Southern Ordos Basin Research Articles

A millimeter-scale insight into formation mechanism of lacustrine black shale in tephra deposition background

To reveal the role of tephra in the deposition of black shale during periods of volcanic activity, we performed lithostratigraphic and geochemical analyses on 14 horizontally sliced samples drilled from a 2-cm-thick black shale interval in the lower Ch7 Member of the Upper Triassic Yanchang Formation, southern Ordos Basin. Results indicate that fewer plankton is preserved during tephra deposition than during periods of volcanic quiescence. With the decrease of volcanic activities and tephra deposition, the abundance of redox-sensitive trace elements (RSTEs) and biolimiting elements increases, while terrigenous elements gradually decrease, resulting in the improvement of organic matter (OM) preservation. Paleoenvironmental proxies suggest that the climate during the Late Triassic was generally warm and humid. However, subsequent intense volcanic eruptions may have caused climatic cooling that affected the water column, resulting in enhanced salinity, primary production, water stratification, and bottom water anoxia, leading to enhanced organic carbon production and preservation. Primary productivity and redox conditions controlled the accumulation of organic carbon. Although physical and chemical reactions relating to the deposition of tephra into water are short-lived, climate change induced by volcanic eruptions is the critical cause of black shale formation.

Ocean–continent transition of the northeastern Paleotethys during the Triassic: Constraints from Triassic sedimentary successions across the Qinling Orogen, central China

To decode the northeastern Paleotethys ocean–continent transition process, a geological profile is constructed across the southern Ordos Basin, central Qinling Orogen, and northern Sichuan Basin. Along the transect, abundant detrital zircon age data, sandstone petrological compositions, and paleocurrent directions taken from the Triassic sedimentary successions across the Qinling Orogen reveal systematic tectonic variations in ocean–continent transition processes. The Early Triassic sedimentary succession was deposited in a foreland basin which was part of the northeastern Paleotethys Ocean and received recycled North Qinling Orogen detritus in a collisional tectonic setting. This evidence implies that the North Qinling region was undergoing intense erosion during the subduction of the Paleotethys oceanic crust. In the Middle Triassic, continued subduction brought the foreland basin in the Huicheng region to an end, and the depocenter migrated westward to the West Qinling and Songpan-Ganzi regions under a convergent collisional tectonic setting. In the Late Triassic, the West Qinling and Songpan-Ganzi regions were still receiving sediment, while the North and South China Blocks entered the continental collision process. The detrital material in the southern Ordos Upper Triassic Yanchang Formation continued to be eroded from the North China Block in a collisional tectonic setting. The sediments in the northern Sichuan Basin were mainly sourced from the South China Block and South Qinling area in a collisional setting, and the Songpan-Ganzi region continued to converge and eventually evolved into a collisional tectonic setting. The Paleotethys ocean–continent transition represents a complete second half of the Wilson cycle, and mountain building preconditioned the paleotopography and influenced the basin formation.

Braided deltas and diagenetic control on tight sandstone reservoirs: A case study on the Permian Lower Shihezi Formation in the southern Ordos Basin (central China)

The Lower Shihezi Formation, situated in the south of the Chinese Ordos Basin, is a tight sandstone reservoir that is generally influenced by its sedimentary facies and diagenesis, corresponding to distinct petrophysical reservoir properties for each lithofacies type. The existing lithofacies types within the Lower Shihezi Formation were determined through observations in field profiles and drill cores that were combined with the analysis of logging data. Diagenetic processes and their effects were analyzed by using different methods, such as conventional thin section, cast thin section, and scanning electron microscope (SEM) analyses. The results of this contribution indicate that the Lower Shihezi Formation in the southern Ordos Basin is dominated by braided delta facies and locally developed lacustrine facies. During diagenesis of this studied formation, compaction and cementation led to a reduction in porosity with compaction probably forming the main factor. Existing cementation is mainly manifested in the form of clay mineral cements, siliceous and carbonate cements, and induced a somewhat lower pore reduction effect on the reservoir compared to compaction. Minor dissolution led to the break-down of rock fragments, feldspars, and other soluble minerals, and thereby may have caused a subsequent formation of secondary pores, thus effectively improving the reservoir property. Consequently, due to the compositional differences and corresponding unique diagenetic effects, the distributary channels and underwater distributary channels display the highest reservoir properties. These are followed by the mouth bars and sand sheets. The lowest reservoir properties are shown by the natural levee microfacies.

Micropore Structural Characteristics and Influence Factors of Triassic Shale Gas Adsorption in the Ordos Basin

Continental shale is investigated for potential shale gas reservoirs in the Tongchuan area, southern Ordos Basin. The kerogen macerals of Upper Triassic Chang 7 Shale are vitrinite 28–43%, exinite 4–7%, and inertinite 52–69%, and Ro is 0.85–0.88%, and the kerogen type is II. The total organic carbon (TOC) content is 9.75–29.92%, free hydrocarbon (S1) is 3.21–12.14%, pyrolysis hydrocarbon (S2) is 41.86–165.39%, and maximum pyrolysis temperature (Tmax) is 439–445°C. The mineral composition is mainly dominated by quartz 3.94–43.27%, followed by feldspar 9.65–30.07% and clay minerals 13.46–59%. Illonite/smectite (I/S) is the main clay mineral, accounting for 47–69%. The liquid nitrogen adsorption curve showed that the pore types were tubular and plate-like. The pores are mainly microporous (<2 nm) and mesoporous (2–50 nm) in size. Isothermal adsorption results show that Langmuir volume (VL) is 2.06–5.57 cm3 g−1, Langmuir pressure (PL) is 2.16–3.28 MPa, and shale adsorption capacity is small and easy to desorb. A quartz content less than 25% is negatively correlated with the specific surface area (SSA); otherwise, it is positively correlated. A clay mineral content less than 45% is negatively correlated with SSA; otherwise, it is positively correlated. The relationship between the carbonate content and pore structure is not obvious. Pyrite content less than 2% is positively correlated with SSA; otherwise, it is negatively correlated. The SSA is positively correlated with TOC, inertinite, and exinite and negatively correlated with vitrinite. The adsorbed gas content is positively correlated with the TOC content, SSA, inertinite content, and exinite content and negatively correlated with vitrinite.

Relations of Uranium Enrichment and Metal Sulfides within the Shuanglong Uranium Deposit, Southern Ordos Basin

Relations of Uranium Enrichment and Metal Sulfides within the Shuanglong Uranium Deposit, Southern Ordos Basin

Petrographic and geochemical characteristics of deep-lacustrine organic-rich mudstone and shale of the Upper Triassic Chang 7 member in the southern Ordos Basin, northern China: Implications for shale oil exploration

Organic-rich lacustrine mudstones and shales in Chang 7 member of the Upper Triassic Yanchang Formation are potential shale oil reservoirs. Petrographic and geochemical data were used to study their lithologic and geochemical characteristics, mineral composition and sedimentary environments. Three different types of deep lacustrine deposits are identified, including unstratified massive mudstone (muddy debrite), normally graded mudstone (muddy turbidite) and laminated shale. Mudstones and shales contain relatively abundant brittle minerals (quartz, feldspar, pyrite and carbonates), which are favorable for the hydraulic fracturing. Terrigenous input and diagenesis conversion process are the main sources of felsic minerals. Laminated shales yield the highest TOC values (average of 7.47%) as well as S1 and S2 values. Most of the mudstones and shales contain Type II organic matter, while contain little Type I organic matter. The Ro and Tmax values combined with OEP versus CPI and C29ββ/(ββ + αα)sterane versus αααC29sterane20S/(20S + 20R) discrimination diagrams and Ts/(Ts + Tm) ratios, indicate that the mudstones and shales are mature. Ph/C18 versus Pr/C17 and Pr/Ph versus gammacerane/C30-hopane discrimination diagrams together with U/Th, V/(V + Ni) and Ce/La ratios show that mudstones and shales reflect a reducing depositional environment. N-alkanes of ΣnC21-/ΣnC22 + values and C27-C29 regular sterane distributions suggest that plankton and terrestrial plants are the parent materials of organic matter. In addition, small amounts of extended homohopanes (>C31) and gammacerane combined with Sr/Ba values support a fresh water depositional environment. These conditions have resulted in high quality and widespread distribution of organic mudstones and shale, which are favorable for shale oil exploration.

Characteristics and formation mechanism of the tight tuff reservoirs of the Upper Triassic Chang 7 member in the southern Ordos Basin, China

Organic-rich shales are commonly associated with tuff layers. Previous studies have mainly focused on the influence of water-laid tuffs on the primary productivity and enrichment of organic matter. Tuffs can also act as tight or low permeability reservoirs or as sweet spots in shales. However, the characteristics and formation mechanism of tight tuff reservoirs are poorly explored. The tuffs in the study area are divided into five lithologies: vitric tuff, crystal-vitric tuff, crystal tuff, tuffite, and tuffaceous sandstone. The pores in these tuffs have various types and sizes, and mainly include (1) intergranular pores, (2) intragranular pores, (3) dissolution pores, (4) seams around grains, (5) organic matter related pores, and (6) fractures. The tuffs form tight reservoir rocks with low porosity and permeability, ranging from 2.33% to 12.62% (average 7.2%) and from 0.000518 mD to 0.281mD (average 0.0783 mD), respectively. The average porosity and permeability of the tuffs are better than those of shales, and the pores in the tuff reservoirs are often filled with oil residues, which indicate that the tuff reservoirs may be favorable for oil and gas accumulations.The tuff reservoir qualities have strong heterogeneity, which are significantly controlled by lithology, grain size, components, devitrification, dissolution, and fluid pathway systems. Tuffs with large grain sizes commonly have high primary porosity with relatively more cracks and seams within and around grains, which can be conducive to the entry of late diagenetic fluids, which cause tuff reservoirs to be more prone to devitrification and dissolution. The devitrification of volcanic glass can produce abundant microcrystalline quartz, feldspars and intergranular pores, and can greatly improve the reservoir quality. Devitrification has generally occurred in the vitric tuffs and crystal-vitric tuffs, and the porosities have a positive covariation with the quartz contents in the two lithologies. The devitrification pores can be further enlarged by dissolution. Devitrification and dissolution were controlled by the tuff compositions and fluid actions. The fluid pathway systems, including scouring surfaces, bedding, fractures, and well-developed primary pores, controlled the fluid movement. Tuffs that contain high volcanic glass contents and coarse particles with abundant beddings and fractures are prone to intense devitrification and dissolution, which results in high reservoir quality.

Mechanical Properties of Lamellar Shale Considering the Effect of Rock Structure and Hydration from Macroscopic and Microscopic Points of View

It is well known that the effect of shale hydration causes wellbore instability due to water phase invasion of drilling fluid to lamellar shale rich formations. This is because of that the mechanical properties (compressive strength, elastic modulus, etc.) of lamellar shale surrounding the borehole, which is rich in clay minerals, will decrease significantly after hydration. In this study, using the lamellar shale in the continental stratum of the southern Ordos Basin, the mechanical properties of lamellar shale were studied by compression tests considering the effect of lamellar structure and hydration from a macroscopic point of view. In addition, the mechanical mechanism was discussed combined with the CT scanning tests results from a microscopic point of view. The results demonstrate the following points. Lamellar shale has stronger anisotropy than bedding shale, the compressive strength (deviatoric stress) and elastic modulus of lamellar shale are both lower than those of bedding shale, and it is more prone to tension fracture. With the increase in the angle (β) between the lamina and the axial direction from 0° to 90°, the compressive strength of lamellar shale decreases when β < 30° and then increases, the elastic modulus of lamellar shale decreases greatly when β < 30° and then tends to flatten. With the increase in hydration time, the compressive strength and elastic modulus of lamellar shale both gradually decrease, and the rates of their decrements reduce. The mechanical properties of lamellar shale are more affected by hydration than those of bedding shale. The hydration of lamellar shale leads to the formation of new fractures and the expansion of existing fractures in the junction area between the laminae and rock matrix, resulting in easy tension fracture along the laminae of shale.

Characteristics and Formation Mechanism of the Tight Tuff Reservoirs of the Upper Triassic Chang 7 Member in the Southern Ordos Basin, China

Characteristics and Formation Mechanism of the Tight Tuff Reservoirs of the Upper Triassic Chang 7 Member in the Southern Ordos Basin, China

Mineralogy and elemental geochemistry of Permo-Carboniferous Li-enriched coal in the southern Ordos Basin, China: Implications for modes of occurrence, controlling factors and sources of Li in coal

As the decisive element promoting the battery industry and a raw material in aerospace Al-Li alloys, the demand for Li is increasing daily. At present, the exploitation and utilization of lithium resources mainly originates from brine (salt lake) deposits and pegmatite deposits. Emerging sediment hosts, including coal-hosted Li deposits, provide a potential source of lithium resources. Based on proximate analyses, including X-ray diffraction analysis (XRD), energy dispersive X-ray spectroscopy (SEM-EDX), X-ray fluorescence spectroscopy (XRF), and inductively coupled plasma mass spectrometry (ICP-MS), the petrological, mineralogical and geochemical characteristics were determined in Permo-Carboniferous Li-enriched coal and Jurassic coal in the southern margin of the Ordos Basin. The results show that Li concentrations in Permo-Carboniferous coal seams range from 16.2 μg/g to 521 μg/g. The maximum Li concentrations occur in parting samples of the Permo-Carboniferous coal seam and are as high as 1822 μg/g. The correlation coefficient of Li with other mineralogical and geochemical parameters shows that clay minerals are the main carriers of Li in coal seams. The discovery of high-temperature minerals (pyroclastic quartz and zircon) and vermicular kaolinite with good crystalline form provide direct evidence for the contribution of volcanic ash to coal. The enrichment of lithium in coal-measure strata is dominated by a Li-enriched source, sealing layer and enrichment place. Li originated from felsic volcanic ash, was captured by clay minerals, sealed in by the coal seam, and preferentially enriched in the coal seam parting. The coal seam itself can also cause some aluminosilicate minerals to be enriched in Li by ionic isomorphism.

Relations between pyrite morphologies and uranium mineralization in the Shuanglong region, northern China

Pyrite is widely distributed in ore-hosting sandstones of the Zhiluo Formation in the Shuanglong uranium deposit in the southern Ordos Basin, northern China. Sandstones from the uranium-bearing stratum was collected to analyze morphologies of pyrite and its relations with uranium mineralization by means of scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS). The results show that colloidal and framboidal pyrite, and pentagonal dodecahedron microcrystalline widely occur in sandstones, and uranium minerals are unevenly distributed around pyrite. According to the occurrences of pyrite, morphologies of pyrite are orderly evolved from microcrystalline, and the formation of pyrite is grouped into four periods. Moreover, the pyrite might preferentially or simultaneously precipitate over uranium minerals. The smaller the diameter of pyrite, the more favorable for uranium enrichment. Additionally, variation trends of U contents are opposed to those of the Fe and S contents as well as the Fe/S atomic ratios. Moreover, the decrease rate of S contents is greater than that of Fe contents. It indicates that S and Fe from pyrite both play roles in uranium mineralization, and the S contribution dominates over the Fe contribution. Additionally, the aforementioned conclusion is also supported by the existences of Fe (III), S, SO32-, and SO42. Compared with carbonaceous debris (CD), pyrite might play a dominant role in uranium enrichment. During the process of enrichment and reduction of uranium by pyrite, CD might provide beneficial reducing environment for precipitation and preservation of pyrite and uranium minerals. The study could provide guide for understanding metallogenic mechanism to some extent.

A method for judging depositional environment of coal reservoir based on coal facies parameters and rare earth element parameters

Coal facies refers to the genetic type and coal rock type of coal formed in a certain depositional environment. Rare earth elements (REE) are not easily disturbed by metamorphism, which is used to determine the provenance characteristics of coal-forming materials and the depositional environment in the coal seam. A total 76 coal samples from Carboniferous, Permian and Jurassic in the southern Ordos Basin were selected to determine the content and distribution characteristics of maceral compositions and REE using microscope observation and ICP-MS analysis. Combining the traditional REE indicator parameters (La/Lu, La/Sm, Gd/Lu, δEu, δCe, and REY) of the sedimentary rocks with the unique facies parameters in coal (Vitrinite content/Inertinite content (V/I), Gelation Index (GI), Texture Preservation Index (TPI), Groundwater Flow Index (GWI), Vegetation Index (VI) and Framework Matrix Ratio (F/M)), we determine the depositional environment, oxidation-reduction condition, coal-forming plants, hydrodynamic conditions and provenance characteristics of coal seams in different periods. The depositional environment is performed with the GI-TPI crossplot. The oxidation-reduction environment is explored by δEu, δCe and GI. The coal-forming environment is judged by a comprehensive combination of GWI-VI crossplot, ∑REY, La/Lu, La/Sm and Gd/Lu. The judgment of provenance characteristics is limited by the diagram of ∑REY and La/Yb and REE parameters. • The petrological features and REE characteristics of Permo-Carboniferous and Jurassic coal seams were explored. • The coal facies parameters suitable for the coal seams were performed. • A method for judging depositional environment of coal seams was established. • The depositional environment and coal-forming environment of coal seams in the southern Ordos Basin were discussed.

Traces of hydrocarbon-bearing fluid and microbial activities and their implications for uranium mineralization in southern Ordos basin, China

The sandstone-type uranium mineralization in the Shuanglong area of the Ordos basin, has been studied from the point view of organic geochemistry and mineralogy. Acidolysis hydrocarbon compositions within sandstones, and occurrences of uranium minerals were analyzed by using optical microscope, gas chromatograph, scanning electron microscopy coupled with energy dispersive spectroscopy, and electron probe. The results show the methane is dominant among acidolysis hydrocarbon components, and the natural gas is oil-type gas. According to the escape time of oil -gas, the main mineralization period, and spatial relations of oil–gas and ore bodies, it is inferred that the hydrocarbon-bearing fluid is mainly from the lacustrine source rocks of the Triassic Yanchang Formation (T3y), and the uranium-bearing stratum might undergo hydrocarbon reduction. Uranium minerals occur in forms of microspheres outlining at the cell lumens of organic matter, and uranyl phosphate minerals are also observed not alongside Fe-oxyhydroxides, indicating that bio-mineralization exits. The fact that carbonaceous debris (i.e., CD) partly underwent biodegradation is evidenced by the front-peak type n-alkanes compounds with the presence of unknown complex mixtures. Besides, numbers of framboidal pyrites with the diameter of 1 µm occur in CD, implying that microbial activities exist in uranium-bearing stratum. The maximum paleogeothermal temperature of the Jurassic Zhiluo Formation (J2z) shows that the geological environment is favorable for the activity of microorganisms. Under the combined actions of deep hydrocarbon-bearing fluid, microbial activities, CD, and pyrites, aqueous uranium is precipitated and enriched in forms of adsorption and reduction, which is helpful for further understanding the mechanism of sandstone-type uranium mineralization, thus providing certain guide for uranium exploration.

Differences of the Pore Structure and Methane Adsorption/Desorption between Vitrain and Durain of Low-Rank Coals: Case Study in the Huanglong Coalfield, Southern Ordos Basin, China

Coal reservoirs are highly heterogeneous, and different macrolithotypes of coal have differences in material composition and pore structure, resulting in different coalbed methane (CBM) adsorption/desorption performance and gas-water migration characteristics. In order to explore the pore development characteristics of different macrolithotypes in low-rank coal and their influence on adsorption and desorption, the low-rank coal of the Yan’an Formation in the Huanglong coalfield is taken as the sample of the research object. Vitrain and durain are collected and then separated. The results show that the porosity of coal is 2.92%–11.81%, and the total porosity is relatively developed. The porosity of durain is slightly higher than that of vitrain, the pore throat of durain is thicker, and the macropores are more developed. The Brunauer-Emmett-Teller (BET) specific surface area (SSA) and Barrett-Joyner-Halenda (BJH) total pore volume (TPV) of vitrain is slightly larger than durain, the micropores are more developed, and most of them are semiclosed and ink-bottle-shaped pores. The pore-fracture connectivity of the durain is better than vitrain via nuclear magnetic resonance (NMR). Vitrain has more developed micropores than durain, with larger specific surface area and stronger adsorption capacity. In the process of methane adsorption/desorption, the phenomenon of desorption hysteresis generally exists. Moreover, the pore connectivity of durain is relatively good, the degree of desorption hysteresis is weaker than vitrain, and the theoretical desorption efficiency is higher than vitrain. Temperature has a significant influence on adsorption and desorption of methane. The increase in temperature is beneficial to desorption and weakens the hysteresis effect of desorption. At the same temperature, the isosteric adsorption heat in the desorption process is greater than the adsorption process, which further explains the incomplete reversibility of the adsorption and desorption process. Under the same conditions, the isosteric adsorption heat of vitrain in the adsorption process is greater than durain, and adsorption capacity of vitrain is stronger than durain. Similarly, the isosteric adsorption heat of vitrain in the desorption process is greater than durain, and desorption needs to absorb more heat. The desorption capacity is weaker than durain.

Quantitative Characterization and Differences of the Pore Structure in Lacustrine Siliceous Shale and Argillaceous Shale: a Case Study of the Upper Triassic Yanchang Formation Shales in the Southern Ordos Basin, China

Quantitative Characterization and Differences of the Pore Structure in Lacustrine Siliceous Shale and Argillaceous Shale: a Case Study of the Upper Triassic Yanchang Formation Shales in the Southern Ordos Basin, China

Features of apparent resistivity anomaly of controlled-source audio-frequency magnetotellurics and prospects of oil shale in the Tongchuan area of the southern Ordos Basin, China

Four controlled-source audio-frequency magnetotelluric (CSAMT) profiles were conducted in early 2011 to investigate the distribution rules of oil shale in the Tongchuan area of the southern Ordos Basin. After terrain correction, we high-pass filtered the resulting images to prepare the 2D CSAMT apparent resistivity profiles for further analysis. Specifically, we have correlated the high-pass-filtered apparent resistivity anomalies to the distribution of oil shale seen in three available wells. The results reveal a close relationship between the oil shale distribution and the anomalously high-apparent-resistivity belt-like anomalies in the Tongchuan area. Our analysis indicates two prospective areas with different depths for oil shale within the study area.

Depositional environment of Middle Triassic organic‐rich shales in the Ordos Basin, Northwest China

The Middle Triassic Tongchuan Formation in the Ordos Basin of northwestern China is a typical lacustrine deposit. A major component of this formation is thick layers of organic‐rich shales that are probably a potential hydrocarbon source and preserve the earliest known Mesozoic‐type lacustrine ecosystem. The exact depositional environment of the shales in the Tongchuan Formation, however, remains unknown. To address this question, we carried out high‐resolution carbon (δ13Corg) and sulphur (δ34Spy) isotope analysis as well as undertook total organic carbon (TOC)/pyrite contents and pyrite morphology investigation, and framboidal pyrite size measurements in shales of the Bawangzhuang section of the southern Ordos Basin. Remarkably high TOC (23 ± 9%) and pyrite (7 ± 3%) contents were obtained from the shales, which indicate a large amount of organic carbon and pyrite burial during shale deposition. Framboids are the dominant pyrite morphology in the pyrite crystals and show large and variable mean diameters (7.0 ± 1.7 μm to 14.3 ± 6.8 μm) across the section, indicating oxic–dysoxic bottom water during shale deposition. δ13Corg and δ34Spy revealed narrow and less variable values, ranging from −31.8‰ to −28.1‰, and −4.1‰ to 4.9‰, respectively. The δ13Corg values suggest balanced and consistent carbon cycles. Integrated with pyrite content and morphological patterns, consistent δ34Spy values probably demonstrate a relatively open environment for the formation of sedimentary pyrite, and thus a shallow chemocline that was quite close to the water‐to‐sediment interface during shale deposition. Overall, the organic‐rich shales of the Tongchuan Formation were probably deposited under oxic–dysoxic bottom‐water conditions. Shallow chemocline depth combined with moderately high sedimentation rate and high primary productivity may have played crucial roles in the deposition and formation of the organic‐rich shales in the Tongchuan Formation. The shallow chemocline also facilitates the fossil preservation in a lacustrine environment.

Trapping of uranium by organic matter within sandstones during mineralization process: A case study from the Shuanglong uranium deposit, China

Carbonaceous debris (i.e., CD), dispersed organic matter (i.e., DOM) within sandstones, has been studied in the Shuanglong uranium deposit of the southern Ordos Basin, northern China. Relations of uranium enrichment and organic matter (i.e., OM) were investigated through maceral analyses, proximate analysis, vitrinite reflectance measurements, and morphology analyses of uranium minerals. The results show that CD widely distributed in sandstones, is associated with pyrite and uranium minerals. Macerals are mostly composed of telinite, collotelinite, collodetrinite, semifusinite, and inertodetrinite, including that collotelinite is dominant (contents more than 60%). The CD is at the stage of medium coalification, and is of low moisture, high ash, high volatile matter, medium fixed carbon, and high sulfur. Uranium occurs in uranium minerals (i.e., pitchblende and coffinite) and amorphous uranium. Moreover, uranium minerals partly or completely replace macerals. Statistical analyses show that porous inertinite and higher organic matter maturity is more advantageous to uranium enrichment. Additionally, uranium is unevenly distributed in macerals along certain directions, which is related to the physicochemical properties of macerals. Porous inertinite could provide channel for uranium-bearing fluid migration and space for uranium preconcentration. The CD at the stage of gas coal is of reducibility to be favorable for the forming of uranium minerals. Moreover, the relative atomic content of uranium (IV) increases with the increasing organic matter maturity. Uranium-bearing fluid could permeate into macerals under certain physical and chemical conditions, and react, i.e., adsorption followed by reduction. It is beneficial to comprehend the mechanism of uranium mineralization, providing certain guides for prospection.

Origin of Phosphate‐bearing Carbonate Concretions in the Upper Triassic Lacustrine Black Shales of the Southern Ordos Basin, China

AbstractCarbonate concretions are conspicuous in organic‐rich shales and are generally related to decomposition of organic matter. The black shales from the Chang 7 Member of the Upper Triassic Yanchang Formation of the southern Ordos Basin host abundant carbonate concretions, which provide a unique record of depositional and early diagenetic conditions of the paleo‐lake sediments. However, little attention has been given to the genesis and growth processes of the concretions in these lacustrine petroleum source rocks. New petrographic observations and geochemical analysis show that the concretions are composed of calcite, phosphate fossil fragments, K‐NH4‐feldspar, quartz, bitumen, and minor Fedolomite. Phosphate minerals, mainly carbonate fluorapatite (CFA), show pervasive replacement by calcite, most of which contains phosphorus, ranging in concentration from 0.26 to 2.35 wt%. This suggests that the phosphate minerals are the precursors for concretion growth. Positive δ13C (+5.6 to +12.4 ‰ V‐PDB) signatures and the absence of pyrite indicate that microbial methanogenesis was the dominant driver for concretion growth, rather than bacterial sulfate reduction. Quartz, bitumen, and Fe‐dolomite are the last cements that occurred, at deep burial depths and high temperatures. The formation of phosphate minerals might have been induced by upwelling of phosphate‐enriched deep water in the Late Triassic paleolake, which promoted phytoplankton blooms and further enrichment of organic matter. Extremely slow sedimentation rates of fine‐grained detrital minerals, relative to dead organism accumulation, led to the high permeabilities of the organic‐rich sediments and rapid concretion growth during shallow burial. The close association of phosphate‐bearing carbonate concretions and organic‐rich shales reflects that upwelling played a critical role in the formation of the high‐quality petroleum source rocks in the Triassic paleo‐Ordos lake.

Pore Type, Pore Structure, and Controlling Factors in the Late Triassic Lacustrine Yanchang Shale, Ordos Basin, China

Organic-rich lacustrine shales in the Upper Triassic Yanchang Formation with thermal maturity mainly in the oil window are the main shale oil and shale gas system in the lacustrine strata of the Ordos Basin, China. Pore systems are important for the storage and transfer of shale oil and gas. The main objectives of this study are to identify the pore types and pore structures and investigate the controlling factors for pore types, pore structures, and total porosities of the lacustrine Yanchang Shale. In this study, organic-rich mudstones, mudstones with siltstone interlayers, siltstone, and sandstones were selected from 15 wells in the southern Ordos Basin. X-ray diffraction, pyrolysis, scanning electron microscopy (SEM), low-pressure nitrogen adsorption analysis, and helium porosimetry were conducted to investigate the mineral compositions, pore types, pore structures, porosities, and controlling factors. Siltstone and sandstone interlayers heterogeneously developed in the Yanchang Shale. The petrology, mineral composition, geochemistry, pore type, pore structure, and porosity of siltstone interlayers are different from those of mudstones. The siltstone and sandstone interlayers usually have more quartz and feldspars, greater detrital grain sizes, and relatively better grain sorting but are lower in clay minerals, total organic carbon (TOC), amount of free liquid hydrocarbons values (S1), and total residual hydrocarbons values (S2), compared to mudstones. Interparticle (interP), intraparticle (intraP) pores, and organic pores (OPs) were developed in both siltstones and mudstones. OPs were observed in samples with lower thermal maturity (e.g., 0.5–0.85%). The inorganic pore size is greater than that of OPs. Additionally, the inorganic pore diameters in siltstone interlayers are also greater than those in mudstones. Organic-rich mudstones generally have higher pore volumes (PVs) of pores with sizes less than 10 nm, pore volumes of pores with sizes between 10 and 50 nm (PV, 10–50 nm), and specific surface area (SSA), but they have lower PVs of pores with sizes greater than 50 nm, total PV, and porosity when compared to siltstone and sandstone interlayers. The dominant pore type in mudstones is OPs and TOC (first order), sources and OM types (second order), and thermal maturity (third order), while the abundances of rigid grains with greater sizes and grain sorting are the main controlling factors of pore structures, SSA and PV. Both inorganic pores and organic pores are abundant in the siltstone interlayers. The pore size distribution (PSD), PV, and porosity of siltstone interlayers are related to the abundance of rigid grains (first order), grain sorting (second order), grain size (third order), and carbonate cement content. The total PV and porosity of Yanchang Shale reservoirs may have increased with the increased abundance of siltstone and sandstone interlayers.