Mizhi intra-platform depression developed on the broad shallow water carbonate platform in Ordos epeiric sea during Early Ordovician.The area of the depression is about 70000 km

2

.It was formd by the combined effect of both the differential subsidence of the platform interior and the rise of sea level.The evolution process can be divided into three periods(lower Majiagou

upper Majiagou and Fengfeng).Each period can be subdivided into three stages.The first stage differential subsidence occurred in the platform interior

and sea level rose;the second stage

intense differential subsidence and evident rise of sea level resulted in formation of Mizhi intra-platform depression;the third stage

differential subsidence weakened gradually

the sea level fell

and the depression was filled.The evolution of Mizhi depression underwent three large-scale transgressive-regressive changes and formed three supercycles and three stratigraphic sequences corresponding respectively to lower Majiagou

upper Majiagou and Fengfeng Formations.Three perfect source-reservoir-cap assemblages were formed from the dark organic-rich micrite at the top of the transgressive limb of each supercycle to the evaporite at the upper part of the regres-sive branch.Mizhi intra-platform depression is a gas-bearing sag because of high organic maturation.Recently

the significant discovery of gas in Fengfeng sequence indicates that intra-platform depression has great potential to accumulate gas.The gas accumulated in porous dolomite reservoirs related closely to palaeokarst.A geological model shows that four types of stratigraphic traps existed in the depression.