Abstract: A significant breakthrough has been made for the first time in the exploration of oil and gas reserves in the footwall of Manas structural belt at the southern margin of Junggar Basin. Well FT1 obtained high-yield oil and gas flow during oil testing in the Cretaceous Donggou Formation. The drilling and testing results of Well FT1 reveal as follows. (1)The condensate oil is derived from Cretaceous and Jurassic source rocks, while the natural gas is sourced from the highly mature Jurassic source rocks. The front belt of Manas thrust nappe is located within the effective hydrocarbon supply area of these two sets of source rocks. (2)The Cretaceous Donggou Formation develops large-scale, high-quality reservoirs. Braid river sand bodies in shallow water are stacked in multiple stages along the vertical profile, with a cumulative thickness of nearly 400 meters. Additionally, for the reservoir at a depth below 6 000 meters, the average porosity is 13.5 %, and the average permeability is 22 mD. Therefore, the footwall of Manas structural belt provides favorable reservoir conditions for high and stable hydrocarbon production. (3)The study establishes a complex overlapping structural model of deep thrust and shallow detachment at the northern foothills of Tianshan Mountains. The front belt, developed in the footwall of the large reverse fault on the structural flank, features large-scale in-situ structural traps with a shallower burial depth compared to the lower combination, offering significant potential for expanded exploration. (4)The front belt provides favorable accumulation conditions, characterized by "source rocks connected through thrust faults and lateral hydrocarbon sealing formed by overthrusts". Deep thrust faults connect with source rocks, facilitating the upward hydrocarbon migration, while overthrusts provide lateral sealing, thus forming a three-dimensional hydrocarbon accumulation pattern in the middle and lower combinations. The exploration breakthrough of Well FT1 has led to the discovery of a new exploration layers in the middle combination of the front belt of Horgos-Manas-Tugulu anticline group in the southern margin of Junggar Basin, confirming the natural gas resource potential of 100 billion cubic meters in the middle combination at the footwall of Manas structural belt. This breakthrough has changed the current exploration status focusing on the lower combination while neglecting the middle and upper combinations, thus expanding the exploration horizons and domains for deep and ultra-deep oil and gas exploitation in the northern foothills of Tianshan Mountains. The favorable exploration targets for the middle combination in the front belt of southern margin of Junggar Basin are located in the footwall of overthrusts in major structures such as Manas, Tugulu, and Horgos. Five similar targets in the front belt have been preliminarily identified, covering an area of 320 km², with the estimated natural gas resources of 4 000×10⁸m³.

Key words: southern margin of Junggar Basin, front belt, middle combination, accumulation condition, exploration direction

摘要： 准噶 尔盆地南缘玛纳斯构造带下盘中组合油气勘探首获重大突破,FT1井在白垩系东沟组试油获高产油气流。FT1井的钻试成果揭示:①凝析油有白垩系和侏罗系2套烃源岩的贡献,天然气为侏罗系高成熟烃源岩的贡献,玛纳斯逆冲前锋带位于2套烃源岩的有效供烃范围内;②白垩系东沟组发育规模优质储层,浅水辫状河三角洲砂体在纵向上呈多期叠置,累计厚度近400 m,6 000 m以深储层的平均孔隙度为13.5 %,平均渗透率为22 mD,在玛纳斯构造带下盘具有高产稳产的储层条件;③建立了天 山北麓深层逆冲、浅层滑脱复式叠置构造样式,其中,构造侧翼大型逆掩断裂下盘发育的前锋带原地构造圈闭具有规模大、埋深浅于下组合的特点,其可拓展勘探的潜力较大;④前锋带具备“逆冲通源、逆掩侧挡”的成藏条件,其中,深层逆冲断裂沟通烃源,油气向上输导,逆掩断裂侧向封堵,中—下组合可立体成藏。FT1井的勘探突破使得准噶尔盆地南缘在霍玛吐背斜群的侧翼前锋带发现了中组合的勘探新层系,证实玛纳斯构造带下盘中组合具备千亿立方米天然气资源潜力,打破了近年来重视下组合而轻视中—上组合的勘探现状,为天山北麓深层—超深层油气勘探拓展了勘探层系及领域。准噶尔盆地南缘前锋带中组合的有利勘探方向为玛纳斯、吐谷鲁、霍尔果斯等主体构造的逆掩断裂下盘,初步识别了前锋带同类型目标5个,勘探面积达320 km²,估算天然气资源量达4 000×10⁸m³。

关键词: 准噶尔盆地南缘, 前锋带, 中组合, 成藏条件, 勘探方向