The Permian Pingdiquan Formation (Pp) in Shishugou Sag, Junggar Basin, Northwest China, is a typical example of mixed sedimentary strata formed in a saline lacustrine environment, characterized by complex lithological assemblages and extensive distribution of organic-rich source rocks. To further investigate the differences in oil-source contribution and hydrocarbon generation characteristics of source rocks with different lithologies, studying the difference of their geochemical characteristics and the formation mechanism of organic-rich source rocks is the top priority. Based on a comprehensive analysis of lithologic types, geochemical characteristics, and hydrocarbon generation potential of source rocks using organic petrology, molecular geochemistry and element geochemistry, this paper reveals the formation background and organic matter enrichment mechanism of the Pp organic-rich source rocks. The results show that the mixed sedimentary rocks of the Pp were deposited under relatively arid and saline lacustrine conditions. Paleoclimatic conditions tended to be more arid and hydrothermal activity became more frequent during the sedimentation process from mudstone, dolomitic mudstone to argillaceous dolomite. Consequently, the water column salinity elevated, leading to an increased proportion of salt-tolerant green algae and slight salt-tolerant cyanobacteria compared to salt-intolerant algae. Frequent changes in paleoclimate and hydrothermal activity control the interbedding deposition of mudstone, dolomitic mudstone and argillaceous dolomite, and the resulting salinity fluctuations contribute to variations in the proportion of salt-tolerant and intolerant algae in source rocks with different lithologies. Furthermore, the source rocks within the mixed sedimentary strata (mudstone, dolomitic mudstone and argillaceous dolomite) all exhibit substantial hydrocarbon generation potential, and there exists heterogeneity among source rocks of the same lithology. The key factor influencing the development of these organic-rich source rocks is the enhancement of palaeoproductivity driven by volcanic activity. Additionally, the water stratification caused by saline water promotes anoxic water columns, also contributing to organic matter enrichment in the source rocks.

中文翻译：

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陆相咸化湖盆混合沉积地层富有机质烃源岩地球化学特征及形成机制——以准噶尔盆地石树沟凹陷二叠系平地泉组为例

准噶尔盆地石树沟凹陷二叠系平地泉组（Pp）是典型的盐湖环境下混合沉积地层，其岩性组合复杂，富有机质烃源岩分布广泛。为进一步研究不同岩性烃源岩的油源贡献和生烃特征的差异，研究其地球化学特征的差异和富有机质烃源岩的形成机制是当务之急。运用有机岩石学、分子地球化学和元素地球化学等手段，综合分析烃源岩的岩性类型、地球化学特征和生烃潜力，揭示了Pp富有机质烃源岩的形成背景和有机质富集机制。结果表明，Pp混合沉积岩是在相对干旱和盐碱湖相条件下沉积的。从泥岩、白云质泥岩到泥质白云岩的沉积过程中，古气候条件趋于干旱，热液活动更加频繁。因此，水体盐度升高，导致耐盐绿藻和轻微耐盐蓝藻的比例高于不耐盐藻类。古气候和热液活动的频繁变化控制着泥岩、白云质泥岩和泥质白云岩的互层沉积，由此引起的盐度波动导致不同岩性烃源岩中耐盐和不耐盐藻类比例的变化。此外，混合沉积地层（泥岩、白云质泥岩、泥质白云岩）内的烃源岩均表现出巨大的生烃潜力，且同一岩性烃源岩之间存在非均质性。影响这些富有机质烃源岩发育的关键因素是火山活动驱动的古生产力的增强。此外，盐水引起的水层化促进了缺氧水柱，也有助于烃源岩中有机质的富集。