东营凹陷深层古流体压力演化过程及其地质意义

渤海湾盆地东营凹陷深层气藏(包括凝析气藏)现今地层流体压力变化大,既有常压,也有超压,未能真实反映古流体压力的变化特征。为明确该区地层压力变化与油气成藏的关系,在分析现今地层压力的基础上,根据流体包裹体均一温度、气液比和流体组分测定等资料,采用VTFLINC古流体压力热动力学模拟软件来拟定古流体压力,结合古埋藏史埋深所对应的古静水压力,得到古流体压力系数和捕获年龄,构建了该区地层古流体压力的演化过程,并剖析了古流体压力演化的两种作用机制及其地质意义。结论认为:1该区深部地层的古流体压力经历超压—泄压—超压和超压—泄压—常压两种演化过程,非平衡压实作用和生烃增压作用是深层流体压力增大的主要原因;2早期欠压实作用保存了储层孔隙,有利于后期凝析气充注,形成流体压力系数高、油气产能高的凝析气藏,此类凝析气藏是该区天然气勘探的重点对象;3早期生烃作用与油气充注形成的古油藏经后期高温作用,重烃裂解形成凝析气藏和常规气藏,该类气藏亦具有重要的天然气勘探价值。

Paleo fluid pressure evolution process in deep layers in the Dongying Sag and its geological significance

The present day fluid pressure in deep gas reservoirs (including condensate gas reservoirs) in the Dongying Sag, Bohai Bay Basin, show a very large variance from normal pressure to overpressure, making it difficult to reflect the true variation of paleo fluid pressure. In order to reveal the relationship between formation pressure change and hydrocarbon accumulation in the study area, we first performed a thorough analysis of the present day formation pressure. And then through the integration of homogenization temperature of fluid inclusions, gas liquid ratio and fluid composition testing data, we studied the paleo fluid pressure by using the VTFLINC paleo fluid pressure thermodynamic simulation software. In combination with the paleo hydrostatic pressure corresponding to burial history and burial depth, we calculated the paleo fluid pressure coefficient and trapping time of fluid inclusion, constructed the evolution process of paleo fluid pressure, and analyzed the two mechanisms of paleo fluid pressure evolution and their geologic significances. The following conclusions were obtained. (1) The paleo fluid pressure of the deep layers experienced two evolution processes, namely overpressure pressure releasing overpressure and overpressure pressure releasing normal pressure. Compaction disequilibrium and hydrocarbon generation are the two major causes of fluid pressure increasing in the deep layers. (2) Reservoir pores were well preserved due to the early undercompaction, which was favorable for the charging of condensate gas in the late stage to form condensate gas reservoirs with high fluid pressure coefficient and high production capacity. This kind of condensate reservoirs is the major target of gas exploration in the study area. (3) The paleo oil reservoirs that were resulted from early hydrocarbon generation and charging experienced high temperature in the late stage, causing the paralysis of heavy hydrocarbons and the formation of condensate gas and conventional gas reservoirs. These two kinds of gas reservoirs are also the important targets of gas exploration.