The Characteristics Of Compositions And Geochemical Of Exhalative Rocks Of The Lower Permian Fengcheng Formation In Urho Area,Northwestern Junggar Basin

The exhalative rocks,also named the hydrothermal sedimentary rocks,attract increasing attention in the geological field.Current researches are mainly focused on marine exhalative rocks.Less continental exhalative rocks are reported,particularly there are the least reported lacustrine exhalative rocks.The hydrocarbon exploration and development has gained exciting achievements in lacustrine “white smoke type” exhalative rocks recently,and it lack of the knowledge about lacustrine “white smoke type” exhalative rocks and their geological significance.Based on the petrology,mineralogy and geochemistry,the genesis of lacustrine “white smoke type” exhalative rocks of Lower Permian Fengcheng Formation in Urho area,Junggar Basin were discussed.The main results were as follows:(1)In the sendimentation stage of Fengcheng Formation is foreland basin,charactered with activity tectonic and volcanic movement,developing deep fracture systems,to provide a good channel for thermal water migration.(2)The rock type in study area are mainly dolomite rocks and salt minerals,including dolomitic mudstone,dolomitic siltstone,tuffaceous dolomite and argillaceous dolomite,silicon boron,shortite,northupite,halite,zwegscheiderite,trona and Pyrite are recognized as salt minerals.(3)Abnormal sedimentary textures and structures were identified in Fengcheng Formation,such as laminated structure,banded structure,.deformation structure,convolute bedding,vein structure,synsedimentary faults,eruptive breccias and intraclastic textures.These structures or textures and their combination were significant evidences of hydrothermal genesis.(4)Different types of rocks in exhalative rock of Fengcheng Formation are highly enriched of Li,Mo,W and U,also have their own enrichment characteristic,that anomalous deposition indicating that deep hot fluids of Fengcheng Formation are multi source.(5)We can concluede from geochemistry analysis that the depositional environment are semi closed-closed reduction environment which are dry and hot,during the period of Fengcheng Formation.(6)Rock types in the study area are mainly dolomite,often mixed with,mud,volcano ash and terrigenous clastics,that are composed of fine-grained silty sand and transported from the sandbody in shallow lakes of fan delta front.And then effected with event deposits(such as storm,flood,earthquake and volcano activity),the gravity flow bring silty sand,mud and volcano ash together to forming matasomatic-inside and outside source of hunji dolomic modle.