Introduction
The remaining conventional oil and natural gas reserves in the Junggar Basin are currently 5.4 billion metric tons and 2.1 trillion cubic meters, respectively [1]. In 2022, the oil production was 14.09 million metric tons, and the natural gas production was 3.75 billion cubic meters, equivalent to 17.48 million metric tons in oil equivalent. The Junggar Basin is considered one of China's important reserve and production bases for oil and gas resources. The Mahu Sag, located at the northwest margin of the basin, is a secondary structural unit. Since 1955, the Mahu Sag and its surrounding areas have cumulatively confirmed conventional oil reserves of 1.79 billion metric tons, making it a globally renowned major oil region [2-3].
The oil-source correlation indicates that the majority of crude oil in the Mahu Sag and its surrounding areas originates from the Permian Fengcheng Formation source rocks [4-5]. Based on evidence from biostratigraphy, lithostratigraphy, structural relationships, and detrital zircon U-Pb dating, the age of the Fengcheng Formation is generally constrained to the late Early Permian [6-7]. However, this period corresponds mainly to the Permian glacial epoch, characterized by a cold climate, with most of the Earth's land and oceans covered by massive ice sheets, leading to a sharp decrease in the diversity and quantity of organisms [8]. Understanding how the Fengcheng Formation developed into a world-class, high-quality hydrocarbon source rock under such extreme climatic conditions remains a pressing question. Recent isotopic dating studies have also raised questions about the traditional views on the age of the Fengcheng Formation [9⇓-11]. Therefore, clarifying the deposition timeframe of the Fengcheng Formation is crucial for understanding the paleoclimate and paleoenvironment at that time and exploring the conditions for the formation of organic-rich shales.
The Fengcheng Formation in the Mahu Sag is recognized as the world's oldest known alkaline lake deposit [5,9]. Despite investigations into its depositional environment [12-13], paleoclimatic conditions [14-15] and the source of alkaline minerals [16-17], the mechanism of alkaline lake (high salinity) environment formation, and the link with the enrichment of organic matter remain unclear. Additionally, examples from the Songliao, Ordos, Sichuan, and Santang Lake basins suggest a close relationship between volcanic activity and the formation of multiple sets of organic-rich shales in these regions [18⇓⇓-21]. During the Carboniferous to Early Permian, volcanic activity was generally frequent in the Junggar Basin [5,22]. However, there is still debate about the number of stages of volcanic activity during the deposition of the Fengcheng Formation and its potential impact on organic matter enrichment.
To address these questions, this study employed SIMS U-Pb dating and carbon isotopic stratigraphy analysis on the Fengcheng Formation. By comparing previous research findings, this study further constrained the age and environment of this stratigraphic unit. In addition, various analyses, including organic geochemistry, petrology, and major and trace elements proxies, were utilized to investigate the primary controlling factors for organic matter enrichment in the Fengcheng Formation.
1. Geological background
The Junggar Basin is located in the northern part of the Xinjiang Uygur Autonomous Region in northwestern China. It is a large composite basin formed on a pre-Cambrian to Carboniferous ancient crystalline basement [5]. The study area, the Mahu Sag, is situated at the northwest margin of the basin and covers an area of approximately 5 000 km2 (Fig. 1a ) [5]. In the Late Paleozoic, the collision between the Central Asian Orogenic Belt and the Junggar Basin caused multiple intra-plate tectonic events, and the Mahu Sag exhibited characteristics of a rift basin [22].
Fig. 1. Tectonic position of the Mahu Sag in the Junggar Basin (a) and lithological column of the Fengcheng Formation (b) (modified from Reference [12]). |
The Fengcheng Formation is primarily a lacustrine deposit, in conformable contact with the underlying Jiamuhe Formation and unconformable contact with the overlying Xiazijie Formation (Fig. 1b ) [12-13]. Based on lithological characteristics, the Fengcheng Formation can be divided into three units from bottom to top, namely Member 1 (P1f1), Member 2 (P1f2), and Member 3 (P1f3). These units correspond to three major stages in the evolution of the ancient lake in the Mahu Sag (Fig. 1b ) [16]. Member 1 inherits the sedimentation and volcanic framework from the underlying Jiamuhe Formation. The lower part is dominated by volcanic rocks, volcanic clastic rocks, and sedimentary pyroclastic rocks, while the upper part includes organic-rich mudstone, dolomite, and dolomitic rocks (Fig. 1b ) [15-16]. During the deposition of Member 2, volcanic activity diminishes, and the lake is at its highest salinity phase, with the central basin accumulating abundant alkaline minerals [14,17]. The lithology of Member 2 mainly consists of interbedded dolomite, sodium-carbonate rocks, and dark shale. Evaporites, identified easily by the high resistivity feature in the well log curves, are present, as shown in Fig. 1b [14,17]. In the deposition period of Member 3, volcanic activity further diminishes, lake salinity decreases, and sodium-carbonate deposition stops. Conversely, the input of terrestrial class gradually increases [15-16]. Member 3 primarily comprises dolomitic rocks within the depression, transitioning to terrestrial clastic rocks at the top [12-13].
2. Samples and experimental methods
2.1. Samples
In this study, a comprehensive analysis was conducted on 198 consecutive core samples from Well Maye-1 in the Mahu Sag, specifically from the Fengcheng Formation. The analyses include total organic carbon content (TOC), organic carbon isotopic composition (δ13C), major and trace elements, and mineralogy analysis. Two samples from Wells Feng-21 and Fengnan-1, representing the Member 1 interval, were taken for in-situ zircon U-Pb isotopic dating analysis. These two wells are located away from volcanic craters, dominated by sedimentary rocks, with large accommodation space and preserving a complete geological record. This allows for a relatively objective investigation of the age of the Fengcheng Formation. Core observations and microscopic rock mineralogy analyses were also performed on samples from wells Fengnan-5, Feng-26, Fengnan-1, and Ke-207 within the Fengcheng Formation, with the spatial distribution of the sample locations shown in Fig. 1a .
2.2. Experimental methods
Zircon U-Pb dating analysis was performed using the Cameca IMS-1280 Secondary Ion Mass Spectrometer (SIMS), with experimental procedures detailed in Reference [23]. TOC was determined using the LECO CS-230 analyzer, following the methods outlined in the standard GB/T 19145-2022 [24]. Carbon isotopic analysis was conducted with the MAT251 Isotope Mass Spectrometer, following the procedures outlined in the standard SY/T 5238-2008 [25]. Major element concentrations were determined using the AB104L Axios-max X-ray Fluorescence Spectrometry method following the standard GB/T 14506.28-2010 [26]. Trace element analysis was carried out using the ELEMENT XR Inductively Coupled Plasma Mass Spectrometer, following the standards GB/T 14506.30-2010 [27] and NY/T 149-1990 [28]. Mercury concentration was measured using the Milestone DMA-80 Mercury Analyzer, following the standard HJ 923-2017 [29]. Scanning Electron Microscopy (SEM) observations were conducted with the FEI Quanta FEG 450 field emission scanning electron microscope, mineral composition analysis was fulfilled using the energy-dispersive X-ray spectroscopy (EDS) probe Bruker XFlash 6 matching FEI Quanta FEG 450, and mineral surface scanning analysis was conducted using the quantitative evaluation of minerals (QEMSCAN) analysis system matching FEI Quanta FEG 450, following the procedures outlined in the standard SY/T 5162-1997 [30]. Petrographic thin-section analysis was carried out using the Olympus BX51 Polarizing Light Microscope, following the methods in standard SY/T 5368-2000 [31]. Cathodoluminescence analysis was conducted with the CITL CL8200 MK5-2 Cathodoluminescence Microscope, configured with the Leica DM4P Polarizing Light Microscope, following the methods in standard SY/T 5916-1994 [32]. Whole-rock X-ray diffraction analysis was performed using the Rigku TTRAX III X-ray Diffractometer, following the procedures outlined in standard SY/T 5163-2018 [33].
3. Experimental results and discussion
3.1. Geological age and climate environment of the Fengcheng Formation
3.1.1. Late Carboniferous age for the middle-lower Fengcheng Formation
The zircon grains in the tuff samples from the top of Member 1 (4 443 m) in Well Fengnan-1 are hypautomorphic to automorphic, with grain sizes ranging from 50 μm to 130 μm and aspect ratios of 1.0 to 2.5. Most zircon grains show weak zoning (Fig. 2a , 2c-2e ). All the 20 analyzed zircon grains have relatively high Th/U values ranging from 0.3 to 0.5, indicating typical magmatic zircons. After excluding five samples with harmonic ratios less than 90% and identified as inherited zircons, the remaining samples have 206Pb/238U ages of 281.4-323.7 Ma, yielding a concordant weighted mean age of (306.0±5.2) Ma, corresponding to the Kasimovian Stage (Fig. 2a ).
Fig. 2. U-Pb age composite chart for the Fengcheng Formation of the Mahu Sag. (a) U-Pb age concordia diagram of tuff in Well Fengnan-1, Member 1; (b) U-Pb age concordia plot of rhyolite in Well Feng-21, Member 1; (c) Core photograph of tuff in Well Fengnan-1, Member 1 (4 443.30 m); (d) Thin-section photomicrograph of sedimentary volcanic tuffaceous texture in Well Fengnan-1, Member 1 (4 443.30 m), with visible basaltic clasts, crossed polarized light; (e) Thin-section photomicrograph of sedimentary volcanic tuffaceous texture in Well Fengnan-1, Member 1 (4 443.30 m), with visible volcanic clasts, plane-polarized light; (f) Core photograph of rhyolite in Well Feng-21, Member 1 (3 373.50 m); (g) Thin-section photomicrograph of rhyolite in Well Feng-21, Member 1 (3 373.50 m), with porphyritic and glassy textures and volcanic clasts, plane-polarized light; (h) Thin-section photomicrograph of rhyolite in Well Feng-21, Member 1 (3 373.50 m), with spherulitic, chicken bone, and rod-shaped glass shards, plane-polarized light. |
For Well Feng-21, the rhyolite samples from Member 1 (3 373 m, 105 m above the base of Member 1) exhibit hypautomorphic to automorphic zircon grains with sizes ranging from 40 μm to 110 μm and aspect ratios of 1.0 to 2.5. Most zircon grains show weak or no zoning features (Fig. 2b , 2f-2h ). All the 22 analyzed zircon grains have relatively high Th/U values ranging from 0.1 to 1.2. After excluding three samples with harmonic ratios less than 90% and identified as inherited zircons, a plot of the remaining samples gives 206Pb/238U ages ranging from 277 Ma to 332 Ma, yielding a concordant age of (303.5±3.7) Ma, corresponding to the Gzhelian Stage (Fig. 2b ). Consequently, the age constraint for the top of Member 1 is further refined to approximately 304 Ma.
The results mentioned above differ by approximately 25 Ma from the conclusions drawn by previous studies based on detrital zircon LA-ICP-MS U-Pb dating, placing the Fengcheng Formation in the middle-late Early Permian [6-7]. Detrital zircons undergo complex geological processes such as weathering, transportation, and metamictization, which may cause lead loss and subsequently result in younger U-Pb ages for the samples. In some cases, individual or multiple detrital zircon ages in the samples may appear younger than the actual deposition age, causing controversies in interpreting the maximum sedimentary age of the formation [34].
Recent studies have supported the findings presented in this paper. Wang et al. constrained the Fengcheng Formation to the Late Carboniferous Kasimovian stage to the Early Permian Artinskian stage, based on LA-ICP-MS U-Pb ages of volcanic ash zircons (305 Ma for Member 1 and (296.8±2.5) Ma for Member 3) [9-10]. Sun et al., using CA-ID-TIMS zircon U-Pb ages from volcanic ash, confined the overlying Lucaogou Formation (previously considered to be the Middle Permian age) to the Sakmarian to Artinskian stages of the Early Permian [11]. Li et al. obtained SIMS U-Pb ages of approximately 309 Ma from basaltic rocks in the Jiamuhe Formation of the Lower Permian in Well Mahu-5 in the Mahu Sag, corresponding to the Late Carboniferous [12]. The stratigraphic chronology evidence from units above and below the Fengcheng Formation indirectly supports the findings of this study.
Carbon isotopic stratigraphy correlation is an effective method for establishing a chronological stratigraphic framework and reconstructing paleoenvironmental conditions. It's worth noting that δ13C not only reflects the chronological characteristics of the samples but is also influenced by thermal maturity and the type of organic matter of the samples. Programmed pyrolysis experiments on 198 samples from the entire section of Well Maye-1 show a predominant Tmax distribution between 430 °C and 450 °C, with the Ro values of 10 samples ranging from 0.87% to 1.12% (with an average of 1.01%) (Fig. 3 ). Additionally, the organic macerals of the Fengcheng Formation in Well Maye-1 are primarily dominated by sapropelic and liptinite, with H/C atomic ratios ranging from 0.84 to 1.49 (average 1.16), and predominantly composed of Type II1 kerogen. As such, there is limited variation in the mudstone/shale thermal maturity and organic matter type throughout the Fengcheng Formation in Well Maye-1, providing a foundation for conducting carbon isotopic stratigraphy studies.
Fig. 3. Geochemical composite column chart for the Fengcheng Formation in Well Maye-1. |
The δ13C curve of the Fengcheng Formation in Well Maye-1 is generally similar to the published δ13C curves for the Carboniferous-Permian boundary sedimentary strata [10,35 -36]. In the Fengcheng Formation of Well Maye-1, the δ13C values exhibit a pronounced negative offset (5.2‰) above the boundary between Member 1 and Member 2. Subsequently, the δ13C values return to a background trend. The negative shift corresponds to the negative carbon isotopic shift during the Kasimovian to Gzhelian stage (Fig. 3 ). In the upper part of Member 2, there is a negative shift of approximately 4.6‰, followed by a positive shift of about 6.5‰. This may be related to the global carbon isotopic shifts at the Gzhelian to Artinskian boundary (Carboniferous-Permian boundary) (Fig. 3 ) [37]. This carbon isotopic trend has been reported in other wells in the Junggar Basin and various global regions, including core data from the Junggar Basin [10], whole- rock data from South China [35], and carbon isotopic composition data from brachiopod shells in the Midcontinent of the United States and the Russian Platform [36] (Fig. 4 ). There are variations in the magnitude of δ13C offsets in different regions, suggesting that the fluctuation range may be influenced by factors such as sedimentary environment, regional events, and changes in surface water conditions [36] (Fig. 4 ). It should be noted that some scholars currently consider the negative shift in carbon isotopes as the boundary between the Carboniferous and the Permian [15]. In the Kongshan and Nanqing sections, the negative carbon isotopic shift is followed by a rapid positive shift [38]. Positive shifts in carbon isotopes often correspond to biotic recovery, while the Carboniferous-Permian boundary witnessed a major biotic extinction, which should correspond to a negative shift in carbon isotopes [38]. Considering that entering the Permian, the climate became cold, and the burial of organic carbon may lead to a decrease in atmospheric CO2 concentration [39], this study designates the location of the positive shift in carbon isotopes as the boundary between the Carboniferous and the Permian.
Fig. 4. Correlation of global carbon isotopic profiles across the Carboniferous-Permian boundary. M—Moscovian; K—Kasimovian; G—Gzhelian; A—Asselian. |
3.1.2. An interglacial warm climate during the early deposition of the Fengcheng Formation
Based on the records of glacial and non-glacial activities in the Carboniferous and Permian of eastern Australia, Fielding et al. identified eight different glaciation periods: four relatively short glacial periods in the Carboniferous (C1-C4) followed by four longer-lasting glacial periods in the Permian (P1-P4) [8]. This study constrains the age of the Fengcheng Formation to be between 297 Ma and 306 Ma. Most of the Fengcheng Formation, including Member 1 and Member 2, was deposited during the Late Carboniferous, contrary to previous assumptions that it is entirely included in the early Permian [6-7]. Therefore, the sedimentation period of the Fengcheng Formation mostly falls within an interglacial period between glacial intervals C4 and P1. The warm climate during interglacial periods plays a crucial role in enriching organic matter in the Fengcheng Formation. It directly influences the paleo-productivity, paleo-redox conditions, and paleo-salinity of the lake, as well as the supply of terrestrial materials.
3.2. Mercury anomaly linked with multiple episodes of volcanic activity
Volcanic activity may bring a significant amount of nutrients to the water, leading to the proliferation of algae and an increase in primary productivity. Additionally, the eruption of large amounts of greenhouse gases such as CO2 can cause long-term climate warming, promoting biological flourishing and enhancing primary productivity. Previous studies suggest the presence of volcanic activity during the deposition of the Fengcheng Formation. However, the specific episodes and intensity remain unclear. Mercury (Hg) anomalies serve as effective indicators for identifying periods of geological history associated with volcanic activity [40]. Volcanic eruptions are the primary mercury source in the natural environment [40-41]. Additionally, the intrusion of magmatic rock into organic-rich sedimentary rocks is a significant mechanism for mercury release [41]. In the samples from Well Maye-1 of the Fengcheng Formation, there is a specific correlation between the Hg content and TOC (Fig. 5a ). To eliminate anomalies in Hg content caused by variations in organic matter content, normalization of Hg content by TOC (Hg/TOC ratio, only for samples with TOC values greater than 0.2%) is typically employed [41]. Several peaks of Hg/TOC ratios were identified in the Fengcheng Formation of Well Maye-1. In the lower part of Member 2 (4 757 m), Hg/TOC ratio rises to 227×10-4 mg/g. At the top of Member 2 (4 676.5 m), Hg/TOC ratio increases to 197.5×10-4 mg/g. Moreover, at the bottom of Member 3 (4 600 m), relatively high Hg/TOC ratio (120-126)×10-4 mg/g was also observed. This suggests three episodes of strong volcanic activities during the deposition of the Fengcheng Formation in the Mahu Sag (Fig. 3 ). It should be noted that mercury in sedimentary rocks may also be associated with sulphides and clay minerals [42]. In the Fengcheng Formation samples from Well Maye-1, there is no significant correlation between Hg and Mo or Al (Fig. 5b-5c ), therefore ruling out the influence of sulphides and clay minerals on mercury content.
Fig. 5. Crossplots of Hg with TOC (a), Mo (b), and Al (c) concentrations in the Fengcheng Formation of Well Maye-1. |
The cores of Fengcheng Formation shale taken from Well Maye-1 and adjacent wells commonly present altered volcanic ash and show a good coupling relationship between organic matter enrichment and volcanic activity (Fig. 6 ). In the representative evaporite sequences situated closer to the depocenter of the alkaline lake, volcanic ash is frequently found in dark shale intervals alternating with layered alkaline salts. Taking the Member 2 of Well Fengnan-5 as an example, the host rock of the dark lithofacies in the evaporite sequence is characterized by fine grain size, low clay content, and high pyrite content, suggesting a potential abundance of volcanic dust. Additionally, white lumps dispersed in the dark mudstone have an irregular morphology (Fig. 6a ) and are primarily composed of microcrystalline felsic particles rather than evaporite minerals, indicating that these white lumps are derived from volcanic glass through alteration (Fig. 6b ). Volcanic ash typically contains a substantial amount of iron, which often precipitates as pyrite during alteration. In the fine-grained dolomitic mudstone of Well Maye-1, such volcanic clasts that have precipitated pyrite microcrystals during alteration are frequently observed (Fig. 6c-6d ). Some altered volcanic clasts are in association with organic-rich laminae (Fig. 6e-6f ). In Fig. 6e , the light-coloured laminated clasts exhibit a rip-up shape without fixed morphology and are composed of microcrystalline felsic particles, indicating that these volcanic clasts were relatively soft during deposition and distributed along the strata. The mudstone coexisting with these torn and altered volcanic clasts has a higher organic content, with TOC and S1+S2 values of 1.68% and 6.27 mg/g, respectively. This suggests that volcanic activity occurred along with organism death and sinking, promoting the burial of organic matter.
Fig. 6. Characteristics of altered volcanic clasts in shales of the Fengcheng Formation, Mahu Sag. (a) Thin-section photomicrograph of tuffaceous mudstone, Well Fengnan-5, 4 072.50 m, Member 2 dark mudstone section, characterized by dispersed white lumps exhibiting no fixed shape; (b) Thin-section photomicrograph of tuffaceous mudstone, Well Fengnan-5l, 4 072.50 m, Member 2 dark mudstone section, XPL (cross-polarized light), showing abundant felsic microcrystalline grains, suggesting light-colored irregular lumps result from volcanic glass through devitrification during diagenesis; (c) Thin-section photomicrograph of tuff-bearing dolomitic mudstone, Well Maye-1, 4 797.10 m, Member 2 with dispersed volcanic clasts within the dolomitic mudstone, PPL (plane-polarized light); (d) Thin-section photomicrograph of tuff-bearing dolomitic mudstone, Well Maye-1, 4 797.10 m, Member 2, with dispersed volcanic clasts within the dolomitic mudstone, macroscopically larger volcanic clasts dispersing in a floating pattern within the dolomitic mudstone, XPL; (e) Thin-section photomicrograph of organic-rich tuffaceous mudstone, Well Maye-1, 4 851.38 m, Member 2, white under PPL; (f) Thin-section photomicrograph of organic-rich tuffaceous mudstone, Well Maye-1, 4 851.38 m, Member 2, filled with felsic microcrystalline grains under XPL. |
3.3. High salinity of Member 2 greatly influenced by hydrothermal activity
Comparative analysis of rock minerals based on core samples, thin sections, and scanning electron microscopy (including energy-dispersive spectroscopy) indicates the development of evaporite minerals in the Fengcheng Formation of the Mahu Sag, which suggests high paleo-salinity. Evaporite minerals in the Fengcheng Formation mainly consist of sodium carbonates, occasionally accompanied by sulfate and halite (Figs. 7-8). The sodium carbonates include various minerals such as wegscheiderite [Na5H3(CO3)4], eitelite [Na2Mg(CO3)2], trona [Na3H(CO3)2·2H2O], nahcolite [NaHCO3], northupite [Na3Mg(CO3)2Cl], pirssonite [Na2Ca2(CO3)3], and magnesite (MgCO3). Among them, trona, eitelite, and pirssonite are the most common (Figs. 7-8). Trona usually occurs in aggregated forms, appearing as medium to thick-bedded in grayish-white to pale gray, exhibiting fibrous or radiating structures (Fig. 7a ). Eitelite typically occurs in very thin to thin bed, discontinuous layers, or patches, presenting a grayish color (Figs. 7 and 8e ). Pirssonite often occurs in discontinuous layers or patches, displaying a grayish color (Figs. 7c and 8e ). These minerals are widely distributed in the Fengcheng Formation of the Mahu Sag, and they are discovered in many well blocks. Experimental studies suggest that the coexistence of nahcolite, trona, and wegscheiderite may indicate the deposition of the Member 2 in an environment with elevated temperature and atmospheric CO2 partial pressure. Sulfates and halite in the Fengcheng Formation are only occasionally observed in scanning electron microscopy (Fig. 7e , 7f ), with some possibly formed by the dissolution and subsequent precipitation of sodium-carbonate rocks during sample preparation. Evaporite rocks in the Fengcheng Formation are commonly associated with calcite and dolomite. Calcite has a limited distribution, often occurring in vein-like patterns within dark shale layers deposited in semi-deep lake to deep lake environments. Dolomite has an extensive distribution and is commonly found in littoral facies siltstone and shallow lake to semi-deep lake facies mudstone, often occurring as very thin to thin beds, laminae, or nodules (Figs. 7b and 8b). Dolomite crystals generally exhibit zonation under cathodoluminescence microscopy (Fig. 8f ).
Fig. 7. Characteristics of different types of evaporites in the Fengcheng Formation, Mahu Sag. (a) Well Fengnan-5, Member 2, 4 068.00 m, trona in the core; (b) Well Feng-26, Member 2, 3 304.50 m, very thin-bedded eitelite in the core; (c) Well Feng-26, Member 2, 3 299.00 m, pirssonite in the core; (d) Well Fengnan-1, Member 2, 4 210.00 m, reedmergnerite with ductile deformation in the core; (e) Well Fengnan-5, Feng Member 2, 4 063.00 m, halite crystals identified by scanning electron microscopy and energy-dispersive spectroscopy; (f) Well Fengnan-5, Member 2, 4 063.00 m, anhedral crystals of mirabilite identified by scanning electron microscopy and energy-dispersive spectroscopy. |
Fig. 8. Microscopic characteristics of sodium-carbonates and associated minerals in the Fengcheng Formation, Mahu Sag. (a) Well Ke-207, Member 2, 4 753.00 m, backscattered scanning electron microscope (SEM) image of a spotted evaporite aggregate; (b) Quantitative evaluation of minerals (QEMSCAN) corresponding to (a), dominated by albite in the matrix with patches of authigenic dolomite; magnesite, pirssonite, and dolomite crystals are visible within the aggregate; (c) Backscattered SEM image of the magnesite and pirssonite in (a); (d) Energy-dispersive spectroscopy (EDS) spectrum of the pirssonite in (c); (e) Well Fengnan-5, Member 2, 4 069.00 m, thin-section photomicrograph of sodium-carbonate deposits, exhibiting spotted aggregate of northupite, pirssonite and reedmergnerite, identified by SEM-EDS; (f) Well Fengnan-1, Member 2, 4 238.00 m, cathodoluminescence microscope image of muddy dolomite showing zonation of dolomite microcrystals; (g) Well Fengnan-5, Member 2, 4 072.00 m, X-ray diffraction pattern of an alkaline minerals bearing rock sample, revealing minerals such as quartz, sanidine, dolomite, trona, northupite and eitelite. |
The B/Ga ratio is an effective indicator for studying the paleo-salinity of water bodies. Research by Wei and Algeo on modern water bodies revealed a strong correlation between sediment B and Ga contents and salinity [43]. Subsequently, B/Ga, as a salinity proxy, has been widely applied in ancient shale formations [44-45]. In modern regions with high primary productivity and organic matter flux, most of the Ga in the water is adsorbed by organic matter. However, there is no correlation between Ga content and TOC in the Fengcheng Formation (Fig. 9a ), indicating that organic matter does not significantly affect Ga enrichment [46]. Ga and Al contents exhibit a strong positive correlation (Fig. 9b ), suggesting that clay minerals are the primary host for Ga enrichment. The second-largest source of B in seawater (after sea salt) is the adsorption onto clay minerals [47]. In this study, the B/Al values of most samples are higher than the B/Al values of the upper continental crust (UCC) (Fig. 9b ), reflecting that B primarily comes from the water. However, there is no correlation between B and Al contents (Fig. 9c ). The B values in the Fengcheng Formation are anomalously higher than the background values, indicating that salts are likely the main hosts for B. Although the presence of high B, non-clay minerals like reedmergnerite in the Fengcheng Formation may hamper the use of the B/Ga ratio as an indicator of ancient salinity, the overall correlation between the elevated B/Ga ratio and increased B content is significant (Fig. 9d ), suggesting that intervals with abnormally high B/Ga ratio still correspond to high salinity.
Fig. 9. Correlation between trace element contents and TOC, Al, B in Fengcheng Formation shale, Mahu Sag. |
In Member 1 and the lower part of Member 2 in Well Maye-1, B/Ga ratio ranges from 3.7 to 57.5, with an average of 18.9 (n=94), indicating relatively low water salinity, possibly due to freshwater influx during glacial melting. In the middle part of Member 2, there is an interval with exceptionally high B/Ga ratio, with an average of 167.1 (n=21) and reaching up to 595, reflecting very high water salinity in this interval. Currently, the main evaporites in the Fengcheng Formation are derived from Member 2, which is closely related to its high salinity water (Fig. 9 ).
The high salinity in the Fengcheng Formation may also be related to hydrothermal activity, as indicated by the widespread presence of reedmergnerite associated with its evaporites (Figs. 7d, 8b, 8e). Such minerals are typically rare and have been reported as trace minerals in the Lower Eocene Green River Formation of Green River Basin and the Holocene deposits of Searles Lake in the United States [48]. The crystals of reedmergnerite in the Fengcheng Formation are usually wedge-shaped, plate- like, or occur as interpenetrating twins, appearing both dispersed and aggregated in clusters, discontinuous layers, or layers (Figs. 7d and 8e). Reedmergnerite in the cores of the Fengcheng Formation is mostly in dispersed, lenticular, or banded forms. The former was primarily formed in the middle-late diagenesis period. In contrast, the latter was primarily formed in the penecontemporaneous period or early diagenesis period and is characterized by plastic deformation (Fig. 7d ). The dispersed or porphyritic reedmergnerite formed in the middle-late diagenesis period is commonly associated with the replacement of carbonate minerals, including shortite, eitelite, northupite, calcite, trona, and rare baricalcite,. Zhao et al. reported δ11B values of reedmergnerite in the Fengcheng Formation from 0.33‰ to 2.13‰, with original brine fluid inclusion temperatures ranging from 100 °C to 116 °C, and suggested that at least part of the boron in reedmergnerite is derived from deep hydrothermal fluids [48].
Compared with other Cenozoic alkaline lake sediments, reedmergnerite is more developed in the Fengcheng Formation of the Mahu Sag. This could be attributed to intense magmatic-hydrothermal activity in the region and the prolonged diagenetic history, leading to a more extensive alteration of reedmergnerite [48]. Therefore, frequent volcanic activities may facilitate the input of deep hydrothermal fluids into the lake basin. These hydrothermal fluids can dissolve a significant amount of minerals and salts, transporting various elements and ions into the lake. Although the salinity of hydrothermal fluids may be lower than that of the lake water, causing a reduction in salinity near the hydrothermal vents, the coupled relationship between the high B/Ga ratio in Member 2 and the occurrence of evaporites, along with associated reedmergnerite, suggests that prolonged hydrothermal activity likely played a crucial role in the high salinity of the lake water.
3.4. Multiple enrichment mechanism of organic matter
The organic-rich shales of the Fengcheng Formation in the Mahu Sag provide the material basis for the large-scale discovery of oil and gas in this region. The formation of the basin is primarily controlled by climatic conditions and tectonic and sedimentary processes. To address the control of climatic conditions in the basin, the author proposed a comprehensive model to describe the climatic background during the deposition of the Fengcheng Formation and the influences of factors such as volcanic activity, redox conditions, biogeochemical cycles, and hydrophysical processes on the enrichment of organic matter (Fig. 10 ).
Fig. 10. Multi-factor organic matter enrichment model for Member 2 (a) and Member 1 and 3 (b) of the Fengcheng Formation. |
The primary productivity in the water body is the basis for the deposition of organic-rich mudstone/shales. Typically, the most significant factors influencing primary productivity throughout geological history are the input of nutrients, with volcanic activity, upwelling currents, and terrestrial clast input being the main contributing factors. As lakes are not influenced by upwelling ocean currents, the input of nutrients brought by volcanic activity and hydrothermal processes may be a key factor causing an increase in productivity.
This study indicates that the Fengcheng Formation was mainly deposited during an interglacial period between C4 and P1, corresponding to the Late Carboniferous period with increased atmospheric CO2 concentration and global warming [39,49]. The warm climate contributes to the proliferation of algae and bacteria, serving as the source of hydrocarbon precursor materials in the water. The warm climate during the interglacial period and high primary productivity are likely the main factors for the enrichment of organic matter in Member 1. The elevated atmospheric CO2 concentration indicated by the negative carbon isotopic shift during the deposition of the Fengcheng Formation, along with the coexistence of nahcolite, trona, and wegscheiderite, may be attributed to frequent volcanic activities releasing significant amounts of greenhouse gases during this period (Fig. 10 ) [50]. The volcanic activity in the Skagerrak large igneous province may have occurred from the Late Carboniferous to the Early Permian [51]. U-Pb and Ar-Ar geochronological research results suggest that the basaltic eruption occurred during 298-300 Ma [51⇓⇓-54]. This may have released a conservative estimation of 1.4×1012 t of CO2 [51⇓-53]. Additionally, magma-related contact metamorphism could release even more CO2 [51].
The widespread occurrence of evaporites in the Fengcheng Formation in the Mahu Sag (Figs. 7 and 8 ) reflects the climatic background of increased atmospheric CO2 concentration and associated global warming during the deposition of the Fengcheng Formation. The absence of typical cold-phase minerals such as natron in the Fengcheng Formation (Figs. 7 and 8 ) also indicates high water temperatures (exceeding 20 °C), as studies suggest that natron typically forms at temperatures below 20 °C under current CO2 pressure [53]. Additionally, the discovery of abundant bisaccate and striate pollen in the Fengcheng Formation, resembling Striatoabieites and Hamiapollenites, suggests that these pollen can adapt well to arid and hot environments [55]. It is also indicated that the Mahu Sag was located in the subtropical/tropical region with a semi-arid climate during the sedimentation of the Fengcheng Formation. Simultaneously, global warming may accelerate continental weathering, increase terrestrial input, and provide more nutrients to the lake, thereby stimulating primary productivity. The high productivity in the Member 2 may result from the combined effects of a warm climate background and frequent volcanic activities.
The nature of the water body is also a crucial factor in the enrichment of organic matter. The increase in organic matter flux enhances oxygen consumption in the water, encouraging the preservation of organic matter. Furthermore, the high salinity of the water body caused by hydrothermal activity in the Member 2 also improved the preservation conditions of organic matter. The abundance of evaporite minerals found in the Fengcheng Formation and high B/Ga ratio suggest that an increased evaporation rate may significantly lead to elevated water salinity during that time (Figs. 7 and 8 ). These processes are influenced by tectonic factors (such as water depth) and climatic conditions (such as precipitation/evaporation ratio), contributing to stagnant water and the formation of oxygen-depleted bottom-water environments that are beneficial for the deposition of organic-rich mudstone/shales (Figs. 3 and 10 ). Overall, the Fengcheng Formation represents the deposition in a deep-lake environment, with (Fe+Al)/(Mg+Ca) content ratio less than 2.5, occasionally showing characteristics of a semi-deep lake environment with (Fe+Al)/(Mg+Ca) content ratio ranging from 2.5 to 5.0 (Fig. 3 ) [56]. The salinity of the lake plays a crucial role in water column stratification. In Member 2 of the Fengcheng Formation, density stratification may occur due to the gradient effect of salinity and temperature, inhibiting water overturn and bottomwater oxygenation and promoting the deposition of organic-rich mudstone/shales (Fig. 10 ).
Member 3 of the Fengcheng Formation exhibits lower B/Ga ratio (ranging from 3.7 to 40.0, with an average of 16.6), indicating a reduction in lake salinity (Fig. 3 ). This decrease may be attributed to a colder climate during the P1 glaciation period of early Permian, which restricted water evaporation and diminished the role of salinity in density stratification (Fig. 10 ). The low TOC in Member 3 may be attributed to various factors, including the decrease in temperature after entering the glacial period, and weakening of volcanic activity leading to a decline in primary productivity. In addition, factors such as reduced salinity, pronounced seasonal variations, a more oxygenated environment in the fresh bottom water, intensified wave mixing, and shallowing of the lake may disrupt the stratification of the bottom water, increase the oxygen content in the water, and compromise the preservation conditions for organic matter (Fig. 10 ).
In summary, volcanic activity, hydrothermal processes, and enhanced continental weathering may increase nutrient input into the water, promoting primary productivity. Additionally, greenhouse gas emissions from volcanic activity and the generally hot and arid climate during interglacial periods, combined with high salinity in the water body, contribute to water stratification and oxygen depletion, improving conditions for organic matter preservation (Fig. 10 ).
4. Conclusions
Based on U-Pb geochronological data, the sedimentation age of the Fengcheng Formation is determined to be approximately 297-306 Ma, corresponding to the Kasimovian Stage to the Artinskian Stage, spanning the Carboniferous-Permian boundary. The carbon isotopic stratigraphy further indicates that most of the Fengcheng Formation, including Member 1 and the majority of Member 2, was deposited during the latest Carboniferous, in the interglacial period between the late Paleozoic ice age events C4 and P1, which is characterized by a warm climate that provided the macroscopic background for the enrichment and preservation of organic matter.
Mercury (Hg) concentration and Hg/TOC anomalies suggest multiple volcanic activities during the deposition of the Fengcheng Formation. Greenhouse gas emission is considered as one of the factors causing the warming climate during interglacial periods. Global temperature rise may lead to increased evaporation in basins, contributing to elevated water salinity, inhibiting the exchange between bottom and surface waters, resulting in bottom-water hypoxia, creating an environment favorable for organic matter preservation. Volcanic activities played a crucial role in the organic matter enrichment process during the deposition of the Fengcheng Formation. Firstly, volcanic activities produced abundant volcanic eruptive materials that could provide abundant nutrients, enhancing primary productivity and promoting organic matter enrichment. Secondly, the hydrothermal input from volcanic activities could introduce various elements and minerals, increasing water salinity and promoting water stratification.
Acknowledgments
During the preparation of this article, Professor Thomas Algeo from the University of Cincinnati provided valuable guidance on interpreting trace element proxies. Sincere thanks are extended to him.
Nomenclature
MSWD—mean square weighted deviation, dimensionless;
n—number of samples, count;
pCO2—CO2 volume fraction, 10-3 mL/L;
Ro—reflectance of vitrinite, %;
S1—free hydrocarbon potential, mg/g;
S2—retained hydrocarbon potential, mg/g;
Tmax—maximum pyrolysis temperature, °C;
TOC—total organic carbon content, %;
δ13C—organic carbon isotopic composition, ‰.