Kenneth C. Hood, Lloyd M. Wenger, Oliver P. Gross, Stanley C. Harrison, Lynne R. Goodoff
Exxon has conducted an integrated, multi-disciplinary study of sources, maturation, and migration pathways in the offshore Gulf of Mexico (GOM). We developed a geological framework from 2-D and 3-D seismic, identified and mapped potential source intervals, and delineated migration pathways to reservoirs and amplitude anomalies. Hydrocarbon compositions from over 2000 oils, 500 gases, and 1000 hydrocarbon-bearing seabottom drop cores constrain source rock characteristics, such as organic matter type, depositional facies, and, to some extent, age, East of the Mississippi River Delta the complete stratigraphic section is seismically visible and wells have penetrated deep source intervals. To the west, correlative organic-rich rocks have been sampled onshore and from sheaths verlying salt diapirs offshore. Integration of these data with the regional geological framework provides a strong basis for hydrocarbon system interpretations.
Major offshore hydrocarbon systems are derived from Lower Tertiary (Eocene), Upper Cretaceous (Turonian), and Upper Jurassic (Tithonian) sources. All Eocene oil types (marine, intermediate, terrestrial) have been tied to source rocks and are consistent with paleofacies distributions for the Eocene deltaic systems. Eocene oils and gases are prevalent on the Texas and Louisiana shelves and extend onshore. Turonian oils have been tied to source rocks offshore (east of the delta) and onshore (e.g., Tuscaloosa, Giddings trends). Offshore we interpret a basinward loss of this source rock based on seismic thinning. Elevated-sulfur oil and associated (cogenerated) gas on the upper GeM slope are interpreted to be Tithonian in age. High-maturity, organic-rich calcareous shales of this age have een penetrated in the eastern GOM, and Tithonian oils occur in Lower Cretaceous reservoirs on the Florida shelf where the Upper Cretaceous and Tertiary sections are immature. Oxfordian carbonate-sourced oils are common across the northern GOM rim, and low maturity hydrocarbons from this source are found in seeps and stains in the deep central GOM.
Widespread oil and gas seepage in the GOM has allowed extension of hydrocarbon systems and maturity maps far beyond well control. Abundant seepage documents that the GeM slope is an actively migrating hydrocarbon system and provides a means of identifying major migration pathways. improved understanding of regional hydrocarbon systems has provided new exploration methodologies and play concepts.
AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995