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Contrasting Biomarker Features of Two Types of Jurassic Mudstones with Different Organic Matter Contents in the Northern Qaidam Basin

YANG Shao-yong1 CAO Jian2 LIU Yun-tian1, 2 HU Kai2BIAN Li-zeng2 WANG Li-qun1 CHEN Yan1 (1. PetroChina Qinghai Oilfield Company, Branch of Petroleum Exploration, Dunhuang, Gansu 736202; 2. Department of Earth Sciences, Nanjing University, Nanjing 210093)  
Two types of Jurassic mudstones with different total organic contents (TOC) from the northern Qaidam Basin were analyzed for biomarker features in this study. It is showed that these two types of mudstones have pronounced different biomarker compositions. The mudstones with high TOC contents show similar features to other Jurassic source rocks and their derived oils in the western China's Jurassic petroliferous basins. In contrast, as to the mudstones with low TOC contents, it is characterized by an n-alkane envelope that peaks at a low carbon number (e.g., nC16, nC17), Pr/Ph around 1.0, abundance of tricyclic terpane and gammacerane, and detection of 25-norhopane series in two samples. Combined with organic petrographic features of mudstones, it is implied that the biomarkers reflect depositional environments and bio-precursors of the mudstones. Bio-precursors of high TOC mudstones consist mainly of algae, cutinites and sporophytes, under relatively anaerobic situations. As to the low TOC mudstones, organic maceral composition is composed mainly of mineral bituminous matrix, which comes from biodegradation of micro-alga by microbe under relatively more anaerobic conditions. Furthermore, based on a comparison between biomarkers of mudstones analyzed in this study and of presently explored oils, the oil source in the study area was discussed.
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