四川盆地东部二叠系茅口组层序地层特征及地质意义

Sequence stratigraphic characteristics and geological significance of Permian Maokou Formation in eastern Sichuan Basin

充分利用40多口探井的岩心、录井和测井等资料,对川东二叠系茅口组进行层序地层划分,剖析层序界面特征,建立等时层序地层格架,在此基础上总结地层充填规律,探讨了层序格架中的沉积古地理格局和有利储集区带分布。识别了茅口组3个Ⅰ型层序界面和3个Ⅱ型层序界面,提出茅口组划分为5个三级层序的新方案。茅口组层序地层总体上具有下部超覆、顶部削蚀的特征,下部的SQ_(1)层序由东北往西南向海西期古隆起和古地貌高地超覆,顶部的SQ_(5)层序因东吴运动有不同程度缺失。层序格架内地层总体上表现出东西分异、厚薄相间的分布趋势:SQ_(3)、SQ_(4)层序在研究区中部均发育一个成带状展布的地层增厚带,可能对应于弱镶边的台缘滩相带,该带以东为地层减薄区,与发育孤峰段泥页岩、硅质页岩的深水沉积区相吻合,展现出从西往东水体逐渐变深、由碳酸盐台地到陆棚的古地理格局。指出层序地层格架内有利储集相带与Ⅰ型层序界面下早成岩期岩溶改造的叠合,有利于茅口组形成富有特色的岩溶型石灰岩储层。

By making full use of the core and logging data of more than 40 exploration wells,this paper divides the sequence stratigraphy of the Permian Maokou Formation in eastern Sichuan Basin,analyzes the characteristics of the sequence boundary,establishes the isochronous sequence stratigraphic framework,summarizes the stratigraphic filling law,and discusses the sedimentary-paleogeographic pattern and the distribution of favorable reservoir areas in the sequence framework.Three type I sequence boundaries and three type II sequence boundaries of Maokou Formation are identified,and a new scheme for dividing Maokou Formation into five third-order sequences is proposed.The sequence stratigraphy of Maokou Formation generally has the characteristics of bottom overlap and top erosion.The bottom of SQ_(1) overlap from northeast to southwest towards Hercynian paleouplift and paleogeomorphic highland,and the top of SQ_(5) is missing in varying degrees due to the Dongwu movement.Generally,the strata in the sequence framework show the distribution trend of east-west differentiation and alternating thickness:a banded stratigraphic thickening zone is developed in the middle of the study area for SQ_(3) and SQ_(4) sequences,which probably correspond to shoal belt of the weakly bordered platform margin.To the east of this zone is the stratigraphic thinning area,which is consistent with the deep-water sedimentary area where mudstone,shale and siliceous shale are developed in Gufeng section.It shows the paleogeographic pattern of gradually deepening from west to east and from carbonate platform to shelf.It is pointed out that the superposition of favorable reservoir facies belt in sequence stratigraphic framework and early diagenetic karst transformation under type I sequence boundary is conducive to the formation of characteristic karst limestone reservoir in Maokou Formation.