摘要： 鄂尔多斯盆地奥陶系马家沟组典型的白云岩-蒸发岩共生体系中白云岩储集层发育,前人围绕该白云岩储集层特征及形成机制开展了大量研究,但缺乏从白云岩-蒸发岩共生体系角度来探索白云石化过程及演化特征的研究。文中选取鄂尔多斯盆地东南缘西磑口剖面发育的2段白云岩-蒸发岩沉积序列(S1、S2)进行高精度连续采样,以沉积学、岩石学分析为基础,结合碳氧同位素、稀土元素及微量元素地球化学分析,识别出2类白云岩-蒸发岩共生组合,并对其白云石化流体性质、来源、演化及白云石化过程进行了详细研究。结果表明: (1)S1采样段发育 Ⅰ 型共生组合,主要由内碎屑颗粒白云岩—膏溶角砾白云岩—泥晶白云岩(含石膏假晶)—泥岩组成,代表强蒸发潮上带环境; S2采样段发育 Ⅱ 型共生组合,主要由薄-中厚层细晶白云岩—中层(颗粒)白云岩—泥质白云岩(含石膏假晶)—泥岩构成,代表潮间带—潮下带环境。(2)2种共生组合类型中白云岩的稀土配分模式与孔隙水的配分模式相似,均表现为中稀土(MREE)富集、Ce弱负异常—正常、Eu弱负异常—正常、δ¹³C和δ18O值无明显差别、Fe_carb和Mn_carb含量较高,推测其白云石化流体来源相同,为海水衍生的弱还原性孔隙水。(3)研究剖面由下至上从 Ⅰ 型共生组合到 Ⅱ 型共生组合沉积序列的垂向演化,反映了蒸发潮坪环境中共生体系的典型白云石化过程,即随着海平面上升,从S1段至S2段由潮上带准同生萨布哈白云石化演化为潮间带—潮下带渗透回流白云石化,Ⅰ 型共生组合中白云岩的MREE和Fe_carb含量高于 Ⅱ 型共生组合白云岩,反映白云石化流体性质从富Fe²⁺-Mn²⁺、MREE向贫Fe²⁺-Mn²⁺、MREE的演化。研究认识丰富了共生体系下白云岩成因的基础理论,可为马家沟组油气勘探提供理论指导。

关键词: 白云岩-蒸发岩共生体系, 白云石化过程, 稀土元素, 马家沟组, 鄂尔多斯盆地

Abstract: The dolomite reservoir is developed in the typical dolomite-evaporite paragenetic system(DEPS)of the Ordovician Majiagou Formation in the Ordos Basin. Many researchers have conducted numerous studies on the characteristics and formation mechanism of dolomite reservoirs in the Majiagou Formation. However,limited research has been done on the dolomitization process and evolution characteristics from the perspective of DEPS. In this study,two dolomite-evaporite sedimentary sequences(S1 and S2)from the Xiweikou section on the southeast margin of the Ordos Basin were selected for high-precision continuous sampling. Through sedimentology and petrology analysis,as well as carbon-oxygen isotopes,rare earth elements(REEs)and trace elements,two types of dolomite-evaporite paragenetic assemblages were identified. A detailed study was conducted on the properties,sources,evolution of dolomitization fluid,and the dolomitization process. The results show that: (1)in the S1 sampling interval,a Type I paragenetic assemblage is present,consisting mainly of intraclast grain dolomite-dissolved gypseous breccias dolomite-dolomicrite bearing gypsum pseudocrystals-mudstone. This indicates a supratidal environment with strong evaporation. In the S2 sampling interval,a Type II paragenetic assemblage is observed,mainly composed of thin-medium thick layers of finely crystalline dolomite-medium(grain)dolomite-argillaceous dolomite bearing gypsum pseudocrystals-mudstone,representing an intertidal-subtidal environment. (2)The REE distribution patterns of dolomite in two types of paragenetic assemblages are similar to those of pore water,both showing enrichment of MREE,with slightly negative anomalies-normal of Ce and Eu. There is no significant difference between δ¹³C and δ¹⁸O,and the contents of Fe_carb and Mn_carb are relatively high. It is speculated that the sources of dolomitized fluid are the same,originating from weakly reducing pore water derived from seawater. (3)The sedimentary sequence in the studied geological section changes from type I to type II from bottom to top,reflecting the typical dolomitization process of paragenetic systems in evaporative tidal flat environments. As sea level rose from S1 to S2,penecontemporaneous sabkha dolomitization in the supratidal zone evolved into seepage reflux dolomitization in the intertidal-subtidal zone. The concentrations of MREE and Fe_carb in dolomite of the type I paragenetic assemblage are higher than those in type Ⅱ,indicating the evolution of dolomitization fluid properties from rich Fe²⁺-Mn²⁺ and MREE to poor Fe²⁺-Mn²⁺ and MREE. The research has enriched the basic theory of dolomite genesis under paragenetic systems and provides theoretical guidance for oil and gas exploration in the Majiagou Formation in the Ordos Basin.

Key words: dolomite-evaporite syngenetic system, dolomitization process, rare earth elements, Majiagou Formation, Ordos Basin

中图分类号: 

• P588.24