北部湾盆地涠西南凹陷C洼湖底扇沉积特征及控制因素分析

Sedimentary Characteristics and Controlling Factors of Sublacustrine Fans in Sag C,Weixinan Depression,Beibuwan Basin

基于岩心、测井和三维地震资料分析,对北部湾盆地涠西南凹陷C洼湖底扇沉积特征及其控制因素进行了系统的分析,建立了研究区湖底扇发育模式。研究区重力流沉积类型主要有滑塌、砂质碎屑流、浊流3种沉积类型,受涠西南低凸起物源、外界触发机制以及古地貌的共同影响,多期重力流在研究区湖盆形成湖底扇。近源区湖底扇沉积以滑塌和厚层砂质碎屑流为主。湖底扇中部主要充填大套砂质碎屑流成因的厚层砂岩。随着流体运移和稀释,砂质碎屑流逐渐转变成远端浊流。在湖底扇末端主要沉积正粒序浊积岩和湖相泥岩。在高精度层序地层格架的约束下,研究区流沙港组一段下亚段湖底扇砂体可划分为5个期次,即U1～U5。受5号断层带差异性活动影响,5期砂体垂向叠置,平面上不断迁移摆动。早期北部分支断层活动较强,砂体主要沉积在分支断层下降盘;后期随着南部主干断层活动性增强以及北部分支断层活动性降低,砂体厚度中心逐渐向南迁移。

Based on cores,logging and 3Dseismic data,the sedimentary characteristics and controlling factors of the Sag C were systematically analyzed,and the sedimentary model of the sublacustrine fan was established for Weixinan Depression,Beibuwan Basin.There are three sedimentary types of gravity flow deposition in the study area,slump,sandy debris and turbidity.Under the control of the provenance of Weixinan low uplift,external triggering mechanisms and paleogeomorphy,multiphase gravity flows generated sublacustrine fans in the study area.Deposits in the proximal part of the sublacustrine fan consist mainly of slump and massive sandy debris flows.Deposits at the middle part of the fan are characterized by massive sandy debris flows.As the fluid migrates and dilutes,the sandy debris flows gradually turn into distal remote turbidity currents.Deposits at the end of the subaqueous lacustrine fans are mainly graded turbidites and lacustrine mudstones.Under the constraint of high resolution sequence stratigraphic frame-work,the sublacustrine fans could be divided into five periods,including U1-U5.Due to the difference activity of the No.5fault zone,the five-stage sand bodies are vertically stacked and constantly migrating and swinging in the plane.In the early period,the sand bodies were mainly deposited on the hanging wall of the northern fault because of the strong fault activity.In the later period,as the activity of the southern main fault increased and the activity of the northern branch fault decreased,the center thickness of the sand body gradually migrated southward.