Ireland, provide a case study in which several different types of digital outcrop data are combined and co-visualized in a 3D model. In vertical sections on opposite sides of the outcrop a small-scale turbidite channel is marked by an erosional base and inclined interbedded sandstones and mud-clast conglomerates. The observed channel margins can be traced through the subsurface using 3D GPR.

Abstract The Nukhul Formation (Suez rift) consists of fluvial and tidally influenced shallow marine strata that were deposited in fault-controlled seaways and tidal embayments during rift initiation. In this study, we create a half-graben-scale, high-resolution (typical grid cell dimensions 20 m x 20 m x <1 m), geocellular outcrop model of the Nukhul Formation. The evolution of the normal fault system in the study area is associated with the development of fault-parallel and fault-perpendicular folds. The changing nature of the structural template, and the resulting geomorphology, during deposition led to complex syn-rift stratigraphic architecture and facies distributions. We use a LIDAR-based digital outcrop approach to map this geological complexity to a high degree of accuracy, for export to reservoir modelling software. Software developed in-house was used to integrate field observations with the digital dataset, aid interpretation, and create realistic surface meshes from outcrop data. Facies modelling used a combination of sequential indicator simulation and object-based modelling approaches. Sedimentary logs were attached to the dataset and used as conditioning data. 2D probability maps, source points, and flow lines constrained the geocellular outcrop model to match the known geology. The approach leads to improvements in three areas: (i) geological knowledge of the study area, (ii) data portability, and (iii) geocellular outcrop modelling. Comparison between the final geocellular outcrop model, outcrop geology, and inferred palaeogeography shows that the geology of the Nukhul Formation is realistically modelled. The final reservoir model can be used as an analogue for similar geological settings. It can be applied to improve the prediction of subsurface geology in analogous reservoirs and to increase the accuracy of static connectivity and flow simulations. Ultimately this will improve knowledge of the impact of facies heterogeneities on reservoir performance and lead to increased efficiency of reservoir drainage.

Abstract Resolving the geological details hidden beneath the resolution of seismic reflection data has been a continuing challenge for decades. Forward seismic modeling of outcrop analogues provides an important scale link between the architectural geometries observed in outcrops and in seismic. Such models can potentially bridge the critical gap in resolution between datasets and provide new and improved insight to the interpretation of petroleum targets. The use of outcrop analogues can produce realistic models where petrophysical properties, lithology distribution, and reservoir architecture are all defined. The synthetic seismic model is compared with subsurface seismic data and adjusted to find the earth model that gives a seismic response that matches the real seismic amplitudes. A four-step workflow on how to build seismic models of outcrops is suggested and detailed in this paper. Information regarding how to construct high-resolution, complex, geologically realistic architectural elements such as mass-transport complexes is also described. Analysis of outcrop synthetic seismic has enabled us to study and understand the anticipated seismic character and imaging of depositional systems at different scales. Seismic models reveal the expected seismic architecture and character of similar geobodies and basin-fill sequences. They also increase the ability to predict hydrocarbon-bearing rocks in unexplored basins. Seismic interpretations of subsurface targets in both Angola and in the Gulf of Mexico have been quality controlled using this technique. This research contributes to better constraints on lithology predictions, fluid response, and geometry distribution in these areas.

Abstract Because of their fast acoustic velocity, their ability to create steep slopes, and their important postdepositional diagenetic modification, carbonate rocks are notoriously more difficult to image and interpret using seismic than siliciclastic rocks. This paper shows how building a 3-D synthetic seismogram based on well-constrained outcrop-based 3-D geocellular models can help in seismic interpretation and seismic-based reservoir characterization. Workflow to populate a 3-D geological model with velocity is presented that is based on building statistical distribution of velocity per facies or lithostratigraphic units or diagenetic features and extrapolating velocity through the model using stochastic Gaussian simulation. A 3-D model built from Lower Permian deep-water carbonate gravity flows is used to demonstrate the complexity of interpreting intricate 3-D geometries using 2-D planar seismic slices and to assess volumetric error associated with the intrinsic resolution loss of seismic. Modern karst morphology is used to assess the seismic response of caves, sinkholes, or karst topography in seismic. Finally, a Permian dolomitized ramp-crest grainstone complex is used to test the sensitivity of prestack techniques to pore-type changes in grainy carbonate rocks. These few examples illustrate the strength of building a well-calibrated 3-D synthetic seismogram based on a 3-D geocellular model so that some of the complexity of seismic response of carbonate rocks might be unraveled.

Abstract During the last decade, the laser scanning of outcrop analogues for the construction of 3D geologic models has become commonplace. Many elements of both laser-scan data collection and photorealistic model creation have dedicated software and are semi-automated or fully automated. However, much of the subsequent geological interpretation process relies heavily on manual digitization. Consequently, only a small fraction of the inherent 3D and 2D information captured within photorealistic models is normally extracted and analyzed. This situation is perhaps most acute in the study of fractured reservoir analogues where hundreds of thousands or even millions of fractures can be captured in multi-kilometer-scale datasets, making full manual digitization of the dataset impractical. In order to maximize and standardize the extraction of quantitative data from digital outcrop models, we sought to develop new technologies specifically for the automated extraction of lithologic information, bedding, and fracture discontinuities from photorealistic data. This is achieved through the adaption of algorithms originally developed for automated fault interpretation from 3D seismic data. Projection of the 2D bedding and fracture discontinuities extracted from digital outcrop images onto 3D outcrop surface, using transform matrices calculated during the construction of a photorealistic model, permits calculation of the strike, dip, and a confidence value for every extracted feature. The high-resolution image-derived discontinuity data is then integrated with larger-scale 3D discontinuities extracted from topologic analysis of laser-scan-derived data. This combined approach maximizes the potential information from both (1) high-resolution 2D digital image data and (2) the 3D laser scan topologic data in order to extract and combine different scales of discontinuities into a single 3D dataset. Independent quality control of the automated bedding and fracture extraction methods is achieved through the third-party collection, analysis, and comparison of structural data collected in the test area. Comparison between automatically extracted and field-mapped and measured fractures demonstrates the accuracy of the methods developed and some limitations. The automated techniques developed are designed not to replace outcrop studies but to augment them. They limit the tedious and time-consuming manual interpretation tasks by providing unbiased, pre-generated geological primitives that can be edited and analyzed with a set of structural-analysis tools. These data and tools allow time to be better spent on the analysis of large numbers of quantitative data and their incorporation into 3D models. The main limitations on quality of the automatic mapping appear to be related to the coverage and quality of the original dataset.

Abstract Prediction of fractures in carbonate reservoirs represents a very significant challenge. We describe the use of a digital outcrop analogue from faulted and jointed Lower Jurassic rocks from Somerset, U.K., that provides exceptional exposure of fractured carbonates. The aims were to gather high-resolution and exact information about the fracture systems and to understand the mechanics of the fracture development. A 2.5 km section of coastline was digitally captured and built into a high-resolution photorealistic model. Faults were hand interpreted in an immersive virtual reality environment. A line sample of the faults in the photorealistic model compares well with a similar line sample taken in the field. The photorealistic data also include large bedding-plane exposures of joint systems. The joints were extracted semi-automatically using a combination of image curvature and ant tracking; ground-truthing of the resulting joint map confirms the validity of the interpretation. By using this semi-automatic technique it is possible to digitize far more joints than would be possible for a human interpreter. The detailed fracture data provide a rich source of data for modelling of fracture systems. However, in order to be predictive in the subsurface, it is not sufficient to have a purely statistical fracture description and so we turn to mechanical modelling. On the assumption that the joint system formed in the perturbed stress system around pre-existing faults, we performed boundary element modelling and were able to match to the joint system in the photorealistic model using an extensional stress regime and fluid-pressure perturbations along the fault plane.

Abstract Channel stacking pattern in deepwater reservoirs has a significant impact on reservoir producibility. These stacking patterns are the result of feedbacks between turbidity currents and associated supply variations and the evolution and aggradation of slope physiography. Turbidite flow events that leave significant physiographic relief due to underfilled within-channel deposition or local erosion tend to have greater influence on subsequent flow events, resulting in organized channel stacking patterns. Turbidite flow events that fill their channels do not confine subsequent flows and result in disorganized channel stacking patterns. High rates of system aggradation result in a greater degree of inter-element disconnection, low rates result in amalgamated elements. Event-based models are amenable to the integration of expert rules related to the influence of channel fill (fraction of active channel fill) and aggradation rate on channel stacking pattern. The event-based approach is part of a new subset of geostatistical modeling that focuses on greater integration of stratigraphic concepts and sedimentological process. In the event-based method stochastic models are constructed as a sequence of depositional events. The sedimentological process and allogenic forcing are approximated as a set of empirical and predictive rules. The resulting numerical laboratory efficiently constructs high-resolution reservoir models and allows calibration of model response to changes in input values. Event-based models are constructed based in part on outcrop examples with various channel stacking patterns. The resulting models are used to explore the relationship between stacking pattern, preservation potential of axis, off-axis and margin facies assemblages and reservoir producibility. In addition, these models can be used to illustrate hierarchical relationships, explore larger issues related to the value of architectural information and to aid in the discovery of new rules by induction combining outcrop observations and models results.

Abstract Studies of turbidite channel complexes in outcrops, wells, and 3D seismic-reflection data suggest a general model of turbidite channel behavior related to three critical measures: (1) the thickness of channel elements; (2)the thickness of abandonment facies within each element; and (3) the thickness of overbank aggradation. These measures constrain channel stacking pattern and can be integrated into event-based geostatistical reservoir models that provide probabilistic predictions of net reservoir volume and element stacking pattern. Although channel and overbank thicknesses are measured routinely, this model provides a predictive framework that also emphasizes the importance of recognizing the presence and thickness of shale-rich abandonment facies at the top of sand-rich channel elements in outcrops. For a given flow composition, the deposits of thick channel elements (thickness of active fill plus abandonment facies) tend to have relatively low sand percentage, abundant bypass facies, and thick abandonment facies (underfilled channels). Underfilled channel elements with high topographic relief, from levee crest to channel thalweg, at the time of abandonment influence the location of subsequent elements, resulting in an organized channel stacking pattern. The deposits of relatively thin channel elements tend to have higher sand percentage, small volumes of bypass facies and thin/absent abandonment facies. Filled channel elements with low topographic relief at the time of abandonment had little influence on the location of subsequent elements, which resulted in a disorganized channel stacking pattern. Channel ‘‘relief ’’ corresponds to the depth of erosion plus the height of the levee crest above the initial sea floor. We observe that erosion relief can correlate strongly with downslope gradient. Flow composition also is critical because the rate of overbank aggradation is strongly influenced by mud volume. Muddy flows tend to produce thick overbank aggradation, high confinement, and under-filled channels with an organized stacking pattern. Sand-rich flows tend to produce relatively low overbank aggradation, low confinement (unless erosion relief is high), and filled channels with a disorganized stacking pattern.

Abstract Fault-related dolomite subsurface reservoirs are formed from fluid circulation that results in significant transformation of the reservoir properties. The geometry and internal organization of such dolomitic reservoirs remain difficult to image with seismics alone. A multi-scale approach is essential to understand and predict the diagenetic processes that control the exact 3D morphology of the dolomite with spatial precision and true dimensions, and consequently the reservoir properties. In this context, we propose an analytical workflow including field work, LIDAR scanning and numerical geology applied to dolomite outcrops in Mesozoic carbonates (SE France). The exposed dolomite-limestone contact exhibits sinuous, irregular and convolute shapes, which are either fault-parallel, bedding-parallel or chaotic. To characterize this complex distribution, we performed LIDAR scanning on 500 m x 150 m cliffs and road cuts with 4.5 cm to 1–1.5 cm average point spacing. The cloud is composed of 22 millions points comprising X, Y, Z, intensity, red, green, and blue attributes. Digitization of the limestone-dolomite boundary was performed in RiscanPro and GOCAD environments, for extracting the true 3D geometry of the dolomite body for further geostatistical and 3D facies modelling. This approach captures the large-scale geometry of the dolomite bodies. However, single RGB or intensity properties do not unequivocally reproduce small-scale (below ∼ 1 m) heterogeneities of the late diagenetic dolomite. Color changes induced by weathering or climatic conditions are of the same size range as the small-scale heterogeneities, thus they are not unique to allow automated tracking on the point set. As a result, the workflow remains time-consuming, and further work is needed to allow calibration of the LIDAR data points with mineralogy.

Abstract Carbonate platforms can have complex internal facies variations and stratal geometries expressed at length scales longer than all but the largest outcrops. The latter commonly form high and relatively inaccessible cliffs, and thus conventional field techniques (logging and photomontages) may not adequately capture the 3D geometry of surfaces and the details of the facies distribution. Because facies and stratal geometry control rock properties and connectivity in carbonate reservoirs, accurate outcrop data can be critical to reservoir and forward seismic modeling. The Gresse-en-Vercors cliffs (southeastern France) provide a seismic-scale slice though a Lower Cretaceous (Barremian) platform margin analogous to Lower Cretaceous reservoirs in the Middle East. The cliffs are 500 m high and extend for 25 km along depositional dip, straddling the transition from shallow-water platform to deeper basin. This paper describes the methodology developed to create a high-resolution stratigraphical digital outcrop model (DOM) integrating field measurements (logged sections, facies mapping) and high-resolution digital data (photomosaic and new LIDAR data acquired by a helicopter survey). Integration of the LIDAR and other point cloud data provide a high-resolution digital elevation model (DEM) on which georeferenced field observations were then posted. The “solid image” technique was used to extract precise x,y,z coordinates of stratigraphic surfaces from the DEM. The resulting numerical geological model allows a coherent restoration of the platform architecture, quantification of component surfaces (shape, angles, dimensions) and geobodies, and a better characterization of the relationship between facies and platform architecture

Abstract Terrestrial LIDAR data were acquired at four sites in the Acacus Formation in southwestern Libya in order to better understand the sedimentary architecture and to create outcrop models to aid ongoing subsurface exploration. The outcrop models comprise up to 8 individual scans merged to produce panels up to 1.2 km wide and c. 200 m high. The panels highlight the gross stratigraphic architecture including shallowing- and coarsening-upwards cycles, major channel bodies, and subtle prograding units. The scan resolution was chosen to resolve the bed-scale stacking that typifies hydrocarbon reservoirs in this formation. Surfaces and facies observed in the outcrops and constrained by the LIDAR data were then used to construct 2-D synthetic seismic panels of the outcrop. These models were parameterized with densities and velocities from subsurface examples and used to gain insight into the seismic resolution of depositional architecture.

Abstract Cross-well or borehole ground-penetrating radar (GPR) tomography is commonly used to map subsurface aquifer heterogeneity; however, the influence of sedimentary architecture and anisotropy on GPR signal transmission is largely unknown. To address uncertainty in GPR tomography interpretation, we developed a method to combine GPR and LIDAR surveying to characterize lateral heterogeneity behind an outcrop of braided-stream deposits. Methods included using black paint to mark a regular grid of GPR shot points on opposing outcrop faces for the transmitter and receiver intervals. A 3D model of the outcrop was then constructed using LIDAR scans to determine distances between shot points. GPR tomography was conducted through the outcrop, and velocity calculations from these data were used for tomographic inversions to determine heterogeneities within the outcrop. We were able to successfully combine GPR shot-points with LIDAR data to calculate distances between transmitter and receiver points, and we were able to see first arrivals through the outcrop. Complications, however, were evident at both short offsets (∼ 3–5 m), when the air wave overprinted the first arrival, and longer offsets (> 8 m) when the signal dissipated before reaching the receiver. Air-time arrivals that were observed compared favorably to model predictions based on LIDAR-derived outcrop geometry, supporting the utility of LIDAR for constructing a digital, 3D outcrop. Furthermore, estimated values of radar velocity for this outcrop of 0.09 to 0.12 m/ns reasonably match known velocities for similar sediment. Had the GPR data acquisition been more successful at this outcrop site, we could have used detailed 3D facies architecture derived from the LIDAR data to assess the influence of sediment heterogeneity and anisotropy on GPR tomographic signals.

Abstract Accurate representation of heterogeneity at varying scales is vital for modeling solute dispersion in groundwater aquifers and petroleum reservoirs. Dispersion, the result of varying velocities in a flow field, is, in part, due to material heterogeneity. In order to represent the influence of heterogeneity at the outcrop scale, a series of terrestrial LIDAR scans at millimeter-scale point spacing were recorded in sediments located in braided stream exposures west of Albuquerque, New Mexico, and outside the Hanford Site in Washington. Scans are projected onto a vertical plane and converted to a high-resolution TIFF image. Using the mean and standard deviation of the ‘‘stacked’’ images, the data are processed through a series of filters to enhance textural information and distinguish between lithologies. The product is converted to a grid with numerical color values for each lithology (e.g., sandstone, gravel). Each lithologic class is assigned reasonable values of hydraulic conductivity. Groundwater flow and transport time are simulated using MODFLOW and MODPATH, respectively. Simulations show that flow and solute transport are focused in the coarser-grained laminae of cross-bedded units. Flow may be focused into some areas in finer-grained beds as well, if the adjacent gravel bed has been cut. Thus, most of the flow may be focused into a smaller volume of the material making up the aquifer. This result shows that terrestrial LIDAR can be successfully applied to produce synthetic stratigraphy for use in fluid flow models.

Abstract A Middle Pennsylvanian deltaic succession is exposed in the Fords Branch road cut located on US Highway 23, south of Pikeville, Kentucky, USA. The outcrop is 1.5 km long and 20 m in thickness and is oriented in an oblique strike orientation with respect to local paleoflow indicators and regional fluvio-deltaic drainage directed broadly to the west in a late Paleozoic Appalachian foreland basin. Sedimentological analysis of the outcrop and interpretation of a panoramic photo montage establishes that the succession in the road cut accumulated in three depositional episodes bounded by sequence stratigraphic surfaces. A basal complex of distributary- mouth bars, channels and levees is cut by and overlain by an incised-valley fill. A capping unit of bay-fill and distributary-mouth-bars sediments completes the exposed succession. Both the basal and capping depositional episodes are interpreted as lower-delta-plain settings. The incised valley is dominated by fluvial channel deposits and exhibits an upward increase in fluvial channel amalgamation. The focus of the study presented here is twofold: (1) the nature of delta-front settings and how best analogs might be chosen, and (2) the evolution of fluvial channels in the outcrop panel, the architecture of which suggests a changing balance of sedimentation rate versus avulsion frequency over time.

Abstract Outcrops are fundamental to everything we hope to achieve in geological understanding. They are gateways to geological processes, earth history and they help ground-truth remote sensing applications. With increasing resolution of subsurface tools and techniques, one could be forgiven in believing that outcrops have had their day and their utility is less than in the past great eras of field mapping and the development of facies models. This premise is far from the truth and this new SEPM volume illustrates how new analytical techniques are revitalizing outcrops and in the process creating a wealth of new data and fresh geological understandings. In this book you will find a compilation of the growing arsenal of outcrop tools and techniques and a consideration of future developments. This collection of papers, delivered at a SEPM Research Conference on the West coast of Ireland in the summer of 2008, is a smorgasbord of case studies, workflows, modeling, and applications which spans clastic and carbonate settings. Whatever your interest in outcrop geology and its application there is something in this volume for you. If you are seeking guidance for using new outcrop tools, looking for efficiencies in data collection or desiring new insights for old and favorite outcrops, this volume is a must have. This volume also makes an excellent reference or textbook for any group of professionals or students working or studying the new technologies that have allowed new insights from outcrops. We also consider this a superbly timed publication because many new outcrop tools are now becoming mainstream via reduced purchase and operating costs. Once you read this volume, and there are reduced prices for SEPM members and students, please share your new experiences with the authors and editors and help continue the revitalization of our shared and continually surprising outcrop library of the earth.

Abstract Seismic reflections interpreted to be top Oligocene, top Wilcox (approximately base middle Eocene), top Cretaceous, top Jurassic, and basement were mapped across portions of the Green Canyon, Keathley Canyon, Walker Ridge, Lund, Sigsbee Escarpment, Amery Terrace, and Lund South OCS areas of the central northern Gulf of Mexico ( Fig. 1 ). 3D Pre-stack depth migrated data were used for mapping the areas covered by allochthonous salt. 2D Pre-stack time migrated data were used for mapping the area on the abyssal plain beyond the Sigsbee Escarpment. These data cover approximately 50,000 km 2 (19,500 miles 2 ). Well control was obtained from data available through the Minerals Management Service. Figure 1. Location map. Black line encloses the area of data coverage. Dashed line marks the transition from 3D prestack depth migrated (PSDM) data to the west and north to 2D pre-stack time migrated (PSTM) data to the east. Numbered segments refer to figures with those numbers. Abbreviations for deep-water OCS areas: AC–Alaminos Canyon; AM–Amery Terrace; AT–Atwater Valley; EB–East Breaks; GC–Green Canyon; GB–Garden Banks; KC–Keathley Canyon; L–Lund; LS–Lund South; MC–Mississippi Canyon; SE–Sigsbee Escarpment; WR–Walker Ridge. Structure maps on the top Oligocene, top Wilcox, top Cretaceous, and basement formed the regional surfaces between which isopach/isochron maps were created to analyze depositional patterns. As might be expected, basement structure displayed the greatest relief and complexity. Outboard from the allochthonous salt of the Sigsbee Escarpment, half-graben structures indicative of rift basin topography were clearly imaged ( Fig. 2 ). Elsewhere on the abyssal plain isolated, sharp-peaked, elevated basement features were observed between more numerous gently sloped highs. These basement structures typically had reflection terminations against their margins or flanks and continuous reflections draping them. Figure 2. 2D Pre-stack time migrated line showing rift basin structure in the basement, Wilcox strata down lapping onto the Cretaceous and thinning to the east, and Oligocene strata down lapping onto the Wilcox and thinning to the north. The vertical scale is in seconds of two-way time (TWT). The horizontal scale is in feet (100,000 feet ~ 18.94 miles or 30.55 kilometers). Abbreviations for horizons: Olig=Oligocene (orange); Wx=Wilcox (blue); K=Cretaceous (green); J=Jurassic (pink); and Bsmt=basement (yellow). The top Cretaceous and top Wilcox surfaces show broad regional similarities and show less structural complexity than the basement. Outboard of the Sigsbee Escarpment, both surfaces are broadly lobate and have relatively gentle inclinations which rise to the east. The main observable differences between the two are: ( A ) the Cretaceous surface has several isolated high points reflecting underlying basement structures and ( B ) the Wilcox surface has a more lobate/interdigitate contour character. The top Oligocene surface is less lobate in appearance than either the Cretaceous or Wilcox surface and rises to the southeast ( Fig. 3 ). Figure 3. Time structure map on the top Oligocene. The contour interval is 50 milliseconds. Isochron maps between the four structural surfaces reflect the underlying structure and depositional trends of the interval. Thus the basement to Cretaceous isochron shows thick Jurassic infill, Cretaceous drape in the grabens ( Fig. 2 ), and thin to no cover over highs in the rifted basement topography. The Cretaceous to Wilcox isochron has a broad lobate form that thins gently from west to east. A very subtle down-lapping pattern is visible within the Wilcox interval on Figure 2 . Deviations from this pattern occur primarily where basement structures produce isolated thins. The Wilcox to Oligocene interval shows a regional gradient of north to south thickening and only a slight influence from deeper structure. Down-lapping and thinning to the north strongly suggest a southerly source for the Oligocene interval. Beneath the allochthonous salt of the Sigsbee Escarpment, all surfaces deepen northward and show much greater local variability. Basement is only occasionally visible as it generally lies below the fifteen kilometer limit of the available PSDM data. The deepest area mapped is in Green Canyon where the top Oligocene approaches twelve kilometers depth, the top Wilcox approaches thirteen kilometers, and the top Cretaceous almost fourteen and one half kilometers. These surfaces shallow to less than eight kilometers deep on the abyssal plain. Three coincident lows roughly oriented north-south suggest preferred sediment pathways and possibly areas of thicker original autothonous salt. A change on the structure and isopach maps from smooth broadly spaced contours on the abyssal plain to highly variable tightly spaced contours suggests the location for the original limits of salt deposition in this area. This location often lies close to but not exactly in line with the present day Sigsbee Escarpment ( Fig. 1 ). Of key interest to hydrocarbon explorationists are any factors that would effect Wilcox deposition. We have observed three factors that influence the deposition and thickness of Wilcox age strata in this area: Pre-existing basement highs have caused the Wilcox to be thin or absent around those structures. Although basement topography is mostly smoothed over by the end of the Cretaceous, a few large structures still influenced deposition in the Wilcox on the abyssal plain beyond the Sigsbee Escarpment. Salt nappes and salt pillows have caused thinning of Wilcox strata over those structures. Our interpretation indicates multiple kilometer thick salt nappes extruded beyond the limits of the original salt basin during the Cretaceous ( Figs. 4 and 5 ). Inflated salt pillows associated with the nappes lay along the boundary of the salt basin. Though now deflated, the presence of these salt pillows and other salt pillows updip are recorded by the depositional thinning of Wilcox strata above them. These allochthonous bodies provided the core structure over which Wilcox and Miocene reservoirs are folded or draped at Chinook, Atlantis, Das Bump, and other important deepwater discoveries. The location of allochthonous salt at the onset of Wilcox deposition is apparently coincident with the pronounced increase in northerly dips of the Mesozoic and Paleogene strata. This relationship is consistent with originally thick autochthonous salt above the deepest mapped basement. Sites of continued salt withdrawal from the autochthonous level into growing salt structures directly affected Wilcox sediment thickness. Such sites would have been primary candidates for the location of Wilcox sediment fairways. Identification and elimination of salt feeders would help in refining/defining these pathways. Figure 4. 3D Pre-stack depth migrated line showing a Cretaceous age salt nappe and its associated deformation front. Both features lie just basinward of the modern Sigsbee Escarpment. Thinned Wilcox and Oligocene strata show where a now evacuated salt pillow once existed. The vertical and horizontal scales are in kilometers. Abbreviations for horizons: Olig=Oligocene (orange); Wx=Wilcox (blue); K=Cretaceous (green); J=Jurassic (pink); and Bsmt=basement (yellow). Figure 5. 3D Pre-stack depth migrated line showing a second Cretaceous age salt nappe. This one lies about thirty kilometers shoreward of the Sigsbee Escarpment. Thinned Wilcox strata and an Oligocene turtle structure show where a now evacuated salt pillow once existed. The vertical and horizontal scales are in kilometers. Abbreviations for horizons: Olig=Oligocene (orange); Wx=Wilcox (blue); K=Cretaceous (green); J=Jurassic (pink); and Bsmt=basement (yellow). Deposition of the Wilcox strata can be broadly divided into two paleogeographic domains: ( A ) a relatively complex north-westerly region characterized by pre-existing, elevated sea-floor, salt-cored structures and sites of contemporaneous salt evacuation, and ( B ) a relatively simple south-easterly region characterized by a near flat and smooth sea-floor rarely punctuated by unburied basement structures. The transition between these two regions should mark changes in Wilcox depositional styles. In the more complex topographic region, Wilcox depositional events were forced to interact with relatively rapid changing sea-floor dips. Whereas in the more simple region to the southeast, a much more unconfined sea-floor presented limited impediment to widespread expansion of depositional events exiting the more complex region to the north-west. Drilling of Wilcox strata to-date has been mainly in the simpler south-easterly region and in the transition zone to the more complex Wilcox geometries towards the north-west. Figure 4 shows an example of one salt nappe and its contractional deformation front that lies in close proximity but basinward of the Sigsbee Escarpment. Thrust relationships suggest that the nappe continued to move/inflate until the end of the Cretaceous. An inflated salt pillow associated with the nappe is present through the Oligocene but then deflates during the Miocene. This interpretation is supported by the thin but depressed Wilcox and Oligocene section behind the nappe today. We predict that the edge of the salt basin lies behind the nappe, below where the Wilcox and Oligocene intervals begin dipping to the north. Figure 5 shows another example of a salt nappe that lies in about thirty kilometers inside of the Sigsbee Escarpment. This nappe does not have a deformational front associated with it. But an inflated salt pillow is associated with this nappe as in Figure 4 . Similar to Figure 4 , the interpretation is supported by a thin but depressed Wilcox section behind the nappe. In contrast, evacuation of the pillow begins in the Oligocene, as evidenced by the Oligocene age turtle structure. Evacuation continues into the Miocene until the pillow is completely deflated. The nappe remnant is all that remains of this salt body. Unique to these two examples, but possibly typical of most salt pillows around the edge of the salt basin, loading has forced salt backwards (updip) into the salt basin. In Figure 4 , the reversal of salt movement is about ten kilometers. In Figure 5 , the reversal of salt movement may be twenty to twenty-five kilometers.