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Abstract

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The Role of Burial Diagenesis in Hydrocarbon Destruction and H2S Accumulation, Upper Jurassic Smackover Formation, Black Creek Field, Mississippi1

Ezat Heydari2

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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°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. 
 
 


Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

1Manuscript received July 13, 1995; revised manuscript received March 11, 1996; final acceptance August 14, 1996.

2Basin Research Institute, Louisiana State University, Baton Rouge, Louisiana 70803.

This 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 W. L. Orr and A. Nicholson provided unpublished data and insightful discussions.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 technical support.

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