Identification Of Low Contrast Oil Layers In Chang 8 And Chang 9 Oil Layers Of Yanchang Formation In Mahuangshan-Gufengzhuang Area,Western Ordos Bain

The largest low-permeability reservoirs in China have been identified within the Triassic Yanchang Formation reservoirs in Jiyuan oilfield of the Midwest Ordos Basin.Significant advances have made in the deep oil exploration of Chang 8 and Chang 9oil layers underlying the main source rock of Chang 7 layer in Yanchang formation,which is becoming a new reservoir series in Jiyuan oilfield.The reservoir size of Chang 8 and Chang 9 oil layers in Mahuangshan-Gufengzhuang is small,the fault and fracture is developed,the pore structure and the type of low contrast reservoirs are complex.Therefore,the accurate identification of oil and water layers is a difficult and hot issue in the exploration and development of Chang 8 and Chang 9 oil formations in this area.Based on the low contrast reservoir study of Chang 8 and Chang 9 layers in Mahuangshan-Gufengzhuang,this thesis mainly(1)studied the sedimentary-reservoir characteristics and its "four-property" relationship,fracture and formation water salinity distribution;(2)figured out the genetic mechanism of multiple types of low contrast reservoir;(3)established the identification methods and evaluation criteria of low contrast oil layers based on different genetic types;(4)predicted the distribution of low contrast oil layers.The thesis mainly obtained the following innovative understanding:The "four-property" relationship which was based on core test analysis and logging interpretation,indicated that(1)the content of sandstone interstitial material in Chang 8 and Chang 9 formations in Gufengzhuang is 12.3% and 8.1%,respectively;(2)the pore types of reservoir are mainly including soluble-intergranular pores and intergranular pores;(3)the average porosity is 13.6% and 14.5%,and the permeability is 2.78 m D and 11.3m D,respectively,it belongs to ultra-low and low permeability reservoir,respectively.And also indicated that(1)the content of sandstone interstitial material in Chang 8 and Chang 9 formations in Mahuangshan is16.1% and 13.6%,respectively;(2)the pore types of reservoir is mainly including soluble-intergranular pore;(3)the average porosity is 9.2% and 9.8%,and the permeability is 0.67 m D and 4.2m D,respectively,it all belongs to ultra-low permeability reservoir.In contrast,the reservoir conditions of Gufengzhuang are better than Mahuangshan,and Chang 9 is better than Chang 8 formation.Meanwhile,the differences are related to the distribution of the well control area in high-resistivity water layer at Gufengzhuang and low resistivity oil layer at Mahuangshan.The genetic mechanism of low contrast oil layer which combined with fault-fracture reservoir,formation water salinity,sandstone granular fluorescence and the bound water that was quantitatively analyzed by core NMR experiment,is indicated that(1)the Chang 8 and Chang 9 formations in the northwest of the research region have a high degree of fault and fracture development,and the area with high linear fracture density is closely related to the distribution area of low salinity formation water.Meanwhile,the QGF parameter of sandstone granular fluorescence shows that the contents of residual asphalt is high,which jointly leads to the development of high-resistivity water layer in Chang 8 and Chang 9 formations;(2)the Chang 8 and Chang 9 formations in the southeast of the research region have low fault and fracture development degree,high salinity of formation water,poor reservoir physical property,high content of interstitials,complex pore structure and high content of bound water.Those factors collectively make the development of low resistivity oil layers in Chang 8 and Chang 9 formations.We constructed that(1)the fluid sensitive parameter method and Fisher discriminant analysis method are used to identify low resistivity oil layer;(2)the optimized saturation parameter method and the resistivity-porosity correlation analysis method for the identification of high-resistivity water layer,on the basis of the "four-property" relationship and the genetic mechanism of low contrast oil layer.Furthermore,the identification standard of low contrast oil layer is established,which based on reservoir physical property,pore structure,fracture linear density and formation water salinity.Also using that standards predicted the distribution of low contrast oil layer(two types are low resistivity oil layer and high resistivity water layer)in Chang 8 and Chang 9 formations within the research region.