Organic Geochemical Evidence of Redox Conditions in the Eagle Ford Formation, Southwest Texas

Abstract

The marine mudrocks of the Eagle Ford Formation in Southwest Texas record variations in organic matter content spanning stratigraphic intervals above and below the Cenomanian to Turonian (C-T) boundary (ca. 95–93 Ma) and the globally distributed interval known as the OAE2. Bulk geochemistry, organic matter biomarkers, and carbon isotope studies of low-mature core reveal that the extractable organic matter is dominantly marine Type II kerogen in both the lower Eagle Ford (LEF) and upper Eagle Ford (UEF), although a minor contribution of terrestrial OM occurs in the UEF. Average TOC values for the LEF and UEF are 7.8%wt and 3.1%wt, respectively. The LEF represents late Cenomanian deposition. The UEF includes the C-T boundary. Redox sensitive biomarkers demonstrate anoxic-euxinic conditions existed in the LEF and persistent oxygenation of the UEF across the C-T transition during OAE2. To assess temporal variations in water-column oxygenation, arylisoprenoids, which indicate photic-zone euxinia, were analyzed in the aromatic fractions of extracted oil. The arylisoprenoids are thought to derive from carotenoids, which are specific to obligately anerobic photosynthetic green sulfur bacteria. Vertical profiles of total arylisoprenoids show concentrations are higher in the TOC-rich LEF, and lower within the less organic-rich UEF and its contained OAE interval. The vertical profile of dibenzothiophene (DBT) in the core samples roughly resembles that of arylisoprenoids, and probably also reflects the presence of H2S implying the presence of sulfur-rich anoxic (i.e., euxinic) waters. These findings support the interpretation that the LEF preserves the greatest evidence for conditions of very low oxygenation whereas the OAE2-associated UEF represents conditions of greater oxygenation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015