Oil-source correlation under the complex geological conditions: A case study of the Chaiwopu Sag, southern Junggar Basin, NW China

Abstract

Slightly (PM 1) to very strongly (PM 9) biodegraded crude oil was discovered in the reservoir of the Tashikula Formation in the eastern Chaiwopu Sag. The origin of oil in the eastern Chaiwopu Sag remains unclear due to different degrees of biodegradation and multi-stage nappe structure. In this study, the geochemical data from crude oil, source rocks and hydrocarbon inclusions are used to establish the correlations between oils and source rocks under complex geological conditions. The characteristics of biomarkers, isotopes and hydrocarbon inclusions reveal two stages of hydrocarbon charging in the eastern Chaiwopu Sag. Hydrocarbons formed during the first charging stage has low maturity and a relatively light carbon isotope composition, and its relative abundance of C 20-23 tricyclic terpanes follows the sequence: C 23 > C 21 > C 20 . Hydrocarbons formed during the second charging stage is characterized by relatively high maturity and heavy carbon isotope composition, and the corresponding abundance of C 20-23 tricyclic terpanes is C 23 < C 21 < C 20 . The biomarkers and isotopic characteristics of crude oil, source rocks and hydrocarbon inclusions reveal the hydrocarbons formed during the first charging stage originated from the Lucaogou Formation in the northern Chaiwopu Sag and the northern Bogeda Mountains, and those formed during the second charging stage originated from the Lucaogou Formation in the central Chaiwopu Sag. The high-quality source rocks and reservoirs of the Lucaogou Formation were deposited in the northern Chaiwopu Sag, which has great potential for tight/shale oil exploration. • The geochemical characteristics of potential source rocks in different regions are revealed. • Degradation grade of oils in the southeastern Chaiwopu Sag are revealed. • Reveal the maturity and accumulation stage of petroleum through the characteristics of hydrocarbon inclusions. • Reveal the origin of petroleum under complex geological conditions through biomarkers, isotopes and hydrocarbon inclusions.