High Resolution Geochemistry Of The Cretaceous Eagle Ford Shale, Bee County, Texas

Abstract

The Eagle Ford Formation of Bee County, Texas is a sporadically laminated carbonaceous dark mudrock. The depositional area of the Eagle Ford Formation stretches across the state of Texas in a northeast-southwest trend. Early studies of the Eagle Ford found the deposits to be rich in organic material but could not fully describe the formation due to the lack of outcrop exposure. Recent studies of the Eagle Ford Formation have begun to explore the sub-surface nature of the formation. Geochemical analyses of the J.A. Leppard #1 core from the southwestern portion of the formation was conducted to further constrain the sub-surface geochemical signatures of the Eagle Ford Formation. Major and trace element compositions were all measured using a hand- held X-ray fluorescence spectrometer. Bulk geochemistry, trace metal enrichments, inferred mineralogy and geochemical relationships were used as proxies to define the depositional paleoenvironment and degree of basin restriction. The Eagle Ford Formation was deposited under mostly anoxic/euxinic conditions with intermittent pulses of oxygenation. The basin was mostly restricted, but with significant periods of a more open marine setting. Dark mudstones associated with similar depositional histories have previously been linked to global Cretaceous oceanic anoxic events (OAEs). Earlier studies inferred that the Eagle Ford Formation preserved in the J.A. Leppard #1 core preserved a record of the Cenomanian-Turonian boundary. However, biostratigraphic evidence questions the original timing of sedimentation at the core location. It is now believed that the chemostratigraphic patterns could be related to marine preconditioning episodes of anoxia/euxinia prior to a major OAE.