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Paint Rock and Southwest Paint Rock Fields, Concho County, Texas: Strawn Analogs of Modern Island Carbonate Facies of Ambergris Cay, Belize

A. M. Reid, S. J. Mazzullo

Lower Strawn (Desmoinesian Goen Limestone) reservoirs at Paint Rock and Southwest Paint Rock fields are a complex of carbonate and associated facies interpreted as having been deposited in various environments on and around large, emergent islands on shallow carbonate shelves. The origin and geometries of the component lithofacies in these fields, and their reservoir diagenetic histories, are similar to those presently accumulating on Ambergris Cay, a linear island complex on the northern shelf of Belize. Paint Rock field originated as a narrow, elongate Chaetetes reef trend that formed the foundation on which the overlying island facies were deposited. As on Ambergris Cay, these reef limestones developed extensive porosity during postdepositional subaerial exposure due t meteoric leaching. In contrast, Southwest Paint Rock field is cored by older island deposits rather than reef limestones. With ensuing stillstand or subsequent sea level rise, beach grainstones were deposited along the windward and leeward margins of the foundation highs in these fields. Tight lagoonal micrites and coals (peat-swamp facies) comprise the inner island facies, and are locally associated with porous supratidal dolomites. These island complexes are transected locally by tidal channels that are filled with nonporous micrites. Repeated sea level fluctuations during the history of these fields resulted in a characteristic cyclic stratigraphy of stacked island facies and reservoirs. The reservoirs in the field are developed in the bedrock or older island cores, as well as in the overlying beach facies and supratidal dolomites. These fields are mappable as linear stratigraphic traps with low-relief closure, and are readily identified by subsurface geologic and facies analyses. Similar shelf island-type fields analogous to these Strawn and Holocene Belizean examples are found throughout the Midland basin and Eastern shelf.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.