Abstract:

The Ordovician Ma5₁₊₂ sub-member in Ordos basin is known to be rich in natural gas resources, which is the key target for oil and gas exploration in Ordovician formation in Jingbian gasfield. Based on the data of core, thin section and reservoir physical property, this paper analyzes and summarizes the diagenesis as well as porosity evolution of Ordovician Ma5₁₊₂ submember reservoir in the southern Jingbian gasfield. A series of restricted-evaporative platform dolomite reservoir is developed in the Ordovician Ma5₁₊₂ sub-member of the southern Jingbian gasfield, which is a low porosity, low permeability reservoir of micro-fracture-pore type with the main reservoir space dominated by gypsum dissolved pores and dissolved fractures. The reservoir has experienced multiple stages of diagenesis including dolomitization, compaction and pressure dissolution, cementing-filling, epigenic karstification, metasomatism and tectonic fracturing. Among them, the syngenetic-penecontemporaneous dolomitization and gypsum precipitation in high salinity seawater provided favorable conditions for epigenic karstification and fracture formation during Caledonian Movement, which were the basis of reservoir formation. During the supergene period, gypsum nodules were dissolved and micro-fractures were enlarged by meteoric freshwater, thus the reservoir properties were significantly improved. During the burial diagenesis period, the cementing-filling of quartz and calcite caused the reduction of pores, displaying a destructive diagenesis. Additionally, the tectonic activities at the late diagenesis period resulted in some open micro-fractures, which improved local reservoir properties, and finally the dolomite reservoir was formed with the reservoir spaces dominated by gypsum dissolved pores and dissolved fractures, and with the micro(dissolved) fractures as the main filtration channels.

Key words: Jingbian gasfield, Ordovician, Majiagou formation, dolomite reservoir, diagenesis, pore evoluation