NW China Research Articles

Tracing of natural gas migration by light hydrocarbons: A case study of the Dongsheng gas field in the Ordos Basin, NW China

Based on the analysis of light hydrocarbon compositions of natural gas and regional comparison in combination with the chemical components and carbon isotopic compositions of methane, the indication of geochemical characteristics of light hydrocarbons on the migration features, dissolution and escape of natural gas from the Dongsheng gas field in the Ordos Basin is revealed, and the effect of migration on specific light hydrocarbon indexes is further discussed. The study indicates that, natural gas from the Lower Shihezi Formation (P1x) in the Dongsheng gas field displays higher iso-C5−7 contents than n-C5−7 contents, and the C6−7 light hydrocarbons are composed of paraffins with extremely low aromatic contents (<0.4%), whereas the C7 light hydrocarbons are dominated by methylcyclohexane, suggesting the characteristics of coal-derived gas with the influence by secondary alterations such as dissolution. The natural gas from the Dongsheng gas field has experienced free-phase migration from south to north and different degrees of dissolution after charging, and the gas in the Shiguhao area to the north of the Borjianghaizi fault has experienced apparent diffusion loss after accumulation. Long-distance migration in free phase results in the decrease of the relative contents of the methylcyclohexane in C7 light hydrocarbons and the toluene/n-heptane ratio, as well as the increase of the n-heptane/methylcyclohexane ratio and heptane values. The dissolution causes the increase of isoheptane values of the light hydrocarbons, whereas the diffusion loss of natural gas in the Shiguhao area results in the increase of n-C5−7 contents compared to the iso-C5−7 contents.

Geological characteristics and exploration breakthroughs of coal rock gas in Carboniferous Benxi Formation, Ordos Basin, NW China

To explore the geological characteristics and exploration potential of the Carboniferous Benxi Formation coal rock gas in the Ordos Basin, this paper presents a systematic research on the coal rock distribution, coal rock reservoirs, coal rock quality, and coal rock gas features, resources and enrichment. Coal rock gas is a high-quality resource distinct from coalbed methane, and it has unique features in terms of burial depth, gas source, reservoir, gas content, and carbon isotopic composition. The Benxi Formation coal rocks cover an area of 16×104 km², with thicknesses ranging from 2 m to 25 m, primarily consisting of bright and semi-bright coals with primitive structures and low volatile and ash contents, indicating a good coal quality. The medium-to-high rank coal rocks have the total organic carbon (TOC) content ranging from 33.49% to 86.11%, averaging 75.16%. They have a high degree of thermal evolution (Ro of 1.2%–2.8%), and a high gas-generating capacity. They also have high stable carbon isotopic values (δ13C1 of –37.6‰ to –16‰; δ13C2 of –21.7‰ to –14.3‰). Deep coal rocks develop matrix pores such as gas bubble pores, organic pores, and inorganic mineral pores, which, together with cleats and fractures, form good reservoir spaces. The coal rock reservoirs exhibit the porosity of 0.54%–10.67% (averaging 5.42%) and the permeability of (0.001–14.600)×10−3 μm2 (averaging 2.32×10−3 μm2). Vertically, there are five types of coal rock gas accumulation and dissipation combinations, among which the coal rock-mudstone gas accumulation combination and the coal rock-limestone gas accumulation combination are the most important, with good sealing conditions and high peak values of total hydrocarbon in gas logging. A model of coal rock gas accumulation has been constructed, which includes widespread distribution of medium-to-high rank coal rocks continually generating gas, matrix pores and cleats/fractures in coal rocks acting as large-scale reservoir spaces, tight cap rocks providing sealing, source-reservoir integration, and five types of efficient enrichment patterns (lateral pinchout complex, lenses, low-amplitude structures, nose-like structures, and lithologically self-sealing). According to the geological characteristics of coal rock gas, the Benxi Formation is divided into 8 plays, and the estimated coal rock gas resources with a buried depth of more than 2 000 m are more than 12.33×1012 m3. The above understandings guide the deployment of risk exploration. Two wells drilled accordingly obtained an industrial gas flow, driving the further deployment of exploratory and appraisal wells. Substantial breakthroughs have been achieved, with the possible reserves over a trillion cubic meters and the proved reserves over a hundred billion cubic meters, which is of great significance for the reserves increase and efficient development of natural gas in China.

Using detrital zircons to constrain the late Paleozoic tectonic evolution of the adjacent tectonic domains of the Tarim Craton, NW China

Late Paleozoic tectonic evolution of adjacent tectonic domains of the Tarim Craton is vital for understanding the basin-mountain coupling and dynamical mechanisms of the Paleo-Asian and Tethys tectonic systems as well as hydrocarbon exploration and development. U−Pb dating and Lu-Hf isotope analyses of detrital zircons of the Late Paleozoic sedimentary successions from three sections across the Tarim basin enable comprehensive identification of provenance changes and reconstruction of the tectonic evolutionary history of the Tarim Basin in the late Paleozoic. A total of 248, 499, and 470 valid ages were obtained for detrital zircon samples from the Kuqa Section in the northern margin of the Tarim Basin; the Xiaohaizi Section in the platform area; and the Aitegou Section in the southwestern margin of the Tarim Basin, respectively. Each age interval corresponds to the geotectonic thermal events of the opening–subduction–closing of the surrounding oceans and the collage/collision of terranes/island arcs. Based on detailed age spectral analysis and other evidence, we propose a tectonic evolutionary model that encompasses the northern and southwestern margins of the Tarim Basin. The South Tianshan Ocean closed completely in the late Carboniferous, resulting in the collision of the Tarim Basin with the Yili–Middle Tianshan Terranes no later than 315 Ma. In the southwestern part of the Tarim Basin, the Tianshuihai Terrane began to approach in the late Carboniferous, gradually amalgamated with the southwestern margin of the Tarim Basin, and finally wholly collided with the Tarim Basin at the end of the Permian, making the Tarim Basin an entirely terrestrial basin. The southwestern Tarim Basin was not an open passive continental margin in the Carboniferous–Permian but a retro-arc basin.

Control of complex lithofacies on the shale oil potential in saline lacustrine basins of the Jimsar Sag, NW China: Coupling mechanisms and conceptual models

The heterogeneity of lithofacies is a pivotal factor influencing the shale oil potential. A comprehensive understanding of the intricate influence of rock complexity on hydrocarbon generation properties, microscopic pore-throat structure, and fluid occurrences is crucial for shale oil exploration and development. The Lucaogou Formation in the Jimsar Sag is a type of saline lacustrine shale oil, which was chosen as a research example in this paper. To unravel the causal linkage and confirm the coupling relationship between lithofacies, pores, and fluid mobility in the sedimentary sequence of saline lacustrine basins, a series of experiments were conducted. These included total organic carbon (TOC), X-ray diffraction (XRD), vitrinite reflectance (Ro), Rock-Eval pyrolysis, field emission-scanning electron microscopy (FE-SEM), low-field nuclear magnetic resonance (LF-NMR), mercury intrusion porosimetry (MIP), and low-temperature nitrogen adsorption (LTN2A). The results revealed significant differences in pores, oiliness, and fluid mobility among the four dominant lithofacies. Employing an innovative comprehensive evaluation method, high-carbon silty dolomite manifests significant potential for hydrocarbon generation, substantial storage capacity, effective oiliness and mobility, heightened brittle mineral content, and facile fracture production. Consequently, it was the most favorable lithofacies. Furthermore, TOC, pore size, and pore types emerged as key factors influencing the movable oil content of different lithofacies. More importantly, a detailed multiscale model illustrated potential linkages across the three different scales of lithofacies associations, pores, and fluid mobility. The study’s outcomes can offer enhanced guidance for accurately assessing shale oil resources, particularly in the identification of crucial “sweet spots”.

Impact of disequilibrium compaction and unloading on overpressure in the southern Junggar Foreland Basin, NW China

Disequilibrium compaction caused by burial or tectonic compression and unloading are the common overpressure mechanisms in foreland basins. However, accurately identifying multiple overpressure origins in such basins is challenging, since it requires distinguishing impact of vertical and horizontal stresses on compaction and overpressure as well as identifying overpressure caused by unloading. This paper proposed a dual compaction factor method (DCFM) to address these issues. A basic DCFM schematic diagram was established through analyzing compaction behaviors before and after tectonic compression to distinguish overpressure derived from two types of disequilibrium compaction and overpressure induced by unloading. In DCFM, a compaction model for mudstone in the foreland basin, which was only controlled by burial, was established based on the relationship between mean deposition rate and compaction factor in weak tectonic compression zone (WTCZ). It was used to determine compaction factor, normal compaction trend (NCT), acoustic transit time, bulk density, and overburden pressure under this compaction condition, as well as to distinguish the impact of burial and tectonic compression on porosity and overpressure at strong tectonic compression zone (STCZ). The DCFM employs mean stress to revise the equivalent depth method (EDM). Pore pressures associated with compaction only derived from burial and derived from both burial and tectonic compression were estimated using conventional EDM and revised EDM, respectively. The total pore pressures were estimated using the Eaton method and were calibrated based on measured ones. Overpressure associated with unloading was determined by comparing total pore pressure with estimated pore pressure generated with disequilibrium compaction. This mode was further applied at two wells from the Sikeshu Sag in the southern Junggar Foreland Basin to reconstruct accurate overpressure profiles with multiple origins, which were consistent with measured pressures and matched well with geological characteristics. Disequilibrium compaction induced by tectonic compression and burial is responsible for overpressure at the Sikeshu Sag. Overpressure transfer is also one of the primary contributors to the excess pressure in the Jurassic and Cretaceous in the Gaoquan anticline at the Sikeshu Sag, with a maximum contribution of 38 %. Plays, consisting of sandstone reservoirs with both transferred overpressure and tectonic-induced overpressure as well as mudstone caprocks with overpressure due to disequilibrium compaction, are favorable for hydrocarbon accumulation. This study can quantify the impact of overpressures with multiple origins on hydrocarbon migration and sealing capacity of caprocks, as well as provide ideas for researching fluid evolution in sedimentary basins.

Subduction zone decoupling and rapid forearc spreading at an active continental margin: Evidence from an SSZ-type ophiolite in Qilian Orogen, NW China

The forearc supra-subduction zone (SSZ) ophiolites are of great significance for understanding the geological processes of subduction systems. They are generally considered to mark the initiation of oceanic subduction. Here we present a well-preserved forearc sequence in the Ebo area, North Qilian Accretionary Belt, NW China. Three rock types are identified from bottom to top, including the forearc basalts (FABs), boninites, and calc-alkaline arc rocks, which are similar to those from the Izu-Bonin-Mariana (IBM) area. The FABs display similar compositions to N-MORB but with lower Ti/V ratios (17.16–20.92) and they were derived from 5 to 10 % melting of depleted MORB mantle in the spinel-stability field at P–T conditions of 1.5–2.3 GPa/1442 ± 56 °C after < 2 % melt extraction in the garnet-stability field. The boninites are characterized by significant geochemical characteristics of subduction-zone fluid/melt involvement, which were the products of flux melting of refractory harzburgitic mantle that interacted with slab-derived fluids/ carbonate melt under the P–T conditions of 0.2–1.2 GPa /1316 ± 38 °C. The calc-alkaline rocks show typical arc geochemistry, suggesting the partial melting of the well-established enriched subarc mantle. Zircon U-Pb dating of the boninite and the arc gabbro yielded ages of 507 ± 10 Ma and 483 ± 4 Ma, respectively. Combined with regional data, we infer that the Ebo FABs and boninites are not the products of subduction initiation but formed during subduction-zone decoupling-retreat and rapid forearc spreading following earlier subduction at the North Qilian continental margin, which is subsequently followed by typical continental arc development.

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.

Pyrite Characteristics in Lacustrine Shale and Implications for Organic Matter Enrichment and Shale Oil: A Case Study from the Triassic Yanchang Formation in the Ordos Basin, NW China.

Pyrite is widely distributed in lacustrine shales and has become a research focus in unconventional oil and gas exploration. Pyrite morphology is useful for identifying different types of organic matter and assessing shale oil enrichment in organic-rich shale. Abundant pyrite is developed in the source rocks from the Chang 7 Member of the Yanchang Formation in the Ordos Basin, NW China. However, the relationship between different pyrite types and the differential enrichment of shale oil still needs to be clarified. The organic geochemistry, petrology, and isotopic composition of the Chang 7 Member samples were analyzed. The significance of pyrite types and sulfur isotopic compositions as indicators of depositional environments and shale oil enrichment was emphasized. The Chang 7 shales contain three pyrite morphologies, framboidal pyrite (type A), spherulitic pyrite (type B), and euhedral and anhedral pyrite (type C), and their aggregates. The sulfur isotopic compositions of pyrite (δ34Spy) in Chang 7 shales with different pyrite types exhibited regular patterns. The δ34S values of types A, B, and C pyrites were sequentially positive overall (average values are -2.739, 2.201, and 7.487‰ in sequence), indicating that type A pyrite was formed during the syn-sedimentary to early diagenetic stage and types B and C pyrites were formed during the early to middle diagenetic stage. Types A, B, and C pyrites showed sequentially increasing kerogen type index values and kerogen carbon isotope values (mean values of -31.59, -28.70, and -26.45‰, successively), indicating that the horizons where types A, B, and C pyrites developed correspond to types I, II, and III organic matter, respectively. Strong correlations between the pyrite content and oil components reveal that pyrite indicates shale oil enrichment. Moreover, variations in pyrite type significantly influenced the enrichment behavior of shale oil. Types A and B pyrites contributing to reservoir space showed shale oil enrichment. They promoted saturated hydrocarbon enrichment at >15% pyrite content, whereas type C pyrite did not indicate shale oil enrichment. These findings provide new insights into the differential enrichment of organic matter and shale oil and valuable guidance for the large-scale exploration and development of shale oil resources.

Mechanism of beryllium mineralization in a granite-pegmatite system: Constraints from ore geology and beryl mineralogy of the large Arskartor Be-Nb-Mo deposit, southern Chinese Altai

The Arskartor Be-Nb-Mo deposit is the second largest Be deposit in the Chinese Altai, NW China, which hosts over 11,000 tons of BeO resource. The muscovite-albite granite and rare-metal pegmatite in the ore district are spatially and temporally coexisted and both are beryllium mineralized. The muscovite-albite granitic stock can be divided into a barren zone and a Be-mineralized zone. The pegmatite shows an well-developed internal zone including the layered muscovite-quartz-albite zone, (lower) beryl-muscovite-quartz zone, quartz core, (hanging) beryl-muscovite-quartz zone, and muscovite-quartz-microcline zone. Beryl is the dominant Be-bearing mineral and mainly occurs in the Be-mineralized granite, the layered muscovite-quartz-albite and the beryl-muscovite-quartz zones of the pegmatite. Subhedral to euhedral beryl in the Be-mineralized granite is interstitial to or intergrown with rock-forming minerals. In contrast, beryl crystals in the pegmatite are coarser in grain size and more euhedral in shape, and mainly coexist with coarse “booked” muscovite and blocky quartz. Concentrations of Li, Cs and Na/Li ratios of beryl are 184–760 ppm, 218–1996 ppm, and 2.13–21.3, respectively. The progressive variations of incompatible elements compositions and Na/Li ratios are consistent with the fractional crystallization mechanism of the granite-pegmatite system. Paragenesis and internal structure of beryl suggest nonequilibrium crystallization of a relatively incompatible elements and fluxes-enriched late granitic melt, generated the coarse-grained Be-mineralized granite. With the progressive enrichment of incompatible and fluxing elements and decreasing temperature, liquidus overcooling was achieved and generated the aplite and immediate zoned pegmatite that hosts abundant beryl. Moreover, the differentiating pegmatitic melt with varying amounts of aqueous fluids at different melt fractions, likely resulted in heterogeneous distribution of beryllium and subsequent variable amounts of beryl crystallization in different internal zones. The exsolution of aqueous fluids from the residual pegmatitic melt resulted in Mo-mineralization and related hydrothermal alteration. Consequently, both protracted fractional crystallization and subsolidus processes contributed the formation of the Arskartor granite-pegmatite system with Be-polymetallic mineralization.

Tectonic fracture prediction for lacustrine carbonate oil reservoirs in Paleogene formations of the western Yingxiongling area, Qaidam Basin, NW China based on numerical simulation

Tectonic fracture prediction for lacustrine carbonate oil reservoirs in Paleogene formations of the western Yingxiongling area, Qaidam Basin, NW China based on numerical simulation

Facies and geochemical characteristics of the Middle-Lower Ordovician Yingshan Formation in the Tarim Basin, NW China: Implications for the high-frequency sequence stratigraphy in shallow-water carbonate platform

Shallow-water carbonates from the Yingshan Formation are important target of hydrocarbon exploration in Tarim Basin. Detailed descriptions of outcrop, core, and thin sections, isotopic composition analysis, and trace element analysis were conducted. Seven lithofacies which consists of two facies belt groups were identified and used to interpret deposition environments of Yingshan Formation: (1) peritidal carbonate, with relatively lower δ13C values from −4.2% to −1.9%, mainly represented by intertidal to supratidal facies and restricted subtidal facies and (2) open-marine subtidal carbonate, with higher δ13C values (−1.5% to −0.3%), mainly consists of shoal facies and interbank sea facies. On the basis of the lithology sets, four types of meter-scale cycle model (types A–D) were grouped into peritidal sequences and subtidal sequences. On the basis of vertical lithofacies, cycle stacking patterns, and accommodation variations in Fischer plots, two third-order depositional sequences (SQ1–SQ2) were recognized. The sequence boundary between SQ1 and SQ2 is not only a surface of a positive shift of δ13C values, but also the transitional zone of cycle stacking patterns. The sequences can be further divided into four fourth-order sequence sets: Sq1–Sq4. The lower sequences (Sq1–Sq2) are dominated by peritidal facies and characterized by a progressive decreasing in accommodation space probably indicates a longer-term fall in sea level. The upper sequences (Sq3–Sq4) are mainly dominated by subtidal facies and characterized by the accommodation space with a progressive increasement, likely indicates a rising sea level with a longer term. The facies-controlled reservoirs are mainly present in the Sq3, related to the sequence boundary between SQ1 and SQ2.

Petroleum accumulations in the tarim basin during various tectonic stages as revealed by U–Pb dating of multi-phase calcite veins in deeply buried carbonate reservoirs

Significant discoveries in deeply buried marine carbonate petroleum exploration have been made recently in the Tarim Basin, NW China. Constraining the evolutionary history of fluid activity and the process of oil charging in the deeply buried carbonate reservoirs is crucial for further hydrocarbon exploration and development. In this study, we conducted an integrated study combining petrography, geochemistry, fluid inclusion analysis, and in-situ U–Pb dating of multi-phase calcite veins collected from the Ordovician carbonate reservoirs, Akeyasu area, Tarim Basin. Four distinct phases of calcite veins were found within the Ordovician carbonate reservoirs. The U–Pb ages of these four phases of calcite veins are as follows: 459.8 ± 4.7 Ma, 418.8 ± 3.7 Ma, 329.7 ± 1.6 Ma to 310.0 ± 3.6 Ma, and 291.7 ± 6.1 Ma to 220.5 ± 7.3 Ma. These ages closely correspond to key tectonic events that influenced the study area, specifically, the Middle Caledonian, Late Caledonian, Middle Hercynian, and Late Hercynian orogenies, respectively. The first phase (C1) calcite veins are believed to have precipitated from meteoric water, closely associated with the first episode of oil charging. Phase Ⅱ (C2) calcite veins primarily formed from Early-Middle Ordovician formation water. Phase Ⅲ and phase Ⅳ (C3 and C4) calcite veins originated from Middle-Late Cambrian formation water and hydrothermal fluids, respectively. Significantly, the second and third episodes of oil charging occurred concurrently with the precipitation of C3 and C4 calcites. Seismic data analysis of Paleozoic strata in the Akeyasu area revealed four stages of fault activity, corresponding to the Early Caledonian, Middle Caledonian, Late Caledonian-Early Hercynian, and Middle-Late Hercynian orogenies. Our findings strongly suggest that the evolution of fluid activity and oil charging was primarily governed by regional orogeny. These large-scale tectonic shifts led to intensified fault activity, ultimately controlling fluid activity and oil charging dynamics. Furthermore, this intensified faulting eased the connection between deep fluid systems and the Ordovician carbonate reservoirs, serving as the primary conduits for fluid migration and oil charging within the deeply buried Ordovician carbonate reservoirs.

Carboniferous magmatic activity in the Aqishan–Caixiashan polymetallic ore cluster, eastern Tianshan, NW China: implications for tectonic evolution and regional metallogeny

Carboniferous magmatic activity in the Aqishan–Caixiashan polymetallic ore cluster, eastern Tianshan, NW China: implications for tectonic evolution and regional metallogeny

Permian tectonic switch of the southern Central Asian Orogenic Belt: Constraints from magmatism in the southern Alxa region, NW China

Abstract Late Paleozoic plutons are widely distributed in the Alxa region, southernmost Central Asian Orogenic Belt, and provided an important clue in constraint for the closing time of the Paleo-Asian Ocean (PAO). In this article, we present new zircon U-Pb ages and whole-rock geochemical data from the Permian Huoersen and Zongnaishan plutons in the southern Alxa region. The Huoersen gabbro (ca. 285 Ma) is enriched in large-ion lithophile elements and depleted in high-field strength elements, similar to the features of continental marginal arc. They were most likely generated by partial melting of depleted mantle that was modified by subduction metasomatic fluids. The Zongnaishan granites (ca. 267 Ma) show characteristics of I-type granites and were generated in a syn-collision setting. The Huoersen granites (ca. 259 Ma) are peralkaline and have positive Ɛ Nd(t) (+1.2 to +1.5) values, exhibiting A2-subtype granites affinities. They were formed by melting of lower crust in post-collision extension setting. Based on geodynamic mechanism, a three-stage evolution model is delineated: subduction, syn-collision to post-collisional extension for oceanic branch of the PAO during the Permian. In general, the rock assemblages indicate a tectonic switch from subduction to post-collision extension regimes and the final closure of the PAO.

Neogene paleomagnetic chronology of the Laolongwan Basin, NW China: New insight for the northeastward expansion of the Tibetan Plateau

The Laolongwan Basin, located on the northeastern margin of the Tibetan Plateau, has a thick layer of Neogene red sediments deposited, recording the northeastward growth process of the Tibetan Plateau. However, due to the lack of high-precision chronology of the late Cenozoic sediments in the basin, the understanding of the growth process of the northeastern margin of the Tibetan Plateau is limited. Based on regional geological surveys, this paper presents a high-precision paleomagnetic stratigraphic chronology study on the Neogene strata of the basin, and establishes a chronological framework for the Laolongwan Basin spanning from ∼ 13.61 Ma to 2.58 Ma. Integrating the paleomagnetic chronological framework with regional geological survey results, the Neogene sedimentary rocks of the Laolongwan Basin are divided into the late Miocene Xianshuihe Formation (∼13.61–8.11 Ma) and the late Miocene to late Pliocene Linxia Formation (8.11–2.58 Ma). Combined with high-precision measured stratigraphic sections, our work indicates a significant change in sedimentary facies at ca. 8.11 Ma, transitioning from siltstone and mudstone deposition to conglomerate deposition, and the sedimentation rate increased remarkably from 256 m/Myr to 544.8 m/Myr, indicating that the northeastward expansion of the Tibetan Plateau affected the Laolongwan Basin at ca. 8.11 Ma.

Retreat and advance subduction processes in the Yamansu–Central Tianshan arc (NW China): insights into the long-lasting accretionary orogenesis and final closure of the Palaeo-Asian Ocean in the southern Altaids

Subduction processes of Eastern Tianshan are crucial to understanding the mechanism of the orogenic evolution of the southern Altaids. To identify whether the tectonic setting of the Late Carboniferous or later was continuous subduction, we present a systematic study on Late Carboniferous magmatic rocks exposed continuously in time and space in the Weiya area, which were mainly derived from crustal materials with involvement of mantle-derived materials in a subduction-related setting. Our newly discovered 319 Ma A-type granites imply an extensional environment; 306 Ma diorites were derived from thickened crust. Combined with published data, we propose that rollback of the southward-subducted North Tianshan oceanic plate induced subduction retreating and tectonic extension in the Yamansu–Central Tianshan arc from 324 to 318 Ma. It was followed by advancing subduction from 315 to 301 Ma, during which the crust of the arc was thickened, and much more crustal material was involved in subduction-related magmatism. In the Early Permian, the arc was in an extensional environment followed by a change in the movement direction of the subducting plate, rather than in a post-orogenic setting. The final closure of the North Tianshan Ocean was likely completed in the Middle Triassic. Supplementary material : Field photographs and photomicrographs, zircon U–Pb and Lu–Hf isotopic data, whole-rock major and trace element data, and compiled geochronological and Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.7038686 Thematic collection: This article is part of the Processes of Pangea construction collection available at: https://www.lyellcollection.org/topic/collections/processes-of-pangea-construction

Sedimentary characteristics of mixed source fine-grained gravity-flow and its significance for shale oil exploration in a lacustrine depression basin: A case study of the Chang 73 Sub-member of the Triassic Yanchang Formation in Ordos Basin, NW China

This study investigates the transport and evolution of fine-grained gravity-flow deposits in lacustrine basins, focusing on the relationship between fine-grained deposits and their respective source–reservoir combinations. Such knowledge is vital for the exploration and development of unconventional oil and gas resources within the source-rock series of continental lake basins in China. We investigated the fine-grained gravity-flow deposits of the Late Triassic Chang73 Sub-member within the Yanchang Formation in the Ordos Basin. Our approach combined core observations, field outcrop evaluations, thin section analysis, scanning electron microscopy, geochemical data, and laboratory measurements. The sedimentary facies, characteristics, controlling factors, and sedimentary model of the fine-grained gravity-flow deposits are investigated. The primary sources of fine-grained sediments are terrestrial clastic and volcanic–hydrothermal fine-grained sediments. These sediments are mainly developed in warm, humid environments with abundant precipitation, favoring a highly reducible hypoxic terrestrial brackish–freshwater environment. Ten lithofacies and eight bed types of fine-grained gravity-flow deposits are categorized. Bed types represent deposits of muddy slide deposits, muddy slump deposits, muddy debris flow deposits, muddy flow deposits, hybrid event beds, fine-grained transitional flow deposits, surge-like turbidity flow deposits, and fine-grained hyperpycnal flow deposits. As a result of paleo-seismicity, volcanic events, and paleogeomorphology, instability and subsequent collapse of the delta front sediments led to the formation of gravity-flow deposits. These deposits formed the “channel–lobe complex” and “sublacustrine fan” sedimentary systems in both the southwestern and northeastern parts of the basin, respectively. Meanwhile, extraordinary flood events formed during the Carnian Pluvial Episode (CPE) played a significant role. It prompted increased fluvial drainage that drove fine-grained hyperpycnal flows through pre-existing valley systems, resulting in the establishment of the “channel–lobe complex” sedimentary system. The Chang 73 Sub-member contains high-quality hydrocarbon source rocks, laying the foundation for shale oil. The introduction of volcanic ash and the influence of hydrothermal activities amplify the enrichment of organic matter within the lacustrine basin. Fine-grained gravity-flow deposits yield siltstone-rich sweet intervals, primarily seen in surge-like turbidity and hyperpycnal flows. The interbedding of sandy and muddy sediments, influenced by hybrid event beds and fine-grained transitional flows, establishes a stratigraphic arrangement in which the upper layer nourishes the underlying reservoir. This structure is particularly beneficial for the development of shale oil and gas sweet intervals.

Mineralogy and 40Ar-39Ar dating of the recently discovered tainiolite occurrences in the Kaerqiaer fluorite Belt, western Altyn Tagh Terrane

The Kaerqiaer Fluorite Belt (KFB) contains the Kaerqiaer, Xiaobaihegou, Kumutashi, Northern Layidan, and Gaijike fluorite deposits in the western Altyn Tagh Terrane of NW China. An occurrence of Li-bearing mica tainiolite (KLiMg2(Si4O10) F2) has been recently discovered, associated with fluorite-bearing alkali feldspar granite. The tainiolite is hosted by fluorite-calcite veins that contain assemblages of tainiolite-fluorite-calcite and apatite-bastnaesite-tainiolite-fluorite-calcite along the contact zone with alkali feldspar granite. The tainiolite occurs as euhedral crystals and has high concentrations in the veins. 40Ar-39Ar dating of the tainiolite from Xiaobaihegou yields an age of 421 ± 2 Ma, and single-crystal X-ray diffraction confirms the tainiolite has a monoclinic 1 M−type with a C2/m (#12) space group. Minor structural variations in the mica are attributed to the substitution of Li for Mg. EPMA and LA-ICP-MS analyses show that the tainiolite has high Li, Rb, Cs, Sn and Nb concentrations, and low Ta, total REE concentrations. The average Li2O content of the fluorite-tainiolite veins reaches 0.36 %, surpassing the economic cut-off grade. We suggest that the fluorite deposits formed from highly fractionated Li-F-P-rich residual melt evolved from the parental magmas of alkali feldspar granite. The tainiolite mineralisation may result from subsequent hydrothermal activities caused by the reactivation of regional faults under the extensional regime between the Middle Altyn and Eastern Kunlun terranes. The discovery of tainiolite occurrences made KFB a potential area for not only F but also Li resources.

The role of fractional crystallization of water-rich magma in forming high Sr/Y intermediate-silicic rocks in the Beishan orogen, NW China

The petrogenesis of intermediate-silicic rocks with high Sr/Y ratios that are closely associated with porphyry Cu-Au mineralization and TTGs remains uncertain. This study presents new geochronological and geochemical data for high Sr/Y granodiorites and associated mafic microgranular enclaves (MMEs) from the Late Devonian–early Carboniferous Suanjingzi batholith in the Beishan orogen, NW China. The MMEs, dioritic and fine-grained, share mineral assemblages with their hosts but contain more amphibole. Similarities in ages and Sr-Nd-O isotopes between the MMEs and their hosts indicate that they share a common magma source. Our geochemical studies and quantitative modeling imply that upper crustal fractional crystallization of water-rich magma, represented by the associated MMEs, played a crucial role in forming the Late Devonian–early Carboniferous high Sr/Y rocks in central Beishan. High H2O contents promote amphibole crystallization and delay plagioclase appearance, giving rise to higher Sr and lower Y contents in evolved magma. Additionally, titanite fractionation can contribute to high Sr/Y ratios by decreasing Y contents. The MMEs, the best available proxy for the parental magma, originated from the partial melting of a mantle wedge metasomatized by subduction fluids. This study emphasizes the significance of upper crustal fractional crystallization of water-rich magma in forming high Sr/Y rocks. The possible increment caused by amphibole-dominated fractionation within the upper crustal level should be considered when employing Sr/Y ratios to quantify crustal thickness. Moreover, we argue that Late Devonian–early Carboniferous magmatism in central Beishan occurred in a continental arc setting, providing a stronger basis for understanding the Paleozoic tectonic evolution of the Beishan orogen.

Mineral transformation, element transport and hydrological impact in weathering at the Bingling Temple Grottoes: Implications for weathering in alkaline environments in NW China

Alkaline environments are common in arid and semi-arid regions in northwestern China and have preserved quantities of rock heritages along the Silk Road. In contrast to the frequently reported weathering mechanism of stone heritages in acidic conditions, mineral weathering mechanism (especially the transformation mechanism of smectite) in natural alkaline environments still remains to be evaluated. To deepen the understanding of weathering in such an environment, research idea from critical zone science was applied to investigate the element transport and mineral transformation from protolith (interbedded sandstone and conglomerate) to regolith in a world cultural heritage, Bingling Temple Grottoes (BTG), in northwestern China. From geochemical and mineralogical characterizations on boreholes, excavated profiles, fractures and cliff surface, we found that dissolution of calcite, plagioclase (anorthite, albite) and K-feldspar (orthoclase) caused major elemental loss of Na, K and Ca (to aqueous solution) in the BTG, and elements of Mg and Fe mostly retained in solid materials. Characterizations on the cliff surface demonstrated higher extents of element depletion and mineral weathering and lower ultrasonic P-wave velocities in sandstone than in conglomerate, and in the bottom sandstone layers (located in the capillary zone and exposure to rain scouring) than in upper sandstone layers (located in the cave and free of precipitation). The transformation of smectite from protolith to regolith follows the sequential reactions: dehydrated smectie → bihydrated smectite → randomly interstratified illite/smectite → illite. Such transformation pathway is favored by alkaline pH and wetting–drying cycles frequently occurred at the BTG. Typical weathering patterns such as clay swelling damage, efflorescence, salt precipitation, etc., are highly-linked to seasonal changes in the hydrological process influenced by both reservoir construction and groundwater system. Our study provides more evidence for the illitization of smectite in natural alkaline environments and highlights the hydrological impacts on weathering patterns of rock grottoes.