Stratigraphic attribution of 7590~8060 m sect of WT1 Well in northeastern Sichuan and its enlightenment to oil and gas exploration

The completion depth of the WT1 Well is 8060 m, refreshing the onshore well depth record of CNPC.However, the attribution of the lower strata of the Dengying Formation has not been determined, and it has been considered to be the Nanhua System due to the influence of oil-base mud.Petrology, detrital zircon U-Pb chronology and geochemical methods were used to study the strata between 7590~8060 m in this well.The rocks include mudstone, siltstone, sandstone and dolomite, with intercalation of siliceous rocks, phosphorous rocks, siliceous and phosphorous bands.The youngest peak ages of detrital zircons are 654.5±7.1 Ma and 644±5 Ma respectively.The CIA(chemical index of alteration) value in cuttings and rocks is great than 60.The results show that the lithologic assemblage in 7590~8060 m sect of WT1 Well is very similar to that of the Doushantuo Formation in the region.The deposition age should be later than 644±5 Ma, and the climate during the deposition period was warm and humid.The research results show that 7590~8060 m sect below the Dengying Formation of Wutan 1 Well belongs to the Doushantuo Formation of Sinian System.This indicates that the existence of a rift developed in the period of Doushantuo, which is conducive to the mineralization of Sinian and Lower Paleozoic oil and gas in northeastern Sichuan.