The Comparative Research On The Characteristic Of South China Organic-rich Shale In The Lower Paleozoic

There are three types and several sets organic-rich shale developed in China in the process of multicycle tectonic-sedimentation,including marine,marine-continental transitional facies and continental facies.South China is the favorable area for marine shale gas.The Lower Cambrian Niutitang formation shale and the Lower Silurian Longmaxi formation shale are two most typical sets organic-rich shale in lower Palaeozoic formations.Niutitang formation is called for Qiongzhusi formation in middle,southwest and northeast of Sichuan Basin,Shuijingtuo formation in the middle Yangtze region and Hetang formation in the lower Yangtze region.Longmaxi formation is called for gaojiabian formation in the lower Yangtze region.The discovery of Jiaoshiba shale gas field marks significant breakthrough in Longmaxi shale gas reservoir,but The Lower Cambrian Niutitang Formation shale gas reservoir has yet to realize the true commercial breakthrough.The huge difference in shale gas production implies different geological characteristics and accumulation and preservation conditions with two sets of shale.The formation of shale gas reservoir is the result of the combined action of hydrocarbon generation condition,reservoir conditions and preservation condition.It is found that the key elements in gas bearing difference of the two sets shale is mainly reflected on the maturity,porosity and permeability,the relative content of clay minerals and roof and floor conditions through fine contrast in various parameters.Among them,the role of maturity is not alone.The high degree of maturity will affect the evolution of the pore and clay minerals,and then affect its gas content.Less content of carbonate minerals in Niutitang Formation is unfavorable for dissolution pore generation.Niutitang shale has a less content of montmorillonite and illite smectite mixed layer mineral(which has a larger surface area)and a more content of chlorite and illite(which has a smaller surface area).Thus,Niutitang shale has a weaker adsorption capacity.At the same time,pore permeability,porosity and permeability are not as good as the Longmaxi shale.In the storage conditions,the roof and floor conditions of Niutitang Formation is worse than the Longmaxi formation,which result a great loss of shale gas generated in the reservoir formation.The maturity of most Niutitang sample has passed the hydrocarbon dead line,which result in hydrocarbon potential depletion.When reflectance of vitrinite has passed 3%,organic matter pores begin to decrease.This phenomenon may be related to organic matter carbonization of the high degree maturity.Compared clay mineral types and their relative contents of two sets shale and found that Longmaxi formation is in the stage of illite / smectite mixed layer to mineral illite transformation,Niutitang shale is in the stage of late diagenetic to metamorphism.The mixed layer minerals of Niutitang shale has been transformed to illite,the specific surface area and pore volume have become smaller drastically and and densification degree increases,thus resulting a reduction of gas bearing property.In the long geological history,high degree of thermal evolution has a serious impact on the Niutitang Formation of microscopic pore structure and diagenesis and thus pore volume and specific surface area were significantly lower than that of the Longmaxi formation shale,which is not conducive to the adsorption and concentration of shale gas.However,roof and floor conditions exacerbated this trend and lead to a worse gas bearing property than that of Longmaxi shale.