Fault Evolution Mechanism And Sealing Evaluation Of Deepwater Anticlines In Niger Delta

This paper is Based on the 3-D seismic data of the OML130 area of the Niger Delta,we apply the theory of gravity deformation of Delta and fault related fold,the structural evolution and period of the study area are defined:(1)from the Tortonian stage to the Messinian stage,Thrust folding developed,forming the main anticlines and the controlling thrust faults,and at the same time,a thrust transfer zone--anticline Egina;(2)From the Messinian stage to the Pliocene epoch,the anticline Egina moved laterally,and it is the key period for structural evolution of anticline Egina;(3)In Quaternary,thrust fold and lateral moving of anticline were no longer active,then compressive stress relaxed and extensional deformation happened weakly?Egina Anticline has moved laterally,and three periods of domino fault triangle bands could be seen on the crest.According to the gravity mechanism of normal faults on crest of deepwater anticline,it's clarified that numerous normal faults on crest of anticline Egina were controlled by gravity and lateral moving of anticline,rather than regional extension.The gravity mechanism of crestal normal faults includes:(1)faults developed in the shallow layers under seabed,the strata are poor-lithified and relatively fine-grained;(2)the faults are controlled by slope angle of top surface of anticline;(3)the faults developed on proper slope angle,but mass movement or degradation developed instead if the slope angle was too steep;(4)the faults developed as arrays,and the faults were relatively large on crest of anticlines,which decreases towards the limbs,showing a "triangle band";(5)faults showed a domino character,dipping from cores to limbs of anticlines;(6)faults always developed on forelimbs,but they may develop on backlimbs or not,which is controlled by the different slope angle.Seven classes in five divisions of fault systems in anticline Egina are classified :(1)thrust faults,(2)thrust transfer faults,(3)sliding step faults by gravity(subdivided into stage ?,?,?),(4)gravity subside faults,and(5)multi-stage faults.The difference of active periods between all kinds of fault systems,can be applied in fault sealing evaluation to raise efficiency.Thrust faults,thrust transfer faults and multi-stage faults tend to be strong sealing,and sliding step faults tend to be weak sealing,and gravity subside faults are leakage.Internal faults separated the oil and water in Egina oilfield.We made use of fault systems to evaluate the fault sealing of Egina: thrust faults,thrust transfer faults and multi-stage faults were very potential for sealing,and sliding fault-step faults were possible for sealing,and gravity subside faults were impossible for sealing.Quantitative Sealing evaluation indicated that there were eight sealing faults in East lobe and nine sealing faults in East channel,dividing the reservoir into seven and nine independent oil and water units respectively.Drilling and injection-production should be deployed for each independent unit,and functionally similar wells deployed in the same unit should be reduced.Additionally,some faults are weakly sealing,which controlled the water fluids along the faults,so the layered water injection should be adopted to improve the recovery factor.