Petrology and element geochemistry and development environment of Yanchang Formation Chang-7 high quality source rocks in Ordos Basin

The core of Yanchang Formation Chang-7 high quality source rock (oil shale) is black,hard,light,and it looks slip-shape after demorphism. Rock slice observation and SEM-energy spectrum test show that the Chang-7 high quality source rocks are characterized by the extraordinary development of organic matter laminas,being rich in framboidal pyrite and abundant cellophane,and crystal fragment and tuff laminae are often observed. Comprehensive test results reveal that the chemical compositions of Chang-7 high quality source rock are characterized by high S2-,Fe,P2O5 contents and relatively low Al2O3,SiO2,CaO and MgO contents. In trace elements,Mo,Cu and U are obviously positive anomalies,V and Pb are positive anomalies,and the others are no or negative anomalies. ∑ REE is low. Low Sr/Ba ratios and B content reflect nonmarine lake-basin deposition feature with little salinity. Some geochemical parameters,such as high V/(V+ Ni) and very high U/Th ratios,indicate anoxic sedimentary environment.