鄂尔多斯盆地西南缘奥陶系马家沟组白云岩成因研究

ORIGIN OF DOLOMITE OF ORDOVICIAN MAJIAGOU FORMATION,WESTERN AND SOUTHERN MARGIN OF THE ORDOS BASIN

基于岩石薄片的系统鉴定,将钻井岩心和地表露头样品相结合,运用多种分析、测试手段对鄂尔多斯盆地西南缘奥陶系马家沟组白云岩的成因进行深入研究,认为该组白云岩主要由准同生白云岩和埋藏白云岩组成。准同生白云岩主要发育于马家沟组一、三、五段,受古地理环境限制明显,由蒸发泵白云石化形成;埋藏白云岩厚度大、分布广、岩石类型多,构成马家沟组白云岩的主体,成因模式包括浅埋藏期的回流白云石化以及中—深埋藏期的压实水流白云石化和地形驱动水流白云石化。热液白云石在盆地西缘的定边地区、南缘的富县、黄陵、麟游均有发现,通常呈脉状充填裂缝,其成因与深部热流体沿断裂系统上涌有关,而且对储层的发育具有指示意义。在上述成因模式研究的基础上,对各类白云石的结构演化进行了讨论。

A large amount of dolomite is developed in the Ordovician Majiagou Formation in western and southern margin of the Ordos Basin.The massive dolomite which has large exploration potential mainly distributes in the northern Tianhuan Depression and Weibei Uplift.Based on the study of outcrop section and borehole core,the lithology and mineralogy characteristics,the geochemical data and sedimentary diagenetic-structural setting,the dolomite in study area can be divided into three types： penecontemporaneous dolomite,burial dolomite and hydrothermal dolomite.Penecontemporaneous dolomite developed in Majiagou Formation Member 1,3 and 5.Its origin is evaporative pumping dolomitization.The genesis of burial dolomite which forms main body of the dolomite of Majiagou Formation can be explained as seepage-reflux dolomitization during shallow-burial stage and compacting flow dolomitization and landform-driven flow dolomitization.Hydrothermal dolomite which fills in fractures is discovered in Dingbian,Fuxian,Huangling and Linyou area.The deep thermal fluid rising along faults and fracture systerm is relate to the origin of hydrothermal dolomite,it plays an important role in forming reservoirs in this area.In addition,this article describes the textural characteristic and evolution of the dolomite,for having an intuitive cognition with the forming process of various types of dolomite.