Biomarkers and C and S Isotopes of the Permian to Triassic Solid Bitumen and Its Potential Source Rocks in NE Sichuan Basin

Abstract

The potential parent source rocks except from Upper Permian Dalong Formation (P 3 d) for Upper Permian and Lower Triassic solid bitumen show high maturity to overmaturity with equivalent vitrinite reflectance (ER ) from 1.7% to 3.1% but have extractable organic matter likely not contaminated by younger source rocks. P 3 d source rocks were deposited under euxinic environments as indicated by the pyrite 34 S values as light as −34.5‰ and distribution of aryl isoprenoids, which were also detected from the Lower Silurian (S 1 l) source rock and the solid bitumen in the gas fields in the west not in the east. All the solid bitumen not altered by thermochemical sulfate reduction (TSR) has 13 C and 34 S values similar to part of the P 3 l kerogens and within the S 1 l kerogens. Thus, the eastern solid bitumen may have been derived from the P 3 l kerogens, and the western solid bitumen was likely to have precracking oils from P 3 l kerogens mixed with the S 1 l or P 3 d kerogens. This case-study tentatively shows that 13 C and 34 S values along with biomarkers have the potential to be used for the purpose of solid bitumen and source rock correlation in a rapidly buried basin, although further work should be done to confirm it.

The geochemical differences and origin of multilayered natural gases in marine strata, the northwestern Sichuan Basin, China

“…P 3 d is contemporaneous heteropic facies of Upper Permian Changxing Fm (P 3 ch) at NW Sichuan Basin. Similarly, the same reason applies to Upper Permian Wujiaping Fm (P 3 w) and P 3 l (Cai et al, 2017a;Chen et al, 2018). It is imperative to give importance to P 3 d-P 3 w/l along with Є 1 q black shale, which are the high-quality muddy hydrocarbon source rock in marine strata (Figure 3A, D and F).…”

“…Due to complex tectonic fracture system, multiple phase uplift (cooling) and subsidence (heating) activities (Li et al, 2022a), insufficient exploration data, the contribution of several sets of potential source rocks was difficult to determine. Geologists hold varying opinions as to origin of these gas, for example, sourced from the Cambrian Qiongzhusi Fm, Sinian Doushantuo Fm or Silurian Longmaxi Fm (Dong et al, 2017;Li et al, 2022b;Xie et al, 2020), derived from Periman Longtan Fm or Periman Maokou Fm (Cai et al, 2017a(Cai et al, , 2017bZhu et al, 2022), mixed gas sourced from Cambrian Qiongzhusi Fm and Periman Maokou Fm (Li et al, 2022c;Lu et al, 2017;Xiao et al, 2020), mixed gas sourced from Periman Maokou/Qixia Fm and Periman Longtan Fm (mixed oil-associated and coal-derived gas) (Huang et al, 1997;Wang et al, 1997). The ambiguity of natural gas sources limits the exploration development in NW Sichuan Basin.…”