川北地区长兴组沉积相和生物礁气藏分布规律

四川盆地北部地区晚二叠世长兴期沉积环境发生了明显分异，依据沉积特征可将其划分为碳酸盐深缓坡相、开阔台地相、海槽相、陆棚（台地）边缘相等单元，并以此建立了川北地区长兴期沉积模式。川东北区块和川西北区块在长兴期已由碳酸盐缓坡相演化为碳酸盐开阔台地相；在开江-梁平海槽相区内原划为下三叠统飞仙关组底部的高伽马低电阻层段应划入大隆组，在其中发现了晚二叠世具深水特征的骨针、放射虫、微体有孔虫及腕足等化石碎片。环海槽陆棚（台地）边缘带是寻找生物礁气藏的最有利相带，其贯穿整个川北地区，延伸数百公里；在二叠系长兴组生物礁相之上常有飞仙关组的鲕粒滩发育，二叠系、三叠系礁、滩叠合边缘相带可供勘探的面积达7300 km²，已获得天然气储量超过6000×10⁸m³，勘探潜力巨大。

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.