Evidence the Eagle Ford Reservoir Principally Produces Migrated Oil on the San Marcos Arch
Marine carbonate source rocks (SRs) typically generate heavy sour oil at low thermal maturity. Does that viscous oil readily flow from a marl reservoir? The Lower Eagle Ford (LEF) Formation is a classic SR reservoir in south Texas; the overlying Austin Chalk contains oil that migrated updip from very mature EF SR beds. Geochemical data measured on oil samples produced from the Austin Chalk and the EF Formation and on petroleum extracted from Austin Chalk and EF core plugs on the San Marcos Arch in Gonzales County indicate migrated oil principally is produced from both reservoirs there: i.e., low-maturity oil generated by LEF kerogen does not efficiently flow. HRGC analyses were performed on 7 produced oil samples, and on extracts obtained at two well locations from 20 core plugs selected in the Austin Chalk, Upper EF (UEF), and LEF marl or clay-shale intervals. ≈29-31°API oil samples produced from the Austin Chalk were generated at higher temperatures than ≈23-27°API oil samples produced from EF reservoirs. C7 temperatures are ≈4°C higher in LEF marl and clay-shale core extracts selected from the well located in a more downdip position. Pristane/phytane (pr/ph) ratios are lower (0.55-0.69) in UEF core-plug extracts obtained from the updip well that contain migrated oil and in the produced oil samples than in most LEF marl extracts (0.79-0.93) obtained from the same well. Pr/ph ratios are even higher (0.94-1.35) in extracts obtained at the downdip well from Austin Chalk, UEF, and LEF marl or clay-shale core plugs. Toluene/nC7 ratios also are lower in UEF core-plug extracts selected from the updip well – and in oil samples produced from nearby wells – than in LEF marl core-plug extracts obtained from that well. A Hierarchical Cluster Analysis using 17 HRGC peak-height ratios confirms that significant differences exist between the composition of: (1) produced oil samples and migrated oil extracted from Austin Chalk and UEF core plugs; and (2) petroleum samples extracted from LEF marl or clay-shale core plugs. Potential end-member samples and mixing lines between extracts and oil samples were identified during a Principal Component Analysis. Oil fingerprinting allocation results indicate that: (1) the migrated oil in UEF core extracts obtained from the updip well is a mixture of LEF marl extracts and some of the produced oil samples; and (2) none of the produced oil samples are commingled mixtures of extracts obtained from LEF marl or clay-shale core plugs.
AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018