Abstract: The thrust belt at the southern margin of Junggar Basin has experienced multiple stages of complex basin-range coupling and transition, as well as complex tectonic deformation, leading to great difficulty in hydrocarbon exploration. Since the discovery of the Neogene Dushanzi oilfield, no large-scale oil and gas fields have been discovered after more than 70 years of exploration and investigation. In recent years, the focus has gradually turned to oil-gas exploration in the lower hydrocarbon accumulation assemblage (lower assemblage) formed by the Lower Cretaceous Tugulu Group mudstone and basal sandstone as well as the Jurassic sandstone. By systematically summarizing the characteristics of geological structure, oil-gas entrapment and accumulation conditions of the lower assemblage in the thrust belt at the southern margin of Junggar Basin, thus making clear the distribution laws and exploration direction of deep clastic oil and gas fields. The comprehensive study shows as follows. (1) The thrust belt at the southern margin develops several sets of source rocks in Paleogene, Cretaceous, Jurassic, Triassic and Permian formations, among which the main Jurassic source rocks are thick, deeply buried and in the stage of high mature to over mature thermal evolution. (2) The lower assemblage in the thrust belt develops 4 inherited provenance systems. In particular, the Cretaceous Qingshuihe Formation develops high-efficient reservoirs with a thickness of 20-80 m, and the Jurassic Karaza Formation, Toutunhe Formation, Sangonghe Formation, and Badaowan Formation develop several sets of large-scale reservoirs with a thickness of 200-400 m. (3) The Cretaceous Tugulu Group in the thrust belt develops ultra-thick mudstone with a thickness of 500-2 000 m, characterized with high pressure and good sealing. (4) The thrust belt develops large-scale inherited anticline groups, and a large area of traps with low exploration degree. At present, 22 target traps have been identified, with a cumulative area of 1 735 km²; according to different tectonic styles of the thrust belt, the hydrocarbon accumulation mode can be subdivided into various types, involving Hutubi anticline, Dongwan anticline, and Tugulu anticline. (5) The lower assemblage in the thrust belt has a good spatial-temporal accumulation and configuration relationship among its source rocks, reservoirs, cap rocks and traps, where traps are early formed, and the tectonic formation period is matched with the peak period of hydrocarbon generation and expulsion of the main source rocks. The tectonic activity in Himalayan stage led to weak reconstruction of the Jurassic reservoir and good preservation conditions, which laid the material foundation for the formation of large oil and gas fields. By integrating various favorable conditions of source rocks, reservoirs, traps, and cap rocks, favorable targets are identified, and Well GT1, Well HT1, and Well TW1 deployed for drilling all have achieved oil and gas discoveries. The thrust belt at the southern margin will become a strategic replacement area for oil-gas exploration in Junggar Basin.

Key words: Junggar Basin, thrust belt at the southern margin, lower assemblage, accumulation condition, exploration potential

摘要： 准噶尔盆地南缘冲断带经历了多期、复杂的盆山耦合和盆山转换过程，构造变形复杂，勘探难度大。自新近系独山子油田发现以来，历经70多年的勘探研究，并未发现大型油气田。近年来，油气勘探逐步转向下白垩统吐谷鲁群泥岩与底砂岩及侏罗系砂岩形成的下部成藏组合（下组合），通过系统梳理南缘冲断带下组合的地质构造特征与油气圈闭成藏条件，明确了深层碎屑岩油气田的分布规律和勘探方向。综合研究认为：①南缘冲断带发育古近系、白垩系、侏罗系、三叠系、二叠系等多套烃源岩，其中，侏罗系主力烃源岩厚度大、埋藏深且处于高成熟—过成熟热演化阶段。②南缘冲断带下组合发育继承性4大物源体系，其中，白垩系清水河组发育高效储层，厚度为20~80 m，侏罗系喀拉扎组、头屯河组、三工河组和八道湾组发育多套规模储层，厚度为200~400 m。③南缘冲断带白垩系吐谷鲁群发育巨厚泥岩，厚度为500~2 000 m，压力高，封堵性好。④南缘冲断带发育大规模继承型背斜群，圈闭具有面积大、勘探程度低的特点，目前已落实目标圈闭22个，累计面积为1 735 km²；根据南缘冲断带不同的构造样式可将成藏类型细化为呼图壁背斜型、东湾背斜型、吐谷鲁背斜型等多种类型。⑤南缘冲断带下组合的烃源岩、储层、圈闭、盖层等在空间上和时间上的成藏配置关系好，圈闭形成早，构造形成期与主力烃源岩的生、排烃高峰期相匹配，喜马拉雅期的构造活动对侏罗系改造弱，保存条件好，具备形成大油气田的成藏基础。综合源、储、圈、盖等诸多有利条件锁定有利目标，先后部署上钻的GT1井、HT1井和TW1井均获得了油气发现。南缘冲断带将成为准噶尔盆地油气勘探的战略接替区。

关键词: 准噶尔盆地, 南缘冲断带, 下组合, 成藏条件, 勘探潜力