Abstract: A significant breakthrough of Permian oil and gas exploration has been made in Duwa Formation of Well Duwa1, located within the Kedong structural belt in the southwestern depression of Tarim Basin. This discovery marks the identification of a new oil-bearing sequence in the southwestern piedmont of Tarim Basin, thereby opening up new prospects for hydrocarbon exploration in this area. Through integrating data from outcrops, shallow drilling, well logging, seismic surveys, laboratory analyses, and regional geological studies, a systematic study was conducted on the hydrocarbon accumulation conditions of the Permian Duwa Formation sandstones around the Kekeya area. The research results show as follows. (1) The Upper Member of Permian Pusige Formation in the periphery of Kekeya area develops semi-deep to deep lacustrine subfacies source rocks. The hydrocarbon generation center covers an area of 1.0×10⁴km², with a maximum thickness of 800 m. The measured values of organic matter abundance range from 0.50 % to 2.23 %, and the types of organic matter are predominantly Type Ⅱ and Type Ⅲ. The measured vitrinite reflectance (R_o) ranges from 0.5 % to 1.8 %, reflecting thermal maturity levels from mature to highly mature stages. (2) During the depositional stage of Duwa Formation, the lacustrine basin in the periphery of Kekeya area experienced shrinkage, thus leading to the development of a sedimentary system that transitions from alluvial fan to braided river, braided river delta, and ultimately to lacustrine deposits, from bottom to top. The shore-shallow lacustrine subfacies are predominant in this region. The reservoirs in the Lower Member of Duwa Formation are primarily composed of underwater distributary channel sands at the braided river delta front, where the single-layer sandbody thickness ranges from 0.89 to 4.2 meters, with a sand-to-shale ratio of 50.72 %. These extensive regional reservoirs are effectively sealed by thick mudstones in the Upper Member of Duwa Formation, forming an optimal reservoir-caprock combination. (3) The Kekeya and its peripheral area, influenced by multiple tectonic events, has experienced two major phases of tectonic deformation since the deposition of Duwa Formation: the Indosinian and Himalayan orogenies. The Paleozoic formation developed imbricate thrust structures, with the Duwa Formation vertically sandwiched between the Jurassic coal-measure source rocks and the Permian lacustrine source rocks. The Indosinian deformation gave rise to several anticlines and faulted anticlines in the periphery of Kekeya area, which were further uplifted and transformed by compression during the Himalayan period. The central horst zone has served as a long-term migration pathway for hydrocarbons from the Paleozoic to Mesozoic strata, making it a favorable target for oil and gas exploration. (4) In Duwa Formation, the crude oil group composition displays δ¹³C values ranging from -30 ‰ to -28 ‰, suggesting an equivalent maturity of 0.7 % to 1.3 %. The methylphenanthrene index (MPI) of aromatic hydrocarbons in the oil is 0.76, corresponding to an equivalent maturity of 0.84 %. These geochemical characteristics indicate that the Duwa Formation oil is a mature product derived from Permian source rocks, with a maturity similar to that of the underlying Pusige Formation source rocks. This supports a hydrocarbon accumulation model of lower-source and upper-reservoir in Duwa Formation. The Permian Duwa Formation traps in the southwestern piedmont of Tarim Basin are distributed in aligned belts, with four traps identified, covering an area of 85.8 km². The estimated petroleum geological resources are approximately 30 million tons of crude oil and 30 billion cubic meters of natural gas, indicating a promising exploration prospects. These traps are expected to become an important replacement fields for increasing reserve and production in Tarim Basin.

Key words: southwestern piedmont of Tarim Basin, Kedong structural belt, Permian Duwa Formation, Well Duwa1, accumulation conditions, exploration breakthrough

摘要： 塔里木盆地西南坳陷柯东构造带杜瓦1井在二叠系杜瓦组获重大油气勘探突破,标志着塔里木盆地西南部(塔西南)山前地区发现一套新的含油层系,进一步开创了塔西南山前地区油气勘探新局面。结合野外露头、浅钻井、钻井、地震、分析化验数据和区域地质的研究认识,对柯克亚周缘地区二叠系杜瓦组砂岩的成藏条件开展了系统研究。研究结果表明:①柯克亚周缘地区二叠系普斯格组上段发育半深湖—深湖亚相烃源岩,其生烃中心面积达1.0×10⁴km²,厚度最大为800 m,有机质丰度为0.50 % ~2.23 %,有机质类型主要为Ⅱ型和Ⅲ型,实测镜质体反射率(R_o)为0.5 % ~1.8 %,热演化程度处于成熟—高成熟阶段。②杜瓦组沉积期,柯克亚周缘地区湖盆萎缩,自下而上发育冲积扇—辫状河—辫状河三角洲—湖泊沉积体系,主体发育滨/浅湖亚相;杜瓦组下段储层主要为辫状河三角洲前缘水下分流河道砂体,单层砂体厚度为0.89~4.20 m,砂地比为50.72 %,是一套分布相对广泛的区域储层,与杜瓦组上段厚层泥岩构成了优质储-盖组合。③柯克亚周缘地区受多期构造运动影响,自杜瓦组沉积以来主要经历了印支期和喜马拉雅期2期构造变形,其古生界形成叠瓦冲断,杜瓦组在纵向上被夹持于侏罗系煤系烃源岩和二叠系湖相烃源岩之间;柯克亚周缘地区在印支期发育多个背斜和断背斜构造,在喜马拉雅期被挤压作用进一步抬升改造,其中部垒带区在古生代—新生代为油气运移的长期指向区,是油气勘探有利区。④杜瓦组原油族组分的碳同位素组成为-30 ‰ ~-28 ‰,推测原油成熟度为0.7 % ~1.3 %,原油中芳香烃的甲基菲指数为0.76,计算原油成熟度为0.84 %,这表明杜瓦组原油与下伏普斯格组烃源岩成熟度相当,为二叠系烃源岩成熟阶段的产物;杜瓦组油藏具有下生上储的成藏组合。塔西南山前地区二叠系杜瓦组圈闭呈成排成带分布,已发现圈闭4个,面积为85.8 km²,其石油地质资源量约为3 000×10⁴t,天然气地质资源量约为300×10⁸m³,具有良好的勘探前景,有望成为塔里木盆地油气增储上产的重要接替领域。

关键词: 塔西南山前, 柯东构造带, 二叠系杜瓦组, 杜瓦1井, 成藏条件, 勘探突破