In the Tarim Basin,it widely distributes thick dolostone in the Lower Proterozoic,in which advantaged source-reservoir-cap rock combinations were well certified with great hydrocarbon exploration potential.Two sets of reservoir-cap rock combinations present two exploration realm,which are below-salts dolostone realm and above-salts dolostone realm,respectively.Based on the analysis of source rocks,reservoir,cap rocks,paleo-uplifts and traps for the successful and failed wells,the critical factors controlling hydrocarbon accumulation were pointed out.Paleo-uplift and distribution of source rocks are main factors affecting oil and gas accumulation in below-salts dolostones,whereas distribution of source rocks and cap rocks and faults control the hydrocarbon accumulation in above-salts dolostones.According to new knowledge on the distribution of source rocks,paleo-uplift and regional cap rocks,three advantaged exploration areas were selected:Two below-salts dolostones areas lying in Badong-Tazhong-Gucheng and Tabei area,and one above-salts dolostone area lying in Manxi-Gucheng area.
In recent years,large amount of hydrocarbon reserves have been discovered in the Tarim Basin.However,selection of advantaged exploration regions becomes more and more difficult because of poor knowledge of the distribution of source rocks.In this paper,outcrop profiles,drilling cores,and seismic materials were studied.The results show mild slope sedimentary models are more suitable for Yu′ertusi Formation,and source rocks mainly distribute in middle-lower mild slope and deep water shelf facies.Based on materials points and seismic profile studies,two sets of source rocks were distinguished: one lies in the northern depression,the other distributes in the south edge of southwestern depression.Distribution of source rocks in the study area coincide with discovered hydrocarbon reserves,providing new evidence for major source rocks.Meanwhile,the distributions of paleo-uplifts were considered.Three exploration regions of Manxi Lower Uplifts,Badong areas,and southwestern depression have been regarded as the most advantaged regions.
The platform margin reef shoal dolostones of Cambrian in the Tarim Basin contain large scale qualitative reservoirs and have a promising exploration prospect.In this paper,the lithostratigraphic and reservoir characteristics of the platform margin dolostones were described,through fine measurement,thin section analysis and porosity test of seven recently found field sections of Xiaoerblak Formation,Lower Cambrian,Kalpin area.Moreover,the inner structure and reservoir lateral distribution of the platform margin belt were discussed and a semi-quantitative reservoir model of the platform margin dolostone was established by section sedimentary facies and reservoir correlation.According to the research results,Xiaoerblak Formation is divided into two members and the upper member is further divided into three sub-members.The lower member is mainly composed of micritic and crystalline dolostones,which indicates a low-energy subtidal environment in a ramp platform margin.The upper member is mainly composed of algal-laminated dolostones,microbial reef dolostones,pelletal dolostones and stromatolitic dolostones,which indicate a high-energy environment in a feebly rimmed platform margin.The dolostone reservoir of platform margin is controlled by sedimentary facies and develops dominantly in the granular beach dolostone of the upper sub-member Ⅱ and Ⅲ.The main reservoir spaces are corroded vug and recrystallized intercrystal pore,and the reservoir porosity ranges from 0.6% to 9.02%,with an average of 5.5%.The average thickness of reservoir is 30m to 40m and the width of reservoir is 25km at least.
Carbonate platform types and features of platform margin belt play an important role in controlling the formation of reservoir and the relationship between reservoirs and caps.Cambrian carbonate platform in the Tarim Basin underwent three evolutionary processes of the Early Cambrian ramp platform,Middle Cambrian edging evaporative platform and the Late Cambrian edging platform,in which,the northern platform margin was the type of deposition,the eastern platform margin was fault controlled and the Lungu-Gucheng had obvious sectional differences.The line from Wushi-Kashi-Maigaiti-Hetian to Minfeng of the southwestern Tarim Basin were ancient lands in Early Cambrian,and they evolved into underwater lows with the rise of sea level in the Middle and Late Cambrian period,and possibly developed into a platform edge deposits in the Late Cambrian.Carbonate platform margin of steep slope and gentle slope can form different reservoir-seal assemblages.The Upper Cambrian aggradation-progradation platform margin reservoir in steep slope of eastern Lungu and Gucheng area is developed with good connectivity and whose caprock is always the key of platform margin reservoir-seal assemblages,so the reef-beach located behind the platform margin belt and near the sea side of the lagoon are favorable reservoirs,and the reservoirs are often overlain by carbonate caprock,which can form good reservoir-seal assemblages.Platform margin belt in gentle slope of Yingmai 7-8 in west of Tabei is a set of dolomite reservoir for algal mound and reef-bank complex with caprock of Middle-Lower Cambrian dolomicrite,gypsum dolomite and mud dolomite.Such dense layers,whose reservoir-seal assemblage is superior to that of eastern Lungu and Gucheng,have better exploration prospects.
The dolomite of Lower Paleozoic in the Tarim Basin is characterized by wide distribution,large thickness and huge resource amount,and it is a potentially important exploration field.But because of old age,great burial depth and complex diagenesis,it will bring a large difficulty to research reservoir and exploration.Using cores,thin sections and outcrops data,we made a systematically study of the types of effective reservoir space in Lower Paleozoic of Tarim Basin.Fabric selective reservoir space includes intracrystalline pore,intergranular pore,intragranular pore,gypsum-dissolved pore,interbreccia pore and algal intraframe pore|part fabric selective reservoir space includes intercrystalline solution pore and vug|none-fabric selective reservoir space includes cave,structural fracture and pressolution seam.According to the position and the distribution characteristics of dolomite reservoir,and combined with the main controlling factors,we put forward a new dolomite reservoir classification scheme.The facies-controlled dolomite reservoir can be divided into Shoal-mound and evaporative tidal flat dolomite reservoir|the sequence boundary-controlled dolomite reservoir can be divided into interlayer karst and buried hill karst dolomite reservoir|the fault-controlled dolomite reservoir can be divided into buried-hydrothermal and fracture dolomite reservoir.This classification scheme has good practicability.
Well Gucheng 6,which lies in the Gucheng region,obtained industrial gas flow at Lower Ordovician dolostones in 2012,and represented a kind of unconventional reservoir-cap combination and gas accumulation type with great importance on gas exploration.Since reservoirs are mainly controlled by pene-contemptuous dolomitization and hydrothermal alternations,and gases accumulate along slide faults,the reservoir-cap rock combination characteristics and hydrocarbon accumulations in Gucheng 6 were studied.Based on the research of reservoir formation and gas accumulation in well Gucheng 6,double cap rocks and slide faults controlling reservoir and reserves model is suggested,i.e.,hydrothermal fluids upwells along slide faults,then transports horizontally beneath cap rocks completing reservoir constructive reformation at early stage,gas inputs along slide faults and accumulates under cap rocks in gas discharging process.According to the distribution study on the Upper Ordovician mudstone,the Middle Ordovician compact limestones and the slide faults,it is redeemed that Manxi-Gucheng area is advantaged distribution district of combined cap rocks and slide faults,and that northern Manxi Uplift and southern Gucheng Uplift are the most practical area for the Lower Ordovician dolostones explorations.
Calculated models of elastic modulus,Poisson′s ratio,strength of dolomite reservoir have been established based on the rock mechanics experiment and the rock components about the complex lithology dolomite reservoir.Meanwhile,the orientation and magnitude of in situ stress have been identified through the wellbore breakouts and induced fractures,and stress profile has been established.On the other hand,natural fracture information about strike and dip has been extracted,and differences between the orientation of principal stress and the natural fractures′ strike have been analyzed.The sliding friction coefficient of natural fracture has been clarified,the shear stress and effective normal stress have been calculated accurately,and the shear slip rates under different injection pressure have been obtained.Therefore,the reconstruction scheme of ultra-deep,dense,gypseous dolomite reservoir has been determined.Finally,the fracturing layer,pump injection program,segmental perforation clusters,operating pressure have been optimized based on stratified stress profile,horizontal stress difference,formation fracability,natural fracture shear slip.
Erlian Basin could be divided into 5 depressions and 4 uplifts based on the distribution of Lower Cretaceous that filled in the faulted-sag in the styles of graben and/or half-graben controlled by NE-NNE striking normal faults.By the configuration of major faults,the complex faulted sag could be categorized into 4 families which are named respectively as serial,parallel,diagonal and interlaced.Each family includes 4 compounded patterns:(1)two or more half-grabens synthetic linked,(2)two or more half-grabens convergent linked,(3)graben and half-graben linked,(4)two or more grabens linked.The distribution of Early Cretaceous faulted-sag was controlled by Hercynian basement soft zone and the depressions extended in different direction.The hydrocarbon-bearing features of Lower Cretaceous have close relationship to both pattern of complex faulted-sags and the mode of rifting.The orthogonal rifting zone consisting of serial,diagonal faulted-sags in small angle with sag axle is beneficial to develop a narrow and deep faulted-lake,and hydrocarbon source rock is found within deep sub-sags.The oblique rifting zone consisting of parallel,diagonal faulted-sag in high angle with sag axle is beneficial to develop a wide faulted-lake,and source rock is found within the slope of faulted-lake.Most of the faulted-sags scattered over the uplift superimposed on the rigid basement,in that steep boundary fault with big displacement will be more favorable to the development of source rock growth in deep lake.
Taking Wulong region in the southeast edge of Sichuan Basin as an example,we have systemly collected Lower Paleozoic-Mesozoic samples and carried out measurement of vitrinite reflectance.Based on the analysis of deposition and evolution history of regional structure,research of division and origin of abnormal vitrinite reflectance zones has been carried out to reconstruct the geothermal gradient and maximum burial temperature of each stratum in the process of earth history by vitrinite reflectance gradient method and maximum burial temperature method.The four zones with abnormal vitrinite reflectance in Lower Paleozoic-Mesozoic in the southeast edge of Sichuan Basin are high-value anomaly zone at the bottom of Upper Triassic Xujiahe Formation,high-value anomaly zone in Middle Permian Qixia Formation,Top-Upper Permian Longtan Formation,high-value anomaly zone in the shale section at the bottom of Wufeng Formation-Longmaxi Formation and low-value anomaly zone of other strata below respectively from top to bottom,which are respectively formed by the thermal fluid movement caused by oil-gas migration,the eruption of Emei basalt in Hercynian period,the increase of temperature at the bottom of shielding layer of hydrothermal fluid and the high pressure compartment of geological fluid.The maximum burial temperature method is more reliable to reconstruct the paleogeotemperature.As shown by the reconstruction results,controlled by geological fluid compartment,the geothermal gradient at Lower Paleozoic is relatively large;the geothermal gradient in Cambrian is relatively low;the average geothermal gradient in Silurian can reach 41.47℃/km;the paleogeothermal gradient in Upper Paleozoic-Mesozoic is 25.10-28.01℃/km,with an average value of 26.18℃/km,and the background of terrestrial heat flow value is 66.78mW/m2.
Research on the filling characteristics and reservoir mechanism of Carboniferous oil and gas in northeast Sichuan Basin helps the understanding of the petroleum system.Based on the dynamics of petroleum accumulation,typical hydrocarbon reservoir analysis,fluid source and its flow pattern,combined with fluid inclusion and tracking hydrocarbon migration path techniques,the regularity of hydrocarbon accumulation have been analyzed through sample analysis and numerical simulation experiment.Results suggest that the organic matter from Silurian is the main source rock;the dolomite from Huanglong Formation is the favorable reservoir|the stratum-structure trap in Yinzhi-Yanshan tectonic period provides a gathering space.There are two filling-periods in 210Ma and 170Ma.The tectonic fissure and S/C-C/P unconformity constitute reticular-passage system.Vertical migration is prominent and lateral migration is minor,which shows the accumulation model of “lower-generation and upper storage” adjustment reconstruction in Xishan period.
The theory and method of constant-rate mercury injection and constant-pressure mercury injection is discussed in the paper.Through the testing and analysis of field rock sample,the difference between constant-rate mercury injection and constant-pressure mercury injection advantages and disadvantages and range of application was contrast,and the impact of experiment pressure on the experiment result is analyzed.The characteristics of constant-pressure mercury injection are fast speed,high experiment pressure and wind range pore radius.So it is suitable to test the majority of samples which are used in production and research.The testing speed of constant-rate mercury injection is very slow,and it can provide parameters of pore and throat and characterize the pore structure of porous medium.The experimental pressure of constant-rate mercury injection is low,so the testing sample pore radius range is small and tight rock parameters are incomplete.The scientific researcher should learn the pore radius range of rock sample and determine a reasonable experimental pressure before choosing the mercury injection method.In general,the permeability of tight rock sample is lower than 1×10-3μm2,so the constant-rate mercury injection should be used carefully.In order to acquire more representative pore characteristic parameters,the experimental pressure of constant-pressure mercury injection should be as big as possible.
The Cambrian deep dolomite reservoir in Tazhong area is deeply buried and complex in lithology.The hydrocarbon pore volume mainly consists of secondary erosional vugs accompanied with fractures.This may cause strong reservoir heterogeneity and complex oil-water relationship.This makes reservoir quantification and fluid identification difficult.A logging evaluation technology which suits Cambrian deep dolomite reservoir in Tazhong area is proposed after researching and practicing.This technology is based on MESI,ECS,APS,Sonic Scanner and combined with MESI Apparent Formation Water Resistivity Spectrum.It can not only finely describe the feature of vugs and fractures developed in dolomite reservoir,but also play well in effectiveness evaluating and fluid confirming of reservoir and plays an important role in discovery and evaluation of Cambrian deep dolomite reservoir.
The exploration of widely and thickly distributed Cambrian dolomite in the Tarim Basin has huge potential.It hasn′t been explored thoroughly because conventional seismic imaging technique is hard to describe the Cambrian dolomite.We analyzed carefully the dolomite reservoir characteristics and investigated the difference of seismic reflection characters between the Cambrian dolomite reservoir and other layers in Yingmai32 area.We researched some kinds of special techniques and formed a series of seismic imaging techniques on the Cambrian dolomite,such as spectrum constraint deconvolution to enhance the frequency,τ-p transform to improve SNR,TTI anisotropy analysis for setting up the velocity model,grid-based tomography inversion to update the velocity model and RTM to improve the seismic imaging.The new techniques help us to obtain top-quality seismic data for structural interpretation and reservoir prediction,and it is significant to further study the Cambrian dolomite.
In the Lower Ordovician of Tarim Basin,there is large amount of bedded dolostones,limestones and transitional rocks,in which the industrial hydrocarbon reserves have been discovered with great exploration potential.Via petrologic and geochemical methods,the transitional rocks occurring in Lower Ordovician were studied in this paper.Based on the petrologic characteristics,the limestone-dolostone transitional rocks were divided into limestone,transitional rocks and dolostones,representing low,middle and high dolomitization parts respectively.Geochemical results show that the oxygen isotope values(δ18O)in dolostones and transitional rocks are about 3.5‰ higher than that in limestones,and the three parts share similar PAAS normalized REE patterns and similar 87Sr/86Sr ratios of paleo-seawater,indicating that the dolomitization fluids come from seawater or evaporated seawater.The transitional rocks and dolostones demonstrate Fe-enriched,U and Mo-depleted,and negative Ce anomaly characteristics,indicating of shallow water and oxidized environments| whereas limestones might have been formed in relatively deep water and reduced environment.Moreover,based on oxygen isotope-temperature meter,the formation temperature of dolostones and transitional rocks is higher than that of limestones,suggesting that it is the temperature that controls the dolomitization.Based on the discussion about fluid source,redox environments,temperature and water depth,the “interbedded dolostones and limestones controlled by bedded seawater temperature model” is proposed.The model demonstrates that dolomitization occurs in shallow water(0-70m)with relatively high temperature(-30℃)environment,such as tidal or shallow shoal facies,whereas limestones commonly occur in relatively deep water with low temperature environment,corresponding to deep shoal facies.
As an oil exploration candidate area,many light crude oilfields have been found in the Southwest Tarim Basin.Research on the light hydrocarbon geochemical characteristics of oil in the whole area has great significance for the oil exploration in this area.Using the GC equipment with PONA capillary column,we analyzed C6—C13 light hydrocarbons of 22 oil samples collected from Southwest Tarim Basin.Based on the composition,component ratio,MCyC6/nC7,and m-xylene/(m-xylene+ nC8+ nC9),22 crude oils were divided into three groups,i.e.,Bashituo light oil groups,Hetian River light oil groups,and Kekeya light oil groups.Results show there are three layers of source rocks at least,and some oils are mixed oils,based on the aromatic parameters and carbon isotope data.
A suite of natural gases from the Eastern Sag of Liaohe Depression were characterized by their molecular and isotopic composition.The gas samples are largely gaseous hydrocarbons,together with high dryness(C1/∑C1-5 from 0.85 to 1.00),δ13C1(-46‰ to -38‰),δ13C2(-30‰ to -25‰),δ2HCH4(-250‰ to -210‰).Isoparaffin is enriched in light hydrocarbons and heptane value(H) & isoheptane value(I) both display a low maturity feature.In contrast with the geochemical characteristics of low maturity gases in the Turpan-Hami Basin,the natural gas in this region mainly belong to low maturity gas because of humic source rock and low maturity.The source rock in the study area is mainly developed in the Eocene Shahejie Formation(Es) and Dongying Formation(Ed).The organic matter of the source rock is humic-type and its vitrinite reflectance(RO ) ranges from 0.5% to 1.0%.The RO of source rocks which were calculated by the empirical relationship between the measured δ13C1 and RO ranges from 0.57% to 0.90%,which is in agreement with the actual RO of source rocks in this region,suggesting that the natural gases are derived from the source rock of the third Member of the Eocene Shahejie Formation(Es3).Additionally,most gas samples display clear geochemical evidence of biodegradation alteration.The majority of gases contain over 90% methane and small quantities of C2+ alkanes,carbon isotopic composition of C2 and C3 in two samples from shallow reservoirs are especially positive when the contents of them microbially decreased to a certain degree.High susceptibility to biodegradation of n-heptane leads to a lower heptane value(H) than the low maturity gas from Turpan-Hami Basin.
Pyrolysis experiments were conducted with low maturity kerogen isolated from typeⅡorganic-rich shale in a vacuum MSSV(Micro scale sealed vessel) system,to simulate gas hydrocarbon generation at two heating rates of 2℃/h,20℃/h.The stable carbon isotopic compositions of generated gas hydrocarbon were measured to investigate their characteristics and influencing factors.At the beginning,δ13C value of methane becomes more negative with increasing temperature until reaching the lightest point,then it changes more positive and the δ13C value of ethane and propane shows a positive trend with elevated temperatures.The main factors impact on carbon isotope of shale gas may include: type of organic matter,maturity of the shale and micro-migration of gas in shale gas systems.Although it is still unclear the reason leading to carbon isotope reversal,we can conclude that isotopic reversal phenomenon wouldn′t happen in a single thermogenic gas reservoir according to our pyrolytic experiment results,thus it must be involved with some other geochemical processes.Carbon isotope compositions of gas from the Fayetteville and Barnett shales show that it may include water-gas reaction and Fischer-Tropsch synthesis during its formation.
Based on the experimental analysis of porosity,pulse permeability,X-ray diffraction of rock minerals,scanning electron microscopy,the characteristics of terrestrial mud shale reservoir of the No.7 members of Yanchang Formation was researched in Ordos Basin.The results show that shale in Yanchang Formation contains rich organic matter with the characteristic of low porosity,permeability and thermal maturity.Organic matter pores are rarely seen.The illite-smectite mixed layer is the main part of clay minerals with smectite-illite ratio of 15%.Macro pores and macro-fissures are the main reservoir spaces.Reservoir physical property has positive correlation with the amount of quartz.High content of clay minerals is the main cause for the low porosity of shale reservoir.Therefore,quartz is important for reservoir space broadening and reservoir fracturing.
Shale thickness,total organic carbon content,brittle mineral content and physical property are key parameter of shale reservoir evaluation,which are obviously controlled by the sedimentary environment.Therefore,it′s significant to study the sedimentary environment and the evolution of the shale formation about gas reservoir evaluation and desserts prediction.This paper systematically tested more than 40 element concentrations of 525 groups of samples in Zhangjiatan shale drilled by well Y1 in the South-Central Ordos Basin.Then we analyzed the evolution of the paleoclimate,the paleobathymetry,the paleosalinity,the paleoproductivity and the oxidation-reduction states of sedimentary environment.The results showed that there are significant periodical changes of the element concentrations and ratios of Zhangjiatan shale from bottom to top,indicating that Zhangjiatan shale was deposited in a warm and humid,freshwater-brackish,hypoxia sedimentary environment,between which,the ④ unit was deposited in a wet sedimentary environment,when the water was deepest and highest reducing with high paleoproductivity.
Based on the shale sedimentary background,geochemical characteristics,reservoir characteristics and gas content etc.,we analyse the formation conditions of shale oil and gas and the reservoir forming pattern,and select the favorable exploration area.The results show that the thickness of member 3 in the study area is large,commonly between 100-500m.The organic carbon mass fraction is in the range of 0.1%-6% with an average of 2.91%.Thermal maturity of organic matter is low with an average value of 0.6%.The reservoir space is mainly inter-granular pore and dissolved pore.The brittle mineral content is high such as quartz and carbonate rocks,so it has certain fragility.Furthermore,the results of isothermal absorption show that shale has high performances of gas absorption,and the total gas content is 0.65-5.11m3/t,with an average value of 2.32%.Finally,through the multi-factors superposition method of shale oil and gas accumulation conditions,we believe the Niuju-Changtan subsag in the north and south of Jiazhangsi-Erjiegou subsag is a favorable area for oil and gas exploration in the Eastern Sag of Liaohe Depression.
In order to reveal methane adsorption capacities influenced by the geological factors in the process of thermal evolution,a shale sample of the Yanchang Formation Chang 7 member in southeastern Ordos Basin was collected to get 7 different simulation burial depth samples through the thermal simulation experiment.The organic geochemical parameters,mineral composition,pore structure and methane adsorption capacities were measured.According to this study,the influence factors on methane adsorption capacity with thermal evolution can be divided into three kinds:Physical factors such as specific surface area and pore diameter,organic geochemical factors such as TOC and thermal maturity,and mineral composition factors such as clay minerals and andreattite.Geological factors have intricate impacts on the methane adsorption capacity and the consideration to combine each factor together may increase the relevance of the adsorption amount and influence factors.Micro-pore,which is the most important factor,has a positive correlation with the methane adsorption capacity.The adsorption quantity increases with the decrease of TOC due to the thermal evolution of organic produced micro-pores that increases the adsorption space.In addition,the adsorption capacity of the shale has a negative correlation to the burial depth,and the deeper it buried the faster the adsorption capacity decreased.
Based on core description,ion beam-scanning electron microscope (FIB-SEM),pore types and characteristics of different scale of continental shale which is represented by the western sag of Liaohe Depression were studied,and the accumulation process of shale oil and gas in the pores was discussed.The results show that the development of large pores like dissolution pores and cracks,tectonic cracks helps the accumulation and migration of oil in the study interval.What's more,inorganic mineral pore,organic pore and micro-cracks were discovered by FIB-SEM,and the formation of the large organic pores is related to the dissolution of the organic acid in the condition of low maturity.Meanwhile,the results show that a variety of micro-cracks are developed in the edge and internal of the organic matter.The extensive development of organic internal cracks is actually the main difference between low maturity continental shale and southern high maturity marine shale.The continental organic in the study area is in the peak of oil generation with a large number of natural gases,and the oil and gas existed at the same time so that the accumulation mechanism is relatively complex.The inner and outer surface of organic and clay plays an important role in the storage of absorbed shale gas,and the dissolved shale gas is mainly stored in the crude oil.The oil-carried gas fills the pores in the shale interval which shares a high oil and gas saturation.Finally,the migration processes are divided into four stages,i.e.,bio-gas accumulation,oil and gas micro accumulation,oil and gas macro accumulation and shale interval accumulation.
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