浅水湖盆三角洲岸线控砂机理与油气勘探意义——以川西坳陷中段蓬莱镇组为例

Mechanism of lake shoreline control on shoal water deltaic sandbodies and its significance for petroleum exploration:a case study of Penglaizhen Formation in the middle part of western Sichuan depression

川西坳陷在蓬莱镇组沉积期,地形平缓,构造活动稳定,水体较浅,发育了一套以浅水三角洲为主的沉积体系,受短期气候旋回的控制,具有"湖岸线控砂"的特点。此次研究,通过大量岩心观察,对蓬莱镇组浅水沉积环境进行了分析,综合利用野外露头和测、录井资料,阐明了在湖平面频繁变化的浅水条件下湖岸线控砂的机理。研究结果表明:1当湖平面处于高水位稳定期,受波浪改造作用,形成第一期平行岸线的河口坝-滩坝叠覆体;当湖平面处于低水位稳定期,形成第二期河口坝-滩坝叠覆体,这2期坝体在湖退体系域末期得以保存;2"河流侵蚀控厚砂,岸线迁移控薄砂"是研究区内浅水三角洲沉积砂体发育的主要特点;34条水位线控制着5个沉积相带发育区,稳定湖盆背景下发育2种岸线控砂模式;4除了分流河道砂体,受岸线控制的河口坝-滩坝叠覆体也可作为优势储层,是下一步勘探的主要目标。

The Late Jurassic Penglaizhen Formation encompasses shoal water deltaic sedimentary system in the middle part of western Sichuan depression,which was deposited under a tectonic-stable,gently-gradient and shoal water environment.Along with high-frequently fluctuated base level,the sandbodies were regularly distributed along the lake shoreline.Using an integrated approach of core,outcrop,and drilling log data,we analyzed the sedimentary environment and proposed the mechanism of shifting lake shoreline controlling shoal water deltaic sandbodies distribution in the study area.The results show that:1In the high stand base level stable period,re-constructed by the wave,the first complex bar was formed along the shoreline,in the low stand base level stable period,the second complex bar was formed along the relatively regressed shoreline,both complex bars were reserved at the end of lake regressive system tract;2 Within the shoal water deltaic sedimentary system,the fluvial controlled sandbodies are thick,while the shoreline controlled sandbodies are thin;3 Four water lines were defined,dividing the study area into five depositional districts,two shoal water deltaic sedimentary models in tectonic stable basin were set up;4 The lake shoreline dominated sandbodies were interpreted to be significant reservoir except for fluvial dominated sandbodies,which will be the main targets in the future exploration.