Subsurface and Outcrop Organic Geochemistry of the Eagle Ford Shale in West, Southwest, Central and East Texas

Abstract

A comprehensive regional organic geochemical study was performed on outcrop and core samples from the Eagle Ford Shale with the aim of determining variations in organic matter source, thermal maturity and depositional environments. A total of 178 samples were subjected to total organic carbon (TOC) and Rock Eval analysis. These data were used to select for vitrinite reflectance and for biomarker and isotope analyses using gas chromatography, gas chromatography-mass spectrometry, and gas chromatography-isotope ratio mass spectrometry. TOC and Rock Eval parameters show that the Eagle Ford Shale has excellent source rock potential and is dominated by Type II kerogen. Distributions of regular steranes, hopanes and monoaromatic steroid hydrocarbons point towards a marine carbonate depositional environment. Aryl isoprenoids suggest the occurrence of intermittent photic zone anoxia. In addition, n-alkanes, steranes distributions, and the tentative identification of gammacerane, suggest deposition of source material under hypersaline conditions in West and East Texas. Biomarker parameters show that in East Texas the Eagle Ford Shale was partly sourced by terrigenous organic matter, reflecting the influence of the Harris Delta. Thermal maturity parameters indicate that the Eagle Ford is immature to marginally mature in West and Central Texas, and show a progressive increase in maturity towards the southeast. In East Texas, the Eagle Ford Shale is in the main oil-window. Geochemical logs show minimal vertical variation within the Eagle Ford Shale. The Lower Eagle Ford has the highest TOC and hydrogen index (HI) values and in particular, the Lozier Canyon Member is the most organic-rich. Pristane and phytane (Pr/Ph) and biomarker ratios suggest the establishment of stronger anoxic conditions during deposition of the Lower Eagle Ford Shale. In East Texas, Pr/Ph ratios indicate source rock deposition under oxic-suboxic conditions. Isotope data indicates a marine organic matter source for the Eagle Ford Shale, but δ13C values do not show significant organic facies, depositional environment or thermal maturity changes.

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