苏里格气田东区马五5储层白云岩成因机理分析

鄂尔多斯盆地苏里格气田东区马家沟组马五5储层白云岩的成因争议较大，通过对该区马五5白云岩沉积背景、地球化学特征和岩石学特征详细研究后，认为该区马五5段白云岩是以潮间潮下带沉积为主的沉积产物，主要发育准同生白云岩和埋藏白云岩两种类型，白云石化流体是高盐度海水与粒间孔隙内海水二者并存的海源流体，形成了准同生白云岩和埋藏白云岩两种成因模式。准同生白云岩以泥微晶白云岩为主，有少部分粉晶白云岩，具有水平纹层、膏模孔和鸟眼构造，常与石膏晶体共生，晶粒以自形为主，有序度较差，微量元素Sr和Na含量高，表明其沉积环境为水体较浅、高盐度及气候干燥的强蒸发环境，属于准同生白云岩成因；埋藏白云岩为粉细晶及中晶白云岩，晶粒总体较大，晶形以半自形为主，晶间孔较发育，有序度较高，与黄铁矿共生，呈团块状或不均匀状分布，微量元素Fe和Mn含量较高，δ¹⁸O_PDB值偏负，反映出埋藏还原条件下形成的产物，属于埋藏白云岩成因。

Mechanism of Dolomite Formation in Member Ma55 of Majiagou Formation,East of Sulige Gas Field

It is a matter of some dispute on the genesis of dolomite in Member Ma₅⁵ of Majiagou Formation of Ordovician, east of Sulige Gas Field, Ordos Basin. Based on the study of Sedimentary background, the characteristics of lithology and geochemistry of dolomite in this area, it is indicated that the dolomite in Member Ma₅⁵ of Majiagou Formation of this area is dominated by intertidal-subtidal sediments. There are two main types of dolomite:penecon-temporaneous dolomite and buried dolomite, and the dolomitized fluid is seawater or relevant sea-source fluid, which were established two dolomitization models:penecontemporaneous dolomite and burial dolomite. The former is maidly dolomicrite and a minority of powder crystal dolomite, it is characterized by horizontally-laminated bed, gypsum mold and bird-eye texture, and it is always coexists with gypsum crystal, mainly idiomorphic crystal, the order degree of grains is bad. The trace element analysis indicates that the contents of sodium and strontium are higher, it shows that the sedimentary environment is a strong evaporation environment with shallow water, high salinity and dry climate, which belongs to penecon-temporaneous dolomite. Buried dolomite often keeps larger particles, mainly fine-grained and medium grained dolomite, subhedron, and intergranular pores are more developed, the order degree of grains is better. It is coexists with pyrite, and it shows agglomerate or uneven distribution, The trace element analysis indicates that the contents of iron and manganese are higher, and the values of δ¹⁸O_PDB display negative distribution. These evidences indicate that the dolomite is formed by buried reduction environment, is belongs to burial dolomite.