鄂尔多斯盆地奥陶系中组合微生物碳酸盐岩分布规律及沉积模式

Distribution and depositional model of microbial carbonates in the Ordovician middle assemblage,Ordos Basin,NW China

以露头剖面、钻井岩心、铸体薄片等资料为基础,系统分析鄂尔多斯盆地中东部奥陶系中组合(马家沟组马五_(10)—马五_(5)亚段)微生物碳酸盐岩的类型、微相组合及分布规律。研究表明,盆地中东部奥陶系中组合发育蓝细菌钙化作用形成的微生物碳酸盐岩沉积,主要由叠层石、凝块石、核形石微生物坪及微生物丘建造组成。其空间展布具有明显的“层控性”及“区位性”特征:微生物坪受层序旋回及沉积相变控制,主要形成于马五_(5)亚段、马五_(6)亚段的潮坪环境,单层厚度为2~3 m,马五_(5)亚段微生物坪发育在海侵初期的碳酸盐岩-膏盐岩转换界面附近,主要分布在盆地东部米脂洼陷;马五_(6)亚段微生物坪发育在碳酸盐岩-膏盐岩相变带附近,呈环带状分布在中央古隆起东侧地区。古地貌对微生物丘分布的控制更为突出,主要形成于马五_(10)—马五_(7)亚段的微生物丘或丘滩复合体,单层厚度可达25 m,主要分布在乌审旗—靖边古隆起。相关微生物碳酸盐岩发育模式表明,碳酸盐岩-膏盐岩相变带及乌审旗—靖边古隆起是微生物碳酸盐岩油气勘探的有利区带。

Based on outcrop profiles,drilling cores,cast thin sections etc.,the types,microfacies combinations and distribution pattern of microbial carbonates in the Ordovician middle assemblage of mid-eastern Ordos Basin have been systematically analyzed.The middle assemblage of Ordovician in the central and eastern Ordos Basin has microbial carbonates formed by the calcification of cyanobacteria,including microbial biostromes and microbial mounds made of stromatolites,thrombolites,and oncolites.The distribution of the carbonates shows obvious“stratum-control”and“regional”characteristics.The microbial biostromes 2–3 m thick each are controlled by sequence cycles and sedimentary facies changes,and were mainly formed in the tidal flat environment during the depositional stages of the Ma5_(6)and Ma5_(5)sub-members.The microbial biostrome in the Ma5_(5)sub-member occurring near the carbonate-evaporite transition interface in the early stage of the transgression is distributed mainly in the Mizhi subsag in the eastern part of the basin;the microbial biostrome in the Ma5_(6)sub-member turns up near the carbonate-evoporite transition zone in ring shape in the east of the central uplift.The ancient landform had noticeable control on the distribution of microbial mounds.The microbial mounds or mound-shoal complexes developing mainly during the depositional stages of Ma5_(7)_Ma5_(10)sub-members are about 15–25 m thick in single layer and distributed largely in the Wushenqi-Jingbian paleouplift.The development model of the microbial carbonate rocks shows that the carbonate-evaporite lithologic transition zone and the Wushenqi-Jingbian paleouplift are favorable exploration zones of microbial carbonates in the Ordovician middle assemblages.