Abstract: Oil and gas production from Qingshankou Formation in Qijia-Gulong sag of Songliao Basin is closely related to formation overpressure. However, there is a lack of asystematic study for the distribution pattern and genesis of overpressure. Based on the measured formation pressure and that predicted from mud density data, this paper analyzes the pressure distribution characteristics across all the sections of Qingshankou Formation in Qijia-Gulong sag. Based on geological-geophysical log response characteristics, an analysis has been performed on the genesis of overpressure and the factors influencing overpressure preservation, followed by exploring the correlation between overpressure formation and oil accumulation in shale reservoirs. The research results show that Qingshankou Formation in Gulong sag, southern and northern Qijia sag exhibits distinct vertical pressure compartment structures of weak-strong overpressure, normal-weak-strong overpressure, and normal-weak overpressure. The maximum formation pressure coefficients of corresponding structure in the Member 1 of Qingshankou Formation are 1.82, 1.66, and 1.38, while those in the Member 2 and 3 of Qingshankou Formation are 1.79, 1.54, and 1.34, respectively. Different regions show significant differences in overpressure genesis. In Gulong sag, overpressure is as a result of pressurization from hydrocarbon generation. In the southern Qijia sag, overpressure is primarily caused by disequilibrium compaction, pressurization from hydrocarbon generation, and overpressure transfer. In the northern Qijia sag, overpressure is mainly generated from disequilibrium compaction. Further, it is found that the contribution rate of hydrocarbon generation-induced expansion to overpressure in Qijia-Gulong sag from south to north decreases from 97 % to 3 % . Furthermore, the weak-strong overpressure compartment structure formed due to hydrocarbon generation-induced expansion is a mark of high-yield oil layers in Gulong shale. The density of faults at the base of Qingshankou Formation have a limited impact on redistribution of the current formation overpressure. Moreover, low sandstone-to-mudstone ratio, fault stability, as well as development of laminar fractures in the mudstone layers provide good preservation conditions for shale oil retention. The north-south differences in overpressure distribution are highly correlated with the hydrocarbon accumulation of light oil, thin oil and black oil zones, highlighting the importance of understanding the genesis of overpressure in shale. To clarify the distribution and genesis of overpressure in Qingshankou Formation can provide a theoretical support for the prediction of tight oil and shale oil resources and the research of accumulation mechanisms.

Key words: overpressure distribution, overpressure genesis, hydrocarbon generation overpressure, Gulong shale, Qingshankou Formation, Qijia-Gulong sag, Songliao Basin

摘要： 松辽盆地北部齐家—古龙凹陷白垩系青山口组油气的产出与地层超压的发育密切相关,但地层压力分布规律和超压成因尚未开展系统研究。基于实测地层压力和利用泥浆密度数据预测的地层压力分析了齐家—古龙凹陷青山口组各段现今的地层压力分布特征,通过地质-测井响应特征分析了地层超压的成因与差异以及超压保存的控制因素,探讨了页岩地层超压与油气成藏的关系。研究结果表明:古龙凹陷、齐家凹陷南部和北部的青山口组在纵向上分别存在弱超压—强超压、常压—弱超压—强超压和常压—弱超压的压力封存箱结构;其对应结构的最大地层压力系数在青山口组一段分别为1.82、1.66和1.38,在青山口组二段＋三段分别为1.79、1.54和1.34。不同区域青山口组超压的成因存在明显差异,古龙凹陷的超压主要由生烃增压作用形成;齐家凹陷南部的超压成因以不均衡压实作用、生烃增压作用和超压传递作用为主;而齐家凹陷北部的超压主要由不均衡压实作用产生。齐家—古龙凹陷由南向北生烃膨胀作用对地层超压的贡献率由97%逐渐降低至3%,生烃膨胀作用导致的"弱超压—强超压"压力封存箱结构是古龙页岩油高产层的标志。青山口组底部断裂的疏密程度对现今地层超压的重新分配影响有限,泥页岩层中的低砂泥比、断层稳定性和页理缝发育为页岩油的滞留提供了良好的保存条件。研究区南、北超压的差异分布与其轻质油带、稀油带及常规黑油带的成藏高度匹配,凸显了判识泥页岩超压成因的重要性。齐家—古龙凹陷青山口组超压的分布及其成因的明确为致密油、页岩油资源预测和成藏机理研究提供了理论依据。

关键词: 超压分布, 超压成因, 生烃增压, 古龙页岩, 青山口组, 齐家—古龙凹陷, 松辽盆地