Diagenesis of Smackover Reservoir Rocks in Southeastern Gulf Coast

Bradford E. Prather

The distribution of Smackover reservoir rocks in the southeastern Gulf Coast resulted from the diagenetic alteration of grain-supported sediments. The distribution of these grain-supported facies was strongly affected by pre-Smackover paleotopography.

Petrographic and geochemical analyses of the grain-supported rocks indicate diagenesis occurred in (1) marine, (2) nearsurface, (3) shallow-burial, and (4) deep-burial environments. Beachrock and submarine cementation were the dominant processes affecting grain-supported sediments during marine diagenesis.

Nearsurface diagenetic environments were present landward of the shoreline during marine diagenesis. Diagenesis in the nearsurface occurred as a result of: (1) evaporitic drawdown of the water table during deposition of a basinal Buckner salt in the Flomaton and Manila basins, (2) storm recharge and percolation of evaporitive brines from overlying Buckner anhydrites, and (3) the establishment of a subregional fresh-water phreatic zone in upper reaches of the Conecuh Embayment. Near-surface diagenesis ended when connate brines seeped into the Smackover pore system during deposition of the upper Haynesville Formation.

Large vuggy pores lined with prismatic calcite cements of shallow-burial freshwater origin indicate that the connate water was flushed by fresh water during Cotton Valley deposition. Dolomitization occurred contemporaneously in the downdip mixed connate and freshwater zone. Diagenesis in this zone profoundly affected the distribution of porosity in Smackover grain-supported rocks.

During deep burial, formation waters in the Smackover evolved as brines were expelled from surrounding sediments. Deep-burial diagenesis produced a mineral assemblage similar to that found in Mississippi Valley type lead-zinc deposits. This mineral assemblage formed during periods of (1) carbonate cementation, (2) sulfate mineralization, and (3) sulfate reduction.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.