鄂尔多斯盆地陇东地区致密砂岩储层碳酸盐胶结物特征及成因机理

Carbonate cementation characteristics and genetic mechanism of tight sandstone reservoirs in Longdong area,Ordos Basin

为研究鄂尔多斯盆地陇东地区细粒致密砂岩储层中碳酸盐胶结物特征及其成因机理,开展了铸体薄片分析、扫描电镜观察、流体包裹体测试等研究工作。结果表明:①鄂尔多斯盆地陇东地区致密砂岩储层中碳酸盐胶结物广泛发育,体积分数平均为4.2%,以铁方解石和铁白云石为主;②碳酸盐胶结物多以孔隙式或加大边形式沉淀,流体包裹体均一温度较高(峰值为100~110-),形成阶段一般较晚;③铁方解石和铁白云石胶结物的钙离子来源与长7砂岩中长石溶蚀、黏土半渗透膜效应密切相关,铁、镁离子的来源则受控于云母、凝灰质蚀变以及白云岩岩屑和变质岩岩屑的溶蚀;④碎屑组分中白云岩岩屑的分布控制了铁方解石和铁白云石胶结物在平面上的差异沉淀,铁白云石胶结物只出现于富含白云岩岩屑的砂岩中;⑤碳酸盐胶结是晚期致密油砂岩孔隙损失的重要因素。强烈压实作用之后,残余孔隙被碳酸盐胶结物逐渐充填,当碳酸盐胶结物体积分数>2%时,胶结减孔效应明显。该研究成果对预测鄂尔多斯盆地陇东地区有利储层分布具有指导意义。

In order to study the carbonate cementation characteristics and its genetic mechanism in fine-grained tight sandstone reservoirs in Longdong area of Ordos Basin,thin section analysis,scanning electron microscopy and fluid inclusion test were carried out.The results show that:(1)Carbonate cements are widely developed in tight sandstones of Longdong area,Ordos Basin,with an average volume fraction of 4.2%,and are mainly composed of ferrocalcite and ankerite.(2)Most of the carbonate cements precipitate in the form of pore or enlarged edge,and the homogenization temperature of fluid inclusions is higher(the peak value is 100-110℃),and the formation stage is generally later.(3)Calcium ion sources of ferrocalcite and ankerite cements are closely related to feldspar dissolution and clay semi-permeable membrane effect in Chang 7 sandstone,while iron and magnesium ion sources are controlled by the alteration of mica and tuffaceous and the dissolution of dolomite debris and metamorphic debris.(4)The distribution of dolomite debris in clastic components controls the differential precipitation of ferrocalcite and ankerite cements in plane,and ankerite cements only occur in sandstones rich in dolomite debris.(5)Carbonate cementation is an important factor for pore loss in late tight oil sandstone.After strong compaction,the residual pores are gradually filled by carbonate cements.When the volume fraction of carbonate cements is more than 2%,the effect of cementation on reducing pore size is obvious.The research results have certain guiding significance for predicting favorable reservoir distribution in Longdong area.