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

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