鄂尔多斯盆地苏东地区马家沟组五段5亚段白云岩成因

以岩石结构分析为基础,结合白云石有序度、微量元素、碳氧同位素等地球化学和包裹体测温与盐度测试,对鄂尔多斯盆地苏东地区马家沟组五段5亚段白云岩成因进行了研究,识别出准同生白云岩和埋藏白云岩2种成因类型。其中具备储集意义的为成岩期交代成因的埋藏白云岩,其岩性主要为颗粒白云岩和晶粒白云岩。准同生白云岩具有鸟眼、石膏或石膏假晶结构,盐度较高(24.43wt%Na Cl);阴极发光呈较暗的棕红色或玫瑰红色,有序度平均0.75,以高Sr、Na为特征;其形成的盐度和温度与微晶灰岩基本一致,形成于高盐度环境和地表条件下(包裹体校正温度26.5℃)。埋藏白云岩常保持原始颗粒灰岩的残余粒屑结构,在较纯的白云岩中晶间孔发育,有序度较高(0.88~0.93),Sr含量低;白云岩形成的温度较高(校正温度81.9℃)。微晶灰岩、准同生白云岩和埋藏白云岩均具有较高的盐度,表明白云石化流体为海水或与海水有关的海源流体。根据岩石学特征和各种地化指标,结合研究区古地理背景,建立了苏东地区马家沟组五段5亚段准同生白云石化和埋藏白云石化模式。

Dolomite genesis in Member Ma55 of Majiagou Formation,Sudong area,Ordos Basin

Based on rock structure analysis in combination with dolomite degree of order, trace element, geochemical characteristics of carbon and oxygen isotopes, inclusion temperature measurement and salinity test, a study was conducted on the genesis of dolomite in Member Ma₅⁵ of Majiagou Formation, Sudong area, Ordos Basin, in which two kinds of genetic types were identified, i.e., burial dolomite and penecontemporaneous dolomite. The former with diagenetic metasomatic geneses is significant for oil and gas reserving, and its lithology is dominated by grain dolomite and crystalline dolomite. The latter is characterized by bird-eye, gypsum and gypsum pseudomorphic crystal texture, a higher salinity of 24.43wt% NaCl, darker palm red or rose red cathodeluminescence, the degree of order averaged at 0.75, high Sr and Na contents, similar salinity and temperature to those of limestone and formation under a high salinity environment and surface condition(calibration temperature of inclusions is 26.5℃). Burial dolomite often keeps residual grain texture of original grain limestone; intergranular pore was developed in pure dolomite, with a high degree of order(0.88-0.93), low Sr content and higher forming temperature(calibration temperature is 81.9℃). Micrite, penecontemporaneous dolomite and burial dolomite all have high salinity, showing that dolomitized fluid is seawater or relevant sea-source fluid. Based on petrological properties and all kinds of geochemical indicators as well as paleogeographical settings of the study area, two dolomitization models were established for penecontemporaneous dolomite and burial dolomite in Member Ma₅⁵ of Majiagou Formation, Sudong area.