ABSTRACT

Over the past five years improvements in 2-D and 3-D seismic resolution and the availability of new well data in the Gulf of Mexico has led to changes in previous interpretational concepts. The enhancement of deeper seismic in some areas, shows evidence for regional decollements, which divide the Tertiary-Quaternary sediments into two separate structural realms. Also being updated is the seismic definition of horizontal salt bodies, and the potential of the underlying sediments as a new exploratory play. Initial tests of sub-salt sediments have already been drilled.

The upper portion of the geologic section down to 15,000 - 18,000 feet is deformed by primary and secondary movements of mobil salt and shale, has numerous concave listric growth faults, and is extensively drilled. Below the decollements surface and base of salt, sediments are more shale prone, structuring is more limited, and the section is penetrated by only a few wells. The age of the surface varies from Pleistocene downdip (Sigsbee Scarp) to Upper Eocene onshore and is seismically defined by a variety of common visual seismic signatures that can be integrated with limited well control.

Exploration potential below the surface is currently still under evaluation due to its depth, presence of abnormal pressures, and risk of reservoir sands. Considerable work is also underway to better understand the relationships to salt injection, hydrocarbon migration, overlying deposition and basin structuring. Current data indicates that the decollements on the southern Louisiana-Texas shelf and out onto the slope correlate with the base of salt where horizontal injection has occurred. Updip they correlate to thin salt and ductile shale.

Until recently, Gulf Coast seismic interpretation of the Tertiary-Quaternary section has presumed the depositional history, sedimentation and salt flowage of the deeper (15,000 - 30,000 feet) portion of the geologic section was similar to the upper, shallower zones. For many years interpretations of sediments along the Gulf Coast and out across the shelf of Louisiana and Texas were dominated by concepts related to shifting depocenters, large extensional listric growth faults, and vertically injected piercement domes rooted in the Jurassic Loanne salt deposits. Earlier updates (Martin, 1980) divided the offshore shelf and slope into three salt styles which were: 1) the tongues along the Sigsbee Scarp; 2) ridges and walls on the slope of Texas and Louisiana; and 3) piercements across the shelf. A re-evaluation of salt injection features based on 180,000 miles of 2-D regional seismic data has expanded previous concepts, and focused greater emphasis on the existence and extent of detachment surfaces, horizontal salt injections and associated weld zones.