四川盆地东北部元坝地区下三叠统飞仙关组储层主要为台地相鲕粒滩灰岩沉积.以研究区6口井岩心观察描述、15块岩心薄片及200块岩屑薄片鉴定为基础,结合扫描电镜及电子探针等分析手段,识别出元坝地区飞仙关组鲕粒灰岩成岩过程中的五种暴露标志:大气淡水方解石胶结物、溶蚀现象(鲕粒内溶孔充填淡水方解石)、含砾屑鲕粒灰岩(砾屑为海滩岩的破碎产物)、渗流黏土及渗流粉砂、褐铁矿化现象.综合区域地质及构造演化,认为飞仙关期二时末期海平面下降,使原沉积区暴露地表,经受大气淡水淋滤和溶蚀.这些暴露现象可作为识别碳酸盐岩层序界面的重要标志,也可以对精细层序的划分进行印证.暴露溶蚀作用易形成大量的次生孔隙,有利于优质储层的形成.

Lower Triassic Feixianguan reservoirs mainly consist of platform oolitic beach limestone in Yuanba area in the northeastern part of Sichuan Basin. On the base of macroscopic observation of core and lithic thin sections from 6 wells, combining with the analysis of SEM and EP, five exposure indications can be recognized in wide-distributing Feixianguan oolitic limestone during diagenesis: 1) meteoric freshwater calcite cementation, 2) dissolution (filling freshwater calcite within oolitic dissolution pores), 3) gravelly oolitic limestone (gravels are the broken beach rock), 4) vadose clay and silt and 5) ferritization of ooids. Combined with the characteristics of regional geological and tectonic evolution, it is shown that sea level decline during the end of Feixianguan 2 Member sedimentation had made the primary sedimentary region expose to surface and undergo meteoric freshwater leaching and dissolution. Contributing to exposure and dissolution, a great amount of secondary pores develop increase which plays an important role in constructing excellent reservoirs. These exposure indications can be also used to recognize the sequence interfaces in carbonate rock and confirm the division of fine sequences.