Petroleum source of oil seepages in the Kalpin area, NW Tarim Basin, China

Abstract

Source rocks in Lower Cambrian (Є1) and Middle-Upper Ordovician (O2+3) have low S2 values, ranging from 0.01 to 0.34 and 0.01–5.95 mg HC/g Rock, respectively. The averages of TOC are 1.07% and 0.84% for Є1 and O2+3 source rocks, respectively. The equivalent vitrinite reflectance values (VREs) indicate that Є1 rocks are in a high-over mature stage for oil generation, and O2+3 rocks are sufficiently mature.Four oil seepages from the Yingshan Formation in the Qingsong quarry of the Aksu area and 34 core samples from Є1 and O2+3 rocks in the Kalpin area were analyzed for stable carbon isotopes. Moreover, 4 oil seepages and 16 core samples were analyzed by gas chromatography-mass spectrometry (GC-MS). Compared with O2+3 rocks, Є1 rocks are characterized by lower values of Gammacerane/C30 hopane (G/H), C28/(C27 + C28 + C29) 20R ααα sterane (C28%), C29/(C27 + C28 + C29) 20R ααα sterane (C29%), C26 20S/C28 20S triaromatic steroids (C26/C28), (C26 + C27)/C28 triaromatic steroids and (C26 20R + C27 20S)/C28 20R triaromatic steroids, and higher values of C21/C23 tricyclic terpane (C21/C23), C27/(C27 + C28 + C29) 20R ααα sterane (C27%), and C35 αβ (22R + 22S) homohopane/αβ C30-hopane (C35 H/H). The two sets of source rocks have a ‘‘V (C27>C28<C29)”-shaped regular sterane distribution pattern, and O2+3 rocks contain triaromatic steranes in abundances as high as those of Є1 rocks. C28C34 25-norhopanes (NHs) indicates that the degree of degradation of oil seepages falls at approximately PM 6, rendering many biomarker parameters ineffective for oil-source correlations, except for triaromatic steriods and carbon isotopic compositions. The values of C26/C28, (C26 + C27)/C28 triaromatic steroids and (C26 20R + C27 20S)/C28 20R triaromatic steroids using 0.50, 1.45 and 1.80 as threshold values, respectively, can differentiate Є1 and O2+3 rocks, and oil seepages show a natural affinity to the Є1 source rocks. The Є1 rocks have lighter δ13C of kerogen and heavier δ13C of group compositions than those of O2+3 rocks. The group compositions of oil seepages are depleted in 13C by approximately 0–1.5‰ compared with kerogens of Є1 extracts, further suggesting that the oil seepages are comparable to Є1 source rocks consistent with the results of biomarker analysis.