塔里木盆地西北部中下寒武统混积岩沉积特征

Sedimentary characteristics of Lower and Middle Cambrian diamict in the northwestern Tarim Basin

近期,塔里木盆地柯坪地区柯探1井在下中寒武统吾松格尔组混积岩中获得工业气流,突破了混积岩勘探的"禁区",但目前对于该区混积岩的沉积特征与油气勘探潜力认识尚不清楚。为了推动该盆地寒武系混积岩深化研究与油气勘探潜力评价,以柯坪邻区阿克苏露头中下寒武统沙依里克组-吾松格尔组大套混积岩为解剖点,结合前人有关区域构造、沉积相和储层方面的研究成果,综合利用露头观察、镜下薄片以及地球化学分析等资料,采用层序地层学、岩石学与沉积微相研究相结合的方法,开展了吾松格尔-沙依克里组沉积相研究,评价了该区混积岩的油气勘探潜力。研究结果表明:①阿克苏露头区中下寒武世主要为蒸发台地、半局限台地及浅海陆棚沉积环境,发育19种混积岩相、3种碳酸盐岩相和2种碎屑岩相,存在着5种微相组合类型,包含4种低孔低渗-特低孔渗的储层类型,局部层段发育优质储层;②阿克苏地区早中寒武世沉积环境具有"开放-闭塞-盐化"的转变过程,包括碳酸盐岩缓坡、蒸发混积局限台地和蒸发局限台地等3大沉积结构层,其中蒸发混积局限台地中的混积岩的发育模式受物源区性质及位置、气候条件及变化速率、蒸发膏盐湖水体深度及温度、台盆古地貌、不同级次层序界面变化速率等控制;③阿克苏地区中下寒武统大套混积岩内发育有机碳碳含量较高的含膏盐云质泥岩和潮坪相泥页岩潜在烃源岩,其中吾松格尔组和沙依里克组混积岩生储盖组合配置关系较好,油气勘探潜力不容忽视。

Recently, Well Ketan 1 in the Keping area of the Tarim Basin achieved an industrial gas flow from the diamict of Wusonggeer Formation, Lower and Middle Cambrian, which breaks through the "restricted zone" of diamict exploration. So far, however, the sedimentary characteristics and oil and gas exploration potential of the diamict in this area have not been understood clearly. In order to promote further study on the Cambrian diamict in the whole basin and the evaluation on oil and gas exploration potential, this paper took the massive diamict of Shayilike–Wusonggeer Formations, Lower and Middle Cambrian in the Akesu outcrop next to the Keping area as the research object. Based on the previous research results on regional structures, sedimentary facies and reservoirs, combined with the data of outcrop observation, thin section analysis and geochemical analysis, the sedimentary facies of Wusonggeer–Shayilike Formations were analyzed using a combination method of sequence stratigraphy, petrology and sedimentation–microfacies study. Then, the oil and gas exploration potential of the diamict in this area was evaluated. And the following research results were obtained. First, sedimentary environments of Lower and Middle Cambrian in the Akesu outcrop area are mainly evaporative platform, semi-restricted platform and neritic shelf, including 19 diamict lithofacies, 3 carbonate lithofacies, and 2 clastic lithofacies. Besides, 5 types of microfacies assemblages are recognized, including 4 types of reservoirs with low to ultralow porosity and permeability. In addition, high-quality reservoirs are developed in regional intervals. Second, the sedimentary environments of Lower and Middle Cambrian in the Akesu area experience a transition process from open to close and then sallification, including 3 major sedimentary structural layers, i.e., gentle slope of carbonate rock, restricted platform of evaporite and diamict, and restricted platform of evaporite. And the development mode of the diamict in the restricted platform of evaporite and diamict is controlled by the property and the location of provenance, the condition and the change rate of climate, the depth and the temperature of evaporative gypsum-salt lake water body, the paleogeomorphology of platform basin, and the interface change rate of different levels of sequences. Third, gypsum-salt bearing dolomitic mudstone with higher total organic content(TOC) and mudstone of tidal flat facies, which are the potential source rocks, are developed in the massive diamict of Lower and Middle Cambrian in the Akesu area. And among them, the source–reservoir–caprock assemblages of Wusonggeer–Shayilike diamict are better in configuration, and their oil and gas exploration potential shall not be ignored.