Geochemical Characteristics\nand Significance of Molecular\nMarkers in the Paleogene-Neogene Crude Oils of the Northwest Qaidam\nBasin

The western Qaidam Basin is rich in oil and gas resources,\nconstituting\nover 90% of the total oil and gas resources in the whole basin. However,\nthe regional distribution characteristics of molecular markers of\ncrude oil and the petroleum system elements relevant to the distribution\nof oil and gas in the northwest Qaidam Basin are still unclear, which\nrestricts further exploration of oil and gas reservoirs. A total of\n38 crude oils were collected from the Xiaoliangshan Sag and Mangya\nSag in the northwest Qaidam Basin and analyzed by gas chromatography–mass\nspectrometry (GC-MS). The indicators of sedimentary environment, organic\nmatter source, and thermal maturity show regular changes from the\nwest to the east in the Xiaoliangshan Sag and Mangya Sag. The <i>n</i>C<sub>21</sub><sup>–</sup>/<i>n</i>C<sub>22</sub><sup>+</sup>, <i>n</i>(C<sub>21</sub> + C<sub>22</sub>)/<i>n</i>(C<sub>28</sub> + C<sub>29</sub>), and C<sub>27</sub>-C<sub>28</sub>-C<sub>29</sub> regular sterane distribution\ncharacteristics indicate that the input proportion of lower aquatic\norganisms in source rocks (E<sub>3</sub>-N<sub>1</sub>) gradually\nincreases, while the input proportion of terrigenous organic matter\ngradually decreases. The triaromatic dinosterane index (TDSI) and\nmethyltriaromatic steroid index (MTDSI) indicate that the input proportion\nof dinoflagellates gradually increases. The gradual increase in the\nPr/Ph ratio indicates a decrease in the reducibility of the sedimentary\nenvironment of source rocks. The gammacerane index (Ga/C<sub>30</sub>H) and methylated-2-methyltrimethyltride-cylchromans (MTTCs) indicate\nthat the degree of stratification and salinity of water body gradually\ndecrease. A total of 23 thermal maturity parameters indicate that\ncrude oil is in a mature evolution stage, and the thermal maturity\nof crude oil increases with depth, indicating that there are the characteristics\nof hydrocarbon accumulation close to source rocks. The variation features\nof sedimentary environment, organic matter origin, and thermal maturity\nindicators from the west to the east in the Xiaoliangshan Sag and\nMangya Sag are controlled by the sedimentary structure characteristics\nof the study area.