Search Results (193)

Effectively separating and utilizing macerals based on their properties is crucial for the efficient and high-value utilization of coal. This study enhances the traditional screening method by employing primary and stepwise crushing techniques to separate Yili coal (YLC) into inertinite-rich (YLI) and vitrinite-rich (YLV) concentrates. The structural characteristics and direct coal liquefaction (DCL) performance of YLC, YLV and YLI are subsequently studied. The results indicate that YLV exhibits the highest yield of oil, asphaltene and gas, a finding closely linked to its elevated content of highly active functional groups and its long aliphatic and bridge chains. Furthermore, the liquefaction oil from YLV contains the highest content of alkanes and phenols, which is attributed to its high content of aliphatic hydrocarbons and phenolic hydroxyl groups. In contrast, YLI exhibits the lowest product yield relative to YLC and YLV, with the highest contents of aromatics, esters, and ketones in its oil, due to its high contents of aromatic and carbonyl carbon. The separation, structural characteristics and DCL studies of macerals from Yili coal offer valuable insights for the efficient separation and utilization of macerals. Full article

The heterogeneity of pore structure in deep coal reservoirs is a critical factor controlling the storage and transport capacity of coalbed methane (CBM). However, the fundamental control exerted by macrolithotypes remains inadequately quantified. This study systematically investigates the No. 8 coal seam of the Taiyuan Formation in the Daniudi gas field, Ordos Basin, using an integrated multi-technique approach including high-pressure mercury intrusion (HPMI), low-temperature N₂ adsorption (LTGA-N₂), and low-pressure CO₂ adsorption (LPGA-CO₂). Results reveal a consistent bimodal pore structure across all samples, dominated by well-developed micropores and macropores, whereas mesopores are relatively underdeveloped. More importantly, a clear macrolithotype control is established: as coal brightness decreases from bright to dull coal, the proportions of micropores and macropores decline significantly, leading to a substantial reduction in total pore volume and specific surface area. Fractal analysis further indicates that dull and semi-dull coals exhibit larger fractal dimensions, reflecting more complex pore structures and stronger heterogeneity compared to bright and semi-bright coals. This heterogeneity shows a positive correlation with ash and mineral contents, but a negative correlation with vitrinite and fixed carbon contents, suggesting that coal composition plays a primary governing role. These findings underscore that bright and semi-bright coals, with their superior micropore storage capacity and well-connected macropore networks, represent the most favorable targets for deep CBM exploration. This work establishes macrolithotype as a practical key indicator for reservoir quality assessment and production strategy optimization in deep CBM plays. Full article

The Qiangtang Basin in the Tibetan Plateau exhibits a paradoxically reversed source-rock maturity pattern (high margins, low center), which presents a challenge to classical basin models. Critically, the unclear genetic mechanism behind this anomaly has impeded hydrocarbon exploration. To address this, this study investigates a north–south-oriented 2D geological section across the central basin. By employing an integrated methodology, the genetic mechanism was elucidated through systematic calculations of paleo-burial depth, paleotemperature, and vitrinite reflectance (Ro) at ten control points (C1–C10). Specifically, tectonic burial history was reconstructed using the backstripping method, while mantle heat flow was corrected by integrating the McKenzie extensional and Royden compressional models. Maturity evolution was quantified using the Easy%Ro model. The results demonstrate that (1) since the Early Jurassic, the basin has undergone five tectono-thermal evolution stages, with the geothermal gradient reaching 30–36 °C/km during the end of the Early Cretaceous); (2) Ro values range from 1.2% to 1.68% at the northern basin margin (C1–C4), are approximately 1.15% in the Central Uplift Zone (C5–C7), and range from 1.45% to 1.6% at the southern basin margin (C8–C10); (3) importantly, the reversed distribution was jointly controlled by three factors: deep burial at the basin margins (5–6 km), early uplift in the central part (initiating from the Late Cretaceous), and local magmatic thermal disturbance. Their estimated contribution ratios are 40–50%, 30–40%, and 10–20%, respectively. Consequently, regions such as the Luxiongcuo Syncline, the Bandaohu–Qingshuihu area, and the Chibuzhangcuo area are identified as having favorable exploration potential. Full article

The formation of hard coal seams is the outcome of multi-stage, complex transformations of organic matter that lead to an increase in carbon content, a decrease in volatile components, and a progressive evolution of the rock’s structure and texture. Diagenetic and metamorphic processes, which underpin coal formation, largely determine its petrographic and geochemical characteristics, but they are not the only factors controlling the final properties of coal. An equally important role is played by the tectonic history of the region in which the coal seams occur. In this study, we carried out an integrated analysis of coal rank, based on vitrinite reflectance measurements (R₀), and mechanical properties, using Vickers microhardness tests (Hv). Coal samples were collected from both sides of a fault plane within a single seam. The results show that the presence of the fault is clearly reflected in the measured parameters. Vitrinite reflectance generally increases towards the fault zone, but in the immediate vicinity of the fault, it exhibits a slight decrease. Subtle yet systematic changes are also observed in microhardness, particularly in the Hv values. The results show that vitrinite reflectance (R₀) and microhardness (Hv) vary in a very similar manner—both parameters decrease as the degree of structural degradation of coal increases within the fault zone. This consistent response of R₀ and Hv to local structural damage suggests that they may serve as sensitive indicators of the presence and extent of influence of small-scale tectonic dislocations. Their combined application provides additional information on the potential occurrence of a fault and on the degree of structural disturbance of coal in its vicinity. Full article

This study integrates microthermometry and laser-Raman spectroscopy of individual fluid inclusions with basin modelling to reconstruct the hydrocarbon-fluid evolution and multistage re-mobilisation of the Permian Longtan Formation transitional marine–terrestrial shale in the YJ-LJ area, southern Sichuan Basin. Systematic analysis of aqueous two-phase, methane-rich, and associated bitumen inclusions hosted in fracture-fill veins and sandy partings identifies four fluid episodes, enabling subdivision of petroleum evolution into five stages. Results show trapping pressure increasing with temperature to a maximum of 107.77 MPa, equivalent vitrinite reflectance (EqVR) between 1.49% and 2.49%, and formation-water salinity that first rises then falls, remaining within the high-salinity continental-leaching brine field. Coupled with thermal-history modelling, shale-oil/gas evolution is divided into: (1) low-pressure slow burial, (2) over-pressured rapid burial, (3) sustained over-pressured deep burial, (4) high-pressure uplift adjustment, and (5) late uplift adjustment. The study demonstrates that over-pressure has been partially preserved, providing critical palaeo-fluid and pressure evidence for exploring transitional marine-terrestrial shale gas. Full article

The pore–fracture system in coal reservoirs is a critical factor controlling coalbed methane (CBM) productivity. This study focuses on deep coal samples from the Benxi and Taiyuan formations in the southeastern margin of the Ordos Basin. Using low-pressure CO₂ and N₂ adsorption experiments combined with fractal theory (Song and FHH models), the pore structure and heterogeneity of micropores (<2 nm) and mesopores (2–100 nm) were systematically analyzed. The results indicate that ash content is the primary inhibiting factor for pore development, showing significant negative correlations with micropore specific surface area, pore volume, and mesopore volume. The influence of macerals exhibits scale-dependent effects: vitrinite is the main contributor to micropore development, while vitrinite and ash content show a synergistic positive correlation with the volume proportion of 10–50 nm mesopores. Thermal maturity has no significant impact on pore volume but notably enhances mesopore heterogeneity. This study reveals an “ash-dominant, vitrinite-assisted” pore development pattern in low- to medium-rank coals, providing a theoretical basis for the efficient development of deep CBM. Full article

Marine organic-rich shales frequently exhibit anomalously high uranium (U) concentrations, yet the mechanisms governing its enrichment in overmature formations like the Wufeng–Longmaxi shales remain unclear. This study examines the distribution and enrichment patterns of uranium in the Wufeng–Longmaxi shales in typical wells through integrated geochemical and geophysical analyses, supplemented by natural gamma spectral logging data. Key findings include: (1) Multiple (up to three) uranium enrichment events are identified within the Wufeng–Longmaxi sequence, consistently corresponding to shale gas sweet spots. (2) Uranium content shows a clear dependence on organic matter (OM) type, with algal fragments being the primary host of uranium, likely due to incorporation during early diagenesis. Pore-water redox conditions and pH further govern the reduction of U (U⁶⁺) and its subsequent sequestration into organic phases. (3) The equivalent vitrinite reflectance (ER_o) of uranium-rich shales is 0.11%–0.17% higher than that of non-uranium-rich shales, suggesting that uranium enrichment may slightly enhance OM thermal maturity. (4) Uranium distribution is collectively controlled by reducing conditions, volcanic eruptions (e.g., tuff layers), and OM type. Additionally, uranium enrichment provides chronostratigraphic markers that may aid in timing marine black shales. These findings thus offer a mechanistic understanding of uranium enrichment in overmature shales, with direct implications for targeting productive intervals in shale gas systems. Full article

Analysis of pyrobitumen in reservoirs can yield key information about hydrocarbon evolution, which may provide vital insights for deep- to ultra-deep hydrocarbon exploration in high- to over-mature petroliferous deep basins. The Ediacaran Dengying Formation in the Penglai area of the Sichuan Basin contains large-scale gas reservoirs, where pyrobitumen is extensively present. To understand the hydrocarbon accumulation and alteration processes in these reservoirs, in this study, we systematically investigated the characteristics of the reservoir pyrobitumen using detailed petrographic analysis and laser Raman spectroscopy. The results indicated that four types of reservoir pyrobitumen are present: pyrobitumen with isotropic (type I), mosaic (type II), fibrous (type III), and honeycomb (type IV) textures. Pyrobitumen in the dolomite reservoirs of the Deng 2 and Deng 4 members of the Dengying Formation often co-occurs with hydrothermal minerals, including saddle dolomite, quartz, and fluorite. The equivalent vitrinite reflectance (Rmc Ro%) calculated indicated that the pyrobitumen is over-mature, with Rmc Ro% values ranging from 3.46% to 3.89%. In addition, significant differences were observed in the Raman parameters between the four types of pyrobitumen: type IV shows the greatest degree of structural ordering, while type II exhibits the highest level of disordering, with types I and III exhibiting intermediate structural ordering. Finally, the spatial distribution of the four types of pyrobitumen indicated that hydrothermal pulses driven by the Emeishan Large Igneous Province toward the end of the Permian Period may be primarily responsible for the extensive cracking of paleo-oil pools, causing the development of types II–IV pyrobitumen and gas generation. Full article

Bedding-parallel fractures represent a crucial flow-path network in shale oil reservoirs, yet their timing of opening and driving mechanisms remain subjects of long-standing debate. This study investigates the origin and opening mechanisms of bedding-parallel fractures within the Paleogene Funing shale oil reservoir of the Huazhuang area, Subei Basin, eastern China. A combination of petrography, fluid-inclusion analysis, PVTx paleo-pressure modeling, hydrocarbon generation history modeling, and reflectance measurements was employed. The results reveal the presence of abundant oil inclusions and bitumen within the bedding-parallel veins, indicating that the initiation of fracture was essentially synchronous with the oil emplacement. The studied Funing shale, with vitrinite reflectance values of 0.85% to 1.04%, is mature, identifying it as an effective oil-prone source rock. Thermal maturity of bitumen is comparable to that of the host shale, suggesting a local oil source. Homogenization temperatures (T_h) of coeval aqueous inclusions record fracture opening temperatures of approximately 100–150 °C, consistent with oil-window conditions. By integrating T_h data with burial history modeling, the timing of fracture formation and coeval oil injection is constrained to the peak period of local hydrocarbon generation, rather than the Oligocene Sanduo tectonic event. This indicates that fracture opening was primarily associated with hydrocarbon generation rather than tectonic compression. Petroleum-inclusion thermodynamic modeling demonstrates that the bedding-parallel fracture opening occurred under moderate to strong overpressure conditions, with calculated paleo-pressure coefficients of ~1.35–2.36. This finding provides direct paleo-pressure evidence supporting the mechanism of bedding-parallel fracture opening driven by fluid overpressure created during oil generation. These oil-bearing, overpressured fluids facilitated the initial opening and subsequent propagation of fractures along the bedding planes of shales. Concurrently, the precipitation of the calcite veins may have been triggered by pressure drop associated with the expulsion of some coexisting aqueous fluids. This study provides evidence addressing the debated mechanisms of bedding-parallel fracture opening in organic-rich shales, highlighting the critical role of oil generation-induced overpressure. Full article

The Northern Tibetan Plateau Basin is the most extensive and least explored Mesozoic marine basin in China and shows considerable potential for oil and gas exploration. This study systematically analyzed the abundance, type, hydrocarbon generation potential, and conversion rate of organic matter within three Jurassic drill core samples from the Biloucuo area of the Northern Tibetan Plateau Basin. The total organic carbon (TOC) content of these Jurassic source rocks was >4%, on average, permitting their classification as excellent source rocks. The average contents of sapropelinite, exinite, vitrinite, and inertinite in kerogen were 74%, 4%, 18%, and 4%, respectively. The H/C and O/C ratios of the kerogen mainly ranged from 0.8 to 1.3 and 0.06 to 0.11, respectively, indicative of type II₁ kerogen. The average S₁ + S₂ content was 15.0 mg/g rock, indicating a high hydrocarbon generation potential. On the basis of the relationship between the quantity of soluble hydrocarbons remaining in the strata and the S₂ and TOC contents, it can be inferred that the hydrocarbon generation conversion rate of these Jurassic source rocks was between 25% and 50%, and partial hydrocarbon expulsion has taken place. It is estimated that the maximum oil generation potential of the formation will reach 20 kg/t rock at a greater depth, which equates to good exploration potential. Full article

Understanding coalbed methane (CBM) enrichment patterns is essential for optimizing production capacity. This study evaluates the CBM reservoir-forming characteristics and exploration potential of the Longtan Formation in the Santang Syncline, Zhijin area, to systematically reveal CBM enrichment and high-production patterns. The investigation integrates regional geology, logging, well testing, laboratory analyses, and drainage production data. Results indicate that coal seam vitrinite reflectance (R_o,max) ranges from 3.20% to 3.60%, with metamorphic grade increasing with burial depth. Coal lithotypes consist predominantly of semi-bright coal, with subordinate semi-bright to semi-dull coal and minor semi-dull coal. Coal seam roofs comprise gray-black mudstone and calcareous mudstone, locally developing limestone, while floors consist of bauxitic mudstone. Pore structure analysis reveals greater complexity in coal seams 6 and 14, whereas seams 7 and 16 display simpler structures. Coal seams 5-3 and 6 demonstrate the weakest adsorption capacity and lowest theoretical gas saturation, while other seams exceed 55% gas saturation. Langmuir volume (V_L) increases with burial depth, reaching maximum values in coal seam 30. Langmuir pressure (P_L) follows a low–high–low trend, with lower values at both ends and higher values in the middle section. Measured gas content is highest in the middle section, moderate in the lower section, and lowest in the upper section. Reservoir condition assessment indicates favorable conditions in coal seams 14, 16, and 21, relatively favorable conditions in seam 7, and unfavorable conditions in seams 6, 30, 32, and 35. Among the three coal groups penetrated, the middle coal group exhibits the most favorable reservoir conditions, followed by the upper and lower groups. Full article

The Baorao Trough of the Jiergalangtu Sag, located in the central Erlian Basin, is rich in petroleum resources. However, due to a lack of systematic geochemical characterization and comparative studies with other source rocks, the hydrocarbon generation potential of its Jurassic strata remains unclear. In this study, 125 samples from the Baorao Trough were analyzed to evaluate their hydrocarbon generation potential, identify organic matter sources and depositional environments, and characterize hydrocarbon generation and expulsion. Results show that source rocks from the first member of the Tengge’er (K₁bt₁) Formation and the Aershan (K₁ba) Formation have high organic matter content, favorable kerogen types, and have reached low to medium maturity. In contrast, Jurassic source rocks are predominantly Type III kerogen and highly mature. K₁bt₁ was deposited in a weakly oxidizing to reducing, brackish environment, while K₁ba formed under weakly reducing, saline conditions. Jurassic source rocks also developed in weakly reducing, brackish to saline settings. Notably, saline and reducing environments promote the development of high-quality source rocks. The lower total organic carbon (TOC) threshold for effective source rocks in the study area is 0.8%, and the hydrocarbon expulsion threshold for vitrinite reflectance ratio (R_o) is approximately 0.8%. Accordingly, K₁bt₁ and K₁ba have undergone partial hydrocarbon expulsion but remain within the oil-generating window, indicating strong oil-generating potential. Jurassic source rocks likely experienced early thermal cracking of Type III kerogen, with generated oil migrating or escaping during early geological activity. However, some gas-generating potential remains. These findings provide significant evidence for assessing resource potential, predicting the distribution of high-quality source rocks and favorable exploration areas. Full article

As critical components in continental shale systems, the composition and evolution of clay minerals are fundamental to their diagenetic processes and petrophysical properties. The Chang-7 shales in the Ordos Basin exhibit abundant clay mineral content, offering a valuable case study for clay mineral research under moderate diagenetic conditions. This study employed XRD analysis to determine the whole-rock mineralogy, clay mineral composition, and the evolution characteristics of illite-smectite mixed-layer minerals (I/S). Comprehensive clay mineral datasets compiled from 13 newly analyzed wells and existing literature revealed distinct lateral distribution patterns. Total Organic Carbon (TOC) analysis and vitrinite reflectance (R_o) measurements provided systematic quantification of organic matter abundance and thermal maturation parameters in the studied samples. The results reveal that the Chang-7 shale exhibits a characteristic clay mineral assemblage, with I/S (average 44.2%) predominating over illite (34.7%), followed by chlorite (15.6%) and limited kaolinite (5.4%). Frequent volcanic activities provided substantial precursor materials for smectite formation, which actively participated in subsequent illitization processes, while chlorite and kaolinite distributions were predominantly controlled by provenance inputs and sedimentary facies, respectively. Inconsistencies exist between diagenetic stages inferred from I/S mixed-layer ratios and R_o values, particularly in low-maturity samples exhibiting accelerated illitization. The observed negative correlation between TOC content and mixed-layer ratios in Well YY1 and YSC Section samples demonstrates the catalytic role of organic matter in facilitating smectite-to-illite transformation. These results systematically clarify the coupled effects of sedimentary-diagenetic processes, offering new insights into the mutual interactions between inorganic and organic phases during illitization under natural geological conditions. The findings advance the understanding of Chang-7 shale oil and gas systems and offer practical guidance for future exploration. Full article

The Junggar Basin contains a large amount of coal resources and is an important coal production base in China. The coal seam in Zhundong coalfield has a large single-layer thickness and high content of inertinite, but large particle Fe-sulphide minerals are associated with coal seams in some mining areas. A series of economic and environmental problems caused by the combustion of large-grained Fe-sulphide minerals in coal have seriously affected the economic, clean and efficient utilization of coal. In this paper, the ultra-thick coal seam of the Xishanyao formation in the Yihua open-pit mine of the Zhundong coalfield is taken as the research object. Through the analysis of coal quality, X-ray fluorescence spectrometer test of major elements in coal, inductively coupled plasma mass spectrometry test of trace elements, SEM-Raman identification of Fe-sulphide minerals in coal and LA-MC-ICP-MS test of sulfur isotope of marcasite, the coal quality characteristics, main and trace element characteristics, macro and micro occurrence characteristics of Fe-sulphide minerals and sulfur isotope characteristics of marcasite in the ultra-thick coal seam of the Xishanyao formation are tested. On this basis, the occurrence state and genesis of large particle Fe-sulphide minerals in the ultra-thick coal seam of the Xishanyao formation are clarified. The main results and understandings are as follows: (1) the occurrence state of Fe-sulphide minerals in extremely thick coal seams is clarified. The Fe-sulphide minerals in the extremely thick coal seam are mainly marcasite, and concentrated in the YH-2, YH-3, YH-8, YH-9, YH-14, YH-15 and YH-16 horizons. Macroscopically, Fe-sulphide minerals mainly occur in three forms: thin film Fe-sulphide minerals, nodular Fe-sulphide minerals, and disseminated Fe-sulphide minerals. Microscopically, they mainly occur in four forms: flake, block, spearhead, and crack filling. (2) The difference in sulfur isotope of marcasite was discussed, and the formation period of marcasite was preliminarily divided. The overall variation range of the δ³⁴S value of marcasite is wide, and the extreme values are quite different. The polyflake marcasite was formed in the early stage of diagenesis and the δ³⁴S value was negative, while the fissure filling marcasite was formed in the late stage of diagenesis and the δ³⁴S value was positive. (3) The coal quality characteristics of the thick coal seam were analyzed. The organic components in the thick coal seam are mainly inertinite, and the inorganic components are mainly clay minerals and marcasite. (4) The difference between the element content in the thick coal seam of the Zhundong coalfield and the average element content of Chinese coal was compared. The major element oxides in the thick coal seam are mainly CaO and MgO, followed by SiO₂, Al₂O₃, Fe₂O₃ and Na₂O. Li, Ga, Ba, U and Th are enriched in trace elements. (5) The coal-accumulating environment characteristics of the extremely thick coal seam are revealed. The whole thick coal seam is formed in an acidic oxidation environment, and the horizon with Fe-sulphide minerals is in an acidic reduction environment. The acidic reduction environment is conducive to the formation of marcasite and is not conducive to the formation of pyrite. (6) There are many matrix vitrinite, inertinite content, clay content, and terrigenous debris in the extremely thick coal seam. The good supply of peat swamp, suitable reduction environment and pH value, as well as groundwater leaching and infiltration, together cause the occurrence of large-grained Fe-sulphide minerals in the extremely thick coal seam of the Xishanyao formation in the Zhundong coalfield. Full article

In comparison to shallow coal seams, deep coal seams exhibit characteristics of high temperature, pressure, and in-situ stress, leading to significant differences in reservoir properties that constrain the effective development of deep coalbed methane (CBM). This study takes the Carboniferous deep 8# coal seam in the Yulin area of Ordos basin as the research subject. Based on the test results from core drilling wells, a comprehensive analysis of the characteristics and variation patterns of coal reservoir properties and a comparative analysis of the exploration and development potential of deep CBM are conducted, aiming to provide guidance for the development of deep CBM in the Ordos basin. The research results indicate that the coal seams are primarily composed of primary structure coal, with semi-bright to bright being the dominant macroscopic coal types. The maximum vitrinite reflectance (R_o,max) ranges between 1.99% and 2.24%, the organic is type III, and the high Vitrinite content provides a substantial material basis for the generation of CBM. Longitudinally, influenced by sedimentary environment and plant types, the lower part of the coal seam exhibits higher Vitrinite content and fixed carbon (FC_ad). The pore morphology is mainly characterized by wedge-shaped/parallel plate-shaped pores and open ventilation pores, with good connectivity, which is favorable for the storage and output of CBM. Micropores (<2 nm) have the highest volume proportion, showing an increasing trend with burial depth, and due to interlayer sliding and capillary condensation, the pore size (<2 nm) distribution follows an N shape. The full-scale pore heterogeneity (fractal dimension) gradually increases with increasing buried depth. Macroscopic fractures are mostly found in bright coal bands, while microscopic fractures are more developed in Vitrinite, showing a positive correlation between fracture density and Vitrinite content. The porosity and permeability conditions of reservoirs are comparable to the Daning–Jixian block, mostly constituting oversaturated gas reservoirs with a critical depth of 2400–2600 m and a high proportion of free gas, exhibiting promising development prospects, and the middle and upper coal seams are favorable intervals. In terms of resource conditions, preservation conditions, and reservoir alterability, the development potential of CBM from the Carboniferous deep 8# coal seam is comparable to the Linxing block but inferior to the Daning–Jixian block and Baijiahai uplift. Full article