Organic geochemical characteristics and origin of solid bitumen and oil sands in northwestern Sichuan

Hydrocarbon compositions extracted from a set of solid bitumen and oil sands collected in the northern Longmenshan Mountain, the northwestern Sichuan, South China, are unique and consistent. It is suggested that the bulk δ¹³C values (<-32‰) for the extracts and each fraction show overall depleted values, which is typical of pre-Cambrian source. The high concentrations of C₂₉ steranes relative to C₂₇ and C₂₈ steranes can be explained by the contribution of cyanobacteria, and the distribution of triaromatic steranes has similar characteristics. 24-n-propylcholestane and 24-isopropylcholestane are abundant, which should be derived from marine chrysophyte algae and demosponges. The high abundance of pregnane and homopregnane could be associated with such specific depositional environment as anoxic to euxinic bottom waters. The absence of rearranged steranes, abundant 30-norhopane, C₃₅ and C₂₄ hopane tetracyclic terpanes, and a wealth of dibenzothiophene compounds, reflect the hypoxic environment and higher carbonate rock content. The features of solid bitumen and oil sands in the northwestern Sichuan indicate that they originated from the Doushantuo Formation of Sinian, and can be compared with foreign crude oils which generated during the same period. The presence of UCM and 25-norhopane proved biological degradation during the preservation and destruction processes of ancient reservoir. The hydrocarbons were generated by sulfur-rich kerogens at the early oil windows with R_c values in the range of 0.57%-0.84%. Tectonic uplift and the following erosion and biodegradation might explain the destruction of ancient reservoir.