Depositional Setting of the Upper Jurassic Smackover Carbonate Source Rock, Northern Gulf of Mexico: Link to Paleoclimate

Roger Sassen
Geochemical and Environmental Research Group (GERG), Texas A&M University, College Station, TX

Laminated lime mudstone of the lower member of the Upper Jurassic Smackover is a source for high-sulfur oil across the northern Gulf Coast and offshore in the gulf continental slope. The Smackover Sea bordered a vast near-equatorial desert with dune sands and a paucity of higher plants. Restricted shallow water and hot winds affected Smackover source rock deposition. The source rock consists of thin laminations with high total organic carbon, sulfur-rich kerogen, diagenetic pyrite, and transported detritus including clay catalysts. Periodically-spaced laminations are separated by barren carbonate indicating repeated oscillations in paleoclimate. Algae and microbes proliferated in the sea after brief, periodic pulses of nutrients. Rapid evaporation of the shallow water limited biodiversity of algae and favored unusual microbes in the water column and sediment. Anoxic density-stratified brine caused efficient preservation of oil-prone kerogen lacking significant higher-plant detritus. Several lines of evidence link harsh Jurassic climate events to source deposition. Carbon isotopes of kerogen and oil are diagnostically enriched in 13C, reflecting the high content in the atmosphere and sea of carbon dioxide from rifting. Moreover, the rocks contain unusual, diagnostic biomarkers consistent with low biodiversity in a stressed environment. The source rock shows great lateral and vertical variation. The best evidence of the harsh Jurassic paleoclimate is found in rocks and oils where low burial depths preserved maximum information from carbon isotopes, biomarkers, and early-generated diamondoids. Oil was often altered by thermal cracking to form gas-condensate and finally methane and solid bitumen, progressively degrading geochemical insight to environment of deposition during deep burial.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas