The carbon isotopic series of alkane gases could be divided into three types:(1)positive carbon isotopic series:δ13C values increase with increasing carbon numbers among the C1-C4 alkanes,which is a typical characteristic for primary alkane gases;(2)negative carbon isotopic series:δ13C values decrease with increasing carbon numbers among the C1-C4 alkanes;(3)partial carbon isotopic reversal,which has no increasing or decreasing relationship between the δ13C values and carbon numbers.Negative carbon isotopic series could be further subdivided into primary and secondary origins.The former is a typical characteristic of abiogenic gases while the later is resulted from the secondary alteration imposed on biogenic gases,which is commonly observed in over-mature shale gas or coal-derived gas.Previous works have proposed several possible explanations for negative carbon isotopic series of secondary origins such as secondary cracking,diffusion,the Rayleigh fractionation of ethane and propane through redox reaction with the participation of transition metal and water at 250-300℃.After a comparative study,the authors found that negative carbon isotopic series of secondary origin for both shale gas and coal-derived gas appeared in the area where source rocks(shales)are at over-mature stage while it was not observed in the area where source rocks(shales)are only at mature-high mature stage.As a result,extremely high maturity(＞200℃)is the main controlling factor for negative carbon isotopic series of secondary origin.Within this maturity interval,secondary cracking,diffusion and Rayleigh fractionation of ethane and propane could happen separately or together.
The Upper Paleozoic strata of Tarim Basin have abundant resources of marine oil and gas.In the Tahe area,Halahatang area,Tazhong area of the basin,many large-scale oilfields have been found.These oilfields have been confirmed with oil and gas reserves more than 2.5 billion tons of oil equivalent and have completed the annual output of more than 14 million tons of marine oil and gas equivalent.The view that the main hydrocarbon source rocks are the Cambrian or Ordovician is still controversial.Geochemists have made much work and effectively guide the oil and gas exploration in Tarim Basin.Because of the complexity of the basin and the limitation of samples,the research work and fine contrast are restricted.In this article,we investigated the Cambrian strata outcrop of Tarim Basin in detail.Through analyzing large amount of outcrops,high-quality hydrocarbon source rocks of Yuertusi Formation have been found in more than 10 outcrop points in Aksu region.The source rocks lithology is black shale,of which total organic carbon (TOC) content are between 2%-16%.Total organic carbon (TOC) of the black shale layer can be up to 4%-16%,especially in outcrops of the Yutixi and Shiairike.This is the best marine hydrocarbon source rock which has found in China by now.The source rocks distribute stably in the Aksu region,the thickness of which is about 10-15m.It is formed in sedimentary environment of middle gentle slope to low gentle slope.Organic matter enrichment is controlled by upwelling currents.Thick strata of dolostone developed in Xiaoerblak Formation are considered as good reservoir of beach and microbial reef in the upper strata of Yuertusi Formation.No hydrocarbon source rocks have been found in outcrop of Xiaoerblak Formation.Thick strata of gyprock and mudstone development are a set of good cap layer in the Lower Cambrian.This hydrocarbon accumulation combination has great exploration potential.
The confirmation of the Ordovician reservoir accumulation model is the key factor to recognize the distribution rule of Ordovician reservoir in the Maigaiti slope. Analyzed from failed well of Ordovician,and combined with areal structural evolution characters,it is considered that Ordovician reservoir accumulation of Maigaiti slope and the northern,middle of Tarim Basin has a certain similarity,has more differences. The weathering crust karst reservoirs of the Yingshan Formation which mainly belongs to platform low energy facies are not well developed;the tectonic evolution process of ‘north to south migration’ of Ordovician in the Maigaiti slope differed from the inherited palaeohigh of the northern,middle of Tarim Basin,the adjusting-migration type of the palaeohigh is not favoured of migration,accumulation and preservation of oil and gas. The Maigaiti slope is far away from the hydrocarbon generation sag of the Maijiaer,while the northern,middle uplift of Tarim Basin is around the sag. The study indicates that there are two main layers in Ordovician for oil-gas accumulation:the oil-gas pool of the Upper Yingshan Formation which develops limestone crust reservoir and mainly distributes in the fracture belt,and that of the Lower Yingshan Formation which develops inside dolomite rock reservoir and stably distributes in this area,too. Autochthonous source rock of Cambrian develops in the Maigaiti slope,and vertical migration is the main type for oil-gas accumulation;the hinge belt that developed during the Ordovician structure adjustment is beneficial for rich accumulation of the secondary oil-gas.
As gas wash affecting oil and gas reservoir,Lungu east reservoir in the Luntai Uplift is characterized by “Dry gas,Heavy oil” property,and its oil and gas phases and saturation show a regional trend from unsaturated gas reservoir,saturated gas reservoir,to saturated oil reservoir from southeast to northwest through the PVT phases analysis.The oil and gas phases have little changes vertically.However,some wells were featured by unsaturated gas reservoir in the upper intervals,while saturated gas reservoir in the lower intervals,which was mainly controlled by the intensity of gas wash,complicated oil and gas accumulation process,fracture and cap rocks.Based on the analysis of Lungu east oil and gas phases,distribution and main controlling factors of oil and gas saturation in the Lungu east area,we observed that the favorable gas accumulation in this area is adjacent to source kitchen.It is concluded that the oils of Triassic reservoir in Jilake area leaked due to the unfavorable sealing conditions in Himalayan movement.Limited gas washing occurred in Triassic oil reservoir for the occurrence of barrier layers,which were resulted from rapid subsidence of Kuqa Formation.We predict that the Jilake field still has an abundance of unsaturation reserves in Ordovician and Caboniferous,based on the analysis of Lungu east oil and gas phases and saturation characters,which is very significant for the exploration of Jilake offshore area.
The origin of natural gas in Changxing-Feixianguan Formations of Longgang Gasfield was studied in detail using geochemical methods in this paper.The gas has a very high dryness coefficient,very low ethane and other heavy hydrocarbons.Apart from a small amount of N2 and CO2,the non-hydrocarbon gases,it generally contains H2SWithin the field location,Changxing-Feixianguan Formations themselves do not have hydrocarbon-generation potential.Near the edge of the Kaijiang-Liangping Trough,there developed Dalong Formation,but it also has very low TOC content in the area of Longgang Gasfield,cannot act as effective source rock.The geochemistry of natural gas is much different from the gases generated by Silurian and Cambrian source rocks.So it is impossible that the gas in Longgang Gasfield is from Silurian and Cambrian source rocks.Gas reservoirs generally contain bitumen,which is considered a product of crude oil cracking.That the carbon isotope fractionation between the bitumen and methane is not distinct indicates that the gas is not directly from oil cracking.The carbon of methane and ethane has isotopically heavy value,which is considered from high-over mature coal-formed gas,mainly from Longtan Formation coal measures.Compared with the gas from high-over mature Xujiahe coal measure source rock in western Sichuan Depression,the methane in Longgang Gasfield has abnormal heavier carbon isotopic value.It is due to the superposition of these two factors together:Higher evolution of source rocks and mixing of gas degassing from water.It is not caused by TSR that most researchers believed at present because the methane carbon isotopic values have no relationship with H2S content.
Analysis of hydrocarbon generation,trap formation,inclusion homogenization temperature,authigenic illite dating and ESR dating were used to understand the hydrocarbon accumulation history and its difference of gas reservoirs in Upper Triassic Formation of Sichuan Basin.The results show the hydrocarbon accumulation mainly occurred during Jurassic and Cretaceous and could be classified into three stages:early hydrocarbon generation accumulation stage and mass hydrocarbon generation accumulation stage before Himalayan Epoch,and parts of hydrocarbon adjustment and re-accumulation during Himalayan Epoch.The second stage is more important than the other two.The Hydrocarbon accumulation histories are obviously distinct in different regions.In west Sichuan Basin,the gas accumulation began at the deposition period of member 5 of Xujiahe Formation and mass accumulation occurred during the Early Middle Jurassic to the end of Upper Cretaceous.In middle Sichuan Basin,the accumulation began at the early Upper Jurassic and the mass accumulation occurred from Middle Lower Cretaceous to the end of Upper Cretaceous.In south Sichuan Basin,the accumulation began at the Middle Upper Jurassic and the mass accumulation occurred from Middle Upper Cretaceous to the end of Upper Cretaceous.The accumulation history of west Sichuan Basin is the earliest,and the south Sichuan Basin is the latest.This paper will help to understand the accumulation process,accumulation mechanism and gas reservoir distribution of Triassic gas reservoirs in Sichuan Basin.Meanwhile,it is found that the authigenic illite origin of deep-burial and its dating is a record of the later accumulation.This suggests that the illite dating needs to fully consider illite origin,otherwise the dating results may be difficult to accurately reflect hydrocarbon accumulation history.
Research of Tongwan Movement gradually became popular for its multi-episode tectonics in Sinian-Early Cambrian.It arrived at a common view that the Tongwan Movement was closely related to the karst reservoirs in Dengying Formation of Sinian.However,it is still controversial on the relationship between the Tongwan Movement and Sinian-Early Cambrian paleo-tectonics.Therefore,a study on the Tongwan Movement is meaningful for its tight connection with hydrocarbon geological conditions.Based on drilling core,outcrop and seismic data in central Sichuan area and combined with geological materials,the stages,the motion of Tongwan Movement and its impact on the Sinian-Early Cambrian paleo-tectonics were studied,leading to the results as follows:(1)The 3-episode Tongwan Movement appeared episodic uplift as a whole.Tongwan Ⅰ and Ⅱ episode influenced the whole of Yangzi Plate,while the effect of Tongwan Ⅲ episode was restricted in limited areas.Even the same episode of Tongwan Movement showed different magnitudes in different areas,which reflected the existence of differential vertical motion in some areas.(2)Tongwan episodic uplifting resulted in differential elevation and subsidence,which accelerated the formation of uplift-depression patterns.The Sinian-Early Cambrian tectonic evolution of Sichuan Basin could thus be divided into 7 stages,including uplift-depression differentiation embryonic stage,uplift-depression differentiation stage,intense uplift-depression differentiation stage,uplift-depression forming stage,uplift-depression adjusting stage and uplift-depression filling-up stage.(3)The interaction of paleohighs and Tongwan episodic uplifting motivated the uplifting and exposure of sedimentary mound-beach in uplifting area,which was beneficial for the development of karst reservoirs.The unconformity of Tongwan Movement could act as dominant channels for migration of ancient oil reservoirs in Sinian and present gas reservoirs.In conclusion,Tongwan multi-episode movement has a great influence on the tectonic evolution,reservoir formation and migration and accumulation of hydrocarbon in Sinian-Early Cambrian.
With the break-through of natural gas exploration of the middle array of Ordovician Majiagou Formation in the west of Jingbian Gasfield,the determination of the origin of natural gases becomes a key issue of the gas exploration direction.Based on the analysis of the geochemical characteristics of natural gas,deepening the understanding of distinguishing the origin of high-over mature gas,the paper investigated the primary source of the natural gas of Ordovician in the west of Jingbian Gasfield by the correlation of gas-gas and gas-source and the analysis of the spacial combination of source-reservoir-cap rocks.The results show that the Ordovician natural gas in the west of Jingbian Gasfield mainly belongs to coal-formed gas and there is some oil-type gas which is characterized by self-generation and self-preservation in local areas.According to the research,the genetic discrimination of highly matured dry gas should take the methane carbon isotope as a main basis.In addition,a kind of new interpretation about the phenomenon that the δ13C2 is apparently low and even lower than δ13C1 has been put forward in this paper.
As an important large intracontinental depression sedimentary basin in China,the Ordos Basin has produced quantity of crude oil from Yanchang Formation.Due to the prospective drilling,more and more interests have been paid to the Chang 9 interval of Yanchang Formation in central Ordos Basin in the last few years.Despite of its optimum oil production,there are two types of Chang 9 sandstone reservoirs with distinct variation of sedimentary properties in western or eastern field.The three aspects of sandstone characteristics including mineral components,sedimentary facies,and sandstone distribution features are compared to demonstrate differences of features respectively in western or eastern fields in this paper.The west field is mostly occupied by those sandstones with following characteristics:Large thickness,predominant sands,and vertically continuous distribution.Correspondingly,those sandstones in the eastern field have some characteristics as following:Small thickness,more barrier mud intervals,and relative isolated distribution of sandstones.The variation mechanism is evaluated by conducting factors of sedimentary environment,flowing energy,and channel stability.They are prevailed by the braided river delta environment in the west field,which contrasts to the meandering river delta setting in the east field.The former type of reservoir,which defined sandstones as “high-energy,continuous,stable-course” in this paper,is dominated by shorter transportation,more powerful hydrodynamic energy,steeper slope of lake bottom bed,and more immobile river channel courses through geological sediment period,compared to the latter defined as “low-energy,intermittent,mobile-course” in this paper,of which sandstone bodies are controlled by longer transportation,less hydrodynamic power,gentle gradient slope,and frequent mobile stream courses.
Well Yutan-1 obtained 34 600m3/d natural gas production in the karst porous fractured reservoir of Kelimoli Formation in western Ordos Basin.According to the geochemical characteristics of source rock and a comparative analysis on carbon isotope of natural gas,it belongs to the oil type gas.This work has the extremely important geological significance for us to rediscover the marine carbonate exploration field of the western Ordos Basin.Through the analysis of the Ordovician paleogeographic evolution,the marine hydrocarbon source rock characteristics,reservoir characteristics and reservoir forming patterns in western Ordos Basin,the article concluded:(1) The western Ordos Basin develops two sets of effective hydrocarbon source rocks:Upper and Lower Paleozoic groups.The Upper Paleozoic marine hydrocarbon source rocks have good hydrocarbon generation ability.(2)This area develops karst porous fractured reservoir,dolomite reservoir and reef -type reservoir.Reservoir performance is better.The karst porous fractured reservoir and dolomite reservoir widely distributed.(3)The western basin develops two sets of effective accumulation patterns,“upper-generation and ldown-preservation type” and “self-generation and self-preservation type”.The western thrust zone and the western part of Tianhuan Sag,belong to self-generation and self-preservation type.The Eastern part of Tianhuan Sag belongs to upper-generation and down-preservation type.Karst porous fractured reservoir and dolomite reservoir are the favorable exploration target area.
Micro-structure evaluation of tight reservoir is the basis of tight oil exploration and development.Taking the tight reservoir of Ek2 in Cangdong Sag as an example,apply the core observation and description,casting thin sections,scanning electron microscopy (SEM),argon ion polishing and scanning electron microscopy (Ar-SEM) analysis,to study the pore structure characteristics of tight sandstone,tight dolomite and shale reservoir.On this basis,using ImageJ software to process and analyze the SEM and Ar-SEM image.The SEM image pseudo color enhancement can highlight the characteristics of pore structure,and a comparative research can be done on these three tight reservoirs such as throat shape,size,distribution and connectivity etc.After two value processing of Ar-SEM images,we can quantitatively evaluate the reservoir pore number,pore size,face rate parameters.
Obvious corrosion characteristics under deep burial environment can be found in the Ordovician carbonate rocks of Bozhong Sag,and secondary pores which were caused by corrosion improve the reservoir property.Systematic analysis of dissolution mechanism in research area were studied using normal thin-sections,cast thin-sections,scanning electron microscope,cathodoluminescence analysis,fluid inclusion analysis and natural gas carbon isotope analysis.The results show the Ordovician reservoir in the study area was dominated by limestone,dolomite and calcite dolomite,the secondary pores formed by multiphase corrosion and fractures are mainly reservoir space.The reservoir has experienced three diagenetic environments including normal sea water,atmospheric fresh water and deep burial environment.The diagenesis is characterized by interaction between cementation and dissolution.Carbonate mineral dissolution in the deep burial environment are mainly controlled by mantle sourced CO2,H2S injected along the deep fault,acidic fluid formed by sulfate thermochemical reaction and organic acids and acidic gas produced in the process of late hydrocarbon accumulation.Comprehensive analysis shows that the southeast side of the study area where is more susceptible to deep fracture and surface dissolution effect is Ordovician carbonate reservoir favorable development area.
Carbonate rock has great heterogeneity,and possesses the characteristics of complex physical property and natural fractures development,thus it is often treated by acidizing to create wormhole.Wormhole dissolution patterns are significantly affected by those characteristics.Hence,a dual-scale continuum model and equivalent seepage theory were used to study,the influence of in-layer heterogeneity,interlayer heterogeneity and natural fractures upon wormhole pattern and acid injection volume when break through (PVBT) were analyzed.The results show that an approximately the same optimum acid injection rate that leads minimal PVBT in different heterogeneity level do exist,and the minimal PVBT firstly increased and then decreased with the increase of heterogeneity level.High permeability zone leads to sharp decrease of PVBT ,but the inference of permeability increase on PVBT decrease becomes weak gradually when the permeability increases to a certain level.The extension direction of wormhole is decided by natural fractures when acid flows into it,and the PVBT decreased sharply by the fractures which are parallel to the acid flow direction.With the increase of fracture length,the acid injection volume tends to decrease linearly.The study is of great significance in guiding matrix acidizing of carbonate reservoirs.
During the development of gas reservoirs with strong bottom or edge aquifer support,how to quantitatively evaluate the risk of water breakthrough comprehensively considering the variety of dynamic and static parameters and make early warning for the wells which the water coning will break through are urgent problems to be solved.Currently,the published literatures on these questions are relatively one-sided.Aiming at the questions mentioned above,firstly,the evaluation index system of static and dynamic parameters associated with water breakthrough in gas reservoirs is established.Then based on the index system,the paper used analytic hierarchy process and fuzzy evaluation to evaluate the risk of water breakthrough.And finally water breakthrough sequence of gas wells can be quantitatively evaluated.The paper took Kela2 abnormal high pressure gas reservoir with bottom and edge aquifer drive for example to illustrate how to use the method to evaluate aquifer influx risk of all gas wells.And all gas wells are classified into 4 types based on the risks of aquifer influx,and water breakthrough sequence of all wells are also determined.Actual gas well performances validate this new method.Furthermore,the evaluation results are very useful for well production rate optimization and gas field effective development.
Based on fracture discretization,applying the potential theory and the superposition principle of infinity formation and the method of matrix transformation,horizontal productivity formulas with a single fracture and multiple fractures were derived out,which considered the fracture conductivity.Productivity equations were solved numerically and the total productivity was obtained by adding the productivity of each fracture.Then the effects of some parameters on the productivity were analyzed.The results show that fractured horizontal well has a high productivity and a sharp drop during initial period,and the productivity tends to be steady.The outer fractures have higher productivities compared with the inner fractures.The productivity is mostly affected by the number of fractures and dimensionless fracture conductivity,and secondly by the fracture length,and least by the angles of horizontal wellbore and fractures.Finally,compared to the previous productivity formulas of fractured horizontal wells,the formula in the paper has a concise form which is easy to be understood,and each item in the formula has a definite physical meaning.The reliability and practicability of the formula in the paper are confirmed in comparison to the well-known source function method.
Permeability is a key factor to reflect seepage ability of reservoirs.The empirical formula of using pore structure parameters to estimate reservoir permeability is based on the idealized model.Tight sandstone reservoirs are mainly characterized by small reservoir pores,fine throats,and high seepage resistance.These properties cause liquids to flow through such ultra-low permeability porous media,which has the characteristic of non-Darcy flow.Compared with core analytical method,there is a great deviation from real permeability using conventional computing method to get permeability of tight sandstone reservoirs,which is not suitable for productivity calculation.Based on the fractal theory and considering the fractal dimension for capillary tortuosity and nonlinear flow characteristics of liquids,a fractal permeability model is derived which considers threshold pressure gradient.Results show that the permeability is a function of porosity,fractal dimension for pore throat,capillary tortuosity and maximum radius of pore throat,which reflects the effects of microscopic pore structure and the fractal dimension on reservoir permeability.Compared with the previous work,the calculated results had less relative error and were consistent with core analysis results.The new model will be helpful to predict permeability of tight sandstone reservoirs.
Since strong compaction and higher conductive component may cause a weak response of continental source rocks on porosity and resistivity curve,using traditional ΔLgR technology to predict the content of organic carbon will have a poor effect.Aimed at this problem,on the basis of continuing the advantages of porosity interference in ΔLgR technology,using natural gamma curve which is more sensitive to deep source rocks instead of the maturity parameters in traditional model,we developed the generalized-ΔLgR technology which employs natural gamma,AC acoustic and resistivity logging curves to predict the content of organic carbon.Furthermore,the generalized-ΔLgR technique was applied to Shahezi Formation of Xujiaweizi Fault Depression.The results show that it can provide a more reliable TOC profile,and the TOC predicting error was reduced by 25.3% compared to the conventional ΔLgR technique.It indicates that the generalized ΔLgR provides a new TOC predicting method for continental source rocks with deep burial depth and strong compaction.
The tight sandstone reservoir in the Xujiahe Formation of western Sichuan Basin has experienced a complex sedimentation,diagenesis and tectonic evolution process.The reservoir physical properties and hydrocarbon content have strong heterogeneity in both vertical and horizontal directions.Strengthening the logging contrast research of dense reservoir gas saturation has great significance for further study of gas and water distribution,looking for high quality reservoir,and contrast between interwell hydrocarbon showings.In this paper,based on the conventional and special logging data of tight sandstone reservoir,we made logging interpretation of gas saturation using the Archie formula and three-phase Biot-type equation (TPE) respectively.It showed that,when the selection of various parameters is appropriate,they can both achieve good evaluation effect.The value of undetermined constant of this tight sandstone reservoir using the Archie formula and its corrected formula can provide a reference for similar research.The effect of evaluation based on the electrical method is greatly influenced by the shale content.When making logging interpretation of gas saturation using the TPE method,obtaining the consolidation parameter α through adaptive method,we found that there’s good power relationship between α and the resistivity log,and we can use this formula to predict α .This acoustic evaluation method is seldom affected by the shale content,but it can be affected by the true gas saturation of formation.When the true gas saturation of formation is lower than 60%,the prediction effect was good,but when it is higher than this value,the prediction result is lower.Because that the high saturation is not the main reason for acoustic dispersion and attenuation,this phenomenon is mainly correlated with the rock itself,such as the matrix mineral composition,internal particles microscopic contact type and the pore-crack structure.Based on the above understanding,we can select appropriate method to make fine logging interpretation of partially saturated tight sandstone reservoir according to the master degree of actual data that we have got.
Permeability refers to that under certain pressure difference,material allows fluid through porous media.It is an important physical parameter to describe porous media permeability.Based on the continuum mechanics and phenomenological theory,the theoretical model of effective stress and damage caused by mechanical and thermal ultrasonic irradiating effects was established,considering the significant influence of secondary damage of ultrasound on granular coal and rock permeability in meso-scale.The permeability of granular coal rock under different gas pressure experiment was carried out,the empirical formula between permeability and gas pressure was fitted,and the relational model of gas permeability,effective stress and secondary damage was established.The results show that the ultrasonic effect can effectively improve the permeability of coal and rock particles system,the gradual increase of effective stress is the main reason for the permeability-increase by ultrasound;the increase of contact particle damage and the significant heating effect is the main influence factor of permeability rate decline.Research work on coal-bed methane permeability-increasing mechanism and optimization of the mining has important scientific significance and practical value in engineering.
By the mercury injection test and low temperature liquid nitrogen adsorption test,combined with ground coalbed methane（CBM） drainage test,the coal pore structure characteristics and the influence on CBM drainage characteristics were studied.The pores in coal are divided into four types:The adsorption pores (pore size <10nm) and free porosity (pore size 10-100nm),diffusion porosity (pore size 100-1 000nm),seepage pore (pore size>1 000nm).The tectonic coal pore system has polorization distribution,that is,the adsorption pores,the free pores and the seepage pores are dominant,and the diffusion pores are minor.The pore types are mainly the cylinder shaped hole,the ink bottle shaped hole and the slit flat.The pore system of tectonic coal determines the characteristics of the more gas reservoir accumulation and the not unobstructed output channel,which leads to the fluctuation characteristics of the gas drainage of CBM ground well.
Based on the geochemistry,reservoir,origin of gas and structure preserving condition,we systematically discuss the accumulation condition of Changde Niutitang shale by using outcrops,drilling and core data.The organic rich shale of Niutitang Formation has good hydrocarbon generation conditions which are characterized by high TOC,great thickness and high thermal evolution degree,good reservoir conditions characterized by 2%-5% effective porosity and high maximum adsorption capacity (1-3.5m 3/t) whose main storage space of organic rich shale is the matrix pore and thick cap rocks (>200m).It has founded out the reason why Changde 1 well produces both gas and water and established the producing gas and water model.The gas and water is mainly from the fractures of Niutitang organic rich shale in the obstructive replacing zone and the gas is a mixture of the gas generating from shale in over mature stage and the air,and may have some deep source gas.We preliminarily analyzes the shale gas accumulation process and dissipation model,and makes clear the main gas content influence factors of organic rich shale of Niutitang Formation are the tectonic movement and the preservation condition and the diffusion of gas is also an important factor.We optimized the west part of well Changde 1 which has great resource potential as the promising exploration target due to good preservation condition for shale gas.
Late Paleozoic mudstone and shale deposited in the marine-terrestrial transitional environments are widely developed in southern and northern China,showing great potential of shale gas,In order to study the process of gas generation,pyrolysis experiment was conducted on immature organic-rich shale samples collected from the section 2 of Shanxi Formation (Shan 2)in Ordos Basin,north China,Pyrolysis results indicate that liquid and gaseous hydrocarbon have been generated at the lowest experimental temperature (T=337.2℃，RO=0.74%),The peak of liquid hydrocarbon generation arrived at 385.3℃(RO=1.08%),where methane’s yield accelerates and dry coefficient increases,As the generation of heavy hydrocarbon reaches peak stage (456.2℃, RO=2.09%),inflection point occurs in the methane-yield curve,Methane is mainly generated from thermal pyrolysis of liquid hydrocarbon,with part formed from thermal pyrolysis of heavy gaseous hydrocarbon,The yield of methane reaches 956.93mL/gTOC at the highest temperature(T=599.7℃,RO=4.45%)with a dry coefficient of 99.75%,leaving small amount of liquid and heavy gaseous hydrocarbon cracking to methane,The process of gas generation could be divided into three stages:(1)Stage of primary oil production at 337.2-385.3℃(0.74%≤RO≤1.08%)with small amount of shale gas;(2)Stage of oil and gas co-existent with wet gas at 385.3-456.2℃(1.08%≤RO≤2.09%);(3)Stage of dry gas formation at 456.2-599.7℃ (2.09%≤RO≤4.45%),Compared with marine Barnett shale,transitional Shan 2 shale in Ordos Basin has the same process of gas generation but higher gas yield,The results of this study indicate that Shan 2 shale has bright gas generation potential.
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