Not only coral reefs, coral stromatoporoid reefs and algal reefs, but also relatively large scale algae bonding rocks and bafflestones are distributed in the Middle Late Ordovician Series in the southwestern margin of the Ordos Basin, and at the same time grain bank facies sediments are widely developed at the bottom and the front of the reefs. The shoal facies deposit connects the relatively independent reefs, forming relatively large scale biohermal complexes. The intrabasinal reefs are mainly layeredalgal reefs, distributed in the west side of the Luliang uplift, the south slope of the Yimeng uplift, and the north side of the centeral palaeohigh. There exist two kinds of ecological characteristics, one is algae growing vertically and forming algal stromatolite, the other is algae binding plaster and forming algae lime mud sphere. Influenced by diagenesis, the origin biological pores and intergranular pores of the reefs nearly disappeared completely. But the dolomitization is common, intracrystalline pores and intracrystalline dissolved pores are well developed, the physical property is good, and the drilled wells have good deliverability. Dolomitized reefs are high\|quality reservoirs and are a new target for the Early Palaeozoic gas exploration.