The Ordovician Ma5 member is mainly carbonate rock, Ordos Basin. In 1989, the Ma5 member of Shancan 1 well obtained high yield gas flow, which marked the discovery of Jingbian large gas field. Gaoqiao area is the southward extension part of Jingbian Gas field, located in jingbian paleo-submerged platform, with stable reservoir distribution. The carbonate reservoir of Ma5 member was formed in an old age and experienced multiple complex diagenetic evolution, with strong reservoir heterogeneity and complex gas and water distribution rule. Therefore, it is urgent to clarify the effect of karst palaeogeomorphology in lower Paleozoic on reservoir transformation, and then study the distribution characteristics of reservoir and the distribution law of gas and water, which has important theoretical significance and application value for carbonate exploration and development of the 5th Member of Ma Formation.

    It is concluded that evaporative tidal flat environment was developed in the Ma 5th member, and seawater was mainly invaded from the east, and mud dolomitic flat, gypsum dolomitic flat and gypsum dolomitic flat sedimentary facies belts were developed in supratidal and intertidal zones. In the west is the paleokarst plateau, in the middle is the karst platform, and in the east is the karst basin, which is cut by three nearly east-west ancient grooves. The karst plateau is dominated by vertical infiltration, while the karst platform is dominated by horizontal undercurrent. The reservoir has experienced Penecontemporaneous karst, supergene karst and burial karst. The remnant hills on karst highland, gentle hills in karst platform and the two sides of grooves are favorable karst reservoir areas. Dolomite is the main reservoir rock type, and dissolution pores and intergranular solution pores are the main reservoir space, and there are four types of pore structures: solution pore-intergranular (solution) pore compound (class ⅰ), intergranular pore-fracture (class ⅱ), fracture-micropore (class ⅲ) and micropore (class ⅳ). Cementation filling and dissolution have great effect on reservoir physical properties. The calcite + siliceous cement-dissolution phase has strong dissolution effect and good physical properties in the Ma 5th member, and is distributed on both sides of the grooves and the gentle hill. Calcite cementation facies and calcalous argillaceous cementation facies have strong cementation and weak diagenesis, and have poor physical properties. They are mostly distributed in low-lying areas such as valley platform and grooves. The distribution of gas and water is macroscopically controlled by palaeokarst landform and diagenetic background, and microscopically controlled by reservoir physical properties and pore connectivity. After adjustment, the natural gas accumulates in high positions on both sides of residual hills, gentle hills and grooves, forming the present distribution pattern of gas reservoirs.