Eustatic Effects on Sandstone and Carbonate Diagenesis in Jurassic Reservoirs of the Mississippi Interior Salt Basin

Ernest A. Mancini, Berry H. Tew

Sedimentation in the Mississippi interior salt basin (MISB) of the eastern Gulf Coastal Plain resulted from rifted continental margin tectonics in association with changes in relative sea level. Unconformity-bounded depositional sequences, which resulted from relative changes in sea level and coastal on lap during the Jurassic, are recognized for the MISB. The LZAGC-3.1 and the LZAGC-4.1 sequences include proven petroleum reservoirs in the Jurassic section: Norphlet continental sandstones highstand systems tract (HST) (LZAGC-3.1 cycle), Norphlet marine sandstones shelf margin systems tract (SMST) (LZAGC-4.1 cycle), lower Smackover carbonates transgressive systems tract (TST) (LZAGC-4.1 cycle), and upper Smackover carbonates and Haynesville marginal marine sandstones (HST- ZAGC-4.1 cycle). The most extensive and prolific reservoirs in the area are associated with HST, while reservoirs associated with the SMST and TST are discontinuous and not as well developed. In general, the SMST and TST deposits in the MISB experienced early cementation, principally due to the influence of the marine phreatic zone. Conversely, the Norphlet and Smackover HST deposits have undergone favorable diagenetic alterations which have acted to enhance primary porosity and permeability through dissolution and dolomitization (Smackover reservoirs) and dissolution (Norphlet reservoirs). Upper Smackover grainstone and dolostone reservoirs are typical of the keep-up carbonate highstand regime; keep-up lithologies are usually grain rich and mud poor and are generally deplete in marine cement. Dolostones are important reservoir lithofacies in the upper Smackover, and these reservoirs result, in part, from diagenesis associated with the development of a freshwater lens at the basin margins and on paleotopographic highs during sea level lowstand. Reflux or hypersaline dolomitization of HST Smackover carbonates also is an important reservoir enhancing mechanism, especially in more basinal areas. Smackover porosity includes interparticulate, grain-moldic, vuggy, and intercrystalline dolomite. Porosity in the Norphlet eolian HST sandstone reservoirs is principall intergranular, consisting of both primary depositional and secondary enhanced resulting from leaching of cements and grains.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995