SEDIMENTARY FACIES OF CHANGXING FORMATION AND DISTRIBUTION OF ORGANIC REEF GAS RESERVOIRS IN NORTHERN SICHUAN BASIN

Depositional environment greatly diversified during Late Permian Changxing age in northern Sichuan Basin.Based on the features of deposition,several sedimentary facies are recognized in Changxing Formation,including deep carbonate ramp,open platform,trough,shelf(platform) margin,and a sedimentary model was established for the Changxing Formation.The sedimentary facies in northeastern and northwestern Sichuan Basin had evolved from carbonate ramp into open carbonate platform in Changxing age.In Kaijiang-Liangping ocean trough facies region,fossil fragments with deep water characteristics,such as specula,radiolarians and micro-foraminifers of the Late Permian Epoch,were identified in the interval with high-gamma rays and low-resistivity that was formerly included in the lower part of the Lower Triassic Feixianguan Formation,thus this interval should be included in the Dalong Formation.The shelf(platform) margin facies around troughs are the most favorable facies belts for exploration of organic reef gas reservoirs and it extends hundreds of kilometers crossing the whole northern Sichuan Basin.The oolitic banks of the Triassic Feixianguan Formation commonly occur on the organic reef of the Permian Changxing Formation.There are 7300 km2 marginal facies available for exploration of reef-bank gas reservoirs in the superimposed area of the Permian and Triassic reefs and banks,and a total of over 600 bcm gas reserves have been discovered and the undiscovered potentials are still large in this area.