Study On Logging Evaluation Method Of Properties Of Tight Sandstone Reservoirs

The reservoir characteristics of tight sandstone are complicated,which can be summarized as complex and variable lithology,physical properties and"rock-electricity"relationship.The basic reason is the multiple influences of sedimentation,diagenesis and tectonism,which affects the effect of reservoir identification and evaluation methods.To solve these problems,this paper takes the tight sandstone reservoirs of Chang 6～Chang 7 member of the Triassic Yanchang Formation in the multi-source convergence area of the central and southern Ordos Basin as an example.The study is based on the complex reservoir characteristics and logging response characteristics.The logging response types of sandstone of chang 6～Chang 7 member in the study area are diverse,which can be divided into three types:conventional sandstone,relatively high radioactive sandstone and intercalated sandstone in source rock.The GR log response characteristics of the relatively high radioactive sandstone of Chang 6 are similar to those of argillaceous rocks and tuff rocks,while the SP log response is similar to that of conventional sandstone.The logging response characteristics of the intercalated sandstone in source rock of Chang 7 are similar to that of the source rock.The relatively high radioactive sandstone and the intercalated sandstone in source rock are the favorable oil-bearing sections and oil-bearing sweet spots of Chang 6 and Chang 7 member respectively,but they are difficult to be recognized by conventional methods due to their special response characteristics,leading to effective reservoir leakage.The difference between SP and GR curve responses among different sandstones is the basis for lithology identification.Among them,SP and GR of conventional sandstones,mudstones and tuff have exponential correlation relationship,while SP and GR of relatively high radioactive sandstones and sandstone trapped in source rocks are not.In addition,GR,estimated gamma ray(GR_V)based on the correlation between SP and GR,and gamma ray curve return amplitude(I_G)can be used to calculate the shale content of conventional sandstone,relatively high radioactive sandstone,and intercalated sandstone in source rock,respectively.Due to the complex and variability of mineral composition and physical properties of sandstone reservoirs in Chang 6 and Chang 7 member in the study area,the sensitivity of logging response to reservoir porosity is relatively weak.Among them,the sensitivity of logging response to porosity is as follows:density is the strongest,acoustic time difference is the second,neutron is the weakest.The weak sensitivity between curve response and reservoir porosity increases the difficulty of reservoir physical property evaluation.Analysis shows that the introduction of variable skeleton parameters,Pe(lithology information)and multiple sensitivity response(porosity-series curve)can improve the applicability of the porosity interpretation model to complex and variable reservoirs to a certain extent.And the applicability of the multi-parameter porosity interpretation model with Pe is better than that of the porosity interpretation model with variable matrix parameters for complex and variable reservoirs in the study area.At the same time,introducing Pe(lithology information)can also improve the applicability of permeability interpretation model to tight sandstone reservoir.The tight sandstone reservoirs of chang 6～Chang 7 member in the study area have double pore structure and the"rock-electricity"relationship is non-Archie characteristic,belonging to multi-component conductive system,which increases the difficulty of reservoir oil-bearing evaluation.Analysis results shows that the tight sandstone reservoir saturation index(n),variable cementation index(m)and pore structure composite index(√?ф)of Chang 6 and Chang 7 reservoirs have a good correlation.Based on the complex pore structure of the reservoir,the model for interpreting the saturation of the dual-pore reservoir with variable rock electrical parameters has a good application effect in the study area.In addition,the method of movable water saturation has a good application effect in the oil bearing evaluation of tight sandstone reservoirs of Chang 6 and Chang 7 member in the study area.The movable water saturation of water layers is more than 30%.