ABSTRACT: Syndepositional, Subtidal Dolomitization Model for the Formation of Some Paleozoic Reservoirs in the Permian Basin
MAZZULLO, S. J., C. S. TEAL, and WILLIAM D. BISCHOFF
Timing of dolomitization in many Ordovician to Devonian reservoirs in the Permian Basin, wherein dolomite has replaced subtidal deposits, is difficult to reconcile by existing dolomitization models. Accordingly, an "a priori" prediction of potential reservoirs is a major risk element in such plays. A newly documented occurrence of Holocene dolomite forming syndepositionally in the shallow-subtidal environment in Belize provides a model that may be applicable to exploration for such stratabound dolomites.
Holocene subaqueous sediments in northem Belize (Cangrejo Shoals) compose a thick (maximum 22 feet) transgressive systems tract of sandy, micritic carbonates deposited in a moderate-energy, normal-salinity environment on the middle shelf. Calcic dolomite composes as much as 30% of the sediment, and occurs as interparticle cements of average 10 micron size. Mean oxygen isotopic composition (delta{18}O = +2 o/oo[PDB]) suggests its precipitation from interstitial fluids of near-normal salinity, inferences that are substantiated by chemical analyses of associated pore fluids. The range in delta{13}C (-0.6 to +8.6 o/oo[PDB]) suggests that dolomitization is promoted by bacterial sulfate reduction and early methanogenesis in these organic-rich sediments. Mg is supplied by circulating seawater driven by wind-induced currents and tidal pumping. This dolomitization model may explain the seeming early formation of some dolomites in analogous subtidal deposits involved in facies-change and subunconformity truncation traps in Ellenburger, Fusselman, and Devonian strata.
AAPG Search and Discovery Article #90947©1997 AAPG Southwest Section Meeting, San Angelo, Texas