Sedimentary Facies And The Law Of Shale Gas Accumulation Of Niutitang Formation Of Lower Cambrian In Western Hubei

Due to the limitation of thermal evolution,structural complexity and burial depth,most of the previous investigations in Western Hubei have focused on the wufeng formation of upper Ordovician and longmaxi formation of lower Silurian.This paper focuses on the niutitang formation of the lower Cambrian in the west of Hubei province.On the basis of full reference to the previous research results,through field profile measurement,sampling,microscopic identification,SEM analysis,mineral composition analysis,shale reservoir physical property analysis and organic geochemical analysis,combined with the theoretical knowledge of sedimentary petrology,lithofacies paleogeography,oil and gas reservoir geology,and so on,it is refined that the sedimentary facies and microfacies of niutitang formation in western Hubei are divided,and the accumulation conditions and rules of shale gas in niutitang formation are studied systematically.The niutitang formation in western Hubei mainly is in shelf environment,and the sedimentary subfacies are from deep-water shelf to shallow water shelf from bottom to top,and the deep-water basin environment is locally developed.The sedimentary microfacies are mainly siliceous shelf,carbonaceous shelf and argillaceous shelf.Horizontally,the water body of niutitang formation is becoming shallower from southwest to northeast,the sedimentary environment is transiting from deep-water shelf to shallow water shelf,and the organic shale is becoming thinner.The lithology is mainly siliceous shale,carbonaceous shale,calcareous shale,silty mudstone and grayish black mudstone.quartz,carbonate and pyrite are the main brittle minerals in organic rich shale.Organic geochemical analysis shows that the type of organic matter in niutitang formation in western Hubei is type I kerogen,and the TOC value of source rock ranges from 2 to 8%,with an average of about 3%.There are two high value areas,Yichang-Zigui area and Hefeng-Enshi area.Ro value ranges from 2.7% to 3.6%,which is in the over mature stage,mainly producing dry gas.The pore types of shale are mainly intergranular pore,intragranular pore,organic matter pore and micro fracture.The pore size is mainly distributed between 0.01-50?m,which is the main space for shale gas reservoir.The upper part of niutitang formation and its overlying Shipai formation in western Hubei are mainly composed of mudstone and limestone,with large thickness,wide range,good sealing property and good conditions of the cap rock;the lower part of niutitang formation is mainly composed of dolomite of Dengying formation,with large thickness and strong sealing property,which can effectively prevent the loss of shale gas and good conditions of the bottom plate.Good roof and floor conditions and caprock conditions,coupled with relatively stable structural environment,reveal that niutitang formation in western Hubei will be a shale gas prospect with great exploration and development prospects.