Abstract Detailed study of exploration wells penetrating the Wilcox interval in the Walker Ridge and Keathley Canyon protraction areas, Gulf of Mexico, support the presence of an extensive, correlative, Wilcox depositional system. They also reveal differences in structural styles and timing of formation. While mobilization of the Jurassic autochthonous salt layer results in many deep structures, their evolution and style varies across the region. South of Green Knoll in Walker Ridge, the structural style exhibits characteristics of compression and inflation, and greatest growth is in the middle to late Miocene. This style transitions into structures along remnant polygonal salt ridges resulting from evacuation of the autochthonous salt layer into the allochthonous salt canopy during the middle to late Miocene. Farther west into Keathley Canyon, earlier salt movement, possibly generated by up-dip Oligocene extension, creates structures along remnant salt ridges associated with salt evacuation and inflation.

Abstract We describe the exploration plays associated with the salt nappe, canopy, and massif systems of Walker Ridge and Keathley Canyon areas in the deep water Gulf of Mexico. Depth imaging of 3D seismic allows definition of the salt emplacement and deformation history, and the associated subsalt trap styles. Salt emplacement in the region follows a simple history: relatively evenly spaced inferred paleo salt stocks have fed salt canopy, nappe, and massif systems. In this region, salt is emanating directly from the Jurassic Louan layer. Mapping of the salt allows division of the present–day salt masses into discrete salt “cells.” Emplacement and extrusion occurred in a series of low-angle and high-angle surfaces, dominantly lateral in the canopy and nappe systems, and dominantly vertical in the massif system. Subsalt structural traps are divided into three major types, from deepest to shallowest: (1) Anticlinal salt–cored folds of Mesozoic and Paleogene strata; (2) Structural inversions (“turtles”) of Paleogene and Neogene strata; and (3) Counterregional dip and truncation of Neogene strata against the vertical salt emplacement to lateral salt emplacement transition. Mesozoic anticlines are located basinward of a regional low in the Middle Cretaceous sequence boundary. These structures are fully detached from the superjacent lateral salt masses. These structures are on–trend with recent significant discoveries in more shallow waters. Structural inversions are associated with paleo salt stocks. Salt truncation traps are more shallow and offset from the crest of the inversion structures. Definition of salt “cells” allows an understanding of the development of each structure in the trend, which may imply subtle differences in structural timing and trap competency prior to hydrocarbon emplacement.

ABSTRACT The lower continental slope of the northwestern Gulf of Mexico is underlain by shallow, allochthonous salt. Keathley Canyon is one of several large canyons that locally define the salt front, extending into the interior of the slope province. We studied multichannel seismic reflection profiles and bathymetric data to evaluate canyon morphology, origin, and evolution. The canyon can be divided into two morphologically contrasting parts: (1) the upper canyon, consisting of a narrow valley, and (2) the lower canyon, consisting of a broad reentrant. Unlike most other canyon systems, it contains no levees, tributary valleys, distributaries, nor the expected sedimentary fan at its mouth. Also, it is too far seaward to be related to rivers that, in other areas, have initiated many submarine canyons during sea-level lowstands. Multichannel seismic reflection profiles provide evidence of salt control on the formation of the canyon. Beneath the upper canyon, laterally moving salt occurs as two large lobes and a smaller one in the middle. These salt wedges uplifted the continental slope and rise sediments and coalesced, resulting in a valley-like feature with walls of uplifted and deformed strata. Once established, the narrow upper canyon was preserved and/or steepened by erosive, gravity-controlled processes. In the downslope position, a broad bathymetric low in the interlobal area between the two large salt lobes forms the lower canyon.

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.

The salt tectonics of the lower slope of the northwestern Gulf of Mexico (GOM) in the Keathley Canyon and Walker Ridge protraction areas (Fig. 1) involves the interaction of allochthonous salt with its attendant cover, creating a range of map-scale geometries due to the kinematics of contraction, extension, and growth. Depth-converted structure mapping on a regional scale documents withdrawal basin morphology and trends formed during the evolution of major salt lobes in the mid to late Tertiary period. These observations aid exploration efforts in regions dominated by mobile salt in the GOM. The Sigsbee salt province serves as a data-rich analog for other salt provinces world-wide.