Research On Bedrock Characteristics And Basement Reservoir In Kunbei Area,Southwestern Qaidam Basin

The basement of Qaidam Basin,including two parts of crystalline basement in pre-sinian and metamorphosed and epimetamorphic basement in Sinian-Late Triassic period,is a bilayer basement.The hypometamorphic rocks of crystalline basement are unconformably covered by the neritic passive continental margin or arc-back basin deposits,which are mainly terrigenous clastic rock and carbonate rock,interbedded with basic volcanic rock and Tuff.Going with volcanic activities of Palaeozoic,the granite intrusion developed widely from East Kunlun Mountains to Qaidam Basin.The intermediate-acid intrusive rocks always form a relatively large-scale batholith.Intrusive activities occurred in the Caledonian Period,Hercynian Period,Yinzhi stage and Yanshn Epoch.The research area locates in Kunbei fault terrace belt,which is the transitional zone of Qaidam Basin and Qimantage mountain in Eastern Kunlun orogenic belt.In recent years,the discovery of basement reservoir in Kunbei fault terrace belt is an important exploring breakthrough in southwestern Qaidam Basin.Based on theories of modern petrology and reservoir geology,the data of experimental analysis,drilling,core logging,thin-section identification,imaging logging and seismics,bedrock characteristics and basement reservoir of the study area are analyzed synthetically and the main achievements obtained in this paper are as follows:The results show that the Tertiary basins in Kunbei area,Southwestern Qaidam Basin have a basement of granites(mainly)and metamorphic rocks(secondary)such as slate and metamorphic sandstone.The northwest of Kunbei area is with underlying metamorphic rocks and granites while the southeast is with granites.Granites are mainly intrusive rocks of the Caledonian and Indosinian,and metamorphic rocks are maily low metamorphic slate.The main components of granites are quartz,K-feldspar,plagioclase and biotite,the secondary ones are amphibole and white mica.The granites,which are of massive structure and half self-crystal fine-mesocrystalline granular eguigranular texture,could be named as monzogranite and moyite according to the minerals composition.Chemical composition analysis shows that the granites belong to the peraluminous calc alkaline series and muscovites-bearing peraluminous granitoid(MPG)for their higher content of SiO2 and Al2O3.The highly incompatible elements(Rb,Ba,Zr)of the granites are enriched strongly while the moderately incompatible elements(Ta,Ce,Nb,Sr,Th)are enriched mildly and the weakly incompatible elements(Y)are enriched.SREE in the rock is mostly among 194.41-231.63(?g/g)with LREE/HREE ratio being 2.31-2.4 and belongs to type of LREE enrichment.The REE distribution patterns show obvious Eu anomaly and mediate Eu depletion,as well as Ce slightly depleted.And Eu/Sm value generally ranges from 0.15 to 0.16.The geochemical characteristics reveal that its source materials might come from the lower crust melting.On the identification diagram of tectonic setting,samples are ploted in the area of the syn-collision stage,showing that the granites were formed during the syn-collision circumstances,with multiphase intrusion.According to core observation,thin section analysis and imaging logs data analysis,the reservoir space can be divided into three categories:tectonic fissure,dissolved fissure and dissolved pore.And the tectonic fissure is the main reservoir space,which plays an important role in improving seepage property.The reservoir type is fracture-dissolution.The analysis of physical property and thin section illustrates that basement reservoir has low permeability and complex reservoir space,but the dissolved pore along the fracture has good physical property,and mainly develop dissolution,filling,cataclasis and weathering.The tendency of bedrock fractures in the area is mainly SEE or NWW,and the trend is mainly NNE to SSW,with a wide spread of dip angle and a consistent dip direction with the largest horizontal stress.The fractures of the effective reservoir in the bedrock mostly have a high angle(the dip angle is larger than 50°).High angle fractures are mostly under the control of tensile stress and shaped with multi-stage net cross cutting,with different fracture openings and extended degrees.Because of the squeezing action,low angle fractures are mostly of small size.The matrix of granite bedrock commonly has porosity less thanl%and permeability below 0.1×10-3?m2.NMR logging data shows that the average porosity of granite reservoir is about 3.3%,with complex reservoir space and low permeability,and the average porosity of the slate is about 2.8%.Basement reservoir is of low porosity and permeability.The bedrocks have a relatively small effective pore throat and extremely complex pore structure.Kunbei fault terrace belt in southwestern Qaidam Basin is equipped with the forming condition of basement oil reservoir.The advantaged key reservoir-forming factors are as follows:1)the high part of continuous developing paleo-uplift is the best place for hydrocarbon accumulation;2)the sealing ability of cap rock is an important condition for forming basement reservoir;3)Frequent tectonic movements make the various reservoir space and the way of oil-gas migration;4)Trap-forming period must be matched with the period of large-scaled oil-gas migration.Moreover,the final Himalayas' movement settled the structure of oil-gas reservoir.According to the sedimentary,structural and accumulation relationships,two types of accumulation models have been established in Kunbei fault terrace belt.One is fault-zone accumulation model resulted from vertical faults and fractures.That is,the oil and gas generated in hydrocarbon source rocks of Qiekelike depression migrates upward along discordogenic fault in Kunbei and accumulates in the bedrock uplift zone which nears discordogenic fault.The other is accumulation model with distant sources in which discordogenic fault is connected with oil source,the unconformity surface migratestoward the sides and the secondary fault aids to migrate.During the upward migration of the oil and gas along Kunbei fault,some of them migrates along the reservoir space within granite reservoirs and then continues to migrate upward when encountered with secondary faults,while the other migrates to the sides along the unconformity surfaces and Tertiary sand bodies for long distances and accumulates into reservoirs when encountered with suitable traps.The bedrock weathering crust developed well in the area,and obviously stratified,semi-weathered layer and unweathered layer remain intact.Semi-weathered layer which can be used as oil and gas reservoir is unconsolidated,and the dissolving pores-fissures develop well.Logging response characteristics of bedrock weathering crust in southwestern Qaidam Basin:The value of gamma ray curve is significantly higher than surrounding rock;Acoustic curve and resistivity curve present as a gradual process from top to bottom that the acoustic curve gradually become smaller.The Formation Micro-Scanner Image(FMI)shows obvious characteristics of fissure and dissolution pore.In the area with better quality of seismic data,there is an obvious seismic anomaly in weathering crust area.The correlation length attribution exhibited moderate energy.Bedrock weathering crust has the characteristics of high absorption coefficient,and low frequency-high energy value,and a medium wave impedance.The coefficient of absorption which presents red yellow represents a high absorption characteristic.Energy of low frequency appears as red yellow shows anomaly area of low frequency energy.Bedrock distribution and favorable development area of bedrock reservoir can be predicted through the combination of logging and seismic technology.