BROOKS, ROBERT O., TGS-CALIBRE Geophysical Company, Houston, TX

ABSTRACT: Upper Tertiary/Quaternary Detachment Surface, Gulf Coast of Texas and Louisiana

Over the past five years, deep seismic resolution in the Gulf of Mexico has improved to the point that extensive horizontal salt bodies have been defined and initial drilling has commenced for subsalt reservoirs. This enhancement of the deeper seismic also shows evidence of a related deep and extensive decollement surface or unconformity that divides the geologic section into two separate but very different structural realms. Upper Pleistocene to Miocene sediments are mixed sand and shale, extensively structured by movements of mobile salt/shale and numerous listric growth faults that generally flatten with depth along a common decollement or detachment surface. Below the decollement zone from 15,000 to over 25,000 ft Lower Miocene through Oligocene mostly shaly sediments exhibit more limited structuring and high-angle faults. Most of the horizontal salt injection on the southern Louisiana shelf occurred within the thick deep-water shales that postdate this surface. Later loading by Pliocene-Pleistocene sands and shales caused extensive additional salt movement. Penetrations into the deeper zones are limited, but include wells in Louisiana, South Marsh Island, and Main Pass. The age of the surface varies from early Miocene to late Eocene and is seismically defined by the flattening of overlying listric faults, discordant dips that in some cases appear as angular unconformities, and higher amplitudes along salt surfaces. The exploration potential is currently questionable due to depth, abnormal pressures, and risk of reservoir sands. There is considerable work to be do e, however, toward understanding the relationships to hydrocarbon migration, Pliocene-Pleistocene deposition, and overlying structuring.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.