北部湾盆地海中凹陷涠三段沉积体系与沉积模式

Depositional systems and model of the third member of Weizhou Formation in Haizhong Sag,Beibuwan Basin

北部湾盆地海中凹陷渐新世涠三段是该凹陷最主要的储集层,因此对其沉积体系发育规律的研究对于该地区的油气勘探具有重要的意义。从研究区钻井岩心精细描述入手,结合测井相、地震相、地震属性特征,确定了涠三段的沉积相并建立了断陷湖盆背景下的沉积模式。研究表明:碟状—席状反射、前积反射、水道充填反射是涠三段最富砂的地震相。涠三段发育滨浅湖相、辫状河三角洲、扇三角洲、冲积扇4种沉积相类型。在滨浅湖背景下,受控于南缓北陡的半地堑湖盆结构,海中凹陷整体呈现出南北分异的沉积特征,表现为在南部缓坡带发育辫状河三角洲,在北部发育辫状河三角洲与扇三角洲;同时坡折带、断裂样式、沟渠古地貌等因素控制着来自三大不同方向物源的三角洲,南部企西隆起物源主要发育辫状河三角洲,并受坡折带控制在平面上划分出平原与前缘,中部涠西南低凸起物源受断裂控制在西侧发育辫状河三角洲与扇三角洲,西北部万山隆起物源的沉积物通过远距离搬运在研究区西北侧湖盆边缘形成辫状河三角洲。

The third member of Weizhou Formation of Oligocene is the most important reservoir in Haizhong Sag, Beibuwan Basin, so it is very important to study the distribution regulation of depositional systems for oil and gas exploration in the area. Based on the fine description of drilling cores in the study area, combined with the characteristics of logging facies, seismic facies and seismic attributes, the depositional facies were determined and the sedimentary models in faulted lacustrine basins is established. The results show that the discalsheeted reflection, progradational reflection and channel filling reflection are the most sand-rich seismic facies in the third member of Weizhou Formation;There are four types of depositional facies: Shore-shallow lacustrine facies, braided river delta, fan delta and alluvial fan in the third member of Weizhou Formation;Under the background of shore-shallow lake and controlled by the half-graben lake basin structure with gentle southern and steep northern slope, the Haizhong Sag as a whole shows the sedimentary characteristics of north-south differentiation, which is manifested by the development of braided river delta in the gentle slope zone in the south, braided river deltas and fan deltas was developed in the north;At the same time, factors such as slope break zone, fault style and gully ancient landform control the deltas from three different directions, and the provenances of Qixi uplift in the south mainly develop braided river deltas. The plain and front are divided on the plane by the slope break zone, and the source of the Weixinan low uplift in the middle of the Beibuwan Basin is controlled by the fault to develop braided river deltas and fan deltas in the west, the sediments from Wanshan uplift in northwest formed a braided river delta by long-distance transport in the edge of Lake basin in the northwest of the study area.