Relationships between hydrocarbon evolution and the geochemistry of solid bitumen in the Guanwushan Formation, NW Sichuan Basin

Recently, the Devonian Guanwushan Formation in the northwestern Sichuan Basin gained industrial attention when solid bitumen was discovered in reservoirs and outcrops. This discovery initiated the exploration and development of the Devonian natural gas. Devonian in the northwestern Sichuan Basin is a new stratum for the exploration and development of Paleozoic marine natural gas in Sichuan Basin. However, the unclear recognition on the genesis, the sources and the hydrocarbon evolution history of solid bitumen and natural gas seriously restricted further exploration and development. Based on the solid bitumen reflectance, carbon isotopes, saturated hydrocarbon chromatography, and solvent extract bitumen biomarkers combined with the component and carbon isotopes of natural gas, comprehensive analyses have been performed on the origin of the solid bitumen and the accumulation and evolution of the natural gases. The results showed that the outcrop solid bitumen is derived from biodegraded oil from the Lower Cambrian source rocks, and is characterised by low bitumen reflectance, low saturated/aromatic hydrocarbon ratios, seriously damaged normal-alkanes, and severe migration of the gas chromatographic baseline. The reservoir bitumen formed through thermal cracking of oil from Lower Cambrian and Lower Permian source rocks, and is characterised by high bitumen reflectance, high saturated/aromatic hydrocarbons, and complete normal-alkanes. The differences of tectonic evolution take control of the two different-genesis bitumen. The Longmen Mountain strata in the northwestern Sichuan Basin were uplifted sharply after the Jurassic and the early generated crude oil was biodegraded during uplifting and formed the outcrop bitumen. However, strata in Well Shuangtan-3 (ST3) continued to subside and the paleo reservoir thermal-cracked during deep burial into dry gas and heavy components gathered to form reservoir bitumen. Revealing the Devonian hydrocarbon evolution in Sichuan Basin is of highly significance and is meaningful to guide further researches on the gas-source correlation of natural from paleo-strata. It is of great significance to reveal the Devonian hydrocarbon evolution in Sichuan Basin and to guide further researches on the gas-source correlation of natural gas from paleo-strata.