Organic matter transformation ratio, hydrocarbon expulsion efficiency and shale oil enrichment type in Chang 7_(3) shale of Upper Triassic Yanchang Formation in Ordos Basin, NW China

The major enrichment type of shale oil in the Chang 7_(3) shale of Upper Triassic Yanchang Formation in the Ordos Basin is unknown.This paper analyzes the organic matter transformation ratio,hydrocarbon expulsion efficiency and roof/floor sealing conditions of the Chang 7_(3) shale,and evaluates the major enrichment type of shale oil in this interval.The average organic matter transformation ratio of the Chang 7_(3) shale is about 45%;in other words,more than 50%of the organic matters have not transformed to hydrocarbons,and the lower the maturity,the greater the proportion of untransformed organic matters.The cumulative hydrocarbon expulsion efficiency of the transformed hydrocarbon is 27.5% on average,and the total proportion of untransformed organic matters plus retained hydrocarbons is greater than 70%.The relative hydrocarbon expulsion efficiency of the Chang 7_(3) shale is 60%on average,that is,about 40% of hydrocarbons retain in the shale.The Chang 7_(3) shale corresponds to Chang 7_(1+2) and Chang 8 sandstones as the roof and floor,respectively,and is further overlaid by Chang 6 shale,where extensive low porosity and low permeability–tight oil reservoirs have formed in the parts with relatively good porosity and permeability.Moreover,the Chang 7_(3) shale is tested to be in a negative pressure system(the pressure coefficient of 0.80–0.85).Therefore,the roof/floor sealing conditions of the Chang 7_(3) shale are poor.The retained hydrocarbons appear mostly in absorbed status,with low mobility.It is concluded that the medium–high mature shale oil is not the major enrichment type of shale oil in the Chang 7_(3) shale,but there may be enrichment opportunity for shale oil with good mobility in the areas where the sealing conditions are good without faults and fractures and oil reservoirs are formed off Chang 7_(1+2),Chang 6 and Chang 8.Furthermore,low–medium mature shale oil is believed to have great potential and is the major enrichment type of shale oil in the Chang 7_(3) shale.It is recommended to prepare relevant in-situ conversion technologies by pilot test and figure out the resource availability and distribution.

Types and microstructures of pores in shales of the Ordovician Wulalike Formation at the western margin of the Ordos Basin, China

Shale samples from the Ordovician Wulalike Formation at the western margin of the Ordos Basin are studied to define the types, microstructures and connectivity of pores as well as the relationships between the pore structures and gas content of the samples by using experimental techniques such as high-resolution field emission scanning electron microscopy (FESEM), mercury injection capillary pressure (MICP), low-temperature nitrogen adsorption (LTNA), CO_(2) adsorption, and focused ion beam scanning electron microscopy (FIB-SEM). The results show that the shale has 10 different lithofacies, typical mixed sedimentary characteristics, and poorly developed pores. The reservoir space mainly consists of intercrystalline pores, dissolution pores, intergranular pores, and micro-fissures, with organic pores occasionally visible. The pore size is mostly within 0.4–250 nm range but dominated by micropores and mesopores less than 20 nm, with pore numbers peaking at pore sizes of 0.5 nm, 0.6 nm, 0.82 nm, 3 nm, and 10 nm, respectively. The pores are poorly connected and macropores are rarely seen, which may explain the low porosity and low permeability of the samples. Samples with high content of organic matter and felsic minerals are potential reservoirs for oil and gas with their favorable physical properties and high connectivity. The pores less than 5 nm contribute significantly to the specific surface area and serve as important storage space for adsorbed gas.

Fluid property identification of the Lower Cretaceous reservoirs with complex oil-water contacts in Deseo Basin,Chad

Recently,exploration breakthroughs have been made in the Lower Cretaceous sandstone reservoirs in the Doseo Basin,but the identification of reservoir fluid property is difficult due to variable reservoir lithology,complex oil-water contact within and faint responses of the oil zone,which causes the lower accuracy of reservoir fluid property identification with conventional mudlogging and wirelogging techniques.Applying the geochemical logging,fluorescent logging,mud logging and cutting logging technology,in combination with formation test data,this paper distinguishes the crude oil types,analyzes the logging response characteristics of oil zone after water washing,and establishes the interpretation charts and parameter standards for reservoir fluid properties.The crude oil can be divided into two types,namely viscous-heavy and thin-light,based on total hydrocarbon content and component concentration tested by mud logging,features of pyrolysis gas chromatogram and fluorescence spectroscopy.The general characteristics of oil layers experienced water washing include the decrease of total hydrocarbon content and component concentration from mud logging,the decrease of S1 and PS values from geochemical logging,the decrease of hydrocarbon abundance and absence of some light components in pyrolysis gas chromatogram,and the decrease of fluorescence area and intensity from fluorescence logging.According to crude oil types,the cross plots of S1 versus peak-baseline ratio,and the cross plots of rock wettability versus fluorescence area ratio are drawn and used to interpret reservoir fluid property.Meanwhile,the standards of reservoir fluid parameter are established combining with the parameters of PS and the parameters in above charts,and comprehensive multiparameter correlation in both vertical and horizontal ways is also performed to interpret reservoir fluid property.The application in the Doseo Basin achieved great success,improving interpretation ability of fluid property in the reservoir with complex oil-water contact,and also provided technical reference for the efficient exploration and development of similar reservoirs.

Types and Characteristics of the Lower Silurian Shale Gas Reservoirs in and Around the Sichuan Basin

This study analyzed the characteristics and types of the Lower Silurian shale gas reservoirs in and around Sichuan Basin through field observations, slices, Ar-ion-beam milling, scanning electron microscopy, and x-ray diffraction analysis of 25 black shale outcrops and samples. Two main types of shale gas reservoirs were determined, i.e., fractures and pores. Fractures were classified into five categories, i.e., giant, large, medium, small, and micro, according to the features of the shale gas reservoirs, effect of fracture on gas accumulation, and fracture nature. Pore types include organic matter pores, mineral pores（mineral surface, intraparticle, interparticle, and corrosional pore）, and nanofractures. The various fracture types, fracture scales, pore types, and pore sizes exert different controls over the gas storage and production capacity. Pores serve as a reservoir for gas storage and, the gas storage capacity can be determined using pores; fractures serve as pathways for gas migration, and gas production capacity can be determined using them.

Basic Types and Structural Characteristics of Uplifts:An Overview of Sedimentary Basins in China

The uplift is a positive structural unit of the crust. It is an important window for continental dynamics owing to its abundant structural phenomena, such as fault, fold, unconformity and denudation of strata. Meanwhile, it is the very place to store important minerals like oil, natural gas, coal and uranium. Giant and large-scale oil and gas fields in China, such as the Daqing Oilfield, Lunnan-Tahe Oilfield, Penglai 19-3 Oilfield, Puguang Gas Field and Jingbian Gas Field, are developed mainly on uplifts. Therefore, it is the main target both for oil and gas exploration and for geological study. The uplift can be either a basement uplift, or one developed only in the sedimentary cover. Extension, compression and wrench or their combined forces may give rise to uplifts. The development process of uplifting, such as formation, development, dwindling and destruction, can be taken as the uplifting cycle. The uplifts on the giant Precambrian cratons are large in scale with less extensive structural deformation. The uplifts on the medium- and small-sized cratons or neo-cratons are formed in various shapes with strong structural deformation and complicated geological structure. Owing to changes in the geodynamic environment, uplift experiences a multi-stage or multi-cycle development process. Its geological structure is characterized in superposition of multi-structural layers. Based on the basement properties, mechanical stratigraphy and development sequence, uplifts can be divided into three basic types-the succession, superposition and destruction ones. The succession type is subdivided into the maintaining type and the lasting type. The superposition type can be subdivided into the composite anticlinal type, the buried-hill draped type, the faulted uplift type and the migration type according to the different scales and superimposed styles of uplifts in different cycles. The destruction type is subdivided into the tilting type and the negative inverted type. The development history of uplifts and their controlling effects on sedimentation and fluids are quite different from one another, although the uplifts with different structural types store important minerals. Uplifts and their slopes are the main areas for oil and gas accumulation. They usually become the composite oil and gas accumulation zones （belts） with multiple productive formations and various types of oil and gas reservoirs.

Origin and Superposition Metallogenic Model of the Sandstone-type Uranium Deposit in the Northeastern Ordos Basin,China

This paper deals with the metallogenic model of the sandstone type uranium deposit in the northeastern Ordos Basin from aspects of uranium source, migration and deposition. A superposition metallogenic model has been established due to complex uranium mineralization processes with superposition of oil-gas reduction and thermal reformation.

Sedimentary Response of Different Fan Types to the Paleogene-Neogene Basin Transformation in the Kuqa Depression,Tarim Basin,Xinjiang Province

A group of alluvial fans formed in the early Paleogene represent marginal sedimentary facies at the foot of the South Tianshan Mountain, Kuqa Depression, Tarim Basin, Xinjiang province. Two types of fans occurred in the middle-late Paleogene Kumugeliemu and Suweiyi formations： one alluvial, and the other fan delta deposited in a lacustrine setting. Within the early Neogene Jidike Formation, coastal subaqueous fans developed, probably in a deeper water lacustrine setting. The three types of fans are stacked vertically in outcrop with the sequence in ascending order： bottom alluvial, middle fan-delta, and top suhaqueous. The subaqueous is a typical coarse-fan deposit occurring in the glutinite member of the Jidike Formation in some wells. Laterally, from the foreland to the lacustrine settings, the distribution pattern of sedimentary facies represents the same three fan types sequentially. The spatial distribution of these fans was controlled by the Paleogene-Neogene Basin transformation, and evolution with different types of fans developed in the Kuqa Depression in response. In the Paleogene, the Kuqa Depression was a rift basin where an alluvial fan was deposited in the foreland setting, which, by early Neogene, became a foreland basin when the lake level changed. With any rise in lake level, fan-deltas migrated from lacustrine to foreland settings, whereas when the lake level fell, fan migration was reversed. In the early Neogene, with increasing slope and rising lake level, fans progressed and covered the previous fan-delta and lacustrine mudstone. Eventually, subaqueous fans developed, forming the present spatial configuration of these three fan types.

Geochemical Characteristics and Genetic Types of Natural Gas in the Xinchang Gas Field, Sichuan Basin, SW China

The molecular compositions and stable carbon and hydrogen isotopic compositions of natural gas from the Xinchang gas field in the Sichuan Basin were investigated to determine the genetic types. The natural gas is mainly composed of methane （88.99%-98.01%）, and the dryness coefficient varies between 0.908 and 0.997. The gas generally displays positive alkane carbon and hydrogen isotopic series. The geochemical characteristics and gas-source correlation indicate that the gases stored in the 5th member of the Upper Triassic Xujiahe Formation are coal-type gases which are derived from source rocks in the stratum itself. The gases reservoired in the 4th member of the Xujiahe Formation and Jurassic strata in the Xinchang gas field are also coal-type gases that are derived from source rocks in the 3rd and 4th members of the Xujiahe Formation. The gases reservoired in the 2nd member of the Upper Triassic Xujiahe Formation are mainly coal-type gases with small amounts of oil-type gas that is derived from source rocks in the stratum itself. This is accompanied by a small amount of contribution brought by source rocks in the Upper Triassic Ma＇antang and Xiaotangzi formations. The gases reservoired in the 4th member of the Middle Triassic Leikoupo Formation are oil-type gases and are believed to be derived from the secondary cracking of oil which is most likely to be generated from the Upper Permian source rocks.

Pore-Throat Combination Types and Gas-Water Relative Permeability Responses of Tight Gas Sandstone Reservoirs in the Zizhou Area of East Ordos Basin, China

With the aim of better understanding the tight gas reservoirs in the Zizhou area of east Ordos Basin,a total of 222 samples were collected from 50 wells for a series of experiments.In this study,three pore-throat combination types in sandstones were revealed and confirmed to play a controlling role in the distribution of throat size and the characteristics of gas-water relative permeability.The type-I sandstones are dominated by intercrystalline micropores connected by cluster throats,of which the distribution curves of throat size are narrow and have a strong single peak(peak ratio>30%).The pores in the type-II sandstones dominantly consist of secondary dissolution pores and intercrystalline micropores,and throats mainly occur as slice-shaped throats along cleavages between rigid grain margins and cluster throats in clay cement.The distribution curves of throat size for the type-II sandstones show a bimodal distribution with a substantial low-value region between the peaks(peak ratio<15%).Primary intergranular pores and secondary intergranular pores are mainly found in type-III samples,which are connected by various throats.The throat size distribution curves of type-III sandstones show a nearly normal distribution with low kurtosis(peak ratio<10%),and the micro-scale throat radii(>0.5μm)constitute a large proportion.From type-I to type-III sandstones,the irreducible water saturation(Swo)decreased;furthermore,the slope of the curves of Krw/Krg in two-phase saturation zone decreased and the two-phase saturation zone increased,indicating that the gas relative flow ability increased.Variations of the permeability exist in sandstones with different porethroat combination types,which indicate the type-III sandstones are better reservoirs,followed by type-II sandstones and type-I sandstones.As an important factor affecting the reservoir quality,the pore-throat combination type in sandstones is the cumulative expression of lithology and diagenetic modifications with strong heterogeneity.

Genesis Types and Diagenesis Compaction Mechanisms of Sandstone Rreservoirs in Dynamic Environments in Oil/Gas Basins in China

The diversity of sandstone diagenesis mechanisms caused by the complex geological conditions of oil/gas basins in China could hardly be reasonably explained by the traditional concept of burial diagenesis. Three genesis types of thermal diagenesis, tectonic diagenesis and fluid diagenesis are presented on the basis of the dynamic environment of the oil/gas basins and.the controlling factors and mechanisms of sandstone diagenesis. Thermal diagenesis of sandstone reservoirs is related not only to the effect of formation temperature on diagenesis, but also to the significant changes in diagenesis caused by geothermal gradients. The concept of thermal compaction is presented. Thermal compaction becomes weaker with increasing depth and becomes stronger at a higher geothermal gradient. At the same formation temperature, the sandstone porosity in the region with a lower geothermal gradient is e^0.077＋0.0042T times higher than that in the region with a higher geothermal gradient. Both sudden and gradual changes are observed in diagenetic evolution caused by structural deformation. Average sandstone compaction increased by 0.1051% for every 1.0MPa increase of lateral tectonic compressional stress, while late tectonic napping helped to preserve a higher porosity of underlying sandstone reservoir. Fluid diagenesis is a general phenomenon. The compaction caused by fluid properties is significant. The coarser the grain size, the stronger the fluid effect on compaction. The greater the burial depth, the weaker the fluid effect on compaction for the specific reservoir lithology and the greater the difference in the fluid effects on compaction between different grain sizes.

Impact of rock type on the pore structures and physical properties within a tight sandstone reservoir in the Ordos Basin, NW China

The pore-throat systems and physical properties of tight sandstone reservoirs are complex,and deposition is thought to be a fundamental control for them.In this study,the impacts of the full ranges of rock types(from pebbly coarse sandstone to fine sandstone) on the pore structures and physical properties of the Permian tight sandstone reservoir in the eastern Ordos Basin were investigated comprehensively through a series of experiments including conventional physical testing,thin-section analysis,scanning electron microscopy,nuclear magnetic resonance analysis and high-pressure mercury injection tests.The results showed that the coarser-grained sandstones tend to have higher feldspar content and lower percentage of cements,leading to strong dissolution,weak cementation and improved porosity and permeability.The medium sandstone has the highest level of quartz and the lowest average content of feldspar,resulting in strong heterogeneity of physical properties.Only those medium sandstone reservoirs with relatively high content of feldspars have better physical properties.Additionally,the coarser-grained sandstones contain relatively large dissolution pores(nearly 200 μm),whereas the finer-grained sandstones have more intercrystalline pores with a relatively more homogeneous pore structure.The pebbly coarse sandstone and coarse sandstone reservoirs are favorable targets with best physical properties.

Study on Water Conservation Capacity of Litter from Different Types of Forest in Caohai Basin

[Objective]The research aimed to understand water conservation capacity of litter from different forest types in Caohai basin. [Method] Current storage amount,water holding capacity and precipitation interception ability of litter from 4 types of forest were investigated and studied. [Result]The order of exist litter amount was grass slope ＆gt;shrub forest ＆gt;mixed broad leaf-conifer forest ＆gt;coniferous forest. The order of natural water holding capacity was mixed broad leafconifer forest ＆gt; coniferous forest ＆gt; grass slope ＆gt; shrub forest. The relevance between water holding capacity of litter and soaking time was in line with this formula： y = kln（ x） ＋ b. During 0-120 min of litter soaking,water holding capacity increased rapidly. After 120 min,the increasing trend remained flat until closing to the maximum water holding capacity. The relevance between water absorption rate of litter and soaking time was in line with this formula： y = a ＋ bx-1. In the beginning of soaking,water absorption rates of litter from different woodlands showed great difference. As soaking time went by,water absorption rates of litter from different woodlands all declined sharply. During 0-120 min,declining velocity of water absorption rate was quicker. After 120 min,the declining trend tended to be slow. The maximum water holding capacity of litter presented the order of shrub forest ＆gt; mixed broad leafconifer forest ＆gt; grass slope ＆gt; coniferous forest. The maximum precipitation interception amount of litter presented the order of shrub forest ＆gt; grass slope ＆gt; mixed broad leafconifer forest ＆gt; coniferous forest. Effective impounding capacity of litter presented the order of shrub forest ＆gt; mixed broad leafconifer forest ＆gt; grass slope ＆gt; coniferous forest. [Conclusion]The research could provide support for the construction of water conservation forest in Caohai basin.

Natural Gas Types,Distribution Controlling Factors,and Future Exploration in the Western Qaidam Basin

The Paleogene and Neogene oil and gas in the western Qaidam basin have a regular distribution in three concentric zones from the edge to the center of the basin. Natural gas mainly occurs in the inner zone, and the gas-oil ratio of the northern area of the basin is significantly higher than that of the southern area. Large amounts of carbon isotope data of natural gas, plotted in X- shaped and comprehensive identification diagrams for the southern area and northern area, respectively, were used to identify the types of natural gas. The large-scale distribution of natural gas is highly consistent with the Ro values of major source rocks, but is poorly correlated with the type of organic matter. This indicates that the main controlling factor of natural gas distribution is organic matter maturity, and the kerogen types act as the basis for the formation of different types of natural gas. Paleouplifts and squeezed anticlines near hydrocarbon generation depression centers, which are major natural gas-rich regions, control the migration directions of natural gas, while hydrocarbon migration pathways and fault systems connecting gas sources are the most important factors for natural gas reservoir formation in the inner basin. Therefore, favorable zones for natural gas distribution can be predicted on the basis of the distribution of thermal evolution and the gas generation intensity of major source rocks as well as the structural map. The Shizigou-Youshashan- Yingdong-Dawusi, Youquanzi -Kaitemilike - Youdunzi, and Xiaoliangshan - Nanyishan - Dafengshan structural belts are favorable zones for natural gas accumulation. This study has important theoretical and practical significance for future natural gas exploration.

Soil Erosion under Different Land Use Types and Zones of Jinsha River Basin in Yunnan Province,China

Severe soil erosion in the middle and upper reaches of Yangtze River has been regarded as a major environmental problem. The on-site impact of soil erosion on agricultural production and the off-site impact on floods and sedimentation in Yangtze Rive are well known. A quantitative assessment of soil erosion intensity is still scanty for developing appropriate soil erosion control measures for different land use types and zones in this region. This article constructs a localized USLE and estimates the average soil loss in the Jinsha River Region in Yunnan Province, one of the priority areas for soil erosion control in the middle and upper reaches of Yangtze River. The estimation is done under different land uses and zones in this basin. The estimation shows that while soil erosion in the cultivated land is the most severe, 36～40% of the garden and forest land suffers from soil erosion of various degrees due to lack of ground cover and other factors. Soil erosion in the pasture is modest when the ground cover is well maintained. It also confirmed that terracing can reduce soil erosion intensity significantly on the cultivated land. Research findings suggest that sufficient attention must be paid to regeneration of the ground cover in reforestation programs. In addition to mass reforestation efforts, restoration of grassland and terracing of the cultivated land should also play an important role in erosion control.

The World＇s Largest Conglomerate Type Oilfield Has Been Discovered in the Junggar Basin of China

The Junggar Basin in the northern part of Xinjiang is the second largest inland basin in China. It is located between the Altai and Tianshan Mountains, which is bounded by the Junggar bounded Mountain in the northwest, the Altai Mountains in the northeast and the North Tianshan Mountains in the south. It belongs to a triangular close inland basin, and extends 700 km in EW and 370 km in NS, covering an area of 38x104 km2. The elevation is about 400 m, high in the east （about 1000 m） and low in the west. The central basin is the Guerbantonggute desert, which accounts for 36.9% of the total basin area.

Genetic type and source of natural gas in the southern margin of Junggar Basin, NW China

Natural gas has been discovered in many anticlines in the southern margin of the Junggar Basin. However, the geochemical characteristics of natural gas in different anticlines haven’t been compared systematically, particularly, the type and source of natural gas discovered recently in Well Gaotan-1 at the Gaoquan anticline remain unclear. The gas composition characteristics and carbon and hydrogen isotope compositions in different anticlines were compared and sorted systematically to identify genetic types and source of the natural gas. The results show that most of the gas samples are wet gas, and a few are dry gas;the gas samples from the western and middle parts have relatively heavier carbon isotope composition and lighter hydrogen isotope composition, while the gas samples from the eastern part of southern basin have lighter carbon and hydrogen isotope compositions. The natural gas in the southern margin is thermogenic gas generated by freshwater-brackish water sedimentary organic matter, which can be divided into three types, coal-derived gas, mixed gas and oil-associated gas, in which coal-derived gas and mixed gas take dominance. The Jurassic coal measures is the main natural gas source rock in the southern margin, and the Permian lacustrine and the Upper Triassic lacustrine-limnetic facies source rocks are also important natural gas source rocks. The natural gas in the western part of the southern margin is derived from the Jurassic coal measures and the Permian lacustrine source rock, while the natural gas in the middle part of the southern margin is mainly derived from the Jurassic coal measures, partly from the Permian and/or the Upper Triassic source rocks, and the natural gas in the eastern part of the southern margin is originated from the Permian lacustrine source rock. The natural gas in the Qingshuihe oil and gas reservoir of Well Gaotan-1 is a mixture of coal-derived gas and oil-associated gas, of which the Jurassic and Permian source rocks contribute about half each.

Types of the Cambrian platform margin mound-shoal complexes and their relationship with paleogeomorphology in Gucheng area,Tarim Basin,NW China

Based on the observation of outcrops and cores,thin section identification,restoration of paleogeomorphology by residual thickness method,fine description of seismic facies,etc.,the coupling relationships between the development patterns of various types of Cambrian platform margin mound-shoal complexes and paleogeomorphology in the Gucheng area of Tarim Basin have been examined.The Cambrian platform margin mound-shoal complex is divided into mound base,mound core,mound front,mound back and mound flat microfacies,which are composed of dolomites of seven textures with facies indication.The different paleogeomorphology before the deposition of mound-shoal complex in each period was reconstructed,and three types of mound-shoal complex sedimentary models corresponding to the paleogeomorphologies of four stages were established:namely,the first stage of gentle slope symmetric accretion type,the second stage of steep slope asymmetric accretion type and the third and fourth stages of steep slope asymmetric progradation type.Their microfacies are respectively characterized by-mound base-mound back+(small)mound core+mound front-mound flat"symmetric vertical accretion structure,"mound base-(large)mound core+mound front-mound flat"asymmetric vertical accretion structure,"mound base-(small)mound core+mound front-mound flat"asymmetric lateral progradation structure.With most developed favorable reservoir facies belt,the steep slope asymmetric accretion type mound-shoal complex with the characteristics of"large mound and large shoal"is the exploration target for oil and gas reservoir.

New Insights into the Depositional Environments of Ordovician Carbonate Formations in the Yubei Area of Tarim Basin Based on Standard Microfacies Types

Objective The Yubei area is located in the mid-east Maigaiti slope of southwestern Tarim Basin, China, with an exploration history of several years. Recent exploration has preliminarily indicated that the Ordovician carbonate formations in this area have some oil and gas potential. Carbonate microfacies provides material basis for reservoir development, seal formation and hydrocarbon generation. Therefore, this work utilized the standard microfacies （SMF） types to study the microfacies of the Ordovician formations in the Yubei area in order to provide theoretical basis for the next exploration.

The Type and Evolution of Neoproterozoic Sedimentary Basin in the Dahongshan Region,Northern Margin of the Yangtze Block:An Insight from Sedimentary Characteristics of the Huashan Group

Objective The Huashan group（composed of the lower Hongshansi Formation and the upper Liufangzui Formation）is an important Neoproterozoic stratigraphic unit along the northern margin of the Yangtze Block.Previous documents have focused on the geochronological and geochemical aspects of the Neoproterozoic sedimentary basin in the Dahongshan region.However.

The Relationship between Jurassic Coal Measures and Sandstone-type Uranium Deposits in the Northeastern Ordos Basin,China

Outcrop and drill hole data show that the Jurassic coal measures in the northeastern Ordos Basin are composed mainly of the Yan’an Formation and the lowstand system tract of the Zhiluo Formation, and there is a regional unconformity between them. The Dongsheng uranium deposit is associated with the Jurassic coal measures. Research data indicate that the Jurassic coal measures in the study area have a certain hydrocarbon-generating capacity, although the metamorphic grade is low （Ro=0.40%–0.58%）. In the Dongsheng region alone, the accumulative amount of generated coalbed methane （CBM） is about 2028.29 × 108 –2218.72 × 108 m3; the residual amount is about 50.92 × 108 m3, and the lost amount is about 1977 × 108 m3. Analysis of the burial history of the host rocks and the evolutionary history of the Dongsheng uranium deposit suggests that the Jurassic coal measures generated hydrocarbon mainly from Middle Jurassic to Early Crataceous, which is the main mineralization phase of the Dongsheng uranium deposit. By the Late Cretaceous, a mass of CBM dissipated due to the strong tectonic uplift, and the Dongsheng uranium deposit stepped into the preservation phase. Therefore, the low-mature hydrocarbon-containing fluid in the Jurassic coal measures not only served as a reducing agent for the formation of sandstone-type uranium deposits, but also rendered the second reduction of paleo-interlayer oxidation zone and become the primary reducing agent for ore conservation. Regional strata correlation reveals that the sandstone-type uranium reservoir at the bottom of the Zhiluo Formation is in contact with the underlying industrial coal seams in the Yan’an Formation through incision or in the form of an unconformity surface. In the Dongsheng region with poorly developed fault systems, the unconformity surface and scour surface served as the main migration pathways for low-mature hydrocarbon-containing fluid migrating to the uranium reservoir.