Based on the investigation of shale gas at home and abroad,this article systematically studies the basic characteristics of shale gas reservoir,analyzes formation mechanism of shale gas reservoir and its controlling factors.In addition,it discusses the matching identification method and resource assessment methods aiming at the shale gas reservoir′s special accumulation characteristics.Studies show that shale gas reservoir generally has characteristics such as self-generation and self-storage,fine particle size,low porosity and low permeability,fracture development,big reservoir area,continuous distribution,wide formation temperature and burial depth,abnormal formation pressure,low capacity,low recovery ratio,long production period and its occurrence mode is mainly adsorption state.Shale gas reservoir formation has transitional characteristic,with reservoir formation mechanism of both source-contacting gas and conventional natural gas.The type,content and maturity of organic matters,the fracture,porosity and permeability,mineral composition,thickness,humidity,burial depth of shale,as well as the temperature and pressure have certain controlling effect on the formation of shale gas reservoir.Shale gas can be identified qualitatively or quantitatively with core analysis method,geophysical method and new logging techniques such as ECS logging and imaging logging.According to the difference of principles,there are mainly four shale gas resource assessment methods for different geological and data conditions,which are analogy method,statistic method,genetic method,comprehensive analytical method. 〖WT5HZ〗Key words〖WT〗:
Through the analyses of shale porosity data from core test and logging interpretation,this paper demonstrates the shale porosity evolution trend and points out the difference of porosity evolution between normal compaction and under-compaction.Three reasons which caused difference in porosity evolution are discussed:(1)For the high quality shale during oil generation peak,the over-pressure which was generated during hydrocarbon generation slows down porosity decline rate,which is the main factor explaining why shale remains relatively high porosity at middle-deep depth.Over 5% of porosity increment was contributed by over-pressure.(2)For the high quality shale during middle-late gas generation period,nano-pores originated in hydrocarbon generation processes is another important factor causing the increment of porosity in organic-rich shale,which contributes 1.8% in average to the porosity evolution in shale.(3)The organic acid dissolution affects less to shale porosity increment.Based on the conclusions above,we have built three-stage normal compaction model,under-compaction model and nano-pore originating in organic matter correction model,analyzed the key parameters of the model and proposed a template for estimation of the face pore ratio in organic matter.
The pore structure of shales plays an important role in the evaluation of hydrocarbon content,gas exploration and development.However,there is little study on the pore structure of shales at home.High-pressure mercury injection and nitrogen adsorption experiments are employed to study the pore structure of shales and the pore structure parameters are also obtained.The effect of pore structure on gas adsorption and movement is analyzed.The results show that micropores and transitional pores are the main pore in shales,which provide the vast majority of pore volume.Micropore in the organic matter is the primary contributor to specific surface area and functions as the main adsorption space.The pore types are complicated and the pore morphology is diverse.There are airtight pores with a close end,air pores,and ink-bottle pores in shales and the pore connectivity is poor.The high content of micropore and transitional pore is beneficial for gas adsorption in shales,but the mesopore and macropore are backward,which goes against for shale gas seepage and production.
According to the gas accumulation mechanism and flow mechanism of shale gas reservoir,a mathematical model of shale gas productivity prediction was established,considering the effect of desorption,diffusion and seepage flow in shale gas reservoir.The pressure distribution and productivity equation were obtained by solving the controlling equation under different boundary conditions.The results show that,the more the desorption quantity is,the slower the formation pressure decrease.The production and the pressure square difference are linearly correlated.The productivity prediction model considering stress sensitivity was solved with new pseudo-pressure function.Production decreases with the increase of stress sensitivity of shale gas reservoir,which guide that controlling the pressure difference reasonably is able to protect the reservoir from damage.The productivity prediction model in this paper provides a theoretical foundation for shale gas reservoir evaluation and development design.
There is a set of dark lacustrine mudstones in the upper part of Chang 10 beds in Wuqi-Zhidan area,Ordos Basin.Evaluation of the geological and geochemical characteristics and hydrocarbon generation potential of this set of source rocks is very important for the oil/gas exploration in Wuqi-Zhidan area.The authors studied the distribution of Chang 10 bed dark mudstones through core and well log data and evaluated the hydrocarbon generation potential of Chang 10 source rock based on the analyses of rock pyrolysis,micro-composition of kerogen,vitrinite reflectance,group composition of solvable organic matter,GC-MS of saturated hydrocarbon,and so on.The study illustrates that the dark mudstones of Chang 10 bed spread widely and its thickness vary from 5m to 20m.Organic matter abundance and hydrocarbon generation conversion of source rock are high.The terrigenous plants which mainly attribute to type II and type III kerogen contribute enormously in source organic matter.Organic matter reached mature oil generation stage.In conclusion,they were determined to be relatively good-good source rocks.The biomarker compound of Chang 10 source rock bed has high Pr/Ph ratio,opposite “L” type distribution of ααα20R configuration C27、C28 and C29regular steranes,which differs distinctly from source rocks of Chang 7 and Chang 9 beds.The source rock of Chang 10 bed,whose hydrocarbon generation quantity is large,is another important efficient source rock of Mesozoic Group in Ordos Basin and its discovery provides scientific evidence for further exploration of the bottom layer of Yanchang Formation in midland of Ordos Basin.
Core data were commonly used in previous study of source rocks in Chongxin area,Ordos Basin.Constrained by the limited cored wells and samples,the organic carbon distribution pattern of source rocks underground is hard to be characterized.Based on logging data and core analysis and test data,using ΔlogR technology,TOC of the study area can be quantitatively calculated.Compared with traditional methods,correlation of the improved ΔlogR technology increases 23%.Satisfactory results have been reached in the study area.Results show Chongxin area has high quality source rocks,with majority TOCcontents ranged from 5%-15%.The eastern region adjacent to center of basin has high TOC contents,mostly above 5%,while the northwestern region suffered from severe erosion,and source rocks were poorly developed.
Through a large number of thin section observations and experimental analyses about Chang 8 interval of Yanchang Formation in Ordos Basin,four main diagenesis types were proposed:mechanical compaction,carbonate cementation,chlorite film cementation and dissolution in Chang 8 interval.Based on the calculation of optic compaction rate,apparent cement ratio and optic dissolution rate,combined with the depositional background of study area,we performed division comparison,quantitatively evaluated the diagenetic effects on porosity of different sedimentary areas.The northeastern sedimentary area was controlled by strong compaction and strong cementation,which resulted in the maximum primary pore loss.The western sedimentary area was controlled by weak compaction and moderate cementation,which resulted in the minimum primary pore loss.The southwestern sedimentary area was affected by moderate compaction and moderate cementation,which resulted in relatively less primary pore loss.The northwestern sedimentary area was controlled by strong compaction and strong dissolution,which resulted in relatively greater primary pore loss.Four types of diagenetic facies have been proposed: chlorite membrane and intergranular pore diagenetic facies,feldspar dissolution diagenetic facies,carbonate cementation diagenetic facies,and weak compaction diagenetic facies.The western sedimentary system belongs to weak compaction diagenetic facies,where compaction degree is lower,particles are in point contact,and physical property is best.The southwestern sedimentary system mainly developed chlorite membrane and intergranular pore diagenetic facies with good physical property.The northwestern sedimentary system mainly developed feldspar dissolution diagenetic facies,where chlorite membrane is relatively not well developed and the physical property is relatively good.The northeastern sedimentary system mainly developed carbonate cement dissolution diagenetic facies,where chlorite membrane is relatively weak,intergranular pore is not developed,and physical property is poor.
The reservoir in well block Zhao 51-Tong 41 of Sulige Gas Field is a typical tight reservoir.Therefore,figuring out the genesis of tight reservoir,searching for the favorable reservoir is one of the main problems for the natural gas exploration and development in this area.Through the analysis of provenance and core observation,combining with slice identification,casting slice,cathode luminescence and scanning electron microprobe studies，we reached the following conclusions：the main lithology of sandstone in this area is lithic sandstone and lithic silicarenite with low grade of maturity.The reservoir pore structures are analyzed,and the results show the dominant types of pore are intergranular pore and intragrannular dissolved pore.Diagenesis of sandstone in this area mainly includes compaction,cementation,corrosion and metasomatism,and the compaction and cementation are the main factors of tight reservoir.Based on the analysis of diagenetic facies,we found that moderate compaction-kaolinite filling-dissolution phase and moderate compaction-calcite filling-kaolinite filling-dissolution phase are the main ones.The conclusion is that small pore space and poor connectivity of pore are the underlying cause of tight reservoir in this study area.
Biogenic methane is an important resource which exists in conventional microbial gas reservoir,and unconventional microbial gas,such as hydrate,shale gas,coalbed methane,and so on.By the analysis of the living environment of microbes,such as temperature,salinity,pH,living space,nutrient substrates,and so on,and combined with the evidence for active microbe,it′s thought that:(1) The appropriate formation water salinity,moderate pH,abundant nutrition are beneficial for the microbial life activities.(2) The phenomenon of GC presenting bulge phenomenon,the detected 25-norhopane,ethene and propene,the δ13C1<-60‰,and the emergence of biogenic gas in the adjacent areas,and so on,is the evidences of the existence of biogenic gas in the area.(3) Influenced by the seasonal variation,the primary biogenic gases of acetate fermentation are formed periodically;the reduced secondary biogenic gases of carbon dioxide are formed in the micro-fractures member of Upper Jurassic.In the Mohe Basin,biogenic gas is not only the important natural gas resource style,but also one of the potential gas sources of hydrate.
At present,the proven reserves of kerogen cracked and oil cracked gases at high thermal evolution stage have accounted for about 26% of the total reserves in China,so it is important to carry out systematic analysis of the formation and distribution characteristics of natural gas.Based on a number of pyrolytic experiments on kerogen and oil at high thermal stage,,combined with the researches on cracked gas,this paper further analyzed the genetic characteristics of cracked gases from oil,marine-lacustrine mudstone and lacustrine coal measures,and proposed five gas accumulation modes.Crude oil cracking was generally thought to occur at temperature more than 150℃.Our experiment proves that it begins to form cracking gas at temperature more than 190℃ in some areas.Gas generation potential of marine mudstone would exhaust when RO approaches 3%.However,lacustrine coal measures still have great methane generation potential when RO is near 2.5%-5%,and the gas amount generated at this stage accounts for more than 20% of the total generated gas.The cracking potential of lacustrine mudstone lies between the two formers,and the coal source rock has the greatest gas cracking potential.Cracking gas mainly has five accumulation modes,including ancient uplift crude oil cracked gas,ancient slope weathering karst cracking gas,dense sandstone coal source rock cracking gas,special reservoir cracking gas,and coal seam-shale source cracking gas.The ancient uplift and slope reservoir,the marine-terrestrial transitional basin dense sandstone reservoir and Meso-Cenozoic special reservoir are important potential exploration direction.
Several methods for quantitatively effectiveness evaluating of carbonate vuggy are discussed.Based on the micro- and macro-pore structure and the analyses of influences on reservoir production,a new method was proposed.In addition to general macroscopic porosity parameter,this method also considers some microscopic pore structure parameters (displacement pressure and average pore throat radius) and the reservoir development degree in vertical.It made up for porosity and increased the accuracy of evaluating reservoir effectiveness.It has been applied to Tazhong and Lungu zones of Tarim Basin,and good effects have been obtained.
Qiangtang Basin is located in the east of Thethys structural domain,in the middle-west part of the Qiangtang-Changdu crustal block,between the two suture zones of Kokoxili-Jiangshajiang and the Bangonghu-Nujiang.Its area is about 18×104 km2 including central uplift belt.It′s the biggest Mesozoic marine residual basin in the Qinghai-Tibet Plateau,and has great exploration potential.There are different ideas about the location in Thethys structural domain in Mesozoic and the basin′s character.Based on comprehensive analysis,this study concludes that the basin is located in the south margin of the Laoya continent,and is a typical superimposed basin.It has undergone 5 evolutionary phases,including foreland basin of north Qiangtang,rift of south Qiangtang,passive margin rift stage,passive margin depression stage and fade phase.
Based on the study of the plannar and profile styles of faults and their activity,the pool forming stage and the hydrocarbon distribution of Neogene,the relationship between fault activity and Neogene reservoir formation in Zhanhua Depression was discussed.The results show that the intensity of fault activity in the study area has a prominent feature of “strong in the west but weak in the east” before the deposition of Es3 ,while it has a contrary feature after the deposition of Es3,and the active phase of the NE trending and nearly EW trending faults matches well with the hydrocarbon filling periods.The periods when fault controls hydrocarbon and when fault controls migration are the two key stages which demonstrate the influence of fault activity on hydrocarbon accumulation.When fault controls hydrocarbon,fault activity provides good material basis for oil and gas accumulation while when fault controls migration,fault activity provides efficient migration pathways for the transportation of hydrocarbon.According to the activity velocity of fault during the two stages,the faults controlling the distribution of petroleum were divided into three degrees named Ⅰ,Ⅱ and Ⅲ.The degree Ⅰ and degree Ⅱ faults lead to richer accumulation of Neogene hydrocarbon in drape structural belt,and the degree of the control of fault on oil determines the petroleum richness of Neogene.
The curve of vitrinite reflectance (RO)-depth (H) is successive across the unconformity on top of Carboniferous-Permian strata in Zhaoyuan area,which means the Carboniferous-Permian buried deep during the later sedimentation.The maximal paleogeothermal gradient of the study area is the Carboniferous-Permian paleogeothermal gradient analyzed by R.P.Suggate standard plate.Based on the occurring positions and homogenization temperatures of fluid inclusions,there are twice hydrocarbon charges.The results show that the Carboniferous-Permian source rocks have a secondary hydrocarbon-generation after later deep burial,and Zhaoyuan area is the favorable area for secondary gas-generation.
To identify the influence of base level cycle on the differences of the reservoir quality among the fluvial reservoir at different stages of base level cycle,the main controlling factors of fluvial reservoir quality in Es 7 2 Formation of 2 block in Shengtuo Oilfield were analyzed based on the sediment dynamic analysis of high resolution stratigraphic sequence and quantitative analysis of core testing and statistical data.The control mechanism of base level cycle on the differences of the reservoir quality among the fluvial reservoir at different stages of base level cycle is identified.The pattern of the reservoir quality among the fluvial reservoir at different stages of base level cycle is confirmed.The results show that the diversity of sedimentary environment of reservoir at different stage of base level cycle was caused by the variation of accommodate space and sediment supply due to the changes of the base level in different scale.On that basis,the base-level cyclic influenced facies controlling porosity-permeability log interpretation model is established.The porosity-permeability logging interpretation precision is improved for the target layer in study area.
The primary source rock of Yabei Sag was coal measures of Yacheng Formation in Oligocene and its distribution was controlled by sedimentary facies.According to the analysis of regional geological setting,paleontology,lithology assemblage,tectonic feature,paleo-topography etc,the sedimentary facies of Yacheng Formation are mainly tidal flat,lagoon and partly fan-delta.The tidal flat grows on the place where the slope is gentle and the topography is broad.Sporopollen analysis showed that there were a lot of mangrove trees in Yabei Sag during the period,which could form coal after death.The relative closed hydrodynamic condition in the sag is favorable for the formation of coal measures.Therefore,tidal flat bears favorable conditions for the formation,accumulation and preservation of coal measures and is a favorable sedimentary facies belt for the development of coal measures source rocks.Tidal flat in the Yabei Sag is mainly developed in the south and southeast gentle slope.Yacheng Formation in the southeast gentle slope has greater burial depth and higher thermal maturation of organic matter,indicating the formation of oil and gas.
Based on the reservoir and hydrocarbon pool characteristics from Lower Ganchaigou Formation to the Upper Youshashan Formation of carbonate hydrocarbon pool in Nanyishan,comparing with lacustrine clasolite hydrocarbon pool,we found there are differences in source,reservoir,cap,source-reservoir-cap assemblage,trap type,type of oil and gas pools,and relation of all the above in Nanyishan lacustrine carbonate oil and gas pools as follows: (1)Nanshishan structure located in the hydrocarbon-generation kitchen,with numbers of source rocks,and hydrocarbon can be captured from the nearby area.The oil source came from Xiaoliangshan Sag,on the southwest of Nanyihshan.Crude oil in E23in the deep was from the source rock in E23,while N 2 was from N1.(2)Four hydrocarbon-bearing series in Nanyishan can be grouped into two types.One is a layered lithologic reservoir controlled by the structure in middle-shallow layer.Lithology of the reservoir is micrite limestone or dolomite,algal limestone or dolomite and grain limestone or dolomite.Intergranular pores and solution pores are storage space.The other was a condensate gas pool in the upper E32,taking marlstone or argillaceous dolomite as reservoir and pore-fissure as storage space.(3)The Nanyishan carbonate reservoir and mudstone formed source-reservoir-cap assemblage frequently interbedded in vertical.(4)Nanyishan pool inN22 had characteristics such as thin reservoir and low permeability.It had elastic drive and solution gas drive as main drive type.Layered lithologic reservoir is controlled by anticline structure.While gas layer of the middle deep condensate gas pool was concentrated in vertical and dominated by the structure and fracture in plane.The productivity capacity was quite different due to fissure storage space.(5)There are three main controlling factors on the formation of Nanyishan pool: Firstly,Nanshishan Structure located in the hydrocarbon-generation kitchen.Hydrocarbon can be captured in the nearby area.Secondly,the fracture and fracture system contributed to the translocating system.Finally,the lacustrine carbonate reservoir played aa key role in the formation of oil and gas pools.(6)Nanyishan structure developed slowly before Pliocene,and the development peak was in Early-Middle Pliocene,but then it stopped .The Lower Ganchaigou Formaion had hydrocarbon generation peak in Early-Middle Pliocene and it formed an authigenic reservoir.The Upper Ganchaigou Formation came into oil generation peak from late Pliocene to Quaternary,and along the northern and southern wing in the structure oil and gas migrated into N1,N12,N22 and the reservoirs formed finally.
Unconventional oil and gas reservoirs are those that cannot use conventional methods and technical means for exploration and development under current economic and technological conditions.The gas reservoirs of Xujiahe Formation in Yuanba Gas Field belong to tight sandstone gas reservoirs.Geochemical data of 20 gas samples from 14 wells of Xujiahe Formation are analyzed for main gas component,gas dryness coefficient and so on in the Yuanba Gas Field.The tight sandstone gas of Xujiahe Formation in Yuanba Gas Field is dominated by methane with content up to 98.39%.The dryness coefficient is basically more than 0.97 for the dry gas with high thermal maturity.δ13C1 values of gas samples in the study area are lower than -30‰,while the majority of the samples is characterized by positive carbon isotope series.In addition to the biogenic origin,the Xujiahe tight sandstone gas mainly consists of self-sourced coal-derived gas and other-sourced oil-associated gas.Secondary alteration has greater influences on oil-associated gases.Gas samples in the study area normally have R/Ra values varying from 0.01 to 0.02.The crust origin helium indicates that tight sandstone gas reservoirs of the study area are mainly located in the tectonically stable area.The widely distributed coal series and dark muddy rocks in the Xujiahe Formation are the main source of coal-associated alkane gas.Part of the tight sandstone gas in the Xujiahe Formation is oil-derived alkane gas,therefore it infers that its existence may be from the underlying strata of the Upper Permian Longtan Formation.
Fractured horizontal well is an effective method to develop low-permeability tight gas reservoirs and shale gas reservoirs.And the productivity of fractured horizontal well is the key factor which can determine whether unconventional gas reservoirs could be developed effectively.However,there are rare methods which can be used to calculate productivity rapidly,effectively and simply.Therefore,under such a circumstance,it is imperative that researchers should make out new effective measures to improve current condition.To solve this problem,based on numerical simulation and an ideal model,this paper established a new method which could predict fractured horizontal gas wells′ productivity in low-permeability tight gas reservoirs rapidly,economically and efficiently.According to the geological characteristics of low-permeability tight gas reservoirs in China,single well′s simulation model has been established.Then a 8×16 orthogonal vector was set up and the processes of revision isochronal well test were simulated.The simulation results show that effective permeability and thickness of the reservoir are the most important two parameters among those which can influence the productivity of gas wells.At the same time,the results can be regressed to relationship equations between binomial deliverability equations′ factors (A and B) and formation capacity.The practical application shows that this method has convenient operation and reliable result,and is well-adopted,economical and time saving.Particularly,these productivity equations are suitable for field application.
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