Abstract: The Lower Ordovician in Tazhong area is dominated by marine carbonate.The Lower Ordovician penetrated by Well TZ5 can be divided into four lithologic members, i.e.limestone, brecciated dolomite, upper and lower dolomite, among which the brecciated dolomite member is a crypto-algae reef composed of stromatolite, lamellar stone, oncolite etc formed by crypto-algae bonding, stromatolite has wave-resistant structure.The crypto-algae organic reef underwent one reef base forming stage, three reef growing stages and two intermission, fore reef and reef-core subfacies alternatively appeared in the reef growing stage.Active oil/gas show in drilling indicates that the crypto-algae organic reef has fair storage capability.

Key words: Tarim basin, Ordovician, Alga, Organic reef, Carbonate rock