Micropetrographic characterization of a siliciclastic-rich chalk; Upper Cretaceous Austin Chalk Group along the onshore northern Gulf of Mexico, USA

摘要

The Austin Chalk Group is a very fine grained carbonate mudrock composed predominantly of microorganisms. To geologically characterize such a unit, the components and depositional and diagenetic features must be examined at the nano- to microscale, using micropetrography. Analytical methods include scanning electron microscopy, energy-dispersive X-ray spectroscopy, and thin-section analysis. Through micropetrographic analysis, we determined Austin Chalk composition to be mainly a matrix of coccoliths and coccolith elements in 20- to 30-micron peloids (probably marine snow). The most common larger components are planktic foraminifers and bottom-dwelling inoccramids. Austin Chalk mineralogy is predominantly calcite and silicidastic components that are mainly clay minerals, quartz, and albite. Grain sizes of Austin Chalk components are generally in the clay- to medium-silt range. Four basic lithofacies define the Austin Chalk: (1) lithofacies 1: highly bioturbated, organic-matter-poor marly chalk, (2) lithofacies 2: highly bioturbated, moderately organic-matter-rich, marly chalk to chalky marl, (3) lithofacies 3: sparsely burrowed, poorly to moderately laminated, organic-matter-rich marly chalk, and (4) lithofacies 4: well-laminated, organic-matter-rich manly chalk to calcareous silicidastic mudstone. These four lithofacies are distinguishable by their different fabrics, as well as by ranges in mineral composition and TOC content. Mean of organic-matter content ranges from 032 to 2.56 wt%, and the kerogen is generally Type I and Type II, except in lithofacies 1, in which Type III is more common. The abundance and type of organic matter suggest that the Austin Chalk can be self-sourcing. This investigation provides the basic nano- to micro-observations that will aid in other studies of the Austin Chalk Group and geologically similar formations relative to rock-strength/brittleness, source-rock potential, and reservoir quality. (C) 2020 Elsevier B.V. All rights reserved.