Unconventional shale systems: A comparative study of the in-source sweet spot developed in the lacustrine Chang 7 Shale and the marine Barnett Shale

The Upper Triassic Chang 7 Shale and the Mississippian Barnett Shale, two remarkable shale systems that are considered promising unconventional shale oil/gas targets in China and the USA, respectively, have different kerogen types that formed under lacustrine and marine environments. In this study, seventy-six and ninety-seven drilled core samples were collected separately from the Zhang 22 Well (Ordos Basin) and the Mesquite #1 well (Fort Worth Basin), respectively, for determining the hydrocarbon generation, retention and migration behaviors as well as the intrasource fractionation in the two shale successions. Geochemical indications of potentially producible oil as indicated by the oil crossover effect and the T-max shift phenomenon as well as the geochemical and molecular compositions of retained hydrocarbon suggest two categories of intervals in the Chang 7 and Barnett shale systems, namely, the in-source sweet spot and the generative unit. Oils retained in the in-source sweet spot have migrated from the adjacent generative unit and have been subjected to stratigraphic fractionation over geological time. These oils are proportionally rich in aliphatic fraction and moderate chain length moieties, which significantly improve the fluid mobility and producibility. The oil quality improved by fractionation, the weak adsorptive affinity of oil to organic matter, and the good shale frackability all provide promise for the exploration and production potential of shale oil from the Chang 7 and the Barnett Shale systems.