Abstract

Organic-inorganic interactions during burial of the Smackover Formation at Black Creek Field, Mississippi, have resulted in nearly complete destruction of hydrocarbons. The formation has been buried to a depth of 6 km, has experienced temperatures of over 200 degrees C, and presently contains 78% H 2 S, 20% CO 2 , and 2% CH 4 . Three distinct stages of burial diagenesis correspond to three phases of organic matter maturation. Pre-oil window diagenesis was dominated by precipitation of prebitumen calcite cement. Diagenesis in the oil window was characterized by precipitation of saddle dolomite and anhydrite in water-filled layers and by formation of solid bitumen in the oil column. Diagenesis in the gas window was dominated by thermochemical sulfate reduction (TSR) resulting in hydrocarbon destruction, anhydrite dissolution, large amounts of H 2 S, CO 2 , and S 0 generation, and postbitumen calcite cementation. During TSR, anhydrite reacted with H 2 S to produce S 0 , which in turn reacted with CH 4 to generate more H 2 S in a self-reinforcing cycle. The lack of metal cations to stabilize H 2 S as metal sulfides, availability of sufficient sulfate to generate H 2 S, and a closed system to prevent H 2 S from escaping resulted in the continuation of the TSR cycle until nearly all hydrocarbons were consumed. In Mississippi, concentration of H 2 S is nearly zero in Smackover hydrocarbon reservoirs that have experienced temperatures of 120 degrees C for more than 50 m.y., suggesting that TSR is not a kinetic (time-dependent) process. High H 2 S concentrations initiate at temperatures above 140 degrees C and increase with temperature, indicating that TSR is a thermodynamic phenomenon. Reported high H 2 S concentrations at low temperatures (80-120 degrees C) from other locations may be explained by the following processes; (1) migration of H 2 S into these reservoirs, (2) high geothermal gradients or local thermal perturbations in the past, (3) a biochemical origin for the H 2 S, or (4) exposure of these reservoirs to temperatures greater than 150 degrees C and a rapid uplift. In Black Creek Field, burial cementation and pressure solution resulted in total destruction of porosity and permeability in limestone reservoirs, but not in dolomite reservoirs, which still possess up to 20% porosity and 100 md permeability. Secondary porosity was not created as a result of hydrocarbon migration. Abundant CO 2 derived during hydrocarbon destruction resulted in calcite cementation rather than carbonate dissolution. Late, secondary porosity development in carbonates may be related to acids generated by metal sulfide precipitation.

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