According to comprehensive research on forming conditions including sedimentary facies,reservoirs,source rocks and palaeo-uplift evolution of Sinian-Cambrian in the Sichuan Basin,it is concluded that:(1)large-scale inherited palaeo-uplifts,large-scale intracratonic rifts,three widely-distributed high-quality source rocks,four widely-distributed karst reservoirs and oil pyrolysis gas were favorable conditions for large-scale and high-abundance accumulation|(2)diverse accumulation models were developed in different areas of the palaeo-uplift.In the core area of the inherited palaeo-uplift,“in situ” pyrolysis accumulation model of paleo-reservoir was developed,while in the slope area,pyrolysis accumulation model of dispersed liquid hydrocarbon was developed in the late-stage structural trap|(3)there were different exploration directions in different areas of palaeo-uplift.Within the core area of palaeo-uplift,we should mainly search for the inherited paleo-structural trap,which was also the foundation of lithological-stratigraphic gas reservoirs,while in the slope areas,we should mainly search for the giant structural trap in the Himalayan Period.
The reason why Sinian-Cambrian gas reservoirs in the Moxi area of the Sichuan Basin can survive well and why there is a great difference between pressure coefficient of Dengying reservoirs and Longwangmiao reservoirs have attracted more and more scientists.A detailed study of gas reservoir caprock characteristics and sealing ability by the combination of macroscopic and microscopic as well as geology dissection and simulation experiment has been carried out.Cambrian shale caprocks,Permian overpressure shale caprocks and Low-Middle Triassic gypsum caprocks are regionally distributed in the study area.The characteristics of caprocks are as follows,porosity is mainly less than 3%,permeability is (0.001-0.02)×10-3μm2,average throat radius ranges from 0.6μm to 1.2μm,pore size is mostly concentrated in 400-500nm,breakthrough pressure is more than 15MPa,and diffusion coefficient ranges from 1.03×10-10cm2/s to 2.36×10-7cm2/s.Caprocks have certain sealing ability,but the reason that Longwangmiao Gasfield can survive for a long history is the result of interaction of capillary sealing and overpressure sealing.Understandings above can be used to interpret formation reason of atmospheric and overpressure gas reservoirs in the study area reasonably.Meanwhile,it can be used for predicting favorable distribution of large gas field.
Based on geological and geochemical methods,geochemical characteristics of natural gas,distribution of genetic types and gas source were studied accurately,and the distribution characteristics of natural gas were identified in the No.5 tectonic zone of Nanpu Sag.The natural gas in the onshore Beipu zone mainly distributed in the Ed Fomation and the beach area in the Es Formation.The natural gas is dominated by wet gas with little amount of dry gas.It is mainly of oil-soluble gas,followed by condensate gas.The types of natural gas are not the same in different areas.The sapropelic natural gas mainly distributes in the Ed Formation in the onshore Beipu zone,while the humic gas distributes in the Es Formation in the beach area.By comparison with geological and geochemical analyses,the humic natural gas and the transited-type natural gas is mainly sourced from the Es3 Formation of beach area in the No.5 tectonic zone of Nanpu Sag,the sapropic natural gas in the onshore Beipu zone is mainly drived from the Es3 Formation of Beipu zone.In general,natural gas in the No.5 tectonic zone of Nanpu Sag are mainly sourced from the middle or deep source rocks of Es3 Formation.Natural gas uauslly migrates and accumulates near the source in the No.5 tectonic zone of Nanpu Sag.
Using data of logging,drilling core and thin sections,this paper studies the different properties of underwater distributary channel sandbodies in the shallow-water delta from Chang 81 between Jiyuan and northern Shaanxi area from the perspective of reservoir sedimentology.The study shows that the sandstone from the two areas is lithic-feldspathic sandstone and feldspathic-lithic sandstone,regarded as fine sandstone and very fine sandstone,but the particle size of sandstone in northern Shaanxi area is finer and better sorted than that in Jiyuan area.Jiyuan area has large thickness of sandbodies with low content of shale,unobvious superposition stages,and good continuity in vertical and longitudinal direction.However,northern Shaanxi area has thin monolayer sandbodies with high content of shale,significant multiply cyclicity,and poor continuity in vertical and longitudinal direction.Chang 81 member in Jiyuan area belongs to shallow-water deposits and closes to the provenance,the provenances are enough and have stable supply ability,the driving force of the water expansion is greater than that of the river flowing into the lake,thereby creating a continuous retrogradation thick sandbody.However,Chang 81 member in northern Shaanxi area also belongs to shallow-water deposits where the water is relatively shallower than that in Jiyuan area.It is far away from the provenance which led to stable but insufficient supplication,and the water power of lake and river was weak,forming a see-saw type thin sandbody.The dominating reservoir pore type of the two areas is inter-granular pore and feldspar-dissolved pore.Overall,the sand reservoir performance of underwater distributary channel sandbodies from Chang 81 of Jiyuan area is better than that of northern Shaanxi area;however,the underwater distributary channel sandbodies from Chang 81 of northern Shaanxi area also have hydrocarbon accumulation capacity.
The thickness and distribution of organic-rich shale are important parts of forming conditions appraising about shale gas and shale oil.The distribution of shale is controlled by sedimentary environments.This study determined the sedimentary environment of dark shale of the Chang 7 Member by field profile and cores.Vertical overlaying features of dark shale were confirmed by field profile and wells.Sedimentary structures and micro-bedding structures were delineated by cores and sections.Differences of total organic content and organic types between dark shale in different sedimentary environments were revealed by testing data and logging interpretation.Dark shale of the Chang 7 member distributes in the subaqueous distributary bay of delta front and semi-deep to deep lacustrine.In semi-deep to deep lacustrine,dark shale has great continuous thickness and sandstones have thin thickness.However,in delta front,dark shale has thin continuous thickness,shale and sandstone interbeddings have similar thickness or thin shale layer and thick sandstone layer.In semi-deep to deep lacustrine,dark shale always has lamellation,horizontal lamination and sandy lamination,whereas lamellation does not develop very well in delta front shales.Organic types of delta front are mainly type Ⅱ2 and Ⅲ,and type I and type Ⅱ1 to semi-deep to deep lacustrine shales,respectively.TOCof semi-deep to deep lacustrine shale are higher than that of delta front shale.Based on these comparisons,we concluded that shales which distribute in semi-deep to deep lacustrine are the more favorable prospecting target for shale gas/oil in the Yanchang Formation.
Interval from the Shanxi Formation to member 8 of Lower Shihezi Formation in the Upper Paleozoic,Ordos Basin,central China,is one of the most important exploration targeted strata,which develops large area of shallow lacustrine deltaic sandstone and bears giant natural gas source.This study investigates the reservoir sandstone development and trys to build a physical model to perform sandstone internal structure analysis in an experimental basin equipped with a subsiding floor.A distorted model characterized by multi-sources,shallow-water and open system,is established which is integrated with tectonic evolution,palaeo-geomorphology,sediment sources and related components and so on.Through the adjusting of experimental parameters,such as inflow water volume,sediment supply,gradient,subsidence value and fluctuation of lake-level etc.during each stage of the experiments,we can reproduce and investigate multi-source big area shallow-lacustrine deltaic deposition.And we use double-direction intensive cross sections to discuss different characters of shallow water deltaic deposits at the position of near-source,middle part and distal.The results indicate that lake-level fluctuation and interplay of multi-direction sources are main controls during the development of large area shallow-lacustrine deltaic deposits.Meanwhile,both abundant sediment supply and suitable subsidence ratio also play an important role in this process.Shallow lacustrine delta front is not similar to traditional deltas with relative uniform foresets,but has complicated internal architecture.Finally,we also compare the physical simulation results with the real subsurface sedimentary strata and point out potential problems.
The Zhangjiatan shale of the Lower Chang 7 member of Yanchang Formation,Ordos Basin is an important exploration target for shale gas.Since the silty beds/laminae in the shale play an important role in gas storage,fluid flow,fracturing and development of shale gas/oil,it is of great significance to ascertain the thickness and the distribution characteristic of the silty beds/laminae.The measurement results from outcrop,core and thin section show that the silty beds/laminae have larger detrital grains than those in the clayey laminae and the silty laminae thickness mainly ranges from 0.5mm to 8mm.Their total thickness can occupy 7%-26% of the shale formation with an average density of 8-40 layers/m,which shows strong vertical heterogeneity.The thickness distribution of silty beds/laminae presents a fractal distribution on a dm to mm scale and the fractal dimension D has scale invariance over the whole thickness range.According to the data from the relatively thick layers of silty beds,the accumulative number of the thinner silty beds/laminae was estimated using the Number-size model.The accumulative thickness of silty beds/laminae and its ratio to the shale formation thickness were calculated.It has been verified that this method can ascertain the thickness of the silty beds/laminae effectively.
Tight sandstone gas has been one of the most important fields for the unconventional oil/gas exploration,especially in the North America.In recent years,China has found and exploited giant tight sandstone gas reservoirs in the Ordos Basin and Sichuan Basin.It is the first time to discover tight sandstone gas reservoir in the west bulge Zhongguai salient of Junggar Basin.Comprehensive studies have shown that these reservoirs are located in the tectonic slope area between structural high and the hydrocarbon-generation sag.It is characterized by the spatial-temporal configuration relationship of source-reservoir,compact reservoir and “sweat points”.These characteristics are typical for compact sandstone gas reservoir.According to the current exploitation situation of the Junggar Basin,compact sandstone gas may be an important exploration subject of remaining natural gas resource and more realistic goal in current unconventional gas and oil resource.The ancient uplift slope belt will be a main exploration area.
With the petroleum exploration and development and comprehensive geological research in northwestern Juggar Basin,high-quality dolomite reservoirs were found in Permian Fengcheng Formation which are characterized by strong heterogeneity,complex and various types of rocks.Through core observation,thin section identification,X-ray,scanning electron microscopy (SEM),physical property data statistics and analysis,the result indicates that high quality dolomitic clastic reservoir mainly include argillaceous silty dolomite,dolomitized mudstone,tuffaceous dolomite and dolomitic tufffaceous shale.Reservoir type is complicated,and characterized by fracture-pore dual medium,with an average porosity of 4.42% and the average permeability is 0.19×10-3μm2.The pore geometry is poor and its throat is tiny,and the reservoir belongs to low porosity and low permeability reservoir.Due to sedimentary environment,dolomitization,tectonism and hydrothermalism have influences on the reservoir physical property and distribution.The reservoir controlling factors was proposed.It is thought generally that depositional environment and sedimentary facies controlled the scale of high quality reservoir and early porosities,while dolomitization,acidic dissolution and hydrothermal fluids influenced secondary pore formation,and the effect of tectonism activity controls the modification effect of reservoir.
Important oil and gas resources have been found in the dolomite reservoir of the lower segment of the Lower Ordovician Yingshan Formation in the Gucheng Lower Uplift.Faults are pathways of oil and gas and play an important role.Based on the detailed interpretation of 3D seismic data and maximum negative curvature attribute along main interface from Gucheng Lower Uplift,Tarim Basin,main fault zones were identified,and its profile characteristics,planar characteristics,active stages,genetic mechanism and influences on hydrocarbon accumulation were also studied.Research shows that two kinds of structural styles were formed,including negative flower structure and parallel high steep fault.On the plane,there were main development of NW,NE and NNE trending.Analysis combined with unconformity,growth index,overlying strata deformation characteristics,the faults were active mainly during the Early Caledonian(the Middle Cambrian),the Middle Caledonian(early in the Late Ordovician) and the Late Caledonian-Early Hercynian(Silurian-Devonian).The Middle Caledonian tension-shear faults developed based on previous basement weak zone,and were influenced by trench-arc-basin tectonic system in the northern margin of Altun.The Late Caledonian-Early Hercynian tension-shear faults developed based on the middle Caledonian tension-shear faults,and were controlled by the dextral strike-slip Guchengbei fault and the sinistral strike-slip Cherchen fault.Fault development plays an important role in the oil and gas accumulation.Mainly in two aspects:fault acts as an important channel to control oil and gas vertical migration|fault is the hydrothermal fluid channel,which will help dolomitization,improve reservoir property.
The prospecting of mid-deep layers of Ledong area in the Yinggehai Basin is at the exploration stage at present.Based on regional geology,seismic data,boreholes as well as geochemical data,the conditions of natural gas accumulation and its controlling factors of the middle-deep layers in Ledong area were analyzed,including the characteristics of source rocks,the styles of reservoirs and traps,petroleum conduction system and accumulation risks.According to the geochemical features of the natural gas in the shallow layers,they belong to mature-high mature coal-derived gas,which is sourced from the mature-high mature humic source rocks in the Miocene Meishan-Huangliu Formations.Moreover,the drilling of well L22-1-7 confirmed the existence of this set of source rocks and it indicated that Ledong area has a good condition of source rocks.Although individual borehole revealed that the development and the reservoir physical properties of the middle-deep layers in Ledong area were not promising,we can′t ignore the whole area.The seismic and geologic data showed that the styles of reservoirs of the middle-deep layers in Ledong area are dominated by low stand sandstones,including coastal sand dams,slope fans,floor fans and channel sandstones,etc.,which were formed in the forced regression environment and should have good physical properties.What′s more,vertical faults and microcracks system resulted from upwarping and piercing forces of diapirs in Ledong area provided high efficient conduction pathways for vertical migration and accumulation of deep hydrocarbons.Though there are many exploration risks and some uncertain factors in the hydrocarbon accumulation of middle-deep layers in Ledong area,especially the key controlling factor on the distribution and physical properties of reservoirs,and nonhydrocarbon gas risks,the middle-deep layer gas accumulation conditions and exploration prospects are unassailable.
Taking “Beads” as exploration objects,Tazhong gas field achieved some effects.With the development of exploitation,we found that “Beads” are not the main features of gas reservoirs,and “Beads” are fewer and fewer.We need to recognize the distribution regularities of gas reservoirs to seek the effective prediction methods.This paper shows that,Tazhong area has been uplifting after the sedimentation of Yingshan Formation,which is influenced by the interaction of differential corrosion and segmentation of strike-slip faults,resulting in the strong heterogeneity of reservoir distribution and the complexity of gas-water distribution.Despite the Yingshan reservoir has complicated distribution and strong anisotropy,through the logging data analysis we found the most sensitive pre-elastic parameters of Yingshan gas reservoirs-Poisson's ratio.AVO modeling shows Yingshan gas reservoirs have three types of AVO abnormal.That means the “Near-Far AVO characterization” and the “Major amplitude & Major frequencies” can predicate the gas reservoir and increase the drilling rate.It is significant in the carbonate exploitation.
Along with the pressure on energy security,the status of shale oil and gas in energy structure of our country is becoming more and more important.As a kind of unconventional hydrocarbon resource,shale oil and gas are characterized by self-generation and self-storage,complex mineral composition,micro-nanometer scale pores,usually ultra-low porosity and permeability,and adsorbed gas and free gas coexistence.The above features have obvious differences from the conventional hydrocarbon resources,so the existing well logging evaluation theories and methods for conventional reservoirs are not applicable.The article guided by the unconventional hydrocarbon geology,aimed at the requirements of exploration and engineering reform,quantitative logging evaluation method of Total Organic Carbon is built,which is applicable to different maturity of continental and marine shale.And quantitative well logging evaluation technology for complex mineral components is formed.Upon the conventional logging data,a new method of rock brittleness evaluation is put forward.And the method of logging classification for shale reservoir quality using multi-parameter weighting is established.Eventually the complementary technologies of logging evaluation for shale oil and gas reservoirs are formed.The above achievements have filled the domestic technological blank of logging evaluation of unconventional shale reservoirs and currently have been widely used in Qing 1 member in Gulong Sag of the Songliao Basin,and Longmaxi Formation and Qiongzhusi Formation in Weiyuan and Changning districts of the Sichuan Basin,which effectively guide the favorable fractured intervals selection of new exploration wells,new horizontal wells and old wells review in the related oilfields.
The process and condition of hydrocarbon generation and expulsion has been simulated on the Cretaceous source rocks from the Songliao Basin, based on the latest experiment platform of hydrocarbon generation and expulsion.New hydrocarbon measurement method is used to acquire accurate data about experimental products including light hydrocarbon.The geological RO are correlated with atomic H/C ratios of pyrolyzed kerogens from experimental solid products.Thus hydrocarbon generation and expulsion efficiency in the thermal evolution process is determined.Results show that, the content of light hydrocarbon is about 5%-29% and about 25% during the peak period of oil generation, and it increases with the increase of thermal maturation.The maximum amount of hydrocarbon generation is 680mg/g TOC, and the absolute hydrocarbon expulsion efficiency at the peak of oil generation is 57%.Analysis of pressure factor indicates that, fluid pressure does affect thermal maturation and hydrocarbon expulsion.Pressure can reduce the maturity in oil generation phase.The decreased value of RO is about 0.2%.Pressure release promotes hydrocarbon expulsion, which results in the decrease of residual hydrocarbon content and light hydrocarbon loss from cores.Light hydrocarbon loss can cause 18% deviation of expulsion efficiency at oil generation peak.The conventional method underestimated the residual hydrocarbon amount in geological conditions, and gas exploration prospect is optimistic.
According to some geological and geochemical data of well Sutan 1 of depth from 2 536.5m to 2 697.0m,including the homogenization temperature of fluid inclusion and the geochemical characteristics of crude oil,the hydrocarbon accumulation history of the studied region was analyzed.The result shows that the crude oil has the properties of high evolution degree with well Sutan 1.The precursor type is mainly high terrestrial plants.Among them three crude oils from depth of 2 620.67m,3 640.57m and 2 697m have been charged with crude oil with aquatic organism.And the crude oil has deposited in weak oxidizing to oxidizing environment with fresh to slight saline water,which is better for preservation and transformation of organics.The characteristics of fluid inclusion with well Sutan 1 show that there are at least three times of filling of oil and one time of filling of gas.And two periods of petroleum accumulation existed in the crude oil of well Sutan 1.The reservoir forming time of the first charge with oil is about 142Ma.The reservoir forming time of the second charge with oil is about 107Ma,which is tectonic inversion of the Yanshanian orogenic movement in Late-Early Cretaceous.
Elaborate and refined geochemical research work has been conducted on the Silurian bitumen sandstone extracts from the Tazhong Uplift and Tabei Uplift.Several explicit conclusions are summarized.Chloroform bitumen (“A”) ranges from 0.477 1% to 1.410 3%, implying the bitumen sandstone is petroliferous enough for crude oil production.The extract of bitumen sandstone shows much more similarity in the triaromatic dinoflagellates sterane distribution than that in most oils from the Tahe Oilfield and the potential source rocks have been regarded as the Middle-Upper Ordovician marls.Whereas chloroform bitumen (“A”) in well TP7 has much lower and different triaromatic steroids and triaromatic dinoflagellates sterane patterns which gives evidence of Cambrian source.Organic petrology data show bitumen in the sandstone is right in “oil window”, which is consistent with the mature information provided by the biomarker fingerprints Ts/(Ts+Tm) vs 4-/1-MDBT.Two accumulation periods are recognized with the reconstruction of burial history of Well Ha9.It is proposed that the abundant oil show and movable oil flow enlighten great potential for the exploration in the Silurian bitumen sandstone.
The Tarim Basin develops several layers of marine shale which have large potential for shale gas resources.Based on large quantities of measured data，the TOC of marine shale was calculated by using logging method.Based on outcrop sample analysis,the favorable shale layers which are rich in organic matters were screened and determined.The classic ΔlogR method is not fit for the TOC calculation of high-over mature shale,while the TOC calculation by using Uranium logging curve works well.The results show that the organic-enriched shale in the Tarim Basin mainly distributes in the Middle-Lower Ordovician Heituwa Formation,the Lower Cambrian Xidashan-Xishanbulake Formation in eastern Manjiaer Depression,and the Middle-Upper Ordovician Saergan Formation in the Keping-Awati Depression area.The Heituwa shale in Tadong area is a favorable layer for the present shale gas exploration,however,considering its small distribution area and shale thickness,it has a small potential for the exploration and development of shale gas.
Desorption and diffusion of methane (CH4)under different moistures in different size pores of coal reservoirs were simulated.The relationships between pore size,moisture,temperature,pressure and desorption/diffusion of CH4 were discussed.The results show that (1)the wall superimposed effect of the pore can significantly affect the adsorption/desorption of H2O molecules,so H2O molecules concentrate distribution in the middle of the pore.With increase of pore size,the wall superimposed effect of the pore gradually transforms into surface effect of two sides of the wall so that H2O molecules concentrate distribution on both sides|(2)With the increase of the moisture content,the influence of the moisture on CH4 desorption capacity increases,and CH4 desorption capacity shows a trend of decrease,but the temperature effect is not obvious|(3)In the 1nm slit pore,the influence of the moisture on CH4 desorption under low pressure is greater than that under high pressure|(4)In the 2nm slit pore,when the moisture content increases form 2.35% to 5%,the moisture impact on CH4 desorption under low pressure is less than that of high pressure|(5)In the 5nm slit pore,with the decrease of pressure,the influence of the moisture content on CH4 desorption capacity becomes not obvious,which explains that when the pressure drops to a certain value,changes of the moisture content will not promote CH4 desorption.When the moisture content is more than 2.35%,the increase of the moisture content still affect CH4 desorption,which explains that 2.35% is not the moisture critical value of the graphite slit pore.However,with the increase of the moisture content,CH4 desorption is less affected gradually,so the critical value may exist|(6)Adsorption heat of water in micropores and adsorption potential from micropores to adsorbates will significantly affect the diffusion coefficient of CH4,while temperature and pressure less affect it.
Tectonic stress is a main influence factor of coalification and coalbed methane(CBM) formation.In order to study the effect and reservoir-controlling significance of different tectonic stress on CBM formation under shallow brittle deformation condition,ⅡⅠ coal samples were collected from tectonic deformation zones induced by compressive,tensile and shear stress according to the typical partitioning features of gravity sliding structure in Western Henan coalfield,and then the nuclear magnetic resonance(NMR) absorption intensity of samples were measured.The results show that the aromatic carbon rate of the three zones decreases successively with values of 0.773,0.730,0.702,while the aliphatic carbon rate increases with values of 0.138,0.167,0.182,and the relative content of each functional group in different zones also changes correspondingly.In addition,the content ratio of carboxyl is 0.061,0.042,0.082,and that of carbonyl is 0.053,0.030,0.016.The NMR structure parameters indicate that tectonic stress is still an important factor of coalification under low temperature,but the effect degree of different stress is different.The action mechanism of anisotropic extrusion stress is complex,and has great promotion in CBM formation|the promotion effect of tensile stress is after extrusion stress,while the promotion effect of shear stress caused by shallow sliding is relatively minimal.The conclusions have some theoretical significance to coal mine safety production and CBM comprehensive exploitation.
For the CBM enrichment and high yield,the reservoir quality,sealing quality of roof and floor and completion quality (three qualities) play a decisive role.According to the log,laboratory analytical data of coal core,logging evaluation methods for main controlling factors of “three qualities” in CBM reservoir are discussed in detail.The quantitative evaluation methods of “three qualities” are established by the main controlling factors of organic fusion mean.Based on these methods,“three qualities” of CBM reservoir in the study area are carried out through quantitative evaluation.The results show that those methods can evaluate “three qualities” of CBM reservoir quantitatively and effectively.The evaluation results are closely consistent with the tests of laboratory analysis and fracturing dynamic drainage results.For the 11# CBM reservoir,the reservoir quality of the central area is much better than that in the northeast and southwest,the one in the northeast is slightly better than the southwest;Overall,the sealing quality of roof and floor block of the 11# coal seam is good.The sealing types are mainly Ⅰ,Ⅱ,the area with good sealing quality locates in the middle of the study area,the poor area locates in the northeast and southwest of the study area;The greater the brittleness index and the stress difference between coal seam and its roof and floor,the smaller the difference coefficient of coal seam stress,and the higher the coal structure index.Then the fracturing of coal seam is strong,the productivity effect is remarkable after fracturing drainage.To obtain high-yield in CBM wells,it requires not only good reservoir quality and sealing quality of roof and floor block,but also the completion quality of fracturing zone,which is an important factor that cannot be ignored.
In order to investigate the nanostructure morphology of shale reservoirs of the Toolebuc Formation,organic geochemical and X-ray diffraction experiments are used to measure the mineral composition and argon ion polishing-SEM,liquid nitrogen adsorption and high pressure mercury experiments are used to analyze the pore types and structures of Toolebuc shale.Result shows that shale of Toolebuc Formation is high abundance marine shale,the content of brittle minerals is 55%,and can be fractured.The average porosity of shale is 15%,while the permeability ranges from 0.002×10-3μm2 to 0.116×10-3μm2.Research shows that pores of shale can be classified into six types: Inter-particle pores between clay minerals,mineral pores in rock skeletons,pores in organic,apertures in paleontology fossils,organic acid solution pores and micro-fractures,of which the most common ones are inter-particle pores between clay minerals and mineral pores in rock skeletons.The pores size distribution of shale is complex,which includes not only predominant mode-pores,but also a certain amount of macro-pores and micro-pores.Macro-pores and mode-pores provide most of pore volume,it is good for the accumulation of free gas,and mode-pores and micro-pores provide most of specific surface area,and it is good for the preservation of absorbed gas.The types and distribution of pores in shale are obviously controlled by the diagenetic evolution and thermal evolution.
In order to optimize the favorable area and strata of shale gas,we investigate the latest literature about organic-rich shale in basins of America and China.According to the comparison and analysis of the regional tectonic background,organic carbon content,sedimentary environment,organic carbon content,reservoir property,gas content and so on of the shale between North America and China,the following results have been reached:(1)Both China marine shale and America shale have similar organic matter abundance and types(typeⅠor type Ⅱ1).The thermal maturities of China′s marine shale are higher than America′s,which indicate that China′s marine shale have already generated adequate hydrocarbon;TOC(total organic carbon)of terrestrial shale is lower than that of America′s.Their organic type belongs to typeⅡ2 or type Ⅲ and maturity varies from 2% to 4.5%.Those indicate that the potential to generate hydrocarbon of terrestrial shale is lower than that of marine shale.(2)With similar content of clay mineral,China′s marine shale contains more brittle mineral,which is conductive to fracturing|In contrast,terrestrial shale contains less brittle mineral than that of America′s.(3)With similar pore types and porosity,China′s marine shale shows as good quality of reservoir as America′s.But terrestrial shale shows relatively lower porosity compared with the marine shale of America′s.(4)Burial depth of china′s marine shale distributes from 1 200m to 5 300m,which is wider than that of America′s(1 300-4 000m).Meanwhile,due to the much more complicated tectonic evolution than the stable geotectonic background of America′s shale,those factors result in poor preservation condition of China′s marine shale and terrestrial shale.Therefore we propose an evaluation criterion of shale gas that suits for the marine/terrestrial shale in China according to the result of comparison and the exploration practice.An evaluation of favorable area and strata is carried out on the base of the shale gas assessment criterion.The result shows that the optimum strata of marine shale are the O3w-S1l of Chongqing and its adjacent area,the ∈1q and S1l of Weiyuan area.And the optimum strata of continental shale are T3y of Ordos Basin and K1q of Songliao Basin.
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