The exploration in Qa idamBasin now has the characteristics of obvious stage and complexity after more than half century's development. Since the beginning of the Tenth Five-YearPlan period, QaidamBasin has focused itself on following the principle of òstrengthening the earlier stage exploration, emphasizing technology, giving prominence to main points, and adopting effectiv e measuresó, liberating exploration concept, a nd relying on advanced technologies and has obta ined notable results in its oil and gas exploration, especially in lithology reservoir explora tion in the western area. With the development of geologic cognition and fundamental work, hydrocarbon accumulation conditions, exploration potential, key target areas for next exploration have been more understood, and the target of the oilfield development has also been laid out. In a ddition, the general exploration concept of the 11th Five-Year Plan has been established, that is, insisting on the three principles of regional: preliminary exploration, risk explora tion, as well as exploration and development integration, emphasizing two key stones including fine clastic rock exploration and strategic breakthroughs in complicated areas, undertaking fine research and optimization of deployment. Oil and gas exploration will be carried out in three stages, i.e., fine exploration, strategic breakthrough, and active preparation. The targets of next exploration are presented as well. With the respect to petroleum exploration, the southwestern area, the foothillbelt of Altun, Mahai-Nanbaxian structural belt in the northern ma rgin, the northern marginal foothill belt, and Delingha depression are the targets of petroleum exploration. In respect of naturalgas exploration, the Sanhu biogas zone, the oil-type gas in the northwestern area, and the coal-type gas in the northern margin are the targets of natural gas exploration

Based on the comprehensive analysis of regional structure background, boundary conditions, basement properties, structura lmov ement, unconformable surfa ce, stratigraphic distribution, sedimentary characters, structural evolution history, it is considered that the Miozoic-Ceozoic structural evolution of Qaidam Basin has undergone two structural cycles of extension and compression, which can be divided into four evolution stages including the Ea rly toMiddle Jurassic extensional rift and depression sta ge, the Late Jurassic to Cretaceous compression depression and uplift stage, the Paleocene(Lulehe age) to Early Miocene(Shangyoushashan age) integer compression depression and the western Qa idam Basin regiona l pull-apart feeble rift stage, as well as LateMiocene(Shizigou age) to Quaterna ry compression inversion stage. The basin structural evolution controls the distribution and evolution of hydrocarbon resource rocks, the formation of traps, as well a s the migration, accumulation a nd preserv ation of oil and gas, and further controls the distribution of oil a nd gas.

Integrated research results show tha t the Tertiary lithologic oil reservoir in south a rea, western QaidamBasin,has favorable reservoir-forming conditions, exhibiting the distribution of lithologic oil reservoirs is controlled by the present structure framework and paleo-structure developed area; The distribution of three depositiona l systems, including Ala 'er fluvia l-delta -la keshore;Alerjin flooding fan-fan delta-half-deep lacstrine facies;Kunlunshan mountains flooding fa n-fluvial-sha llow la keshore depositional system, and the fa vorable sa ndbodies formed in fluvial,delta and fan delta regressiv e system tracts controlled the enrichment of lithologic oil reservoirs; The oil-enrichment sa gs and the direction of oil-ga s migration a nd accumula tion controlled the forma tion of the lithologic oil reservoirs. After the exploration practice in recent years,it is proposed that a set of exploration method suited to the geologic features of lithologic oil reservoir in this area￡?including the analysis on sedimentary ba ckground, stratigraphic sequence division and correlation,sequence bounda ry tracing, comprehensiv e analysis of sedimenta ry facies, target ev alua tion and analysis on the regula rity of reservoirforming. Explora tion results show that the sanbodies, formed in the fan-delta depositional system in foreland Alerjin, Hongliuquan-Yuejin lake depositional system a nd fluvial delta depositiona l system inYuejin area, has the adva ntages to form the lithologic oil reservoirs and will become the promising target for next exploration in this area.

The northern ma rgin a rea in Qa idam Basin has been explored for many years, where some oil and ga s fields such a s Lenghu,Nanbaxian and Mabei ,whose hydrocarbon source rocks are mainly Jurassic sources, were discovered.Through comprehensive analysis of petroleum geological conditions in this area,it is considered that the main control factors of Jurassic oil and gas reservoir-forming in northern Qaidam Basin might be fea tured by a s followings: a.The distribution of oil-gas phase state are controlled by the distribution of hydrocarbon source rocks, hydrocarbon evolution and hydroca rbon-generating center; b.Key time of oil-gas migration and accumula tion in northern ma rgin of Qaidam Basin is Later Eogene and Middle Neogene, the important structural factors that controlled oil-gas migration and accumulation a re the paleouplift developed in Yanshania n period and structural belts developed in Eogene;b.The large-scale regional unconformity face developed between Mesozoic and Cenozoic eras and regiona l permea ble sandstone distributed in Tertia ry offer the favorable condition for oil-gas la tera lmigration;d. Violent activity of main-controlled faults and their derived fa ults in Later Himalaya period is helpful for oil-ga s vertical migration.

According to the high-resolution and high-a ccuracy analysis on the paleomagnetic dating for profile and the seismic growth sequence, it is believed that at least 10 stages of structura lmovement and three structura lepisodics hav e occurred in the Altun slope area of western Qaidam Basin since the Cenozoic era , indica ting that the Altun area has undergone three a ccelerate-uplift sta ges. In general, this area has foreland characteristics. The Paleogene is the main developed area of hydrocarbon source rocks in western Qaidam Basin, of which thrusting is the main structura l feature and the sedimentary and subsidence centers are relativ ely stable. Strike-slip flexuring is the main structural feature of the Neogene and its depositional and subsidence centers obv iously migrate from southwest to northeast along Altun fault. The vertica l basin-oriented Altun Orogenic belt can be divided into the high fault-nose belt in the front of mounta in a rea , the low fault-nose belt in the middle pa rt, the lea ding thrusting belt a nd the recoiling belt in the Pa leogene of Qa idam Basin. It is the superimposition of multistage thrusting, strike-slip, and tilting uplift that formed the current structural traps in the Altun slope area of western Qaidam Basin. The well-developed structural traps, unconformity traps, and lithological traps are the main structural trap types in this region and they ha ve certa in potential for oil and gas exploration.

According to the a nalysis of the ga s-bea ring seismic anomaly a nd old-well log interpreta tion, the current conditions of biogas exploration in the eastern Qaidam Basin and its technical problems are illustrated. Solutions, such a s 2D seismic reprocessing, reinterpreta tion or recollection, implementing variable-speed mapping, carrying out 3D seismic exploration, are proposed to solve the problems of the low relief structure hard to find out and the thin sandbody hard to identify under the special geologic conditions of the little strata dip, the low reflection v elocity, the thin interbedded of sandstone-mudstone, and saturation formation wa ter. It is hoped that such solutions may provide an instruction to the further deployment of biogas exploration in the eastern region and the explora tion research in similar regions

For more than 50 yea rs of exploration research, it is revealed that two series of hydrocarbon source rocks of Qaidam Basin are mainly distributed in Tertiary and Jura ssic. The hydrocarbon source rock of Tertiary,mainly distributed in the western region of the basin, has higher hygrocarbon conversion rate, but the richness and thermal evolution of its organic matter are poor in general. While the source rock of Jurassic, mainly distributed in the northern margin of the basin, is of rich organic matter a nd the IIIII2 organic matter is the most common type. There are two kinds of reservoir including detrital rock and carbonate rock, of which the detrital reservoir is the main oil-bearing reservoir in the basin. It is vertically distributed in Jurassic, Palaeocene, Eocene, Lower Xiaganchaigou Formation, and Youshashan Formation. Moreover, it is widely distributed in the area, of which the sandstone reservoirs with different sedimentary origins such as fluvial delta, beach, embankment, and sub-water fun are the most common depositional env ironments. T he total oil resources in Qaidam Basin are 2.15 billion tons with the proven rate being 15.3 percent. It shows that a large potential for petroleum resources exploration still exists in the basin. Based on comprehensive analysis, three key pre-prospecting zones are proposed for next exploration, that is, the structural lithologic zone in the southwestern Qaidam Basin, the deep structural belt in Shizigou-Youshashan, the buried structural belt in the northern margin of Qaidam Basin

Qaidam Ba sin is the fourth largest gas-bearing region in China, of which the natural gas can be classified into oil-type gas, coal-type gas and biogas according to the carbon and hydrogen isotopic index. There are three suites of gas-bearing systems developed in the basin including the oil-type gas systemin the northwestern area, the coal-type gas systemin the northern margin, and the biogas systemin the eastern area. Based on the research on the basic characteristics of the natural gas in the basin and the analysis of its resources potential and exploration degree, the thinking for next exploration is proposed as òputting the foothold on biogas and making a breakthrough in pyrolysis ga s. It is suggested that the water-soluble gas and shallow gas in the eastern region and the pyrolysis gas in the northwestern region be the key targets for exploration

According to the study on field geological survey, comprehensiv e seismic data interpretation, and general evaluation of exploration ta rgets, Delingha depression, along the ea stern segment of the block-faulted zone in the northern ma rgin of Qaidam Basin, is considered as a n extensional depression developed in the Middle a nd Late Jurassic-Early Cretaceous of Mesozoic. After undergoing the stages of transtension sag in Tertiary and compressive sag in Quaternary, the hydrocarbon source rocks from Member four to sev en of the Dameigou Formation in Middle Jurassic were stably distributed in Delingha depression and potentialhydrocarbon generation rocks of Carboniferous were developed in the depression as well. Reservoirs are distributed in all layers, in which the quartz sandstone of Member six in Dameigou Formation and the fluvial deltaic sandstones of Cretaceous are the most common reservoirs of Mesozoic.Traps a re ma inly distributed in sa lient belts, and nappe structural belts are developed in the front of mountain area. It is considered tha t the forming time of structures matches the time of oil and gas reservoir formation, making it possible to form reservoir-seal a ssemblage featured as self-generation and self-storage and lower-generation and upper storage. According to the assessment on the exploration potentia l and prospects of the oil and gas resources in Delingha depression, it is believed that the self-genera tion and self-storage oil-gas reservoir forming assemblage in the Middle Jurassic is the most rea listic exploration a rea. What's more, Tuonan stage benches, the thrusting belt of Zongwulong, and the Aimunike uplift zone are considered to be the major sa gs for hydrocarbon generation, which have certain exploration prospects.

The signal-to-noise of seismic data is relatively low for complex mountanious region in Qaidam Basin affected by complicated surfa ce condition and underground structure, as result of that, this region is bla nk of seismic explora tion or seismic data, directly influencing on the exploration potentia l eva lua tion and fa vorable ta rgets selection. With the continuously expanding of seismic exploration in the basin, it is carried out to tackle key problem of mountainous seismic technology, ba sed on the old data analysis a nd exploration experience,through implementing a series of measures including observation system design aimed at the ta rget,fine investigation for surface structure,sampled pa rameters test , well depth design by selecting rock and bed a nd other acquisition techniques, a series of more ma ture exploration technique suited for complex mountainous region ha s been developed a nd the break through has been made in quality of data acquired field, providing the strong technique support for petroleum exploration in complex mountainous region.

With complicated surface a nd underground geologic conditions, sev eral problem that Qa idam Basin has been facing with includes not-idea l sta tic correction and low signa l-to-noise while seismic data processing. T hrough the research on the technology of static correction in mounta inous region and disturbing wave denoising, it is proposed tha t the method integra ting static correction, migrated ima ging with denoising is used to improv e the quality of seismic profile in complicated structure area to reflect the underground geologic feature of complicated area. The primary effect has been obta ined through applica tion of this method in No.7 Lenghu a nd Shizigou areas.

From the perspective of geophysical field, applied integra ted geophysical a nd geochemical explora tion techniques reveal that Qaidam Basin has double basement structures. The upper is the folded basement and the crystallized basement exists in the deep part. The thickness v ariation of the crysta llized basement controls the upwa rpingdownwarping pattern of the ba sin. The folded ba sement lithology has obvious zoning cha racteristics and it va ries a lot in different zones. As a result, different zones have different deformed cha racteristics. The zonality is determined by the fracture of deep structure. The front belt of mounta in is a n important area for oil and ga s exploration in Qaidam Basin, however, there are still a lot of limita tions on the cognition of it due to its sophisticated topographic and geologica l conditions. The structura l cha racteristics of the foothill belt have been basically known, some piedmont depressions a nd overthrust nappe zones ha ve been discovered, and the exploration area of Qa idam Basin has been widened as well on the basis of the application of the integrated geophysical and geochemical exploration techniques

The reservoir lithology of upper Xia ga nchaigou oil reservoir in Ga sikule a rea of Qingha i Oilfield dominates in a lga limestone, with strong a nisotropism a nd change of physica l property.The stora ge spa ce types include primary porosity, secondary porosity and fra cture. Due to FMI technique ha ving the advantages of highresolution and a lot of geological information, it ca n be used to recognize, differentiate and evaluate alga limestone, siltstone a nd limestone reservoirs. The FMI image of alga limestone reservoir features in floc or cloud-like, outer boundary being not straight, a symmetry interna l structure, disorder bedding, loca l dissolution; T he FMI ima ge of siltstone reservoir and calca reous siltstone features in bright-yellow, brown-yellow strip; and The FMI image of limestone and ma rlite features in white,bright-yellow a nd white bla ck interbedded strips. Open fracture a nd highly conductive fracture show as dark sinusoidal curve on the FMI image, and high resistivity fracture shows a s brightyellow and white sinusoidal curve on the FMI image. More developed fractures, better permeability of reservoir has.The dissolution fracture shows a s locally enla rging a long the fracture, dispersed stellar or beaded dark spot on the FMI image.

Technical countermea sures for well Kun-2 are drawn up to solve the key difficulties in deep exploration well in the northern marginal zone of Qaidam Basin. A suite of optimal a nd fast drilling techniques suitable to the northern marginal zone of the basin are dev eloped on the basis of the optimization of drilling equipment, wellbore structure, drilling bits and tools assembly, a s well a s drilling parameters and properties of drilling fluid, and the resea rch a nd application of oil-gas reservoir protection techniques. As a result, the completion drilling depth of wellKun-2 rea ches 5,950 meters.As the well depth increases by 888 meters and the big-sized well hole gets longer, the cycles of drilling and well completion of well Kun-2 are reduced by 98.75 days and 124.2 days respectively, with the speed of each drilling rig increasing by 317 meters monthly a nd the a verage drilling rate increasing by 0.82m per hour. An optima l composite and temporarily blocked plan is established to protect the oil-gas reservoir. By applying this plan, a new kind of drilling fluid is made up, with its restorable permea bility rea ching more than 87.5 percent and having better fluidity and lower filtration loss, obviously improving mud-ca ke qua lity a nd enhancing sidewall stability.

The reservoir in the western region of Qaidam Basin in Qinghai Province is a typica lmarlstone reservoir with low porosity and low permeability, in which ca rbona te rock, cla stic rock a nd mudstone account for the same proportion respectiv ely. It is difficult to improv e such a kind of reservoir for its difference from pure carbonate rock reservoirs a nd pure sa ndstone rock reservoirs. By a pplying the techniques of gel acid fracturing, hydraulic sand a nd flowback, alkyd a cidizing, and oil base fra cturing, 43 times of technical tra nsformation ha ve been carried out in various exploration wells of the region including 12 times of acidizing, 20 times of acid fracturing, and 11 times of sand fracturing. The success rate of acid-fracturing and acidizing operation is far higher than that of sand fracturing, a nd the field operation parameter and the design coincidence ra te a re both kept a t more than 85 percent. The results show that 80 percent of the explora tion wells see an increasing fluid production ba sed on the reservoir reconstruction, with the avera ge fluid flowback rate being 65 percent

The data transmission and production management of exploration and production in QinghaiOilfield always depend on microwave transmitter-receiver,which can not meet the demand of rapid exploration, blocking the transmission and intercommunion of information. In addition, field operation of oil-gas exploration features in high mobility, fixation communication systemis not set up,which making production management be not in time and lack veracity. Through the star communication network of VSAT Ku wave band was set up and linked with distance-controlled telephone network at the base, so as to realize the free communication between production unit field and management department at base, at same time, by using the information system of exploration production, it is realized that the specific data processing such as log profile along with drilling well, well logging curves, pla yback of log second data , real time data of log instrument, and oil-testing data , geophysica l da ta a s well a s web releasing function, with the geological data, the daily report of drilling well and oil-testing released onsite together, the dynamic information platform of exploration management has been established, which provides leaders a t various level, research personnel and managers with an integrated, abundant data, dynamic information platform along with drilling

In the process of investigation a nd practice of exploration project, QinghaiOilfield Company insists closely a iming at the explora tion targets, paying much attention to prophase preparation and exploration benefit, resolves the problems such as project goal being not clear and not carefulma nagement, increases the geophysical exploration load and strengthening the explored well location argumentation while project arranging and implementing, introduces and applies in time some availa ble technique and method to ensure the effect of exploration, and as a result, a passel of litho-stratigraphic traps such as the southea stern slope of Gasi, Qigequan and Hongliuqua n have been discovered

T he a ppearance of fossil Cyprideis was demarcated on the magnetic biostratigraphic column in Qaidam Basin. T he eruptible time of Cyprideis is at the later Middle Miocene, a bout 12Ma ago, which is the important evidence to divide the boundary between Shangyoushashan Formation and Xiayoushashan Formation, and also key mark that seismic T2 reflect interface is used to trace and validate whole basin. The eruptible point of Cyprideis can be recognized clearly in Tarimand Junggar Basins, which has been pla yed a n importa nt role in the stra tum classification and contra sting of Neogene inside in these two basins as well as comparing with that in Qa idam Basin. It is suggested that the boundary between Shangyoushashan and Xiayoushashan Formation would be determined basica lly through sampling few core and much debris a nd drawing the abundant curve of Cyprideis existed in debris.

T he mistake of strata comparison between designed boundary and the real boundary by well completion of wellYHD-1 in 2002 in Qaidam Ba sin was made, its reason would be the result of continuing to use the wrong data of stra ta division from well Yanshen-1(1959) and well Yan-14(1995). Agip's summary report of well completion said tha t the aim bed drilled would be Pa leocene-Eocene Lulehe Forma tion, but the authors keep opposed opinion tha t the a bov e boundary is still drilled into Miocene Xiayoushasha n Forma tion. T he a ge value of stratum a t well bottom estimated by both parties ha s la rger difference, which is at lea st the discrepancy of 26Ma, accounting for second fifth of Cenozoic era(65Ma ).On the basis of analysis on the cause of false comparison byAgip Company, it sets forth that the comparison of Cenozoic strata of Qa idam Ba sin, which fea ture in complex sedimentary structure and multiple material sources, should be adopted comprehensive correla tion method, especially integrating seismic stratigra phy with biological stra tigraphy method.