珠江口盆地西部文昌G区渐新统恩平组扇三角洲沉积特征与储层主控因素

Sedimentary characteristics and main controlling factors of the fan delta of Oligocene Enping Formation in Wenchang G block,Pearl River Mouth Basin

针对珠江口盆地西部文昌G区下渐新统恩平组多物源、多期次扇三角洲沉积纵向频繁叠置、横向非均质性强的特点,从构造-沉积充填演化特征入手,分析了断裂体系对每期扇三角洲的影响,并与沉积序列的复杂性相结合,分析了垂向相序变化和平面储层展布规律。纵向上,恩平组共划分为上、下2个三级层序,5个准层序组;平面上,上层序沉积早期发育的一系列NEE向断裂体系组成自南向北发育的一组断阶,这在一定程度上改变了当时的构造-沉积格局,从而影响了低位期三角洲的展布,使其分布范围最广。沉积机制和水动力条件是影响储层平面差异性的重要因素:首先,沉积相带是影响储层物性的先决条件,扇三角洲前缘水下分流河道和河口坝砂体较扇根部位砂体的储层物性明显变好,且多以原生粒间孔隙为主,次生孔隙为辅;其次,上层序湖侵期广泛发育的厚层泥岩形成相对超压,对深埋藏储层的孔隙具有一定的保护作用。厚层泥岩盖层之下的、受断阶影响的大型低位扇三角洲,为本区"甜点"储层的有利勘探方向。

Wenchang G block in the western part of the Pearl River Mouth Basin is adjacent to Shenhu uplift.During the early Oligocene Enping period,affected by the continuous activity of Zhusannan fault,multi-stage fan delta sedimentation with multiple sediment sources developed.This set of sedimentation has the characteristics of frequent vertical superposition and strong horizontal heterogeneity.Starting from the evolution of structure-sedimentary filling,this paper analyzes the influence of fault system on each stage of fan delta,and analyzes the vertical facies sequence change and plane reservoir distribution law considering the complexity of sedimentary inner.Vertically,Enping formation is divided into two sequences(upper and lower)and five subsequence sets.On the plane,a series of NEE faults developed in the early stage of upper sequence form a group of fault terraces developed from south to north,which changed the structure-sedimentary pattern at that time to a certain extent,and affected the distribution of delta in the lowstand stage,making it the most widely distributed.The sedimentary mechanism and hydrodynamic conditions are the important factors that affect the differences of reservoirs.Firstly,the sedimentary facies belt is the prerequisite to affect the physical properties of the reservoirs.The physical properties of the underwater distributary channel and mouth bar sandbody in fan delta front are obviously better than those in fan root of fan delta,with dominant primary intergranular pores supplementing by secondary pores.Secondly,the overpressure due to the thick mudstone of upper sequence would be helpful to the deep buried reservoir.It is indicated that the lowstand fan delta sealed by mudstone in Enping Member-1 would be the favorable exploration target.