涠西南凹陷陡坡带流一段上亚段异重流沉积新发现

.New discovery of hyperpycnal flow deposits in the El1x section of the steep slope belt in the Weixinan sag

以涠西南凹陷北部陡坡带中段流一段下亚段为研究对象,综合利用三维地震、测井、录井、钻井岩心及分析测试等资料,对研究区异重流成因深水扇的沉积特征、控制因素及沉积模式进行了研究。结果表明,断陷盆地陡坡带至湖盆底部可发育洪水型异重流沉积,依次为顺直水道、弯曲水道和朵叶体,其延伸距离约为15km,宽度为50~150m。其中,水道地震反射明显强于周围深水泥岩,其内部沉积以砂岩和含砾砂岩为主;朵叶体沉积主要为细砂岩或粉砂岩,富含陆源植物碎屑,其粒度概率曲线以上拱弧式及宽缓上拱式为主,样品点分布大致平行于C=M基线,指示重力流沉积动力特征。垂向上,单层砂岩一般表现为逆粒序与正粒序成对出现,中部粒度最粗,可见泥质碎屑呈叠瓦状排列,且发育层内微侵蚀面。此外,异重流沉积内部发育块状层理、平行层理、波状交错层理及爬升砂纹层理等沉积构造。“源-汇”系统决定了异重流的沉积特征与展布,万山隆起东部岩浆岩等物源输入是异重流形成的物质基础,强构造运动、陡坡折带、湿润-半湿润气候及低湖水密度是控制异重流发育程度的主要因素。研究区异重流的发现不仅丰富了断陷湖盆陡坡带沉积相类型,也为陡坡带进一步油气勘探提供了理论依据。

In a case study in the lower part of the first member of the Liushagang Formation(El1x)in the middle of the steep slope zone in the Weixinan sag,we examined the sedimentary characteristics,controlling factors and distribution features of deep-water fan formed by hyperpycnal flow through analyzes of 3 D seismic information,well logging and core drilling data and laboratory measurements.The results show that hyperpycnites are distributed from the steep slope to the bottom of lacustrine basin,where hyperpycnal flow developed sequentially straight and meandering channels and lobes which expand 15 km long and 50-100 m wide.The seismic reflections of hyperpycnites are obviously stronger than that of the surrounding deep-water mudstones.Hyperpycnites are mainly composed of fine-grained sandstone,siltstone and small amounts of pebbly sandstone;terrigenous biolastics(plant material)are also common.The grain size accumulation curve is mainly overarching and the sample point distribution is roughly parallel to the baseline of C=M,showing the characteristics of gravity flow deposits.A bed of sandstone is usually composed of paired inverse and normal grading sequences,with the coarsest grain depositing in the middle of the bed.Rip-up mud clasts are typical with imbrication.The intrasequence weak erosional contacts are also obvious.In addition,hyperpycnal flow developed sedimentary structures of massive and parallel bedding,Hummocky crossstratification and climbing sand ripples by bed-and suspended-load transportation.Since sedimentary characteristics and distribution of hyperpycnites are determined by source-to-sink system,magmatic rocks originated from eastern Wanshan uplift provided the material basis of hyperpycnites,while strong tectonic movements,steep slope,wet-half humid climate and relatively low density of water are the main controlling factors for the development of hyperpycnites.Our discovery of hyperpycnal flow deposits in the study area not only enriches our knowledge of sedimentary environment in the steep slope belt in lacustrine basin,but also provides theoretical guidance for further hydrocarbon exploration in steep slope regions.