The Character Of Deep Clastic Reservoir Of Xujiaweizi Fault Depression In The North Of Songliao Basin

Xujiaweizi fault depression is located on the east of central palaeohigh belt in Songliaobasin; the area is about 5310 km2. It is the main exploration targets for deep natural gas andhigh-content inorganic generated CO2 type gas coming from deep source rock in the basin. Atpresent, there are 70 wells drilled for deep natural gas and therein 22 wells have reachedcommercial gas flow standard in this region. Clastic rock and volcanic rock are more importantreservoirs. But the diagenesis of this kind of clastic rock is rather serious and the preservingconditions of original pores are much poorer so the reservoirs problem is one of the mainproblems to keep the control of the breakthrough of deep natural gas exploration. For example,the studies on pore genesis, preserving mechanism and well-developed belt of secondary poresof deep good reservoirs are rather skin-deep and limited. And furthermore the controlling factorstudies of reservoir properties are not thorough and the classifying standards that are suitable fordeep natural gas exploration are not enough, so it is difficult to make a favorite reservoirdistribution prediction.In view of the above problems, guided by reservoir geology and geochemical theory, thefollowing studies such as reservoir litholigy, physical properties, pore types, secondary poregenesis and its distribution, the influencing factors on reservoir physical properties and so on areconducted with the help of comprehensive applying latest microscopic testing techniques andmethods for this kind of reservoirs. In addition, the above study has still integrated withmacroscopic reservoir study, basin sedimentary history and heat history studied results.On the basis of detail checking regional geologic setting and studying petroleum geologicbackground, by means of the following combined studies including systematical thin-sectionanalysis, X-diffraction analysis, scanning electron microscope, microscopic analysis by cathodeluminescence and so on, the classification of the studied area's clastic rock can be determinedand divided into two kinds: one is sandstone; the other is conglomerate. The main types areclastic arkose and arkose sandstone. The debris content below Yingcheng formation is obviouslyhigher than the one above Denglouku formation. The maturation of mineral compositions islower. The interstitial filling materials are mainly muddy mixed matrix and siliceous, carbonate,laumonite authigenic cements and so on. Sandstone reservoirs are generally distributed in No.1and 2 intervals of Quantou formation and moreover Denglouku formation. The gravelcompositions in conglomerate are rather complicated, magmatic rock, metamorphic rock andsedimentary rock are can be seen, but the former takes absolute advantage. The mixed matrix iscomposed of mud and sand level clastic materials; the cement is mainly made up of calcite,laumonite and quartz (secondary outgrowth). The type, number and filled model of infillingmaterials often determine the pore type and reservoir properties of conglomerate formation. Theconglomerate is located in the following three formations: Denglouku, Yingcheng and Shahezi,the conglomerate reservoir distributed in No.4 interval of Yingcheng formation is the mostimportant. The reservoir space of deep sandstone is mainly located in various secondary pores that aregenerated by solution and metasomatism, the favorite reservoirs are buried between 2600m to3100m, they are controlled mutually by sedimentation and diagenesis; the beneficial reservoir inthe conglomerate is generally distributed in No.4 interval of Yingcheng formation, whose poretypes are mainly intergranular pore partially left by intergravel muddy material after compactionand fractures existing within and along the conglomerate. Diagenesis is the dominant controlling factor over deep layer's physical properties, but dueto the differences resulted from original characteristics of sandstone mineralogy in distinctsedimentary facies, there are much more obvious deference in cement porosity of sandstone andorganic-inorganic diagenesis. The compressive resistance of sandstone is directly controlled bydifferent sedimentary facies: the cement porosity of sandstone in fluvial subfacies is more thanone in offshore shallow lake facies in nearly same depth, but the later is pretty higher than one inthe sandstone of flood plain subfacies. That is to say that the ability of compressive resistance ofthe former is greatly stronger and the original porosity after compaction is still much higher.Different sedimentary facies still have a certain influence on sandstone diagenesis facies andpore types. A certain controlling effects of different sedimentary facies on sandstone compositionand its structure can result in the action on migration of pore fluid and the interaction amongclastic particles. Finally it acts on diagenesis facies, pore origin and types. The study of secondary pores possesses much more important significance for deepreservoir description. The paper has established a recognizing mark for the secondary pores indeep clastic rock. The above pores can occupy 20%～90% of the total pores, ones of goodreservoir can reach 3%～7% that is equal to the lower limit of the porosity, so thewell-developed belt of secondary pores has the most possible potential to be high-productivitypetroleum reservoir. The recognizing method of secondary pores is suitable to other region andbasin, so it owns much high value of popularization. Organic acid and phenol in deep layer are critical factors of the formation of secondarypores, the distribution of high-porosity belt has something to do with the belt ofhigh-concentration organic acid and phenol. Dark mudstone and coal layers are well spread indeep layer and furthermore well rich in Type Ⅲkerogen that produces a large amount ofacid—one number lever higher than Type Ⅰand Ⅱkerogen, thus it provides abundant source ofacid. Under proper geological conditions, after decarboxylation, kerogen can generate organicacid. After dehydration, clay mineral can be a carrier and migrate to sandstone along faults orthrough high-permeability layers, finally can result in partial solution of soluble compositions inlaumonite and feldspar and furthermore produce new reservoir space. Later generated oil and gascan migrate into reservoir through the channel of acid water and moreover discharge acid water,in the end secondary pore petroleum reservoirs can be formed. Fault T4 possesses control actionover the generation of secondary pore in Denglouke formation. In detail, Fault T4 generated inNo.1 interval of Qingshankou formation at the period of sedimentation is the more beneficial tothe production of secondary pore and the accumulation of oil and gas. The critical time ofsecondary pore formation is between 97Ma～96Ma. Predicted by the inclusion with brine solution at the same time with petroleum enclosure,the time of petroleum accumulating in Denglouku formation is between 96Ma and 85Ma, thereare much better match relation of the developing period of secondary pores with the geologicaltimes of petroleum migration into the reservoir, therefore the match relation model has beenestablished between the generation period of secondary pores and the times of petroleummigration into the reservoir, thus it has enriched and improved the theories of reservoir geologyand reservoir geochemistry. The lower limit determination of deep tight gas sandstone and conglomerate reservoirsdirectly influences the calculation of effective thickness and reserves of these reservoirs. Fromthe point of view of reservoir microscope, the paper studies the characteristics of sandstonereservoirs with different productivity and their controlling factors, finally the porosity lowerlimits (separately 5.5% and 4.0%) in tight sandstone and conglomerate gas reservoirsexploration have been determined. In addition, the classification standards of tight sandstone andconglomerate have been established and furthermore the classified evaluation and prediction ofthese reservoirs have been done. The porosity of sandstone reservoir is mainly 2%～12%; andthe permeability is 0.01×10-3μm2～50×10-3μm2. The best physical properties regions in horizonrefer to Shengping-Wangjiatun and South Cangde, thus it is coincident with the well-developedarea of secondary pores. The secondary pores in No.3 and 4 intervals of Denglouku formationare located nearby main fault T4, secondary pores are likely developed beside the fault. Theporosity and permeability of conglomerate are respectively 1%～8% and 0.01～5×10-3μm2.Original sedimentary conditions and later diagenesis have great affect on the physical propertiesof conglomerate. The content and feature of sandy filling material among gravels directlyinfluence the physical properties, but the difference of gravel composition has not so muchinfluence. The conglomerate reservoirs with high sand content and low content of muddy,calcium and so on infilling materials are generally considered to be good reservoirs. Reservoirproperties are greatly improved just due to the strong solution of partial laumonite and feldspar.Current exploration shows that the individual fine-medium conglomerate in interval No.4 ofXingcheng region is pretty thick, its accumulative thickness is generally more than a hundredmeters and furthermore distributes in large area. In addition, the limit of conglomerateproductivity is desirably lower, so it is can be called much more favorite target play. During the studies, the paper has set up and applied a new technique to research pore 3Dstructures with the help of laser common focusing microscope. Much better results have beenobtained in the study of pore structure of conglomerate in Xushen No.6 well. This technique has...