北部湾盆地涠西南凹陷流二段三角洲的沉积特征及油气勘探意义

Sedimentary characteristics and oil-gas exploration significances of the delta in Member Liu-2 of Weixinan Sag in Beibuwan Basin

长期以来,涠西南凹陷流二段一直被认为半深湖—深湖相沉积,岩性为单一的泥岩,缺乏良好的砂岩储层,此认识束缚了该区油气勘探。利用岩心观察、粒度分析和地震等资料进行综合分析,发现流二段中部存在三角洲相沉积。研究认为:三角洲相发育三角洲前缘分流河道、河口坝、席状砂等沉积微相;沉积物粒度概率曲线分为“跳跃—悬浮”2段式和“滚动—跳跃—悬浮”3段式两种类型;单井相上见多期三角洲朵叶叠置,地震相上见明显的大型下切河道和大型席状砂;物源供应量增加和湖平面降低是流二段中部发育三角洲的两大主控因素。三角洲砂岩被流二段厚层泥岩包裹极易形成岩性圈闭,并且砂岩储集层与烃源岩直接接触,砂体直接从烃源岩获得油气充注从而形成岩性油气藏。

Over a long time,Member Liu-2 in Weixinan Sag is considered all the time as semi-deep and deep lacustrine sedimentation,the lithology is the single mudstone and there is no much better sandstone reservoirs,this understanding has restrained the oil and gas exploration in this area.With the help of the comprehensive analysis of the core observation,grain size,seismic data and so on,the delta deposition in Middle Member Liu-2 was revealed.The study shows that the delta is well developed by the following sedimentary micro-facieses:delta-front distributary channel,mouth bar,sheet sand and son forth;the probability curve of the sediment grain size is divided into the following two types:two-segment“bouncing-suspension”or three-segment“roll-bouncing-suspension”;the multi-period delta lobe superimposition is shown in the single well facies,the large incised channels and sheet sand appear in the seismic facies;the provenance supporting amount increase and lake surface reduction are two main controlling factors of the development of the delta in Middle Member Liu-2.The delta sandstone is surrounded by the thick mudstone in Member Liu-2,and then the lithologic trap is extremely prone to form,the sandstone reservoirs are contacted directly by the hydrocarbon source rock and directly poured by the oil and gas from the source rock,and finally accumulated into lithologic reservoirs.