Low-maturity soft bitumen (or biodegraded heavy oil) and higher maturity solid bitumen are present in Palaeozoic siliciclastics at Tianjingshan in the NW Sichuan Basin, southern China. The origin of these bitumens of variable maturities was investigated. Samples of low-maturity bitumen from Lower Devonian sandstones and high- and low-maturity bitumen from Upper Cambrian siltstones were analysed to investigate their organic geochemistry and stable isotope compositions. Lower Cambrian and Upper Permian black shales were also investigated to assess their source rock potential, and the burial and maturation history of potential source rocks was modelled using PetroMod. Liquid and gaseous hydrocarbon fluid inclusions in the Devonian sandstones were analysed.
Results suggest that both the soft and solid bitumens are derived from crude oil generated by Lower Cambrian organic-rich black shales. Reservoir rocks at Tianjingshan have experienced two separate oil charge events – in the early-middle Triassic and early-middle Jurassic, respectively. The first oil charge was generated by Lower Cambrian black shales in a kitchen area located in the hanging wall of the Tianjingshan fault. The later oil charge was also derived from Lower Cambrian black shales, but the kitchen area was located in the footwall of the fault. Movement on the Tianjingshan fault resulted in progressive burial of the Lower Devonian sandstone reservoir rocks until the end of the middle Triassic, and the “early” charged oil was thermally degraded into high-maturity solid bitumen. The later-charged oil was altered into soft bitumen of lower maturiy by biodegradation during uplift of the reservoir after the Jurassic.