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°C, and presently contains 78% H2S, 20% CO2,
and 2% CH4. 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 H2S, CO2, and S°
generation, and postbitumen calcite cementation. During TSR, anhydrite
reacted with H2S to produce S°, which in turn reacted with
CH4 to generate more H2S in a self-reinforcing cycle.
The lack of metal cations to stabilize H2S as metal sulfides,
availability of sufficient sulfate to generate H2S, and a closed
system to prevent H2S from escaping resulted in the continuation
of the TSR cycle until nearly all hydrocarbons were consumed.
1997. The American Association of Petroleum Geologists. All rights reserved.
received July 13, 1995; revised manuscript received March 11, 1996; final
acceptance August 14, 1996.
Research Institute, Louisiana State University, Baton Rouge, Louisiana
research was supported by the Applied Carbonate Research Program, Basin
Research Institute, and the Department of Geology and Geophysics at Louisiana
State University. Phillips Petroleum Company generously provided the cores
used in the study. Exchange of ideas with
W. J. Wade
throughout the study improved the clarity of concepts developed. W. L.
Orr and A. Nicholson provided unpublished data and insightful discussions.
The manuscript benefited greatly from editorial comments by
J. S. Hanor,
C. H. Moore, W. L. Orr, and by AAPG reviewers J. J. Dravies, I. Hutcheon,
and C. Schreiber. I also thank W. LeBlanc for x-ray diffraction analyses,
K. Lyle for photography, and P. O'Neill, J. Kropog, and B. Simpson for