珠江口盆地荔湾井区珠江组深水扇沉积特征

Deepwater fan sedimentary characteristics of Zhujiang Formation in Liwan area of Zhujiang River mouth basin

珠江口盆地白云凹陷荔湾深水区为油气勘探有利区域,通过岩石学和古生物组合特征研究,认为该区珠江组下部属于典型的深水扇沉积,可划分为早期盆地扇和晚期斜坡扇两种类型,两类深水扇都以发育颗粒流、砂质碎屑流及低密度浊流等顺坡重力流沉积为主,同时夹有少量内波和等深流等深水牵引流改造沉积。砂质碎屑流为两类深水扇水道砂体的主要沉积类型,发育有逆-正粒序层理、平行层理和水平层理,粒序层内可见各种丰富的液化变形和生物逃逸构造,而水平层内发育有更多的生物钻孔和扰动现象。两类深水扇的沉积构造和古生物特征有明显差异,其中盆地扇水道砂岩中普遍含硅质小砾石,水道间泥岩中含有较多保存完好的抱球虫化石,斜坡扇水道砂岩以富含炭泥屑为典型特征,水道间泥岩含有更丰富的颗石藻和抱球虫化石,其中部分抱球虫和颗石具有遭受海底溶蚀作用的现象,指示斜坡扇相对盆地扇有更大的水体深度。平面上两类深水扇具有相似的重力流沉积的分带性,都具有自陆架坡折向盆地方向由颗粒流沉积逐渐向砂质碎屑流和近源高密度浊流、远源低密度浊流等单向演化的特点。

Liwan deepwater area located in Baiyun Sag of Zhujiang River mouth basin is a potential area for oil and gas exploration.The Lower Member of Zhujiang Formation in the study area is regard as a deepwater fan based on a detailed study of petrologic and paleontological assemblage characteristics.The deepwater fan in Liwan area can be divided into two types,i.e.,early basin fan deposit and late slope fan deposit.Both types of the deepwater fan have developed granular flow,sandy debris flow,low-density turbidity flow and other gravity currents along the slope,with the development of a little internal wave and contour current deposit.Sandy debris flow is the mainly sedimentary type of channel sand bodies of the two deepwater fans,and there are also reverse grading,positive grading,parallel bedding and horizontal bedding.In addition,many liquefaction and fluidization structures and biological escape trace in graded bedding as well as abundant burrow pores and bioturbation in horizontal bedding are observed.The two types of deepwater fan have different sedimentary and paleontological characteristics.There is silicious pea gravel in channel sandstone and globigerina fossil in interdistributary bay mudstone of the basin fan.Slope fan deposit contains rich coccoliths and globigerina fossil,and is characterized by carbon mudstone scraps in channel sandstone.Coccoliths and globigerina fossil in the slope fan have obvious characteristics of submerged corrosion,which means that the slope fan has deeper water than the basin fan.The two types of deepwater fan have similar distribution of gravity currents,i.e.,the evolution from proximal granular flow to sandy debris flow,then to high-density turbidity flow and finally to low-density turbidity flow;in other words,from continental shelf slope turned directly to the basin.