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Source Rock Potential Evaluation of the Eagle Ford—Austin Chalk Transition in San Antonio, Texas


Source rock potential evaluation of the Eagle Ford – Austin Chalk transition in San Antonio, Texas

Garrett Velko, Aman Gupte, John Cooper, and Alexis Godet

The University of Texas at San Antonio

Sea level changes, subsea volcanism, and local to global anoxic events shaped Late Cretaceous sedimentation in South Central Texas. Near the Turonian – Coniacian boundary, organic-rich calcareous mud rocks of the Eagle Ford Group transition into finely-grained facies of the Austin Chalk Group. The objective of this research is to understand what syndepositional and post-depositional parameters favored the preservation of organic matter and could account for fluctuating kerogen types. Two outcrops in the San Antonio area were investigated to develop an integrated depositional model based on a sedimentological and geochemical approach. We aim to improve our understanding of organic matter production, preservation and source rock potential during the late Turonian – early Coniacian.

The two studied outcrops are located on the north-east and the north-west side of the city of San Antonio. Their detailed sedimentological description suggests a deeper, more complex depositional setting to the north-east, where organic material was preferentiality preserved as suggested by dark, finely laminated mud rocks. Moreover, the occurrence of an iron-stained layer caped by levels with iron-rich ooids constitutes an additional difference compared to the second study location where ripples and small scale hummocky-cross stratifications highlight a higher energy depositional environment. Geochemical analyses (X-ray fluorescence spectrometry, rock-eval pyrolysis) of samples from both locations provide additional information on the nature of the organic matter preserved. On the north-east side of San Antonio, a deeper, suboxic depositional environment (enrichment factor in vanadium of up to 18) favored the preservation of organic material produced by an enhanced primary productivity stimulated by an increased nutrient supply (high phosphorus content). This is not the case on the northwest side of San Antonio where wave activity might have promoted oxidation of organic material.

Van Krevelen and Pseudo Van Krevelen diagrams reveal the presence of moderately mature type I to less mature type II kerogens to the northeast, and of degraded type III kerogen to the northwest, suggesting a stronger influence of neighboring landmasses and the development of small-sized intrashelf basin where marine organic matter has been preserved to the northwest and northeast parts of San Antonio, respectively.