Abstract

The geochemistry of oils and gases, as well as sediments from which they are derived, is fundamental knowledge. The current study produces a subregional to regional characterization of the geochemistry of Eagle Ford oils and sediments in the context of a meaningful stratigraphic framework. The study area includes the main and most important producing areas of the Eagle Ford shale oil play.

The lower part of the Eagle Ford is shown to be the organically richest part of the group. This is demonstrated by the general literature, reference to work completed by colleagues of this volume, and presentation of data for a core from an important Eagle Ford producing area. This interval is lower-middle Cenomanian in age. It depositionally predates the Oceanic Anoxic Event 2 (OAE2) that occurs at the Cenomanian–Turonian boundary. Elevated organic richness in the lower Eagle Ford that varies along strike suggests organic accumulation is partly controlled by localized, semipermanent circulatory patterns.

Multivariate statistical classification using biomarkers and carbon isotopes from a large number of oils in Cretaceous reservoirs closely related to the Eagle Ford resulted in the identification of eight compositionally distinct families, three of which occur in the main part of the Eagle Ford shale oil-producing area: Family 2, Family 3, and Family 7. Average data for each family are compared to a large set of global oils representing a variety of depositional environments and depositional times. Comparison of the south Texas oils to the cosmopolitan dataset indicates that Family 3 oils were derived from shales deposited in distal marine settings. Family 7 oils compare favorably with oils derived from carbonate-rich source rocks and Family 2 oils from compositionally intermediate marl-rich sediments.

Maturity-sensitive data from the oil families were submitted to principal component analysis. Seventy-five to ninety-four percent of the variability in these datasets was contained in the first or primary principal component (Factor 1). The level of correlation suggested these Factor 1 values could be converted to equivalent vitrinite reflectance values (%VRE). This was accomplished and the VRE for the oils mapped. Oil maturities obtained by this process are consistent with maturity trends obtained from regional considerations.

When assessing source rock thermal maturity using pyrolysis techniques (e.g., Rock-Eval), it is useful to measure pyrolysis parameters both before and after solvent extraction, especially at or near peak oil maturity levels.

The certitude that oils in this study are derived from the Eagle Ford, as opposed to the Austin Chalk or some third source, comes from several observations. Some Family 2 oils come directly from completions in the Eagle Ford. Family 7 oils come from the First Shot field area and Family 3 oils from Giddings are derived from Eagle Ford/Boquillas Shales based on positive oil-source correlations. Several source scenarios can be imagined given two proven Eagle Ford sources (lower-middle Cenomanian and Turonian) and three organic facies represented by oils. It is possible that one or more organofacies are active sources within each chronostratigraphic interval.