CONTROLS ON THE ACCUMULATION OF ORGANIC MATTER IN THE EAGLE FORD FORMATION IN CENTRAL TEXAS

The Upper Cretaceous Eagle Ford Formation is an organic-rich source rock that has recently become a target for oil and gas exploration. The goal of this study is to investigate the depositional controls on the accumulation of organic matter in the Eagle Ford outcrops of central Texas. A composite section was generated using 9 outcrops from which 166 samples were collected and analyzed for the abundance of trace elements that are redox sensitive (e.g. Mo, U, V) or indicators of paleoproductivity (e.g. Cu, Ni, Zn). The concentration and isotopic character of organic matter was also measured.

The lower member of the Eagle Ford is composed of dark, thinly laminated mudrocks with interbedded bentonites and limestones. The mudrocks develop an average Total Organic Carbon (TOC) content of around 5.7% and contain enrichments in redox sensitive trace metals immediately following the transition from the underlying Pepper Shale (Woodbine Formation equivalent). The interbedded limestones are depleted in trace metals and contain much lower TOC values. The δ¹³C_org is consistently around -27.50‰ for all samples with the exception of a 3‰ positive excursion that occurs at the transition to the upper member of the Eagle Ford. This excursion is identified as the oceanic anoxic event at the Cenomanian/Turonian boundary: OAE-2. The onset of this event corresponded with a sudden depletion of redox sensitive metals and TOC. Above this section the TOC increases slightly, though redox sensitive trace element concentrations remain low. This upper part of the Eagle Ford is characterized by variable TOC (average 2.35%) as well as lower concentrations of redox sensitive elements. In this part of the section TOC is not positively correlated with redox sensitive trace metals.

These data suggest anoxic conditions generally persisted through much of the lower Eagle Ford leading up to OAE-2, where the trace metal abundance may have been depleted from the water column due to basin restriction. Fluctuating 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.