PARAGENESIS OF THE AUSTIN CHALK FORMATION IN SOUTHWEST TEXAS

Author

Mitchell C. Sherry

Year of Award

2024

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Geosciences

Committee Chair

Marc S. Hendrix

Committee Co-chair

Michael H. Hofmann

Commitee Members

Marc S. Hendrix, Michael H. Hofmann, Kent D. Sugden

Keywords

Diagenesis, Chalk, Texas, Reservoir Quality

Publisher

University of Montana

Subject Categories

Geology

Abstract

Presented in this study is a detailed investigation of the paragenesis and diagenetic processes of the Austin Chalk Formation in southwest Texas, utilizing a combination of core-based, petrographic, mineralogic, SEM, and geochemical based analyses. This study is conducted on a single whole rock core from an oil and gas producing well in Webb County, Texas. At this distal shelf location within the Maverick Basin, the Austin Chalk Formation consists of foraminiferal wackestones, siliceous wackestones, foraminiferal mudstones, bentonites, and foraminiferal packstone facies. These facies were deposited below wave base through pelagic, hemipelagic, and gravity driven processes. Petrographic and SEM analyses reveal a complex diagenetic history of the Austin Chalk Formation. This includes early formation of framboidal pyrite, early intragranular calcite cement, mechanical compaction, chemical compaction, authigenic albite formation, precipitation of authigenic dolomite and high-Mg calcite, and formation of late-stage euhedral pyrite. Mineralogic data from XRD analysis shows that clay-minerals in the Austin Chalk Formation were subject to illitization. Through the integration of petrographic, SEM, and geochemical data, clear associations between diagenetic products and lithofacies were delineated. The abundance of clay-minerals stands out as a major driver of a number of the diagenetic products observed. Abundances of both authigenic dolomite and albite show positive correlations to the abundance of clay-minerals. This suggests that these products are genetically related to illitization, and the cations released during this process. The degree of chemical compaction and associated stylolitization also increases with increased clay-mineral content. This resulted in decreased reservoir quality in the lithofacies with elevated clay-mineral content and subsequent increased pressure solution. This study highlights the complex diagenetic history of the Austin Chalk Formation. The findings contribute to a better understanding of the reservoir quality and hydrocarbon potential of the Austin Chalk Formation in an understudied area of the Austin Chalk trend in south Texas, with implications for exploration and production strategies in similar geologic settings.

Recommended Citation

Sherry, Mitchell C., "PARAGENESIS OF THE AUSTIN CHALK FORMATION IN SOUTHWEST TEXAS" (2024). Graduate Student Theses, Dissertations, & Professional Papers. 12322.
https://scholarworks.umt.edu/etd/12322