Geochemical characteristics and genesis of crude oils from Gasikule oilfield in Western Qaidam Basin, China

The Gasikule oilfield, including a deep layer E31 and a shallow layer N21 pool, is the largest one in the Qaidam Basin and its crude oil output accounts for 80% of total oil output in the basin. In order to understand the genetic mechanism of crude oils from this oilfield, crude oils from both E31 and N21 pools in the Gasikule oilfield were collected and analyzed using gas chromatography and gas chromatography-mass spectrometry. The geochemical characteristics of biomarkers in the crude oils were synthetically studied. The biomarker compositions indicate that the crude oils were derived from strong anoxic and saline-hypersaline lacustrine environment with a dominance of algal organic matter. It was also observed that the ancient salinity of the sedimentary environments of the source rocks for the N21 crude oils was slightly higher than that for the E31 crude oils. The E31 crude oils were derived mostly from bacteria whereas the N21 crude oils originated mostly from plankton. The maturity parameters of biomarkers reflect that the crude oils were low mature. The maturity of the E31 crude oils was slightly higher than that of the N21 crude oils. These results, together with research of crude oil formation, show that the N2 and E31 crude oils were yielded from free biological lipids, extractable organic matters and kerogen in the Tertiary source rocks deposited under an anoxic and saline-hypersaline environment at the stage of low maturity.