Controls on H₂S Concentration and Hydrocarbon Destruction in Smackover Formation, Southwest Alabama

William J. Wade, Jeffrey S. Hanor, Roger Sassen

H₂S generated by thermal sulfate reduction and oxidation of hydrocarbons in deeply-buried Smackover reservoirs is preferentially destroyed by reaction with metal ions to form sulfide minerals in the underlying Norphlet Formation. Resulting H₂S concentration differences can be described by calculated molecular diffusion profiles within the Smackover Formation. Theoretical H₂S diffusion coefficients extrapolated for 45 Alabama Smackover fields and measured H₂S concentrations from those fields are in agreement with model steady-state profiles. Factors controlling reservoir H₂S concentration in this model are porosity, permeability, tortuosity, and thickness of the Smackover Formation. Lesser factors are nature of pore ph se (oil, gas, or formation water), temperature (in excess of critical reaction temperature), and pressure.

Although calculated H₂S diffusion profiles can successfully describe or predict H₂S concentration gradients, rates of molecular diffusions are insufficient to account for observed reservoir concentrations of H₂S. It is thus probable that advective dispersion resulting from convective overturn is the means by which the inferred steady-state profiles are maintained.

The rate of destruction of hydrocarbons by thermal sulfate reduction is partly dependent on H₂S flux, which may be estimated from the H₂S concentration gradient, convection rate, and temperature. Economic basement for Smackover reservoirs therefore varies. Reliable estimates of porosity, permeability, and thickness trends allow (1) prediction of H₂S concentrations in the Smackover Formation with reasonable accuracy, and (2) estimation of local economic basement for Smackover reservoirs.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.