四川盆地中部地区侏罗系沙溪庙组沉积体系演化及砂体发育特征

Sedimentary system evolution and sandbody development characteristics of Jurassic Shaximiao Formation in the central Sichuan Basin

四川盆地侏罗系沙溪庙组是致密气勘探的重点领域,但其沉积体系和砂体发育特征研究薄弱,制约了天然气的勘探开发。为进一步弄清沙溪庙组沉积体系演化和砂体发育特征,以四川盆地中部地区(以下简称川中地区)沙溪庙组为研究对象,综合利用岩心、薄片、测录井和地震资料,系统研究沙溪庙组沉积相微相和砂体成因类型。研究结果表明:①受东北部大巴山和米仓山逐渐隆升影响,四川盆地沙溪庙组沉积时期为北东—南西向的前陆盆地,盆地内地势宽缓,古水系主要呈北东—南西向,北部的大巴山和南部物源联合控制川中地区沙溪庙组沉积格局;②川中地区砂岩多见海绿石矿物,表明沙溪庙组沉积中心位于盆地中部,沙溪庙组气候干湿交替频繁,湖盆水体较浅且动荡,地形较为宽缓,具备形成浅水三角洲的沉积背景;③沙一段半干旱气候条件下发育的浅水三角洲多呈朵叶状,沉积相带展布宽广,平面表现为多个朵叶体叠合而成的复合体,砂体叠置连片发育,单期河道砂体宽度大,常见河口砂坝;④沙二段干旱气候条件下主要发育枝状分流河道型浅水三角洲,多呈鸟足状或树枝状,砂体顺河道呈条带状分布,河道较窄且相互切割,河口砂坝仅仅分布于河道的末端,单个规模较小。结论认为,系统分析了川中地区沙溪庙组沉积相的演化与分布规律,总结了沙溪庙组的沉积特征及砂体发育模式,对下步川中地区沙溪庙组致密气勘探具有重要指导意义。

The Jurassic Shaximiao Formation in the central Sichuan Basin is an important field of tight gas exploration,but its sedimen-tary system and sandbody development characteristics are less researched,which restricts the exploration and development of natural gas in this area.In order to further clarify the sedimentary system evolution and sandbody development characteristics of Shaximiao Forma-tion,this paper takes the Shaximiao Formation in the central Sichuan Basin as the research object to systematically study the sedimentary microfacies and sandbody genetic types of Shaximiao Formation by using core data,thin section analysis,logging and seismic data com-prehensively.And the following research results are obtained.First,due to the gradual uplifting of the Daba Mountain and the Micang Mountain on the northeast,the Sichuan Basin was a NE-SW trending foreland basin during the sedimentation of Shaximiao Formation,the topography in the basin was wide and gentle and the ancient water system was mainly in the NE-SW direction,so the sedimentary pattern of Shaximiao Formation in the central Sichuan Basin is under the joint control of the Daba Mountain on the north and the prove-nance on the south.Second,glauconite minerals are common in the sandstone of the central Sichuan Basin,indicating that the depocenter of Shaximiao Formation is located in the center of the basin.Shaximiao Formation is characterized by frequent alternation of dry and wet climate,shallow and turbulent lake basin and wide and gentle terrain,and has the sedimentary background to form shallow water deltas.Third,the shallow water deltas developed in the first Member of Shahejie Formation under the condition of semi-arid climate are mostly in the shape of lobate,with a wide distribution of sedimentary facies.In the plane,they are presented as a complex of multiple lobates,sandbodies are overlapped and developed continuously,single-phase channel sandbodies are wide and river mouth bars are common.Fourth,branched distributary channel-type shallow water deltas are mainly developed in the second Member of Shahejie Formation under the condition of arid climate,and they are mostly in the shape of bird’s foot or branch.Sandbodies are distributed in strips along chan-nels,which are narrow and cut each other.River mouth bars are only distributed at the channel end,with a small single scale.In conclu-sion,this paper systematically analyzes the evolution and distribution laws of sedimentary facies of Shaximiao Formation in the central Sichuan Basin and summarizes the sedimentary characteristics and sandbody development models of Shaximiao Formation,which is of great significance to the exploration of Shaximiao Formation tight gas in the central Sichuan Basin.