Geochemical characteristics and source of crude oil from the eastern Shawan Sag, Junggar Basin

The Shawan Sag is one of the important exploration areas in the hinterland of the Junggar Basin. In order to better clarify the source of crude oil from the Permian, Jurassic and Cretaceous reservoirs in the sag and identify hydrocarbon accumulation processes and enrichment pattern, the authors conducted a carbon isotope and biomarker analysis on the typical crude oil samples from the eastern Shawan Sag in this study, and further carried out crude oil classification and oil-source correlation based on the analysis of geochemical characteristics of crude oil. The results indicate that the crude oil from different strata in the eastern Shawan Sag can be divided into three types. The first type mainly occurs in the Permian and Jurassic reservoirs, and its whole oil δ¹³C value and Pr/Ph ratio range from -31.0‰ to -29.0‰ and from 1.0 to 2.0, respectively. The relative contents of αααR regular sterane display the characteristics of C₂₇ < C₂₈ < C₂₉, and the terpanes are mainly characterized by C₂₀ < C₂₁>C₂₃ TT and C₂₄TeT/C₂₆TT < 1, with the gammacerane index lower than 0.30. This type of crude oil is mainly derived from the source rocks in the Middle Permian Lower Wuerhe Formation. In the oil samples from the Upper Permian Upper Wuerhe Formation, Ts content is below the detection limit, the β-carotane/C₃₀ hopane ratio is greater than 1, and the tricyclic terpane distribution displays the characteristics of C₂₀>C₂₁>C₂₃TT. The distribution fraction of methyl phenanthrene reflects that the crude oil is in the stage of high-maturity evolution. All these indicate the mixing of crude oil generated in the Lower Permian source rocks. The second type occurs in the Middle Jurassic reservoirs. The whole oil δ¹³C value and individual n-alkanes δ¹³C value are higher than -29.0‰. The Pr/Ph ratio is relatively higher, ranging from 2.0 to 2.5, with C₂₄TeT/C₂₆TT>1, which indicate that this type of crude oil is mainly derived from the Jurassic source rocks. The third type occurs in the Lower Cretaceous reservoirs, and its whole oil δ¹³C value and Pr/Ph ratio are lower than -31.0‰ and 1.0, respectively. The αααR regular sterane contents display the characteristics of C₂₇≈C₂₈ <C₂₉, with the gammacerane index higher than 0.50. The individual n-alkanes δ¹³C value gradually decreases with the increase of carbon number and is commonly lower than -31.0‰. This type of oil is demonstrated to be mainly derived from the Lower Cretaceous source rocks.