Research On Mechanism Of The Hydrocarbon Migration And Accumulation In Chengbei Step-fault Zone, Qikou Depression

Based on the data of geology,geochemistry and geophysics,adopting the modern theories and methods of oil and gas accumulation,this paper gives a comprehensive research on the hydrocarbon accumulation characteristics and main factors influencing hydrocarbon accumulation in Chengbei step-fault zone. The results show that there is not abnormal pressure in the Cenozoic strata of the step-fault zone near the uplifted area,but low amplitude abnormal pressure is general in the Paleogene strata of the step-fault zone near the depression.Low pressure means that better reservoir conditions are necessary for the hydrocarbon accumulation. The hydrocarbon migration has two-direction characteristics,namely from the Qikou depression to south and from Qinan depression to north-east. Connected sand bodies,faults and unconformable surfaces are the basic migration pathways,and the faults are the most important one which has determined the vertical migration and multilayered distribution of oil and gas. The connected sand body (unconformable surface)- fault - connected sand body (unconformable surface)- fault transportation system constitute a complicated migration path for the hydrocarbon migration. The first stage happened at the end of Ed depositional stage,the second stage was at the end of Nm depositional stage to Quaternary Period,the latter is the key period which has determined the current distribution pattern of oil and gas. By macro-structural setting,regional seal,reservoir distribution,master fault sealing,trap-source distance effects,the hydrocarbon accumulation characteristics have significantly differences in each step-fault zone and layers. Fault sealing and regional seals are the key factor to control the vertical distribution of oil and gas. The fault transportation system in bedrock hills is the dominant factor to control the hydrocarbon horizontal migration and accumulation. Special tectonic setting,good reservoir conditions,long-term succession faults and complex transportation system jointly control the pattern of hydrocarbon accumulation in the area. Three kinds of hydrocarbon migration and accumulation models are classified:the migration model in sand lens in source rock,the horizontal migration model in connected sand body or unconformable surface and the step-migration mode,and the last one is the dominant model in this area.