油源断裂输导油气时期厘定方法的改进及其应用--以渤海湾盆地歧口凹陷南大港断裂为例

Improvement of method for timing of hydrocarbon migration along source rockrooted faults and its application:A case of the South Dagang fault in Qikou Sag,Bohai Bay Basin

油源断裂输导油气时期对于准确认识油源断裂在油气成藏中的贡献至关重要。以往其厘定要么是仅仅依据油源断裂活动时期,要么是把油源断裂活动时期与烃源岩排烃时期相结合,所得结果往往难以反映地下的实际情况。为此在油源断裂输导油气机理研究的基础上,对以往的方法进行了改进,提出了通过确定油源断裂活动时期与烃源岩内、外油气聚集时期的叠合时间来厘定油源断裂输导油气时期的方法,并以渤海湾盆地歧口凹陷南大港断裂为例对改进前、后方法的应用效果进行了对比。结果表明:利用改进后方法厘定出的南大港油源断裂输导油气时期为新近系明化镇组沉积时期,明显短于利用改进前方法厘定出的新近系馆陶组-明化镇组沉积时期。与改进前方法相比,改进后方法给出的油源断裂输导油气时期可更合理地解释南大港断裂附近古近系沙河街组一段目前已发现的油气分布,因而更符合地下油源断裂输导油气机理。

Timing of hydrocarbon migration along source rock-rooted faults is critical for understanding the contributions made by the faults to hydrocarbon accumulation.The previous timing has been based only on the activating periods of the related faults,or on both the related faults’activity period and hydrocarbon expulsion period,and the results obtained often cannot reflect the actual situation underground.In the study,we improve the method for timing of hydrocarbon migration along the source rock-rooted faults by defining the overlapping time of the faulting and hydrocarbon accumulation inside and outside the source rocks,coupled with study on hydrocarbon migration mechanism of source rock-rooted faults.Then a case study on South Dagang fault in Qikou Sag,Bohai Bay Basin,is carried out to comparatively analyze the effects of both the original and improved methods.The results show that the South Dagang fault is estimated as being active in hydrocarbon migration during the depositional period of the Neogene Minghuazhen Formation according to the improved method,which is much shorter than the deposition period of Guantao Formation to Minghuazhen Formation of the Neogene as confirmed to be hydrocarbon migration period of the fault by the original method.Compared to the original one,the hydrocarbon migration period of the South Dagang fault determined with the improved method is more reasonable in explaining the oil and gas distribution discovered in the first member of the Paleogene Shahejie Formation(Es 1)near the South Dagang fault.It proves that the improved method is better consistent with the hydrocarbon migration mechanism of the source rockrooted faults underground.