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26 March 2026

Anomaly in Methane Concentrations on Co To Island (Northern Vietnam): Results from the 2024 Underground Water Research

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1
V.I. Il’ichev Pacific Oceanological Institute FEB RAS, 690041 Vladivostok, Russia
2
Institute of Earth Sciences (IES), Vietnam Academy of Science and Technology (VAST), Hanoi 100000, Vietnam
3
Department of Earth Sciences, Graduate School of Science, Chiba University, Chiba 263-8522, Japan
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Author to whom correspondence should be addressed.
Geosciences2026, 16(4), 138;https://doi.org/10.3390/geosciences16040138
This article belongs to the Section Geochemistry
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Abstract

The northern Vietnam shelf, particularly the area adjacent to the Red River Fault Zone, is characterized by complex geology and active neotectonics. However, the patterns of degassing and the origins of hydrocarbon gases in this region remain poorly understood. In particular, the potential links between deep-seated fluid migration, fault systems, and gas anomalies in island groundwater systems have not been systematically investigated. This study presents preliminary results of dissolved methane, its homologues (C2–C5), helium, hydrogen, and carbon dioxide measurements in groundwater from Co To Island (Northern Vietnam), with the aim of identifying gas origins and assessing structural controls on fluid migration. A significant methane anomaly was discovered, with concentrations reaching up to 10% by volume in the northwestern part of the island. The hydrocarbon homologous series is traced up to pentane (C5), and CO2 content is also elevated, with a maximum of 5.4%. The average He concentration of 10.8 ppm significantly exceeds atmospheric equilibrium values, with maximum recorded concentrations of 18 ppm for He and 34.5 ppm for H2. Stable carbon isotope analysis of methane (δ13C-CH4 values ranging from −50.2‰ to −49.7‰ VPDB), combined with the presence of a complete C1–C5 hydrocarbon series and elevated mantle/crustal tracers (He, H2), indicates a predominantly thermogenic/metamorphogenic origin for the gases, ruling out a purely biogenic source. The spatial distribution of anomalies is structurally controlled, closely associated with the NE-SW trending Co To Fault system and its intersections with subsidiary faults, as corroborated by recent electrical resistivity tomography data. These findings indicate intensive, focused gas leakage from a deep-seated source, likely related to thermogenic/metamorphic processes and active fault-mediated degassing. The results highlight the significant hydrocarbon potential of the region and underscore the critical role of neotectonic activity in controlling fluid migration pathways in island aquifer systems.

1. Introduction

The South China Sea (SCS) is a key region for understanding complex geophysical, geological, oceanographic, climatic, and biological processes in the western Pacific margin. Although the general geological structure, formation history, petroleum potential, and oceanography of the region have been described [1,2], significant knowledge gaps persist. Certain areas, intrinsically linked to Vietnam’s offshore natural geo-resources, have been less studied compared to the shallow shelf, particularly the Red River sedimentary basin system (Song Hong/Yinggehai and Qiongdongnan basins) [3,4]. These regions exhibit strong indications of hydrocarbon and other mineral potential. A comprehensive study of these areas will enable comparison of the current state of the seabed and water column with data from the Far Eastern seas of Russia.
The Song Hong Basin is one of the largest Cenozoic sedimentary basins containing oil and gas, located on the continental shelf of the Socialist Republic of Vietnam [5]. Hydrocarbon exploration in the basin began in the early 1960s but was initially focused on onshore areas [2,6,7]. With growing interest in the hydrocarbon resources of northern Vietnam, understanding the genesis and distribution patterns of background and anomalous gas-geochemical fields has become particularly relevant [8,9]. Understanding the gas-geochemical and tectonic features of the region will be crucial for assessing hydrocarbon potential and guiding exploration efforts.
Globally, the study of methane and associated gases (helium, hydrogen, carbon dioxide) in diverse geological contexts has provided fundamental insights into Earth’s degassing processes, fluid migration pathways, and the dynamics of petroleum systems. In active tectonic margins such as the Nankai Trough [10,11] and the Cascadia Margin [12,13], methane emissions are closely associated with accretionary prisms and the presence of gas hydrates, with faults serving as primary conduits for fluid expulsion. Similarly, extensive research in sedimentary basins with proven thermogenic gas generation—including the Gulf of Mexico [14,15], the North Sea [16], and the Po Basin in Italy [17]—has demonstrated the critical role of structural traps and fault systems in hydrocarbon migration and surface seepage.
Beyond conventional petroleum systems, significant gas emissions have been documented in metamorphic and ultramafic terrains. Studies in the Oman Ophiolite [18] and the Canadian Shield [19] have revealed that abiogenic methane and hydrogen can be generated through low-temperature water–rock interactions (serpentinization) and radiogenic processes, providing analogs for understanding deep gas sources in crystalline basement rocks. Furthermore, major strike-slip fault zones and rift systems, such as the Dead Sea Transform [20], the San Andreas Fault [21,22], and the Baikal Rift [23], serve as large-scale conduits for mantle-derived helium and deep-seated CO2 and methane, highlighting the intimate connection between neotectonic activity and focused degassing.
The link between gas flux distribution and modern geodynamic processes is well-established [16,24,25,26]. Geodynamically active deep faults serve as channels for Earth’s degassing, manifested in marine environments as gas-geochemical fields with anomalously high concentrations of helium, hydrogen, methane, and other gases in the water column and sediments. Temporal variability in these fields reflects the dynamics of tectonic processes and changes in seismicity [27]. Seismic activity can act as a trigger for gas emissions in the Vietnamese sector of the SCS [28]. Phases of tectonic deformation are distinctly expressed in the Co To Island of the Tonkin Gulf [29], where high concentrations of hydrocarbon gases, carbon dioxide, helium, and hydrogen were detected in water sources [30].
Within this global context, the Red River Fault Zone and its offshore extension in the Bac Bo Gulf represent an exceptional natural laboratory for studying tectonically controlled fluid migration in a complex strike-slip and extensional setting (Figure 1). Co To Island, situated at the intersection of multiple fault systems, offers a unique opportunity to investigate how neotectonic reactivation of Paleozoic basement structures controls the focused degassing of deep-sourced hydrocarbons and associated gases. This study builds upon the global understanding of gas-geochemical processes and applies it to a previously understudied but highly prospective region of the South China Sea.
Figure 1. (A) Overview map of northern Vietnam indicating the position of the study area; (B) Co To Island within the Tonkin Gulf (Bac Bo Gulf); (C) Detailed map of Co To Island showing the topography and sampling points (sample number in Table 1).
The aim of this study is to establish the main features of hydrogen, helium, methane, and other dissolved gas distribution in the ground waters of Co To Island, located on the northern Vietnamese shelf of the South China Sea.

1.1. Geographical Setting and Tectonic Evolution

The Co To archipelago consists of three major islands—Co To Lon Island, Co To Nho Island, and Thanh Lan Island—located at the northeastern margin of the Red River–Bac Bo Gulf fault system. This region occupies the transitional zone between the North Vietnam block and the South China block, at the eastern edge of the Red River sedimentary basin. The area preserves a long and complex geological evolution history from the Paleozoic to the Cenozoic, characterized by successive tectonic regimes ranging from intraplate stability to collisional and extensional deformation. During the Ordovician–Silurian period, a stable continental tectonic setting controlled the deposition of deep marine sediments and the emplacement of S-type granites. In contrast, by the Late Paleozoic (Devonian–Late Permian), the region was strongly influenced by orogenic convergence, leading to the formation of regional metamorphic belts representative of classical collisional systems [31,32,33]. In the Cenozoic, particularly since approximately 50 million years ago, the collision between the Indian and Eurasian plates resulted in the southeastward extrusion of the Indochina Block. This process facilitated the development of major strike-slip fault systems such as the Red River Fault Zone [34,35,36]. Seismic profiles and offshore drilling data from the Bac Bo Gulf reveal that the Red River Fault Zone was tectonically active between approximately 30 and 5.5 million years ago. This activity is manifested in the formation of pull-apart extensional basins, flower structures, and minor transpressional features during the Late Miocene [7,37,38,39,40,41,42]. After 5.5 million years ago, offshore tectonic activity declined significantly, although vestiges of earlier deformation remain well-preserved in surface topography and structural fabric [38]. Within this tectonic context, the Co To Islands hold particular geotectonic significance: they represent a well-preserved segment of the ancient uplifted margin within the Red River Fault system and distinctly illustrate the transitional nature from Paleo-deformation to neotectonic expressions along the northern margin of the Bac Bo Gulf.

1.2. Lithological Composition and Sedimentary Characteristics of the Co To Formation

The Co To Formation, dating from the Late Ordovician to Early Silurian, constitutes the principal geological unit forming the structural foundation of the island, locally overlain by unconsolidated Quaternary sediments. It represents a typical deep-marine sedimentary sequence deposited within a foreland basin setting, formed during an early intraplate phase characterized by basement subsidence and erosion of pre-existing orogenic terranes. The lithological assemblage is diverse, including polymictic sandstone, siltstone, mudstone, tuffaceous rocks, and conglomerates, interbedded with weakly metamorphosed sericite–chlorite phyllites [32]. Field exposures reveal typical features of slope-apron sedimentation in a deep-marine environment, with abundant soft-sediment deformation structures such as cross-bedding, slumping folds, flame structures, load casts, and sand-injection dykes. These features reflect gravity-driven sedimentation processes such as slumps, debris flows, and turbidity currents [43,44]. The high-energy depositional environment also indicates the influence of mild but sustained tectonic activity during the Early Paleozoic.

1.3. Sedimentary Facies, Fault Structures, and Neotectonic Features

The sedimentary facies on Co To Island represent a well-developed ancient submarine fan system, spanning from the proximal slope to the distal lobe. Coarse-grained beds composed of sandy–gravelly material containing broken clay clasts and angular rock fragments are interbedded with finer-grained silt–mud layers exhibiting parallel and ripple laminations, indicative of Bouma-type depositional sequences. The dominant sediment transport direction trends northeast–southwest, consistent with a paleoslope dipping from an ancient uplifted block toward the center of the depositional basin. On the 1:10,000-scale geological map [30], the Co To Formation is subdivided into three distinct members that exhibit a topographically controlled spatial distribution, exposed at elevations ranging from 5 to 100 m. These units are dissected by fault systems trending northwest–southeast, northeast–southwest, and near-meridional directions [45,46]. Fault zones, typically 10–15 m wide, are commonly associated with quartz veins crosscutting the host rocks, indicating the reactivation of deeper fault structures during the Cenozoic. Quaternary cover sediments are predominantly found in low-lying areas and consist of fine to coarse sand, silty mud, gravel fragments, and marine shell debris. These deposits accumulated under late Holocene transgressive conditions and reflect post-glacial sea-level fluctuations [43].

1.4. The Co To Fault System and Modern Structural–Geodynamic Features

Co To Island is situated within a tectonic domain strongly influenced by the late Paleogene–Neogene extensional regime, which developed in response to sinistral strike-slip motion coupled with crustal stretching associated with the Ailao Shan–Red River Shear Zone (ASRRSZ). This area marks a transitional zone between nearshore basins such as the Kenh An–Thuy Nguyen depression and the main Red River–Bach Long Vi extensional basin, where a transtensional tectonic setting prevailed from the Eocene to Oligocene [40,47]. The Co To Fault—interpreted as a modern strike-slip shear zone—cuts across the island along a northeast–southwest trend, clearly visible on geological maps with a high fault density within Ordovician–Silurian units (O3S1ct, O3S2ct). Several faults coincide with stratigraphic boundaries and have undergone reactivation, resulting in intense weathering observed in drill cores, suggesting ongoing neotectonic activity (Figure 2).
Figure 2. Schematic fault system of Co To Island, adopted from [45,46] and sampling points of 2025. 1—marine deposits, Upper Pleistocene (mQ2); 2—Lagoon-marine deposits, Upper Pleistocene–Holocene (mbQ2–3); 3—Marine-aeolian deposits, Upper Pleistocene–Holocene (mvQ2–3); 4—Alluvial-proluvial deposits, Quaternary (dpQ); 5—Upper sub-formation of Co To Formation, Upper Ordovician–Lower Silurian (O3-S1ct2); 6—Co To Formation undifferentiated, Upper Ordovician–Lower Silurian (O3-S1ct); 7—sample points; 8—Area 1, 2 and 3.
The presence of asymmetric half-graben structures and oblique fracture systems reflects a strike-slip extensional (transtensional) regime. The excellent preservation of syn-rift strata and the abundance of organic matter in sandstone layers indicate significant hydrocarbon prospectivity. The spatial distribution of faults also suggests the presence of rotated fault blocks, consistent with evidence for counterclockwise block rotation in this area [29]. Late-stage uplift and mild deformation of the Quaternary cover indicate that modern tectonic activity is still ongoing, contributing to elevated risks of coastal geohazards.
Recent geophysical investigations using electrical resistivity tomography (ERT) by Trung et al. (2025) [45] have provided high-resolution images of the subsurface structure in the northwestern part of Co To Island. Their results reveal a three-layer resistivity structure: (1) a thin (0–12 m) Quaternary porous aquifer with high resistivity; (2) a weathered and fractured zone of the O3-S1ct formation (13–45 m thick) with variable resistivity; and (3) a high-resistivity basement of solid, slightly fractured rocks. Importantly, Trung et al. (2025) [45] identified several vertical low-resistivity zones (20–40 m wide, ~49 Ωm) cutting through the high-resistivity basement, which they interpret as fault zones corresponding to the NE-SW and NW-SE trending fault systems shown in Figure 2. These fault zones, characterized by enhanced fracturing and fluid content, provide direct geophysical evidence for the existence of permeable pathways that can serve as conduits for deep fluid migration.
The paper [46] deals with geological exploration results of Co To Island. Its basement rock composition includes coarse terrigenous rock formations interspersed with fine-grained terrigenous rocks from the Upper Ordovician-Lower Silurian period, formation Co To (O3-S1ct). These rocks exhibit heterogeneous lithology, characterized by an alternation of coarse-grained sedimentary units (e.g., coarse-grained sandstones containing fragments of acid volcanic rocks) and fine-grained units (e.g., siltstones) with well-developed banded structures. The formation demonstrates significant lateral and vertical variability in bed thickness, reflecting both its depositional setting as a deep-marine turbidite system and subsequent polyphase tectonic deformation. The Co To formation can be divided into three sub-formations: the upper sub-formation contains multi-mineral sandstone with medium to small grain size, siltstone, claystone, and banded structures, interbedded with a few layers of tuff sandstone and coarse-grained multi-mineral sandstone. Sometimes there are layers of sericite-chlorite shale and conglomeratic lenses. The middle sub-formation consists of two members:
  • Member 1: This member is composed mainly of coarse-grained, multi-mineral sandstone with thick layers of gray gravel, interspersed with lighter-colored, small-grained multigrain sandstone and layers of silty sand and gray to gray-black siltstone. The thickness of this member ranges from 90 to 135 m
  • Member 2: This member contains siltstone, claystone, and sericite-chlorite layers interbedded with some layers of medium-grained multimineral sandstone. The thickness of member 2 ranges from 60 to 110 m
The lower sub-formation is composed of coarse-grained multiphase sandstone, poorly sorted, alternating layers of siltstone and clay, banded structures, a few layers of tuff sandstone, tuff conglomerates, and multi-mineral conglomerates. There may also be a layer of sericite-chlorite shales in this formation.
The dominant fault systems are NE-SW, meridian, and NW-SE (Figure 2). Faults in the northwest-southeast and sub-meridional directions often have steep slopes (65–70 degrees), creating fracture zones that are sometimes accompanied by hydrothermal quartz veins with a width of 10–15 m [46]. These formations are directly covered by Quaternary unconsolidated sedimentary deposits of various origins: eluvial-proluvial, marine-aeolian, and lagoon deposits, with thicknesses ranging from a few meters to tens of meters.
Marine sediments of the Upper Pleistocene-Holocene (mQ2–3) are composed of medium- to coarse-grained sand mixed with shell fragments, with an observed thickness exceeding 5 m. Marine-aeolian deposits (mvQ2–3), formed by coastal wind reworking of marine sands, consist mainly of sand, occasionally mixed with clayey mud. Lagoon deposits (mbQ2–3) are primarily comprised of pebbles, sand, silt, and clay, with a typical thickness of 2–4 m. Alluvial and proluvial deposits (dpQ2–3) consist of pebbles, sand, and clay, and vary in thickness from 2 to 3 m.

1.5. Gas Geochemical Characteristics of the Bac Bo Gulf Region

The northern sector of the Bac Bo Gulf, including Co To Island, is recognized as one of the most active and geochemically diverse gas-emission regions in the Bien Dong (South China Sea). Over the past two decades, geochemical surveys have revealed widespread occurrences of methane, helium, and hydrogen within seabed sediments as well as in both surface and bottom water layers [30,48,49]. Methane concentrations in sediments range from 1.010 to 8.000 nL/dm3, while helium and hydrogen levels in seawater can reach several thousand nL/L, particularly in areas associated with active rifting.
These gases have a mixed origin, derived from microbial degradation of organic matter (methanogenic), deep metamorphic reactions, and deep-seated magmatic activity (magmatogenic) [50]. This reflects a complex geological setting characterized by multiphase processes and significant natural gas potential [51]. Strongly negative δ13C-CH4 values (ranging from −25.5‰ to −40.2‰) have been recorded in coastal seawater and in thermal springs along the Red River Fault Zone, further supporting the presence of both biogenic and thermogenic methane [30]. Notably, gas distribution follows a non-random, geologically-constrained pattern, with concentrations observed in structural depressions, proximal to Paleogene–Neogene source rocks and major tectonic intersections, where conditions are favorable for temporary gas accumulation [52,53]. In addition, the high porosity and permeability of Cenozoic sandstone, conglomerate, and siltstone units play a crucial role in gas storage and migration, especially under the influence of ongoing neotectonic activity [54].

1.6. The Role of Fault Systems in Controlling Geochemical Gas Migration

The fault systems in the northern Bac Bo Gulf region play a central role in controlling the generation, accumulation, and migration of geochemical gases. Major northwest–southeast-trending faults such as the Red River, Song Chay, and Song Lo faults, along with associated linked rift structures, have been demonstrated to act as efficient conduits for gas transport from deeper crustal levels to the surface, as evidenced by the spatial correlation between gas anomalies and tectonic zones [3,55]. High-resolution shallow seismic profiles have revealed features indicative of active gas migration, including gas migration chimneys, amplitude-chaotic seismic zones, and acoustic wipeout zones—typical signatures of fluid escape along fault-related fracture systems. In addition, helium and methane anomalies are frequently associated with neotectonic features such as uplift, rupture of shallow Cenozoic strata, and thermo-hydrochemical deformation, suggesting an ongoing coupling between tectonic activity and gas release dynamics [56]. In locations with favorable lithological conditions—such as interbedded sandstone, conglomerate, and mudstone—gases may become temporarily trapped, forming transient structural traps [57]. This highlights the dual role of faults not only as migration pathways but also as contributors to the formation of localized gas accumulations [58,59]. Overall, the fault system exerts a comprehensive control on the entire fluid system in the region, from gas source and migration routes to surface emission points.

2. Materials and Methods

The studied area of North Vietnam is directly located in the zone of influence of the Red River rift. In the modern relief, the Red River rift is a narrow and deep intermountain valley. Its continuation in the Gulf of Tonkin, the Shong Hong basin is filled with deposits of mainly terrigenous rocks of the Eocene-Quaternary age with a thickness of up to 15–16 km [60]. The sedimentary cover is underlain by a folded carbonate basement complex of Late Paleozoic age (Devonian-Carboniferous), from which industrial oil inflows and numerous oil and gas occurrences were obtained [61].
In October 2024, sampling was conducted on Co To Island in northern Vietnam. A total of 28 water samples were collected from wells for dissolved gas analysis (see Table 1 and Table 2).
All 28 water samples were collected from existing wells (both dug wells and drilled boreholes) distributed across Co To Island (Figure 2). No natural springs were sampled. The wells tap two main aquifer systems: the shallow Quaternary porous aquifer and the deeper O3-S1ct fractured bedrock aquifer, with depths ranging from approximately 1.5 to 70 m based on previous hydrogeological investigations [30] and recent geophysical surveys [29].
According to the detailed hydrogeological characterization of the study area by Trung et al. [29], water levels in the dug wells range from 1.1 to 3.1 m below ground surface, with depths typically ranging from 2 to 8 m. Drilled wells in the fractured bedrock aquifer have greater depths, from 19 to 45 m, with water levels varying from 0.7 to 6.3 m below ground surface depending on topography [29]. Electrical conductivity (EC) values in the study area show a wide range: most groundwater samples from the northern part of Dong Tien Commune have low EC values (161.6–503.7 μS/cm), corresponding to fresh water, while a highly salinized zone in the southern part exhibits EC values up to 6740 μS/cm (TDS 4359.7 mg/L) [29]. Temperature measurements during our fieldwork ranged from 25.2 to 29.0 °C (Table 1), consistent with shallow groundwater temperatures in tropical coastal settings.
The headspace equilibrium concentration method was used to determine methane, helium, and hydrogen concentrations in all samples [62]. Water from the bathometer was collected using the “triple overflow” method into sterile 100 mL glass bottles, which were then hermetically sealed with rubber stoppers to prevent atmospheric contamination. Excess water was removed using syringe needles, and 10 mL of atmospheric air was injected into each bottle to create a gas phase for methane analysis. Further, the samples were intensively mixed. Before the analysis, the gas phase was equilibrated and extracted with a syringe to insert the sample into the gas chromatograph.
Methane was determined in the laboratory of the POI FEB RAS using a “Kristall 9000” gas chromatograph (JSC “Chromatec” Co., Ltd., Yoshkar-Ola, Russia). The analysis for the content of helium and hydrogen was also carried out in the POI FEB RAS laboratory on a gas chromatograph “Chromatec-Gazochrom 2000” (JSC “Chromatec”, Yoshkar-Ola, Russia), equipped with high-sensitivity thermal conductivity sensors. The minimum detectable concentration for helium is 1–2 ppm.
Isotopic analysis of carbon in methane samples was conducted at Chiba University (Chiba, Japan). The gaseous components within the gas sample container were transferred to a glass vial, and water vapor was then removed using a cold trap. The CH4 was oxidized to carbon dioxide, and its stable isotope ratios were measured using a GC-Isotope Ratio Mass Spectrometer (IsoPrime 100 with vario EA System, Isoprime Ltd., Cheadle, UK). The results are normalized with the VPDB standard and expressed in the delta notation, and the experimental errors (±0.2‰) were calculated by repeated measurements of the laboratory standard.

3. Results and Discussion

The study identified a localized area in the northwestern part of Co To Island with elevated water temperatures (up to +28.3 °C) and extreme methane concentrations reaching 10% by volume (Table 1, Sample 8). This value is comparable to, or exceeds, methane emissions from known hydrocarbon accumulations off Sakhalin Island [63].
We also found that background methane concentrations were elevated, with exceptionally high values in the northwestern part of the island (Figure 3). The average content was 8.193 ppm, with a minimum of 1.96 ppm and a maximum of 105 004 ppm. At the same time, heavy hydrocarbon gases such as pentane were detected in all the samples (Table 2). The presence of hydrocarbons up to pentane in most samples (Table 2) confirms a thermogenic origin rather than purely biogenic methane. The presence of a complete homologous series is a strong indicator of mature hydrocarbon source rocks.
Figure 3. (A) Spatial distribution of methane (CH4) in water on Co To Island; (B) spatial distribution of carbon dioxide (CO2) in water on Co To Island. 1—methane (CH4) concentrations, %; 2—carbon dioxide (CO2) concentrations, %; 3—sampling points; 4—isobaths; 5—faults.
Table 1. Methane, carbon dioxide, helium, and hydrogen concentrations in water samples from Co To Island.
Table 2. Heavy hydrocarbon gas content (%) in water samples from Co To Island.
The island is also characterized by elevated helium content. The average value was 10.8 ppm with a minimum of 8.8 ppm and a maximum of 18 ppm. The elevated background and maxima significantly exceed typical atmospheric equilibrium concentrations (~5.24 ppm), indicating a possible crustal/mantle-derived component.
The average hydrogen concentration was 3.7 ppm, with a minimum of 1.4 ppm. The maximum value of 34.5 ppm (sample 18) is particularly noteworthy, as H2 is a sensitive indicator of active geodynamic processes [64,65].
CO2 concentrations in water ranged from 0.2% to 5.4%, with an average of 2.3%. High CO2 levels can result from thermogenic degradation of organic matter, carbonate metamorphism (decarbonation), or magmatic degassing [66].
The spatial distribution of gas anomalies is not random but shows a clear structural control (Figure 3, Figure 4 and Figure 5). The highest concentrations of all gases (Area 1) are clustered along the NE-SW trending Co To Fault system and its intersection with subsidiary NW-SE faults (see Figure 2). This interpretation is strongly supported by recent electrical resistivity tomography data [45], which revealed vertical low-resistivity zones cutting through the high-resistivity basement of the O3-S1ct formation. These zones, interpreted as fault-related fracture systems with enhanced permeability, correspond precisely to the fault traces mapped in Figure 2. The coincidence of our gas anomalies with these geophysically defined permeable zones provides compelling evidence that active fault systems serve as the primary conduits for focused vertical migration of deep-sourced thermogenic fluids.
Figure 4. (A) Spatial distribution of helium (He) in water on Co To Island; (B) spatial distribution of hydrogen (H2) in water on Co To Island. 1—helium (He) concentrations, ppm; 2—hydrogen (H2) concentrations, ppm; 3—sampling points; 4—isobaths; 5—faults.
Figure 5. The CH4-He-CO2 content ternary scatter plot shows sampling points with their numbers.
Samples 2, 7, 8, 12, and 14—which show the highest integrated gas content on the ternary diagram (Figure 5)—are all located within this structurally complex zone, further confirming the role of faults as fluid pathways.
Based on the integrated gas content, the study area can be subdivided into three distinct zones (Table 3). Area 1, which encompasses samples 1 to 19, exhibits the highest average concentrations of methane (1.203%), carbon dioxide (2.71%), helium (11.00 ppm), hydrogen (4.39 ppm), and heavy hydrocarbon homologues (0.000039%). This zone corresponds directly to the intersection zone of the Co To fault system. In contrast, Areas 2 and 3, located outside this main fault zone, show significantly lower gas concentrations, with average methane contents of 0.001% and 0.012%, respectively, and only trace amounts of heavier hydrocarbons (Table 3). This stark contrast in gas-geochemical signatures between the fault-bounded Area 1 and the background Areas 2 and 3 underscores the critical role of tectonic structures in controlling fluid migration and accumulation.
Table 3. Average gas concentrations within areas 1, 2, and 3.
The close spatial correlation between the gas anomalies and the Co To Fault system underscores the critical role of neotectonically active faults as conduits for focused fluid flow. The fault zones, often accompanied by quartz veins and intense weathering, represent zones of enhanced permeability [67,68,69].
The coexistence of high methane with a full suite of heavier homologues (C2–C5), elevated He, and H2 strongly suggests a thermogenic gas origin [16,24]. The high He content (up to 18 ppm) argues against a purely shallow biogenic source and suggests a contribution from basement rocks or mantle degassing facilitated by deep faults [3,55]. The high H2 concentrations may be linked to water–rock interaction in ultramafic rocks at depth, a process associated with active tectonic settings [27].
To further constrain the origin of the gas, stable carbon isotope analysis of methane (δ13C-CH4) was conducted for three samples from Area 1 (Samples 2, 8, and 14). The obtained values range from −50.2‰ to −49.7‰ VPDB (Table 1). While these values fall within the conventional overlapping zone between biogenic and thermogenic methane [70,71], their interpretation in isolation can be ambiguous. Therefore, a multi-proxy approach is essential for a reliable genetic assignment. In the context of the comprehensive gas-geochemical dataset from Area 1, these isotopic values strongly support a thermogenic origin. This conclusion is substantiated by: (1) the presence of a complete homologous series of hydrocarbon gases up to pentane (C5) in the same samples (Table 2, Sample 7), which is a diagnostic feature of thermogenic gases formed during catagenesis and is absent in purely microbial gases [72]; (2) the systematic co-occurrence of methane with elevated concentrations of helium (up to 18 ppm) and hydrogen (up to 34.5 ppm), which are established tracers of deep crustal/mantle degassing and active geodynamic processes [24]; and (3) the tight structural control exerted by the Co To fault system, which provides focused, high-permeability conduits for the vertical migration of deep-sourced fluids [30,69]. Collectively, this evidence confirms that the methane anomaly originates from a deep-seated source, likely involving thermal maturation of organic matter or metamorphic reactions, and the δ13C-CH4 values are entirely consistent with a high-maturity thermogenic or metamorphic gas in this tectonic setting.
The data provide direct geochemical evidence for the active migration of mature hydrocarbon gases to the surface. This “gas chimney” effect indicates the presence of a functioning petroleum system in the subsurface of the Co To archipelago and the adjacent Bac Bo Gulf.
Similar gas-geochemical anomalies (CH4, He, H2) were previously identified on Cat Ba Island, located further west along the same tectonic trend [29,30]. This regional consistency confirms that degassing along the Red River Fault system and its splays is a widespread phenomenon, controlled by the regional stress field and tectonic history. The main reasons for the formation of gas permeability channels in ancient (Paleozoic) rocks on Cat Ba Island and adjacent territory are likely to be shear dislocations, determined by the tectonic compression regime [42].
Large-scale regional geochemical and geological studies have been conducted for the first time. These studies have resulted in unique data on the distribution of hydrocarbons, helium, and hydrogen in the northern shelf of Vietnam and the Cat Ba Archipelago. The features of the degassing process along tectonic faults and the release of hydrogen as one of the geodynamic indicators have been considered.
Gases of thermogenic–metamorphic origin detected in hydrogeological wells on both Cat Ba and Co To Islands, together with methane, hydrogen, and helium anomalies in the subsurface, indicate the upward migration of natural gases through zones of enhanced permeability [8,30]. The structural control of these anomalies by the Red River Fault system and its subsidiary splays has been demonstrated in previous studies [3,42,55], confirming that active fault zones serve as primary conduits for deep fluid expulsion. Similar fault-controlled fluid migration has been documented in other regions, where faults act as high-permeability pathways for vertical gas transport [24,73,74]. The data obtained, which include high concentrations of hydrocarbon gases, hydrogen, and helium; a heavy carbon isotope composition in methane and carbon dioxide, indicate the gas condensate potential of the Bac Bo sedimentary basin. The contribution of deep gas hydrocarbons and hydrogen fluids can be significant. Based on gas-geochemical conditions, Bac Bo Bay, as well as the entire Shong Hong Trough, has the potential to form large gas condensate and oil deposits.

4. Conclusions

Expedition results reveal an extreme local anomaly of dissolved gases in the groundwater of northwestern Co To Island. Methane concentrations reach 10% by volume, accompanied by high levels of CO2 (up to 5.4%), helium (up to 18 ppm), and hydrogen (up to 34.5 ppm). The presence of a complete homologous series of hydrocarbons up to pentane, together with methane carbon isotope values (δ13C–CH4) ranging from −50.2‰ to −49.7‰ VPDB, confirms a thermogenic origin and rules out a purely biogenic process. The gas composition and its structural control suggest a mixed origin involving deep crustal processes and the degassing of a mature hydrocarbon system. The results provide strong direct evidence for the hydrocarbon potential of the Co To area and the northern Bac Bo Gulf.
The spatial distribution of gas anomalies is structurally controlled and correlates directly with the NE-SW trending Co To Fault system and its intersections with other fault systems. This indicates that active fault zones serve as the primary conduits for focused vertical migration of deep-sourced fluids. The ongoing neotectonic activity, evidenced by fault reactivation and Quaternary deformation, plays a key role in driving modern degassing processes, highlighting the dynamic interplay between tectonics and fluid flow in this region.

Author Contributions

Conceptualization, N.S. and A.K.; Data curation, R.S., L.D.A., T.H.Y., and T.H.T.; Formal analysis, N.S., A.V., and E.M.; Funding acquisition, L.D.A. and R.S.; Methodology, A.K., H.T., E.K., L.D.N. and I.I.; Validation, R.S., N.B.H.; Visualization, A.K. and E.K.; Writing—original draft, A.K.; Writing—review & editing, N.S. and R.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by a grant from the Russian Science Foundation No. 24-47-04001 (QTRU06.04/24-26) and as part of the State Program for Basic Scientific Research (No: 124022100078-7).

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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