Geochemical Characteristics of Meso-Neoproterozoic-Cambrian Potential Shale Source Rocks and Oil Source Correlation for Paleo-Reservoirs in the Cambrian Formations of the Zhongtiaoshan Area in Southern East of the Ordos Basin.

Abstract

Meso-Neoproterozoic to Cambrian oil fields have been discovered in several regions, including Australia and the Middle East. These findings suggest the presence of potential source rocks, despite their long evolutionary history. In the Zhongtiaoshan area of the southeastern Ordos Basin, several outcrop samples were collected from the Xuzhuang, Cuizhuang, and Baicaoping formations spanning the Meso-Neoproterozoic to Cambrian. Geochemical analyses indicate that the Cuizhuang Formation (Mesoproterozoic) exhibits significant hydrocarbon generation potential, characterized by relatively high total organic carbon (TOC) and <i>S</i> ₁ + <i>S</i> ₂ values. In contrast, the other formations appear to have lower source rock potential. Gas chromatography-mass spectrometry (GC-MS) analysis of rock extracts reveals a low Pr/Ph ratio (<1) and a normal alkane distribution, suggesting an anoxic depositional environment. The high distribution of tricyclic terpanes and elevated gammacerane ratios indicate a saline lake facies with substantial algal contributions. Additionally, C₂₇ steranes dominate the sterane distribution, and the C₂₇-C₂₈-C₂₉ sterane pattern follows an "L" distribution, further supporting an algal and aquatic microbial origin. A high sterane/hopane ratio, a low hopane index, and the relative scarcity of homohopanes (e.g., C₃₅ hopanes) suggest minimal bacterial input. The source rocks were likely deposited in a lagoonal facies under low sea levels, which favored organic matter preservation. During sea-level rises, increased oxygen availability may have led to the oxidation of organic matter. Based on key geochemical biomarker indicators including thermal maturity, depositional environment, and organic matter input, the bitumen in the Xuzhuang Formation was likely derived from Cuizhuang Formation shale source rocks.