断陷盆地缓坡区控砂控藏机制与勘探发现——以歧口凹陷歧北缓坡带为例

为揭示断陷盆地缓坡区的控砂控藏机制,以高品质的800km~2三维地震和91口钻井资料为基础,层序地层和沉积演化分析表明,受控于断块掀斜和差异沉降作用,歧口凹陷歧北斜坡南侧在古近纪表现为具挠曲坡折的缓坡带。该带层序界面标志清晰,包括削截、顶超和上超等反射终止类型,由此将古近系划分为3个二级层序和11个三级层序。三级层序的结构样式类似于具陆架坡折的被动大陆边缘,其低位体系域和湖扩体系域普遍发育不同类型的岩性圈闭。其中,沙河街组三段—沙河街组二段沉积期,研究区断陷作用强烈,挠曲坡折带的坡度较大,层序的低位体系域主要发育大型低位扇;随后,拗陷作用增强,挠曲坡折带的坡度逐渐减小,至东营组沉积期转变为以低位三角洲沉积为主。湖扩体系域的岩性圈闭发育在沙河街组二段—沙河街组一段和东营组,发育湖相碳酸盐岩滩坝和碎屑岩滩坝。研究区挠曲坡折带明显控制了地层-岩性圈闭的发育和分布,成为缓坡带油气勘探的有利场所。通过勘探实践,新增三级储量近4×10~8t,探井成功率达80%,展示了挠曲缓坡带地层-岩性油气藏良好的勘探前景。

Sand and reservoir controlling mechanism and exploration discovery in the gentle slope of fault basin:a case study of Q ibei slope in Q ikou sag

To reveal the sand and reservoir controlling mechanism in the gentle slope of fault basin, the sequence stratigraphy and sedimentary evolution were analyzed based on high-quality 3D seismic data covering an area of 800 km² and the drilling data of 91 wells, indicating a control from block tilting and differential sedimentation, and the south side of Qibei slope in Qikou sag was identified as the flexure slope break belt in the Paleogene. The sequence boundary marks of this zone are clear, including truncation, toplap, onlap and other reflection termination types. On this basis, the Paleogene was divided into 3 second-order sequences and 11 third-order sequences. The structural pattern of third-order sequence is similar to the passive continental margin with a shelf slope break, where different types of lithologic traps are commonly developed in the low-stand system tract and lacustrine transgression system tract. During the sedimentary period of Member 3 and Member 2 of Shahejia Formation, the faulting is intensive in the study area, the flexure slope break zone has a great gradient, and the large low-stand fans are mainly developed in the low-stand system tract of the sequence. Subsequently, the depressing effect is enhanced, and the gradient of flexure slope break zone is gradually reduced. Till the sedimentary period of Dongying Formation, the low-stand deltas are dominant. The lithologic traps of lacustrine transgression system tract are developed in the Member 2 and Member 1 of Shahejie Formation and Dongying Formation, where lacustrine carbonate and clastic beach bars exist. The flexure slope break zone in the study area is able to control the development and distribution of stratigraphic-lithologic traps significantly, providing a favorable site for oil and gas exploration in the gentle slope. Through exploration, the newly-added 3P reserves are approaching 4×10⁸ tons with a drilling success rate of 80%, showing the favorable exploration prospects of flexure slope break zone and lithologic reservoirs.