井研—犍为地区麦地坪组—筇竹寺组沉积相特征

Sedimentary Facies Characteristics of Maidiping-Qiongzhusi Formation in Jingyan-Qianwei Area

井研—犍为地区麦地坪组—筇竹寺组是四川盆地下寒武统页岩气勘探的重要阵地,但沉积特征研究相对薄弱。通过岩心描述和测井资料分析对研究区麦地坪—筇竹寺组进行沉积相特征分析及总结。麦地坪组发育深灰—灰黑色磷质细晶白云岩、灰黑色—黑色含粉砂质页岩与硅质磷块岩互层,硅质结核及水平层理发育,顶部见灰色—深灰色岩屑石英粉砂岩与灰黑色粉砂质泥岩互层,泥岩与粉砂岩突变接触,发育冲刷面、包卷层理、泄水构造以及粉砂岩中的泥粒定向排列,为典型重力流沉积;筇竹寺组发育灰黑色—深灰色页岩、粉砂质页岩与灰色—深灰色钙质细粒岩屑砂岩、灰质粉砂岩、泥质粉砂岩不等厚互层,块状及水平层理,顶部见波状层理和小型交错层理,黄铁矿发育,三叶虫和高肌虫化石发育。建立了2个三叶虫化石带:尖头遵义盘虫(Tsunyidiscus acutus)—阎家寨遵义盘虫(Tsunyidiscus yanjiazhaiensis)带和始莱得利基虫(Eoredlichia)—武定虫属(Wutingaspis)带,以及3个高肌虫化石带:梁山虫(Liangshanella)—小遵义虫(Tsunyiella)化石带、峨眉虫(Emeiella)—粗脊昆明虫[Kunmingella(Validocostatella)fusulus]化石带和朵氏昆明虫(Kunmingella douville)—卵形川滇虫[Chuandianella ovata(Lee)]—简单虫(Mononotella)化石带。研究区受兴凯地裂运动控制,位于坳拉槽西侧缓坡带中,为陆棚相沉积。识别出浅水陆棚、深水陆棚和坳拉槽3个亚相,以及滩坝、砂泥质浅水陆棚、浊积砂、砂泥质深水陆棚、泥质深水陆棚和坳拉槽底共计6个沉积微相。麦地坪组主要发育构造作用下的深水沉积,其顶部发育重力流沉积,富含磷元素,含少量黄铁矿;筇竹寺组发育较稳定构造背景下的陆棚沉积,具有以泥岩沉积为主,磷元素含量低,局部富含黄铁矿等特征。

Maidiping-Qiongzhusi formation in Jingyan-Qianwei area is an important position in the Sichuan basin for lower Cambrian shale gas exploration.But the study of sedimentary characteristics of this formation in the area is relatively weak.In this paper,the sedimentary facies features of Maidiping-Qiongzhusi formation in the study area are analysed and summed up by core description and analysis of conventional logging data.Mai Diping formation is comprised of the dark gray and gray black phosphorus fine dolomite,the interbed of greyblack black containing silty shale and siliceous phosphorite,siliceous nodules and horizontal bedding.The top of Maidiping formation is comprised of the interbed of grey and dark grey lithic quartz siltstone and grey silty mudstone.The contact relation between mudstone and siltstone is abrupt.The scour surfaces,convolute beddings,water discharge structures and directional mud particles are typical in gravity flow deposit.Qiongzhusi formation is comprised of the unequally thick interbed of black and dark grey shale or silty shale and gray dark gray calcareous fine lithic sandstone,siltstone or argillaceous siltstone,massive and horizontal bedding.There are wavy beddings,small cross beddings,pyrites and fossils of Trilobites and Bradoriids in the top of Qiongzhusi formation.Two Trilobites fossil zones are divided：Tsunyidiscus acutusTsunyidiscus yanjiazhaiensis and Eoredlichia-Wutingaspis.The Bradoriids fossil zones are divided into three parts：Liangshanella-Tsunyiella,Emeiella-Kunmingella（Validocostatella） fusulus and Kunmingella douville-Chuandianella ovata（Lee）-Mononotella.The research area is located in the west slope zone in the aulacogen which is controlled by xing Kai taphrogeny.It is the shelf facies deposits.In this area,there are three sedimentary subfacies including shallow shelf,deep shelf and aulacogen.Additionaly,there are six microfacies which are beach bar,sand and mud of shallow continental shelf,turbidite sand,sand and mud deep shelf,mud deep shelf and bottom of aulacogen belong to the three subfacies.The Maidiping formation mainly developed deepwater sedimentary structures under the action of the top developed gravity flow deposits,rich in phosphorus,and a small amount of pyrite.In the other part,Qiongzhusi formation stable shelf sedimentary tectonic background,with mudstone sediments.phosphorus content is low.And there is a characteristics of the rich pyrite in local in this formation.