Geochemistry of oil shale from Chang 7 reservoir of Yanchang Formation in south Ordos Basin and its geological significance

Trace elements, rare earth elements and organic geochemical parameters are of important significance for revealing oil shale rock sedimentary environment, paleoclimate, prediction of oil and gas as well as oil shale resource potential. Based on geochemical parameters obtained from the oil shale samples of Chang 7 reservoir in southern Ordos Basin, The authors have obtained the following results:Comparing with mean value of the earth's crust and other areas, W, Mo, As, Sb, U are obvious concentrated, and Sc, Cr, Ni, Cl show significant depletion; The oil shale exhibits lower total REE, enrichment of LREE, and depletion of HREE. The rare earth element is significantly different than that of chondrites, but the difference is not obvious in comparison with the North American shale; The types of organic matter in oil shale are of type Ⅰ and type Ⅱ₁, dominated by Ⅱ₁ type; organic matter abundance has reached the best level, maturity is low mature to mature. Through the further analysis of these geochemical parameters and characteristics, the following two aspects are obtained:First, the oil shale of Chang 7 oil reservoir was formed under the condition of warm and humid climate and in a deep-semideep lake reducing environment, which is conducive to the enrichment and preservation of organic matter in oil shale; Second, this area does not have the potential of formation of oil and gas, but has potential of forming high grade and large oil shale deposits.