Groundwater Systems and Resources in the Ordos Basin,China

The Ordos Basin is a large-scale sedimentary basin in northwestern China. The hydrostratigraphic units from bottom to top are pre-Cambrian metamorphic rocks, Lower Paleozoic carbonate rocks, Upper Paleozoic to Mesozoic clastic rocks and Cenozoic deposits. The total thickness is up to 6000 m. Three groundwater systems are present in the Ordos Basin, based on the geological settings, i.e. the karst groundwater system, the Cretaceous clastic groundwater system and the Quaternary groundwater system. This paper describes systematically the groundwater flow patterns of each system and overall assessment of groundwater resources.

Assessment of recoverable oil and gas resources by in-situ conversion of shale——Case study of extracting the Chang 7_(3) shale in the Ordos Basin

The purpose of this study is to investigate the entire evolution process of shales with various total organic contents(TOC)in order to build models for quantitative evaluation of oil and gas yields and establish methods for assessing recoverable oil and gas resources from in-situ conversion of organic matters in shale.Thermal simulation experiments under in-situ conversion conditions were conducted on Chang 7_(3) shales from the Ordos Basin in a semi-open system with large capacity.The results showed that TOC and R_(o) were the key factors affecting the in-situ transformation potential of shale.The remaining oil and gas yields increased linearly with TOC but inconsistently with R_(o).R_(o) ranged 0.75%—1.25%and 1.05%—2.3%,respectively,corresponding to the main oil generation stage and gas generation stage of shale in-situ transformation.Thus a model to evaluate the remaining oil/gas yield with TOC and R_(o) was obtained.The TOC of shale suitable for in-situ conversion should be greater than 6%,whereas its R_(o) should be less than 1.0%.Shales with 0.75%(R_(o))could obtain the best economic benefit.The results provided a theoretical basis and evaluation methodology for predicting the hydrocarbon resources from in-situ conversion of shale and for the identification of the optimum“sweet spots”.The assessment of the Chang 7_(3) shale in the Ordos Basin indicated that the recoverable oil and gas resources from in-situ conversion of organic matters in shale are substantial,with oil and gas resources reaching approximately 450×10^(8) t and 30×10^(12)m^(3),respectively,from an area of 4.27×10^(4) km^(2).

Reservoir characteristics and formation model of Upper Carboniferous bauxite series in eastern Ordos Basin,NW China

Through core observation,thin section identification,X-ray diffraction analysis,scanning electron microscopy,and low-temperature nitrogen adsorption and isothermal adsorption experiments,the lithology and pore characteristics of the Upper Carboniferous bauxite series in eastern Ordos Basin were analyzed to reveal the formation and evolution process of the bauxite reservoirs.A petrological nomenclature and classification scheme for bauxitic rocks based on three units(aluminum hydroxides,iron minerals and clay minerals)is proposed.It is found that bauxitic mudstone is in the form of dense massive and clastic structures,while the(clayey)bauxite is of dense massive,pisolite,oolite,porous soil and clastic structures.Both bauxitic mudstone and bauxite reservoirs develop dissolution pores,intercrystalline pores,and microfractures as the dominant gas storage space,with the porosity less than 10% and mesopores in dominance.The bauxite series in the North China Craton can be divided into five sections,i.e.,ferrilite(Shanxi-style iron ore,section A),bauxitic mudstone(section B),bauxite(section C),bauxite mudstone(debris-containing,section D)and dark mudstone-coal section(section E).The burrow/funnel filling,lenticular,layered/massive bauxite deposits occur separately in the karst platforms,gentle slopes and low-lying areas.The karst platforms and gentle slopes are conducive to surface water leaching,with strong karstification,well-developed pores,large reservoir thickness and good physical properties,but poor strata continuity.The low-lying areas have poor physical properties but relatively continuous and stable reservoirs.The gas enrichment in bauxites is jointly controlled by source rock,reservoir rock and fractures.This recognition provides geological basis for the exploration and development of natural gas in the Upper Carboniferous in the study area and similar bauxite systems.

Ordovician reservoirs in Fuxian area:Gas accumulation patterns and their implications for the exploration of lower Paleozoic carbonates in the southern Ordos Basin

In recent years,the Fuxian area in the southeastern Ordos Basin has undergone significant exploration,with industrial gas flow tested in wells drilled into the Ordovician marine carbonates.Despite this,the gas accumulation patterns of this area are not fully understood,posing challenges for further exploration.Our analysis of geological conditions indicates that the Ordovician Majiagou Formation in this area hosts two gas plays:one found in weathering crusts and the other found in interior of the formation.We investigated various typical gas reservoirs in the area,focusing on differentiating the geological conditions and factors controlling gas accumulation in the weathering-crust and interior gas reservoirs.The results suggest three primary gas accumulation patterns in the Majiagou Formation in the Fuxian area:(1)upper gas accumulation in weathering crusts,present in the high parts of landforms such as residual paleo-hills or buried paleo-platform(Pattern I);(2)the stereoscopic pattern with gas accumulation in both weathering crusts and strata interior,arising in high parts of landforms such as residual paleo-hills or buried paleo-platforms(Pattern II);(3)lower gas accumulation in strata interior,occurring in the upper reaches and on both sides of paleo-trenches(Pattern III).This study will serve as a geological basis for future exploration deployment in the Fuxian area.

Conditions for the enrichment of karst hydrothermal resources in Bohai Bay Basin

Drilling for karst hydrothermal resources in eastern China has posed challenges,including disparities between the temperature and yield of geothermal water.It is evident that relying solely on geothermal anomalies or indications of karst reservoirs is inadequate for the exploration of karst hydrothermal resources.This study seeks to elucidate the cause of geothermal sweet spots by analyzing the interplay between geothermal anomalies and karst reservoirs and the underlying geological conditions for karst hydrothermal enrichment.Key findings include:(1)the Bohai Bay Basin has been geologically favorable for the development of karst hydrothermal resources since the Mesozoic era;(2)the karst hydrothermal enrichment varies significantly between the basin’s margin and its interior.On the basin margin,the enrichment is largely driven by groundwater activity and faults,particularly where faults facilitate the upwelling of geothermal water.In contrast,within the basin’s interior,karst hydrothermal resources are predominantly influenced by buried hills and are especially enriched in areas facilitating the discharge of deep geothermal waters.

Experimental analysis of matrix moveable oil saturation in tight sandstone reservoirs of the south Ordos Basin,China

Tight oil reservoirs in the south Ordos Basin are characterized by fractured,heterogeneous oil-bearing strata(an oil saturation of less than 55%on average),normal pressure(0.8±)and extra-low permeability(less than 0.3 mD).In the Chang 8 tight sandstone reservoir in Honghe oilfield,micro-and nanopores,especially those with a pore-throat radius of less than 1 mm,account for more than 90%.Fluid flow in the matrix is non-linear and crude oil flow rates are very low under normal pressure gradients.An improved understanding of oil mobility in a tight matrix is key to further development of normalpressure tight-oil resources in the continental basin.In this study,constant-velocity and high-pressure mercury injection experiments were conducted using samples of typical tight sandstone cores obtained from the south of Ordos Basin.A new method for reconstructing the full-scale pore-throat distribution characteristics of tight sandstone reservoirs was established successfully,based on which multistage centrifugal tests,tests of low-pressure differential displacement of oil by water,and nuclear magnetic resonance tests were conducted in order to obtain the distribution characteristics of moveable fluid in different pores.The moveable oil saturation(MOS)and degree of oil recovery(i.e.ratio of accumulative oil production to producing geologic reserves)of the core samples under different differential pressures for displacement were determined.As for the tight oil reservoirs in the south Ordos Basin,the moveable fluids are mainly stored in sub-micron(0.10-0.5 mm)pores.For Type I reservoirs(k>0.1 mD),the volume percentage of moveable fluid in pores with a radius larger than 0.5 mm is relatively high(greater than 40%).The degree of oil recovery of water flooding serves as the basis for forecasting recoverable reserves for tight oil reservoirs.Recoverable reserves under water flooding,mainly occur in pores with a radius greater than 0.5 mm.The contribution of Type I reserves to oil production is observed to be greater than 60%,and the degree of oil recovery reaches up to 17.1%.These results help improve our understanding on the evaluation and classification of Chang 8 tight sandstone reservoirs in Honghe oilfield and serve as theoretical basis for pilot tests to explore effective injection media and development methods to improve the matrix-driven pressure differences and displacement efficiency for oil.

Extreme massive hydraulic fracturing in deep coalbed methane horizontal wells:A case study of the Linxing Block,eastern Ordos Basin,NW China

Deep coal seams show low permeability,low elastic modulus,high Poisson’s ratio,strong plasticity,high fracture initiation pressure,difficulty in fracture extension,and difficulty in proppants addition.We proposed the concept of large-scale stimulation by fracture network,balanced propagation and effective support of fracture network in fracturing design and developed the extreme massive hydraulic fracturing technique for deep coalbed methane(CBM)horizontal wells.This technique involves massive injection with high pumping rate+high-intensity proppant injection+perforation with equal apertures and limited flow+temporary plugging and diverting fractures+slick water with integrated variable viscosity+graded proppants with multiple sizes.The technique was applied in the pioneering test of a multi-stage fracturing horizontal well in deep CBM of Linxing Block,eastern margin of the Ordos Basin.The injection flow rate is 18 m^(3)/min,proppant intensity is 2.1 m^(3)/m,and fracturing fluid intensity is 16.5 m^(3)/m.After fracturing,a complex fracture network was formed,with an average fracture length of 205 m.The stimulated reservoir volume was 1987×10^(4)m^(3),and the peak gas production rate reached 6.0×10^(4)m^(3)/d,which achieved efficient development of deep CBM.

Enrichment conditions and resource potential of coal-rock gas in Ordos Basin,NW China

To reveal the enrichment conditions and resource potential of coal-rock gas in the Ordos Basin,this paper presents a systematic research on the sedimentary environment,distribution,physical properties,reservoir characteristics,gas-bearing characteristics and gas accumulation play of deep coals.The results show that thick coals are widely distributed in the Carboniferous–Permian of the Ordos Basin.The main coal seams Carboniferous 5~#and Permian 8~#in the Carboniferous–Permian have strong hydrocarbon generation capacity and high thermal evolution degree,which provide abundant materials for the formation of coal-rock gas.Deep coal reservoirs have good physical properties,especially porosity and permeability.Coal seams Carboniferous 5^(#)and Permian 8^(#)exhibit the average porosity of 4.1%and 6.4%,and the average permeability of 8.7×10^(-3)μm^(2)and 15.7×10^(-3)μm^(2),respectively.Cleats and fissures are developed in the coals,and together with the micropores,constitute the main storage space.With the increase of evolution degree,the micropore volume tends to increase.The development degree of cleats and fissures has a great impact on permeability.The coal reservoirs and their industrial compositions exhibit significantly heterogeneous distribution in the vertical direction.The bright coal seam,which is in the middle and upper section,less affected by ash filling compared with the lower section,and contains well-developed pores and fissures,is a high-quality reservoir interval.The deep coals present good gas-bearing characteristics in Ordos Basin,with the gas content of 7.5–20.0 m^(3)/t,and the proportion of free gas(greater than 10%,mostly 11.0%–55.1%)in coal-rock gas significantly higher than that in shallow coals.The enrichment degree of free gas in deep coals is controlled by the number of macropores and microfractures.The coal rock pressure testing shows that the coal-limestone and coal-mudstone combinations for gas accumulation have good sealing capacity,and the mudstone/limestone(roof)-coal-mudstone(floor)combination generally indicates high coal-rock gas values.The coal-rock gas resources in the Ordos Basin were preliminarily estimated by the volume method to be 22.38×10^(12)m^(3),and the main coal-rock gas prospects in the Ordos Basin were defined.In the central-east of the Ordos Basin,Wushenqi,Hengshan-Suide,Yan'an,Zichang,and Yichuan are coal-rock gas prospects for the coal seam#8 of the Benxi Formation,and Linxian West,Mizhi,Yichuan-Huangling,Yulin,and Wushenqi-Hengshan are coal-rock gas prospects for the coal seam#5 of the Shanxi Formation,which are expected to become new areas for increased gas reserves and production.

Microfacies and diagenetic alteration in a semi-deep to deep lacustrine shale: The Yanchang Formation in the Ordos Basin, China

The mineralogical development and diagenetic sequence of lacustrine shales in the Chang 7 Member of the Yanchang Formation in the Ordos Basin are detailed studied.A model of their depositional system and a diagenetic diagram are proposed in this study.Through detailed petrographic,mineralogical,and elemental analyses,four distinct shale types are identified:argillaceous shale,siliceous shale,calcareous shale,and carbonate,clay,and silt-bearing shale.The main diagenetic process in argillaceous shale is the transformation of illite to smectite,negatively impacting shale porosity.Siliceous shale undergoes carbonate cementation and quartz dissolution,contributing to increased porosity,particularly in mesopores.Calcareous shale experiences diagenesis characterised by carbonate formation and dissolution,with a prevalence of siderite.In carbonate,clay,and silt-bearing shale,the dissolution of K-feldspar contributes to illitization of kaolinite.Argillaceous shale,characterised by more clay minerals and lower mesopore volume,is identified as a potential hydrocarbon seal.Siliceous shale,with the highest pore volume and abundant inter-mineral pores,emerges as a promising shale oil reservoir.These findings contribute to a comprehensive understanding of shale properties,aiding in the prediction of shale oil exploration potential in the studied area.

A tectono-thermal perspective on the petroleum generation,accumulation and preservation in the southern Ordos Basin, North China

The southern Ordos Basin has excellent petroleum exploration prospects.However,the tectono-thermal history and the controls on petroleum generation,accumulation and preservation conditions in southern basin are unclear.In this study,we analyzed the present geothermal field,paleo-geothermal gradient,maturity of the hydrocarbon source rocks,uplift and cooling process and tectono-thermal evolution history.In the study area,for the Ordovician,Permain and the Triassic strata,the present temperature is low(3070℃)in the southeastern area but high(80140℃)in the northwestern area.The paleogeothermal gradient varied from 24℃/km to 30℃/km with a heat flow of 58—69 m W/m^(2)(i.e.,a medium-temperature basin).The paleo-temperatures are higher than the present temperatures and the maximum paleo-temperatures controlled the thermal maturity of the source rocks.The vitrinite reflectance(Ro)values of the Triassic strata are>0.7%and the thermal maturity reached the middlemature oil generation stage.The Rovalues of the Permian-Ordovician strata are>1.8%and the thermal maturity reached the over-mature gas generation stage.The southern Ordos Basin has experienced the multiple tectonic events at the Late Ordovician Caledonian(452 Ma),Late Triassic(215 Ma),Late Jurassic(165160 Ma),End-Early Cretaceous(110100 Ma)and Cenozoic(since 40 Ma).A large-scale tectonothermal event occurred at the End-Early Cretaceous(110100 Ma),which was controlled by lithospheric extension,destruction and thinning.This event led to the highest paleo-temperatures and thermal maturities and coeval with the peak period of petroleum generation and accumulation.The southern Ordos Basin has undergone rapid and large-scale uplift since the Late Cretaceous due to expansion of the northeastern margin of the Tibetan Plateau,uplift of the Qinling orogenic belt and thrust faulting in the Liupanshan tectonic belt.The southern Ordos Basin experienced tectonic overprinting that was strong in the south and weak in the north.The strongest overprinting occurred in the southwestern part of the basin.The large-scale uplift,denudation and faulting led to oil and gas loss from reservoirs.The petroleum generation,accumulation and preservation in the southern Ordos Basin were affected by deep lithospheric structures and the tectono-thermal evolution.This work provides a novel tectono-thermal perspective on the petroleum generation,accumulation and preservation condition of the southern Ordos Basin.

Connotation,pathway and significance of carbon neutrality“super energy system”:A case study of the Ordos Basin,NW China

Super oil and gas basins provide the energy foundation for social progress and human development.In the context of climate change and carbon peak and carbon neutrality goals,constructing an integrated energy and carbon neutrality system that balances energy production and carbon reduction becomes crucial for the transformation of such basins.Under the framework of a green and intelligent energy system primarily based on“four news”,new energy,new electricity,new energy storage,and new intelligence,integrating a“super energy system”composed of a huge amount of underground resources of coal,oil,gas and heat highly overlapping with abundant wind and solar energy resources above ground,and a regional intelligent energy consumption system with coordinated development and utilization of fossil energy and new energy,with a carbon neutrality system centered around carbon cycling is essential.This paper aims to select the traditional oil and gas basins as“super energy basins”with the conditions to build world-class energy production and demonstration bases for carbon neutrality.The Ordos Basin has unique regional advantages,including abundant fossil fuel and new energy resources,as well as matching CO_(2)sources and sinks,position it as a carbon neutrality“super energy basin”which explores the path of transformation of traditional oil and gas basins.Under the integrated development concept and mode of“coal+oil+gas+new energy+carbon capture,utilization and storage(CCUS)/carbon capture and storage(CCS)”,the carbon neutrality in super energy basin is basically achieved,which enhance energy supply and contribute to the carbon peak and carbon neutrality goals,establish a modern energy industry and promote regional green and sustainable development.The pioneering construction of the world-class carbon neutrality“super energy system”demonstration basin in China represented by the Ordos Basin will reshape the new concept and new mode of exploration and development of super energy basins,which is of great significance to the global energy revolution under carbon neutrality.

Fault characteristics and their control on oil and gas accumulation in the southwestern Ordos Basin

3D seismic data recently acquired from the Ordos Basin shows three sets of regularly distributed fault systems,which overrides previous understanding that no faults were developed in this basin.Seismic interpretation suggests that the faults in the southwestern Ordos Basin have three basic characteristics,namely extreme micro-scale,distinct vertical stratification,and regularity of planar distribution.These NS-,NW-,and NE-trending fault systems developed in the Meso-Neoproterozoic e Lower Ordovician strata.Of these,the NS-trending fault system mainly consists of consequent and antithetic faults which show clear syndepositional deformation.The fault systems in the Carboniferous e Middle-Lower Triassic strata are not clear on seismic reflection profiles.The NW-and NE-trending fault systems are developed in the Upper Triassic e Middle Jurassic strata.Of these,the NW-trending fault system appears as a negative flower structure in sectional view and in an en echelon pattern in plan-view;they show transtensional deformation.A NE-trending fault system that developed in the Lower Cretaceous e Cenozoic strata shows a Y-shaped structural style and tension-shear properties.A comprehensive analysis of the regional stress fields at different geologic times is essential to determine the development,distribution direction,and intensity of the activity of fault systems in the Ordos Basin.Current exploration suggests three aspects in which the faults within the Ordos Basin are crucial to oil and gas accumulation.Firstly,these faults serve as vertical barriers that cause the formation of two sets of relatively independent petroleum systems in the Paleozoic and Mesozoic strata respectively;this is the basis for the‘upper oil and lower gas’distribution pattern.Secondly,the vertical communication of these faults is favorable for oil and gas migration,thus contributing to the typical characteristics of multiple oil and gas fields within the basin,i.e.oil and gas reservoirs with multiple superimposed strata.Finally,these faults and their associated fractures improve the permeability of Mesozoic tight reservoirs,providing favorable conditions for oil enrichment in areas around the fault systems.

Tectono-sedimentary evolution and oil-gas geological significance of first to third member of Ordovician Majiagou Formation in Ordos Basin,NW China

Based on logging,core,thin section and geochemical analysis,the tectonic-lithofacies paleogeographic pattern of first member to third member of Ordovician Majiagou Formation(O_(1)m_(1)–O_(1)m_(3)for short)in Ordos Basin is reconstructed,and the tectono-sedimentary evolution characteristics and oil-gas geological significance are discussed.The results are obtained in four aspects.First,a set of marginal argillaceous dolomites with high gamma ray value developed steadily and diachronously at the bottom of Majiagou Formation,which distributed over the Huaiyuan Movement unconformity,with δ^(13)C values positive drift characteristics comparable to global transgression of the Early Ordovician Floplian.Second,the global sea level rose and the ancient land was submerged into the underwater uplift in O_(1)m_(1)to O_(1)m_(2),and the central uplift was deposited for the first time in the Ordovician,forming a tectonic pattern of“one uplift and two depressions”.Subsequently,the subduction and extrusion outside the basin and the differentiation of uplift and depression in the basin of O_(1)m_(3)resulted in the activation of the Wushenqi–Jingbian bulge.Third,the evolution of the tectonic pattern had a significant impact on the sedimentary paleoenvironment.The O_(1)m_(1)overlaps westward,and saline lagoon is formed in eastern depression and influenced by the transgression.The transgression continued in O_(1)m_(2)and resulted in communication with the wide sea,and the large-scale grain shoal developed around eastern depression,and the late dry shrinkage formed a small scale evaporite lagoon in upper part.Under the influence of highland sealing in O_(1)m_(3),the water body gradually differentiated into dolomitic gypsum and saline lagoons to the east,and the grain shoal spread along the highs around sag.Fourth,the source rocks developed diachronously at the bottom of Majiagou Formation form a favorable source-reservoir assemblage with the shoal facies reservoir distributed around the slope of O_(1)m_(2)–O_(1)m_(3),and they have certain exploration potential for natural gas.

Dynamic evaluation on sealing capacity of caprocks of the Meso-Neoproterozoic reservoirs in Ordos Basin,China

The Meso-Neoproterozoic is a new play in the Ordos Basin.A deeper understanding about the dynamic relationship between the caprocks and the source rocks is needed.Based on the comprehensive analysis of hydrocarbon source development characteristics of the Meso-Neoproterozoic and its overlying strata,as well as the formation contact relationships,lithology characteristics and exploratory drilling data,it is recognized that the Meso-Neoproterozoic contains two types of petroleum accumulation assemblage,that is,the“self-sourced indigenous”and“upper source rock-lower reservoir”assemblages.The former is mainly controlled by the development and distribution of source rocks of the Changcheng System,with the Lower Cambrian shale sequence as its caprock.The later is controlled by the superposition between the Meso-Neoproterozoic and its overlying source rocks and this assemblage is mainly distributed in Hangjinqi and Pingliang areas with the Carboniferous-Permian shale sequence as its caprock.The dynamic evaluation on the displacement pressure serves to reconstruct the displacement pressure history of the caprock.The results show that the shale sequence of the Cambrian Maozhuang Formation in well XY 1 in the southern Ordos Basin has possibly acquired the ability of sealing natural gas since the early of Late Triassic.Its displacement pressure increased rapidly up to 20 MPa during the Late Triassic-Jurassic and keeps at 9.2 MPa at present,indicating fair sealing ability.The Carboniferous-Permian caprocks in Hangjinqi area could have acquired the ability to seal natural gas in the Late Jurassic-Early Cretaceous,and the present-day displacement pressure is 9e12 MPa,indicating good sealing ability.The upper Paleozoic caprock in Pingliang area has been able to seal natural gas since the Early Jurassic,with a maximum displacement pressure of 23 MPa during the Cretaceous period and a current value of 17 e20 MPa,indicative of strong ability to seal natural gas.The sealing ability of caprocks of both the“selfsourced indigenous”and“upper source rock-lower reservoir”assemblages has come into being earlier than or at least no later than the peak gas generation of the source rocks and therefore the caprocks are dynamically effective in geohistory.The Meso-Neoproterozoic reservoirs in the Ordos Basin are well preserved and probabally of better potential for exploration in terms of the caprock-source rock combination.

Two stages of subsidence and its formation mechanisms in Mid-Late Triassic Ordos Basin,NW China

Based on a large number of newly added deep well data in recent years,the subsidence of the Ordos Basin in the Mid-Late Triassic is systematically studied,and it is proposed that the Ordos Basin experienced two important subsidence events during this depositional period.Through contrastive analysis of the two stages of tectonic subsidence,including stratigraphic characteristics,lithology combination,location of catchment area and sedimentary evolution,it is proposed that both of them are responses to the Indosinian Qinling tectonic activity on the edge of the craton basin.The early subsidence occurred in the Chang 10 Member was featured by high amplitude,large debris supply and fast deposition rate,with coarse debris filling and rapid subsidence accompanied by rapid accumulation,resulting in strata thickness increasing from northeast to southwest in wedge-shape.The subsidence center was located in Huanxian–Zhenyuan–Qingyang–Zhengning areas of southwestern basin with the strata thickness of 800–1300 m.The subsidence center deviating from the depocenter developed multiple catchment areas,until then,unified lake basin has not been formed yet.Under the combined action of subsidence and Carnian heavy rainfall event during the deposition period of Chang 7 Member,a large deep-water depression was formed with slow deposition rate,and the subsidence center coincided with the depocenter basically in the Mahuangshan–Huachi–Huangling areas.The deep-water sediments were 120–320 m thick in the subsidence center,characterized by fine grain.There are differences in the mechanism between the two stages of subsidence.The early one was the response to the northward subduction of the MianLüe Ocean and intense depression under compression in Qinling during Mid-Triassic.The later subsidence is controlled by the weak extensional tectonic environment of the post-collision stage during Late Triassic.

Prediction of favorable geological storage areas based on sedimentary characteristics:A case study on the northeastern Ordos Basin,China

The Ordos Basin is a large cratonic basin with stable deposition in Northwest China.Given its mostly stable subsidence background and the urgent need for saline water disposal and carbon dioxide storage in the coal mining and coal chemical industries,the Ordos Basin has been chosen as a pilot demonstration site for carbon dioxide and saline water storage in China.However,few studies have been made to evaluate the sedimentary and sequence stratigraphy characteristics of this region,as well as their influence on carbon dioxide and saline water storage potential.To address this research gap,we conducted a sedimentary study of the Lower Triassic Liujiagou Formation in the northeastern Ordos Basin utilizing the stratigraphy theory,laboratory test analysis,and pilot project demonstration,to evaluate the area's viability for the deep geological storage of gas and liquid waste.We studied the tectonic setting,petrological features,and sedimentary characteristics of the favorable strata and predicted favorable areas based on sequence stratigraphy theory.The lithology predominantly consists of feldspathic graywacke,with a fine grain size and mostly fine-to-medium-grained sandstone.The distribution of thick-grained sandstone and fine-grained sediment was identified by dividing the sequence,and a favorable reservoir-cap assemblage configuration was formed.It's concluded that the Lower Triassic Liujiagou Formation exhibits suitable characteristics for the deep geological storage of carbon dioxide and saline water.This study demonstrates the importance of basic theory in guiding practical applications and provides a reference for the scientific selection of favorable areas for deep basin storage.

Main controlling factors and exploration enlightenment of aluminous rock series gas reservoirs in Ordos Basin,NW China

Based on the data of outcrop,core,logging,gas testing,and experiments,the natural gas accumulation and aluminous rock mineralization integrated research was adopted to analyze the controlling factors of aluminous rock series effective reservoirs in the Ordos Basin,NW China,as well as the configuration of coal-measure source rocks and aluminous rock series reservoirs.A natural gas accumulation model was constructed to evaluate the gas exploration potential of aluminous rock series under coal seam in the basin.The effective reservoirs of aluminous rock series in the Ordos Basin are composed of honeycomb-shaped bauxites with porous residual pisolitic and detrital structures,with the diasporite content of greater than 80%and dissolved pores as the main storage space.The bauxite reservoirs are formed under a model that planation controls the material supply,karst paleogeomorphology controls diagenesis,and land surface leaching improves reservoir quality.The hot humid climate and sea level changes in the Late Carboniferous–Early Permian dominated the development of a typical coal-aluminum-iron three-stage stratigraphic structure.The natural gas generated by the extensive hydrocarbon generation of coal-measure source rocks was accumulated in aluminous rock series under the coal seam,indicating a model of hydrocarbon accumulation under the source.During the Upper Carboniferous–Lower Permian,the relatively low-lying area on the edge of an ancient land or island in the North China landmass was developed.The gas reservoirs of aluminous rock series,which are clustered at multiple points in lenticular shape,are important new natural gas exploration fields with great potential in the Upper Paleozoic of North China Craton.

Tracing of natural gas migration by light hydrocarbons:A case study of the Dongsheng gas field in the Ordos Basin,NW China

Based on the analysis of light hydrocarbon compositions of natural gas and regional comparison in combination with the chemical components and carbon isotopic compositions of methane,the indication of geochemical characteristics of light hydrocarbons on the migration features,dissolution and escape of natural gas from the Dongsheng gas field in the Ordos Basin is revealed,and the effect of migration on specific light hydrocarbon indexes is further discussed.The study indicates that,natural gas from the Lower Shihezi Formation(Pix)in the Dongsheng gas field displays higher iso-C5-7contents than n-C5-7contents,and the C6-7light hydrocarbons are composed of paraffins with extremely low aromatic contents(<0.4%),whereas the C7light hydrocarbons are dominated by methylcyclohexane,suggesting the characteristics of coal-derived gas with the influence by secondary alterations such as dissolution.The natural gas from the Dongsheng gas field has experienced free-phase migration from south to north and different degrees of dissolution after charging,and the gas in the Shiguhao area to the north of the Borjianghaizi fault has experienced apparent diffusion loss after accumulation.Long-distance migration in free phase results in the decrease of the relative contents of the methylcyclohexane in C7 light hydrocarbons and the toluene/n-heptane ratio,as well as the increase of the n-heptane/methylcyclohexane ratio and heptane values.The dissolution causes the increase of isoheptane values of the light hydrocarbons,whereas the diffusion loss of natural gas in the Shiguhao area results in the increase of n-C5-7contents compared to the iso-C5-7contents.

Paleoenvironment of marine-continental transitional shales in the lower Permian Shanxi formation,southeastern Ordos Basin,China

The paleoenvironment of shales can be reconstructed to some extent using the combinations or concentrations of elements that correlate strongly with environmental conditions.In this study,we analyzed rare earth elements(REEs),major elements,and trace elements in the marine-continental transitional shales(transitional shales for short)of the Shan 2^(3)submember of the Shanxi Formation in the southeastern Ordos Basin.The purpose is to deduce the paleoenvironmental conditions of the shales,encompassing paleoredox,paleoclimate,paleoproductivity,and paleo-provenance.The Shan 2^(3)submember comprises four sections,namely Shan 2^(3)-1,Shan 2^(3)-2,Shan 2^(3)-3,and Shan 2^(3)-4.The Ba/Al,P/Al,and Cu/Al ratios,along with biogenic barium(Babio),indicate that the paleoproductivity of the submember peaked during the Shan 2^(3)-1 deposition and exhibited a downtrend upward in other sections.Trends in the Uau and the Ni/Co,V/Cr,U/Th,and V/Sc ratios suggest that suboxic conditions prevailed during the Shan 2^(3)-1 deposition,with the oxidation level gradually increasing from Shan 2^(3)-1 to Shan 2^(3)-4.C-value and the Sr/Cu vs.Ga/Rb cross-plot indicate a warm and arid paleoclimate during the Shan 2^(3)-1 deposition,which transitioned to cooler,drier conditions during the deposition of other sections.Indicators sensitive to paleoclimate,such as the K/Rb and Th/U ratios,along with the ICV,PIA,and Chemical Index of Alteration(CIA),highlight elevated weathering from Shan 2^(3)-2 to Shan 2^(3)-4,with Shan 2^(3)-1 exhibiting the weakest weathering during its deposition.As suggested by the REE data,the Zr/Sc vs.Th/Sr cross-plot,provenance discriminant functions,and the cross-plots of Hf vs.La/Th,Th vs.Hf-Co,and∑REE vs.La/Yb,the sedimentary provenance for the transitional shales of the Shan 2^(3)submember is of multiple origins,with significant contributions from the Upper Continental Crust(UCC).Discriminant diagrams,including those of Th-Co-Zr/10,Th-Sc-Zr/10,La-Th-Sc,and K_(2)O/Na_(2)O vs.SiO_(2),suggest that the transitional shales of the Shan 2^(3)submember were primarily deposited under tectonic settings such as continental island arcs(CIAs)and passive continental margins(PCMs).

Evaluation of source rocks and prediction of oil and gas resources distribution in Baiyun Sag,Pearl River Mouth Basin,China

By conducting organic geochemical analysis of the samples taken from the drilled wells in Baiyun Sag of Pearl River Mouth Basin,China,the development characteristics of hydrocarbon source rocks in the sag are clarified.Reconstruct the current geothermal field of the sag and restore the tectonic-thermal evolution process to predict the type,scale,and distribution of resources in Baiyun Sag through thermal pressure simulation experiments and numerical simulation.The Baiyun Sag is characterized by the development of Paleogene shallow lacustrine source rocks,which are deposited in a slightly oxidizing environment.The source rocks are mainly composed of terrestrial higher plants,with algae making a certain contribution,and are oil and gas source rocks.Current geothermal field of the sag was reconstructed,in which the range of geothermal gradients is(3.5–5.2)℃/100 m,showing an overall increasing trend from northwest to southeast,with significant differences in geothermal gradients across different sub-sags.Baiyun Sag has undergone two distinct periods of extensional process,the Eocene and Miocene,since the Cenozoic era.These two periods of heating and warming events have been identified,accelerating the maturation and evolution of source rocks.The main body of ancient basal heat flow value reached its highest at 13.82 Ma.The basin modelling results show that the maturity of source rocks is significantly higher in Baiyun main sub-sag than that in other sub-sags.The Eocene Wenchang Formation is currently in the stage of high maturity to over maturity,while the Eocene Enping Formation has reached the stage of maturity to high maturity.The rock thermal simulation experiment shows that the shallow lacustrine mudstone of the Wenchang Formation has a good potential of generating gas from kerogen cracking with high gas yield and long period of gas window.Shallow lacustrine mudstone of the Enping Formation has a good ability to generate light oil,and has ability to generate kerogen cracking gas in the late stage.The gas yield of shallow lacustrine mudstone of the Enping Formation is less than that of shallow lacustrine mudstone of the Wenchang Formation and the delta coal-bearing mudstone of the Enping Formation.The numerical simulation results indicate that the source rocks of Baiyun main sub-sag generate hydrocarbons earlier and have significantly higher hydrocarbon generation intensity than other sub-sags,with an average of about 1200×10^(4)t/km^(2).Oil and gas resources were mainly distributed in Baiyun main sub-sag and the main source rocks are distributed in the 3^(rd)and 4^(th)members of Wenchang Formation.Four favorable zones are selected for the division and evaluation of migration and aggregation units:No.(1)Panyu 30 nose-shaped structural belt,No.(3)Liuhua 29 nose-shaped uplift belt and Liwan 3 nose-shaped uplift belt,No.(2)gentle slope belt of Baiyun east sag,and No.(8)Baiyun 1 low-uplift.