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Controls on the Accumulation of Organic Matter in the Eagle Ford Formation in Central Texas

Kenneth S. Boling
Baylor University, Waco, Texas

The Upper Cretaceous Eagle Ford Formation is an organic-rich source rock that has recently become a target for oil and gas exploration. This study investigated the controls on the accumulation of the organic matter in this formation using organic and inorganic geochemical data collected from outcrops in central Texas.

The lower Eagle Ford displayed an average Total Organic Carbon content of 5.70% with enrichments in redox sensitive trace metals immediately following the transition from the underlying Woodbine Formation. Interbedded limestones were depleted in redox metals and contained lower TOC than the thinly laminated mudstones. The δ13Corg was consistently around -27.50‰ for all samples with the exception of a 3‰ positive excursion. This was identified as the isotopic signature of the oceanic anoxic event at the C/T boundary: OAE-2. The onset of this event corresponded with a sudden depletion of redox metals and TOC. Above this section in the upper Eagle Ford, TOC increased while some redox metals remained depleted, marking the only section where high TOC was not positively correlated with redox trace metals.

These data suggest anoxic conditions generally persisted through much of the lower formation leading up to OAE-2, where the trace metal abundance may have been depleted from the water column due to basin restriction. Small scale changes in redox conditions may have been influenced by sea level changes and storm events, represented by an increased occurrence of interbedded limestones. Oxic conditions may have evolved during deposition of the upper Eagle Ford, though productivity remained high.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013