Nitrogen heteroatoms in shale systems strongly influence hydrocarbon quality, catalyst performance, and the release of harmful byproducts, yet their transformation pathways during thermal maturation remain poorly constrained. In particular, investigations into the coevolutionary relationship between solid-phase (kerogen) and liquid-phase (shale oil) nitrogen species during thermal maturation of sedimentary organic matter remain scarce. In this study, we quantitatively evaluated the structure and hydrocarbon-generation behavior of lacustrine type I kerogen across the oil window (vitrinite reflectance, Ro = 0.5 similar to 1.6%) to determine the compositional evolution of organic nitrogen functionalities within the kerogen matrix, the thermal transformation pathways of basic nitrogenous compounds in generated shale oils, and the coupled-evolutionary dynamics between kerogen-bound and free nitrogen species during thermal maturation. Xray photoelectron spectroscopy N 1s spectra identified five organic nitrogen moieties in kerogen, while high-resolution mass spectrometry determined that basic N1 class compounds were the predominant heteroatomic species in shale oils. During prepeak oil generation, rising thermal maturity released pyrrolic nitrogen and nitro groups while forming protonated pyridinic nitrogen associated with hydroxyl/carboxyl groups. During postpeak oil generation, the breakdown of these protonated complexes released pyridinic nitrogen and hydroxyl/carboxyl groups, with liberated oxygen-containing groups facilitating the formation of nitro groups. Graphitic nitrogen emerged exclusively above 1.4% Easy%Ro. Quinoline derivatives in shale oils were primarily formed at the onset of the oil window and progressively decreased with increasing maturity. Pyridine derivatives in shale oil formed continuously up to 1.4% Easy%Ro through the release of pyridinic nitrogen from kerogen and partial cracking of quinoline derivatives. Subsequent thermal alteration generated low-molecular-weight pyridines and significantly increased the relative abundance of nitrogenous polycyclic aromatic hydrocarbons. These results establish a coevolution framework linking kerogen-bound nitrogen moieties to nitrogenous compounds in shale oil, offering mechanistic insights into the staged transformation and release of nitrogen species. This framework provides predictive value for shale oil quality assessment, refining challenges, and mitigation of nitrogen-related emissions during exploitation of lacustrine Type I kerogen.

Shale oil has emerged as an increasingly important unconventional resource in the global oil and gas market. It is defined as hydrocarbons retained within the nano- to micro-scale pore-throat networks of shale, comprising a complex mixture of multi-component hydrocarbons, non-hydrocarbons, and semi-solid to solid organic matter. Despite its substantial resource potential, the efficiency of enhanced oil recovery (EOR) remains limited in shale due to highly variable fluid mobility and complex nano- to micro-scale pore-throat networks. Therefore, it is crucial to improve EOR efficiency by unraveling the factors controlling shale oil mobility, referred to herein as the mechanism of Component Flow (CF). In this study, we combined geochemical experiments and molecular dynamics simulations to investigate the behavior of different oil components and identify the factors governing shale oil mobility. Our results demonstrate that the variations in organic matter composition, in conjunction with complex mineralogical characteristics, result in a highly heterogeneous distribution of pore-throat structures that affects the mobility of shale oil. The shale oil formed in freshwater lacustrine environments is enriched in light-to-medium hydrocarbons that exhibit favorable fluid mobility. These hydrocarbons are preferentially retained in fine pore-throat structures in shale reservoirs and are subject to strong adsorption effects due to high clay mineral abundance. In contrast, the oil formed in saline lacustrine settings contains a higher proportion of heavy and non-hydrocarbon fractions, which are retained in larger pore-throat structures and have a weaker adsorption owing to relatively low clay mineral contents. Furthermore, molecular simulations and experimental results show that with increasing formation temperature and pressure, light hydrocarbons act as natural solvents to dilute heavy fractions and disaggregate asphaltenes. This process reduces oil viscosity and enhances the miscible flow of shale oil. Based on case studies from the Permian Lucaogou Formation in the Junggar Basin, the Paleogene Kongdian Formation in the Bohai Bay Basin, and the Cretaceous Qingshankou Formation in the Songliao Basin, we conclude that exploiting the CF mechanism can effectively enhance cumulative per-well production. Our study underscores the significance of the CF mechanism in deciphering the critical factors and conditions for optimizing EOR in shale oil systems.

Significant hydrocarbons have been discovered in Cambrian strata of Sichuan and Tarim Basins, China. These Cambrian reservoir rocks, which are crucial for petroleum exploration, were deposited in a distinct carbonate platform setting, inner-platform in Sichuan and platform-margin in Tarim. Two 3D seismic surveys, GM in Sichuan and GC in Tarim, provide valuable data to characterize facies and reservoir architectures through seismic sedimentology and multiple discipline data. Modern analogs, such as San Andreas Fault system and the Florida Keys, aid in geomorphologic and facies interpretation, as well as analyzing the reservoir distribution in both basins. In the GM, facies include shoal crest, shoal margin, intershoal, and lagoonal settings. Reservoir quality of GM is enhanced by high-energy shoal crest and shoal margin facies and characterized by thick, well-developed grainstone with favorable porosity and permeability. Conversely, the GC exhibits a facies framework comprising middle ramp, outer ramp, and basinal deposits, where reservoir development is linked with tidalchannel-modified shoals, channel-mouth deltas in middle ramp, and slope fans transitioning into basin-floor fans in deeper-water settings. These findings have significant implications for hydrocarbon exploration and production strategies. The seismic sedimentological approach offers valuable insights for exploring analogous hydrocarbons in the carbonate systems worldwide.

The flow mechanism of continental shale oil in clay-rich micro-nano pores, differ from marine shale oil with high carbonate content and poor clay in North America, is still unclear, which restricts the effective development of continental shale oil resources. Based on shale oil production data and experimental studies, this study proposed the concept of “component flow”, which represents a miscible state of light, medium, heavy hydrocarbons within micronanopores of underground shale according to similarity and intermiscibility principles. This phenomenon entails the “suspension” of molecular aggregates of immobile components, such as heavy hydrocarbons, in light and medium hydrocarbons, thereby decreasing shale oil viscosity and enhancing fluidity and outflows. A high content of light and medium hydrocarbons in shale oil is essential to initiating the “component flow” of shale oil in micropores/nanopores. High formation temperatures can lower the viscosity of heavy hydrocarbons, such as highmolecular-weight wax, which is conducive to improving fluidity. The formation of “component flow” is significant for enhancing well productivity and EUR of shale oil, which merits great attention for favorable area prioritization and economic exploitation of shale oil.

Characterizing the sedimentary architecture of successions of shallow-water deltas fed by braided rivers is crucial for the prediction of sandbody development in potential field. However, the complex distribution and diverse sedimentary characteristics of these deltas have led to insufficient insight into the controlling factors for different types. Based on high-resolution seismic data and well-logging data from the P Oilfield in the Bohai Bay Basin, two zones (I1 and III2) within the Neogene upper member of Guantao Formation were selected. Utilizing palynological analysis, elemental geochemical analysis, and frequency-division seismic-attribute fusion technology based on an ensemble-machine-learning algorithm, paleoclimate analysis and detailed architectural characterization of the studied stratigraphic interval were conducted. The findings indicate that: (1) Two palynological assemblages were identified in the upper member of Guantao Formation: Assemblage I suggests a subtropical-temperate monsoon climate, while Assemblage II indicates a continental semi-arid climate. (2) Zone I1 represents a lobate shallow-water braided-river delta, whereas zone III2 represents a lobate shallow-water braided-river delta with bar-finger. (3) Stable discharge conditions and a high bedload fraction under a subtropical-temperate monsoon climate facilitate the formation of the lobate shallow water delta. A low bedload fraction provides a favorable condition for the development of bar-finger, but flood events under a continental semi-arid climate enhance channel-mouth deposition driving the formation of the lobate delta with bar-finger.

Considering the complex occurrence environment and significant compositional variation of continental shale oil, as well as the uncertainties in its mobility and producible amount, this study employs geochemical analysis and production monitoring to investigate the “component flow” phenomenon of shale oil during production from the Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin. It is clarified that the miscibility of different hydrocarbon components and non-hydrocarbon substances improves the flowability of multi-component hydrocarbons and non-hydrocarbons, thereby effectively enhancing the production of shale oil. Research indicates that the “lower sweet spot” has a relatively high content of light and medium hydrocarbon components and strong formation energy compared to the “upper sweet spot” of Lucaogou Formation, resulting in higher density and viscosity of the produced crude oil, which can be regarded as evidence of “component flow” of retained hydrocarbons. The “upper sweet spot” exhibits two scenarios. In areas far from faults with good preservation conditions, the high content of light and medium components in retained hydrocarbons and a high formation pressure coefficient make component flow more likely to occur. Consequently, the produced crude oil has a higher density, and the estimated ultimate recovery (EUR) per well is also higher. In areas near faults with poor preservation conditions, although the produced crude oil has a light density, the EUR per well is relatively low, indicating that the conditions for component flow of retained hydrocarbons underground have deteriorated. The study also demonstrates that preservation conditions (preventing light hydrocarbon escape and maintaining formation energy) and production strategies (controlling production pressure differential and maintaining stable operations) are important factors in regulating the occurrence and continuity of “component flow” to maximize EUR per well. These new insights can be applied to the evaluation of economically productive “sweet spots” and provide guidance for achieving optimal EUR per well in shale oil production.

Mixed shales occur in continental saline lacustrine basins in China,exhibiting complex lithologies and various types of pores.The occurrence and distribution characteristics of multicomponent hydrocarbons within shale pores affect shale oil mobility and estimated ultimate recovery(EUR)per well.Focusing on the 2nd member of the Kongdian Formation(also referred to as the Kong 2 Member)in the Cangdong Sag,Bohai Bay Basin,we analyze the geochemical characteristics,biomarker variations,and reservoir fluorescence characteristics of shale oil produced at different recovery stages.Accordingly,the occurrence characteristics of shale oil within micro-and nano-scale pores and their impacts on shale oil mobility are systematically analyzed.The results indicate that the hydrocarbon components of shale oil exhibit cyclic variations during the development process.A higher frequency of these variations is associated with more rapid replenishment of light hydrocarbons,as well as a more stable miscible flow regime and a higher flow rate of multicomponent hydrocarbons.During the recovery of shale oil at great burial depths,hydrocarbons produced in the early stage show higher maturity,while those recovered in the late stage exhibit lower maturity.This pattern indicates that shale oil generated in different periods occurs in independent micro-and nano-scale pores without mixing.In contrast,for shale oil at shallow burial depths,no significant differentiation in hydrocarbon maturity is observed throughout the oil recovery.Hydrocarbons with higher maturity occur primarily within the micron-scale macropores in felsic laminae,whereas those with lower maturity are primarily confined to nano-scale micropores.Saline lacustrine basins are characterized by early and multi-stage hydrocarbon generation,coupled with multi-stage hydrocarbon accumulation and enrichment.Consequently,shale oil with different hydrocarbon compositions occurs in subsurface micro-and nano-scale pores.This facilitates hydrocarbon component flow,thereby enhancing the expulsion of heavy components from micro-and nano-scale pores and ultimately increasing the flow rate of shale oil.

The Linhe Depression in the Hetao Basin represents an important area for potential breakthroughs in hydrocarbon exploration of medium-and small-sized basins and ultra-deep strata in China.Although the Jilantai and Bayan oilfields have been discovered in recent years,multiple exploratory wells in this depression have failed to deliver expected results.The failure analysis suggests that insufficient oil sources represent a critical constraint on hydrocarbon exploration in the depression.Based on the latest exploration achievements and laboratory data,we systematically investigate the characteristics of source rocks therein and calculate their resource potential using an integrated approach that combines geological assessment,basin simulation,and resource prediction.The research results indicate that the Linhe Depression in the Hetao Basin experienced an evolutionary pattern during the Meso-Cenozoic characterized by multi-stage tectonic subsidence and staged migration of subsidence centers,with the Naoxi sub-sag acting as a long-term inherited subsidence center.The depression contains four suites of source rocks:source rocks of the Guyang,Wulate,Linhe,and Wuyuan formations.The source rocks of the Linhe Formation,among others,predominate,characterized by substantial thicknesses,extensive distribution,high organic matter abundance,and favorable kerogen types.Furthermore,these source rocks exhibit low activation energy for hydrocarbon generation,offering advantages including early maturity,early hydrocarbon expulsion,a wide oil window,and high efficiency of organic matter conversion.These characteristics and advantages are closely related to the deposition of the source rocks in a highly reducing,saline lacustrine sedimentary environment and the enrichment of algae and sulfur within organic matter.Basin simulation results identify the Pliocene-Quaternary as the major oil generation period of source rocks in the Linhe Depression,accounting for 83.1%of the total oil generated.This result aligns well with the late-stage tectonic activity and contributes to the formation of a rapid late-stage hydrocarbon accumulation model.Hydrocarbon resource evaluation indicates that the Linhe Depression has a total amount of generated oil of 31.19×109 t and petroleum resources of 2.299×109 t.Given that the current cumulative proven reserves account for merely 1.2%of the total petroleum resources,the Linhe Depression holds considerable potential for hydrocarbon exploration.Since the oil generation centers are concentrated in the Naoxi sub-sag and the central fault zone,future exploration efforts in the depression should focus on tectono-lithologic composite traps within the Linhe Formation in the central fault zone(including the Xinglong,Nalinhu,and Ulan Buh structural zones),as well as lithologic traps and shale oil in the Linhe and Wuyuan formations in the sub-sag.

In-situ heating conversion is the most practical recovery method for lacustrine low-to-medium maturity shale oil.However,the energy output-input ratio must exceed the economic threshold to achieve commercial development.This paper systematically investigates the mechanism of super-rich accumulation of organic matter in continental shale,sweet spot evaluation,optimal heating windows,and appropriate well types and patterns from the perspectives of enhancing energy output and reducing energy input.(1)The super-rich accumulation of organic matter in lacustrine shale is primarily controlled by the intensity,frequency,and preservation of external material inputs,and is related to moderate volcanic and hydrothermal activities,marine transgressions,with total organic carbon content greater than or equal to 6％.(2)The quality of organic-rich intervals is related to the type of source material and hydrocarbon generation potential.The in-situ conversion-derived hydrocarbon quality index(HQI)is established,and the zones exhibiting HQI＞450 are defined as sweet spots.(3)Considering the characteristics of the organic matter conversion material field and seepage field,the temperature interval 300-370 ℃ is recommended as the optimal heating window for the Chang 73 sub-member of the Triassic Yanchang Formation in the Ordos Basin.Based on the advantages of thermal conductivity,permeability,and hydrocarbon expulsion efficiency along the bedding direction during in-situ heating,the"horizontal well heating+vertical well development"scheme is proposed,which has demonstrated significant enhancement in both recovery factor and energy output-input ratio,making it the optimal in-situ conversion process.The research findings provide a theoretical and technical foundation for the economical and efficient development of low-to medium-maturity shale oil.

Volcanism is an important controlling factor for organic matter enrichment and sedimentary evolution in proximal continental lakes. Numerous studies have focused on revealing the intrinsic mechanisms of high organic carbon burial regulated by volcanism, while the understanding of the dynamic evolutionary processes of volcanically induced lacustrine depositional systems remains unclear. We integrated core, logging, and geochemical data from more than 20 fully cored wells spanning all sedimentary facies belts of the Chang 73, to reconstruct the paleoenvironment and establish sedimentary models during volcanism. By combining sequence stratigraphy, carbon isotope chemostratigraphy, zircon U–Pb dating and astronomical cycle techniques, we established a high-precision isochronous stratigraphic framework, spanning ∼241.36 Ma to ∼239.7 Ma. This study presented the first detailed record of sedimentary evolution in response to volcanism in the Ordos Basin: Early volcanism led to a transient, relatively dry-cold climate and low weathering intensity; Volcanic ash supplied nutrients to the lake, and the accumulation of loose pyroclastics induced overloading and instability of the delta front, triggering slump-type gravity flows. These factors jointly promoted the formation of tuffaceous shale; And sustained volcanism induced a greenhouse effect, resulting in an extremely humid climate and enhanced weathering, thus inducing hyperpycnal flows and appropriate-intensity hyperpycnal flows facilitate felsic shale formation. Furthermore, the climatic background modulated by astronomical cycles amplifies the sedimentary evolutionary responses to volcanism. Multistage siltstone–muddy siltstone assemblages with good reservoir properties, which act as favorable shale oil “sweet intervals”. This study provides new perspectives for understanding the controlling factors of continental lacustrine basin depositional systems and offers guidance for the spatial prediction of shale oil “sweet spots” in the Chang73.

This study investigates the effects of different types of primary organic matters on hydrocarbon generation and expulsion of source rocks. Samples representing three typical source rocks from the Lucaogou Formation in the Jimusaer Sag were collected and analyzed by hydrous pyrolysis, total organic carbon, Rock-Eval, gas chromatography-mass spectrometry, organic petrology, and scanning electron microscopy. Distinct differences in crude oil biomarkers were observed between telalginite- and lamalginite-rich source rocks. Telalginite-rich source rocks exhibit higher abundances of pristane, phytane, beta-carotane, gammacerane, and C29 regular steranes, while lamalginite-rich source rocks are characterized by enrichments of C24 tetracyclic terpane, C29 hopane, and C28 regular steranes. These biomarkers provide insights into the primary types of organic matter and their depositional environments with high water salinity of telalginite and low salinity of lamalginite. Hydrous pyrolysis results reveal that telalginite-rich source rocks demonstrate early hydrocarbon generation and a wider oil window than lamalginite-rich source rocks. Compared to lamalginite-rich source rocks, telalginite-rich source rocks produced better quality of crude oil, exhibit a smaller specific surface area of organic matter, and greater development of organic-inorganic pores, which contribute to their higher oil expulsion. These findings are helpful to the understanding of the constraints imposed by different primary organic matters on hydrocarbon generation and expulsion of the source rocks, and the insights have significant implications for exploration and development of the shale oil resources.

Based on the analysis of surface geological survey,exploratory well,gravity-magnetic-electric and seismic data,and through mapping the sedimentary basin and its peripheral orogenic belts together,this paper explores systematically the boundary,distribution,geological structure,and tectonic attributes of the Ordos prototype basin in the geological historical periods.The results show that the Ordos block is bounded to the west by the Engorwusu Fault Zone,to the east by the Taihangshan Mountain Piedmont Fault Zone,to the north by the Solonker-Xilamuron Suture Zone,and to the south by the Shangnan-Danfeng Suture Zone.The Ordos Basin boundary was the plate tectonic boundary during the Middle Proterozoic to Paleozoic,and the intra-continental deformation boundary in the Meso-Cenozoic.The basin survived as a marine cratonic basin covering the entire Ordos block during the Middle Proterozoic to Ordovician,a marine-continental transitional depression basin enclosed by an island arc uplift belt at the plate margin during the Carboniferous to Permian,a unified intra-continental lacustrine depression basin in the Triassic,and an intra-continental cratonic basin circled by a rift system in the Cenozoic.The basin scope has been decreasing till the present.The large,widespread prototype basin controlled the exploration area far beyond the present-day sedimentary basin boundary,with multiple target plays vertically.The Ordos Basin has the characteristics of a whole petroleum or deposition system.The Middle Proterozoic wide-rift system as a typical basin under the overlying Phanerozoic basin and the Cambrian-Ordovician passive margin basin and intra-cratonic depression in the deep-sited basin will be the important successions for oil and gas exploration in the coming years.

In addition to the organic matter type, abundance, thermal maturity, and shale reservoir space, the preservation conditions of source rocks play a key factor in affecting the quantity and quality of retained hydrocarbons in source rocks of lacustrine shale, yet this aspect has received little attention. This paper, based on the case analysis, explores how preservation conditions influence the enrichment of mobile hydrocarbons in shale oil. Research showns that good preservation conditions play three key roles. (1) Ensure the retention of sufficient light hydrocarbons (C1–C13), medium hydrocarbons (C14–C25) and small molecular aromatics (including 1–2 benzene rings) in the formation, which enhances the fluidity and flow of shale oil; (2) Maintain a high energy field (abnormally high pressure), thus facilitating the maximum outflow of shale oil; (3) Ensure that the retained hydrocarbons have the miscible flow condition of multi-component hydrocarbons (light hydrocarbons, medium hydrocarbons, heavy hydrocarbons, and heteroatomic compounds), so that the heavy hydrocarbons (ΣC25+) and heavy components (non-hydrocarbons and asphaltenes) have improved fluidity and maximum flow capacity. In conclusion, in addition to the advantages of organic matter type, abundance, thermal maturity, and reservoir space, good preservation conditions of shale layers are essential for the formation of economically viable shale oil reservoirs, which should be incorporated into the evaluation criteria of shale oil-rich areas/segments and considered a necessary factor when selecting favorable exploration targets.

The Middle-Late Triassic Ordos Basin has gained increasing attention due to its significance in recording the earliest known recovery-radiation of terrestrial aquatic ecosystems following the Permian-Triassic mass extinction (PTME) and its abundant non-renewable resources. Despite numerous attempts to elucidate the mechanisms for high organic carbon burial, the pattern of carbon cycle and its associated environmental variations remain largely unknown, probably ascribed to multi-periodic deposition of volcanic ashes and complex lacustrine depositional architectures. Herein, we reconstructed environmental variations within basinal and global contexts and identified the formation of metalimnetic oxygen minimum area in paleo-lacustrine environments. Our results demonstrate a shift of oxygen-deficient and organic matter hyper-enriched areas from the central basin in the lower part of Chang 7-3 (interval I) towards the periphery in the upper part (interval II). This transition is ascribed to enhanced weathering intensity but weakened volcanic activity, which is contemporaneous with terrestrial bio-evolution across the Mid-Late Triassic boundary. In addition, the contemporary variations in carbon isotopes and relatively sea and lacustrine levels around the Paleo-Tethys Ocean, along with the rare occurrence of high redox metal abundances in lacustrine environments, indicate a (intermittent) water connection between the Ordos Basin and Paleo-Tethys Ocean during the latest Middle Triassic to early Late Triassic. We hypothesize that mild to moderate volcanic activity induced genetic disturbance and stimulated bioirradiation in the late Ladinian. In contrast, intensified weathering resulted in biological proliferation but also transiently elevated morality in the early Carnian, which appears to be the prelude of spectacular Triassic terrestrial radiation that dominates modern ecosystems.

Since the past decade, a super-large gas field with reserves of similar to 2 trillion cubic meters has been discovered in the Ediacaran-Cambrian Formations (referred to as Ediacaran-Cambrian hereafter) of the Sichuan super gas basin. In carbonate rocks of the Ediacaran-Cambrian, the geological conditions and reservoir formation issues of natural gas have drawn extensive attention, leading to numerous research advances. However, key geological aspects-such as the genesis of the Deyang-Anyue aulacogen, origin and source of natural gas, process of oil and gas formation, mechanism of gas-reservoir pressure formation and evolution, main controlling factors of natural gas enrichment, and calculation method of oil-cracking gas resources-remain unclear. This study systematically reviews the advancements in research on the Ediacaran-Cambrian of the Sichuan Basin, including the geochemical characteristics and genesis of natural gas and solid bitumen, geological conditions responsible for natural gas accumulation, cracking and gas generation processes in paleo-oil reservoirs, accumulation models, and controlling factors of natural gas accumulation. Results revealed that the understanding of the genesis of the Deyang-Anyue aulacogen has been unified, and the genesis and sources of natural gas have been clarified based on extensive geochemical data of natural gas. In addition, based on a detailed explanation of the geological conditions responsible for the formation of large gas reservoirs in the Ediacaran-Cambrian, the review elucidates the formation of oil and gas along with the formation and evolution mechanisms of gas-reservoir pressure. Further, it establishes an accumulation model and identifies the main controlling factors of the accumulation. Moreover, a calculation formula for the natural gas dissipation coefficient is established to address the deficiencies in existing methods used for calculating the resource quantity of oil-cracking gas. Using a genetic method and PetroMod three-dimensional basin simulation software, the resource quantity of cracking gas is estimated following the technical route of oil formation from kerogen, formation of paleo-oil reservoirs, and subsequent cracking to generate gas. In addition, the oil generation history of the basin and five tectonic units is studied, yielding the resource quantity of paleo-oil reservoirs. Finally, the directions for ultradeep natural gas exploration was further pointed out further indicate. Hence, this study is critical for the oil and gas geological theory and exploration activities of ultradeep, ancient strata. Moreover, this study offers a reference, which is expected to promote the exploration of Ediacaran-Cambrian natural gas in domestic basins (such as Sichuan, Ordos, and Tarim), and holds substantial scientific and practical importance for pioneering the exploration of natural gas in ancient strata from the Middle-Upper Proterozoic to the Lower Paleozoic eras worldwide.

Natural radioactive radiation emitted by uranium (U) in sedimentary basins continuously transforms organic matter and thus its degrading impact overprints the burial-related thermal maturation process. However, the systematic pathway for irradiated alteration of sedimentary organic matter and its influence remain poorly understood. This omission hinders a comprehensive understanding of organic matter evolution after burial. Our study investigated the artificially and naturally irradiated alteration of organic matter in the U- and organic-rich Cambro-Ordovician Alum Shale of Northern Europe and Triassic Chang 7-3 shale in the Ordos Basin of China. After exposure of samples to a Co-60 source, radiation accelerated the conversion of bitumen to natural gas with a lower gas dryness (C-1/C1-5 < 0.8), while kerogen is converted to natural gas with a higher gas dryness (>0.8). Moreover, enhanced radiation dose facilitates the transformation from resins and asphaltenes to aromatics and leads to enrichments of carbon-13 isotope, when the radiation dose reaches the thresholds of approximately 100 and 2000 kGy, respectively. Natural radiation exposure in the Chang 7-3 shales, identified by spatial distribution and affiliated phases of U, may have influenced bitumen structures through accelerating transformation of macro- to micro-organic compounds and generation of wet gas that could have enhanced hydrocarbon mobility. Moreover, an addition to the standard model for organic matter burial and transformation is proposed with integration of natural radiation exposure. Based on the calculated Proterozoic-Phanerozoic radiation dose in sedimentary rocks, we hypothesize that the radioactive processes might have consistently modified the characteristics of retained bitumen in uranium-enriched shales, thereby affecting carbon cycle on Earth's surface environments.

The Late Cretaceous Nenjiang Formation in the Songliao Basin presents a unique setting to examine how climate change and sea-level rise influenced organic matter accumulation. This study combines TOC analysis, Rock-Eval pyrolysis, GC-MS, GC-MS-MS, and elemental geochemistry on core samples from two wells to assess organic matter deposition before and after transgressive events. TOC values range from 0.18 to 14.63 wt%, with significant variations in hydrocarbon potential and thermal maturity. Periodic warm and cool climates triggered intermittent seawater intrusions that created anoxic conditions conducive to marine diatom and lacustrine dinoflagellate proliferation. Extended warm periods, however, suppressed dinoflagellate development and reduced paleo-productivity. The activity of methanogenic bacteria further contributed to the degradation of sedimentary organic matter, hindering its accumulation. While warm climates facilitated flood events that transported terrigenous nutrients, enhancing dinoflagellate blooms and expanding the oxygen minimum zone. These findings highlight the bio-environmental interactions that governed organic matter accumulation during transgressions, offering insights for exploration in similar sedimentary environments.

The Upper Cretaceous Qingshankou Formation black shales, deposited in the late Turonian (LTB shales), are the main source rocks of the Songliao Basin. The origins of organic matter enrichment of the shales is a contentious subject fuelling many ongoing debates. This study investigates the genesis of the organic matter-rich shale by using molecular geochemistry. The LTB shales can be divided into three sections. The Section I shales were deposited in saline, stratified and anoxic water conditions, which are related to seawater incursion events. At least three episodic and periodic seawater incursion events were recognized during Section I shale deposition. The Section II shales deposited in brackish to fresh and deep lake-level conditions with high primary productivity, which are related to lake-level transgression. The Section III shales were deposited under fresh and slightly oxidized water conditions, which are related to lake-level regression. Two organic matter enrichment models for the LTB shales are identified, that is, the seawater incursion model and the maximum lake-level transgression sedimentation model, which act on different shale sections, both playing significant roles in the enrichment of organic matter.