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Study of the occurrence and formation of sedimentary organic matter in the Sichuan Basin, southern China, provides valuable insights into the biological sources and compositions of Neoproterozoic to Lower Cambrian shales. Rock samples from two sets of shales in the Sichuan Basin and adjacent areas, including the Ediacaran Doushantuo shale and the Lower Cambrian Qiongzhusi shale in the Gaoshiti-Moxi and Ziyang-Weiyuan areas were collected and geochemically analyzed in this study. The distribution of biomarkers, stable carbon isotope compositions and microfossil characteristics reveal that eukaryotic algae including chlorophytes, pelagophyceae, dinoflagellates, diatoms (or their ancestors) and sponges proliferated widely during the Late Neoproterozoic to Early Cambrian in the Sichuan Basin. Cyanobacteria, some α-Proteobacteria, rhizobiales and aerobic methanotrophs also made a contribution to organic matter in the studied shales. Aerobic methanotrophs that utilize 13C-depleted substrates such as CH4 as carbon sources may have been the cause of anomalous 13C-depleted kerogen in the study area.

Pyrobitumen has been discovered in the dolostone reservoir of the Central Sichuan Basin, southwest China, accumulates coevally with hydrothermal minerals in pores. The pyrobitumen has strong anisotropy with an evolutionary sequence from mosaic to a fibrous texture. A similar sequence of anisotropy has also been observed in mesophase pitch during the process of coking coal. Reflectance of the pyrobitumen shows a positive correlation with anisotropic evolution and increasing depth. Hopane and sterane biomarkers are abundant in extracts of highly anisotropic pyrobitumen with smooth, rough, hyperplastic surface and honeycomb micropores. Methylpyrene and methylchrysene maturity indicators and the reflectance values of pyrobitumen show a positive linear correlation in Cambrian samples, but show random correlation in Sinian samples. Fluid inclusion microthermometry of hydrothermal minerals, together with the burial and heat flow history of the reservoir indicate an abrupt thermal event that may have impacted the generation and evolution of pyrobitumen. This appears to be confirmed by the reflectance values of pyrobitumen in the source rock and reservoir. The pyrobitumen in source rock was less heated by the hydrothermal fluid due to its poor porosity and have lower reflectance values. However, the pyrobitumen in weathering crust reservoir was more heated by the hydrothermal fluid and have higher reflectance values, because the reservoir rocks have higher porosity. The occurrence and characteristics of pyrobitumen indicate an in-situ pyrolysis of oil, which was triggered by an abrupt thermal event in the reservoir. The reconstruction of burial history and heat flow, as well as the hydrothermal fluid minerals of the dolomite reservoir all indicate the thermal event of the Sichuan Basin might correlate with the Emei mantle plume in Late Permian.

The lower Cambrian sediments including the Qiongzhusi and Maidiping formations in the Sichuan Basin are generally considered as the main source rocks of Precambrian-Cambrian petroleum system. A comprehensive study integrated with multiple geochemical methods was conducted on 119 lower Cambrian drillcore samples collected from the upper Yangtze Platform. The results show that the fluctuation of sea level and climate during the deposition of the Qiongzhusi and Maidiping formations act as the critical factors controlling the watermass conditions and thus OM accumulation. During the deposition of upper Maidiping Formation (Є1m), the paleoclimate was arid and sea level was relatively low in the study area. The depositional environment indexes (UEF-MoEF, gammacerane index, β-carotane and VEF) suggested that the upper Є1m was deposited in a strong restricted marine setting with high water salinity and anoxic water conditions in Ziyang-Weiyuan area. However, during the early Qiongzhusi Formation period, the paleoclimate became humid and sea level rose in the study area. High paleoproductiviy caused by high concentrations of nutrients brought by seawater and freshwater plays an important role in OM accumulation in the basal part of the Qiongzhusi Formation (Є1q). Furthermore, the model of paleoproductivity promotion caused by the coordinated control of climate and extensive transgression in the lower part of Є1q may also be applicable to interpret the origin of high organic carbon abundance in the strata deposited in the same period with the Є1q (such as Shiyantou, Shuijingtuo and Niutitang formations) in other parts of the upper Yangtze Platform.

After analyzing the molecular and stable isotope compositions of 45 natural gas samples in the Changxing and Feixianguan Formations in northern Sichuan basin, significant differences have been observed between the samples in the northwest Kaijiang-Liangping trough and those in the northeast. The δ13C2 values of the gas in the northeast of Kaijiang-Liangping trough (δ13C2=−25.0‰∼−32.3‰ with an average of −28.2‰)are lighter than those in the northwest of Kaijiang-Liangping trough (δ13C2=−21.3‰∼−28.9‰ with an average of −24.8‰). The δD1 values of the gas in the northeast (δD1=−114‰∼−129‰ with an average of −121‰) are also lighter than those in the northwest (δD1=−107‰∼−115‰ with an average of −113‰). By analyzing the kinetic fractionation of carbon isotope in methane and ethane, we find that gas in the northwest of Kaijiang-Liangping trough originated from the P2l coal measures while those in the northeast were sourced from the thermal cracking of oil, which are most likely generated from the marine source rocks in the Upper Permian Longtan formation(P2l) and the Silurian. The CO2 content varies from 0 to 30.20% with the δ13CCO2 values ranging from −2.9‰ to 3.3‰, which suggests that the CO2 may possibly originate from the carbonate reservoirs dissolved by acidic fluids. Thermochemical Sulfate Reduction (TSR) has resulted in the richness of deuterium in the methane.