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Core-Based Characterization of the Upper Cretaceous Austin Chalk Group in Western Louisiana; Comments on Depositional Setting, Lithofacies, Organic Matter, and Pore Networks


The Austin Chalk in Louisiana is an active exploration trend especially in the downdip area south of the paleo-Stuart City shelf margin. There are no up-to-date published studies on the Austin Chalk Group in Louisiana and only rare limited older studies. Little is available relative to lithofacies, depositional setting, origins of organic-rich and organic-poor cyclicity, pore networks, etc. We have investigated all publicly available cores (five cores) in Louisiana to address these lack of data. These cores has been characterized for sedimentology, diagenesis, XRF geochemistry, organic matter, mechanical strength, and micropetrology for texture, fabric, and pore types. The Chevron No. 1 Burton (API# 17079203690000) wireline log in Rapides Parish was selected as the type well for regional correlations. The Austin Chalk was deposited on the drowned Lower Cretaceous paleoshelf. Deposition was in relatively deeper water below storm-wave base. The cyclic character of the Austin Chalk, between burrowed organic-poor lithofacies and laminated organic-rich lithofacies, suggests bottom conditions varied between aerobic and dysaerobic/anaerobic. We are speculating that these cycles are related to Milankovitch cycles. Four dominant lithofacies are defined: (1) Argillaceous burrowed lime wackestone to lime packstone (chalky marl); deposited under aerobic conditions (mean TOC = 0.34%), (2) Very argillaceous burrowed lime wackestone to lime packstone (chalky marl); deposited under aerobic to dysaerobic conditions (mean TOC = 0.97%), (3) Poorly laminated (some burrows) argillaceous wackestone to packstone (chalky marl to calcareous argillaceous mudstone); deposited under dysaerobic to anaerobic conditions (mean TOC = 1.38%), and (4) Laminated argillaceous wackestone to packstone (chalky marl to argillaceous calcareous mudstone); deposited under anaerobic conditions (Mean TOC = 2.02%). The lighter colored burrowed lithofacies are the leanest and the laminated lithofacies are the richest in TOC. Pseudo van Krevelen plots indicate that the most argillaceous and laminated lithofacies are composed of Type I and II organic matter. In the laminated lithofacies, the organic matter is in wavy seams up to 5 microns thick and are interpreted as anaerobic bacterial mats. The dominant nanopore types are interparticle pores between coccolith hash and OM pores in bitumen. Minor intraparticle pores exist. This study provides an overview of Austin Chalk geology in western Louisiana.