| The FulaNorth oilfield is located in the central tectonic belt of the Fula Depression in the Muglad Basin,Sudan.The oilfield is affected by the Central Africa Shear Zone and its subordinate geological structure.The Lower Cretaceous Bentiu Formation is the main exploitation layer of the oilfield,followed by the Aradeiba Formation,and there are mainly heavy oils in the two oil reservoirs.The proven geological reserves of the FulaNorth oilfield are larger,and it is one of the important oilfields of CNPC overseas.The Bentiu reservoir is the main target layer of the FulaNorth oilfield,characterized by shallow burial,reservoir changed great in the longitudinal and lateral direction,and with strong reservoir heterogeneity.The drilling and geological analysis confirmed that the oil-water contact of the reservoir is inclined.In the early stage of oilfield exploitation,the geological understanding of the reservoir is mainly based on the hydrodynamic reservoirs,and made a natural energy-based exploitation program.However,the oil exploitation data show that the oil rate decreased rapidly,and the pressure decreased quickly,showing natural energy shortage and low recovery level,which are quite different from the hydrodynamic reservoirs results in previous studies,and bring a series of problems to the later oilfield development.It is important that the geological data are needed to analyze deeply in order to discuss the main influencing factors of the oil-water contact in the Bentiu Formation,assess reserves and provide geological information and support for the oilfield exploration and development.Therefore,based on the geological data analysis,well correlation,focusing on the study of sedimentary characteristic,reservoir characteristic and oil reservoir characteristic,this paper analyzed the main mechanism of oil-water contact tilt in Bentiu formation,and one set of assessment methods for assessing such special reservoirs was proposed.The main achievements of this paper are as follows:1.Three secondary sequence interfaces were identified in FulaNorth oilfield by logging and seismic data,and a sequence stratigraphic framework was established.By establishing standard wells,this study determined 4 marker layers,and the Bentiu formation is divided into 3 sands groups,Bentiu 1,Bentiu 2 and Bentiu 3 respectively,and furthermore,the main oil sand group,Bentiu 1,is divided into 5 layers.2.The Bentiu Formation belong to braided river sedimentary,and there are mainly three sedimentary microfacies,such as channel bar,channel and alluvial flat,and the channel bar microface is the most developed.The interbeds are mainly composed of muddy interbeds in Bentiu,with large distribution density and maldistribution.The wide spread and stability developed interbeds can play a block effect in a certain range,which is favorable for oil and gas sealing.The reservoir physical property is better and the reservoir heterogeneity is medixm-strong.Among them,B1b layer is of the strongest heterogeneity.3.The structure trap type of Bentiu is mainly of reverse fault(broken nose),which is a heavy oil reservoir with low sulfur content,low wax content and low freezing point under normal temperature and pressure.The oil-water contact in the reservoir titled obviously,with the inclination angle around 0.68°(slope of 12 m/km).4.In this paper,more than 40 titled oil-water contact oilfields are analyzed by the characteristics of hydrodynamic conditions,physical property and structure and so on.The statistics results show that the formation mechanism of the tilted oil-water contact can be grouped into four main interpretations:the first,affected by the hydrodynamic system;the second,affected by the capillary pressure and interbeds;the third,affected by the neotectonics movement;the fourth,affected by the late exploitation of an oilfield.Based upon statistical analysis,it is suggested that the degree of the inclination of the oil-water contact be divided based on the dip of oil-water contact(DipTOWC)-The tilted oil-water contact is divided into three categories:large dip(DipTOWC?55 m/km),medium dip(4 m/km?DipTOWC<55 m/km),and small dip(DipTOWC<4 m/km).The tilted oil-water contact of Bentiu is of medium dip level,and the principal formation mechanism is the complex distributed interbeds.5.Based on the main influencing factors and formation mechanism of the titled oil-water contact,8 classification schemes of reserve evaluation units considering the controlling mechanism of the titled oil-water contact in the volume method were proposed,and the reserve evaluation process of the conventional volume method was optimized.6.Based on the analysis of geological conditions of Bentiu,the characteristics of reservoir with inclined oil-water contact,this paper carried out three kinds of reserves evaluation methods based on conventional volume method,volume method considering the titled oil-water contact and three-dimensional geological modeling.The results show that the volume method based on the titled oil-water contact,it is reliable for the reserves evaluation of the titled oil-water contact reservoir,which divides the reservoir into different calculation units,and then carries on the reserve calculation method based on the traditional volume method.Moreover,it is concluded that titled oil-water contact distribution characteristics of Bentiu reservoir has been well detected,and the main influencing factors,tilt angle and oil-bearing distribution characteristics of the titled oil-water contact were clarified,so the estimated reserves are confirmed reserves(P1)without probable reserves(P2)and possible reserves(P3).Comprehensive evaluation is made to the oilfield,that it is medium-sized heavy oil field with high porosity and high permeability,high reserves abundance.The research ideas of this paper,together with the technical methods and technical processes,as well as the main understanding of the study,are meaningful and some guidance for reservoir exploration and development with complex geological conditions,especially for the tilted oil-water contact reservoir oilfield characteristics and reserves evaluation. |
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