ABSTRACT: Origin and Variability of H2S Concentrations in Siliciclastic and Carbonate Reservoirs--Smackover and Norphlet Formations of Central and Eastern Mississippi

SHEW, ROGER D., Shell Development Company, Houston, TX

The Jurassic Smackover and Norphlet formations of the Gulf Coast contain variable amounts of H2S, which is important to reservoir evaluation and economics. Several fields in central Mississippi are sour gas productive from siliciclastic reservoirs within the Smackover Formation. The H2S concentrations range from 25% to 42%. Direct and indirect evidence for a thermochemical sulfate reduction (TSR) origin include (1) sulfur isotope values of the H2S (+16.1 to +17.3o/oo CDT) and sulfur (+16.1o/oo CDT) are similar to those of the anhydrite (+16.3 to +17.3o/oo CDT), (2) bedded nodular and secondary pore-filling anhydrite are present as a sulfate source, (3) elemental sulfur is locally present, and (4) higher H2S concentrations with increasing temperature are observed for several of the sou gas fields; all reservoir temperatures >175 degrees C.

South State Line, eastern Mississippi, is gas productive from Smackover carbonates and Norphlet arkosic sandstones. The Smackover contains 25% H2S while the Norphlet contains +/- 1.8%. The Smackover is the hydrocarbon source for both formations so a similar gas mixture might be expected, particularly since both reservoirs have undergone the same thermal history. Possible reasons for the difference may be (1) anhydrite available to react in the Smackover was not available to the Norphlet or (2) the reaction of H2S with abundant iron-rich minerals may have removed H2S through the formation of pyrite. The latter is at least locally important as large amounts of pyrite have been observed in core. The sulfur isotope values of the pyrite (+15.8o/oo CDT) are similar to the anhydrite and gase (+15.9o/oo to +19.8o/oo CDT). The higher H2S concentrations remaining in the Sour Gas Area of central Mississippi may be a result of the sandstones being iron poor and "acting" similar to carbonate reservoirs. Mineralogical associations and reservoir models are used to suggest the likely occurrence and reasons for the variable concentrations.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)