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

Abstract

The Gasikule oilfield, including a deep layer E 3 1 and a shallow layer N 2 1 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 E3 1 and N2 1 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 N 2 1 crude oils was slightly higher than that for the E3 1 crude oils. The E3 1 crude oils were derived mostly from bacteria whereas the N2 1 crude oils originated mostly from plankton. The maturity parameters of biomarkers reflect that the crude oils were low mature. The maturity of the E 3 1 crude oils was slightly higher than that of the N 2 1 crude oils. These results, together with research of crude oil formation, show that the N2 and E3 1 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.