四川盆地东北部下三叠统飞仙关组碳酸盐蒸发台地沉积相

Sedimentary facies of evaporative carbonate platform of the Feixianguan Formation of Lower Triassic in northeastern Sichuan Basin

四川盆地早三叠世飞仙关早期的环境基本上继承了晚二叠世长兴期末的格局。在川东北地区演化为碳酸盐蒸发台地。飞仙关组蒸发台地为一套含硫酸盐的富白云岩的蒸发潮坪沉积层系。下部为薄层的含石膏、硬石膏晶体、团块及肠状石膏层的泥晶灰岩、微晶白云岩组成的小潮差蒸发潮坪层序。中部为层状鲕粒白云岩、鲕粒灰岩的潮下鲕粒滩相与萨布哈环境的富层状石膏岩的膏坪相、含石膏岩的膏云坪相组成的大潮差蒸发潮坪层序。上部为中薄层状富泥的含石膏质夹层的泥质泥晶灰岩、微晶白云岩组成的旋回性清楚的似蒸发潮坪层序。这三种层序的纵向组合在蒸发台地相区可以很好对比,尽管各层序厚度有明显变化。强烈白云石化的鲕粒白云岩是四川盆地飞仙关组大中型气藏的主要储层,依据鲕粒岩的相序特征可将其分为有障壁性质的台缘鲕粒坝和台内鲕粒滩两类。蒸发台地西缘的台缘鲕粒坝在飞仙关期基本稳定。东缘的台缘鲕粒坝在飞仙关中后期大幅度向盆地方向迁移。

The sedimentary environment of the Early Feixianguan Age of Early Triassic in northeastern Sichuan Basin mainly inherited the pattern of the Late Changhsingian Age of Late Permian. The northeastern Sichuan area is evaporative carbonate platform. The Feixianguan Formation of evaporative carbonate platform is a set of sulfate-bearing evaporative tidal sediments with aboundant dolomite.The lower part is a micro-tidal range evaporative tidal sequence consisting of thin-bedded gypsum-bearing, muddy-limestone and dolomicrite. The middle part is a set of large tidal range tidal sequence including oolitic shoal facies which is made up of bedded-oolite dolomite and oolite limestone, gypsum-tidal facies in sabkha environment which consists of bedded gypsolyte, and gypsolyte-bearing gypsum-dolomite tidal facies. The upper part is a set of evaporite quasi-tidal sequence with clear cycles which is made up of middle-thin-bedded, gypsum-bearing muddy limestone and dolomicrite.Vertical combination of these three sequences can be well correlated in evaporitive platform area even though thicknesses of each sequence are obviously different. Strongly dolomized oolite dolomites are the main reservoir of mediumlarge scale gas reservoir of the Feixianguan Formation in Sichuan Basin. According to its facies sequence features, the oolitic rocks are deposited in platform marginal oolitic bar and inner-platform oolitic bank. Platform marginal oolitic bar along the western margin of the evaporite platform was static during the Feixianguan Age, while the platform marginal oolitic bar along the eastern margin greatly shifted towards the basin during the Middle-Late Feixianguan Age.