Abstract: The Niutitang Formation in northwestern Hunan Province,eastern margin of Sichuan Basin is regarded as having great potential and one of the major shale gas exploration targets with its wide distribution,large thickness,favorable burial depth,high content of organic matter,high maturity and etc,in south China.An parameter well,the Huaye-1,in the Formation of the area,was chosen to be studied with systematic means including observation of thin sections,XRD mineral analyses,physical property tests,SEM imaging and low-temperature N₂ adsorption/desorption experiments with core and drilling cutting samples,to probe into the characteristics and influencing factors of the reservoirs.The deep shelf facies of the Niutitang Formation developed dark carbonaceous shale and siliceous shale rich in carbonaceous and siliceous material,as well as pyrite,but lean in clay minerals (mainly illites).The shale reservoirs are of ultra-low porosity and permeability.The SEM images and low-temperature N₂ adsorption/desorption experiments show that the reservoirs contain mostly cylindrical pores (organic pores),narrow parallel-plate pores (clay mineral interlayer pores),tapered tube pores with openings around (intergranular pores among clay minerals) and tapered plate pores (micro-crack).The mesopores (sized between 2 to 50 nm) contribute an average of 60.3% of BJH volume.The development of the micropores is found to be affected by multiple factors:the deep-water muddy continental shelf provided the material basis for the pores,the high content of TOC facilitated the growth of organic pores (especially organic macropores),the composition and content of minerals dominated the types and amounts of the pores,and the thermal evolution of organic matter promoted the development of organic pores and micro-cracks.

Key words: shale,reservoir,Niutitang, Formation,northwestern, Hunan, province,Sichuan, Basin