The 3

rd

Member of Xujiahe Formation(T

3

x

3

)has great petroleum exploration potential and its predicted gas resource volume is a hundred billion cubic meters in Yuanba area

Northeast Sichuan Basin.However

the characteristics and gas accumulation control factors are not clear.Based on cores

thin sections and physical properties

the characteristics and gas accumulation control factors of T

3

x

3

were studied through correlation between well and seismic data.The results indicate that the dominant sedimentary facies are braided river delta plain and delta front.The calcarenaceous sandstones are characterized by high carbonate debris content and low quartz content

and low natural gamma value and high resistance.The calcarenaceous sandstones are of low porosity and extra-low permeability.The reservoirs have a variety of pore types

which are dominated by dissolution pores and authigenic mineral intercrystal pores

followed by micro-fractures.The dissolution pores mainly occur in between calcarenaceous grains and have good connectivity.They are the main pore paces of high-quality reservoirs.The prior factors of gas accumulation and high production are the existence of conglomeratic calcarenaceous sandstones

calcarenaceous sandstones and glutenites in distributary channels of delta plain and delta front.The horizontal and low-angle interlayer fractures and the dissolution pores along the fractures are the determinant factors for the gas accumulation.In addition

structures and faults are also important factors.