ABSTRACT

Dolomitization is critical to the development of reservoir porosity in the Smackover Formation of southeastern Alabama, as seen in the Appleton and Big Escambia Creek fields. Dolomitization is concentrated in permeable facies on depositional highs. In both fields, the dolomitizing fluids were geochemically similar to the waters in which deposition occurred. Deposition and dolomitization in the vicinity of the Appleton Field (near the updip limit of the Smackover Formation) took place in more saline waters than basinward in the Big Escambia Creek Field. The Smackover Formation in the Appleton Field was deposited in supratidal, intertidal, and shallow lagoonal environments. Abundant blue-green algal structures and the absence of normal-marine fossils suggest the lagoonal waters were hypersaline. Appleton Field dolomites have stable carbon and oxygen isotope compositions (¹³C = 4.4 to 5.7, PDB; ¹⁸O = 1.4 to 4.5, PDB) indicative of formation in hypersaline waters. Evaporative pumping in supratidal flats may have provided the hydraulic gradient to move dolomitizing fluids through the Smackover Formation during or very shortly after deposition.

In the western part of Big Escambia Creek Field, dolomitization is concentrated in the upper Smackover Formation in ooid-peloid grainstones which accumulated on a syndepositional high. Bioturbated peloid packstones and wackestones accumulated in deeper water off the syndepositional high and are not pervasively dolomitized. Rhodolites and calcareous fossil fragments indicate deposition in or near seawater with normal marine salinities. Stable isotope compositions of Big Escambia Creek dolomites (¹³C = 5.2 to 6.0; ¹⁸O = - 1.8 to - 0.2) are consistent with dolomitization in warm seawater with normal marine salinities. Evaporative drawdown in a restricted lagoon north of Jay and Big Escambia Creek fields may have caused the dolomitizing seawater to move through ooid-peloid shoals.