C_(24) tetracyclic terpanes and their origin in crude oils and source rocks from the Zhu 1 Depression,Pearl River Mouth Basin
BAO Jian-ping;ZHU Cui-shan;CHEN Xi-wen;WANG Fang;Key Laboratory of Oil and Gas Resource and Exploration Technology;Geochemistry Department, Yangtze University;
Four C_(24) tetracyclic terpanes have been detected in crude oils from the Zhu 1 Depression, three of them are identified as 10b(H)-des-A-oleanane(A), 10b(H)-des-A-lupane(B1), and 17, 21-secohopane(C), respectively. However, compound B is an unknown biomarker. Its mass spectrum indicates that it is a C_(24) tetracyclic terpane because of a molecular ion at m/z 330. In addition, its base peak is at m/z 191, which is derived from the D/E ring cleavage in pentacyclic triterpane, and a key fragment is at m/z 287, indicating an isopropyl group in its structure. Like lupanoids, there is an isopropyl group in the E ring of C30 hopane, therefore, this compound is likely derived from degradation of C30 hopane or its precursor. The relative abundance of compound B is negatively correlated with terrigenous triterpanes and their degraded compounds, such as oleanane, 10b(H)-des-A-oleanane, 10b(H)-des-Alupane, and bicadinane, but positively correlated with C30 ab-hopane, indicating bacteria input in crude oils. These relationships imply that compound B has a microorganism source and is tentatively identified as C_(24) des-A-hopane. In addition, compound B is abundant in lacustrine oils and source rocks rich in C30 4-methylsteranes, but very low or almost absent in coaly oils and carboniferous mudstone having higher Pr/Ph ratios(6.0) and predominant C29 diasteranes, regular steranes, and other terrigenous triterpanes. This implies that compound B, and its relative abundance, have an important role in indicating origin and source of crude oils in the study area. It is well-known that lacustrine source rocks are deposited under relatively reducing environments, advantageous to the growth of anaerobic microbes, but the coal-containing sequence formed under relatively oxic conditions, with strong aerobic microbe activity, which resulted in the different distributions and compositions of C_(24) tetracyclic terpanes in geological samples. Therefore, the different correlations between compound B, C and Pr/Ph ratios suggest that their formation requires different geochemical conditions, such as anaerobic or aerobic and related microbial populations.