Physical Properties and Whole-Oil GC Geochemical Characterization of Produced Crude From Eagle Ford Shale

Abstract

A total of 56 oil samples were collected from the self‐sourced Eagle Ford Shale. These samples cover a wide range of API gravity ranging from 30° to 63° and are geographically distributed in a major shale‐oil production area in the west and east parts of the San Marcos Arch. Oil physical properties, sulfur and nitrogen content and SARA fractions (saturate, aromatics, resins, and asphaltene) were measured. Quantification of the hydrocarbon concentration for individual group compounds with the same carbon number were conducted in whole‐oil GC analysis. Empirical correlations between oil chemistry and bulk‐oil physical properties were obtained for understanding the key controls on the heterogeneity of Eagle Ford crudes. This study produced a regional geochemical characterization of the Eagle Ford oils. A negative correlation occurs between oil API gravity and sulfur content in Eagle Ford crude. sulfur content ranges from 0.009% to 1.43%—the wide variation directly relating to oil type. The concentration of light‐hydrocarbon components (C6–C10) in oil greatly affects oil quality, and an empirical trend of exponential increase in the concentration of C6–C10 light hydrocarbons in oils with API gravity was achieved. Thermal maturity is the principal controlling factor of oil quality. Oil maturities (%Roc) are calculated on the basis of MPI‐1 aromatic parameters and show a comparable correlation with oil API gravity. Particularly in the self‐sourced Eagle Ford unconventional system, the maturity of oils is considered equivalent to the source‐rock maturity level at the time of oil generation because little compositional fractionation caused by migration and expulsion. The empirical equation between %Roc and API gravity can be considered a reference for oil and source correlation from Type II kerogen‐ dominated marls (for Eagle Ford?). Carbon preference index (CPI) and pristine:phytane ratio (Pr:Ph) are two indicators of organic‐ matter source input and redox conditions during deposition of Eagle Ford sediments on the San Marcos Arch. Oil samples from the east part of the arch are characterized by higher CPI and Pr:Ph values, indicating a mixing of terrestrial organic‐matter source input and less reducing environment. The low and constant Pr:Ph value (≤ 1) of oils in the west part of the arch indicate a marine‐reducing environment. CPI and Pr/Ph values significantly increase when the maturity reach oil cracking stage in, and, as a result, neither can be used as reconstructing parameters for depositional environments or OM source input.

AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019