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Models for Hydrocarbon Generation and Destruction, Eastern Jurassic Smackover Formation

Roger Sassen, J. A. Nunn, C. H. Moore, F. C. Meendsen, V. K. Iliff

A regional source rock model contributes to explaining the distribution of oil and gas in the Smackover trend of Mississippi, Alabama, and Florida. The source rock model is based on organic geochemical analysis of an extensive suite of Smackover cores and crude oils and calculation of subsidence and temperature histories.

Although the Smackover is a significant source rock across the study area, source potential is variable and localized with the lower member of the formation. Oil-prone kerogen is preserved in laminated carbonate mudstones of the lower Smackover that were deposited at subtidal depths under anoxic and hypersaline conditions. Organic richness of the Smackover source facies is modest, but (1) expulsion of hydrocarbons is efficient, and (2) migration is focused by evaporite seals that often occur above and below Jurassic source and reservoir rocks.

Thermal maturity is a major factor that controls the generation of oil in the lower Smackover, and the eventual destruction of hydrocarbons in upper Smackover and Norphlet reservoir rocks. Thermal maturity measurements across the eastern Smackover trend show variations that are not solely the result of differences in burial history. Geothermal gradients have changed with time, but also appear to vary regionally. In the Mississippi Salt basin and Conecuh embayment, differences in maturity can be attributed to lateral changes in thermal conductivity of basin sediments. The relatively advanced maturity of the Smackover near the Wiggins arch appears to be the result of heat flow from adjacent basins.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.