Reservoir solid bitumen-source rock correlation using the trace and rare earth elements–implications for identifying the natural gas source of the Ediacaran-Lower Cambrian reservoirs, central Sichuan Basin

Abstract

The hydrocarbon-source correlation in highly-mature and over-mature stages is a challenging task in natural gas exploration. The Ediacaran-Lower Cambrian natural gas in the central Sichuan Basin is typical oil-cracking gas. With a burial depth of more than 4650 m, the gas is now in the highly-mature to the over-mature stage (equivalent Ro>1.8%) with apparent alteration by thermochemical sulfate reduction (TSR), leading to difficulties for gas-source correlation. Conventional hydrocarbon-source correlation mainly relies on organic geochemical methods, taking media such as biomarkers and carbon isotope for correlation. However, in the highly-mature and over-mature stages, the characteristics of conventional biomarkers tend to be homogenised. Furthermore, the gas components and carbon isotope of natural gas are frequently altered by TSR in the deep burial stage. Thus, conventional organic geochemical methods are significantly restricted. This paper explored the source of highly-mature natural gas from the DY and LWM Formations in the central Sichuan Basin by studying the inorganic geochemical information (trace elements and rare earth elements) of the reservoir bitumen and potential source rocks. The results show that the depositional environment of the potential source rocks (the QZS and the D-3 source rocks) in the central Sichuan Basin were different. The redox-sensitive parameters (V/(V + Ni), Mo/Ni, δCe and Ce/Y) and the provenance-sensitive parameters (La/Co and La/Sc) of the source rocks have apparent differences, which can be used as the basis for gas-source correlation. The correlation results show that the LWM gas was derived from the QZS source rock, while the D-2 and D-4 gases were mainly derived from the QZS source rock, with a partial contribution of the D-3 source rock. During geologic evolution, the crude oil generated from the two sets of source rocks were thoroughly mixed in the paleo-oil reservoir of the D-4 Member. As a result, the characteristics of the D-3 source rock were obscured by the QZS. In contrast, the solid bitumen in the D-2 paleo-oil-reservoir recorded the contribution of the D-3 source rock. However, this contribution was limited within the range of 50 m adjacent to the D-3 Member by a more significant oil generation from the QZS source rock located in Deyang-Anyue Rift Trough. This study clarified the gas source differences between the D-2 Member, D-4 Member and LWM Formation, providing a new insight for in-depth study on the gas accumulation of the Ediacaran-Lower Cambrian in the central Sichuan Basin.