Progressive rupture of cap rocks and step-wise hydrocarbon migration in gentle slope zone around Bozhong area
As exploration in the uplift area of the Bohai Bay Basin intensifies,the gentle slope zone has become an important subsequent exploration area.However,the exploration success rate in the gentle slope zone is relatively low,and accurate prediction of hydrocarbon migration patterns and pathways is a crucial factor that determines the success or failure of exploration.The Bozhong area was studied,and the relationship between cap rock rupture and hydrocarbon migration under the condition of multi-layer transport systems in the gentle slope zone of a faulted basin was analyzed.The corresponding hydrocarbon migration patterns were summarized,and three-dimensional hydrocarbon migration simulations were conducted based on the migration pattern,achieving precise prediction of hydrocarbon migration pathways in the gentle slope zone.The research findings are as follows:①The gentle slope zone around the Bozhong Sag generally exhibits a"cap rock-fault double-control stepwise"hydrocarbon accumulation pattern,which mainly includes two types of oil and gas reservoirs:Paleogene proximal and Neogene distal.Hydrocarbons in proximal reservoirs are directly sourced from source rocks and migrate along the first transport layer of the Paleogene system under the control of the regional cap rock in the Dongying Formation.Hydrocarbons in distal reservoirs originate from injection points undergoing secondary adjustment and migration.After breaking through the cap rock in the Dongying Formation along faults,hydrocarbons migrate in a stepwise manner along the multi-layer transport system of the Neogene system.②The cumulative cap rock juxtaposition thickness of less than 450 m is the triggering condition for the rupture of underlying cap rocks and the injection into shallow transport layers.③ Controlled by the coupling of cap rock thinning and fault activities,different structural parts of the gentle slope zone exhibit different stepwise zones of hydrocarbon migration and accumulation.The spatial distribution of hydrocarbons can be predicted through"cap rock-fault coupling"analysis.Based on this idea,innovative three-dimensional stepwise hydrocarbon migration simulations were conducted,achieving a comprehensive prediction success rate of 88％in the PL19-3 gentle slope zone.④Due to the dense development of normal faults at the slope break zone where the slope platform transitions,coupled with significant variations in cap rock thickness and juxtaposition thickness,the outer slope zone below the slope break zone typically favors proximal hydrocarbon accumulation,and hydrocarbon exploration should focus on the middle-deep layers.In the inner slope zone above the slope break zone,hydrocarbons usually accumulate in distal reservoirs,and hydrocarbon exploration should prioritize shallow layers.This research plays a significant role in predicting favorable directions for hydrocarbon exploration in the gentle slope zones of faulted basins.
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