Molecular markers, carbon isotopes, and rare earth elements of highly mature reservoir pyrobitumens from Sichuan Basin, southwestern China: Implications for PreCambrian-Lower Cambrian petroleum systems.

Highlights • Unique geochemistry of high mature reservoir pyrobitumens in Lower Cambrian. • Parallel application and cross-validation of REE, carbon isotopes and biomarkers. • Multiproxy study of oceanographic conditions in the Lower Cambrian. • Variations of inorganic element ratios in probitumens along migration pathways. • A potential means through analysis of bitumen to study paleo-oil reservoirs. Abstract The Moxi–Gaoshiti Bulge of the Sichuan Basin in southwestern China is unique because it provides insights into the geochemistry of Lower Cambrian reservoir bitumens and their geological significance for paleo-oil reservoirs. Analyses of biomarkers, carbon isotopes, trace elements (TE) and rare earth elements (REE) were conducted on reservoir bitumens and the associated source rocks to constrain sedimentary environments and bitumen/paleo-oil sources. Based on biomarkers, carbon isotopes, TE ratios and REE distribution patterns, both of the Cambrian Qiongzhisi (Є 1 q) and Sinian Dengying (Z 2 dn) source rocks can be hydrocarbon sources for the Cambrian Longwangmiao (Є 1 l) bitumens in the MG Bulge (Lower Cambrian Doushantuo Formation not discussed here). Abundant n -alkanes relative to isoprenoids, significant C 21 and C 22 steranes, predominance of dibenzothiophene (DBT), and low ratios of biomarkers, such as Pr/Ph, DBT/P, C 22 TT/C 21 TT and C 35 H(22S)/C 34 H(22S) indicated that the source rock was characterized as shale from an overall highly reducing marine depositional environment. The average δ13C values from the compositions of the solid bitumens show an abnormal order: saturate hydrocarbons < asphaltenes < N-S-O compounds < aromatic hydrocarbons, suggesting that high temperature might affect the normal fractionation among different organic matter fractions. The reservoir bitumens are enriched in Mo, Tl, Pb and Ba, strongly depleted in Bi Compared to PAAS, and mostly well correlated in the REE distribution patterns. In the primary migration pathways suggested by the DBT concentration, the parameters ∑REE and Zr/Cu show an overall increase, and the ratios V/Ni, Ni/Co, V/Zr and V/Cr have decreasing trends. The comprehensive analyses of molecular biomarkers, carbon isotopes, TE and REE in pyrobitumens of the highly mature reservoirs in this study suggest a potential means to study paleo-oil reservoirs, and can thus provide valuable information for deep oil and gas exploration in Precambrian – Lower Cambrian petroleum systems. [ABSTRACT FROM AUTHOR]