Rare earth elements geochemistry characteristics and their geological implications of lacustrine oil shale from Chang 7 oil layer in southern Ordos Basin, China

摘要

Located in the middle-western area of China, Ordos Basin is a large continental basin with vast petroleum resources. Oil shale of Chang 7 oil layer from Triassic Yanchang Formation represents the typical lacustrine oil shale in China. Oil shale samples were collected from Chang 7 oil layer to study the characteristics of elements geochemistry. The rare earth elements (REEs) concentration of oil shale samples varies from 138.51 to 206.36g/g with an average of 162.64g/g, close to the average REEs content of the North American shale Composite and slightly higher than the upper continental crust. The chondrite-normalized and North American shale composite-normalized REEs distribution patterns explain that oil shale samples, together with silty mudstone samples, may have been derived from a similar terrigenous source. The oil shale deposited mainly in early diagenetic stage B period and the paleoclimate condition was warm and humid with moderate chemical weathering. The whole sedimentary rate of oil shale was low. The source rocks of oil shale were mainly from the felsic rocks and deposited in the continental island arc tectonic setting. The total REE of oil shale samples show positive correlations with Al, Si, K, and ash yield concentration and negative correlations with oil yield, Fe, P, and total sulfur concentration, illustrating that REEs concentration in clay minerals is more than that in organic matter. In addition, the light REEs and heavy REEs are both present in clay minerals and controlled by land-derived detritus.