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Carbon-Sulfur Dynamics in the Cretaceous Eagle Ford Formation

Abstract

The Cretaceous Eagle Ford Formation is an important carbonate-rich source rock producing oil and gas, extracted both in situ from the Eagle Ford (an ‘unconventional’ reservoir) and after migration to the Austin Chalk Formation. The unit was deposited in a complex and highly dynamic shelf environment, as evident from pronounced fluctuations in organic carbon and other key geochemical variables.

The abundance and phase association of sulfur has profound effects on source rocks, affecting the preservation and maturation of organic matter, the quality of the extracted oil/gas, and H2S-associated risks/costs during extraction and refining. The interplay between carbon, sulfur, and iron, and the impact this has on sulfur speciation, is not yet well understood; nor is the influence of climate and depositional environment on this coupling.

This study focusses on the partitioning of sedimentary sulfur, as a result of interactions between organic matter, iron, and sulfur inputs, to understand the underlying processes that lead to high-S and organic-S-rich source rocks.

We sampled four intervals (Lower, Upper, and OAE2 Eagle Ford) at a sliding resolution (1/2” to 1’) from a core in the western Maverick Basin, to study the partitioning of sulfur into organic and inorganic forms. With analytical work in progress, we show the first outcomes from a high resolution geochemistry study, identifying distinct fluctuations in sulfur abundance (0.2-3.3% based on XRF scanner logs), sulfur partitioning, TOC (0.4-5%), and iron speciation, in combination manifesting environmental change at orbital and shorter timescales in the rock record. These initial results are placed in an environmental context using biomarkers. SEM-EDX element mapping is in progress, finally linking geochemistry to the fine-scale sedimentological record, supporting interpretations of both mineralogy and pyritization processes.

A direct comparison of intervals in the Maverick Basin core should permit the influence of evolving paleoceanographic conditions on iron-sulfur-carbon dynamics to be examined, with a second core from the San Marcos Arch (ACC core) providing a more proximal depositional perspective of OAE2. In combination, this project will enhance our understanding of the competition between organic matter and iron sulfurization, with differing reaction rates and possibly loci of sulfurization, which has direct implications on key properties on petroleum production (e.g. S-rich kerogen and H2S hazards).