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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 39 (1989), Pages 309-320

Controls on H2S Concentration and Hydrocarbon Destruction in the Eastern Smackover Trend

William J. Wade (1), Jeffrey S. Hanor (2), Roger Sassen (1)

ABSTRACT

H2S concentrations in deep Smackover reservoirs may reflect local steady-state conditions attained when generation of H2S is balanced by flux of H2S out Of the system. H2S generated during thermochemical sulfate reduction within upper Smackover reservoirs is preferentially destroyed by reaction with metal ions to form sulfide minerals in the underlying Norphlet Formation. H2S concentrations are highest in and near Smackover reservoirs and decrease with depth to very low concentrations in the Norphlet Formation. Key factors controlling equilibrium H2S concentrations in Smackover reservoirs are porosity, tortuosity, and thickness of carbonates, nature of pore phase (oil, gas or formation water), and temperature (in excess of about 112°C).

Calculated diffusion-dispersion profiles successfully describe observed H2S concentration gradients. However, rates of molecular diffusion are insufficient to control H2S concentrations in Smackover reservoirs. It is possible that advective dispersion resulting from convective overturn is the means by which observed concentration gradients are maintained.

Because the rate of destruction of methane by thermochemical sulfate reduction is partly dependent on flux of H2S from Smackover reservoirs to the underlying Norphlet Formation, economic basement for Smackover reservoirs varies with H2S flux. The rate of methane destruction can be estimated from the H2S concentration gradient, convection rate, and temperature. Reliable estimates of porosity, permeability and thickness trends allow: (1) prediction of H2S concentrations in potential Smackover reservoirs with reasonable accuracy, and (2) estimation of local economic basement for Smackover reservoirs.


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