ABSTRACT Exploration for oil in the Norphlet reservoir in the deep-water Gulf of Mexico began in 2003 at prospect Shiloh (DC269). The well found oil but not an economic volume. The second prospect, Vicksburg (DC353), was drilled in 2007. This well found a larger in-place volume of oil, but with an immovable solid hydrocarbon component within pore spaces, there was great uncertainty as to the potential producible volumes. Two subsequent wells (Fredericksburg [DC486] and Antietam [DC268]) were dry and had a very small amount of oil, respectively. Finally, in late 2009, the fifth well (Appomattox [MC392]) was a significant discovery of high-quality oil in a thick aeolian Norphlet sandstone.

Published estimates for the original volume of Mid-Jurassic Louann evaporites found throughout the entire Gulf of Mexico Basin vary widely. Volume totals derived from both map data and actual volume numbers, range from about 10,500 km 3 (2,500 mi 3 ) to 839,000 km 3 (200,000 mi 3 ), an 80 fold variation. Little new information has been published during the past twenty-five years to address this disparity. But gaining knowledge of the present day volume of salt would be an important metric if debates concerning the origin of the salt and the nature of the Gulf of Mexico Basin during salt deposition are to be reconciled. A methodology now exists to estimate more accurately and quantitatively the volume of salt present in a given area. Multiple, recent generations of 3-D seismic depth volumes in the offshore Gulf of Mexico require that salt velocities be inserted. This vital processing step includes a systematic picking and interpretation of the tops and bases of all salt bodies encountered. The resulting models of salt velocity allow salt volume in the 3-D data sets to be calculated. Combining the salt volumes calculated from multiple seismic surveys offers new stratigraphic insights across large portions of the original salt basin. A comparison of salt volumes derived from seismic data cubes and volumes derived from published maps can now be made. The comparisons should give some suggestion as to the accuracy of the map data. By extrapolation it should also give a more accurate and quantitative estimation of the original salt volume deposited in the Gulf of Mexico basin.

Abstract Interpretation of ION-GXT long-offset 2D depth-imaged and other seismic data suggests that outer continental margins collapse and tilt basinward rapidly as rifting yields to seafloor spreading. The outer continental margin, already thinned by rifting processes, can be viewed as the hanging wall of a mega-half-graben associated with a landward dipping shear zone. The footwall of the shearmega is the serpentinised or serpentinising sub-continental mantle that commonly becomes exhumed as it rises from beneath the thinned and embrittling continental margin. We call this shear zone between the continental crust and the rising subcontinental mantle the “outer marginal detachment.” At magma-poor margins, the outer continental margin collapses to tilt basinward (“outer marginal collapse”) to depths up to 3-3.5 km sub-sea at the continent-ocean transition, so that it normally lies deeper than the adjacent oceanic crust (accreted later by seafloor spreading). We use the term “collapse” because of the rapidity of the deepening, which seems to be <3 Ma, and possibly <1 Ma. Salt deposition or rapid clastic (deltaic) sedimentation at non-magmatic margins, or SDRs and less organised magmatism at magmatic margins, may accompany the collapse, with salt thicknesses rapidly reaching 5 km and volcanic piles reaching 15-20 km, as controlled by isostasy. This mechanism of rapid salt deposition allows some megasalt basins ( e.g ., Gulf of Mexico) to be deposited on synrift sections at global sea level, although other salt basins may form in air-filled depressions below sea level. Outer marginal collapse is “postrift” from the perspective of the continental crust, but of tectonic, and not of thermal, origin, owing to shear on outer marginal detachments. We examine how outer marginal collapse can migrate diachronously along strike, much like the onset of seafloor spreading, as is required in models of ocean creation. We suggest that backstripping estimates of lithospheric thinning (beta) at outer continental margins may be excessive, because they probably attribute outer marginal collapse to thermal subsidence.

Abstract The petroleum geology of the Mississippi Canyon, Atwater Valley, western DeSoto and western Lloyd Ridge protraction areas, offshore northern Gulf of Mexico, is controlled by the interaction of salt tectonics and high sedimentation rate during the Neogene, and has resulted resulting in a complex distribution of reservoirs and traps. Seventy-eight fields/discoveries are evaluated and comprise structures with four-way closures (18), three-way closures (46), and stratigraphic traps (14). Three of these discoveries are in Upper Jurassic eolian reservoirs, the remainder are in Neogene deep-water reservoirs. The tectonic-stratigraphic evolution of the area is analyzed at eleven discrete intervals between 24 Ma and Present. The analyses show how the allochthonous salt systems evolved over time, and their effect on sedimentation patterns and sub-basin evolution. The study area includes some of the largest fields in the northern deep Gulf of Mexico. Thunder Horse produces from an anticlinal (turtle) structure that developed with a basement-controlled allochthonous system. The greater Mars-Ursa sub-basin has nine fields with > 1.5 BBBOE EUR, including Mars, Ursa and Princess, that developed with a counterregional allochthonous salt system. The remaining fields have considerably smaller reserves, which are controlled by the area within closure and number of reservoir intervals. Many of the smaller fields are produced from one well subsea tiebacks. Most of fields in the study area are contained within sheet-like or wedge-shaped stratigraphic intervals and have four-way or three-way trapping configurations. These findings reflect the profound effect that mobile salt has had on the petroleum geology of the region.

Abstract We summarize four emerging concepts in salt tectonics in the deepwater Gulf of Mexico, selected from a longer list of concepts that have advanced significantly in the last decade. Squeezed salt stocks are common in orogenic forelands, in inverted basins and at the toe of salt-bearing passive margins. Modelling suggests that during early shortening, an inward salt plume from the source layer inflates the diapir and arches its roof. After further shortening, diapiric salt is expelled as an outward plume back into the source layer. Salt canopies are conventionally thought to advance by glacial extrusion. However, almost all modern salt canopies are now buried and can only advance by frontal thrusting. Thrusting allows the salt canopy and its protective roof to advance together, minimizing salt dissolution. Advance is by a roof-edge thrust rooted in the leading tip of salt or by thrust imbricates forming accretionary wedges. Minibasins can sink into salt if the average density of the overburden exceeds that of salt. This requires 2–3 km of burial of siliciclastic fill, yet most minibasins first sink when much thinner. Three alternative mechanisms to negative buoyancy in the deepwater Gulf of Mexico address this paradox of initiation. First, squeezed diapirs inflate, leaving the intervening minibasins as depressions. Second, when a diapir's salt supply wanes, the overlying dynamic salt bulge subsides, allowing a minibasin to form. Third, differential loading causes the thick end of a sedimentary wedge to sink faster into the salt, creating a sag. Spreading salt canopies can transport their dismembered roof fragments tens of kilometres basinward. These exotic fragments are up to 25 km in breadth and comprise anomalously old Mesozoic through Miocene sequences. Strata of the same age underlie the salt canopy or its welded equivalent, signalling lateral transport by thick salt.