--> Abstract: Controls on Thermochemical Sulfate Reduction and Alteration of Smackover Crude Oils, Alabama and Arkansas, by W. J. Wade, R. Sassen, C. H. Moore, and R. D. Snelling; #91012 (1992).

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ABSTRACT: Controls on Thermochemical Sulfate Reduction and Alteration of Smackover Crude Oils, Alabama and Arkansas

WADE, WILLIAM J., Louisiana State University, Baton Rouge, LA, ROGER SASSEN, BP Exploration, Houston, TX, and CLYDE H. MOORE and RON D. SNELLING, Louisiana State University, Baton Rouge, LA

Reactions between reduced sulfur species and hydrocarbons in sour Smackover reservoirs can result in alteration or destruction of hydrocarbons at relatively shallow depths. Although onset of thermo-chemical

sulfate reduction (TSR) is dependent upon temperature, pressure, and original sulfur content of crude oils, the degree of alteration may be more critically controlled by availability of dissolved sulfate and proximity to Fe-rich siliciclastic strata, which remove dissolved H2S by precipitation of metallic sulfides. H2S concentrations in some Smackover reservoirs apparently reflect local steady-state conditions in which generation of H2S is balanced by flux of H2S out of the system. Incorporation of sulfur derived from Jurassic sulfates (Sulfur 34 ~ +16o/oo CDT) commonly results in a trend of progressive enrichment in Sulfur 34 in organosulfur compounds and inorganic sulfur-bearing species above a temperature of approximately 115 degrees C. However, where the rate of TSR is limited, su fur isotopic fractionation between organosulfur compounds, sulfides, SO4, and H2S may be significant. Smackover crude oils that react with abundant TSR-derived sulfur show preferential cracking of heavy saturates and relative preservation of aromatics. Higher H2S concentrations, progressive lowering of C15+ saturate contents, decreased saturate/aromatic ratios, and heavy Sulfur 34 values of organosulfur compounds serve to differentiate altered hydrocarbons, in which effects of TSR predominate, from sweet crude oils and condensates, in which thermal cracking is the dominant process of hydrocarbon maturation.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)