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

摘要： 玛湖凹陷是准噶尔盆地重要的生烃凹陷,发育下二叠统风城组等优质烃源岩,已发现的原油主要来自风城组。为了指导下一步勘探实践,在前人研究的基础上,应用大量有机地球化学和无机地球化学分析化验资料,开展了风城组烃源岩生烃潜力、沉积环境和有机质富集等方面的分析。研究结果表明：①玛湖凹陷风城组烃源岩主要由Ⅱ₁—Ⅱ₂型干酪根构成,以生油为主,总体属于中等—极好烃源岩。风城组二段和风城组三段烃源岩的有机质类型和丰度要好于风城组一段。烃源岩的成熟度主要处于生油窗内。2风城组主力烃源岩沉积期,气候炎热干旱、水体相对较深,水动力条件较弱,为高盐、强还原环境,耐盐的藻类和细菌大量发育。受研究区内火山作用影响,富含基性火成岩的风化产物。3炎热干旱的气候条件为风城组高盐碱湖的形成提供了宏观背景,耐盐生物大量繁殖,火山灰携带的大量营养物进一步促进了生物的勃发。盐度导致水体分层,加之古水深较大,形成了高pH值、低Eh值的强还原环境,促进了有机质的保存。研究成果对提升玛湖凹陷风城组烃源岩的成因机理认识和指导页岩油勘探有重要借鉴意义。

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