The Distribution and Geochemical Significance of Triaromatic Steroid Series in Crude Oils from the Chepaizi Area, Junggar Basin, NW China

The complex physical and chemical properties and geochemical characteristics of crude oils in the Chepaizi area have long made the types and sources of crude oils controversial, plus biodegradation widely exists in crude oils, thus, triaromatic steroid series with strong biodegradation resistance and source significance can be used for oil-oil and oil-source correlation in the Chepaizi area. The saturated and aromatic fractions in crude oils and source rocks in the Chepaizi area were analyzed by gas chromatography-mass spectrometry. The results show that triaromatic steroid series are detected in all investigated oils and four sets of source rocks from the Chepaizi area. Based on the considerable differences in their compositional features, three families of crude oils are distinguished. Family I crude oils show the highest ratio of C26/C28 20S TAS and abundant triaromatic dinosteroids, which are sourced from Qingshuihe Formation of Lower Cretaceous. Oil Family II has the lowest C26/C28 20S TAS, C27/C28 20R TAS and C26 + C27/C28 TAS ratios and highest 4-MTSI ratio, considering to be related to Jurassic source rocks. Oil Family III displays very similar distribution characteristics of C26–C28 triaromatic steroids and C27–C29 methyltriaromatic steroids to Permian source rocks, and can be subdivided into two subtypes based on C26/C28 20S TAS, C27/C28 20R TAS and C26 + C27/C28 TAS ratios, both of these two subtypes are mainly originated from Permian Fengcheng Formation source rocks due to the lack of triaromatic dinosteriods, while Family III-1 possibly mixed with a minor contribution from Wuerhe Formation because of the coexistence of n-alkanes with 25-norhopanes. The studied oil samples are found to be mainly in the mature and high mature stage based on the triaromatic steroid series parameters. The findings of this paper suggest that triaromatic steroid series may provide valuable molecular marker evidence for geochemical study in complex oil and gas exploration fields, at least be well applied in the Chepaizi area.