Depositional Lithofacies and Diagenetic Overprints of Pennsylvanian Lower Cisco Shelf Margin Carbonates, Wolf Flat Field, Motley Co., Texas

Abstract

The Wolf Flat field produces from Pennsylvanian aged shelf margin carbonates that were subaerial exposed to fresh water diagenesis during the late Pennsylvanian. About 400 feet of cores from five wells were studied to understand the reservoir's lithofacies and diagenesis relationship to porosity. Eight lithofacies types were identified in the reservoir section which is overlain by black glauconitic prodelta shale. The lithofacies are: Oolitic/oolitic skeletal/oolitic pisolitic grainstone; peloid crinoid skeletal grainstone; peloid skeletal mud rich and grain rich packstone; phylloid skeletal mud rich packstone; phylloid fusulinid / fusulinid phylloid/fusulinid/fusulinid crinoid mud rich and grain rich packstone; crinoid peloid wackestone; crinoid bryozoan and bryozoan sponge mud rich packstone; as well as skeletal wackestone/mud rich-grain rich packstone. Generally, the wackestone and mud rich packstone lithofacies constitute the bases of shallowing upward cycles that are capped by grain rich packstone or grainstone. A crinoid peloid wackestone and a skeletal wackestone mound were observed at the bottom of cycles in two of the sample wells. The cycles in the individual wells represent long term regression marked by progressive reduction in accommodation space and predominantly shallow shelf facies. The long term regression resulted in the subaerial exposure of the reservoir facies followed by fresh water diagenesis. Freshwater diagenesis was typified by karst features, owing to intense leaching, with their associated cave structures and collapse breccias. Other diagenetic overprints include replacement dolomites, fractures, stylolites, pyrite, calcite cement, anhydrite cement, saddle dolomite and sediment infill. Most of the original intergranular/intragranular porosity related to depositional facies were occluded by spar calcite cements. New porosity types; cavernous, vuggy, moldic, fracture and breccia porosities; were also partially occluded by calcite and anhydrite cements as well as saddle dolomite and sediment infill. Although fresh water diagenesis and ensuing karstification created abundant cavernous/vuggy and breccia porosities in the mud rich lithofacies, most of the new porosity are occluded by cements and sediment infill. Replacement dolomitization of the crinoid peloid wackestone mound and a significant part of its overlaying bryozoan crinoid phylloid mud rich packstone lithofacies, and the skeletal wackestone mound created significant vuggy porosity in the mud rich lithofacies. Most of the cycle capping grainstone and grain rich packstone lithofacies have high porosity irrespective of whether they have experienced replacement dolomitization or not. The exception to this are where the grainstone or grain rich packstone is in close proximity to the overlying prodelta shale. Here, most of the primary intergranular/intragranular porosity of the grainstone or grain rich packstone are occluded by spar calcite cement and the breccia porosity created by their exposure prior to the deposition of the prodelta shales are occluded by percolating prodelta shales.

AAPG Datapages/Search and Discovery Article #90250 © 2016 Southwest Section AAPG Annual Convention, Abilene, Texas, April 9-12, 2016