Abstract: The dolomite underlying gypsum-salt layer in the 5^th member of Ordovician Majiagou Formation in the eastern Ordos Basin proves to be of great gas potential by latest exploration practice.Thus we conducted systematic study on its petrological features,reservoir space,porosity and permeability by core observation,thin section identification,physical property analysis and geochemical analysis.The results show that the reservoirs underlying the gypsum-salt layer in the Ordovician are of porous dolomite type in the Ordos Basin,and they occur vertically in O₁m⁶₅,O₁m⁷₅ and O₁m⁹₅.The litho- logies of the O₁m⁶₅ reservoirs are mainly granular dolomite and porphyritic dolomite with very fine-crystalline,while the lithologies of O⁷₁m₅ and O₁m⁹₅ reservoirs are dominated by granular dolomite and dolomite with very fine-to fine-crystalline.The reservoir spaces for the three reservoir intervals are primarily dissolved pores or cavities,with residual intergranular pores,intercrystalline dissolved pores,intercrystalline pores and micro cracks ranked second.The average porosity for each is 5.84%,6.81% and 7.03%.Sedimentary paleo-geomorphology,sea level eustacy and penecontemporaneous meteoric water dissolution collectively control the development and distribution of the dolomite reservoir underlying gypsum-salt layer.It is concluded that the central paleo-uplift belt and Yulin-Hengshan uplift belt,located outside the boundary of the gypsum-salt layer,are relatively higher in topography,and the deposits of grain shoal facies are relatively thicker,so they are prone to undergo meteoric water dissolution during penecontemporaneous and shallow burial stages.Its internal pores are not easily subject to filling of gypsum salt.All in all,they are the favorable areas for the growth of dolomite reservoirs underlying the gypsum-salt layers inside the basin.

Key words: sea level, dissolution, paleo-geomorphology, gypsum-salt layer, dolomite, Majiagou Formation, Ordos Basin