Hydrocarbon Migration Dynamic Conditions And Accumulation Models Of Lower Paleozoic In Chexi Area

Chexi area is located in the western part of the Chezhen sag and characterized by the dominance of rich oil in-place reserves reservoired in buried-hill traps of Lower Paleozoic Cambrian-Ordovician marine carbonates.Lower Palaeozoic hydrocarbon accumulation dynamics and accumulation pattern is one of the key problems to be solved,however,with the lack of systematic research and the insufficiency of interpretation to the dynamics conditions and the process of petroleum migration,which greatly influence the understanding of distribution pattern of petroleum.On the basis of previous research results of characteristics of tectonic and hydrocarbon geological conditions,this paper taking Chexi area of Lower Paleozoic as the research object,has carried out comprehensive research to target area,including the characteristics of different types of hydrocarbon lose conductor,the spatial relationship of source-pathway-reservoir,the possible ways of migration of hydrocarbon,the quantitative research of the formation pressure evolution,the transfer and leakage process of overpressure along the fault zone in the key accumulation period,The possibility of hydrocarbon migration and accumulation in different transport mechanism and combination model of conductors using mathematical modeling methods,and the model of hydrocarbon migration and accumulation combined with the geological analysis of oil reserviors.Through research papers,mainly we made the following understanding:Lower Paleozoic reservoirs in Chexi area are dominated by décollement-structure and block-structure buried hills reservoirs,with complex relationship of oil and water.The reservoirs are layered inside the buried hills reservoirs rather than massive reservoirs.Lower Paleozoic reservoir can be divided into four sets.The shale and isolated sand body of the Es3-Es4 section in the Chexi area developed abnormally high pressure,resulting mainly from compaction and hydrocarbon generation.The utmost excess pressure of Lower Es3 during the main hydrocarbon generation and expulsion period is 15~20MPa,reducing gradually to the surrounding formation,whereas Upper Paleozoic and Lower Paleozoic are atmospheric pressure systems.Second Step Fault breaking up to the closed overpressure system of the Lower Es3,can open as the vertical migration pathway potentially during the accumulation period under the action of tectonic stress and fluid pressure.However,in a single episodic activity of faults,the overpressure of Lower Es3 can release and disperse quickly along the fault zone and the permeable formations of Es4-Pz2,which could hardly delivere to Pz1,and be the driving force of downward vertical migration over long distances.The most likely model of hydrocarbon migration and accumulation of Chegu 20 buried hill in the Futai area is that the Es3 hydrocarbon driven by buoyancy migrate and accumulates laterally along fault/fracture zone,rather than the downward migration driven by overpressure,faults of Che 571 Block in Futai area are the main migration pathways.The migration and accumulation model of Chegu 1 hill in the Taoerhe area are the Futai hill Hydrocarbon Migration?the lateral conduction along with weathering karst belt on the top of Pz1?buoyancy driven.The migration and accumulation model of Chegu 53 hill and Chegu 9 hill is stepped lateral migration along with E sand-weathering karst belt on the top of Pz1-fault.