鄂尔多斯盆地中部奥陶系马五盐下沉积环境与海平面变化*

鄂尔多斯盆地中部奥陶系马家沟组赋存有丰富的天然气资源,前人研究多集中于顶部与风化壳相关的储集层。文中基于岩心、薄片及阴极发光分析,将鄂尔多斯盆地中部奥陶系马五盐下碳酸盐岩-蒸发岩划分为10种岩石类型。根据各岩类的宏观、微观特征以及它们在纵横向上的组合与分布特点,利用岩石类型组合的方法,将研究区划分为局限-蒸发潟湖、台内滩、滩间海、微生物丘和台坪5种沉积环境。根据各亚段岩石类型发育的差异性及统计结果发现,研究区马五盐下具有蒸发岩与碳酸盐岩间互的旋回性沉积演化与海平面升降特征,每个亚段都代表一次海侵或海退的旋回沉积。其中,马五6、马五8和马五10亚段为海退期沉积,以纹层状云质膏岩、泥晶云岩构成的局限-蒸发潟湖沉积为主,蒸发岩矿物含量高,反映沉积环境相对闭塞、能量较低且盐度较高;马五7和马五9亚段为海进期沉积,以相对高能的台内滩或微生物丘沉积为主,并与相邻的局限-蒸发潟湖和台坪沉积构成较完整的向上变浅米级沉积旋回,表明该阶段水体循环相对更好,整体沉积环境开阔且能量较高。

Sedimentary environment and sea level change of the subsalt interval of Member 5 of Ordovician Majiagou Formation in central Ordos Basin

The Ordovician Majiagou Formation in the central Ordos Basin is rich in gas resources. However,previous studies generally focused on the palaeo-weathered crust reservoirs in the uppermost part of the Majiagou Formation. Based on cores,thin sections and cathodeluminescence data,we studied the subsalt interval of Member 5 of the Majiagou Formation in the central Ordos Basin. The carbonate-evaporite sediments of the subsalt interval can be divided into ten rock types. On the basis of the macro- and micro-features of all the rocks and their vertical and horizontal combination and distribution,five types of sedimentary environment were recognized in the study area,namely restricted-evaporative lagoon,bank,shallow marine,microbial mound and platform flat. The difference of rock types in each submember and their statistics indicate that the subsalt interval is characterized by cyclical deposition of carbonates and evaporites as well as eustatic sea level change,and each submember represents a transgression or regression. Consequently,the Ma5₆,Ma5₈ and Ma5₁₀ submembers are regressive sediments in restricted-evaporative lagoon,mainly consisting of laminated dolomitic gypsum and micritic dolostone,which reflects a restricted,low-energy and high salinity environment. The Ma5₇ and Ma5₉ submembers are transgressive sediments of bank and microbial mound,which usually make up a complete descending meter-scale sedimentary cycle in vertical together with the adjacent restricted-evaporative lagoon and platform flat,indicating good water circulation and a relatively high-energy open environment.