北部湾盆地涠西南凹陷雁列断层变换带发育特征及其控储作用--以古近系流沙港组一段重力流沉积为例

Development pattern of en echelon fault transition zone in Weixi′nan Sag,Beibu Gulf Basin and its control on hydrocarbon accumulation--A case study of gravity flow deposits in the Palaeogene Liu 1 Member

北部湾盆地涠西南凹陷二号断裂带由一组雁列式正断层组成,断层相互叠接部位形成大型变换带,前人对该大型变换带的控储作用一直缺乏系统的研究。以三维地震和钻井资料为研究基础,对二号断裂变换带的展布特征、演化过程及控储作用开展了系统分析。二号断裂主要受新生代两期伸展活动的影响:早始新世,受北西-南东向伸展应力控制,基底断层复活,形成北东向的单一、连续大断层;渐新世中、晚期,伸展应力转为近南北向,基底断层活动减弱,上部盖层内发育分段式、雁列式断层,断层联接部位形成变换斜坡,并控制了B洼流沙港组一段的重力流沉积体发育。西北物源大型三角洲经由各分段断层间的变换斜坡进入B洼,在重力作用下发生二次搬运形成富砂湖底扇,钻井证实B洼流沙港组一段重力流沉积体呈北东向平行于二号雁列式断裂分布,沉积类型主要为砂质碎屑流。该认识为深洼带流沙港组一段岩性圈闭的勘探提供了理论依据,并取得了较好的勘探成效。

The No.2 fault zone of the Weixi'nan Sag in the Beibu Gulf Basin is composed of a group of en echelon normal faults,with a large transition zone formed at the contact parts of faults in overlapping.However,there has been a lack of systematic study on the effect of the transition zone on hydrocarbon accumulation.Based on 3D seismic and drilling data,this study systematically analyzes the distribution pattern and evolution of No.2 fault transition zone,and its role in controlling hydrocarbon enrichment.As affected by two stages of extensional activities since the Cenozoic,the development of No.2 fault transition zone featured resurrection of basement faults in the early Eocene as controlled by NW-SE transtensional stress,resulting in a continuous single fault of large scale;while in the mid-to-late Oligocene,the transtensional stress turned to strike SN,accompanied by weakened basement fault activities,resulting in segmented en echelon faults developed in the upper caprock,and the contact parts of faults growing to be transition slope serving to control the development of gravity flow deposits in the first member of Liushagang Formation(Liu 1 Member)of B low.At the sedimentary period of Liu 1 Member,the large-scale delta sourced from the NW entered B low through the transition slopes of the segmented faults and formed sand-rich sub-lacustrine fan under the secondary transportation with gravity.In addition,drilling data confirm that the gravity flow deposits in B low is mainly of sandy debris flow,trending NE in parallel with the No.2 fault zone in en echelon arrangement.The understanding provides theoretical guidance in the exploration of lithologic traps in the study area with good effects achieved in exploration.