碳酸盐岩中地开石的分布特征及其成因

塔里木盆地巴楚-麦盖提地区石炭系小海子组碳酸盐岩中存在大量地开石，利用X射线粉晶衍射对该矿物进行了鉴定，利用阴极发光显微镜和扫描电镜对地开石的形态与分布进行了研究。地开石晶体为短柱状，晶体大小约3~13 μm，分布在溶蚀孔、白云石晶间孔和缝合线的溶解段中。经岩石薄片和阴极发光观察，发现在地开石出现的岩样中存在萤石。地开石与萤石组合的出现，表明该地区小海子组存在着热液活动。经统计，地开石和萤石通常富集于某一层，可追踪热液流体来源的深度。由于地层中未见地开石的前身矿物，并且在温度低于100℃的地层中也出现了地开石，推断该区地开石为热液成因。热液流体不仅为地开石形成创造了条件，还导致了小海子组碳酸盐岩产生大量溶蚀孔隙。热液与岩石间的水岩反应导致萤石交代白云石，溶孔壁白云石重结晶。通过对储层物性数据的分析，表明热液溶蚀能明显增加孔隙度，但萤石和地开石的形成对孔隙度起破坏作用。热液溶蚀是小海子组碳酸盐岩储层形成的重要机理，对指导该区油气勘探具有实际意义。

The Distribution and Origin of Dickite in Carbonate

Dickite was detected in carbonate of Xiaohaizi Formation at Bachu-Markit area,in Tarim Basin in this study.Identification of dickite was made by X-Ray diffraction.Through thin section and CL observing,the morphological character and distribution of dickite were studied.Dickite was blocky,and the size of its crystal was 3-13μm,which distributing widely in the dissolved pores,intercrystalline pores of dolomite,and in the dissolved segment of stylolites.The thin sections in study area were observed under cathode luminescence,and we found that there were some fluorites coexisting with dickite.The mineral association of dickite and fluorite illuminated that there was hydrothermal activity in Xiaohaizi formation in study area.Generally,dickite and fluorite were rich in certain layer,which can be use to trace the depth of hydrothermal fluid.Because of absence of a precursor mineral,and dickite occurring in the formation with temperature less than 100℃,it was proposed that dickite was hydrothermal origin.Hydrothermal fluid not only created the condition of dickite formation,but also led to form many dissolved pores in carbonate of Xiaohaizi Formation.The reaction between hydrothermal fluid and rock led to dolomite was replaced by fluorite,and dolomite recrystallized along the dissolved pore wall.Base on physical properties data of reservoirs,it’s obvious that the hydrothermal corrosion can make porosity increase.However,fluorite and dickite filled in pores could destroy porosity.As a result,we suggest that hydrothermal corrosion was the important mechanism of carbonate reservoir formation in Xiaohaizi Formation,which can guide the oil and gas exploration in the future.