Abstract: The carbonate gas reservoirs of the Changxing Formation in the Yuanba Gas Field are characterized by complex pore structure and strong heterogeneity.A comprehensive study has been taken through test,experimental analysis and experience statistical method for the lower limit of effective porosity of the Changxing Formation.The research result shows that the lower limit of effective porosity is 2.0%.The first stage of proven reserves has been calculated on this basis.