Junggar Basin Research Articles

Study on the Mechanical Parameters and Permeability of Coal Reservoirs at Different Temperatures.

Deep coal reservoirs, as opposed to their shallower counterparts, exhibit characteristics of higher temperatures and pressures. These conditions affect the fracture structure and mechanical properties of coal, which in turn controls permeability. Substantial studies have been conducted to determine the effects of overburden pressure on permeability, but the correlation between the temperature and mechanical parameters/permeability of coal remains unclear. This study focused on low-rank bituminous coal from the southern edge of the Junggar Basin in Xinjiang. Using experiments conducted on seepage and mechanics at different depths (considering effective stress and temperature), the study investigated how temperature affects the mechanical parameters and permeability of coal column samples. A permeability prediction model was established incorporating temperature, mechanical parameters, and effective stress. The results show that from 20 to 80 °C, the elastic modulus of coal column samples decreases by 31.0%, and the Poisson ratio increases by 72.0%. Permeability decreases between 48.37 and 90.12% under different depths. The stress sensitivity coefficient under various temperature conditions decreased exponentially as the effective stress increased, and the temperature sensitivity coefficient under various effective stress conditions decreased with increasing temperature. The permeability was more sensitive to a temperature below 40 °C. In the permeability prediction model, the fracture compressibility coefficient is bifurcated into two coefficients, each controlled by temperature and effective stress. The permeability prediction error of the model was 12.7% under constant effective stress and 17.2% under varying effective stress and temperature conditions. The study could provide guidance for fracturing and coalbed methane production in deep coal reservoirs.

Experimental investigation of hydrocarbon generation, retention, and expulsion of saline lacustrine shale: Insights from improved semi-open pyrolysis experiments of Lucaogou Shale, eastern Junggar Basin, China

The organic-rich shale deposited in a saline lacustrine setting contributes massive conventional and unconventional petroleum reserves. However, the hydrocarbon generation, retention, and expulsion of saline lacustrine shale with increasing thermal maturity were rarely reported. Predicting the quality and quantity of generated, retained, and expelled hydrocarbons under various maturity stages for a given shale before drilling persists as a vital issue in exploration. Here, this study conducted two improved semi-open pyrolysis experiments to investigate the characteristics of the hydrocarbon generation, retention, and expulsion of Lucaogou Shale. Solid residues, adsorbed oil, free oil, expelled oil, and hydrocarbon gases were carefully quantified during the pyrolysis experiments. The results reveal that the composition and quantity of generated hydrocarbons are determined by the maturity and degree of openness of the shale system. The Tasmanites of Lucaogou Shale rapidly were converted to abundant oil in the main oil stage, promoting oil retention and expulsion. Meanwhile, the yield of adsorbed and free oil reached its peak at the VRo of 1.01 %. In the late oil window, the previously retained oil was further expelled from the shale as thermal stress increased. High oil retention significantly promoted the generation of extensive wet gas due to secondary cracking within the gas window. However, in a more open system, most of the generated oil was expelled within the oil window. The Lucaogou Shale generates relatively small amounts of hydrocarbon gas through kerogen cracking within the gas window. Hence, the extent of oil transformation into gas is controlled by the residence time and quantity of retained oil in shale. In addition, the maturity range of the oil saturation index greater than 100 mg oil/g TOC in Lucaogou Shale is relatively wide (VRo of 0.9 %-1.5 %), consistent with the renowned Woodford Shale. Expanded geochemical data of natural shale and artificial maturation experiments indicate the distribution of potential hydrocarbon resources. These valuable insights enhance the possibility of searching for commercial shale oil and conventional hydrocarbon resources. Collectively, these findings provide critical new insights into the hydrocarbon generation, retention, and expulsion of saline lacustrine shale, and the results could be a valuable analog for other saline lacustrine petroleum systems worldwide.

Formation mechanism and implication of analcime in the sandstone reservoirs of the Permian Jingjingzigou formation in the Jinan sag, southern Junggar basin, NW China

Analcime plays a significant role in sandstone reservoirs as an authigenic diagenetic mineral in the Junggar Basin (northwestern China). However, the origin and controls on the reservoirs have received remarkably little attention. This study investigates the formation mechanism of analcime in the Middle Permian strata in the Jinan sag (southern Junggar Basin) through petrography and geochemistry. The results show that analcime is formed through early alkaline hydrolysis of volcanic materials under specific temperature and pressure conditions. The reservoir rocks primarily consist of various lithic sandstones, including volcanic debris such as basalt, andesite, and tuff. Analcime is characterized as rich in aluminium and poor in sodium, classified as low-silica analcime with a low Si-Al ratio (1.98–2.38). Furthermore, various other diagenetic minerals, such as glauconite, chlorite, albite, and calcite have been identified. The primary reservoir space chiefly consists of intragranular dissolved pores of analcime, while secondary pores are formed by intragranular pores of feldspar and lithic, along with some remaining intergranular pores. Cementation of analcime during early diagenesis changes primary pore structures and reduces reservoir properties. The low-silica analcime dissolves due to acidic pore fluids associated with three stages of oil and gas charging, transforming into albite and creating numerous secondary pores, thereby enhancing reservoir quality.

Comparative study on pore-connectivity and wettability characteristics of the fresh-water and saline lacustrine tuffaceous shales: triggering mechanisms and multi-scale models for differential reservoir-forming patterns

Although particular attention has been paid to responses of hydrocarbon storage and percolation capacity to the devitrification concerning lacustrine tuffaceous shale reservoirs in recent years, there is still a lack of systematical and comparative investigation on differential patterns and potential triggering mechanisms concerning development of the pore-microfracture systems and characteristics of surface wettability between the fresh-water and saline lacustrine settings, which is of considerable importance in fully understanding of genesis and spatial distribution of dessert reservoir intervals of tuffaceous shale reservoirs, and to provide further conceptual basis for deciphering shale-oil movability of saline lacustrine fine-grained mixed sedimentary sequences. In this study, tuffaceous shales from both the Upper Triassic Yanchang Formation in the Ordos Basin and Middle Permian Jingjingzigou Formation in the Junggar Basin are targeted to unravel the differential behavior of tuff devitrification and potential impacts on reservoir wettability and pore connectivity concerning fresh-water and saline lacustrine settings, and we present new results here from integrated analyses and combined interpretation of FE-SEM, Image Pro Plus (IPP) software image processing, contact angle and spontaneous imbibition experiments. In view of comparative analysis from representative samples, the tuffaceous shales from saline lacustrine environments are characterized by well-developed intergranular-intercrystalline and dissolution pores, and inorganic microfractures, generally yield a higher plane porosity of representative pore-fracture spaces and spontaneous imbibition slopes, a relatively lower average of n-decane contact angles and corresponding wettability parameters. The saline lacustrine tuffaceous shales are thus suspected to have undergone more intense devitrification resulting in a higher amount of devitrification and associated dissolution pores, and a relatively better connectivity between isolated micropore systems with adjacent microfractures. This would significantly facilitate the interface wettability reversal and occurrence of movable hydrocarbon fluid in microscopic reservoir spaces. Finally, a comprehensive and conceptual model is established illustrating the effects of differential devitrification on reservoir-forming patterns concerning tuffaceous shales developed in the fresh-water and saline lacustrine settings, respectively. These findings are of great theoretical and practical significance to enrich theory of high-quality reservoir formation and shale-oil accumulation in saline lacustrine tuffaceous shale reservoirs, and lay the foundation for guiding efficient exploration of continental fine-grained mixed sedimentary sequences.

A novel approach to understanding the genesis and distribution of widespread reservoirs at the basin edge: A case study of gas-bearing sandstone in the Sangonghe Formation, Junggar Basin, Western China

A 20 m thick sandstone drilled in the second member of the Sangonghe Formation in the Qianshao area of the Junggar Basin has been identified to be a high-quality gas-bearing reservoir. However, recent missions reveal an uncertainty in reservoir distribution and bring high risk to the subsequent gas development, although the gas-bearing reservoir has been encountered by several drillings and proved to be widespread. To investigate the accurate distribution and reveal the evolution of the reservoir, we reconstruct the stratigraphic framework and depositional stages by a high-revolution sequence stratigraphic method, effectively identify the gas-bearing reservoir by seismic attributes refusion and seismic stratigraphic slices, and finally map the distribution strata distribution at different stages. The results indicate that the landward onlap phenomenon in the seismic data indicates a retrogradational stratigraphic structure during the water level rise rather than the equal-thickness stratigraphic pattern under stable water conditions. According to the retrogradational stratigraphic structure, the real driver for the widespread characteristic of a gas-bearing reservoir is the backward staggered movement of multistage sand bodies during the transgressive process rather than single-stage deposition. The fusion of seismic acoustic impedance and main frequency attributes provides an effective survey to identify the accurate distribution of the reservoir, by which interpretation of a stratigraphic slice along the vertical centerline of the reservoir extracted from the multiattribute fusion survey can present the distribution and evolution of the three stages on only one map. Achieved by an appropriate interpretation on the map, two sandy debris flow bodies are identified as being deposited at the bottom successively during the first two transgressive stages. They are completely staggered in the source direction with a clear boundary between well Qs9 and Qs7. Subsequently, the subsequent rise of the water level led to the deposition of a large-scale multibranch shallow-water delta, which exhibits the characteristics of multistage, multisource, and multibranch. The results not only advance our comprehension of widespread sand bodies but also furnish fundamental insights for the development of lithologic condensate gas reservoirs in the Qianshao area.

Single-grain luminescence dating of Manas Lake paleoshorelines reveals late quaternary glacial meltwater forced lake level highstand in arid Central Asia

The spatiotemporal patterns of late Quaternary lake evolution, along with their responses to climatic changes and glacial meltwater in the westerlies-dominated Central Asia, remain unclear primarily due to the lack of well-dated records spanning across glacial and interglacial cycles. In this study, we investigated five well-preserved paleolake shoreline sequences, 15–27 m above the modern lake basin of the now-dry Manas Lake, a representative terminal lake in the Junggar Basin of arid Central Asia. Both single aliquot and single-grain K-feldspar post-infrared infrared stimulated luminescence (pIRIR) dating protocols were applied to 26 shoreline samples to reconstruct a lake level variation for Manas Lake over the past 90 kyr. The reliability of the K-feldspar pIRIR dating was tested through assessment of luminescence characteristics and comparison of single-grain and single-aliquot K-feldspar pIRIR Des. The results indicate that the highest water levels (∼25 m deep) occurred during late Marine Isotope Stage (MIS) 5 (∼80 ka) and MIS 3 (31–27 ka). A lake level 20 m above modern lake basin (a.m.l.b.) occurred from the last deglaciation to the early Holocene (14–10 ka) and again in the late Holocene (3.4–0.3 ka). The lake level changes of Manas Lake are decoupled from observed Westerlies precipitation changes over the past 90 kyr. This decoupling suggests enhanced glacial meltwater sourced from the high Tianshan Mountains, triggered by higher mean summer temperatures, worked together with Westerlies precipitation drove periods of lake level highstand in Manas Lake during late MIS 5, late MIS 3, and early Holocene periods.

Late-Holocene environment evolution revealed by multi-proxy analyses of peat sequence in the Sawuer Mountains, northern Xinjiang of China

The Sawuer Mountains, located in the western Junggar Basin within northern Xinjiang of China, are considered as a distinct watershed between the Xinjiang inland water system and the Arctic Ocean water system. The suitable climate has fostered the widespread development of peat in the Sawuer Mountains, but our understanding about peat development and their environmental evolution remains limited. This study conducted the detailed investigation of peat developments and their environmental evolution in the Sawuer Mountains inferred from two cores using multi-proxies (peat accumulation rates, loss-on-ignition, total nitrogen, total phosphorus, humification and n-alkanes). The dating results present that peat began to develop at ∼3.1 cal kyr BP. The multi-proxies reveal that the climate exhibited a dry environment at ∼3.1-∼2.24 cal kyr BP and an increasing moisture at ∼2.24-∼1.42 cal kyr BP with the past 1420 years being characterized by an overall drying trend, agreeing well with other records from northern Xinjiang. The moisture reconstruction was both modulated by the NAO (North Atlantic Oscillation) phase and the solar forcing in the late Holocene.

The Role of Hydrocarbons in the Formation of Uranium Mineralization, Louzhuangzi District, Southern Junggar Basin (China)

In recent years, there have been important breakthroughs in the exploration for sandstone-hosted uranium (U) deposits in the Louzhuangzi district of the southern Junggar Basin. Between 2020 and 2023, a medium-sized sandstone-hosted uranium deposit production area was identified in the region. Only a few investigations have been conducted at the Louzhuangzi U deposit, including those analyzing its geological–tectonic evolution, basic geological features, hydrogeology, and ore-controlling factors. It is generally believed that uranium mineralization at the Louzhuangzi U deposit is controlled by a redox zone. Organic matter (referred to as OM hereafter) consisting of bitumen and carbonaceous debris is very common in the uranium ores (especially in high-grade ores) at the Louzhuangzi U deposit. However, the characteristics of the OM and its contribution to uranium’s mineralization have not been studied in detail. In this study, OM-rich U-ores, altered sandstone, and barren sandstone samples were collected for petrography, mineralogical, micro-spectroscopy, carbon, and sulfur isotope studies. The results of this study show that the distribution of U minerals and metal sulfides (pyrite, sphalerite, etc.) was strictly controlled by bitumen at the Louzhuangzi U deposit. The bitumen may have been formed by hydrocarbon-rich and U-rich ore-forming fluids, which were formed after hydrocarbon generation and expulsion in the underlying Jurassic coal-bearing source rocks. The fluids contained U, Zn, Fe, and other metal elements, which migrated together and then precipitated into the oxidized Toutunhe Formation sandstone through cracking and differentiation processes. Therefore, the results indicate that migrated hydrocarbons were involved in U mineralization, in addition to oxidation–reduction processes, in the Louzhuangzi district, south of the Junggar Basin (China).

Characteristics and controlling factors of Lucaogou formation shale reservoir in the northern edge of Bogda Mountain, the Junggar Basin, China

The practice of oil and gas exploration has confirmed the existence of recoverable shale oil resources in the Permian Lucaogou Formation on the northern edge of the Bogda Mountains. However, previous research on the development characteristics and main controlling factors of shale oil resources in this area is relatively limited. In order to elucidate the development characteristics and principal controlling factors of the shale deposit in the Lucaogou Formation, the characteristics, physical properties, diagenesis, and influencing factors of the shale ore were investigated utilising data derived from outcrop, drilling, seismic, and geochemical analysis. The findings indicate that the shale of the Lucaogou Formation is prevalent and extensive. The deposit’s lithology is predominantly composed of dark grey and grey-black mud shale, interspersed with thin layers of dark grey and grey-black sandy mud shale and dolomite mud shale. The most prevalent minerals are carbonate minerals, followed by feldspar and quartz, with a notable proportion of brittle minerals. The deposit is primarily composed of dissolution pores, bedding fractures, and structural fractures, with a porosity of 1.23–3.26% and permeability of 0.012–0.076 mD, which are characteristic of ultra-low porosity and ultra-low permeability deposits. Among the three deposit types, the sandstone type exhibits the most favourable physical properties, followed by the dolomite type and the shale type, which displays the least favourable properties. The shale of the Lucaogou Formation is currently in the middle diagenetic phase, which is characterised by compaction, cementation (carbonate cementation, mudstone cementation, pebble cementation), and dissolution. The destructive effect of compaction and cementation on the physical properties is counterbalanced by the constructive effect of dissolution. The diagenetic environment has gradually changed from an alkaline environment to a slightly alkaline, slightly acidic stage.

Temporal and spatial evolution of wildfires during the Jurassic: From regional to global scale

With ongoing global warming, the frequency of wildfires has increased in many regions worldwide. To fully understand the manifold interactions between warming climates and wildfires it is necessary to study wildfires during Earth's history, preferably during periods characterized by warm climates. The Jurassic was a significant greenhouse period in Earth's history, and although evidence of combustion has been widely reported from deposits of Jurassic age, the spatio-temporal patterns and processes of Jurassic wildfires have not been studied on a global scale. In this study, evidence for wildfires occurring in deposits of the Early Jurassic Sangonghe Formation within the Junggar Basin is reported for the first time. It was found that the range of burning temperatures of these wildfires decreased from the late Early Jurassic to the early Middle Jurassic. Additionally, a comprehensive database, consisting of 196 published records, of Jurassic combustion products, including fossil charcoal, pyrogenic inertinite (fossil charcoal in coal deposits), and pyrogenic polycyclic aromatic hydrocarbons (PAHs), was compiled from the literature and analyzed for this study. Spatially, the database shows a high concentration of published evidence for Jurassic wildfire records in warm temperate climate zones in the mid-latitudes of the Northern Hemisphere. Temporally, trends in the Jurassic wildfires may be related to changes in atmospheric oxygen levels, climate change and fuels. However, like other global compilations on this topic for different periods of Earth's history, these results have to be seen with some care, as various taphonomic biases, including researcher bias and preservation issues, cannot be ruled as complicating factors.

Bedding-parallel fracture density prediction using graph convolutional network in continental shale oil reservoirs: A case study in Mahu Sag, Junggar basin, China

The natural fractures, e.g., the bedding-parallel fractures (BPF), have a significant impact on the storage space and horizontal permeability of continental shale oil reservoirs. However, the conventional logging response of BPF is complex. To solve the problem of BPF density prediction, we propose the Edge Generation Weighted Graph Convolutional Network (EG-WGCN) method, which is an improved Graph Convolutional Network (GCN). The new method integrates the relationship information between the fractures and lithology into the edge generation method, and adds the generated edge weight information into the message passing process, thereby effectively improving the accuracy of fracture density prediction. The prediction process is divided into three steps: first, using conventional logging sampling points as vertices, establish vertex sets for each lithology and single lithologic layer. Second, based on the depth sequence of a single lithologic layer and the Euclidean distance of the same lithologic vertex sets, the connection between vertices is established and overlapping edges generated by the above methods are given higher weights. Third, the vertices in the constructed graph are classified through graph convolutional layers, which are integrated into the edge weight information. This method is applied to the fracture density prediction of continental shale oil reservoirs in the Mahu Sag of the Junggar Basin, western China. The results show that the accuracy of our proposed EG-WGCN method in testing data is 95.13%. Compared to the GCN (boundless) without using the edge generation method and EG-GCN with the edge generation method but without the weighted aggregation mechanism, model accuracy is improved by 14.48% and 1.22%, respectively. In summary, this method proves that if a proper ML model is used, conventional logging data can become valuable for predicting BPF density with high accuracy even when cores are missing.

Geochemical characteristics of the upper carboniferous source rocks in a terrestrial-marine transitional environment from the shuangjingzi area, Junggar Basin: Insights into palaeoenvironment, palaeoclimate, and palaeovegetation

The Upper Carboniferous organic-rich sediments record the depositional transition from terrestrial to marine environment across the Shiqiantan Formation-Batamayineishan Formation boundary. The geochemical characteristics of source rocks deposited in such terrestrial-marine transitional environments are not clear. The Late Carboniferous palaeovegetation, palaeoclimate and palaeoredox conditions are comprehensively reconstructed by combining organic petrology, molecular biomarkers, stable carbon isotopes and elemental composition. The molecular composition characteristics and stable carbon isotope compositions suggest a major terrestrial organic matter input with a lesser contribution of algae. Oxic to suboxic conditions for the Late Carboniferous sediments can be inferred from the elemental composition and biomarker proxies. A wet, cool, and seasonal dry climate during the Late Carboniferous can be deduced based on biomarkers, macerals, and spore and pollen records. The Late Carboniferous period is characterized by the enhanced burial of terrestrial organic carbon, coinciding approximately with the intensification of the Late Paleozoic Ice Age. The expansion of terrestrial floras in the Late Carboniferous is likely associated with the Late Paleozoic Ice Age.

Reconstruction of sedimentary paleoenvironment of Permian Lucaogou Formation and its implications for the organic matter enrichment in south-eastern Junggar Basin, China

Reconstruction of sedimentary paleoenvironment of Permian Lucaogou Formation and its implications for the organic matter enrichment in south-eastern Junggar Basin, China

The whole petroleum system with ordered coexistence of conventional and unconventional hydrocarbons: Case from the Junggar Basin, China

The whole petroleum system with ordered coexistence of conventional and unconventional hydrocarbons: Case from the Junggar Basin, China

Hydrocarbon-generating potential of the Middle Permian Lucaogou source rock in the eastern Junggar Basin, China

The Middle Permian Lucaogou Formation is the most significant source rock in the eastern Junggar Basin. Previous studies have confirmed its excellent hydrocarbon-generating potential in the Jimsar Sag. However, its potential in other areas of the eastern Junggar Basin remains uncertain. Based on total organic carbon and pyrolysis, organic petrology, hydrocarbon simulation experiments, basin simulation, and combined well-seismic coupling interpretation, this study systematically compares the hydrocarbon-generating potential of the Lucaogou source rock in the Jimsar Sag with other areas of the eastern Junggar Basin. It discusses the sedimentary environment of high-quality source rocks and depicts the distribution of practical source kitchens. The Lucaogou source rocks in the eastern Junggar Basin are oil-prone, dominated by type I–II kerogen, and generally classified as good to excellent source rocks. Nowadays, the area of the Lucaogou source rocks that have entered the main oil-generating window is approximately 11 × 103 km2. Except for the bulge area, the Lucaogou source rocks in the eastern Junggar Basin successively entered the hydrocarbon-generating threshold during the Jurassic and the main oil-generating window in the Cretaceous. The Lucaogou source rocks in the Jimsar Sag and other parts of the eastern Junggar Basin share similar biomarker fingerprints, characterized by relatively low ratios of Pr/Ph, Pr/n-C17, Tm/C30 hopane, C19/C20 tricyclic terpene, and C24 tetracyclic terpene/C26 tricyclic terpene, and high β-carotene content, gammacerane index, and Ts/Tm ratios. These characteristics reflect deposition in a strongly reducing brackish lacustrine environment with parental sources dominated by lower organisms such as algae and bacteria. Generally, the Lucaogou source rocks in the eastern Junggar Basin have an oil-generating intensity of more than 3 × 106 t/km2. Several oil-generating centers with an intensity of more than 5 × 106 t/km2 have developed in the front of the Bogda Mountain, Jimsar Sag, Dongdaohaizi Sag, Wucaiwan Sag, and Shazhang Fault Zone, covering a total area of approximately 12,500 km2. These characteristics of the Lucaogou source rocks promise favorable potential for forming large and medium oil fields. The results further consolidated the oil and gas resources in the eastern Junggar Basin and provided valuable references for exploring future oil and gas fields.

Phase Behavior of the Mid-deep Carboniferous Near Critical Condensate Gas Reservoir in Junggar Basin, China

Phase Behavior of the Mid-deep Carboniferous Near Critical Condensate Gas Reservoir in Junggar Basin, China

Source‐To‐Sink Analysis in the Mesozoic SW Junggar Basin, Central Asia: Evidence From Detrital Garnet and Tourmaline Geochemistry

AbstractSource‐to‐sink systems respond to and therefore potentially record topographic and tectonic changes. North of the Tian Shan Belt, the Sikeshu subbasin of the SW Junggar Basin transitioned from active extension in the Triassic to post‐extensional subsidence in the Jurassic‐Cretaceous. Sediment in the Sikeshu subbasin has been shown to be derived from the Tianshan. However, the details of the source‐to‐sink system remain unclear and discrepancies exist between proxy records. The heavy minerals in the Middle Triassic in the Sikeshu subbasin are dominated by garnets. To investigate the garnet sources and decipher the Mesozoic source‐to‐sink evolution, we conducted petrological and sedimentary analysis and detrital garnet and tourmaline geochemistry. We found that the geochemistry of garnets in the Middle Triassic sandstone is most consistent with that of the skarns in the Yili Block (YB) in Tianshan, while the geochemistry of 55%–84% of garnets in other Mesozoic sandstones is consistent with that of garnets in amphibolites in the YB. The geochemistry of the tourmalines since the Upper Triassic is consistent with that of the meta‐sedimentary rocks in the YB and Central Tianshan Block. The dominance of garnets sourced from skarns in the Middle Triassic probably indicates a near‐source point provenance and the broader range of garnet compositions from the Upper Triassic–Lower Cretaceous suggests multiple sources. We infer that the point source changed to multiple sources, which is consistent with the zircon spectra changing from unimodal to multimodal. This change reflects the expansion of the drainage that accompanies a change from active rifting to a post‐rift stage.

Corrigendum: Stratigraphic-tectonic evolution and characterization of the carboniferous in the Karamay-Baikouquan fault zone in the Northwestern Margin of the Junggar Basin, Northwest China

Corrigendum: Stratigraphic-tectonic evolution and characterization of the carboniferous in the Karamay-Baikouquan fault zone in the Northwestern Margin of the Junggar Basin, Northwest China

Paleo-Sedimentary Environment and Formation Mechanism of the Organic-Rich Shale of the Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, China

The Jimsar Sag is an important shale oil exploration target area in the Junggar Basin, northwestern China. The Permian Lucaogou Formation, with a thickness of 200–300 m, is the primary exploration target. High-frequency variation in lithology is a typical feature of the Lucaogou Formation, reflecting the fluctuation of the depositional environment and organic matter enrichment. The evolution of the depositional environment and accumulation mechanism of organic matter still need to be elucidated for the Lucaogou Formation. High-resolution sampling of the entire Lucaogou Formation was applied to a 248 m long core from Well JX in the Jimsar Sag to examine the depositional environment and organic matter enrichment. The findings unveiled that the Lucaogou Formation was deposited under a hot and arid climate, within the confines of a closed saline paleo-lake, where sediments endured an extended period of anoxic conditions, displayed periodic oscillations in paleo-temperature and paleo-salinity values over time, alongside a continuous rise in paleo-water depth. The predominant source lithology of the Lucaogou Formation is felsic igneous rock. Small-scale transgression and hydrothermal sedimentation occurred during the deposition of the Lucaogou Formation. The prevailing hot climate and enduring reducing environment fostered ideal circumstances for the enrichment of organic matter in the Lucaogou Formation. Due to different sedimentary environments and enrichment mechanisms, organic matter is enriched in two modes in the Lucaogou Formation.

Strontium Isotopic Variations of Authigenic Calcite in Clastic Strata Record Its Sediment Provenance and Fluid−Rock Interactions

Abstract Strontium isotopes of authigenic carbonate potentially record sediment provenance, fluid sources, and fluid–rock interactions, little was studied on this topic in clastic strata. This study investigated clastic rocks containing authigenic calcite in the Lower Triassic Baikouquan Formation in the Junggar Basin, northwestern China. Mineral compositional and fluid inclusion analyses were conducted to constrain the precipitation processes of authigenic calcite, and the Sr contents and isotope ratios of the calcite were also measured. The authigenic calcite was precipitated at 80–140°C as the final product of thermochemical oxidation of hydrocarbons and thus has high Mn contents and highly negative δ13CVPDB values (as low as −70‰). The calcite also exhibits anomalously low 87Sr/86Sr values (0.704827, 0.706612), which are lower than contemporaneous seawater and published 87Sr/86Sr values of carbonate cements in clastic sediments, and also much lower than 87Sr/86Sr values (0.722027, 0.736750) of alkali feldspar in the strata. These low 87Sr/86Sr values record the low 87Sr/86Sr of the dominant rocks in the provenance area, such as volcanic rocks. During diagenesis, especially mesodiagenesis, the charging of hydrocarbon-bearing fluids promoted abundant dissolution of orthoclase in the alkali feldspar detritus, releasing radiogenic 87Sr into the pore waters, and eventually increasing the 87Sr/86Sr values in the late-stage calcite that precipitated after this reaction. This inference is consistent with the positive correlation between the calcite 87Sr/86Sr ratios and the dissolution intensity of orthoclase. In regions that do not undergo hydrocarbon-charging and where orthoclase remains stable, the lower 87Sr/86Sr ratios of the calcite generally record the provenance. For authigenic calcite associated with intense fluid–rock interactions, the higher 87Sr/86Sr ratios reflect the enhanced dissolution intensity of 87Sr-rich minerals such as orthoclase. Therefore, combined with a petrological study, Sr isotopes of authigenic carbonate in clastic sediments can trace sediment provenance and intensity of fluid–rock interactions.