Abstract: Industrial oil flow has been discovered in hydrocarhon exploration from the Permian to the Jurassic in the central Mahu sag, showing promising indications of oil and gas. However, the level of overall exploration in the study area remains relatively low. In addition, there is a notable absence of systematic research on the geological conditions for hydrocarbon enrichment, accumulation mode and primary controlling factors. A deep understanding of accumulation conditions and the distribution of favorable zones within the central Mahu sag is crucial to improving the efficiency of hydrocarbon exploration efforts. Based on recent data from core, logging, well tie and seismic profiles, and microscopic rock thin section observation, this paper investigates the characteristics of source rocks in the central Mahu sag, as well as the sedimentary characteristics and reservoir-controlling factors of key target strata, including the Permian Xiawuerhe Formation, Triassic Baikouquan Formation and Baijiantan Formation, and Jurassic Badaowan Formation. On this basis, this study determines the locations of multi-layered oil-gas target strata and identifies three-dimensional accumulation modes, thus providing a forward-looking prediction for the next phase of exploration. The study indicates that a widespread strike-slip fault system facilitates communication with the underlying Permian source rocks. Thus, oil and gas can be transported vertically and laterally to the Permian Xiawuerhe Formation and Triassic Bakouquan Formation through faults and unconformities, and further to shallow formations through faults for oil-gas accumulations. The extensive hydrocarbon accumulation in Mahu sag lies in the good reservoir-cap assemblage of widespread fan delta front facies mud-poor gravel reservoirs and retrogradational fan delta facies sediments with the late massive set of mudstone in Xiawuerhe Formation and Bakouquan Formation. The shallow oil-gas accumulation is associated with the favorable reservoir-cap assemblage of the thin-bedded glutenite reservoirs with massive mudstone in Baijiantan Formation and Badaowan Formation. A comprehensive analysis suggests that the fan delta front facies sandbodies of the Xiawuerhe Formation and the upper submember of Member 1 of the Baikouquan Formation in the central Mahu sag can develop high-quality reservoirs, and possess conditions for the large-scale development of stratigraphic traps. Moreover, braided fluvial delta front facies thin-bedded glutenites of the Member 2 of Baijiantan Formation and the Member 1 of Badaowan Formation in the central Mahu sag can develop high-quality reservoirs and provide favorable conditions for the formation of lithological traps. All the above are key strata for further oil-gas exploration in the central Mahu sag.

Key words: Mahu sag, Baikouquan Formation, Xiawuerhe Formation, Baijiantan Formation, Badaowan Formation, accumulation mode

摘要： 玛湖凹陷中部(玛中)地区油气勘探在二叠系—侏罗系获得了工业油流和良好的油气显示,但目前其整体勘探程度较低,油气富集条件、成藏模式及主控因素的系统研究较为薄弱,深入认识玛中地区多层系油气的成藏条件与有利区分布,是提高油气勘探成效的关键。基于最新岩心、测井、连井和地震资料分析以及岩石薄片观察等,系统论述了玛中地区的烃源岩特征、主要目的层系(二叠系下乌尔禾组、三叠系百口泉组和白碱滩组以及侏罗系八道湾组)的沉积特征和储层控制因素,明确了多层系油气富集层位及油气的立体成藏模式,为下一步勘探提供了前瞻性预测。研究结果表明,广泛发育的走滑断裂体系沟通了下伏二叠系烃源岩,断层和不整合面将油气垂向或侧向输运到二叠系下乌尔禾组和三叠系百口泉组,并进一步通过断层输送至浅层形成油气聚集;下乌尔禾组和百口泉组中,呈广覆式分布的扇三角洲前缘相贫泥砾岩储层和退积型扇三角洲相与晚期大套泥岩组成的良好储-盖组合是玛湖凹陷获得大面积油气成藏的关键;白碱滩组和八道湾组的薄层砂砾岩储层与大套泥岩组成的良好储-盖组合是玛中地区浅层油气成藏的关键。综合分析认为,玛中地区百口泉组一段上亚段及下乌尔禾组扇三角洲前缘砂体可发育优质储层,具备形成大型地层圈闭的条件;玛中地区白碱滩组二段和八道湾组一段辫状河三角洲前缘薄层砂砾岩发育优质储层,具备形成岩性圈闭的条件;以上均为玛中地区下一步油气勘探重点层系。

关键词: 玛湖凹陷, 百口泉组, 下乌尔禾组, 白碱滩组, 八道湾组, 成藏模式