关键词: 烃源岩, 二叠系, 风城组, 生烃潜力, 准噶尔盆地, 玛湖凹陷, 沉积环境, 富有机质页岩

Abstract: Mahu sag is an important hydrocarbon-generating sag in Junggar Basin, with the development of high-quality source rocks such as the Lower Permian Fengcheng Formation. The discovered crude oils mainly originate from the Fengcheng Formation. Based on previous achievements, extensive datasets from organic and inorganic geochemical analyses and tests were applied to explore the hydrocarbon generation potential, sedimentary environments and organic matter enrichment mechanisms of Fengcheng Formation source rocks, aiming to guide the further exploration practice. The research results show as follows. (1) The source rocks of Fengcheng Formation in Mahu sag are predominantly composed of kerogen types ranging from Ⅱ₁ to Ⅱ₂, mainly generating oils, and are generally classified as fair to excellent source rocks. The organic matter types and abundances of source rocks in Member 2 and 3 of Fengcheng Formation are better than those in Member 1. The source rocks are generally in the oil generation window. (2) During the depositional period of the primary source rocks in Fengcheng Formation, the hot and arid climate, deep-water settings, and weak hydrodynamic conditions resulted in a hypersaline anoxic environment, where a large number of salt-tolerant algae and bacteria were developed. Affected by the volcanic activity in the study area, the sediments are rich in the weathering products derived from basic igneous rocks. (3) The hot and arid climate conditions provided a macro-environmental setting for the formation of a high salinity and alkaline lake in the depositional period of Fengcheng Formation. This environment supported the proliferation of salt-tolerant organisms. The substantial influx of nutrients carried by volcanic ash further promoted the prosperity of organisms. The water stratification caused by salinity, combined with the large paleo-water depth, promoted reducing conditions of high pH and low Eh, thus facilitating the preservation of organic matter. These results provide critical insights into understanding the genesis mechanism of source rocks in Fengcheng Formation of Mahu sag and guiding the exploration of shale oil.

Key words: source rock, Permian, Fengcheng Formation, hydrocarbon-generating potential, Junggar Basin, Mahu sag, sedimentary environment, organic-rich shale