Characterizing petroleum in source-rock core samples using HRGC data

Solvent extracts obtained from center-cut horizontal core plugs selected in Upper Wolfcamp (UW) and Eagle Ford source-rock (SR) beds contain unaltered volatile (i.e., gasoline-range) HC compounds because they were extracted in a closed vial. Therefore, a C7 source parameter, a C7 maturity parameter, and pristane/phytane ratios were used to compare the source and thermal maturity of these petroleum samples and oil samples produced from nearby wells landed in the same SR reservoirs. Five distinct pay zones previously identified in the UW SR reservoir using geological criteria each contain slightly different kinds of petroleum generated at different levels of thermal maturity. A thick overlying carbonate reservoir contains the kind of petroleum generated by the kerogen present in one underlying SR pay zone. The same source and maturity parameters demonstrate that the oil-prone kerogen present in Eagle Ford SR beds in core plugs selected from wells located ˜7.5 mi (12 km) apart on the San Marcos Arch in South Texas formed in different depositional environments. It is difficult to allocate commingled oil samples using only core-plug extracts because solvents extract the producible oil plus a component that does not readily flow from SR reservoirs because it is sorbed in kerogen and/or on clay minerals. However, because only saturate HC compounds were used to determine C7 source and maturity parameters, they provide valuable insights about the nature of the free oil present in SR reservoirs and in commingled oil samples.