Contents of Natural Gas Geoscience, No.4, 2017
Crust-derived helium is generated from the radioactive decay of uranium,thorium and other helium-derived elements in geological bodies.Compared with conventional natural gas,helium is a typical weak source gas as a result of extremely slow generation rate and absence of generation peak.It is associated with methane or carbon dioxide reservoirs frequently and related to groundwater closely.Helium can reach the industry standard with 0.1% in volume fraction.In order to study the accumulation mechanism of helium,the previous results on Henry’s coefficient and solubility of helium,nitrogen and methane are summarized and the key roles of Henry’s Law in the helium migration,accumulation and preservation are discussed by simulating calculation in Weihe Basin.According to the Law,the gas solubility in dilute solution is controlled by the gas partial pressure and Henry coefficient.Compared with the carrier gases,the Henry constant of helium is high,with striking difference in low and high temperature and the helium partial pressure is greatly different in helium source rocks and gas reservoirs,resulting in the great differences of helium solubility in the two places.The accumulation progresses are as follows.Firstly,helium can dissolve into water and migrate out of helium source rocks due to the high partial pressure and high temperature,causing high solubility.Secondly,when water soluble helium is transported to the shallow gas reservoir,it is prone to be out of solution and into reservoir due to the extremely low partial pressure and low temperature.Meanwhile part of carrier gas dissolves into water,as if the helium is “replaced” out.Furthermore,the low concentration funnel of dissolved helium is formed near the gas reservoir and other dissolved helium continues to migrate towards the gas reservoir,which greatly improves the helium accumulation coefficient.Finally,when entering the gas reservoir,helium is difficult to dissolve into water of cap layers due to the low partial pressure,resulting in the preservation of helium in reservoir.The results show that the helium enrichment mechanism in the aspects of migration,accumulation and preservation,providing theoretical basis for helium resource exploration.
The sedimentary environment in Longfengshan area is very complicated,the comparison of reservoir and the research of the regularity of high production are very difficult.In order to define the sedimentary characteristics and controlling factors of reservoir development in this area,based on regional sedimentary background,combined with the core observation and data analysis of the well logging,we used sedimentary facies analysis of unified method of macro geological understanding and micro geological evidence and found this region mainly developed fan delta front sedimentary system.The favorable sedimentary microfacies is the channel.Through analysis of the distribution characteristics of the sedimentary facies,the lower part of Ying Ⅳ formation developed channel with vertical superimposition at multiple depositional stages.The upper part of Yingcheng Ⅳ formation developed multiperiod distributary channel of sand-mud interbed.By the experimental analysis of fluid PVT,combined with the testing data,Longfengshan Gasfield is high oil-bearing property and has nearly saturated condensate gas reservoir.The distribution of effective reservoir is controlled by sedimentary factor and structure factor.Finally,the distribution range of gas is determined.According to the favorable area and producing reserves,well location deployment was carried out.So the researches provide a technical support for the productivity construction.Based on the analysis of the sedimentary characteristics and the influencing factors of gas reservoir distribution,the drilling success rate and the capacity compliance rate are 100%.
In reservoirs,shale gas flows within a wide range of differential pressure and pressure gradient.The flow process is influenced by several mechanisms such as Knudsen diffusion and slippage effect.Flow characteristics vary with flow mechanisms.By experiments of nitrogen and helium flow in 9 shale samples,the variation of apparent permeability within a certain pressure range at isothermal condition could be obtained.The results show that apparent permeability of shale where Knudsen diffusion occurs is tens of times higher than conventional gas logging permeability.The actual flow ability of reservoirs increases because of Knudsen diffusion under the condition of small pressure difference or pressure gradient.When the pressure level or pressure gradient is relatively low,diffusion is an important mechanism for mass transfer.The conclusions are significant to further research on basic flow rules of shale gas and guidance to improve shale gas production.
Khuff Formation of Permian is the most important gas production zone in Middle East.From 1970′s,the Khuff Formation exploration works had been carried out in UAE and few traps had been discovered.However,the exploration result is not good:all the onshore traps test are dry or with water;the offshore trap test have limited gas with lots of NHCG.It′s very difficult for the future exploration work for Khuff reservoir.Khuff reservoirs have good source,reservoir and cap.The Qusaiba hot shale is the source of Khuff reservoir;the Sudair muddy limestone and middle anhydrite in Khuff are both good cap rock for Khuff reservoir.Based on the analysis of comprehensive information,the key point of Khuff gas reservoir effectiveness is the structure development and the generation and migration of source.The pool forming pattern shows:the offshore trap was developed with salt swell in the early stage and effected by Acadian,Hercynian and Zagros structure movement in the late stage.The offshore Khuff trap kept integrated.In Jurassic,oil migrated to the offshore trap and accumulated;In Late Cretaceous,oil cracked into gas.Then the gas reservoir of offshore Khuff trap is effective.However,as the burial depth of Khuff Formation becomes more and more,more and more H2S and CO2 are generated due to TSR.The development of onshore traps began in Oligocene and was affected by Zagros structure movement.The trap formation time is later than that of the formation and migration of source,so the onshore Khuff traps have no gas.
Based on comprehensive analysis of internal factors (i.e.,organic matter type and organic matter maturity) and external factor (i.e.,biodegradation and migration) on characteristics of butane,the application of geochemical characteristics of butane was studied.The paper cleared the effects of internal factors on characteristics of butane.The coal type gas derived from humic organic matters,which had the higher iC4/nC4 ratio and δ13C4 value than the oil type gas from sapropellic organic matters.The iC4/nC4 ratio decreased with the increase of maturity,and it decreased quickly at the mature stage and slowly at the high-mature stage.The iC4/nC4 ratio would increase as the biodegradation increases,and the stronger the biodegradation,the higher the iC4/nC4 ratio.Whether the migration would affect the iC4/nC4 ratio and how it affects,both are uncertain.It is found that the butane can provide information about the biodegradation,genesis and maturity of natural gas.The iC4/nC4 ratio can be used to effectively determine the gas is biodegraded or normal.The iC4/nC4 ratios of biodegraded gas were higher than 2 and most of them were higher than 3.If the iC4/nC4 ratio is higher than 3,the natural gas is biodegraded gas.The iC4/nC4 ratios of normal gas were lower than 3 and most of them were lower than 3,if the iC4/nC4 ratio is lower than 2,the natural gas is normal gas.For the gas suffered less influence by external factor,the oil type which has iC4/nC4 ratios lower than 0.8 and δ13C4 value lower than -26‰,the coal type gas which has iC4/nC4 ratios higher than 0.8 and δ13C4 value higher than -26‰.The iC4/nC4 ratios can help analyze the maturity of natural gas.For the iC4/nC4 as trace index,there are still more controversial and poor effectiveness,and should be carefully to choose it for tracing migration.
In order to investigate the characteristics,genesis and origin of the natural gas of the Xujiahe Formation in the central Sichuan Basin,we analyzed the geochemical compositions and carbon and hydrogen isotopes of gas samples from Guang’an Gasfield and made comparison with surrounding areas.The natural gas is mainly composed of methane.It is characterized by wet gas with a dryness coefficient generally lower than 0.95,except at Yuanba-Tongnanba area.The dryness coefficient is mainly controlled by the maturity of source rocks.The δ13C1 values range from -43.8‰ to -29.3‰,δ13C2 values range from -35.4‰ to -24.3‰,and δDCH4 values range from -179‰ to -152‰.The identification of gas origin and gas-source correlation indicate that the natural gas of the Xujiahe Formation in most gas fields is mainly derived from coal-measure source rocks in the Xujiahe Formation.At most areas,partial reversal of the carbon isotope series(δ13C3>δ13C4) is found in only a few gas samples,which is ascribed for the mixing of gas from the same source but different stages.
Core samples of source rocks and oil sands from Yanchang Formation in southern Ordos Basin were collected systematically and the hydrocarbon generation characteristics,sedimentary environment and oil-source correlation of which were studied utilizing various analytical methods.The results show that source rocks in Chang 6 to Chang 9 members are at mature stage of evolution.Among the studied samples,source rocks in Chang 73 member have the best quality with average TOC of 13.99% and mainly kerogen type of typeⅠ.Source rocks in Chang 62 member have the second-best quality with average TOC of 4.12% and kerogen type of typeⅠ+Ⅱ1.Source rocks in Chang 9 member have average TOC of 2.85% and kerogen type of type Ⅱ1+Ⅱ2.The source rocks of Chang 6 to Chang 9 members are all formed in the typical fresh lacustrine environment.The source rocks in Chang 8 to Chang 9 members deposited in weak oxidizing to oxidizing environment with swallow water and the organic matter of which was mainly derived from higher plants.The source rock in Chang 6 to Chang 7 members deposited in reducing environment with relatively deep water and the organic matter of which was derived from a source mixed by hydrobios and higher plants.The quality of source rocks in the studied area is closely related to its sedimentary environment.The source rock deposited in reducing sedimentary environment with more hydrobios input in its organic matter source has the best quality.Oil sands in Chang 7 to Chang 8 members are sourced from source rocks in Chang 73 member according to the hierarchical cluster analysis.
Maigaiti Slope,Bachu Uplift is one of important tectonic units in the west of the Tarim Basin.The Well Luosi 2,which was placed on the Maigaiti slope belt,succeeded in the Penglaiba Formation at the bottom of the Ordovician making a significant breakthrough in oil and gas exploration in new layer.In this paper,the geochemical characteristics of oil and gas of the Well Luosi 2 are studied systematically to discuss the sources and the genetic characteristics of the oil and gas.The results reveal that the crude oil in Well Luosi 2 has a very low level of biomarkers constrained by thermal evolution such as steroidal terpenes and so on,and could not be used as an identification parameter to identify the origin or other geochemical characteristics.Meanwhile,the matured parameters such as adamantane and methyl phenanthrene of crude oil indicate that the mature degree of the crude oil is high that is similar with the Cambrian source rock in this area.The carbon isotope of the whole oil is heavy.Based on the geochemical characteristics of crude oil and the comparison with hydrocarbon source rocks,it should be derived from the Upper Cambrian source rock.For the natural gas of Well Luosi 2,the results reveal that it has a high drying coefficient and the carbon isotope of natural gas is heavy and it should also be derived from the Middle-Upper Cambrian hydrocarbon source,according to the correlation with the crude oil.
Due to the fact that the influence of CO2 and CH4 mixed gas on the rock electrical parameters is not clear,calculation accuracy of water saturation is low.Through the simulation of reservoir conditions rock electric experiment,respectively with different content of CO2 and CH4 mixed gas coreflood,with determination of theb,n and m values,the effects have been analyzed of mixture of CO2 and CH4 on the electrical properties of rock.Different CO2 content of mixed gas dissolved in the formation water,the a and m values basically have no difference;with the increase of CO2 content,b value increases, n value decreases.A prediction model was established to the b and n values variance with the content of CO2.The water saturation calculated by this method is in good agreement with the core analysis results,and the reliability of the method is confirmed.
Coordinate rotation technique can be used to establish new elastic parameters.Finding proper rotation angle can maximize the ability of distinguishing the physical and gas-bearing properties of the reservoirs by using the newly constructed elastic parameters.Fusion technique can fuse different kinds of seismic attributes,which contributes to improve the accuracy of reservoir prediction.By petrophysical analysis,we use coordinate rotation technique to construct sensitive elastic parameters of physical and gas-bearing properties,and then use prestack simultaneous inversion to calculate these parameters.From the seismic attributes extracted from different prestack and poststack seismic data,we select those which have good relevancy with “sweet spot” factor.Finally,we fuse the inversion results of elastic parameters and the selected seismic attributes to identify and predict the reservoir “sweet spot”.The result matches the well data and has a higher reliability than that of using one single method.
This research quantifies the logging identification characteristics of the mudstone containing siderite and establishes its identification template accordingly using the method of logical analysis and the spider diagram analysis,based on the drill core observation and the logging data from Benxi Formation to Taiyuan Formation in the northeast area of the Ordos Basin.It is analyzed that the mudstone containing siderite has the features of high Photoelectric Absorption Coefficient(PE),high Density(DEN),high Natural Gamma(GR),low Interval Transit Time(AC),low Resistivity(M2RX) and low Neutron Porosity(CNCF).The results of siderite identification of 12 drills in the research area show that the mudstone containing siderite is unevenly distributed with the total thickness of 3.9-25.4m and the total layers of 8-28.It is concluded that mudstone containing siderite has the characteristics of low porosity,low permeability and high breakthrough pressure according to the quantitative analysis of the logging data.The average permeability,average effective porosity,and average breakthrough pressure of the mudstone containing siderite is 2.29×10-8μm2,1.20%,and 12.32MPa,respectively,which specifies its function as the impermeable gas barrier.Based on the spatial development position,sedimentary environment and forming process of the siderite containing siderite,four stable gas barriers are identified from Benxi Formation to Taiyuan Formation in the research area.The strata from Benxi Formation to Taiyuan Formation are divided into 5 independent Coalbed-Methane Systems combining the gas survey data.The boundary of each independent Coalbed-Methane System is the interlayer which develops a certain scale of rock stratum mainly consisting of mudstone containing siderite vertically and has the characteristics of stable area distribution and good continuity.
Finding out favorable CBM development sections in developing fault areas can deploy wells placement accurately and reduce the blind investment.According to the data on the southern of Shizhuang area,the floor traces were recovered about 3# coal seam after tectonic movement like stage of Yanshan,the early stage of Himalayan and the late stage of Himalayan by tectonic leveling method and wave superposition theory,the areas were divided into 18 blocks.According to the gas/water differentiation phenomenon,structural curvature method,coal structure observation method,the differences about the coal reservoir pressure and permeability after the tectonic movement effects were obtained.Then the favorable block and favorable block of CBM development were evaluated.The results show that:the escaping gas and formed coal near normal faults are the main reasons for low gas production.The relative high value blocks in the floor elevation in faults interval zones are of low permeability and low reservoir pressure,and gas potential is small;the relative low value blocks in the floor elevation in faults interval zones are of low permeability and relatively high reservoir pressure,and gas potential is medium;the relatively middle value blocks in the floor elevation in faults interval zones are of better permeability,and gas potential is better.The actual gas production about CBM wells verified the accuracy of theoretical analysis.The studying results can provide idea and reference for optimum favorable segments in developing fault areas.
As coal metamorphism is a key factor controlling the physical properties of coal reservoirs,the development characteristics of pores and fractures show significant differences in different ranks.In this paper,based on the methods of scanning electron microscopy(SEM),fluorescence microscopy observation,mercury intrusion,low-temperature liquid nitrogen adsorption,nuclear magnetic resonance(NMR) and computed tomography(CT),taking the coal seam of Shanxi Formation in eastern Ordos Basin,the characteristics and variation of coal reservoirs in different ranks are studied and the controlling regulation of metamorphism on the development of pores and fractures in coal are revealed.The results show,as the coal rank increases,the quantity of plant tissue pores and intergranular pores in coal decreases while the quantity of gas pores increases.The porosity of the coals varies in a wavelike fashion of ‘decreasing-increasing-decreasing’ as the coal rank increases.The proportion of mesopores and macropores decreases at first and then tends to be stable and the proportion of micropores and transition pores shows opposite variation with coal rank increasing.During coal metamorphism,the BET surface area and BJH total volume of adsorption pores decrease,the average pore diameter increases,the surface of adsorption pores becomes rough at first and then smooth,and the pore structure tends to be homogenized.As the coal rank increases,the quantity of fractures decreases firstly and then increases,their length increases and the fractures arranges regularly.
A number of independent coalbed methane(CBM) systems are often superimposed in coal-bearing strata with multiple coal seams,and severely impede the multi-layer commingled CBM production rate.So we take Bide-Santang Basin,western Guizhou as examples,variable coefficient of gas content gradient and water head pressure coefficient are used to represent development degree of independent CBM systems quantitatively,and the geological controlling factors and mechanism are analyzed from perspectives of structure,sedimentary and hydrogeology,respectively.Following conclusions are obtained:the marginal faults and the resulted triangle structure framework control the stress-strain characteristics and coal accumulation of the study area.From western marginal fault zone to basin internal part,the ground stress and strain weaken,coal thickness and stability increase and mudstone roof is more developed.Sedimentary facies distribute in NE direction,with the sea influence increasingly enhanced,the deposited characteristics of coal-bearing strata change regularly from west to east in the basin:The sand/mud ratio reduces,sedimentary cycles occur at higher frequency,and authigenic minerals,such as pyrite and kaolinite,tend to be more developed and fill the pores.Groundwater flows from NW to SE,with the increasing flowing resistance,resulting in weak hydrodynamic conditions and groundwater stagnation in the SE direction.Under the coupling controls of structure,sedimentary and hydrogeology conditions,the independent CBM systems are increasingly developed from NW to SE in the basin.The study area is partitioned into four subareas with different development degrees of superposed CBM systems,which are verified by the CBM exploration and development practice,to a certain extent.
Shale gas exploration in Sichuan Basin has gained great success.In south and southeast of Sichuan Province,Lower Silurian marine shale gas achieved commercial discovery,and Lower Jurassic lacustrine shale gas in east and northeast of Sichuan saw industrial gas flow.Characteristics of the shale gas in Sichuan Basin have been revealed including multi-flied,multi-layer and multi-type.Using Argon-ion polishing-SEM,MIP-adsorption analysis,pulse test and X-diffraction test,features between marine and lacustrine mud shale such as sedimentary facies,hydrocarbon generation conditions,reservoir characteristics,fracability and enrichment have been systematically investigated and compared.Results indicate that for marine facies,the high quality shales in Longmaxi Formation mainly developed at the deep-water shelf within a craton interior depression.The black shales have wide distribution,consistent thickness,high organic carbon,high organic porosity,well-developed lamellation and rich organic silicate content.All these features are favorable for marine shale gas generation,enrichment,storage,and reservoir stimulation.There are three different types in terms of shale gas enrichment (ultra-high pressure (UHP) in the basin,overpressured shale gas at the inner edge of the basin,underpressured shale gas outside the basin).The most favorable types for shale gas exploration are intra-basin UHP and overpressured shale gas type;for lacustrine facies at Daanzhai interval,favorable lithology combination is interbedded shale and limestone which mainly developed in an alternate deposition changing between storm beach and mud of the carbonate lake slope at highest lake level.With large accumulated thickness,higher organic carbon,more grain-edge pores and fractures,high permeability and more brittle minerals (silica and calcium),these shales are considered as potential targets for lacustrine shale gas exploration.Based on the accumulation characteristics of different shale gas types in both marine and lacustrine facies,carrying out research on adaptive exploration and development technology is the key to realize an effective and economical shale gas development in various fields.
According to the Lower Cambrian black shale outcrop observation and drilling core description in southern Sichuan-central Guizhou,based on the method of argon ion polishing-scanning electron microscopy,geochemical isothermal adsorption method and geophysical data,this paper discussed the conditions of shale gas enrichment.In the study area the sedimentary facies of the organic-rich shale is deep water shelf facies and middle shelf facies; the organic carbon content is above 2% in general,and the maturity is 1.32%-4.78%,averaging at 2.85%.Type I kerogen is the dominant organic materials,and the reservoir types are mainly micron-nano porosity,with developed fractures.The shale property statistics show that porosity ranges between 0.24%-11.9% and permeability ranges between (0.000 087-0.535)×10-3μm2,and the average content of gas capacity from isothermal adsorption test is 2.74m3/t.The content of gas of Huangye-1 well from field analysis is between 0.16m3/t to 0.99m3/t with an average of 0.52m3/t.The content of gas of Jinshi-1 well from field analysis is between 1.51-2.41m3/t,with an average of 1.8m3/t.The research shows that shale in the Lower Cambrian in Sichuan Basin has good experience for shale gas accumulation of long shallow burial-long uplift-rapid deep subsidence-rapid uplift of the burial history and thermal evolution history and the structure is relatively stable,so it results in good shale gas preservation conditions,but outside the area of the basin,the hydrocarbon generation and expulsion,which experienced many times,also suffered from the transformation of tectonic movement,and the preservation conditions of shale gas reservoirs were destroyed in different degrees.Therefore,the superior material basis,preservation condition are the main controlling factors for shale gas enrichment.The exploration area should go to the Sichuan Basin,the constituency to find conducive to shale gas reservoirs of the burial history and thermal history,and at the same time we should take the principle which is “high to find low,strong to find weak,good to find thick”.
Exploration of shale gas in Silurian in China mainly concentrates in the Sichuan Basin and around,new achievements are acquired constantly in recent years,i.e.,explorations in Jiaoshiba and other regions in Sichuan Basin have made major breakthroughs.Shale-gas reservoirs in those areas are characterized by overpressure.Shale gas reservoirs out of the Sichuan Basin are characterized by normal pressure because of strong tectonization.Based on practices of exploration,this study researched comprehensive geology,including the shale sequence stratigraphy,sedimentary microfacies,geochemistry,reservoir characteristics,tectonization,etc.Three major factors controlling the accumulation of shale gas with normal pressure in the southeast of Chongqing was established.Firstly,development of high quality shale in deep continental shelf is the base of enrichment of shale gas.The shale gas accumulation static evaluation index of Wufeng-Longmaxi Formations in the research area are superior.The high-quality shale is 20-40 meters thick,the TOC is 2%-4%,and RO is 2%-3%.Secondly,preservation condition is the key factor to the shale gas accumulation and high yield.Through the comparative analysis between drilling results of Wells PY1,NY1,RY1 and faults,burial depth,distance from the denudation area,etc.,it indicates that the intension of tectonic reworking,the distance to the fault and outcrop,moderate burial depth are the key conditions of shale gas enrichment.Thirdly,development of organic pore is important to the high yield of shale gas.Using SEM,specific surface area and pore size analysis,nano CT,thermal simulation experiment method,etc.,it demonstrates that the organic pore is the primary reservoir space of shale gas occurrence,thermal evolution of organic matters,compaction,and formation pressure are key element to influence organic pore.Standards for dessert optimization of normal pressure shale gas in the southeast of Chongqing were established.
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