The Characteristics And Controlling Factors Of Subsalt Dolomite Reservoirs In The Fifth Member Of Majiagou Formation Of Ordovician In Central Ordos Basin

In recent years,studies on the reservoirs of middle Ordovician majiagou formation in Ordos basin have made important progress,but its exploration focus more on the upper M51?5 sub-member,known as weathering crust reservoir.Exploration degree on the dolomites of subsalt layer is relatively low.However,the subsalt layer of the Ordovician in the Ordos basin has huge exploration potential which can not be ignored,because its status as the reserve of Ordos basin is becoming increasingly prominent.Therefore,evaluating the reservoir characteristics and controlling factors of subsalt layer of the Middle Ordovician of Ordos basin has important practical significance for the oil-gas perspective in the research area.According to the coring and logging data,we studied the development rules as well as the characteristics of the reservoir.Results showed that the research area is rich in the types of reservoirs,but granular and crystal grain dolomites play a major.The reservoir space includes intergranular pore,intergranular solution hole,intercrystalline pore,intercrystalline solution hole,dissolved inter-grain pores and small caves.The research area typically develops pore-cave reservoirs with low to medium porosity and permeability.Dolomites of M56?10 sub-member have experienced long-time diagenesis modification.The types of diagenesis is enormous.Research on the controlling factors of subsalt layer reveals to us that psammitic banks are the best favorable facies belts of reservoirs;pene-sedimentary karst plays obviously constructive role in the forming of the reservoirs,which is also the core enabler of pore-cave reservoirs;the contribution of penecontemporaneous dolomitization mainly relys on dolomites' better ability to resist compaction and pressure-solution than limestones,benefiting to the saving and the late adjustment of residual intergranular pore.