Evaluation of Subsurface Porosity in the Austin Chalk Group, South Texas




Hancock, Andrew Morgan

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The Upper Cretaceous Austin Chalk Group of South Texas was deposited as a highly fossiliferous chalk along a distally steepened ramp, in environments ranging from a shallow shelf to a deeper basin. Previous exploration efforts have focused on a highly faulted and fractured trend running along the northern portion of the subsurface Austin Chalk Group. Recent advances in hydraulic fracturing have shifted the focus of exploration towards targeting matrix porosity in portions of the Austin Chalk Group outside of the fracture trend. This study evaluates the evolution of porosity in the subsurface of South Texas, moving from outcrop in the northwest toward the shelf margin in the southeast portion of the study area. A high-resolution stratigraphic framework based on the correlation of 906 well logs constrains the thickness of formations constitutive of the Austin Chalk Group and permits to identify three main depocenters. Density porosity logs are gridded across the study area to create the model used to evaluate trends in porosity. A relationship between porosity, stratigraphic thickness, and structure of the basin is examined. An overall trend of porosity loss with burial depth is observed in the northern portion of the study area. Preservation of porosity is related to early diagenetic processes such as dissolution and reprecipitation of aragonite as low magnesium calcite that strengthened initial porosity of carbonate deposits, which were more resistant to further loss in porosity. The subsequent migration and emplacement of hydrocarbons counterbalanced the effect of burial diagenesis and associated cementation that could have reduced porosity.


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Austin Chalk, Chemostratigraphy, Diagenesis, Isotope, Porosity