Petrophysical Characteristics And Logging Evaluation Methods Of Complex Reservoirs

In this paper,the low-contrast reservoirs of Ed II oil formation in LD27-2 oilfield of Bohai Bay Basin and the zeolite bearing high resistivity reservoirs of the Permian Lower Wuerhe formation in eastern slope of Mahu sag of Junggar Basin are taken as research objects,and the petrophysical characteristics,log response mechanisms and logging reservoir evaluation methods of complex reservoirs in these study areas are studied.The resistivities of some oil layers of Ed II oil formation in LD27-2 oilfield are less than the lower limit value or even close to the resistivities of water layers,which makes it difficult to identify reservoir fluid in logging evaluation process.For lack of core analysis data,this paper analyzed formation zoning characteristics,rock structure characteristics,porosity and pore structure characteristics through conventional logs,formation microscanner image（FMI）logs,combinable magnetic resonance（CMR）logs and some early experimental results.The results show that the difference of sedimentary conditions in different periods of Ed II oil formation results in the change of reservoir lithology and physical properties in the vertical direction,which is followed by the decline of resistivity in the vertical direction according to the change of reservoir lithology.Zeolite cements are ubiquitous in the Permian Lower Wuerhe Formation reservoirs in Madong area,resulting in obvious increase of reservoir resistivity and complex logging response characteristics.The logging response characteristics and genetic mechanism of zeolite bearing reservoirs are unclear,and it is difficult to to identify and calculate zeolites in reservoirs.The conventional physical properties,P wave and S wave velocities,resistivitis at different salinities and NMR T₂ spectra of natural zeolite mineral samples have been measured in this paper.On this basis,the log response characteristics of zeolite bearing reservoirs and zeolite free reservoirs are compared combined with the analysis results of mineral composition of the rock.It is found that,affected by zeolite minerals,the zeolite bearing reservoirs in the study area have low natural gamma,low density and high neutron porosity log response characteristics.Besides,the acoustic transit time of the zeolite bearing reservoirs are similar to that of zeolite free reservoirs,and the short relaxation component of the nuclear magnetic T₂spectra in zeolite bearing reservoirs are wider.In addition,based on the experimental results of the CEC（Cation Exchange Capacity）and resistivity measurements of zeolite minerals,it is concluded that even though zeolite has high cation exchange capacity in powder state,its unique framework crystal structure makes it has no additional conductivity when it exist as cement.The correlation between zeolite contents and porosities indicates that the degree of cementation and dissolution of zeolite mineral during diagenesis controls the physical properties of reservoirs in the study area.According to the influence of zeolite cement on reservoir physical properties during diagenesis,this paper carried out the log response characteristics simulation analysis of the zeolite bearing reservoir which under different diagenesis processes and fluid properties through physical model of bulk volume rock.Based on the log response characteristics and log response mechanisms,the targeted logging evaluations of these two types of complex reservoirs were carried out in this paper.For low-contrast reservoirs of Ed II oil formation,based on the difference of log response characteristics among reservoirs with different lithologies,the corrected resistivity–acoustic transit time cross plot method for Ed II oil formation in LD27-2oilfield is established.This method combined with the resistivity-porosity curve correlation analysis method and water saturation–irreducible water saturation overlap method can improve the reliability of the evaluation result.For the zeolite bearing reservoirs of the Lower Wuerhe formation in Madong area,three qualitative identification methods and three zeolite content calculation methods were put forward respectively based on the logging response characteristics of zeolite minerals.Among them,qualitative identification methods include acoustic-density apparent porosity overlap method,NMR T₂ spectrum characteristic method and multi-parameter neural network discriminant method while zeolite content calculation methods include density–NMR apparent skeleton density method,apparent porosity difference method and multi-mineral model optimization method.The processing results of logging data verifying the reliability and effectiveness of these logging evaluation methods in each area.