Basin Formation Research Articles

Age and sources of the kodar group rocks of the udokan complex (Aldan shield): results of geochemical, U–Th–Pb (LA-ICP-MS) geochronological and Nd–Hf isotopic studies

The paper presents the results of geochemical and Nd whole rock isotopic studies, as well as U–Th–Pb (LA-ICP-MS) geochronological and Hf isotope studies of detrital zircon from metaterrigenous rocks of the Kodar Group of the Udokan Complex, Aldan Shield. It has been established that the rocks of the Kodar Group have an age of 1.99–1.91 Ga, and the rocks of the Chinei and Kemen groups of the Udokan Complex are in the range of 1.90–1.87 Ga. This allows us to raise the question of identifying the Kodar Group as an independent stratigraphic unit. Archean igneous and metamorphic rocks of the Chara-Olekma Geoblock and, probably, the Kalar and Kurulta blocks of the Stanovoy suture zone, as well as Paleoproterozoic (2.04–1.99, 2.08, 2.20 and 2.30 Ga) complexes of active continental margins or ensialic island arcs in the western–northwestern and southern (in modern coordinates) framing of the Chara-Olekma Geoblock, not identified in the region on the modern erosion level, were the sources of terrigenous rocks of the Kodar Group. Erosion of rocks of the igneous arcs and the continental slope led to the accumulation of sediments of the Kodar Group in the retro-arc foreland basin setting, and the subsequent collapse of the orogen and the formation of an intracontinental extension basin led to the accumulation of terrigenous rocks of the Chinei and Kemen groups. Obtained data indicate widespread previously unidentified Paleoproterozoic continental crust formation at about 2.04–1.97 Ga in the western part of the Aldan Shield.

Mesozoic Evolution of the Southeastern Tibetan Plateau From Sediment Provenance of the Qamdo Basin: Implications for Proto‐Plateau Development

AbstractProvenance analysis of the Qamdo basin reveals convergent tectonic evolution of the southeastern Tibetan plateau region from initiation as a Late Triassic foreland basin to a final phase as a Cretaceous intermontane basin. Sandstone composition and detrital zircon ages consistently reflect recycled sedimentary sources throughout the Mesozoic, with only minor evidence for contributions from magmatic sources. Detrital zircon with Mesozoic ages are rare (≲10% of all ages), with no <160 Ma zircon observed; instead detrital zircon ages form five main age populations (230–330 Ma; 400–500 Ma; 650–1,200 Ma; 1,600–2,100 Ma and 2,400–2,600 Ma). These results indicate that during Late Triassic‐Early Jurassic time, sediment was transported from sources in the Yidun terrane across the Jinsha suture into the Qamdo depocenter as it formed a south‐facing foreland basin. Middle‐Upper Jurassic strata of the central Qamdo basin record local input from volcanic arcs of the Bangong‐Nujiang suture zone as the Meso‐Tethys ocean closed and a north‐facing foreland developed in the Qamdo region. The youngest Cretaceous deposits reflect coarse‐grained alluvial deposition synchronous with development of east‐vergent growth‐strata, reflecting intermontane basin formation resulting from inboard stress transmitted from closure of Neo‐Tethys oceanic lithosphere due to convergence with the Lhasa block. By this time, the Qamdo basin region was a high‐standing depocenter developed within orogenic uplands that preceded formation of the high topography of the “Lhasaplano,” and is consistent with models suggesting proto‐plateau formation prior to Himalayan orogenesis.

Development characteristics of joints in the Chang 6-Chang 9 reservoirs of the Yanchang Formation in southern Ordos Basin, China

As the most important source rock of the Ordos Basin, China, the Chang 6-Chang 9 reservoirs of the Yanchang Formation have developed abundant of joints. Understanding the formation, geometry and distribution is very important for flow modelling and shale gas exploration in the reservoir. To this end, we investigate the development characteristics of joints in the Chang 6-Chang 9 reservoirs based on field joint mapping and statistical analysis. A total of 1574 joints were mapped across 43 outcrops, which can be divided into three groups: NW-SE, N-S, and NEE-SWW. The distributions of joint spacing ( s ) and trace lengths ( l ) and their dependence on bed thickness ( t ) are statistically analyzed. We find that the distributions of joint trace lengths follow exponential distributions, while joint spacing distributions are well expressed by lognormal distributions. Besides, both the s - t and l - t relationships fit linear distributions. Here, a total of 12 sandstone layers are selected as representative layers to study the condition of joint saturation. Statistical analysis shows that the joint spacing distribution of the sandstone gradually transforms from a skewed lognormal distribution to a normal distribution as s/t decreases from 1.51 to 0.54, indicating that the development of joints is gradually approaching saturation.

A high‐resolution climatological study of explosive cyclones in the Mediterranean region: Frequency, intensity and synoptic drivers

AbstractIn the complex context of climate change and in a heavily populated area like the Mediterranean region, understanding and studying explosive cyclones (ECs) is crucial because of their potential to cause extreme weather events, including strong winds, heavy rainfall, and significant disruptions to human activities and infrastructure, as happened in 2018 with the Vaia storm. The genesis and persistence of ECs, characterized by a rapid drop in central mean sea level pressure exceeding 12 hPa in 12 hours, are influenced by various physical processes and while water vapour convergence in the mid‐low troposphere is crucial, mid‐upper tropospheric cyclonic vorticity advection and upper tropospheric divergence also play significant roles. In this study, we used ERA5 hourly data of mean sea level pressure from 1979 to 2020 to analyse EC occurrences in the Mediterranean. The identification of EC events followed the 12‐hr deepening‐rate criterion introduced by Zhang et al. (2017), as shorter deepening rates definitions proved unreliable. A total of 80 ECs were identified and classified based on their spatial occurrence and magnitude, 34 of which were concentrated in the northwest of the basin, likely due to orographic deformation caused by the Alps on baroclinic waves originating from the Atlantic. The study also identified recurring synoptic patterns associated with the persistence of ECs over the sea, including the presence of potential vorticity (PV) streamers and mid‐tropospheric dry‐air intrusions. Empirical Orthogonal Function (EOF) analysis highlighted the Scandinavian pattern as the primary driver, with blocking conditions over western Russia and Scandinavia, and composite analyses revealed the intrusion of PV to various atmospheric levels, extending up to 500 hPa. In summary, this study offers valuable insights into the complex mechanisms governing EC formation in the Mediterranean Basin, shedding light on the synoptic patterns, atmospheric dynamics, and potential impacts on regional weather systems.

High-resolution cyclic framework for the Songliao Basin in northeastern China, and its implications for sedimentation and organic matter enrichment

The study of fine-grained sedimentation has consistently concentrated on investigating the mechanisms and principles governing the enrichment of organic matter. However, the lack of unified stratigraphic framework has always existed as fine-grained sedimentation covers two distinct grain-size grades, namely, mud and silt, which has impeded the progress of subsequent production research. This study exemplified this issue by analyzing the first member of the Qingshankou Formation in the southern Songliao Basin. We established reconstructed gamma and density curves that mitigated filter noise interference, integrated high-resolution sequence results with astronomical cycle divisions, and created a high-frequency isochronous stratigraphic framework for clastic fine-grained sedimentation by leveraging the weak sensitivity of sandstone density curves and the robust stability in eccentricity cycle extraction. This approach addresses the inconsistencies in stratigraphic division methodologies and mismatched outcomes stemming from the use varying techniques to delineate mud and silt components within clastic fine-grained sedimentary sequences. Furthermore, it elucidates how tectonic-scale variations in sediment supply coupled with potential accommodation changes dictate macroscopic stacking patterns within strata, whereas climate fluctuations on orbital time scales govern sand-mud progradation degrees within these layers, culminating in periodic rhythmic characteristics characterized by vertical sand-mud interbedding. A model for stratigraphic development pertaining to lake delta systems constrained by a “synchronous heterotopy” paradigm is proposed for the southern Songliao Basin. The organic matter enrichment pattern aligns with its filling dynamics, indicating an “overfilling” type developmental pattern at lower strata levels where organic material predominantly originates from terrestrial plant debris external to the basin; this material accumulates primarily within silty zones along layers—with areas exhibiting heightened enrichment values slightly lagging behind short-eccentric maxima positions. In contrast, under an upper “balanced filling” type developmental framework, sources of organic matter are derived both internally and externally relative to the basin—exhibiting substantial heterogeneity—and regions marked by elevated organic matter concentrations are directly associated with locations identified as short-eccentric maxima.

Three-dimensional spatial structure and heterogeneity characterization of #5 coal of Shanxi Formation in eastern Ordos basin

Based on macroscopic qualitative observation, microscopic quantitative analysis, low-temperature adsorption, high-pressure mercury intrusion and three-dimensional CT scanning, the spatial structure and heterogeneity of the #5 coal from the eastern Ordos Basin, Shanxi Formation, are quantitatively characterized. The following observation were studied: 1) The main body of the #5 coal was intact structural coal seam, with the density of cleats gradually increasing from north to south. 2) The microscopic composition of the #5 coal is dominated by desmocollinite, which contains small amounts of inertinite and inorganic minerals, with the inorganic component mainly composed of carbonate rocks and a small amount of clay. The carbon content and the content of its inorganic minerals in the #5 coal gradually decrease toward the inner basin, caused by transport distance and depositional environment. 3) At the micrometer scale, no significant pores were observed, and the #5 coal bed is primarily composed of micro-mesopores smaller than 40 nm, with a small number of 50∼100 nm macropores. Simultaneously, micro-fractures in the coal bed were well developed; by using mercury intrusion data of the bimodal feature of the pore throat, indicates the coexistence of micrometer-scale micro-fractures and nanometer-scale mesopores within the coal, with pronounced differences and interconnected complexity. 4) The porosity and permeability of microfractures, as well as the number of microfracture bars and permeability, indicate that the connectivity of coal bed is primarily caused by the microfracture system. By reconstructing the three-dimensional spatial structure of the #5 coal through multiple tests, the heterogeneous characteristics are quantitatively characterized.

The osteology of Triisodon crassicuspis (Cope, 1882): New insights into the enigmatic "archaic" placental mammal group "Triisodontidae".

Following the end-Cretaceous mass extinction, mammals underwent an increase in body size, taxonomic diversity and ecological specialization throughout the Paleocene, exemplifying their adaptability. One especially enigmatic group is the "Triisodontidae", medium- to large-sized ungulate-like placentals from the Paleocene which are best known from their teeth that exhibit adaptations towards carnivory. The "triisodontids" were the first large carnivorous mammals and pre-date, and may have given rise to, Mesonychia, a group of more specialized placental carnivores. The "triisodontids" have been well-described from dental material, although very little is known about their postcrania. Here, we describe the postcrania of Triisodon crassicuspis-the most completely represented species of the genus to date-from a specimen (NMMNH P-72096) recovered from basal Torrejonian strata of the Nacimiento Formation in the San Juan Basin, New Mexico. Anatomical comparisons reveal that the forelimb long bones of Tri. crassicuspis are robust relative to its size, more so than other "triisodontids". Attachment sites on the ulna are evidence of well-developed muscles involved in powerful extension and flexion of the manus. In Tri. crassicuspis, the range of pronation-supination was limited as evident from the humeroradial morphology. Qualitative functional assessment of osteological features of the forelimb of Tri. crassicuspis is suggestive of terrestrial locomotion with at least moderate digging ability. Re-analyses of the dentition confirmed that Tri. crassicuspis had specializations for carnivory, and provide a body mass estimate of ca. 32-44 kg based on dental proxies. In summary, Tri. crassicuspis was a relatively large and powerful terrestrial animal, and one of the first known placentals to fill a largely carnivorous niche.

Constraining timing of Proterozoic fault movement in the Capricorn Orogen, Western Australia

Ancient faults dissect low-grade Mesoproterozoic sedimentary rocks of the 1673–1455 Ma Edmund Basin and the 1455–1075 Ma Collier Basin in the Capricorn Orogen, Western Australia, and reveal a protracted history of basin formation and subsequent deformation. K–Ar dating of illite from fine-grained fault gouge separates (ideally <0.1 µm) provide robust dates of the most recent fault movement. From the Godfrey–Mount Vernon Fault, a sample of siltstone from the Kiangi Creek Formation (1590–1514 Ma) in the Edmund Basin, which was collected adjacent to faulting, provided a whole-rock date of 1506 ± 30.2 Ma. This is interpreted as a mixed age, encompassing modification of the depositional age by subsequent faulting during the extensional opening of the Edmund Basin. A fault gouge sample dated at 1141.4 ± 58.6 Ma, also from the Godfrey–Mount Vernon Fault, is consistent with the tectono-thermal 1321–1171 Ma Mutherbukin Tectonic Event identified in underlying high-grade Gascoyne Province basement rocks. Fault gouge sample dates of 885.5 ± 45.9 Ma from the Godfrey–Mount Vernon Fault and 782.2 ± 40.1 Ma from the Quartzite Well Fault define the younger newly identified crustal shortening during the Kuparr Tectonic Event. Concomitant tectonic activity in eastern Australia and South China has been considered as a possible precursor of the breakup of Rodina. In southern Africa, the Zambezi Supracrustal Sequence has been dated at 880–820 Ma and the Munali Intrusive Complex at 862–857 Ma also identified as rift-related.

Mg-C-O isotopes and elements reveal the origin of dolostone in the Middle Jurassic Buqu Formation

The origin of dolostone in the Middle Jurassic Buqu Formation of the Plateau Basin has been a subject of prolonged debate. This study combines detailed petrological observations with analyses of Mg-C-O isotopes and elements to constrain the origin of dolostones in the Buqu Formation. Petrography and cathodoluminescence (CL) examination identified three types of matrix dolostones: very finely to finely crystalline dolostone (D1), finely to medium crystalline dolostone (D2), and medium to coarsely crystalline dolostone (D3). The analysis of the diagenesis sequence reveals that D1 originated from the dolomitization of grainstone in the early diagenetic phase, whereas D2 and D3 resulted from the recrystallization of D1 during the later burial phase. The presence of high Na (>100 ppm), low Fe (<1000 ppm), low Mn (<250 ppm), positive Ce anomaly, LREE enrichment, stable δ26Mg (−2.28‰ to −2.04‰), and δ13C (1.02‰ to 2.95‰) indicates that the early dolomitization fluid was oxidized seawater. As the crystal size increases (D1→D2→D3), the progressively rising Mn content and significantly negative δ18O (−10.72‰ to −7.81‰) suggest that the dolostone has experienced modification and alteration by buried pore water in the later stages. The fluctuations in relative sea level during the sedimentary deposition of the Buqu Formation were reconstructed through the utilization of Na, Sr/Cu, Sr/Ba, Rb/Sr, ∑REE, and δ13C. It was observed that the δ26Mg of dolostone closely mirrored the variations in sea level. The consistent trend of change confirms that sea level fluctuations control the formation and distribution of early dolostone. Frequent sea level rise and fall prompted the limestone deposited on the carbonate platform to be continuously transformed into dolostone, which accumulates over a long period to form large-scale thick dolostone. After the formation entered the burial stage, under the combined action of high Mg/Ca ratio pore water, high temperature, and high pressure, the early dolostone experienced the adjustment of burial dolomitization. This research offers a typical case study on the application of Mg-C-O isotope and elements to determine the origin of dolostone. This will aid in a more comprehensive understanding of the formation process of dolostone in ancient rock records.

Evolution of the West African fold belts: review, new geochronological data, new correlations and new geodynamic hypothesis.

Several synthetic papers have been published about the geological structure of the threeWest African fold belts (Rokelides, Bassarides and Mauritanides), which were attached to the western margin of the West African Craton. Owing to the paucity of radiometric ages, some orogens and tectonic events have previously been confused and little consensus exists regarding their geodynamic evolution. Matching previous geological geochronological and geophysic data with nine new radiometric U-Pb ages on zircon, allows us to propose a new geodynamic model. Apart from ages of Mesoproterozoic events, four different orogens have been distinguished: Pan-African I (ca. 900 to 640 Ma), Pan-African II (ca. 640 to 520 Ma), Mali-Rokel River (ca. 490 to 450 Ma) and Variscan (ca. 450 to 300 Ma). The pan-African I orogeny is characterized by a complex evolution, terminating with a subduction process to the West, including the formation of a volcanic arc and a collisional event between ca. 660 and 640 Ma. The pan-African II orogen corresponds to a subduction process to the East leading to the building of a new volcanic arc and the formation of a back-arc basin to the East. Thenewly evidencedMali-Rokel River orogenic event led to the formation of extensive intra-continental N-S directed basins. The latter were folded and thrusted before the deposition of early Palaeozoic molasses. In the course of the Variscan Orogeny the Palaeozoicbasins setting in Mauritania and Northern Senegal were deformed during the Carboniferous Pangaea assembly. These two later orogensare intra-continental in the area but likely linked to far-field oceanic subduction to the West.For each orogenic period in the belts there are contemporaneous sedimentary deposits on the West African Craton and, of course, related unconformities between them. These data have facilitated correlations between the West African Belts and with the belts setting in adjacent areas like the Appalachians to the West and the northern Brazilian belts to the South.

FIRST RECORD OF UPPER CRETACEOUS (CAMPANIAN?) LARGER BENTHIC FORAMINIFERA FROM THE YIGEZIYA FORMATION OF THE SOUTHWESTERN TARIM BASIN, CHINA

Larger benthic foraminifera are described for the first time from the Upper Cretaceous Yigeziya Formation of the southwestern Tarim Basin, China. They occur in middle-outer ramp carbonates with debris of rudists, coralline algae, echinoids, pithonellids, and rare dasycladalean green algae. The material allows an open taxonomic approach for a few taxa. The assemblage includes Charentia cf. cuvillieri Neumann, Cuneolina cf. parva Henson, Nezzazatinella cf. picardi (Henson), orbitolinid gen. et sp. indet., cyclamminid indet., Tetrataxiella? floriforma Schlagintweit &amp; Sanders, Suturina? sp. and the new species Dictyopselloides? tarimensis n. sp. Lacking any resilient stratigraphic data in our material, a Campanian age is tentatively assumed. The benthic association is typical for Neotethys assemblages but also shows conspicuous overlap with the Campanian Tielongtan Formation of western Tibet.

Data-driven machine learning approaches for precise lithofacies identification in complex geological environments

ABSTRACT Reservoir characterization is a vital task within the oil and gas industry, with the identification of lithofacies in subsurface formations being a fundamental aspect of this process. However, lithofacies identification in complex geological environments with high dimensions, such as the Lower Indus Basin in Pakistan, poses a notable challenge, especially when dealing with limited data. To address this issue, we propose four common data-driven machine learning approaches: multi-resolution graph-based clustering (MRGC), artificial neural networks (ANN), K-nearest neighbors (KNN), and self-organizing map (SOM). We utilized these proposed approaches to assess their performance in scenarios with varying core sample availability, specifically evaluating their effectiveness in identifying lithofacies within the Lower Goru formation of the middle Indus Basin. The study reveals that in scenarios with a limited number of core samples, MRGC is the preferred choice, while KNN or MRGC is more suitable for larger datasets. The results demonstrate the superior performance of MRGC and KNN in lithofacies identification within the specified geological environment, with SOM following closely behind, and ANN exhibiting comparatively lower efficacy. The accurate identification of lithofacies from the selected model is complemented by the application of the truncated Gaussian simulation method for facies modeling. Comparative results confirm the excellent agreement between the model identification of lithofacies from well logs and electro-facies obtained from the truncated Gaussian simulation electro-facies volume. This study highlights the crucial role of selecting the right machine learning approach for precise lithofacies identification and modeling in complex geological environments. The comparative analysis provides practitioners in the petroleum industry with insights into the strengths and limitations of each method, enhancing existing knowledge. In conclusion, this research emphasizes the significance of comprehensive research and method selection for advancing lithofacies identification in diverse formations or study areas, ultimately benefiting the broader field of subsurface characterization in the petroleum industry.

Diagenesis of marine-continental transitional shale from the Upper Permian Longtan Formation in southern Sichuan Basin, China

Abstract Upper Permian Longtan Formation transitional shale has become an important exploration layer, but the Longtan Formation shale (LFS) has a complex mineralogical composition, which affects the subsequent diagenesis and diagenetic evolution, and restricts the subsequent geologic exploration of shale gas. In this article, the observation of drilling cores, argon ion polishing-scanning electron microscope, Vitrinite reflectance, and X-ray diffractometer were used to analyze the type and characteristics of diagenesis of the LFS and clarify the stage of diagenetic evolution. The results show that the main diagenesis in the LFS is compaction, cementation, thermal maturation of organic matter (OM), dissolution, and transform of clay minerals. Among them, OM hydrocarbon generation, clay mineral transformation, and dissolution are pore-enhancing diagenetic events. Compaction and cementation are pore-reducing diagenetic events. The transitional and marine shales have similar characteristics of diagenesis, but there are big variations in the diagenesis of OM hydrocarbon, authigenic quartz, and siderite. The complex depositional environments of the marine-continental transition environment have resulted in a variety of rock types, which in turn influenced the diagenesis types and diagenetic evolution process. Compared with the transitional shales of the Shanxi and Taiyuan formations in the South China North Basin, the LFS are characterized by high clay content, low quartz content, complex mineral compositions, and a higher degree of thermal evolution.

Evidence of a 4.33 billion year age for the Moon’s South Pole–Aitken basin

AbstractThe Moon’s farside South Pole–Aitken (SPA) basin is the largest and oldest visible impact basin in the inner Solar System. Determining the timing of this catastrophic event is key to understanding the onset of the lunar basin-forming epoch, with implications for understanding the impact bombardment history of the inner Solar System. Despite this, the formation age of the SPA basin remains poorly constrained. Here we show that the chemical composition of the lunar meteorite Northwest Africa 2995 is in good agreement with lithologies exposed within the southern region of the SPA basin. Radiometric dating of a range of mineral and rock components in Northwest Africa 2995 yielded consistent dates of ~4.32–4.33 billion years old. We interpret these dates as the age of SPA basin formation, inferring that this event occurred ~120 million years before the formation of the main cluster of lunar impact basins between ~4.2 Ga and 3.8 Ga. This weakens support for a narrow period of lunar late heavy impact bombardment and also implies that the earliest formed impact basins on the Moon (that is, &gt;4.33–4.5 Ga old) were erased either by the SPA impact itself when its formation caused massive resurfacing of the lunar surface or through other geological processes.

West Siberian sedimentary basin. Crustal subsidence caused by rock contraction in its lower part due to prograde metamorphism

The history of the crustal subsidence in the Mesozoic and Cenozoic in the West Siberian Basin – the largest sedimentary basin in the world – is considered. Most researchers associate its formation with post-rift subsidence of the crust, which followed an episode of strong lithospheric stretching about 250 million years ago near to the Permian to the Triassic transition. A characteristic feature of post-rift subsidence is a decrease in its rate in time. During the Mesozoic-Cenozoic history, in Western Siberia the rate of crustal subsidence should have slow down by an order of magnitude. However, the analysis of long (700–900 km) seismic profiles in the north of Western Siberia and in the Southern Kara Sea shows that since the beginning of the Mesozoic in these regions, on average, there has been an acceleration of the crustal subsidence. Under such circumstances, lithospheric stretching in them could be responsible for only a small part of the total subsidence of the crust of 6–7 km. In Western Siberia, the Earth’s crust is close to the isostatic equilibrium. Then, in the absence of strong stretching, the accumulation of thick sedimentary sequences in the basin could only have been caused by rock contraction in the lower crust due to prograde metamorphic reactions. To obtain the above results, we used, for the first time, some simple methods to analyze the structure of the sedimentary cover in the West Siberian Basin. Detailed seismic profiles have been published for many other deep basins on all the continents. The methods of their interpretation implemented in this paper can be easily applied to determine the role of lithospheric stretching in the formation of deep sedimentary basins on the global scale.

Mineralogical and geochemical studies of shales from Kopili Formation, Dima Hasao district Assam, North East India: Insights into diagenesis, deposition and provenance

The Eocene Kopili Formation in the Assam foreland basin records sediment sources, tectonic activity and depositional environments following the India-Eurasia collision. Mineralogical and geochemical analysis of these shales exposed near Garampani, Assam, NE India was conducted to study the diagenesis, palaeoweathering, palaeosalinity, redox conditions, tectonic settings and provenance. The Kopili Formation is composed of mainly shale with thin beds of limestone, black shale and sandstone and these units overlie the Upper Sylhet Limestone. The presence of goethite-rich phosphatic nodules, secondary precipitation within the fissile planes of shale and the occurrence of anatase, talc, smectite and chlorite suggest diagenesis. The prevalence of kaolinite and mean Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Al2O3/MgO and Rb/Sr values of 86.73, 97.26, 16.36 and 1.32, respectively indicates extensive source rock weathering in humid tropical climate. The Index of Compositional Variation (ICV) values ranging between 0.47 and 2.38 also signify high weathering and tectonically active basin. The Sr/Ba ratio 0.34 ± 0.27 (mean±2σ) suggest overall freshwater basin. The Ni/Co (2.84 ± 2.29), V/Cr (1.65 ± 0.45) and Ce/Ce∗ (0.97 ± 0.63) suggest fluctuating redox environment. The samples predominantly cluster in continental island arc domain on the Th-Sc-Zr/10, La-Th-Sc and Th-Co-Zr/10 discriminant diagrams. Elemental ratios of Th/Sc, La/Sc, Cr/Th with average 0.91, 3.18, 10.13 respectively and La/Th-Hf plot suggest a felsic source rock. The Kopili shale was deposited in freshwater, continental island arc basin with fluctuating redox conditions, receiving sediments from weathered Himalayan granites and gneisses under a warm and humid climate. Subsequently, they underwent diagenetic alteration by low-pH fluid(s).

Hydrocarbon accumulation and reconstruction in the Ediacaran Dengying carbonate reservoirs of the southeastern and central Sichuan Basin, SW China

The carbonate reservoirs of the Ediacaran Dengying Formation (Z2dn) in the southeastern Sichuan Basin (SESB) and central Sichuan Basin (CSB) experienced different tectonic movements during the Caledonian, Yanshanian, and Himalayan orogenies, resulting in distinct hydrocarbon accumulation processes in these two regions. Based on a detailed analysis of diagenetic sequences, pore-fluid pressure evolution, hydrocarbon charge history, and regional tectonic structural analysis, the hydrocarbon accumulation processes in the Dingshan and Ziyang areas can be divided into three stages, namely Triassic to Jurassic, Jurassic to Cretaceous, and from Late Cretaceous to the present. The first stage was characterized by the formation of paleo-oil pools, marked by an episode of oil charge in the Z2dn reservoirs, as evidenced by solid bitumen and bitumen-bearing inclusions. During the second stage, pore fluid pressure increased from normal to overpressure due to oil cracking in the favorable preservation conditions, leading to the formation of paleo-gas pools in the SESB and CSB. The timing of gas charge in the Dingshan and Ziyang area was at 173 Ma and 165 Ma, respectively, according to the trapping temperature of gas inclusions. The third stage was marked by a pressure drop from overpressure to normal pressure, possibly due to gas dissipation during the Yanshanian-Himalayan uplift. The destruction of paleo-gas pools in the Dingshan area, accompanied by gas leakage through faults and fractures, was caused by intense tectonic movement in the SESB. In contrast, the low gas productivity in the Ziyang area of the CSB was attributed to the migration of natural gas into the Weiyuan area. The fluid evolution of carbonate reservoirs and various tectonic characteristics reveal similar hydrocarbon accumulation but different reconstruction processes in the SESB and CSB, providing greater insight into the mechanism of hydrocarbon enrichment in the Sichuan Basin and other multi-cyclonic composite basins.

Structural analysis of fossilized tree trunks from the Miocene Mydlovary Formation of the South Bohemian Basin (Czech Republic)

Tree trunks were found in situ in Miocene strata of the Mydlovary Formation of the South Bohemian Basin. Our study presents microscopic and chemical tissue analyses of these tree trunks. The trees were fossilized and flattened. However, they are well preserved, which made the analyses feasible. Good preservation of the tissues is indicated by the high Tissue Preservation Index, the high content of cellulose, and the slightly gelified state of the wood. The Gelification Index indicated that there was only limited alteration during diagenesis. Huminite macerals prevailed over liptinite and inertinite macerals, and two varieties of ulminite and textinite were distinguished, differing in colour reflection, light fluorescence, and reflectance values. The biomarker composition in the extracts of the fossil wood obtained by gas chromatography-mass spectrometry, included terpenoid signatures, supporting a relationship to the Family Cupressaceae s.s.. Gradual burial of the trees is indicated by the fact that they lay crosswise on top of each other. The character of the sedimentary environment and the physiological differences of the trees could explain the deformation and the high degree of preservation.

Lithofacies and diagenetic controls on thrombolitic dolomite reservoir development from the Precambrian Dengying Formation in the central Sichuan Basin, SW China

Microbialite reservoirs are of great importance in oil and gas exploration. However, there is still a lack of comprehensive studies on the formation mechanisms of thrombolitic reservoirs, a specific type of microbialite. This research focuses on the oldest thrombolitic dolomite reservoir located within the Precambrian Dengying Formation in the central Sichuan Basin, southwestern China. A multi-disciplinary approach was employed to characterize different thrombolite facies and elucidate the formation mechanism of thrombolitic dolomite reservoirs and their controlling factors, involving core observation, thin-section analysis, cathodoluminescence, scanning electron microscope (SEM) microscopy, elemental analysis using LA-ICP-MS, and carbon and oxygen stable isotope analysis. Based on variations in texture, four types of thrombolite were identified: 1) distinct clotted thrombolite, 2) diffuse and regular clotted thrombolite, 3) diffuse and irregular clotted thrombolite, and 4) composite clotted thrombolite. Notably, the diffuse clotted thrombolitic dolomite is the prevalent lithology in the reservoir. Through modification by meteoric water, organic acid fluids, and hydrothermal fluids, a reservoir with dominant porosity in the form of primary growth-framework pores, dissolution pores, and vugs was formed. This resulted in the development of two high-quality reservoir intervals within the Second Member and at the top of the Fourth Member of the Dengying Formation. The growth-framework porosity of the thrombolites, epigenetic karstification, and tectonic fracturing were mainly conducive to reservoir development. However, various types of cementation have reduced porosity and connectivity within the reservoir. Overall, this study is a valuable example of the methodology required to understand meso- and microstructures of deep-buried thrombolitic dolomite reservoirs, including their heterogeneities and diagenesis, as the original structures influence diagenesis.

Rhenium-platinum group elements reveal seawater incursion induced massive lacustrine organic carbon burial

Organic carbon burial in ancient lacustrine settings is a crucial source of petroleum resources. Unlike the marine environment, the dynamics of organic carbon burial in the terrestrial realm are more complex due to the interplay of global and regional climate-tectonic factors. There appears to be a potential linkage between seawater incursion events (SWIEs) and the generation of lacustrine source rocks. However, reliable proxies to reconstruct the frequency and extent of SWIEs are currently lacking. Here, we explore the potential of rhenium-platinum group elements (Re-PGE) system as a novel proxy for determining SWIEs in the Nenjiang Formation of the lacustrine Songliao Basin in China that is noted for its high-quality source rock. By comparing marine and non-marine intervals, we validate the utility of Re-PGE fractionation patterns and osmium (Os) isotope compositions. Moreover, the Re/Ir ratios demonstrate two main episodes of quantitative seawater-lake water exchange. The comparison of variable indicators shows that the Re-PGE system is more sensitive to the changes in water sources, thus providing detailed information of frequency and exchange amount. The inverse variation between seawater contribution and total organic carbon content further implies that the massive sulfate influx from SWIEs facilitated bacterial sulfate reduction in the sediment pile, which had the effect of recycling nutrients (e.g., phosphorous) back into the water column. The SWIEs-triggered eutrophication induced a positive feedback loop between productivity and hypoxia, creating ideal conditions for the preservation of organic carbon. Our data reveals the detailed mechanism of SWIEs-triggered organic carbon burial and emphasizes the significant role of SWIEs in generating economically important hydrocarbon reservoirs.