塔里木盆地柯坪地区寒武系微生物白云岩特征、成因及意义

生物礁(丘)碳酸盐岩是重要的油气储层.塔里木盆地关于生物礁的报道主要集中在奥陶系,而在寒武系中却少有发现.通过柯坪地区上寒武统丘里塔格下亚群中的叠层石白云岩和凝块石(礁)白云岩的野外产出特征和微观结构构造分析,认为凝块石白云岩主要形成于潮下带水动力条件较强的沉积环境,而叠层石白云岩则形成于潮坪-潮间水动力相对稳定的沉积环境.叠层石白云岩中发育纳米级球形颗粒及其组成的哑铃状、链状结构,并发育管状-片状结构.上述特殊的结构在成分上均为白云石,与微生物培养实验中沉淀的白云石在形态上基本一致,说明微生物调制作用是研究区寒武系白云岩重要的成因模式之一.丘里塔格下亚群中未见任何蒸发岩和黄铁矿,说明叠层石白云岩形成流体盐度未达到沉淀石膏的临界点,白云石沉淀流体中含有SO₄^2-,并且未发生明显的细菌硫还原作用.因此,前人提出的SO₄^2-抑制白云石沉淀的作用可能被夸大,结合白云岩成因研究新进展,认为白云石成因机理研究应更多考虑Mg²⁺的水合作用.此外,对叠层石白云岩和凝块石白云岩的储集特征初步研究发现,其有可能成为天然气的潜在储层,值得引起重视.

Characteristics,origin and geological implications of the Cambrian microbial dolomite in Keping area,Tarim Basin

Reefal (biohermal) carbonate rocks are important petroleum reservoirs. In Tarim Basin, the occurrences of reefal facies in the Ordovician sequences have been well documented, whereas that in the Cambrian strata have been rarely reported. In this study, the occurrences of stromatolite dolomite and reefal dolomite in the Cambrian Qiulitage Formation at Keping area, Tarim Basin were reported. Based on careful field observations, the micro-structures of these microbial dolomite were investigated by using conventional microscope and scanning electron microscope (SEM). It is suggested that the microbial reefs formed in shallow subtidal environment where the water was turbulent, the deposition rate was relatively high and the platform was relatively open. In contrast, the stromatolite dolomites were developed in tidal flat-intertidal facies. Nanoscale spherical dolomite formed nanocrystal aggregats including dumbbell and chain-like structures. In addition, tubular and sheet-like aggregates were suggested to be in close association with the mineralization of extracellular polymeric substances (EPS). All the above nanostructures resemble the morphology observed in microbial culture experiments. These evidences support the conclusion that microbial mediation might have played an important role in the formation of the Cambrian dolomite. During the formation of microbially mediated dolomite, sulfate reducing bacteria did not play an important role because no pyrite was observed, then SO₄^2- may not serve as an important kinetic inhibitor for the precipitation of dolomite. Thus it is proposed that the hydration of Mg²⁺ should be emphasized in the future study on the origin of dolomite.