Abstract

Some hydrocarbon reservoirs in upper Pennsylvanian and Permian rocks in the Permian Basin are mainly skeletal carbonate sands that were deposited in relatively low-energy, inner shelf environments. Examples include some fields in the “Bough” members (Wolfcampian) in the Tatum Basin, the Strawn in the Midland Basin and on the Eastern Shelf, and the Guadalupian in the Midland Basin and on the Central Basin Platform. Typical reservoir geometries are narrow and elongate (parallel to platform margins), and reservoir zones are thin, and locally, vertically stacked and/or laterally juxtaposed. Modern analogs of such deposits, as models for their origin, are poorly described.

Holocene (<6000 years) carbonate deposits of the microtidal inner shelf region (Chetumal Bay) in northern Belize are a modern analog of such reservoirs. These deposits include foraminferal sands and muds deposited in keep-up and catch-up fashion during decelerating post-Pleistocene sealevel rise. Hence, they are shallowing-upward and also coarsening-upward transgressive systems tract deposits. Inherently high-porosity sands are deposited mostly in shallow water (<1 meter) over bedrock and/or antecedent depositional highs, and they are mainly of linear geometry. The numerous linear sand shoals are topographic highs that cap subjacent muds or tabular sands, and their long axes are mutually parallel and at a constant angle to prevailing trade winds. Interparticle and intraparticle pores dominate in these sediments. In contrast, lower-permeability muds and sandy muds are restricted to adjoining deeper-water areas (1-2 meters) overlying bedrock lows. Although early porosity reduction via compaction typically is expected to occur in such muddy deposits, their potential for ultimately developing reservoir-grade secondary porosity may be high owing to syndepositional dolomitization. The dolomite composes as much as 65% of these organic-rich sediments, and apparently is forming as a result of bacterial sulfate reduction and possibly also methanogenesis in anoxic pore-water environments.