| Reservoir assemblage in the bed rocks are the predominant oil reservoirs in the Kunbei area,southwestern Qaidam basin,previous works mainly focus on the passageway and accumulation of oil,as well as potential evaluation of oil productivity;the geological features,vertical zoning and four property relationship for the reservoir assemblage in the bed rocks are still unknown.In addition,eorrosion pores in the bed rocks are crucial factors that control the oil productivity.This thesis mainly focus on the geological features of the corrosion pores and structure fracture,logging identification standards,distribution patterms and factors for the reservoir assemblage in the bed rocks in the Kunbei area.These results can provide important insights into the oil exploration and productivity in the Kunbei area.Corrosion pores in the weathered granitic rocks can promote the reservoir properties of the bed rocks,this thesis have carried out the following works:(1)according to the zircon LA-ICP MS U-Pb dating,whole-rock geochemistry of the granitic rocks in the Kunbei area,determine their formation ages,distribution patterms and geological implication;(2)based on detailed drill core observation,petrography,nuclear magnetic resonance and mercury injection experiment,analysis the geological features of the pores and fractures in the bed rocks,reservoir space structure,variation patterm of the T2 spectral line,vertical zoning and variations patterns of the reservoir properties in different sections;(3)according to the detailed comparison between core observation and data from well logging,evaluate the four property relationship of the bed rocks from the Kunbei area.In addition,by means of high resolution imaging logging and nuclear magnetic resonance(NMR)to determine geological features,distribution patterns,Spectrum bandwidth,peak values and percentage of secondary pores in the bed rocks.Constraints the standard lower limit of property and oiliness for the bed rocks from the Kunbei area.Detailed zircon LA-ICP MS U-Pb dating reveal that the granites from the four drilling cores(q 404,q 406,q 1612-8,q 1613-8)have identical ages of 251±3Ma,256±4Ma,247±2Ma and 251±6Ma,respectively,these ages are identical with the Permian-Triassic granites from the eastern Qaidam basin.Detailed geochemical analysis indicate that these granites display typical affinities of highly-fractioned I-type granites,in combination with the geological background,we propose that the Permian-Triassic highly-fractionated I-type granites were resulted from partial melting of intra-crustal mafic rocks,the primitive I-type granitic melts undergone significant fractional crystallization of feldspars.The occurrence of highly-fractionated I-type granites in the southwestern Qaidam basin suggested a Permian-Triassic active continental margin in the northern margin of the East Kunlun orogenic belt.The granites from the Q603 has zircon U-Pb age of 430±2 Ma,which contain K-feldspar megacrysts and have high A/CNK values,suggesting S-type affinity,which was considered to be formed by muscovite dehydration melting in collisional setting.The bed rocks in the Kunbei area can be subdivided into four sections from the top to the bottom:(1)? residual weathering crust,which is consisted by clay stone with limited pores and fractures in it,it is cover layer in some area;(2)? corrosion section,the reservoir space in this section are mainly pores in inter-crystals or inter-grains,their T2 spectral line display high-left and low-right shape,NMR porosity concentrate upon a small area,the high irreducible water saturation indicate high contents of fine-grained minerals;(3)? fractures section,the reservoir space in this section are mainly structure fracture and corrosion pores,TZsPeetral line display symmetrical shape,NMR porosity concentrate plotted in a dispersed area,their low irreducible water saturation indicate the motion reservoir space place predominant role in this section;(4)IV unweathered granitic rock,the rocks in this section are fresh granites with limited fractures and none corrosion pores;the bed rocks in the ? and ? section are seriously weathered,some features of fresh granites were also preserved in these two sections,the reservoir space change from"pore-type" to "fracture-type" from the ? to ? section.The cross-plot of various well logging parameter is effective to discrimate different rock type is the bed rocks.Our detailed works indicate that cross-plot of natural gamma(GR)and interval transit time(?t)is the most effective way the discrimate the bed rock with different weathered features:residual weathering crust GR<282(API),?t>196(us/m);corrosion section GR<282(API),?t<196(us/m);fracture section:282<GR<358(API);unweathered granites:GR>358(API);the natural gamma(GR)and Elemental Capture Spectroscopy(ECS)display perfect "Three ladder" shape in the vertical section,which indicate that the potassium(K)contents was controlled by weathered degree.According to the detailed analysis of 16 drill cores,Q16 and Q4 region preserve the most complete weathered crust,while residual weathering crust in the Q 16 and Q6 have been erosion.By means of magnify the logging response characteristics in fracture-pore developed sections,we can construct new parameters to discrimate the location and type of fracture-pore in the bed rock,discuss the distribution patterns of fracture and pores in the bed rocks.The results indicate the pore-bearing section have low values of wave impedance and high values of comprehensive parameters of crack.The fracture density display positive correlation with the distance to faults,while the fracture frequency display dispersed correlation with the distance to the faults.Imaging logging indicate that the fractures in the bed rock display NNE direction,with NWW inclination,the dip angle ranging from 24 to 54 degree.Frequency analysis of corrosion pores and T2 spectral line from NMR indicate that the secondary corrosion pores are the predominant type(up to 80%)in the weathered bed rocks.According to these results,we found a new logging quantitative evaluation system that mate the bed rocks in the Kunbei area,southwestern Qaidam basin.The data from well logging and oil test indicate that the oil-bearing section has high natural gamma(GR)and interval transit time(AC)values,low resistance values in high resistance background,radial amplitude difference in high and low electrical resistivity.Some oil penetration and patch can be found in the oil-bearing section.The well that have data of nuclear magnetic resonance(NMR),we calculate the oil saturation by means of the pseudo capillary pressure method and electrical resistivity methods,respectively,the two methods give similar results.To the well that have no data of nuclear magnetic resonance(NMR),we calculate the oil saturation by means of electrical resistivity method,which is based on rock electrical parameters.The final results indicate that property lower linit for the reservoir assemblage in the bed rocks is porosity greater than 5%,oil saturation greater than 50%. |
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