Early oil charging from Sinian shales in Ordovician carbonates of the Tabei Uplift, Tarim Basin: Evidence from biomarkers

Abstract

Despite over 40 years of exploration focusing on the deep to ultra-deep Ordovician carbonates as major hydrocarbon targets in the Tarim Basin, the identification of their source rocks remains elusive. Based on biomarkers, carbon and sulfur isotopes of hydrocarbons, the primary source for the Ordovician petroleum system has been attributed mainly to the lower Cambrian shales, although some oils were likely contributed from the Lower Ordovician source rocks. However, the current understanding of the evolution of the Ordovician petroleum system remains rudimentary, largely due to the complex interplay of multi-source (i.e., the widespread Precambrian shales) hydrocarbon inputs, diagenetic alterations, and tectonic processes over geological time. This study systematically investigates the molecular geochemistry of reservoir bitumen within the Ordovician carbonates from Tabei uplift, coupled with bitumen from the Sinian units at the western edge of the Tarim Basin. Our results indicate that the ion chromatography-mass spectrometry spectra and the saturated to aromatic hydrocarbons ratio of Ordovician reservoir bitumen closely resemble those of Sinian bitumen, as well as the published data of Sinian shales. This coupling linkage is revealed by cross-plots and ternary phase diagrams of various biomarker parameters, which can effectively distinguish the Sinian sources from other sources, i.e., the Lower Cambrian and Ordovician, for the Ordovician reservoir bitumen. Specifically, the ∑n-C₂₁^-/∑n-C₂₂⁺, Pr/Ph, G/C₃₁H22S, C₂₃/C₂₁TT ratios are effective indexes to differentiate these source rocks. By compiling the published organic geochemistry data of oils, it appears to infer that approximately 8.1% of the present-day oils produced from the Ordovician carbonates likely contain some proportion of Sinian-sourced oils. The recognition of Sinian sourced oils contributing to the paleo- and present-day Ordovician petroleum system offers valuable insights for the exploration of deep-ultra deep carbonates in the Tarim Basin, emphasizing the need to consider Precambrian shales as a significant hydrocarbon source.