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A Type Cored Section for the Upper Cretaceous Austin Chalk Group in South Texas; Getty No. 1 Lloyd Hurt Well, LaSalle County, Texas

Abstract

The fractured Austin Chalk reservoir trend is still very active in South Texas. There is very little published on the geology of this unit and there is no rock-based type section with regional correlation markers. The 575-ft long core from the Getty (Express Oil and Gas) No. 1 Lloyd Hurt well (API# 42283303050000) in the southern section of the Pearsall field in northwestern LaSalle County, Texas includes the upper Eagle Ford section, the complete Austin Chalk section, and the lowermost section of the Anacacho Limestone. This core has been characterized for sedimentology, diagenesis, XRF geochemistry, organic matter, mechanical strength, and micropetrology for texture, fabric, and pore types. The core is divided into correlatable units that are derived from the Austin Chalk Pearsall field. The Austin Chalk was deposited on the drowned Lower Cretaceous paleoshelf. Deposition was in relatively deep water below storm-wave base. Most of the biota lived in the shallow-water column, such as coccospheres, planktic foraminifers, and calcispheres. Hard-shelled bottom dwellers were dominated by inoceramid clams. Both horizontal and vertical burrows are present, but horizontal burrows are most abundant. The cyclic character of the Austin Chalk, between burrowed organic-poor lithofacies and laminated organic-rich lithofacies, suggests the bottom conditions varied between aerobic and dysaerobic/anaerobic. We are speculating these cycles are related to Milankovitch cycles. Two major rock types are present: argillaceous burrowed chalky marl and laminated argillaceous chalky marl to calcareous argillaceous mudstone. The more argillaceous and laminated rocks are TOC rich (0.62% to 2.00%). The very argillaceous and laminated lithofacies are composed of Type I and II organic matter, whereas the lighter, lower argillaceous burrowed lithofacies is composed of Type III organic matter. The matrix is relatively tight. A modified gas-expansion method was used to measure porosity and permeability. Porosity ranges from 1.2% to 5.5% and permeability ranged from 3.8 nd to 397 nd. The nanopore network is composed of interparticle pores between coccolith hash; however, many of these pores are plugged with solid bitumen. Some OM pores are in the solid bitumen. Other pores (rarer) are intraparticle nanopores within clay platelets, calcite grains, and pyrite microframboids. This type core should be a valuable reference for understanding and exploring in the Austin Chalk Group.