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April 2021 Bulletin

The development of porosity in the upper Cretaceous Austin Chalk Group of Texas
Miguel Bernardo and Alexis Godet
Department of Geological Sciences, The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249
The original mineralogy of the upper Cretaceous Austin Chalk Group of Texas controls its early compactional history and is responsible, at least in part, for its diagenetic history and the evolution of its porosity. This study aims to assess the relationship between depositional facies and differential diagenetic pathways within the Austin Chalk Group to constrain the parameters that favored the formation of porosity. Outcrops at Del Rio, San Antonio, and Dallas are analyzed for their depositional setting, diagenesis, micro-texture, and porosity. This research tests the hypothesis that different facies environments and different burial depths across the study area controlled the development of porosity. The petrographic analysis of rock slabs and thin sections allows the description of fossil content, depositional texture, and matrix to inform on ecological changes and are interpreted as changes in sea level. Cathodoluminescence microscopy helps constrain the chronology of diagenetic events that promoted or hampered the formation of porosity. Scanning electron microscopy is used to assess the microtexture and to measure pore throat diameters. Finally, porosity values are obtained for each sample by calculating the volume displacement and difference between dry and water-saturated weight. Porosity and pore throat values are compared to facies and microtextures to further understand the evolution of porosity. The highest porosity values found in the Dallas area are due to a shallower depositional setting and less influence of burial diagenesis compared to San Antonio and Del Rio, where the lowest porosity values are measured. As a result, this study helps exploration efforts that focus on chalk series deposited above basement or salt-related topographic highs to identify intervals with better reservoir properties.
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