--> ABSTRACT: Late Burial Diagenesis Driven by Thermal Degradation of Hydrocarbons and Thermochemical Sulfate Reduction: Upper Smackover Carbonates, Southeast Mississippi Salt Basin, by Ezat Heydari, C. H. Moore, and R. Sassen; #91030 (2010)

Datapages, Inc.Print this page

Late Burial Diagenesis Driven by Thermal Degradation of Hydrocarbons and Thermochemical Sulfate Reduction: Upper Smackover Carbonates, Southeast Mississippi Salt Basin

Ezat Heydari, C. H. Moore, R. Sassen

Deeply buried (6 km) upper Smackover grainstones exhibit a complex physical and chemical diagenetic history, starting shortly after burial and continuing to the present. Paragenesis prior to crude oil migration is represented by early marine cementation and postcompaction complexly zoned calcite followed by saddle dolomite, anhydrite, and highly luminescent calcite cements. Oxygen isotopic compositions are thought to indicate precipitation temperatures of 40°-80°C prior to hydrocarbon migration. Carbon isotopic compositions of pre-hydrocarbon migration carbonates remain constant, but carbonate oxygen becomes isotopically lighter. Although crude oil migration initially retarded chemical diagenesis, this process resumed during subsequent deeper burial as a consequ nce of thermal cracking of crude oil to form gas-condensate, abundant solid bitumen, and ultimately methane and nonhydrocarbon gases. Thermochemical sulfate reduction and hydrocarbon oxidation generated hydrogen sulfide and carbon dioxide resulting in precipitation of metal sulfides (pyrite, sphalerite, and galena), replacement of anhydrite by calcite, and late precipitation of calcite whose highly negative carbon isotopic composition indicates a significant contribution of carbon from thermal oxidation of hydrocarbons. Oxygen isotopic compositions of post-hydrocarbon migration carbonates remain constant, but carbonate carbon becomes isotopically lighter. Sulfide sulfur isotopic compositions and burial history models suggest that rapid thermochemical sulfate reduction and hydrocarbon oxi ation commenced in the 110°-120°C range, and continues at the present time in the deep Smackover (200°C).

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.