台内微生物丘沉积特征及其对储层发育的控制——以塔里木盆地柯坪—巴楚地区下寒武统肖尔布拉克组为例

Sedimentary characteristics of intra-platform microbial mounds and their controlling effects on the development of reservoirs: A case study of the Lower Cambrian Xiaoerbulake Fm in the Keping-Bachu area, Tarim Basin

塔里木盆地柯坪—巴楚地区下寒武统肖尔布拉克组广泛发育一套微生物碳酸盐岩,前人针对该组地层台缘微生物礁的沉积、储层特征进行了大量研究,但对台内微生物丘的研究则较少。为此,在野外剖面实测、岩心观察、薄片鉴定、阴极发光和物性测试的基础上,对该组地层台地内部微生物丘的沉积演化和储层特征进行了系统分析。结果表明:1台内微生物丘主要发育在开阔台地相区,宏观上呈明显的丘状正凸起,自下而上依次发育凝块石云岩、残余颗粒粉—细晶云岩→叠层石云岩→泡沫绵层云岩→蓝细菌颗粒云岩,构成完整的丘基—丘核—丘盖沉积组合;2储层孔隙类型主要有泡沫绵层体腔孔、凝块间孔、粒间孔、针状溶孔和顺层状溶蚀孔洞等,储层质量明显好于层状微生物建造;3沉积对微生物丘储层的发育具有明显的控制作用。结论认为:台内微生物丘储层作为肖尔布拉克组一种重要的储层类型,对于塔里木盆地寒武系盐下油气勘探领域由台缘向台内扩展具有重要的意义。

A set of microbial carbonate rocks are widely developed in the Lower Cambrian Xiaoerbulake Fm in the Keping-Bachu area, Tarim Basin. Previous studies focused on the sedimentary and reservoir characteristics of the platform margin microbial reefs. However, the intra-platform microbial mounds were less investigated. In this paper, therefore, the sedimentary evolution and reservoir charac- teristics of intra-platform microbial mounds in the Xiaoerbulake Fm were analyzed systematically by means of field outcrop measure- ment, core observation, thin section analysis, cathode luminescence examination and physical property test. Based on these studies, the following results are obtained. Firstly, intra-platform microbial mounds are mainly developed in the zones of open platform facies and hummocky highs are obviously presented in the macroscopic scale. Thrombolite dolomite and residual powder grain-fine crystalline do- lomite → stromatolite dolomite → foam spongy dolomite → cyanobacteria grain dolomite are developed successively from bottom to top and a complete sedimentary assemblage of mound base-core cap is constituted. Secondly, its reservoir quality is much better than that of laminated microbial formations and its pore types include mainly foam spongy visceral pore, interclot pore, intergranular pore, needle dissolved pore and bedded dissolved pore. Thirdly, sedimentation plays a vital role in the development of microbial mound reservoirs. It is concluded that the intra-platform microbial mound reservoir, as an important type of reservoir in the Xiaoerbulake Fm, is significant to the extension of the Cambrian subsalt oil and gas exploration in the Tarim Basin from the platform margin to the internal platform.