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

Abstract Source rocks in Lower Cambrian (Є 1 ) and Middle-Upper Ordovician (O 2+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 Є 1 and O 2+3 source rocks, respectively. The equivalent vitrinite reflectance values (VREs) indicate that Є 1 rocks are in a high-over mature stage for oil generation, and O 2+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 Є 1 and O 2+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 O 2+3 rocks, Є 1 rocks are characterized by lower values of Gammacerane/C 30 hopane (G/H), C 28 /(C 27  + C 28  + C 29 ) 20R ααα sterane (C 28 %), C 29 /(C 27  + C 28  + C 29 ) 20R ααα sterane (C 29 %), C 26 20S/C 28 20S triaromatic steroids (C 26 /C 28 ), (C 26  + C 27 )/C 28 triaromatic steroids and (C 26 20R + C 27 20S)/C 28 20R triaromatic steroids, and higher values of C 21 /C 23 tricyclic terpane (C 21 /C 23 ), C 27 /(C 27  + C 28  + C 29 ) 20R ααα sterane (C 27 %), and C 35 αβ (22R + 22S) homohopane/αβ C 30 -hopane (C 35  H/H). The two sets of source rocks have a ‘‘V (C 27 >C 28 29 )”-shaped regular sterane distribution pattern, and O 2+3 rocks contain triaromatic steranes in abundances as high as those of Є 1 rocks. C 28 C 34 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 C 26 /C 28 , (C 26  + C 27 )/C 28 triaromatic steroids and (C 26 20R + C 27 20S)/C 28 20R triaromatic steroids using 0.50, 1.45 and 1.80 as threshold values, respectively, can differentiate Є 1 and O 2+3 rocks, and oil seepages show a natural affinity to the Є 1 source rocks. The Є 1 rocks have lighter δ 13 C of kerogen and heavier δ 13 C of group compositions than those of O 2+3 rocks. The group compositions of oil seepages are depleted in 13 C by approximately 0–1.5‰ compared with kerogens of Є 1 extracts, further suggesting that the oil seepages are comparable to Є 1 source rocks consistent with the results of biomarker analysis.