Tectonic Subsidence, Crustal Thinning, and Petroleum Generation in Jurassic Trend of Mississippi, Alabama, and Florida

B. W. Driskill, J. A. Nunn, R. Sassen

The Atlantic-type passive margin of the northeastern Gulf of Mexico consists of a thick section of Mesozoic and Cenozoic sedimentary strata. Tectonic subsidence curves, calculated by backstripping stratigraphic columns acquired from deep-well data in Mississippi, Alabama, and Florida, compare well with theoretical subsidence curves for uniform extension of the lithosphere. Results support an interpretation of rift-related lithospheric extension during the Early to Middle Jurassic, followed by a long period of thermal subsidence. Subsidence patterns indicate that the lithosphere is unattenuated north of the peripheral fault zones and thins southward. Crustal thinning reached its maximum in the deep Mississippi Salt basin and in state waters west of Mobile Bay. Present-day crustal thicknesses estimated from tectonic subsidence curves agree well with seismic refraction data and show that thick transitional crust underlies the northeastern Gulf margin.

Using the one-dimensional heat-flow equation and including effects of sedimentation rate, compaction, and radiogenic heat production, the thermal history of the sediment fill was determined. Constraints for the model were provided by geochemical data, bottom hole temperatures, and heat flow measurements. Model results show that the Jurassic Smackover Formation, the regional source rock, entered the oil window during Early to early Late Cretaceous. Calculated thermal maturities suggest that the Smackover may reach maturity levels sufficient to generate liquid hydrocarbons at burial depths as shallow as 3 km and become overmature at a depth of approximately 5 km.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.