Production of Migrated Oil From Horizontal Wells Landed in the Eagle Ford Formation on the San Marcos Arch

Abstract

Migrated oil produced from the Austin Chalk and the Buda Formation in south Texas was generated by and expelled from deeper Eagle Ford source-rock beds. The origin of oil samples produced from Austin Chalk, Eagle Ford, and Buda reservoirs on the San Marcos Arch was determined by comparing their composition to the composition of extracts obtained from conventional core plugs selected in the Austin Chalk and Eagle Ford Formation at two nearby wells. Lower Eagle Ford (LEF) marl core samples are very good source rocks that contain oil-prone kerogen at VRE ≈0.70. Upper Eagle Ford (UEF) and LEF clay-shale core samples are good source rocks that contain oil + gas-prone kerogen. Leaner Austin Chalk core plugs contain only gas-prone or inert kerogen. HRGC data were used to calculate oil source and maturity parameters, classify the oil samples, and allocate commingled samples. The API gravity of the produced oil samples is controlled by the temperature at which they were generated. Oil extracted from core plugs selected in a deeper well was generated at a slightly higher temperature than oil extracted from the same stratigraphic intervals in a shallower well. Two geochemical source parameters that utilize only saturate compounds indicate that the extracts obtained from the shallower well and the produced oil samples were generated by the same kind of oil-prone kerogen (which is different than the oil-prone kerogen that generated extracts obtained from the deeper well). But extracts obtained from LEF marl core plugs selected in the shallower well are assigned to a different family than the produced oil samples using HRGC peak-height ratios that include aromatic compounds because all Eagle Ford core-plug extracts are enriched in aromatic compounds compared to the produced oil samples. Allocation results using the produced oil samples indicate a 30°API oil sample obtained from the Austin Chalk is a migration mixture of medium-gravity oil generated locally by the LEF marl, and much lighter oil that migrated laterally and updip. The oil produced from a horizontal well drilled into the UEF is a commingled mixture of end-member oil samples obtained from the Austin Chalk and from the UEF at a nearby vertical monitor well. In contrast, the oil produced from a horizontal well drilled into the LEF is a commingled mixture of end-member oil samples obtained from the LEF marl and the Buda Formation. Allocation results for extracts obtained from several UEF core plugs indicate the basal portion of that reservoir contains some oil that was generated locally and expelled by underlying LEF marl source-rock beds. UEF SR beds have not efficiently charged the overlying Austin Chalk reservoir on the San Marcos Arch because they have not generated a large amount of oil at low thermal maturity.