Source characterization of highly mature pyrobitumens using trace and rare earth element geochemistry: Sinian-Paleozoic paleo-oil reservoirs in South China

Understanding the relationships between migrated hydrocarbons and source rocks under highly mature conditions (i.e., pyrobitumen-source correlation) is a critical and challenging issue in the field of petroleum geology and geochemistry. Based on a case study of three Sinian-Paleozoic paleo-oil reservoirs from Yunan and Guizhou provinces, South China, we attempt to provide further understanding of this issue using trace and rare earth element (REE) geochemistry, which can characterize the depositional setting, organic input and rock provenance of source rocks. Results show that the bitumens from the three paleo-oil reservoirs can be distinguished from each other using 'Post-Archean Australian Shale (PAAS)'-normalized trace element distribution patterns, REE distribution patterns, hierarchal cluster analysis of element concentrations and redox- and provenance-sensitive elemental ratios (e.g., V/Ni, Ni/Co, Ni/Mo, Zr/Cr, La/Sc and La/Co). Pyrobitumen in the Devonian Xiaocaoba paleo-oil reservoir is derived primarily from Devonian source rocks, representing a self-sourced petroleum system and one stage hydrocarbon charging. Similarly, pyrobitumen in the Sinian Jinsha-Yankong paleo-reservoir also received a single fluid charge, but the source is different and likely generated by contributions from both lower Cambrian and Sinian sources. In contrast, the bitumen in Ordovician Kaili-Luomian paleo-reservoirs are indicated to have contributions from two different hydrocarbon sources and thus experienced two stages of hydrocarbon charging: an early charge from lower Cambrian source rocks and a later charge from Permian source rocks. The new trace and rare earth element method used in this study appears to be effective at providing a better understanding of the hydrocarbon-source correlation under highly mature conditions. The results improve our understanding of the complex hydrocarbon accumulation-alteration evolution in the marine strata of South China, which is a potential target for petroleum exploration.