Microscopic Characteristics Of Triassic Reservoirs In Lunnan Oilfield And Its Surrounding Areas

The Lunnan Oilfield is located in Lunnan Uplift.Triassic reservoir is one of the most important reservoir in the study area.Although the reservoir buried deep,the physical properties of it are good.At present,the factors that have influence on formation of the deep-buried reservoir with good physical properties are not clear yet.And it is hoped to predict the distribution of favorable reservoirs by analyzing factors that effect the reservoir.Used core,slice and other analysis datas comprehensively to study lithology characteristics of the target section in the work area.The types of lithofaces were divided by sedimentary structure and grain size.According to the combination of lithofacies,sedimentary microfacies were divided.The differences of lithological characteristics among sedimentary microfacies were analysed as well.Combined with logging datas,diageneitc facies were determined,and the diagenetic facies distributions of single well and flat were analysed.Using the experimental methods of mercury porosimetry and NMR,the characteristics of pore structure were analysed and the physical properties of the reservoir were summarized.At last,analyzed factors that affect physical properties,then made the prediction of favorable reservoirs by taking advantages of various factors.Trissic reservoir in Lunnan Oilfield and its surrounding areas is mainly composed of feldspathic lithic sandstone and lithic feldspar sandstone,component maturity of the reservoir is low.The grain size distribution is wide,sorting poor,in a word the structural maturity is low.Main microfacies in the target area are subaqueous distributary channel microfacies,mouth bar microfacies and inter branch channel microfacies.It is studied that the diagenetic evolution of target strata is weak.Now the reservoirs are in A stage of middle diagenesis.Porosity and permeability of reservoirs are effected obviously by compaction,cementation and dissolution.Combined with logging curves,the target strata can be divided into four types of diagentic facies: carbonate cemented diagenetic facies,strong compacted facies,main clay minerals cemented facies and unstable components dissolution facies.The main pore type of Triassic reservoir is primary pore,and the content of dissolution pores is low.The coarser the grain size is,the higher the primary content is.There is little content of primary pores and dissolved pores in siltstones.Reservoir physical properties are effected by sedimentary,diagenesis,overpressure and burial history.The coarser the grain size is,the reservoir property is worse.The poorer the sorting is,the reservoir property is worse.Compaction and cementation result in the decrease of porosity and permeability of reservoirs and dissolution improves the physical properties.Therefore,the unstable components dissolution facies is the best.And the porosity and permeanbility of carbonate cementation phase are extremely low.Overpressure develops in the study area,and the reservoir is undercompacting.The reservoir experienced a long-term shallow burial in low temperature.Therefore,the diagenetic evolution is weak.Under the influence of many factors,the reservoirs with high porosity and permeability are formed.Based on the influences of sedimentation and diagenesis,the reservoirs can be divided into five types.Physical properties of type I and II are excellent,properties of type III are medium.Type IV and type V reservoirs are poor.And the distribution of each type was predicted.It was found that overall properties of the study area are good.And the poor reservoirs are small and discontinuous.