富油凹陷洼槽区油气成藏机理与成藏模式--以冀中坳陷饶阳凹陷为例

Hydrocarbon accumulation mechanism and model of sub-sags in hydrocarbon-rich sag:a case study of Raoyang sag in Jizhong depression

随着正向构造带勘探难度的增加,洼槽区逐渐成为渤海湾盆地富油凹陷增储上产的重要领域,而成藏机理的特殊性制约了洼槽区勘探的深入。以冀中坳陷饶阳凹陷为研究对象,通过对洼槽生烃能力、油气成藏时期、成藏动阻力条件以及输导体系等分析,探讨了富油凹陷洼槽区油气藏的形成机理及模式。研究表明,饶阳凹陷富油洼槽区有效生烃范围大,热演化程度较高,存在东营组沉积末期和新近纪明化镇组沉积期2期主要生烃及成藏过程;东营组沉积末期油气充注范围较小,储层物性好,成藏阻力小;而明化镇组沉积充注时期,储层物性差,成藏阻力大,但剩余压力大,油气成藏动力强。洼槽中心区的油气多沿源岩层系中的砂体、裂缝和小断层等进行短距离运移聚集形成岩性为主的油气藏;洼槽边缘区往往发育深切入烃源岩的油源断层,成为油气垂向运移的重要通道,油气沿断层运移聚集形成构造为主的油气藏。富油洼槽区存在"源内-两期成藏-隐蔽输导-短距离运移"与"源外-单期成藏-断裂输导-垂向运移"2种油气成藏模式,成藏动、阻力条件控制着洼槽区有效烃源岩层系内油气成藏的物性下限,而断层为主体的输导体系分布特征及有效性控制着洼槽区源外油气的分布和富集程度。

As the exploration difficulty in positive structral zone increases,sub-sags have gradually become the important area to increase reserve and production in hydrocarbon-rich sag,Bohai Bay Basin.However,the special accumulation mechanism restricts further exploration in subsags.Taking Raoyang sag in Jizhong depression as the research object,the reservoir formation mechanism and hydrocarbon accumulation model of sub-sags in hydrocarbon-rich sag were discussed through analysis of hydrocarbon generation capacity,accumulation stage,hydrocarbon accumulation dynamics and resistance,pathway system,etc.Research shows that the hydrocarbon-rich sub-sags are characterized by a wide range of effective hydrocarbon generation and high degree of thermal evolution as well as two stages of hydrocarbon generation and accumulation,i.e.,late Dongying period and Neogene Minghuazhen period.In the late depositional stage of Dongying Formation,oil and gas charging occurred on a small scale;reservoirs had good physical property and small accumulation resistance.In the charging stage of Minghuazhen period,reservoirs had poor physical property and great accumulatioin resistance,while possessing great excessive pressure and strong accumulation dynamics.Oil and gas in the center of sub-sags mainly migrate in a short distance through sand bodies,fractures,small faults,etc.in source rocks,accumulate and generally form lithologic reservoirs.Oil source faults deep cutting into source rocks are often developed on the margins of sub-sags,thus becoming an important channel for hydrocarbon vertical migration;hydrocarbon migrates along faults and mainly forms structural reservoirs.There are two accumulation models of＂intrasource-two stages of accumulation-subtle pathway-short migration distance＂and＂extrosource-one stage of accumulation-fault pathway-vertical migration＂in hydrocarbon-rich sub-sags.Meanwhile,the lower limit of physical property for hydrocarbons accumulation in effective source rocks is controlled by accumulation dynamic and resistance,while the distribution and enrichment degree of hydrocarbon outside source rocks is controlled by the distribution characteristics and effectiveness of fault-dominating pathway system.