Diagenetic Numerical Modeling Of Low-Permeability And Tight Reservoir

The research purpose of this paper is to determine the reservoir quality difference genesis,distribution rule and controlling factors of the reservoir“sweet spots”in the deep low-permeability and tight sandstones in the few wells offshore area,which is conducive to well deployment and efficient development of gas reservoirs.Taking the H3,H4,and H5sandstones in the N gasfield in the Xihu Sag of the East China Sea Basin as an example,based on geological data,logging data,seismic data,and analytical test data,the diagenesis evolution model of"sedimentary burial diagenesis,compaction dominated,and multiple diagenetic pathways"of the H3,H4,and H5 sandstones was established.Firstly,the diagenesis numerical simulation study based on compaction was carried out to reproduce porosity evolution and high-value areas.Secondly,according to the diagenetic evolution pathways related to compaction,dissolution and cementation,the diagenetic numerical modeling study with diagenetic facies as the simulation object was carried out to reproduce the evolution and spatial distribution of porosity.Finally,the evolution and spatial distribution of reservoir"sweet spots"of the H3 and H4 sandstones were predicted.The following conclusions and understandings were obtained:The sandstone types of the H3,H4,and H5 members are mainly feldspathic lithic quartzose.The content of quartz mineral ranges from 62.85%to 75%.The diagenesis types of these sandstones mainly included compaction,clay mineral transformation,quartz cementation,calcite cementation,and dissolution.The five-stage diagenetic events of the H3,H4,and H5members were determined through the combination of petrological characteristics,geochemical characteristics,and burial history-thermal history.These diagenetic events included（1）the formation of calcite cement and chlorite coat in connate alkaline environment of the Oligocene sandstone,（2）dissolution of feldspar,rock fragment and carbonate cement in the late Oligocene meteoric water weak acidic diagenetic environment,（3）the dissolution of feldspar,rock fragment and carbonate cement,the formation of secondary pores,and the precipitation of quartz cement and authigenic kaolinite in the early-middle Miocene organic acid and CO₂ acidic fluids diagenetic environment,（4）the precipitation of authigenic quartz under the condition of lower reservoir temperature in the late Miocene,and（5）the formation of ferrocalcite,illite and chlorite in alkaline diagenetic environment from late Miocene to present day.These diagenetic events and diagenetic environment controlled the type and spatial distribution of diagenetic facies.Based on the burial history-thermal history,organic matter evolution and diagenetic evolution sequence,the diagenetic facies classification scheme of H3 and H4 sandstones was established.These diagenetic facies included chlorite-coated facies,calcite-cemented facies,dissolution facies,quartz-cemented facies,and tightly compacted facies.In the case of few offshore wells,the method of combining well-seismic data was used to predict the spatial distribution of diagenetic facies.Through statistical analysis,it was found that the p-wave impedance,shear modulus,and porosity data volumes had a good indication effect on diagenetic facies.The p-wave impedance and shear modulus had a negative correlation with the measured porosity.The more developed the pores in the rock,the lower the p-wave impedance value.The greater the effective stress and the degree of compaction,and the greater the shear modulus.Based on this,the supervised learning method was used to establish two discriminant functions between core-derived diagenetic facies and shear modulus,p-wave impedance,and porosity data volume,and the seismic data volumes X and Y were obtained.The seismic data volumes X and Y were merged into a new diagenetic facies comprehensive index seismic data volume through a linear function.The distribution ranges of the diagenetic facies comprehensive index of the dissolution facies,chlorite-coated facies,quartz-cemented facies,and tightly compacted facies are 1.0～6.0,-0.15～1.0,-2.35～-0.15,and-8～-2.35,respectively.The stratal slicing method in seismic sedimentology was used to extract the stratal slice data of 19 sublayers in the H3 and H4 members.Based on the constraints of core-based diagenetic facies,typical stratal slices and sedimentary microfacies,the seismic diagenetic facies planar maps of 19 sublayers in the H3 and H4 members were accurately interpreted.The chlorite-coated facies mainly developed in the channel bar and a small amount of subaqueous distributary channel sedimentary microfacies.The dissolution facies mainly developed in the channel bar and subaqueous distributary channel sedimentary microfacies near the fault.The quartz-cemented facies mainly developed in the subaqueous distributary channel and a small amount of channel bar sedimentary microfacies.The tightly compacted facies mainly developed in the subaqueous interdistributary bay and a small amount of subaqueous distributary channel sedimentary microfacies.For compaction,a compaction numerical simulation method that integrated mechanical and chemical compaction processes was proposed.The duration age of mechanical compaction porosity loss of sandy conglomerate,fine sandstone,and medium sandstone in the H4 and H5members ranged from 27Ma to 7Ma,and the duration age of chemical compaction porosity loss was 10.6Ma～0.0Ma.The mechanical compaction porosity loss of sandy conglomerate,fine sandstone,and medium sandstone in the H4 and H5 members mainly occurred in the slow subsidence and rapid subsidence stages,while the chemical compaction porosity loss mainly occurred in the uplift erosion and regional steady subsidence stages.Sandstones with porosity of 9%～11%were scattered in the H4 and H5 members,and sandstones with porosity of 6%～9%were distributed in contiguous areas.For multiple diagenetic facies,a diagenetic numerical modeling method combining seismic diagenetic facies and diagenetic model was proposed.This method used the diagenetic facies as modeling object,which can better reproduce the diagenesis evolution process,evolution process and spatial distribution of the porosity for low-permeability and tight sandstone in the few wells offshore area.The statistics of the porosity spatial distribution show that the porosity distribution ranges of 5 wellsites in the H3b1,H3b2,H3b3,H3b4,H3b5,and H3b6 sublayers are 7.03%～7.62%,9.01%～10.66%,9.91%～11.97%,9.7%～14.61%,10.13%～15.43%,and 10.03%～14.49%,respectively.The porosity distribution ranges of 5 wellsites in the H3c1,H3c2,and H3c3 sublayers are6.31%～6.97%,8.16%～14.99%,and 8.02%～14.79%,respectively.The porosity distribution ranges of 5 wellsites in the H4b1,H4b2,H4b3,H4b4,H4b5,and H4b6 sublayers are5.63%～6.12%,7.83%～11.39%,7.68%～11.45%,7.57%～11.29%,9.18%～13.18%,and9.64%～12.81%,respectively.The porosity distribution ranges of 5 wellsites in the H4c1 and H4c2 sublayers were 7.26%～9.25%and 7.03%～9.03%,respectively.Based on the measured core porosity and the log interpretation porosity,the distribution rule of the reservoir“sweet spots”at the 4 wellsites was determined.On this basis,the probability volume of type I"sweet spot"in the H3b and H4b sub-members were obtained by using pre-stack inversion technique and probability analysis technique.Based on the verification of type I"sweet spot"probability volume and well-site reservoir“sweet spot”data,the evolution and spatial distribution of reservoir"sweet spots"of 19 sublayers were accurately reproduced by using reservoir“sweet spot”classification criteria and porosity evolution history.The H3b2,H3b3,H3b4,H3b5,H3b6,H3c2,H3c3,H4b2,H4b3,H4b4,H4b5,and H4b6 sublayers mainly developed type I and type II₁ reservoir"sweet spot"in the H3 and H4 members.There are 58type I reservoir“sweet spots”,19 type II₁ reservoir“sweet spots”,9 type II₂ reservoir“sweet spots”,and 12 type III reservoir“sweet spots”in the H3 member.There are 48 type I reservoir“sweet spots”,39 type II₁ reservoir“sweet spots”,14 type II₂ reservoir“sweet spots”,and 10type III reservoir“sweet spots”in the H4 member.At the end of this paper,the distribution rules and controlling factors of the different types of reservoir"sweet spots"were elaborated,and the reservoir"sweet spots"distribution pattern of the low-permeability and tight sandstone was established.Type I reservoir"sweet spot"mainly developed in the area of chlorite-coated facies and dissolution facies of medium-to coarse-grained sandstone in the channel bar and braided channel sedimentary microfacies.Type II reservoir"sweet spot"mainly developed in the area of quartz-cemented facies,chlorite-coated facies,and a small amount of dissolution facies of medium-to fine-grained sandstone and sandy conglomerate in the braided channel,subaqueous distributary channel,and channel bar sedimentary microfacies.Type III reservoir"sweet spot"mainly developed in the area of tightly compacted facies,calcite-cemented facies,and quartz-cemented facies of fine-grained sandstone,siltstone,and a small amount of sandy conglomerate in the subaqueous distributary channel and sheet sand sedimentary microfacies.