Abstract: The ultra-deep Mesozoic volcanic trap group in the surrounding area of Bozhong sag has great exploration potential for forming large and medium-scale volcanic oi-gas reservoirs. In recent years, both Well BZ8-3S-A and Well BZ8-3S-B deployed in the Bozhong8-3 South structure in the southwest low uplift zone of the sag have achieved high-yield oil-gas flow from volcanic reservoirs. This breakthrough marks the new exploration progress of large and medium-scale volcanic hydrocarbon reservoirs in the deep subsags of Bohai Sea. Based on the analysis of the core, thin section and source rock, in combination with well logging and seismic interpretation data, the paper systematically investigates the formation conditions and accumulation characteristics of large-scale volcanic oil-gas reservoirs in Bozhong sag, and also explores the seismic prediction methods of favorable volcanic reservoirs. The Mesozoic Yixian Formation volcanic rocks in Bohai Sea consist of 4 eruption cycles, and the large-scale acidic volcanic reservoir represented by the Bozhong8-3 South structure are mainly constructed by the third eruption cycle. The Bozhong8-3 South structure exhibits favorable hydrocarbon accumulation conditions as below. (1) The Mesozoic volcanic buried-hill reservoir is adjacent to three sets of high-quality Paleogene source rocks in the Member 1 and 3 of Shahejie Formation and Dongying Formation, which provide superior oil-gas supply conditions. (2) Four laterally stacked large acid volcanic edifices suffer from the strong superimposed transformation of weathering, tectonic activities and fluid flows, forming large-scale volcanic rock reservoirs. (3) There is a multi-dimensional hydrocarbon charging mode based on the combination of the lateral direct source-reservoir contact and unconformity migration, and the source rock formations with an overpressure coefficient of 1.5 to 1.8 can not only provide sufficient hydrocarbon migration driving force, but also enhance the sealing capacity of the mudstone overlying volcanic rocks. (4) The Bozhong8-3 South structure develops massive condensate-rich gas reservoirs with large gas-bearing formation thickness, high gas column height, high temperature and high pressure, late to ultra-late accumulation. Through the analysis of the seismic response characteristics of the drilled wells, the maximum amplitude and arc length attributes were optimized to predict the distribution patterns of weathering crust. Additionally, the multi-attribute cluster analysis was applied to predict reservoir development zones. On this basis, a seismic prediction technology was established for large-scale favorable volcanic reservoirs in the Bozhong8-3 South structure. The research methods and understandings are of important significance for guiding the exploration of ultra-deep volcanic reservoirs in Bohai Sea.

Key words: Bozhong sag, Bozhong8-3 South structure, volcanic buried hill, reservoir prediction, accumulation mode

摘要： 渤海海域环渤中凹陷超深层中生界火山岩圈闭群具备形成大—中型火山岩油气藏的巨大潜力。近年来,部署在渤中凹陷西南低隆起带渤中8-3南构造上的BZ8-3S-A井和BZ8-3S-B井在火山岩储层中均获得高产油气流,打开了渤海海域深洼区超深层大—中型火山岩油气勘探的新局面。基于井壁取心、岩石薄片分析和烃源岩测试分析数据,结合测井和地震资料,系统研究了渤中凹陷大型火山岩油气藏的形成条件与成藏特征,探讨了火山岩有利储层的地震预测方法。渤海海域中生界义县组火山岩由4个喷发旋回构成,以渤中8-3南构造为代表的大型酸性火山岩建造主要由旋回3构成。渤中8-3南构造具备良好的油气成藏条件:中生界火山岩潜山紧邻古近系沙河街组一段、沙河街组三段和东营组3套优质烃源岩层,具有优越的油气供给条件;4个侧向叠置分布的大型酸性火山机构受风化作用、构造作用、流体作用等多元要素的强烈叠加改造,可形成规模性火山岩储层;发育源-储侧向直接对接和不整合面运移相结合的多元立体油气充注模式,且烃源岩的地层压力系数达1.5~1.8,超压不仅提供了充足的油气运移动力,也增强了火山岩上覆泥岩的封盖能力;形成了渤中8-3南构造“气层厚度大、气柱高度大、高温高压、晚期—超晚期成藏”的高含凝析油的块状凝析气藏。通过已钻井地震响应特征分析,优选最大振幅属性和弧长属性预测风化壳的分布规律,并利用多属性聚类预测储层发育带,建立了渤中8-3南构造火山岩规模性有利储层的地震预测技术。研究方法与认识对于指导渤海海域超深层火山岩油气藏勘探具有重要意义。

关键词: 渤中凹陷, 渤中8-3南构造, 火山岩潜山, 储层预测, 成藏模式