Research on palaeokarst and its reservoir-controlling mechanism of Jingbian gasfield

Secondary holes, caverns and fractures are developed in carbonate reservoirs of Jingbian gasfield, and Ordovician palaeokarst is very important for the forming of secondary holes, caverns and fractures. The palaeokarst features and its controlling factors are researched and the controlling mechanism of palaeokarst on reservoirs are furtherdiscussed by use of the core observation data, thin section analysis data and various well test data. The results indicatethat the reservoirs in Jingbian gasfield experienced three periods of karst consisting of interstratal karst under the environment of syndeposition-intermittent exposure, weathering crust karst and fracture-cavern system karst. The firstperiod of palaeokarst is controlled by the sedimentary environment, the second period of palaeokarst is controlled bythe pre-Carboniferous palaeogeomorphology and the thirdperiod of palaeokarst is under the control of paleostructure;multi-stage palaeokarst controls the distribution of gas reservoirs; M511-531 gas reservoir is the product of the superposition of multiphase palaeokarst (Ⅰ+Ⅱ+Ⅲ), while M541gas reservoir is the product of the superposition of (Ⅰ+Ⅲ)palaeokarst which is controlled by the sedimentary microfa-cies and local nose lifting structure.