Source Rocks Research Articles

Organic and inorganic geochemical cyclicity of a Maastrichtian oceanic open-shelf carbonate source rock

Organic-rich source rocks are not only crucial for hydrocarbon exploration and production but also serve as valuable archives of past environmental conditions. This study investigates the Upper Cretaceous (Maastrichtian) source rocks present in the Al-Lajoun Basin of central Jordan, to identify geochemical compositional variability corresponding to the paleo-environmental conditions during deposition. To this end, a multifaceted approach using Rock-Eval, SGR, XRD, XRF, ICP-OES, SEM-EDX, and thin-section petrography is utilized to understand bulk organic and inorganic geochemical proxies. Based on the results, the Jordan source rock is characterized as organic-rich, Type IIS kerogen, and thermally immature source rock, representing three distinct cycles of organic matter distribution. Cycle 1 is characterized as organic-rich carbonate mudstones with an average total organic carbon (TOC) content of 17 wt.%. This cycle represents high organic matter productivity, anoxic bottom water conditions, and episodic detrital influx (clays and detrital quartz). Cycle 2 is characterized as silica-rich mudstones to wackestones with an average TOC of 15 wt%. This cycle reflects a shift from carbonate-dominated to silica-dominated biota, likely driven by increased nutrient supply and changing climatic conditions. These conditions resulted in high bioproductivity and highly reducing anoxic/euxinic bottom water conditions during deposition. Cycle 3 represents foraminiferal wackestones to packstones with an average TOC of 12 wt.%. This cycle is characterized by a relatively high detrital sediment input, with comparatively low organic matter productivity and anoxic bottom water conditions. The identified organic and inorganic geochemical variability between these cycles implies changing climatic conditions over the open shelf setting, which in turn implies changes in ocean currents impacting the upwelling system of the Tethys margin. Understanding this relationship between ocean currents, climate, and the geochemical composition is crucial for efficiently exploring and exploiting organic-rich source rocks. A regional correlation of these cycles and their geochemical signatures could provide a powerful tool to trace ocean currents and associated climate change along the Tethys margin during the early Maastrichtian.

Characteristics and Mechanisms of Hydrocarbon Accumulation in Tight Oil Reservoirs of the Carboniferous Kalagang Formation, Malang Sag, Santanghu Basin, China

ABSTRACTAs hydrocarbon exploration progresses in the Santanghu Basin, tight oil reservoirs in Carboniferous volcanic rocks have become a focal point for exploration and development efforts. This study aims to account for significant industrial oil flow from the tight tuff layers of the Kalagang Formation in well blocks Ma 71 to 33 in the Malang Sag within the basin. Through experiments including organic carbon analysis, rock pyrolysis, and nuclear magnetic resonance (NMR), this study systematically investigated the principal characteristics and determinants of the tight oil reservoirs, culminating in the development of hydrocarbon modes, thus providing a reference for tight oil exploration and production in the study area. The results indicate that the Kalagang Formation is composed predominantly of tuff, especially vitric tuff. The storage spaces of this formation include dissolution pores, tectonic fractures, and dissolution fractures, with devitrification pores less common. Compaction plays a crucial role in rendering the tuff reservoirs tight. Nevertheless, differentiated diagenetic alterations have improved the microscopic storage spaces and their interconnectivity. Well blocks Ma 3301 and Ma 33, having undergone significant weathering (leaching), exhibit superior reservoir physical properties and connectivity compared to well blocks Ma 71 and Ma 73. In contrast, well blocks Ma 71 and Ma 73 experienced more significant alterations due to organic acid dissolution and devitrification under the influence of source rock types and hydrocarbon generation. The comprehensive analysis indicates that tight oil reservoirs in the Malang Sag were formed due to the close superimposition of source rocks and reservoirs, along with the effective connection of source rock‐rooted faults. This results in the formation of two sets of proximal tight tuff reservoirs with the ‘lower‐source rock and upper‐reservoir’ and ‘self‐sourced reservoir’ modes.

Characteristics and genetic model of dolomite reservoirs in the Qigebulak Formation, Upper Sinian, Tarim Basin, China

In recent years, oil and gas exploration has expanded into ultra-deep fields. Thick primary dolomite is developed in the Qigebulak Formation of the Tarim Basin, but there is little research on the petrological characteristics and reservoir genetic mechanisms of the primary dolomite. The Qigebulak Formation consists of the upper, middle, and lower members, and the dolomite can be divided into crystalline dolomite, grain dolomite, and algal framework dolomite. The analysis of the petrological and geochemical characteristics shows that dolomites in the middle member retain the geochemical characteristics of the original deposition. The δ13C of member Ⅱ ranges from 3.2‰ to 4.6‰ (VPDB), and δ18O range from −1.7‰ to 2.1‰ (VPDB). These values are consistent with the distribution range of carbon and oxygen isotopes in seawater during deposition. The REE distribution patterns of the three members of the Qigebulak Formation are relatively flat and right-inclined, with REE < 100 ppm. The LREE/HREE ratio ranges from 6.34 to 23.83, with an average of 13.82, indicating LREE enrichment and HREE relative loss. Overall, the dolomite in the Qigebulak Formation is formed by seawater deposition. Sedimentation is the foundation and premise of reservoir development. Effective reservoirs are mainly developed in the grain banks and microbial mounds, with the reservoir spaces mainly consisting of algal framework pores and intergranular pores. Supergene karstification plays a key role in the formation of the reservoir, and the upper Qigebulak Formation experienced strong surface dissolution. During the burial stage, high temperature and high pressure caused the source rocks of the Yuertusi Formation to dissolve and form organic acidic fluids. The dissolution of these acidic fluids promoted the improvement of the reservoir’s physical properties. Based on a comprehensive analysis of the controlling factors for reservoir development, a reservoir genetic model is established. The primary pores are developed in early sedimentary microbial mounds and grain banks, serving as the foundation for reservoir development. During the Late Sinian period, influenced by the Keping movement, the Tarim Basin experienced tectonic uplift, leading to extensive supergene karst development under the influence of meteoric water. In the burial phase, the degradation of organic matter produced acidic fluids that dissolved dolomite, thereby further enhancing porosity. The dolomite reservoir of the Qigebulak Formation developed under the control of sedimentation, supergene dissolution, and burial dissolution. At present, industrial oil and gas have been discovered in the Sinian system in the Tabei Region, but the overall exploration level is low. The well-developed paleokarst reservoirs suggest that the Sinian system holds promise for oil and gas exploration.

Mechanisms of lithium and cesium enrichment in the Semi-Dazi geothermal field, Qinghai-Xizang Plateau: insights from H–O–Li–Sr isotopes

Hot springs in the southern Qinghai-Xizang Plateau show anomalous lithium (Li) and cesium (Cs) enrichment, but the mechanisms driving this enrichment remain poorly constrained. Using multi-isotope tracers (H, O, Li, Sr), we investigate the Semi-Dazi geothermal field, which hosts the Plateau’s highest recorded geothermal Cs concentrations. The system comprises two geographically separated geothermal areas: Semi and Dazi, spaced ~ 15 km apart, displaying distinct hydrogeochemical signatures. Semi hot springs show significantly higher Li (34.2 to 35.6 mg/L) and Cs (49.8 to 52.7 mg/L) concentrations than Dazi (Li: 11.4 to 21.1 mg/L; Cs: 21.5 to 37.7 mg/L). Isotopic contrasts further differentiate the areas: Semi exhibits elevated δ7Li (1.53 to 1.91 ‰) and lower 87Sr/86Sr (0.739 to 0.741), whereas Dazi shows δ7Li values of − 0.25 to 1.24 ‰ and 87Sr/86Sr ratios of 0.742 to 0.759. Two key processes govern enrichment: (1) atmospheric recharge infiltrates Li–Cs-rich strata, where high-temperature water–rock interactions (217 °C at Semi and 197 °C at Dazi reservoirs) mobilize these elements into geothermal waters; (2) phase separation during ascent causes differential steam loss (Semi: 24%, concentration factor 1.32; Dazi: 8 to 21%, 1.08 to 1.26). Secondary processes (cold water mixing, conductive cooling, mineral adsorption) further modify surface hot springs geochemistry. Semi-Dazi geothermal field illustrates how a shared geothermal system can yield chemically distinct fluids from separate reservoirs characterized by differing hydraulic connectivity and circulation pathways. Geyserite deposits and high reservoir temperatures suggest that a crustal partial melt layer adds extra heat, intensifying water–rock reactions. The occurrence of Li–Cs-rich springs on the Plateau is intrinsically linked to elevated concentrations of these elements in underlying crustal source rocks and spatially associated with deep, extensive fault systems, particularly at fault convergences. These findings underscore the necessity of multi-isotope models for interpreting geothermal Li–Cs anomalies in continental collision zones, with implications for strategic mineral exploration.

Nonlinear Seismic Inversion of Total Organic Carbon for Hydrocarbon Source Rocks

Seismic prediction of hydrocarbon source rocks critically relies on total organic carbon (TOC) estimation from seismic data, which represents a critical yet challenging task for resource potential assessment. Common TOC inversion methods rely on statistical correlations between TOC and seismic parameters, lacking a direct physical connection to seismic data. To address this gap, a seismic reflection coefficient equation incorporating TOC is derived by integrating a published rock-physics model of hydrocarbon source rocks and the Gray approximation of the Zoeppritz equation. The rock-physics model quantifies relations between elastic properties and TOC of hydrocarbon source rocks at varying maturity stages. Gray approximation links P-wave reflection coefficient to reflectivity of rock elastic properties, assuming small property differences at interface. To ensure the accuracy of the seismic reflection coefficient equation incorporating TOC and to guarantee the reliability of TOC inversion results, the derivation circumvents approximations to the rock-physics model. Theoretical model tests and comparisons confirm that the seismic reflection coefficient equation incorporating TOC effectively portrays the quantitative relationship between seismic data and TOC. Building upon the seismic reflection coefficient equation incorporating TOC, a nonlinear seismic inversion of TOC is proposed. Within a Bayesian framework, assuming the prior probability density function conforms to the mixed Gaussian model and the likelihood function conforms to the Gaussian model, the posterior probability density function of TOC is deduced. Given the nonlinearity of the posterior probability density function of TOC and the independence of each posterior probability density component, a differential evolution Markov chain Monte Carlo algorithm is employed to simulate different posterior probability density components of TOC. Synthetic data tests and field data applications validate the efficacy and stability of the proposed nonlinear seismic inversion method in TOC prediction. The resultant TOC inversion profiles finely delineate the spatial distribution of hydrocarbon source rocks.

Instantaneous petroleum charge in the Shunbei reservoirs, Tarim Basin

Instantaneous petroleum charge is uncommon in sedimentary basins, particularly in the Tarim Basin, where petroleum has accumulated from multiple source rocks with multiple episode charge stages. In the present study, an instantaneous petroleum charge was identified within the No.3 fault zone of the Shunbei reservoirs in the Tarim Basin. The oil from Well Shunbei 3 (SHB3 oil) is considered as an end member close to the stratigraphic maturity of source rock at a certain maturity level with evidence from n-alkanes, isoprenoids, steroids, terpenoids, diamondoids and a series of aromatic hydrocarbons. In comparison to oil samples from the No.1 fault zone, SHB3 oil did not receive the early-charged oil whereas it has not undergone petroleum charging during the very late maturation stages. The end member of instantaneous petroleum charge in Shunbei reservoirs provides valuable insights into petroleum generation, expulsion and charge histories of ultra-deep Ordovician reservoirs in the Tarim Basin.

Application of image analysis to quantify maceral composition of source rocks: Examples from the Devonian New Albany Shale and Marcellus Shale

Application of image analysis to quantify maceral composition of source rocks: Examples from the Devonian New Albany Shale and Marcellus Shale

Source Rock Evaluation in the "Idea" Field, Bintuni Basin, West Papua: A Geochemical Approach

The Bintuni Basin is one of the largest hydrocarbon-bearing basins in Eastern Indonesia, although only a limited number of oil and gas fields are currently. Consequently, further investigation into its petroleum system, particularly the potential of its source rocks. In hydrocarbon exploration, characterizing source rock is critical to evaluating the presence of organic-rich strata capable of generating hydrocarbons. Geochemical analysis is a widely used method for assessing source rock potential, utilizing total organic carbon (TOC), rock-eval pyrolysis, and vitrinite reflectance data. In this study, geochemical data were obtained from one well and two outcrop samples to evaluate the quantity, quality, and thermal maturity of organic matter. The geochemical assessment of rock samples from four stratigraphic formations indicates that the Permian Ainim Formation exhibits the highest source rock potential. TOC values range from 1% to 80% with hydrogen index (HI) values range between 13 and 431 mg HC/g TOC classifying the formation as a good to excellent potential. The dominant organic matter consists of kerogen type II/III suggesting the potential for both oil and gas generation. Thermal maturity analysis indicates that the Ainim formation has reached the oil and gas generation window at depths of 8,075–8,420 feet. These findings demonstrate that the Ainim Formation represents a significant source rock within the Bintuni Basin, contributing valuable insights into the region's petroleum system and hydrocarbon prospectivity.

The Bakken Model: Deposition of Organic-Rich Mudstones and Petroleum Source Rocks as Shallow-Marine Facies Through the Phanerozoic

Many organic-rich marine mudstones, which are key hydrocarbon sources, were deposited on continent margins in mid-water oxygen-minimum zones (OMZs) that expanded and intensified during oceanic anoxic events (OAEs). Other marine hydrocarbon sources include platform and forearc black shales that record trans-continental, long-erm anoxic/dysoxic environments with no modern analog. Their explanation as recording deep-water, Black Sea-type basins or low-oxygen upwelling is not satisfactory for occurrences on shelves that lack significant epeirogenic activity, while modern studies show that upwellings do not cross the shelf break. The alternative is the Bakken model, which concludes that regionally extensive shelves and forearc organic-rich mudstones are shallow-water facies. These Bakken facies reflect hyper-warming conditions with high sea-levels, high water temperatures with increased insolation and low oxygen solubility, turbid water due to algal blooms and mud eroded from orogenic highlands, and possible LIP activity. Early Paleozoic black shales indicate that increased nutrients presumed to accompany the Devonian appearance of forests with deep roots that enhanced weathering simply cannot explain older Cambrian–Ordovician shelf anoxia/dysoxia. Shallow-marine deposition by the Bakken model is mandated by black shales deposited on subaerial unconformities that show high-energy facies (wave cross beds, HCS) and common bioturbation. The Bakken model explains shallow anoxia/dysoxia with high Paleozoic sea levels and tropical distribution of large continents. It is based on the Upper Devonian–lower Mississippian Bakken Formation (western U.S. and adjacent Canada). Rising temperatures, diminished oxygen solubility, and eustatic rise with deglaciation accompany modern climate change and mean that near-future platform seas will feature the reappearance of low-oxygen Bakken facies and environments.

Tight Sandstone Reservoir Characteristics and Sand Body Distribution of the Eighth Member of Permian Shihezi Formation in the Longdong Area, Ordos Basin

The eighth member of the Permian Shihezi Formation is one of the main tight sandstone gas layers in the Longdong Area of Ordos Basin, and the source rocks are dark mudstones and shales located in the Shanxi Formation and Taiyuan Formation of the Permian. The tight muddy sandstone at the top provides shielding conditions and constitutes traps. The lithology is mainly lithic quartz sandstone, followed by lithic sandstone. The reservoir space is mainly dissolved pores, inter crystalline pores, intergranular pores and so on. Clay minerals are the main interstitial materials, and chlorite has the highest content in it, a product of alkaline, moderate- to high-temperature, reducing conditions, effectively inhibited quartz cementation and enhanced secondary porosity development during mesodiagenesis. The average porosity of the reservoir is about 4.01%, and the average permeability is about 0.5 × 10−3 μm3, which is a typical low porosity and ultra-low permeability tight reservoir. The thickness of the sandstone reservoir in the study area is from 5 m to more than 25 m, mainly in the NE direction. The sand bodies are distributed in lenses on the plane.

Improving the Raman Model for Dravite and Schorl Tourmalines by μXANES Analysis of Iron Oxidation States

ABSTRACTTourmalines, a complex borosilicate mineral supergroup, are significant in geological studies due to their chemical and mechanical stability across various temperature and pressure conditions, making them useful as source rock indicators. A major issue in the characterization of tourmaline composition is the consideration of the iron oxidation state, which can significantly influence the distribution of elements at each site. This study reports the enhancement of a previous model that correlates Raman spectral parameters of dravite–schorl tourmalines with their composition, taking into account the Fe valence state in Y and Z sites measured through micro‐x‐ray absorption near edge structure (μXANES) spectroscopy. Raman spectroscopy was employed in a prior study on these two tourmaline species by correlating peak positions and intensities with differences in the magnesium–iron ratio. However, it was assumed that all iron was in the ferrous oxidation state (Fe2+), which led to a misrepresentation of the Fe3+ content in certain samples. The model has been thus implemented in this work by using μXANES, enabling the accurate quantification of Fe2+ and Fe3+ in dravite–schorl minerals, hence refining Mg/(Mg + Fe2+) ratios for Raman spectral analysis. Results demonstrate the validity of the correlation between Raman peaks in both the fingerprint and OH stretching regions and the magnesium–ferrous iron ratio. Our research confirms that Raman spectrum analysis is an effective method for recognizing tourmalines from the dravite–schorl series and evaluating their composition, including now the evaluation of the Fe2+ and Fe3+ occupancy. By integrating μRaman and μXANES techniques, one can acquire insights into the oxidation state of iron in tourmalines from the dravite–schorl series, thereby enhancing the accuracy of the Mg/(Mg + Fe2+) ratio. The observed linear correlations for P2 peak position, P1/P2 relative intensities, and WOH(3) peak position in the Raman spectrum enable the rapid identification of dravite and schorl tourmalines, as well as the retrieval of relative Mg and Fe2+ contents.

Geochemical characteristics of elements in sedimentary environment and its geological significance

Elements, especially inorganic elements, are geochemical tracers widely used to study the evolution of various geological processes, material sources and envi-ronmental changes, especially in the fields of ocean, environment, mineral de-posits and so on. The geochemical characteristics of elements in the sedimen-tary environment are of great significance for tracing the provenance of sedi-ments. The content of elements changes with the change of rock type, sedimen-tary process and sedimentary environment. Some elements with relatively sta-ble chemical properties are basically not affected by the weathering, denudation, redeposition and diagenesis of rocks, and can well reflect the provenance char-acteristics of source rocks. Other elements have different characteristics in dif-ferent sedimentary environments. Some elements are enriched in marine sedi-ments, while others are enriched in terrigenous sediments, and are affected by sedimentary fluid environment in this process. For example, the enrichment degree of some metal elements will also change in different oxidation-reduction environments. With the process of sedimentary evolution, trace elements will also be dispersed and enriched during the formation, sedimentation and burial of sedimentary organic matter. Therefore, the analysis of the geochemical char-acteristics of elements in the sedimentary environment is of great significance for identifying the provenance and tectonic setting of sediments.

Unravel subseafloor hydrothermal leaching and magmatic degassing during chimney formation at Kolumbo volcano

Hydrothermal chimneys are the upmost expression of fluids, metals and ligands transfer from the subseafloor to the hydrosphere, eventually forming seafloor massive sulfides. In volcanic arc settings, both magmatic and hydrothermal fluids occur together. While each fluid reflects different metal mobilizing mechanisms (i.e. magmatic degassing and hydrothermal leaching of subseafloor lithologies), it is unclear which metals they respectively provide to the budget of the chimneys. We investigate the metal sources and mobilizing mechanisms associated with a gold-rich hydrothermal field from Kolumbo volcano (South Aegean Volcanic Arc, Greece) by comparing Pb isotope ratios of ore minerals from a chimney with those of potential source rocks. Four key findings result from our study: (1) Kolumbo volcanic rocks are the main source of Pb for the chimneys; (2) Magmatic assimilation of Cycladic Basement allows to track magmatic differentiation and identify metal mobilizing mechanisms for Pb and metals with similar behavior. At Kolumbo, magmatic degassing mobilizes As, Ag, Au, Cu, Hg, Sb, Sn and Zn along with Pb, while hydrothermal leaching of rhyolite provides Tl and likely some base metals to the chimneys; (3) Magmatic fluids contributed to galena and Sb-Pb sulfosalts formation while pyrite formed from hydrothermal fluids leaching rhyolite; (4) Galena growth zones in pyrite reveal episodic pulses of magmatic fluids during the chimney growth. The combined use of Pb isotopes on ore minerals and source rocks provides an additional tool to discriminate between magmatic and hydrothermal fluids contribution during seafloor massive sulfide formation, especially in arc settings where magmatic assimilation of crustal material with distinct isotopic signature is more likely to occur.

Using detrital zircon to reconstruct Neoproterozoic crustal thickness variation in the northwestern margin of the Yangtze Block

Trace element geochemistry and chronology of zircon are reliable tools for reconstructing sediment provenance and crustal evolution, particularly in contexts where early crustal and rock records are sparse. We hereby employ in-situ U-Pb dating of Neoproterozoic detrital zircons to refine our understanding of sediment sources, tectonic settings, and crustal evolution along the northwestern margin of the Yangtze Block. Detrital zircons from the Doushantuo and Dengying formations exhibit similar Neoproterozoic age distributions (700–950 Ma). The lithology of the zircon source rocks correlates with the bimodal volcanic rocks extensively developed in the Micangshan–Hannan region along the Yangtze Block’s northwestern margin. Using Eu/Eu* ratios derived from detrital zircons, we reconstructed crustal thickness variations in the northwestern Yangtze Block during the Neoproterozoic. The crustal thickening from 1000 to 850 Ma, thinning between 850 and 730 Ma, and thickening between 730 and 539 Ma. The U/Yb-Nb/Yb, Nb/Hf-Th/U, U/Yb-Hf, and U/Nd ratios of the Neoproterozoic detrital zircons mainly suggest island arc or orogenic features. The zircons younger than 850 Ma suggest progressively depleted mantle-type characteristics and extensional intra-plate. The fluctuated Th/U ratios exhibit a general trend of increase during 1000–730 Ma, which is followed by a decrease. The U/Yb ratios (mostly > 0.1) show a decrease from 1000 to 820 Ma and an insignificant change from 820 to 730 Ma followed by an increase. The estimated crystallization temperatures of the dated detrital zircons, calculated using the Ti-in-zircon geothermometer equation, reveal a general temperature increase during 1000–730 Ma, followed by a gradual decrease. The Ce/Nd ratios (oxygen fugacity) show a fluctuation but a general increase between 820 and 730 Ma possibly due to heat influx from the subduction slab rollback and sediment melting. All the trace element analyses of detrital zircons indicate that subduction along the northwestern margin of the Yangtze Block persisted until 730 Ma and the slab rollback around 850–730 Ma. It also supports the hypothesis that the South China Plate was situated at the periphery, rather than the interior, of the Rodinia supercontinent. After 730 Ma, the Yangtze Block experienced internal extension and rifting, forming rift basins. However, the northwestern margin of the Yangtze Block continued to collide with multiple microcontinents, resulting in crustal thickening. After ca.625 Ma, crust extension led to a decrease in crustal thickness, which is also consistent with the subduction environment indicated by the cumulative distribution function (CDF) plot of detrital zircon ages.

Unique bimodal oil generation of alkaline-saline lacustrine source rock: Evidences, model and mechanism of organic-inorganic interactions

Unique bimodal oil generation of alkaline-saline lacustrine source rock: Evidences, model and mechanism of organic-inorganic interactions

Pore–Fracture Structure and Fractal Features of Carboniferous Taiyuan Formation Hydrocarbon Source Rocks as Investigated Using MICP, LFNMR, and FESEM

The pore structure of reservoir rocks was a crucial factor affecting hydrocarbon production. Accurately characterized the micropore structure of different types of rock reservoirs was of great significance for unconventional natural gas exploration. In this study, multiple observation methods (field emission scanning electron microscope (FESEM) and low-field nuclear magnetic resonance (LFNMR)) and physical tests (mercury injection capillary pressure (MICP)) were employed, and double logarithmic plots for fractal fitting were illustrated. The fractal dimension of 15 samples was calculated using fractal theory to systematically investigate the pore–fracture structure and fractal characteristics of hydrocarbon source rock (limestone, mudstone, and sandstone) samples from the Late Carboniferous Taiyuan Formation in the Huainan coalfield. MICP experiments revealed that sandstone reservoirs had larger and more uniformly distributed pore throats compared to mudstone and limestone, exhibiting superior connectivity and permeability. The T2 spectrum characteristic maps obtained using LFNMR were also consistent with the pore distribution patterns derived from MICP experiments, particularly showed that sandstone types exhibited excellent signal intensity across different relaxation time periods and had a broader T2 spectrum width, which fully indicated that sandstone types possess superior pore structures and higher connectivity. FESEM experiments demonstrated that sandstone pores were highly developed and uniform, with sandstone fractures dominated by large fractures above the micrometer scale. Meanwhile, the FESEM fractal dimension results indicated that sandstone exhibits good fractal characteristics, validating its certain oil storage capacity. Furthermore, the FESEM fractal dimension exhibited a good correlation with the porosity and permeability of the hydrocarbon source rock reservoirs, suggesting that the FESEM fractal dimension can serve as an important parameter for evaluating the physical properties of hydrocarbon source rock reservoirs. This study enriched the basic geological theories for unconventional natural gas exploration in deep coal-bearing strata in the Huainan coalfield.

Geochemical characteristics and origins of natural gases in Yinggehai and Qiongdongnan Basins of South China Sea

DF13-2 and YC13-1 are two large gas fields found in Yinggehai and Qiongdongnan Basins, respectively, and the gas-source correlation is a very challenging scientific exercise due to the similarity of their hydrocarbon isotoperatios and gas dryness. In this study, the chemical and isotopic compositions of natural gases (including rare gases and bulk gases) and the biomarkers of condensate were comprehensively integrated to identify their origins. The gases from the two fields are composed primarily of methane (87–91%), and isotope ratios of hydrocarbon compounds exhibit a thermogenic origin, with δ13C1 values ranging from − 40.7 to 35.0‰, and δ13C2 from − 27.0 to − 25.4‰. There are small yet distinct differences in the genetic characteristics of CH4 He, Ar, CO2, N2 and condensate between the samples from two regions. The δ13C1 values of DF13-2 (− 36.8 to − 33.7‰) are slightly larger than those (− 40.7 to − 35.5‰) of YC13-1, suggesting a slightly higher thermal maturity in DF13-2 field. In addition, DF13-2 gases contain lower Ar (0.0096–0.016%) and He (11–15 ppm). The 3He/4He ratios range from 3.31 × 10−8 to 7.72 × 10−8, indicating a typical crustal origin, likely representative of pristine source-rock signatures. The gases have higher N2 (8.4–9.66%) but lower CO2 contents (0.10–0.36%), which are co-generated with the thermogenic hydrocarbon gases. The condensate coexisted with gas contains abundant terrestrial-derived oleanane but a low abundance of bicadinanes. These characteristics correlate well with the Miocene neritic shales in Yinggehai Basin. Fractures associated with the diapiric activity provide key conduits for gas up-migration into the Huangliu Formation reservoir, implying that the flank of a diapiric structure is a favorable site for gas accumulation. In contrast, the gases from YC13-1 field show lower N2 but higher CO2 amounts. He and Ar abundances are both higher with a small amount contribution (2–9%) of mantle-derived He. The elevated amounts of mantle-derived 3He may come from mantle-enriched groundwater circulating in the petroleum system. The coexisted condensate is rich in terrestrial biomarkers such as oleanane and bicadinane, which is significantly different from DF13-2 condensate, and has a close affinity with the coal-bearing source rocks of the Oligocene Yacheng Formation. Unconformities and faults serve as important conduits for lateral and vertical migration from the source rocks to the traps. This suggests that short-distance migration and source facies control the distribution of the natural gases in Qiongdongnan Basin. This study provides novel insights into the origin and accumulation model of natural gases in Yinggehai and Qiongdongnan Basins.

Geothermal Regime and Its Implication on Hydrocarbon Accumulation in the Northern Junggar Basin

ABSTRACTThe geothermal regime of a sedimentary basin plays a critical role in the formation and distribution of hydrocarbon resources. However, the lack of measured temperature data has hindered a comprehensive understanding of the geothermal regime, complicating the analysis of hydrocarbon accumulation processes in the northern Junggar Basin. This study validated and applied a predictive method based on mantle and crustal heat flow to estimate heat flow in regions without measured temperature data. By integrating data from existing and pseudo boreholes, the present geothermal regime of the northern Junggar Basin was analysed. Thermal histories were reconstructed for different regions, and the maturation processes of source rocks were further examined. Correlation analysis confirmed that the mantle and crustal heat flow method provides a reliable approach for predicting heat flow in areas lacking direct measurements. The heat flow of the northern Junggar Basin varies between 31 and 59 mW/m2, with an average of 43 mW/m2. Spatially, the highest heat flow values were found in the southeast, followed by the west, with the lowest in the north. The spatial distribution of the geothermal gradient and deep formation temperatures aligns with heat flow patterns and is primarily influenced by basement topography and orogenic activity. Basin modelling results revealed that the southeastern region experienced higher temperatures, leading to the maturation of source rocks during the Permian and their progression into the gas generation stage. In contrast, source rocks in the western region remain in the oil generation stage. Similarly, lower temperatures in the northern region have preserved source rocks in the oil generation stage. This study provides a detailed understanding of the thermal regime in the north of Junggar Basin and its impact on hydrocarbon accumulation processes, offering valuable insights for future hydrocarbon resource exploration and evaluation.

Adsorption-induced negative carbon isotope sequence in over-mature coal-type gas from the southwest Ordos Basin

The carbon isotope sequence of alkanes is a key indicator used to distinguish organic from inorganic gas. A negative carbon isotope sequence (i.e., δ13C1 > δ13C2 > δ13C3 > δ13C4) is a characteristic feature of inorganic gas. Some gas samples from the Qingyang gas field in the southwestern Ordos Basin exhibit a negative carbon isotope sequence, but the geological conditions necessary for the development of inorganic gas are not present. There is currently no reasonable explanation for this phenomenon. This study takes into account the geological background of the Ordos Basin and comprehensively investigates the causes of the anomalous carbon isotope sequence of alkanes in the Qingyang gas field; this is done through an analysis of natural gas geochemical characteristics and adsorption/desorption experiments on high-rank coal. Our results show that: (1) Natural gas from the Qingyang gas field is over-mature coal-type gas derived from Carboniferous‒Permian formations. Its negative carbon isotope sequence is mainly related to the adsorption of gases by coal. During the over-mature stage, the content of heavier hydrocarbon gases (C2+) is very low, and the adsorption capacity of coal for C2+ gases is stronger than that for methane. Heavier hydrocarbon gases (e.g., ethane), with lighter carbon isotope signatures, preferentially desorb, resulting in a relatively light observed carbon isotope composition. Owing to its high abundance, the isotopic composition of methane is impacted relatively little by adsorption. (2) The anomalous geochemical characteristics of over-mature coal-type gas result in the failure of the negative carbon isotope sequence method for identifying inorganic gas; this also invalidates the criterion for classifying oil-type gas and coal-type gas based on thresholds of δ13C2 = − 28‰ and δ13C3 = − 25‰. Additionally, previously proposed empirical formulae (e.g., δ13C2‒Ro and δ13C3‒Ro) are not applicable to over-mature natural gas. Furthermore, the δ13C2−δ2H1 cross-plot method, used for determining the type of source rock, is rendered ineffective because over-mature samples deviate from the coal-type gas range. (3) The methane carbon isotope signature (δ13C1) and gas dryness coefficient (C1/C1–5) are reliable indicators of source rock maturity.

Extensive Early Cretaceous arc-like magmatic rocks in central Tibet manifest subduction rollback of the Neo-Tethyan ocean

Spatiotemporal distribution of magmatism in continental arcs is generally accompanied by compositional change of igneous rocks. However, it remains unclear whether and how the variation footprint and geochemical affinities of magmatism in fossil magmatic arcs are effective for reconstructing subduction polarity. In this study, new geochronological, mineralogical, geochemical, and Sr-Nd-Hf isotope data are presented to characterize the Early Cretaceous (ca. 130−110 Ma) bimodal volcanic rocks of the Zenong Group in central Lhasa, central Tibet. Our data show that the Zenong Group volcanic rocks are dominated by rhyolite and dacite, subordinate basalt, and local andesite. The basalts have enriched Sr-Nd-Hf isotopic compositions and arc-like trace elements, suggesting a lithospheric mantle source in the spinel stability field with minor asthenospheric mantle contributions. The andesites have similar Mg# values and isotopic compositions, indicating a fractionation origin from the basaltic magmas. The coeval dacites and rhyolites display relatively low Mg# values and variable isotopic compositions, pointing to an ancient lower crust source with minor mantle contribution. The bimodal compositional characteristics and contrasting magma sources of the volcanic rocks indicate an extensional setting. We propose that the northward migration of the Early Cretaceous (ca. 130−110 Ma) magmatism in central Lhasa is the result of backarc rifting associated with slab rollback of the subducting Neo-Tethyan oceanic plate. The backarc rifting model helps to reconcile ca. 130−110 Ma sedimentary-magmatic evolution records in broader region, including the Xigaze forearc spreading, magmatic lull in the Gangdese arc, and volcanic−sedimentary rocks in the central Lhasa subterrane.