Shelf-Interior Carbonate Grainstone Shoals: Lower Cretaceous Pearsall Formation, South Texas
Shoal-water carbonate complexes developed over a low-relief structure (Pearsall arch) on the Lower Cretaceous shelf of South Texas in the Sligo, Pearsall, and Glen Rose Formations. One of these complexes in the Pearsall Formation, the Cow Creek Limestone, was penetrated and cored in the Tenneco No. 1 Sirianni well, Frio County. The Sirianni well encountered the thickest part of the Cow Creek complex.
Three shoaling-upward cycles were encountered. The lower cycle contains echinoid-mollusk wackestones and packstones, deposited under low-energy subtidal conditions, overlain by echinoid-mollusk grainstones, deposited in an intertidal shoal environment. The second cycle is similar to the lower cycle except that ooid grainstones are well developed in the upper part of the grainstone unit. The upper cycle comprises another wackestone to ooid-skeletal grainstone upward shoaling sequence which is capped by boundstones and grainstones deposited in a patch-reef complex.
Porosity is facies controlled in that 95% occurs in the grainstones. Facies maps, therefore, serve also as porosity-distribution maps.
Figures & Tables
Carbonate Sands-A Core Workshop
Carbonate sands, both skeletal and non-skeletal, have been studied by geologists as intensely as carbonate buildups. The underlying reason for the studies is the importance of those sands as significant hydrocarbon reservoirs. This core workshop is intended to provide a “hands on” look at the subsurface geologic record of carbonate sands with emphasis on lithofacies, stratigraphy of the sands and surrounding deposits, geometry of the sand deposits, diagenesis and porosity evolution, and wireline log data.