Enrichment feature of thorium element in tuff interlayers of Upper Triassic Yanchang Formation in Ordos basin, China

Tuff interlayers distribute widely in Yanchang Formation of Ordos basin. The data of natural gamma spectrometry logging shows that the thorium content is 25×10-6~35×10-6 in tuff layer. The results of element geochemical test indicate that the content of thorium is from 15×10-6 to 80×10-6, and average is 36.6×10-6, which is obviously higher than the average of any other lithospheres and the upper and lower layers of tuff interlayers. There are two factors may cause the enrichment of thorium element in tuff interlayer of Yanchang Formation. Firstly, the thorium content of original magma is very high, and the geochemistry attribution of thorium is inactive, so the migration of thorium is relatively not obvious; the other factor is tuff had experienced intensive alteration at the process of diagenesis, and the primary products are clay minerals, which might have adsorbed thorium element from upper and lower layers. The enriching characteristic of thorium element can be a sign of recognizing the tuff interlayer in the study area, which is favorable for more delicate strata correlation. At the same time, the ratio of Th/U reflects the elements emigration and palaeo-environmental changing caused by volcanic tuff sedimentary process, and it might play an important role during the developing of excellent petroleum source rock of Yanchang Formation.