Abstract: The oil and gas exploration in the non-hydrocarbon-generating subsags at basin margins is usually restricted by relatively poor pathway systems,and often regarded as the "forbidden area" for exploration.In order to examine various hydrocarbon migration and accumulation patterns in non-hydrocarbon-generating subsags at basin margins and expand the realm of hydrocarbon exploration,we have taken the Sanhecun subsag of Zhanhua Sag,Bohai Bay Basin as an example to analyze its hydrocarbon migration and accumulation pattern based on a systematic review of previous knowledge and data including drilling,logging,seismic analysis of Sanhecun subsag of Zhanhua Sag.The results show that Sanhecun subsag has undergone two hydrocarbon charging periods.These are the early-stage hydrocarbon accumulation during the sedimentation of Dongying Formation and the middle-late-stage hydrocarbon accumulation during the sedimentation of Guantao-Minghuazhen Formation.In the early-stage accumulating period,the hydrocarbon-generating Bonan sag was connected to Sanhecun subsag,and hydrocarbon from Bonan sag accumulated in the Shahejie Formation of Sanhecun subsag through lateral migration.In the middle-late-stage accumulating period,through combined migration and accumulation mechanisms of "meshwork-carpet" and "unconformity",hydrocarbon generated in Bonan sag was charged to the Dongying Formation and shallower strata in the Sanhecun sag.The speculation of hydrocarbon migration and accumulation mechanisms in a non-hydrocarbon-generating subsag at basin margins has a theoretical significance by extending hydrocarbon exploration realm,and has led to the discovery of Sanhecun oilfield in practice.

Key words: negative structural zone, hydrocarbon migration and accumulation model, non-hydrocarbon-generating subsag, Zhanhua Sag, Bahai Bay Basin