Abstract:

Take the geochemical indicators of boron,equivalent boron,adsorptive K⁺+Na⁺,Sr and the ratio of Sr/Br etc.as palaeosalinity indicators,the palaeosalinity of Chang6 oil reservoir set in Ordos Basin is analysed and calculated by using the formula advanced by T.D.Adams and E.L.Couch.The results indicate that the Late Triassic Ordos Basin is not an inland fresh water lake as recognized for a long time,but a brackish water to semisalt water marginal lake caused by intermittent seawater ingression.The paleosalinity of water body of the reservoir set was 0.940%～01.016% during its deposition.The contents of K⁺+Na⁺ and the ratio of K/Na in the above mentioned paleosalinity do not correspond with that of Chang 26 reservoir set;the contents of boron,equivalent borom, adsorptive K⁺+Na⁺,Sr and Sr/Ba ratio are reliable identifying indicators of the paleosalinity.After the regression analysis of the identifying results of various indicators,it is suggested that the results calculated by E. L. Couch formula are more reliable.The identificatin of paleosalinity could not only distinguish the water type of paleolake,but also could know the evolution of source rock.

Key words: Ordos Basin, Chang26 reservoir set, paleosalinity, equivalent boron, adsorptive K⁺+Na⁺, ratio of Sr/Ba