Abstract:

Based on lithological statistics and core observation of a large number of wells, sedimentological analysis of outcrops, and seismic facies analysis, four lithofacies paleogeography maps with a system tract as a unit were prepared for Feixianguan Formation of Induan Age of Lower Triassic in Sichuan Basin and adjacent areas in a 3rd-order sequence stratigraphic framework by synthesizing regional geological data such as plate tectonic evolution. These demonstrated that Kaijiang-Liangping Basin mainly deposited in the 1st and 2nd Members of Feixianguan Formation, and evolved into intra-platform depression in the 3rd Member of Feixianguan Formation. In addition, the maps showed distribution and migration characteristics of oolitic shoals in the basin during different periods, and platform margin oolitic shoals were found in the outcrops of Mianzhu(periphery of the basin) and Lichuan(Hubei Province). Together these findings provided evidences for oolitic gasfield exploration in Sichuan Basin. The oolitic shoals were widely distributed in each system tract of Feixianguan Formation in Sichaun Basin, with largest oolitic shoals found in the main gas-producing zones, i.e., TSQ1-HST(2nd Member of Feixianguan Formation) and TSQ2-TST(3rd Member of Feixianguan Formation). The oolitic shoals showed continuous migration from the intra-platform to the basin facies. In the TSQ1-TST(1st Member of Feixianguan Formation), oolitic shoals were mainly distributed in the intra-platform paleotopographic highland and littoral zones, with small-scale oolitic shoals(bars) formed on the rimmed platform margin along both sides of Kaijiang-Liangping Basin. In the TSQ1-HST, oolitic shoals(bars) were concentrated along both sides of Kaijiang-Liangping Basin and platform margins of southwestern Chengkou-E’xi Basin. In the TSQ2-TST, large-scale oolitic shoals were developed in the northern and eastern Sichuan. In the TSQ2-HST(4th Member of Feixianguan Formation), oolitic shoals were locally distributed and formed through the sedimentation of restricted platform and evaporite platform. The main favorable exploration zones of the Lower Triassic Feixianguan Formation in Sichuan Basin are the dolomitized platform margin shoal facies along Kaijiang-Liangping Basin, and the dolomitized platform margin shoal facies along the southwestern Chengkou-E’xi Basin and dissolved intra-platform shoal facies.

Key words: sequence stratigraphy, lithofacies paleogeography, favorable exploration zone, Feixianguan Formation, Lower Triassic, Sichuan Basin

摘要：

通过大量钻井岩性统计、岩心观察、露头沉积相研究与地震相分析，结合板块构造演化等区域地质资料，以体系域为单元编制了4张三级层序地层格架下的四川盆地及邻区早三叠世印度期岩相古地理图，明确了开江-梁平盆地相主要发育于飞仙关组一段及二段沉积期，在飞仙关组三段沉积期已演化为台内洼地，以及不同时期盆地内部鲕粒滩体的展布与迁移特征，且在盆地周缘绵竹、湖北利川等露头发现了台地边缘鲕粒滩，为四川盆地飞仙关组鲕滩气藏的勘探提供依据。四川盆地飞仙关组各体系域都有鲕粒滩体分布，其中层序TSQ1高位体系域（相当于飞仙关组二段）及层序TSQ2海侵体系域（相当于飞仙关组三段）沉积期鲕滩规模最大，也是主要产气层段，滩体展布具有由台地内部不断向盆地相区迁移的特征。层序TSQ1海侵体系域（相当于飞仙关组一段）沉积期，开江-梁平盆地相两侧的镶边台地边缘鲕粒滩（坝）规模不大，鲕粒滩体主要分布于台内古地貌高地以及滨岸地带;层序TSQ1高位体系域沉积期，鲕粒滩（坝）沿开江-梁平盆地相两侧及城口-鄂西盆地相西南缘台地边缘集中分布;层序TSQ2海侵体系域沉积期，川北、川东地区发育鲕粒滩体且规模较大;层序TSQ2高位体系域（相当于飞仙关组四段）沉积期，滩体分布较少，主要为局限台地和蒸发台地沉积。四川盆地下三叠统飞仙关组天然气有利勘探区带主要是环开江-梁平盆地白云石化台缘滩相、城口-鄂西盆地西南缘白云石化台缘滩相以及溶蚀的台内滩相。

关键词: 层序地层, 岩相古地理, 有利勘探区带, 飞仙关组, 下三叠统, 四川盆地