Geochemical Studies of Crude Oil Generation, Migration, and Destruction in Mississippi Salt Basin

Roger Sassen, C. H. Moore, J. A. Nunn, F. C. Meendsen, Ezat Heydari

The main source for crude oil in the Mississippi salt basin is the laminated lime mudstone facies of the lower Smackover. Crude oil generation and migration commenced at a level of thermal maturity equivalent to about 0.55% vitrinite reflectance. Short-range lateral migration of crude oil was focused on upper Smackover and Norphlet reservoirs, but vertical migration also charged some overlying Cotton Valley, Rodessa, lower Tuscaloosa, and Eutaw reservoirs.

Following migration from the lower Smackover, thermal maturity history of reservoir rocks controls the preservation of crude oil, gas condensate, and methane. Slow thermal cracking of crude oil occurred in deep upper Smackover reservoirs, resulting in formation of gas condensate and precipitation of solid bitumen. The maximum thermal maturity for preservation of condensate is equivalent to about 1.3% vitrinite reflectance. Only methane, pyrobitumen, and nonhydrocarbon gases, including hydrogen sulfide, persist at higher levels of thermal maturity. Early destruction of methane in deep upper Smackover reservoirs near the Wiggins arch is driven by thermochemical sulfate reduction. Lesser availability of sulfate in Norphlet reservoirs could account for methane preservation at higher level of thermal maturity.

One basic geochemical strategy for further exploration of the Mississippi salt basin is to focus exploration effort on traps with reservoirs in the thermal maturity window for hydrocarbon preservation. Another strategy is to avoid drilling traps with overmature reservoir rocks.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.