Relationship between element geochemical characteristic and organic matter enrichment in Zhangjiatan Shale of Yanchang Formation, Ordos Basin

Some tests were carried out on organic carbon, sulfur content, major and trace elements of 64 shale samples from well core in Zhangjiatan shale of Yanchang Formation, south-center of Ordos basin. Based on the analysis of the characteristics of major and trace element contents, the paleoclimates, redox conditions and paleoproductivity of Zhangjiatan Shale were studied. Moreover, the major controlling factors for organic matter enrichment in Zhangjiatan Shale were explored. Research results show that the organic carbon content was high (2.14%~11.44%). Compared with organic-poor shale, the Na2O, CaO, U, V, Ba, Cu contents of organic rich shale are apparently higher, but the Al2O3, TiO2, K2O and MgO are lower. Palaeosedimentary environment proxies show that the Zhangjitan shales were mostly deposited in humid-warm to semi-arid climatic conditions. The bottom water was suboxic to anoxic in the sedimentary stage of Zhangjiatan shale. The paleoproductivity proxies reflect a middle to high paleoproductivity. TOC content has a positive correlation with redox proxies, suggesting that the changes in organic matter content is predominantly controlled by redox environment. Further research shows that the organic matter enrichment in Zhangjiatan shale is mainly controlled by the degree of water anoxic resulting from the change of paleoclimate. © 2017, Editorial Office of Journal of China Coal Society. All right reserved.