Geochemical characteristics of crude oil from lower Kalpintag Formation in SH9 well area, northern slope of middle Tarim Basin

In the lower Kalpintag Formation in SH9 well area in the northern slope of the middle Tarim Basin, medium-light black oil has been discovered for the first time and is characterized by low viscosity, low sulphur content, low pour point and high wax content. The total hydrocarbon content and the saturated/ aromatic hydrocarbon ratio are high, while the carbon isotope is light, usually less than -31‰. Normal alkane totally shows the features of single peak and front peak types. The preservation of normal alkane is relatively complete with slight “drum kits” (UCM peak), showing slight degradation characteristics. The CPI value is 1.04 ~ 1.05 and the Pr/ Ph value is 0.84 ~0.94. Normal alkane has no odd-even predominance and has apparent phytane advantage. Regular sterane shows “V” type distribution. The relative content of C₂₈20R sterane is less than 25% and the content of C₂₉ sterane is higher than that of C₂₇ sterane. The relative contents of rearranged sterane, ascending pregnane and pregnane is relatively higher. The abundance of tricyclic terpane series is higher than that of hopane series, and the C₂₃ tricyclic terpane has abundance advantage in tricyclic terpane. The content of gammacerane is also higher. In the crude oil, sulfur fluorine is rich while triaromatic dinosterane and dinosterane are lack. Maturity parameter shows that the oil is mature. The Middle and Upper Ordovician serve as the main sources while the Middle and Lower Cambrian contribute little.