Abstract Regional mapping and maturity modeling show distinct patterns that are characteristic of the complex petroleum system in the deep water portion of the northern Gulf of Mexico (GOM). Maturity for source rocks within the Cretaceous and Jurassic sections tends to increase from the abyssal plain to the salt canopy province as the overlying section thickens. One striking exception to this trend is the Cuba fracture zone, which extends southeast from South Pass to the southern GOM. Observed as a strong magnetic anomaly in basement maps, the Cuba fracture zone shows other impressive anomalies. Heat flows tend to increase approximately 25% along the zone relative to calibration points on either side, which suggests that it is an important crustal feature. Empirically, the Cuba fracture zone appears to be a major dividing point in the north central GOM, where on its northeast side gas appears to be much more prominent than on the southwest side. Mapping the GOM on a regional scale required the integration of 2D and 3D seismic data, gravity and magnetic data, and large-scale velocity models that include salt for proper depth conversion. Basement maps were generated from the integration of gravity and magnetic data with acoustic basement mapping from seismic in the abyssal plain. A variety of key chronostratigraphic horizon depth maps were generated from a regional velocity model that included salt and was applied to multiple time horizons. Probably the most difficult mapping task was to make accurate correlations between areas with enormous amounts of data (e.g ., 3D seismic) and those with a paucity of data (e.g ., 2D seismic in subsalt sections). Developing maturity models required an accurate set of stratigraphic depth maps, calibration and mapping of heat flow on a large scale, and the appropriate choice of source rock horizons and associated properties to evaluate. Results from this regional evaluation indicate that there is a definite relationship between source rock maturity and major oil and gas discoveries. The timing of hydrocarbon generation and migration relative to the timing of structuring is critical to each successful discovery. This can be evaluated on a regional scale when maturity results are placed in context with general structural trends.

Abstract The first test of the Bell Aerospace gravity Gradiometry Survey System (GSS) for geologic applications was conducted in April 1994 in collaboration with the U.S. Navy. The GSS is a recently declassified gravity sensing system that contains the world's only moving-base gravity gradiometer. The system measures both gravitational acceleration and gravity gradients, yielding six measurements that define the local gravity field and its gradients in three dimensions a technologic advance in measuring gravity analogous to the advance from 2-D to 3-D seismic profiling through the towing of multiple rather than single hydrophone arrays. The gravity gradiometry test survey was conducted over a buried salt structure southsoutheast of New Orleans in water depths of ˜1500 m. The quality of the survey data is excellent. In declassified grids of the data at 2-km wavelengths, gravity gradients are resolved to 0.5 and gravity to 0.07 mGal. Simple models are used to illustrate the power of this data in subsurface structure definition. The potential utility of gravity gradiometry in oil and gas exploration then is demonstrated through application of the survey data in improving a geologic model of a part of the survey area derived from 3-D seismic data.

ABSTRACT The Perdido fold belt, located in the northwest part of the Gulf of Mexico basin, is defined by a series of large-scale fold structures that extend southwest into Mexican waters and northeast beneath the Sigsbee salt nappe. Within the Alaminos Canyon OCS lease area, the fold belt consists of northeast-southwest trending, sub-parallel, concentric, box folds cut on one or both of their flanks by high-angle reverse faults. The folds are slightly asymmetric and verge both landward and basinward, a geometry typical of contractional fold belts formed above a weak detachment layer. The folds uplift the regional middle Cretaceous sequence boundary (MCSB) by up to 3 km, with a basinward decrease in height and amplitude of the folds. Detailed structural mapping has led to a model for the structural evolution of the Perdido fold belt that is consistent with sequence stratigraphic analysis of the seismic data. Minor salt movement occurred during the Late Jurassic and Early Cretaceous, as indicated by onlapping and thickness variations within the relatively thin overlying section at that time. Salt mobilization before the main phase of shortening led to early growth of some fold structures during the Eocene-early Oligocene. The main phase of compressional deformation occurred during the late Oligocene-early Miocene by gravity sliding on a detachment within the Jurassic Louann Salt. The basinward limit of autochthonous salt deposition defined the southeastern margin of the foldbelt. Detailed analyses of onlapping middle to upper Miocene strata indicate that separate folds had different evolutionary histories and developed variable along-strike geometries. Subsequent Pliocene to present-day reactivation of the highest-relief structures further modified the fold geometries. Topographic relief over the highest folds has in turn influenced the evolution of the allochthonous Sigsbee salt nappe. The advancing salt nappe has been deflected around the highest fold structures, resulting in a complex allochthonous salt sheet geometry. Previous studies of the Perdido fold belt have produced conflicting interpretations for the evolution of the fold geometries. These include salt or shale-cored interpretations and the development of the fold geometries by imbrication and fault-bend folding. Our interpretation favours an origin as salt-cored detachment folds, with late modification by re-mobilization of salt in the cores of the folds.