基于成藏过程重建研究渤中凹陷西北缘陡坡带油气差异聚集机理

渤海湾盆地渤中凹陷西北缘陡坡带油气分布“西贫东富”,成藏条件复杂。利用储层定量荧光技术、傅立叶红外光谱技术、原油地球化学参数、流体包裹体岩相学,结合埋藏史、热史及构造演化,从古流体和现代流体特征角度进行系统定时定量化成藏过程恢复,研究差异成藏原因。研究表明,陡坡带经历了两期油气充注过程,第一期开始于明化镇组沉积早期(距今7 Ma),来自东南部沙河街组烃源岩的低熟油经历短距离运移在东区古近系成藏,该期原油充注量较少。第二期发生在明化镇组沉积中期(距今5.3 Ma),较高成熟度的烃类由东向西,由深向浅运移,在东、西区古近系形成大规模古油藏。明化镇组沉积晚期(距今3.7 Ma),边界断层活动性增强,西区断层活动速率强于东区,断层作用由封闭作用转为输导作用,西区古近系古油藏大规模破坏,东区东三段古油藏油水界面向上迁移,油气向浅层聚集成藏。第四纪沉积期(距今2.2 Ma),断层活动性减弱,逐步形成现今油藏。距离供烃源岩的远近、经历成藏期次的不同以及控藏断裂活动的强弱差异共同导致了渤中凹陷西北缘陡坡带油气差异分布的格局。

Mechanism of Differential Oil-gas Distribution in the Steep-slope Zone (Northwestern Margin of Bozhong Sag): Evidence from Reconstruction of Hydrocarbon Accumulation

Hydrocarbon distribution is low in the western part but high in the eastern part of the steep-slope zone in the northwestern margin of Bozhong sag (Bohai Bay Basin), and the reservoir-forming conditions are complex. Based on fluid inclusions petrographic observation, quantitative grain fluorescence technology, Fourier transform infrared spectroscopy (FTIR), and crude oil geochemical properties, and combined with the thermal, and burial history and tectonic evolution, the hydrocarbon accumulation process was unraveled from the paleo- and modern fluid characteristics. Besides, the reason of differential oil-gas distribution was also discussed. Our results define two hydrocarbon charging stages in the study area: stage one started at the early deposition of the Minghuazhen Formation (7 Ma), when the lowly-mature oil was sourced from the Shahejie Formation in the southeast. It has experienced short-range migration and formed reservoirs in the Paleogene sequences in the eastern part of the area. The charging scale is low at this stage. Stage two occurred in the middle deposition stage of the Minghuazhen Formation (5.3 Ma). The highly-mature hydrocarbons may have ascended from east to west, and formed large-scale paleo-oil reservoirs in the Paleogene sequences. During the late deposition stage of the Minghuazhen Formation (3.7 Ma), the activity of boundary faults was strengthened, and the activity rate in the western part of the area was higher than that in the eastern part. The faults may have changed from being hydrocarbon seal to fluid conduits. The Paleogene paleo-oil reservoirs in the western part of the area were largely destroyed, and the oil-water interface of the paleo-oil reservoir in the Dongying Formation (3^rd member) moved upward, with the hydrocarbon ascending to shallow formation. During the Quaternary sedimentation (2.2 Ma), the fault activity weakened and the current reservoirs were gradually formed. The hydrocarbon distribution pattern in the steep-slope zone of the northwestern margin of Bozhong sag are likely controlled by the distance from the source rock, the different reservoir formation stages, and the deformation intensity difference of the boundary faults.