鄂尔多斯盆地东南缘长8油层组碳酸盐胶结物成因分析

Origin analysis of carbonate cements in Chang 8 reservoir in southeastern margin of Ordos Basin

碳酸盐胶结物是鄂尔多斯盆地东南缘长8油层组最重要的自生矿物,主要类型为铁方解石,另有少量的白云石和铁白云石。镜下观察、包裹体测温、碳氧同位素分析及铁氰化钾染色实验分析证实,方解石胶结作用从早成岩阶段持续到晚成岩B期。早成岩阶段形成少量的无铁方解石,到晚成岩期,因有机质在热催化作用下脱羧而形成大量铁方解石。这种后期形成的铁方解石以半基底式—基底式胶结充填剩余粒间孔隙,大大降低了储层的储集性能。

Carbonate cements are the most important authigenic minerals in Chang 8 reservoir in southeastern margin of Ordos Basin,and they are mainly ferrocalcite,with some dolomite and ferrodolomite.Based on thin slice and SEM inspection,inclusion enclave temperature measurement,carbon and oxygen isotope analysis and potassium ferricyanide coloring analysis,it is proved that calcite cementation started from the early diagenetic stage and lasted to the late diagenetic stage of B stage.In the early diagenetic stage,there generated light calcites,while to the late diagenetic stage,there generated a great quantity of ferrocalcites because of the organic material decarboxylation under thermocatalysis.Ferrocalcite generated in late diagenetic stage filled the remained intergranular pores with a semi-basal or basal cementation,which enormously lowers the accumulation capacity of the reservoir.