Provenance analysis of Chang 7 Member of Triassic Yanchang Formation in Ordos Basin

Provenance analysis is the basis and prerequisite of depositional system and palaeogeography, and also provides to some extent the significant reference for forecasting and evaluation of sand body reservoirs. The analysis of source rock features and provenances is becoming more and more important in the exploration of Chang 7 reservoirs. During the period of Triassic, the ancient lands around Ordos Basin, including Yinshan Old Land on the north, Alxa Old Land on the northwest, Longxi Old Land on the southwest and Qilian-Qinlin Old Land on the south, were the main sources of the detrital sediments in the basin. With the help of the methods based on analysis of detrital minerals, cuttings, heavy minerals, trace elements and rare earth elements, four source directions can be found: northeast, northwest, west-southwest and south. The comprehensive analysis of light and heavy mineral combination characteristics shows that the source rocks of the northern study area are Archean to Palaeozoic mesometamorphic to hypometamorphic rocks and intermediate to basic igneous rocks, and those of the south and southwest areas are Proterozoic and Palaeozoic epimetamorphic rocks, carbonate and clastic rocks. The analysis of trace elements and rare earth elements indicates that the source rocks are mainly felsic rocks derived from the upper crust, and also a quantitative of mixed rock of meta-intermediate-basic igneous rocks with high feldspar such as andesite and alkaline basalt. Archeozoic strata possess a considerable proportion in the source rocks.