Characteristics of reservoirs of Changxing Formation in Langzhong-Nanbu area, northern Sichuan Basin

Abstract：Located in the ramp region between the Central Sichuan uplift and Northern Sichuan depression, the Langzhong-Nanbu area possesses superior accumulation conditions. The reservoir characteristics of the Changxing Formation in the study area were studied by means of core observation, debris description, microscopic observation of cast thin sections, analysis of mercury intrusion and geophysical interpretation. The results show that favorable facies belts for the development of reservoirs are interbank area and bank in the platform as well as platform marginal reef flat. The rocks are dominated by bioherm limestone limestone, calcarenite and micritic limestone. In various kinds of reservoir space such as pores, holes and seams，the pores are mainly intergranular pores and dorsal foramen. In massive micro-throats and little fine throats, most of the throats are confined. As the average porosity is 1.16% and the average permeability is 0.5415×10-3μm2, the reservoirs are classified as extra low-porosity and extra low-permeability reservoirs. Seismic response displays the features of low frequency, weak amplitude and chaotic seismic reflection. The well-logs for the gas reservoirs exhibit the properties of high neutron porosity, low density and low resistance.