川东北地区WT1井7590~8060m段地层归属的确定及其对油气勘探的启示

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

WT1井完钻井深8060 m,刷新了中国石油陆上井深纪录。但其灯影组下伏地层归属问题一直悬而未决,前期由于油基泥浆的影响一直被认为是南华系。利用岩石学、碎屑锆石U-Pb年代学及地球化学方法,对该井7590~8060 m段地层进行对比研究。WT1井为泥岩、粉砂岩、砂岩夹白云岩组合,见硅质岩、磷块岩及硅磷质条带等夹层。碎屑锆石最年轻的年龄峰值分别为654.5±7.1 Ma、644±5 Ma。岩屑及岩石的化学蚀度指数(CIA)大于60。结果表明,WT1井7590~8060 m段地层岩石组合与区域上陡山沱组岩石组合极相似;其沉积时代应晚于644±5 Ma,且沉积时期的气候属温暖潮湿型。研究结果综合限定了WT1井灯影组之下7590~8060 m段应属震旦系陡山沱组,预示着川东北地区可能存在陡山沱期的裂谷,有利于打开川东北地区震旦系—下古生界的油气勘探局面。

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.