Vertical zonation of weathered crust ancient karst and reservoir evaluation and prediction—A case study of M55–M51 sub-members of Majiagou Formation in gas fields, central Ordos Basin, NW China

The M55–M51 sub-members of the Middle Ordovician Majiagou Formation in the gas fields of the central Ordos Basin were taken as a case to analyze the vertical zonation characteristics of weathered crust ancient karst and to evaluate and predict reservoirs through plenty of core and thin section observation, with geological background and petrological characteristics as main clue. According to detailed underground geological surveys, description of the typical karst section, systematic sampling, and laboratory test, there mainly developed karst formation rock and transformation rock in the ancient weathered crust in the top of the Majiagou Formation, which were divided into surface karst zone, vertical vadose zone, horizontal underflow zone and deep slow flow zone from surface to subsurface. Karst intensity in each zone has great effect on the storage and flow capacity of the reservoir. In the strong karstification subzone of the horizontal underflow zone with over strong karst, the parent rock was extensively damaged and formed karst caves filled with karst formation rock rarely with pores. In the deep slow flow zone with over weak karstification, there are sedimentary and filling activities. The medium karstification subzone of the horizontal underflow zone and the vertical seepage zone had a moderate karstification, which not only created storage and flow space but also prevented collapse, and were most beneficial to reservoir development.