Several wells have tested moderate-to-high industrial gas flow from the calcareous coarse clastic reservoirs the third Member of Xujiahe Formation in Yuanba Area

Northeastern Sichuan Basin

revealing its great exploration and development potential.However

the reservoir characteristics and main controlling factors are still unclear.Core

thin section and testing data are integrated to study the rock types

reservoir space

physical properties and controlling factors.The reservoirs are dominated by calcareous sandstone and limy conglomerate which featuring in low content of quartz

feldspar and high content of carbonate debris.And the reservoir space is dominated by intergranular dissolved pores

intragranular dissolved pores and fractures.The calcareous sandstone reservoirs are obviously different from the limy conglomerate reservoirs.The former is of fracture-pore type reservoir

while the latter is of pore-fracture type reservoir with low porosity and low permeability.The development of the reservoirs was mainly controlled by sedimentary microfacies

rock types

dissolution and fractures.Besides

for the development of high-quality reservoir

favorable sedimentary microfacies and reservoir rock types are the basis and dissolution and fractures are the key factors.The medium-coarse grained calcareous sandstone in underwater distributary channel is the most favorable faces

as the high content of calcareous component can improve the development of high quality reservoirs through dissolution and fracture modification at late stage.