Middle Permian Research Articles

Types, composition and diagenetic evolution of authigenic clay minerals in argillaceous limestone of sepiolite-bearing strata: A case study of Mao-1 Member of Middle Permian Maokou Formation, eastern Sichuan Basin, SW China

The types, occurrence and composition of authigenic clay minerals in argillaceous limestone of sepiolite-bearing strata of the first member of the Middle Permian Maokou Formation (Mao-1 Member) in eastern Sichuan Basin were investigated through outcrop section measurement, core observation, thin section identification, argon ion polishing, X-ray diffraction, scanning electron microscope, energy spectrum analysis and laser ablation-inductively coupled plasma-mass spectrometry. The diagenetic evolution sequence of clay minerals was clarified, and the sedimentary-diagenetic evolution model of clay minerals was established. The results show that authigenic sepiolite minerals were precipitated in the Si4+ and Mg2+-rich cool aragonite sea and sepiolite-bearing strata were formed in the Mao-1 Member. During burial diagenesis, authigenic clay minerals undergo two possible evolution sequences. First, from the early diagenetic stage A to the middle diagenetic stage A1, the sepiolite kept stable in the shallow-buried environment lack of Al3+. It began to transform into stevensite in the middle diagenetic stage A2, and then evolved into disordered talc in the middle diagenetic stage B1 and finally into talc in the period from the middle diagenetic stage B2 to the late diagenetic stage. Thus, the primary diagenetic evolution sequence of authigenic clay minerals, i.e. sepiolite–stevensite–disordered talc–talc, was formed in the Mao-1 Member. Second, in the early diagenetic stage A, as Al3+ carried by the storm and upwelling currents was involved in the diagenetic process, trace of sepiolite started to evolve into smectite, and a part of smectite turned into chlorite. From the early diagenetic stage B to the middle diagenesis stage A1, a part of smectite evolved to illite/smectite mixed layer (I/S). The I/S evolved initially into illite from the middle diagenesis stage A2 to the middle diagenesis stage B2, and then totally into illite in the late diagenesis stage. Thus, the secondary diagenetic evolution sequence of authigenic clay minerals, i.e. sepiolite–smectite–chlorite/illite, was formed in the Mao-1 Member. The types and evolution of authigenic clay minerals in argillaceous limestone of sepiolite-bearing strata are significant for petroleum geology in two aspects. First, sepiolite can adsorb and accumulate a large amount of organic matters, thereby effectively improving the quality and hydrocarbon generation potential of the source rocks of the Mao-1 Member. Second, the evolution from sepiolite to talc is accompanied by the formation of numerous organic matter pores and clay shrinkage pores/fractures, as well as the releasing of the Mg2+-rich diagenetic fluid, which allows for the dolomitization of limestone within or around the sag. As a result, the new assemblages of self-generation and self-accumulation, and lower/side source and upper/lateral reservoir, are created in the Middle Permian, enhancing the hydrocarbon accumulation efficiency.

Petroleum System Analysis and Burial History of Middle Permian Source Rock in Turpan–Hami Basin, NW China

The pre-Jurassic in the north depression of the Tuha depression is the most favorable replacement strata to obtain new reserves in the Turpan–Hami Basin. (Pre-Jurassic, in this paper, refers to the Permian and Triassic.) The main source rocks are the Taodonggou Group, of which the burial history and hydrocarbon generation potential remain unconfirmed. The investigation of the burial and thermal history is vital for the basin analysis and hydrocarbon exploration. Therefore, in this paper, by using the acoustic time difference method, vitrinite reflectance method, stratigraphic trend method and PetroMod-1D software, the differential characteristics of denudation thickness, burial history and thermal evolution history of different tectonic units in different periods of Taibei Sag in the Turpan–Hami basin are studied, and their influence on the petroleum system is analyzed, and then the zones with exploration potential are optimized. The results show that the Taibei Sag has experienced multiple tectonic uplift events. The Late Indosinian movement has profound effects on the Taodonggou Group source rocks. The rather large uplift amplitude postpones the maturation of source rocks. In addition, the Turpan–Hami Basin is a typical cold basin. Therefore, the thermal maturity of the source rocks is relatively low, with respect to the relatively deep burial. The thermal histories of the different sub-sags in the study area are slightly differentiated from each other. The Taodonggou Group source rocks in the Taibei Sag generally became mature during the Mid–Late Jurassic epoch, except for those in the Central–Southern Shanbei sub-sag, represented by Well LT-1, which reached the mature stage during the Late Triassic epoch. The study area has well-developed reservoir rocks, and effective reservoir bodies are formed in the slope zone and near the Tainan Sag, due to the higher porosity and permeability of reservoir rocks. The statistics related to the faults and an analysis of the structural styles of oil reservoirs indicate that the structural slope and anticline of the Huobei, Lianbei and Shanbei sub-sags are favorable for increasing reserves and production of hydrocarbons.

Urzhumian Pedocomplexes of the Kazan Volga Region (Eastern Europe): An Example of Paleoclimate and Paleolandscapes of the Subtropics of the Continental Pangea in the Middle Permian

The paleosol profiles of the Urzhumian from the section on the right bank of the Volga River near the village of Grebeni were studied by field and laboratory methods. Two paleosol profiles on red clayey siltstones were identified. They were classified as strongly developed Vertic Calcisols based on their pedogenic features such as horizonation, carbonate nodules, slickensides, etc. The climate during their formation was found to be seasonally wet and warm. The mean annual precipitation was calculated from the geochemical indices of the paleosols and amounted to 522 mm/year for the Late Urzhumian. The reddish-brown soils in the dry subtropics of the Mediterranean or Australia can be considered as modern analogs of these paleosols.The upper paleosol profile is a pedocomplex consisting of four single soils separated by nonpedogenic carbonate layers in the middle part. The pedocomplex structure contains one lacustrinepalustrine and two lacustrine-playa sedimentary microrhytms, which indicate the setting of lacustrinealluvial plains and make it possible to estimate the duration of its formation spanning thousands to tens of thousands of years.A pedocomplex with a similar structure was identified in the upper part of the Urzhumian of the reference section in the Cheremushka Ravine. Both pedocomplexes have very similar mineralogical, chemical, and granulometric composition of their siliciclastics, as well as the isotopic composition of pedogenic nodules. The biostratigraphic position of the pedocomplex in the Cheremushka Ravine shows that it is coeval with the pedocomplex of the Grebeni section. The correlation between them is direct, making them a useful pedostratigraphic marker. It is suggested to label this marker horizon as the “Crimson Clay pedocomplex”.

Final suture of the Palaeo-Asian Ocean domain: Insights from the geochemistry and geochronology of the early to Middle Permian Elitu Formation volcanics

The consumption of the Palaeo-Asian Ocean domain triggered substantial resources and environmental effects, but the final suture timing remains debated. This study presents the zircon U-Pb age and Hf-O isotope, whole–rock element and Sr–Nd–Pb isotope data from the Elitu Formation trachyandesite and rhyolitic tuff in the Bainaimiao arc belt. The zircon U-Pb ages (260–280 Ma) indicate that the Elitu Formation volcanism erupted at the Kungurian to Capitanian. Trachyandesites exhibit an arc-like whole–rock trace element composition and have enriched Sr–Nd–Pb isotopic composition (ISr(t) = 0.7062–0.7065; εNd(t) = −8.8 to −9.4; (206Pb/204Pb)t = 17.14–17.31) and high zircon δ18O (6.08‰–7.59‰) values, which are derived from the partial melting of the lithosphere mantle metasomatised by ancient sediment-derived melts. Rhyolitic tuffs have variable εHf(t) (−7.15 to +10.47) and low δ18O (4.99‰–6.21‰) values and exhibit affinities with I-type felsic rocks. They mainly originate from the partial melting of the basement of the Bainaimiao arc belt. Our compiled geochemical and geochronological data indicate that there were significant differences between the Early to Middle Permian Elitu Formation volcanic rocks and intrusive counterparts and the Early to Middle Triassic magmatic rocks from the Bainaimiao arc belt, and the Late Permian was the key tectonic transition period. The Late Permian transition was compatible with the regional crustal-shortening deformation, greenschist facies metamorphism and shift of deposition environment. This study argues that the Elitu Formation volcanism should be subduction-related and predate the final suture of the Palaeo-Asian Ocean domain at the Changhsingian. The suture of the Palaeo-Asian Ocean domain was not the original trigger of the Kungurian climate change in the North China Craton.

Multistage dolomitization influenced by the Emeishan large igneous province: Petrographical and geochemical evidence

The formation of large igneous provinces (LIPs) seems to require a series of conditions conducive to massive dolomitization; however, the specific mechanisms remain unclear. Based on petrographic, geochemical and fluid-inclusion microthermometry analyses, this study identifies the types of dolomites in the Maokou Formation (Middle Permian) in the eastern Sichuan Basin. It analyses the related dolomitization fluids and discusses the influencing mechanisms of the Emeishan large igneous provinces (ELIP) on dolomitization at different stages. Three types of dolomite were identified: (1) fabric-retentive, very finely crystalline, planar-s to nonplanar-a dolomite (MD1); (2) fabric-retentive, fine to medium crystalline, planar-s to nonplanar-a dolomite (MD2); and (3) medium to coarsely crystalline, nonplanar (saddle) dolomite (SD). MD1 exhibited dark-red luminescence under cathodoluminescence and rare earth element patterns that are similar to those of lime mudstone, indicating that its dolomitization fluid was medium-salinity Permian seawater. MD2 displayed orange-red luminescence under cathodoluminescence, with a high Ba content (mean: 397.5 ppm) and positive Eu anomalies (mean: 2.34), indicating that its dolomitization fluid was a mixture of shallowly buried seawater and a small amount of external hydrothermal fluid. SD showed red luminescence under cathodoluminescence, with the highest Ba content (mean: 9820.4 ppm) and significant positive Eu anomalies (mean: 3.165), indicating that its dolomitization fluid was mainly ELIP-related hydrothermal fluid with a small amount of residual seawater. Before the ELIP activity, no dolomite occurred in the study area. In the initial stage of ELIP activity, upwelling of the Emeishan mantle plume led to a shift in sedimentary patterns, promoting the occurrence of reflux dolomitization and the formation of MD1. In the intermediate stage of ELIP activity, the activation of basement faults promoted the occurrence of thermal convection dolomitization, resulting in MD2 formation. In the peak stage of ELIP activity, large-scale eruption of Emeishan basalt led to the development of hydrothermal dolomitization, forming SD. This study provides an approach to the investigation of the influences of LIPs on dolomitization processes.

The Upper Permian tubular fossils from South China and their possible affinity to sabellid polychaetes

ABSTRACT This study analyses intriguing tubular fossils derived from the Upper Permian deposits of southern China. The fossils’ structure is diagenetically altered by silicification precluding an unambiguous systematic assignment. However, the general morphology and tube architecture strongly imply a relationship with tube-dwelling organisms of different affinity such as tentaculitoids and tube-dwelling polychaetes. Due to the simple morphology of the tubes lacking any ornamentation, as well as the encrustation mode of some individuals, the investigated specimens most likely belong to early sabellids. Although calcareous sabellids persisted since the Middle Permian, their distribution and abundance during the Permian were confined to a single occurrence. If the fossils studied indeed represent sabellids, the global range and abundance of this group during the late Palaeozoic was likely more extensive than currently assumed.

Reservoir identification of marine facies carbonate shoals in the Middle Permian in the Sichuan Basin

Controlled by the base level change cycle, the carbonates exposed from the periodically shallowed water body at the top of the high-frequency sequence develops multiple sets of thin reservoirs. Such reservoir identification has always been tricky in the prediction of carbonate reservoirs. Based on the Middle Permian strata from the Sichuan Basin, this paper established a forecasting approach of high-frequency-sequence-controlled carbonate thin reservoir. The steps are as follows: (1) Combine the measured results of samples from the typical outcrops with changes of carbon and oxygen isotopes, thus to construct a division program for the 3rd-order sequence of the Middle Permian strata. (2) Compare the carbon and oxygen isotope changes and the global sea level change trend, determine the controlling factors of the 3rd-order sequences. (3) Carry out spectrum analysis based on the natural gamma energy spectrum logging (ln (Th/K)) curves of Wells Long-17 and Anping-1 to establish a high-frequency sequence framework division program, and then apply the logging curve characteristics to correct the top interface positions of the high-frequency sequences, which is the potential location of carbonate reservoir. The research results suggest that the Middle Permian in the Sichuan Basin can be divided into three 3rd-order sequences, which are PSQ1, PSQ2 and PSQ3 from bottom to top. Among which, PSQ1 is mainly affected by global sea level (GSL) changes, and the changes originated from its tuning curve can be directly applied to the division of high-frequency sequences and reservoir prediction. PSQ2 and PSQ3 are jointly controlled by global sea level changes and regional tectonics. In cope with the influence of regional tectonics, the positions of the four-level sequence interface established according to the tuning curve need to be fully considered, and good results were achieved through systematical correction. This method can effectively reduce the risk of reservoir prediction caused by marine carbonate heterogeneity, and provide scientific foundation for the prediction of high-frequency cyclic reservoirs of platform facies and platform margin subfacies in other marine carbonate sedimentary basins.

Geochemical and mineralogical characteristics of shales from the early to middle Permian Dohol Formation in Peninsular Malaysia: Implications for organic matter enrichment, provenance, tectonic setting, palaeoweathering and paleoclimate

The early to middle Permian Dohol Formation is characterized by a significant presence of shale deposits. While these shales exhibit a low potential to generate hydrocarbons, there is a need to ascertain the possible reasons for the low hydrocarbon generation potential. Also, there are several unidentified properties and attributes associated with these shales in terms of their inorganic geochemical characteristics and their mineralogy. This study is focused on using XRF, ICPMS, and SEM with EDX to determine the mineralogical and geochemical characteristics of these shales and use these data to discuss their provenance history and tectonic setting and interpret the paleoclimatic and paleoweathering conditions. The inorganic geochemical analysis shows that the shales from the Dohol Formation are from a felsic igneous source. The shales were also identified to be from a passive margin based on the bivariate plot of SiO2 vs log (K2O/Na2O) and several multidimensional diagram plots. The CIA and CIW data, as well as the A–CN–K plot, all point to a significant degree of chemical weathering, ranging from mild to intense. The Sr/Cu ratio and C-value, combined with various other geochemical proxies, indicate that the shales were formed in warm-humid climatic conditions. The SEM analysis shows that the samples are mainly composed of kaolinite and illite, and this result was supported by the EDX elemental composition. The high terrigenous influx of sediments, the oxic to sub-oxic conditions in which the sediments were deposited, and finally low marine productivity were found to be the reasons for the low TOC in the shales from the Dohol Formation.

A composite water saturation model of continental mixed shale oil reservoirs based on complex lithology identification

The complex lithology, frequent interbedding and strong heterogeneity of the shale oil reservoir in Lucaogou Formation of the Middle Permian in Jimsar Sag lead to the complexity of litho‐electric parameters and the low accuracy of conventional reservoir saturation evaluation methods. Therefore, in this paper, automatic lithologic identification was firstly performed with the optimization inversion method of mineral components and K‐means clustering algorithm, and then, litho‐electric experiments were carried out to determine the litho‐electric parameters of different lithologies. Furthermore, the shale oil reservoir saturation calculation models of different lithologies were optimized and determined. Then, a composite water saturation model (Composite‐SW) based on complex lithology identification was constructed. Finally, the model was verified and analysed with the experimental results of water saturation. The coefficient of correlation between the calculation results of the Composite‐SW model and the core experiment results reached .88, indicating that the model was much better than conventional models. The model can be widely applied in logging evaluation of the continental mixed shale oil reservoir in the study area. The study provides a new idea for oil saturation evaluation of continental shale oil reservoirs and is significant for evaluating other types of shale oil reservoirs.

Age and fluid source constraints of the Haoyaoerhudong orogenic gold deposit, North China: Evidence from geochronology and noble gas isotopes

The Haoyaoerhudong gold deposit, located in the northwestern part of the North China craton (NCC), has produced over 120 metric tonnes (t) of gold since 2007. It has a total reserve of > 240 t at average gold grade of 0.62 g/t, making it one of the largest open pit gold mines in China. The steeply dipping, large-tonnage, low-grade, vein- or veinlet-type gold orebodies are hosted in strongly-deformed Mesoproterozoic carbonaceous schist of the Bayan Obo Group. The laminated/boudinaged veins/veinlets in the sinistral ductile–brittle shear zones are dominated by quartz, biotite, gold-bearing löllingite, pyrrhotite, (arseno)pyrite, with minor native gold, titanite and xenotime. In this paper, we present new in situ U–Pb geochronological data on magmatic zircon from the pre-ore dikes, on metamorphic and hydrothermal xenotime, and on hydrothermal titanite from the hydrothermally altered carbonaceous schist and auriferous quartz–sulfides veins/veinlets, as well as He-Ar isotopic analysis on gold-bearing (arseno)pyrite in the syn-ore stage. The metamorphic xenotime U–Pb age of 426 ± 6.0 Ma (2σ) records a regional metamorphic event, possibly related to the accretion of the Bainaimiao arc onto the NCC. Two pre-ore andesitic dikes yielded similar emplacement ages at ∼ 278 Ma constrained by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb zircon data. Hydrothermal xenotime grains from the altered carbonaceous schist and auriferous quartz–sulfides veins yielded U–Pb ages of 256.0 ± 4.1 Ma (2σ) and 254.4 ± 2.1 Ma (2σ), respectively, overlapping with that of the hydrothermal titanite at 255.4 ± 0.8 Ma (2σ) from the laminated quartz–sulfides veinlets. This indicates that the gold mineralization occurred at ca. 255 Ma. The ∼ 255 Ma gold mineralization age is much younger than the previously reported Early–Middle Permian regional magmatic activity (ca. 291 Ma to 268 Ma), and may be associated with the regional sinistral strike-slip event in the late orogenic cycle related to the collision between the Siberian craton and the NCC. The 3He/4He (R/Ra) and 40Ar/36Ar values of the gold-bearing (arseno)pyrite are 0.04 to 0.09 (average = 0.07) and 375.8 to 2023 (average = 1045), which reveal the ore-forming fluids dominantly originated from the crustal rocks, with limited involvement from the mantle. Collectively, our new geochronological data, noble gas isotopic analyses, and geological evidence support a typical orogenic gold deposit model at Haoyaoerhudong.

The Geochemical Characteristics of Trace Elements in the Magnetite and Fe Isotope Geochemistry of the Makeng Iron Deposit in Southwest Fujian and Their Significance in Ore Genesis

The Makeng iron deposit in southwest Fujian is a significant iron polymetallic deposit containing various types of iron ore, including garnet magnetite, diopside magnetite, and quartz magnetite. The metallogenetic type of the deposit has been a subject of debate, particularly in relation to the genesis of magnetite and the source of iron. In situ microanalysis of trace elements in magnetite from different ores shows relatively low levels of V, Ti, Cu, and Zn, with higher concentrations of Ca and Si, indicating the characteristics of a skarn type deposit. The δ57Fe values of the magnetite range from −0.091‰ to 0.317‰. Combining these data, whole-rock iron isotope analyses, including Juzhou and Dayang granites, diabase, and the Lower Carboniferous Lindi Formation sandstone, suggest that Fe in the magnetite primarily originates from granitic pluton, with potential contributions from diabase and the Lower Carboniferous Lindi Formation sandstone. Combined with field work, these results indicate that Makeng iron deposit is a skarn-type magnetite deposit associated with Yanshanian granitic intrusions. Therefore, the initial ore-forming fluid is postulated to be a high-temperature magmatic hydrothermal fluid with high oxygen fugacity. This fluid infiltrates spaces such as interlayer fracture zones between the Upper Carboniferous Jingshe Formation–Middle Permian Qixia Formation carbonate rocks and the Lower Carboniferous Lindi Formation sandstone, resulting in diverse magnetite ores due to metasomatism. The mineralization process of the Makeng iron deposit is basically the same, as it is composed of typical skarn deposits. Magnetite was mainly formed during calcic skarn formation stage, and this process persisted until the initial phase of the retrograde alteration of skarns. In contrast, sulfide minerals, including molybdenite, sphalerite, and galena, precipitated during the quartz–sulfide stage.

Predatory synapsid ecomorphology signals growing dynamism of late Palaeozoic terrestrial ecosystems

Terrestrial ecosystems evolved substantially through the Palaeozoic, especially the Permian, gaining much new complexity, especially among predators. Key among these predators were non-mammalian synapsids. Predator ecomorphology reflect interactions with prey and competitors, which are key controls on carnivore diversity and ecology. Therefore, carnivorous synapsids may offer insight on wider ecological evolution as the first complex, tetrapod-dominated, terrestrial ecosystems formed through the late Palaeozoic. Using morphometric and phylogenetic comparative methods, we chart carnivorous synapsid trophic morphology from the latest Carboniferous to the earliest Triassic (307-251.2 Ma). We find a major morphofunctional shift in synapsid carnivory between the early and middle Permian, via the addition of new feeding modes increasingly specialised for greater biting power or speed that captures the growing antagonism and dynamism of terrestrial tetrapod predator-prey interactions. The further evolution of new hypo- and hypercarnivorous synapsids highlight the nascent intrinsic pressures and complexification of terrestrial ecosystems across the mid-late Permian.

Early-middle permian terrigenous deposits of South-Western Primorye: material composition, source areas and formation settings

The study presents the results of studying the material composition of terrigenous rocks from the Early – Middle Permian deposits of the Reshetnikovka formation of the Laoeling-Grodekovo terrane of South-Western Primorye. Studies were carried out in order to reconstruct paleogeodynamic environments for the accumulation of studied deposits, as well as to determine the tectonic type and composition of source rocks of sources areas. It has been established that in terms of mineral and geochemical parameters, the sandstones of the formation are petrogenic or “first cycle” rocks, correspond to arcoses and, only partially, subarcoses and lithite arenites, are characterized by a fairly high degree of maturity of clastic material, and their formation was due to the destruction of largely weathered parent rocks of sources areas. Paleogeodynamic interpretation of the obtained data indicates that in the Early and Middle Permian sedimentation occurred in the basins of the passive continental margin, which are intra-and intercontinental rifts and aulacogens. The main influence on sedimentation processes was exerted by continental sources areas: cratons and uplifted basement blocks, which were projections of the crystalline basement framed by rift zones. Mainly acidic igneous and metamorphic rocks were eroded with the participation of ancient sedimentary formations. U–Pb isotopic dating of detrital zircons made it possible to establish the age and possible location of magmatic complexes, due to the destruction of which formation deposits were formed.

The evolutionary history of Coleoptera (Insecta) in the late Palaeozoic and the Mesozoic

AbstractRecent progress in beetle palaeontology has incited us to re‐address the evolutionary history of the group. The Permian †Tshekardocoleidae had elytra that covered the posterior body in a loose tent‐like manner. The formation of elytral epipleura and a tight fit of elytra and abdomen were important evolutionary transformations in the Middle Permian, resulting in a tightly enclosed subelytral space. Permian families were likely associated with dead wood of gymnospermous trees. The end‐Permian extinction event resulted in a turnover in the composition of beetle faunas, especially a decline of large‐bodied wood‐associated forms. Adephaga and Myxophaga underwent a first wave of diversification in the Triassic. Polyphaga are very rare in this period. The first wave of diversification of this suborder occurs in the Jurassic, with fossils of Elateriformia, Staphyliniformia and Cucujiformia. The Cretaceous fossil record has been tremendously enriched by the discovery of amber inclusions. Numerous fossils represent all major polyphagan lineages and also the remaining suborders. Improved analytical methods for documenting and placing extinct taxa are discussed. Different factors have played a role in the diversification of beetles. The enormous number of species associated with flowering plants, and timing and patterns of diversification in phytophagous lineages indicate that the angiosperm radiation played a major role in beetle macroevolution. Moreover, the evolution of intimate partnerships with symbionts and the acquisition of novel genes—obtained from fungi and bacteria via horizontal gene transfers—facilitated the use of plant material as a food source and were key innovations in the diversification of plant‐feeding beetles.

Hydrocarbon Source Rock Evaluation of the Lucaogou Shale in the Periphery of Bogeda Mountain (SE Junggar Basin, China) and Its Implications for Shale Oil Exploration: Insights from Organic Geochemistry, Petrology, and Kinetics Pyrolysis

Since the discovery of the vast Jimusaer shale oilfield in the southeastern Junggar Basin in 2012, there has been considerable interest in neighboring areas around Bogeda Mountain that have shale oil potential. The primary productive interval in the basin, the Middle Permian Lucaogou Formation (P2l), is well-developed in the areas of Qitai, Mulei, Shiqiantan, Chaiwopu, and Miquan. In this study, we conducted an assessment of the hydrocarbon generation potential of the P2l in these five areas and compared it with that of the P2l in the Jimusaer oilfield, which were determined by GC-MS, total organic carbon (TOC) and vitrinite reflectance (Ro) measurements, Rock-Eval pyrolysis, and organic petrology to investigate the type, origin, thermal maturity, hydrocarbon potential, and oil/gas proneness of organic matter in the P2l. Additionally, we applied open-system pyrolysis of hydrocarbon generation kinetics to explore differences in hydrocarbon generation and expulsion across various P2l mudstone/shale in the southeastern Junggar Basin. The findings of this study revealed that the P2l shale in Qitai and Miquan areas contains more abundant and lower thermally mature organic matter (early mature–mature stage), characterized by primarily Type II1–I kerogen, similar to that found in the P2l shale of the Jimusaer oilfield. Conversely, the P2l shale in Mulei, Shiqiantan, and Chaiwopu contains less abundant and more thermally mature organic matter (mainly mature–highly mature stage), dominated by Type II2–III kerogen. Consequently, shale in these areas is considerably less desirable for oil exploration compared to the Jimusaer shale. The semi-deep to deep lake facies in Miquan and Qitai exhibit the most promising exploration potential. This study can serve as a guide for shale oil exploration in the southeastern Junggar Basin.

Reservoir quality of the Lower–Middle Permian Shan 2 and He 1 members in the Ordos Basin, China: Implications for depositional and diagenetic processes and the role of volcanic tuffaceous sediment in tight sandstones

The Permian Shan 2 and He 1 members (Shan 2-He 1 Mbr) are the main tight gas-producing intervals in the Ordos Basin. This reservoir mainly comprises low-permeability and ultra-low-permeability fluvial–deltaic sandstones. However, the formation of tight sandstone reservoirs, particularly those associated with volcanic tuffaceous sediment, remains poorly understood. Therefore, in this study, we aimed to identify the most favourable mechanism for reservoir formation using petrological, mineralogical, scanning electron microscopy with energy dispersive spectroscopy, X-ray diffraction, and porosity and permeability analyses to investigate the characteristics and porosity changes of the Shan 2-He 1 Mbr. The reservoir consists predominantly of lithic and lithic quartz sandstones, with an average porosity of 6.07% and average permeability of 0.434 mD. Depositional processes were crucial in controlling reservoir formation. Different sedimentary microfacies exhibited varying physical properties, of which coarse-grained braided bars and channel fill that formed in high-energy environments exhibited superior petrophysical properties. Rapid early compaction was the main cause of reservoir densification. Additionally, illite cement, secondary quartz grain enlargement, and late-stage carbonate cement negatively affected the reservoir quality. Secondary pores formed through the dissolution of terrigenous clasts, volcanic tuffaceous sediment (VTS), and carbonate cement. Diagenetic processes introduced heterogeneity and modified the reservoir, resulting in an effective reservoir space that comprises dissolution and intercrystalline pores, with limited primary pores. The reservoir physical properties were influenced by the sedimentary facies, siliceous mineral content, diagenetic processes, and the presence of VTS. Therefore, a comprehensive understanding of these factors is vital for identifying favourable reservoir formation conditions in the Shan 2-He 1 Mbr.

Episodic hydrothermal alteration on Middle Permian carbonate reservoirs and its geological significance in southwestern Sichuan Basin, SW China

To analyze the episodic alteration of Middle Permian carbonate reservoirs by complex hydrothermal fluid in southwestern Sichuan Basin, petrology, geochemistry, fluid inclusion and U-Pb dating researches are conducted. The fractures and vugs of Middle Permian Qixia–Maokou formations are filled with multi-stage medium−coarse saddle dolomites and associated hydrothermal minerals, which indicates that the early limestone/dolomite episodic alteration was caused by the large-scale, high-temperature, deep magnesium-rich brine along flowing channels such as basement faults or associated fractures under the tectonic compression and napping during the Indosinian. The time of magnesium-rich hydrothermal activity was from the Middle Triassic to the Late Triassic. The siliceous and calcite fillings were triggered by hydrothermal alteration in the Middle and Late Yanshanian Movement and Himalayan Movement. Hydrothermal dolomitization is controlled by fault, hydrothermal property, flowing channel and surrounding rock lithology, which occur as equilibrium effect of porosity and permeability. The thick massive grainstone/dolomites were mainly altered by modification such as hydrothermal dolomitization/recrystallization, brecciation and fracture-vugs filling. Early thin–medium packstones were mainly altered by dissolution and infilling of fracturing, bedding dolomitization, dissolution and associated mineral fillings. The dissolved vugs and fractures are the main reservoir space under hydrothermal conditions, and the connection of dissolved vugs and network fractures is favorable for forming high-quality dolomite reservoir. Hydrothermal dolomite reservoirs are developed within a range of 1 km near faults, with a thickness of 30–60 m. Hydrothermal dolomite reservoirs with local connected pore/vugs and fractures have exploration potential.

Characteristics and main controlling factors of intra-platform shoal thin-layer dolomite reservoirs: A case study of Middle Permian Qixia Formation in Gaoshiti–Moxi area of Sichuan Basin, SW China

Based on the study of the distribution of intra-platform shoals and the characteristics of dolomite reservoirs in the Middle Permian Qixia Formation in the Gaoshiti–Moxi area of the Sichuan Basin, SW China, the controlling factors of reservoir development were analyzed, and the formation model of “intra-platform shoal thin-layer dolomite reservoir” was established. The Qixia Formation is a regressive cycle from bottom to top, in which the first member (Qi1 Member) develops low-energy open sea microfacies, and the second member (Qi2 Member) evolves into intra-platform shoal and inter-shoal sea with decreases in sea level. The intra-platform shoal is mainly distributed near the top of two secondary shallowing cycles of the Qi2 Member. The most important reservoir rock of the Qixia Formation is thin-layer fractured-vuggy dolomite, followed by vuggy dolomite. The semi-filled saddle dolomite is common in fracture-vug, and intercrystalline pores and residual dissolution pores combined with fractures to form the effective pore-fracture network. Based on the coupling analysis of sedimentary and diagenesis characteristics, the reservoir formation model of “pre-depositional micro-paleogeomorphology controlling shoal, sedimentary shoal controlling dolomite, penecontemporaneous dolomite benefiting preservation of pores, and late hydrothermal action effectively improving reservoir quality” was systematically established. The “first-order high zone” micro-paleogeomorphology before the deposition of the Qixia Formation controlled the development of large area of intra-platform shoals in Gaoshiti area during the deposition of the Qi2 Member. Shoal facies is the basic condition of early dolomitization, and the distribution range of intra-platform shoal and dolomite reservoir is highly consistent. The grain limestone of shoal facies is transformed by two stages of dolomitization. The penecontemporaneous dolomitization is conducive to the preservation of primary pores and secondary dissolved pores. The burial hydrothermal fluid enters the early dolomite body along the fractures associated with the Emeishan basalt event, makes it recrystallized into medium–coarse crystal dolomite. With the intercrystalline pores and the residual vugs after the hydrothermal dissolution along the fractures, the high-quality intra-platform shoal-type thin-layer dolomite reservoirs are formed. The establishment of this reservoir formation model can provide important theoretical support for the sustainable development of Permian gas reservoirs in the Sichuan Basin.

Early–Middle Permian Terrigenous Rocks in Southwestern Primorye: Material Composition, Provenances, and Formation Settings

Early–Middle Permian Terrigenous Rocks in Southwestern Primorye: Material Composition, Provenances, and Formation Settings

Timing of rifting of the Dongkaco microcontinent (Central Tibet) and implications for Neo-Tethyan evolution

Rifting of continental blocks away from northern Gondwana resulted in the development of several Tethyan oceanic branches, the number of which and the timing of tectonic processes are actively debated. Within the Mesozoic Bangong-Nujiang suture, locally exposed Paleozoic strata are potential witnesses of the early development of Tethyan basins, yet their origin and tectonic significance remain poorly understood. In this study, Paleozoic limestone and sandstone units exposed in the Dongkaco area are identified as part of an independent Dongkaco microcontinent, differentiated from the South Qiangtang terrane by the Dongqiao suture on the north and from the Lhasa Block by the Beila suture on the south. The Upper Paleozoic deltaic to shallow-marine feldspatho-quartzose and quartzose sandstones within the microcontinent are interbedded with glaciogenic sequences and exhibit provenance characteristics that align with the Lhasa Block. These stratigraphic and provenance features suggest that during the Carboniferous to Early Permian the Dongkaco microcontinent was attached to the Lhasa Block. The discovery of Middle Triassic (Anisian) radiolarian chert associated with the Beila ophiolite indicates that sea-floor spreading was active within the Beila Ocean at that time. Data presented here show that the detachment of the Dongkaco microcontinent from the Lhasa Block resulted in the Beila Ocean, a branch of Bangong-Nujiang Ocean that expanded through Middle Permian to Early Triassic times. These findings shed new light on the paleogeographic evolution that preceded India-Asia collision and final amalgamation of the Himalayan-Tibetan orogen.