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Article

The Implications of Seeping Hydrocarbon Gases in the Gunsan Basin, Central Yellow Sea, off the Southwest of Korea

by
Jin-Hyung Cho
,
Seung-Yong Lee
,
Seok Jang
,
Nam-Do Jang
,
Cheol-Ku Lee
,
Seung-Hun Lee
,
Byung-Cheol Kum
,
Bo-Ram Lee
* and
Seom-Kyu Jung
Marine Domain & Security Research Department, Korea Institute of Ocean Science and Technology, Busan 49111, Republic of Korea
*
Author to whom correspondence should be addressed.
Geosciences 2024, 14(9), 230; https://doi.org/10.3390/geosciences14090230
Submission received: 27 July 2024 / Revised: 25 August 2024 / Accepted: 26 August 2024 / Published: 27 August 2024
(This article belongs to the Section Geochemistry)
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Abstract

:
A detailed analysis of high-resolution (3.5 kHz) chirp seismic profiles acquired in the Gunsan Basin of the central Yellow Sea revealed that hydrocarbon gases are actively seeping via the formation of many plumes. The uppermost sedimentary layer was acoustically confirmed to be fully or partially charged with gases. Somewhat favored by the low-tide period, episodic gas seepage is mainly associated with the underlying fault systems of Cretaceous-Cenozoic sedimentary strata in the southwestern part of the basin. Catastrophic gas expulsion seems to have formed a crater at the sidewall of a sedimentary ridge and two diapirs. Here, methane is poorly concentrated but rich in the heavy carbon isotope (δ13C, −52.6‰ to −44.7‰ The Vienna Peedee Belemnite [VPDB]), indicating that methane formed mainly through biodegradation of heavy oils at depth remains in the shallow sediments following its expulsion. Episodic rapid upward advection of porewater is also manifest by unmixed heavy methane trapped in the upper part of the primary biogenic methane (δ13C, about −90‰ VPDB)-filled sediment core. These findings imply that the Gunsan Basin fulfills the requirements for possible generation and preservation of oil and gas, like the petroliferous basins of eastern China and the Yellow Sea.

1. Introduction

In the Gunsan Basin, the eastern part of the Northern South Yellow Sea Basin (NSYSB), various research focused on oil exploitation has been conducted throughout the last 40 years [1,2,3,4]. The research mainly includes seismic stratigraphy and biostratigraphy performed to delineate source and reservoir strata of oil and gas and efforts to discern the basin evolution based solely on seismic profiles, well data, or their combination [5,6,7,8]. The Gunsan Basin consists of three intracontinental rift subbasins filled with a thick (~6 km) Mesozoic–Cenozoic nonmarine sedimentary succession, which is similar to the strata of other basins [Bohai Bay, North Yellow Sea–West Korea Bay, Subei (Jiangsu)–Southern South Yellow Sea] within and around the Yellow Sea that presently produce oil and natural gas at various rates [3,9,10,11]. However, hydrocarbon resources have not yet been discovered in this basin. Recent reinterpretation of seismic profiles and well data suggests that hydrocarbons might have been generated from late Eocene source rocks and trapped in anticlinal structures, faults, and some igneous complexes of the middle Miocene age [12].
Seepage of natural hydrocarbon gases is common in oil and gas-bearing basins and provides very important information about the origins, chemical characteristics, degradation, and migration pathways of hydrocarbons from deep source and reservoir strata [13,14,15]. In marine environments, a combination of geophysical prospecting by acoustic detection and geochemical analysis of seeping gases and gases charged into shallow sediments has been adopted as a simple but highly effective means for perceiving the potential of hydrocarbon resources at depth [16,17]. In shallow-water sediments, gases are generally dissolved in porewater and partly adsorbed onto sediment particles, and these gases often form bubbles that cause the sedimentary layer to appear acoustically transparent or turbid [18,19]. Gas seepage into the water column is easily recognized by the acoustic properties of compositely stacked hyperbolic and columnar reflectors, such as plumes, attributed to the large density difference between gas bubbles and water molecules [13,20,21]. The plumes are generally associated with pockmarks (or craters), diapiric domes (mud volcanoes), and carbonate fields on the seafloor and in shallow depths of the sedimentary layer. They can be traced downward on deep seismic profiles to gas migration pathways, such as chimney structures, bright spots, faults, etc., from their reservoirs and generation sites [15,21,22]. Seismic activities may trigger the gas seepage, seasonal changes in the overlying seawater temperature, variations in gas pressure caused by tidal cycles, and oil and gas production or migration [23,24,25]. Constrained by their source materials and the thermal and pressure conditions at the generation site and during migration, the compositional and isotopic characteristics of seeping gases make it possible to delineate their origins (primary and secondary biogenic, thermogenic, and rarely abiogenic) and mixing and degradation rates [26,27,28]. Primary biogenic gases contain the light carbon isotope in larger amounts because of their lower maturity and shorter residence time of organic matter at shallow sediment depths of within ~1 km [19]. Secondary biogenic gases formed through biodegradation of oils and bitumen at depth are isotopically similar to thermogenic gases [10,29,30]. Among the gases, methane is the main target for geochemical analysis because of its highest mobility and lowest molecular weight, which make it most suitable for obtaining information about the deep substrate [28,31,32]. Recently, direct observation and sampling of seeping gases by using autonomous underwater vehicles and other submersibles, along with conventional geophysical techniques, have enhanced understanding of the physical and biogeochemical processes of gas seepage [22,33,34].
In the Gunsan Basin, high-resolution (3.5 kHz) chirp seismic profiles show that hydrocarbon gases, probably related to the underlying geologic structures, are actively seeping from the uppermost sedimentary layer. Here, the implications of the gas seepage are examined based on geophysical observations of the gas seepage activities and geochemical characteristics of the methane and sediments. For this study, methane was extracted from the sediments in gravity cores and analyzed for its carbon stable isotope ratios, and seafloor morphologies possibly made by gas seepage activities were delineated. This study will facilitate understanding of the potential hydrocarbon resources at depth in the Gunsan Basin, off the southwestern coast of Korea.

2. Materials and Methods

The Gunsan Basin, located in the central Yellow Sea, comprises the eastern part of the NSYSB (Figure 1). The Yellow Sea is a semi-enclosed, shallow continental shelf sea surrounded by China and Korea that has been submerged since the postglacial sea-level rise [35]. It extends from Bohai Bay in the north to the East China Sea in the south. The sea is under the strong influence of a semi-diurnal tide that increases progressively northeastward, oblique to the Korean Peninsula, with a spring tide of ~5 m off of the city of Gunsan [36]. Even in the offshore region deeper than 70 m, tidal currents are strong with velocities as high as 50–80 cm/s near the seafloor [37]. The water circulation is reinforced by monsoonal winds, which blow toward the northwest in summer and southeast in winter [38]. Especially in summer, storms generally occur in a northerly or northeasterly direction several times each year. The Gunsan Basin is rather smooth within the water depth of 60–90 m and shoals progressively toward the east and the west. In the eastern part, ubiquitous sand ridges of various scales (~10 m in height and 3 km in length) are oriented in a NE–SW direction, oblique to the west coast of Korea due to the strong tidal currents [35,39]. The basin seafloor comprises the uppermost sedimentary layer of pre-Holocene palimpsest sands that have been actively reworked by transgressive migration processes since the sea-level rise. According to the dispersal patterns of clay minerals and the seafloor topography, the basin presently receives sediments from Korean rivers and streams flowing westward [8,35], whereas the western part of the NSYSB receives sediments from the Yellow River [40].
The Yellow Sea contains several sedimentary basins, the Bohai Bay and North Yellow Sea in the north, the Northern South Yellow Sea in the central part, and the Southern South Yellow Sea in the south (Figure 1). The North Yellow Sea Basin includes the West Korea Bay Basin in the east; the NSYSB includes the Gunsan Basin in the east; and the Southern South Yellow Sea Basin includes the Subei Basin in the west, which extends onto land in China [3,41,42]. As revealed by analyses of deep seismic and well data, the Gunsan Basin formed through intracontinental extension accompanied by transtension and faulting since the early Cretaceous [8]. It contains three depressed subbasins formed by the reactivation of preexisting normal and transpressional faults in the early Miocene, when the basin contracted thermally and subsided after the basin extension. The Miocene faults are superimposed on Cretaceous–Eocene faults that are concentrated in the southwestern part of the basin. The basin is filled with a thick (up to 6 km) Cretaceous–Cenozoic nonmarine sedimentary succession [6,8], as are the other basins of the Yellow Sea [3,41,42]. Other than the Gunsan Basin, the Bohai Bay Basin is a petroliferous, large-producing basin, accounting for around as much as one-third of the total oil production in China [11]. Ref. [12] recently suggested that the hydrocarbons generated from the shallow Late Eocene source rock might have been trapped in the anticlinal structures and associated faults formed during the Middle Miocene in the southern part of the basin. Volcanic activities and structures such as stocks, laccoliths, dikes, and others also might have aided the processes of insulating, maturing, migrating, and preserving the hydrocarbons [7].
A composite geophysical survey of multi-channel and high-resolution (3.5 kHz) chirp seismic profiling was performed in the Gunsan Basin from the R/V Onnuri (Figure 2). Survey navigation was done using the Differential Global Positioning System at a vessel speed of approximately 3–4 knots. The chirp seismic profiles were analyzed for geological and acoustic characteristics indicating gas accumulation in the uppermost sedimentary layer and seepage into the water column along the total survey line of 3622 km. The relationship between gas seepage and tide variation was obtained by analyzing the number and height of plumes together with the time sequence variation of the tide off of the city of Gunsan [36]. The height was assumed to indicate the intensity of gas seepage.
Sediment cores were collected onboard the R/V Onnuri using a 3 m long gravity corer at two sites near a crater and mud diapirs that were selected after analysis of the chirp seismic profiles (Figure 2). During this survey, the crater and diapirs were mapped with a multi-beam echosounder. In the southeastern part of the basin, a sediment core was recovered to compare its gas and sediment characteristics to those of the previous study. The cores penetrated the sedimentary layer to a depth of 280 cm (Table 1). After recovering the cores, the sediments were sampled into glass tubes (volume, 5.5 mL) through pre-punctured holes (diameter, 1.5 cm) at 10 cm intervals along the core liners. The sediment-filled glass tubes were placed into bottles filled with seawater tightly sealed with a silicon rubber stopper and tape. Headspace was made by extracting seawater through the stopper with a syringe. The samples were stored below 0 °C to minimize the likelihood of microbial activity until the gas could be extracted in the laboratory.
After the cores were halved, sediment subsamples were obtained around the core depths where the sediments for gas analysis were sampled. Sediment grain sizes were determined using a Sedigraph 5100D (Micromeritics, Inc., Norcross, GA, USA) after removing carbonate and organic materials with 10% HCl and 10% H2O2 solutions, respectively. Total organic carbon (TOC) and total nitrogen (TN) contents of dried sediment powders were analyzed using a CHN analyzer (Carlo-Erba, Cornaredo, Italy) after removing inorganic carbon with 10% HCl. The analytical error, as checked by analyzing the 4 PCS-1 and 3 BCSS-1 standards together with the samples, was less than 5%.
The concentration of methane (C1) was analyzed by capillary gas chromatography with flame ionization detection (GS-Q) and GC columns (GasPro, Santa Clara, CA, USA). The analytical error was less than 4%, as confirmed by analyzing replicates at least twice with standard samples. The stable carbon isotope ratio of methane (δ13C1) was analyzed at Hokkaido University, Japan, using an isotope ratio-monitoring gas chromatograph/mass spectrometer. The amounts of ethane were too small to analyze. The accuracy of the isotopic analysis was confirmed by analyzing the standard gas to the detection limit of 200 pmol for isotope ratios within a standard deviation of 0.3‰.

3. Results

3.1. High-Resolution (3.5 kHz) Seismic Profiles

The chirp seismic profiles in the Gunsan Basin showed that the seafloor is generally flat, except in the eastern portion, where large sedimentary ridges undulate with long wavelengths (~3 km) and small height (less than 20 m) (Figure 3). The ridges are often partially scoured and covered with sand waves with wavelengths of ~100 m and heights of a few tens of centimeters. These bedforms are attributed to differential deposition of palimpsest sands eroded and re-deposited from pre-Holocene sediments by tidal currents since the latest sea-level rise in the Yellow Sea [8,35,39]. The sedimentary layer in the central Yellow Sea shelf comprises various seismic facies. The lower part sedimentary layer shows folding structure containing many faults (Figure 3). Also, large-scale paleo-channels are observed and fill high-amplitude seismic reflectors, regarded as incised channel fill deposits (Figure 3). The hummocky seismic facies indicate a relatively lowland area around the paleo-channel in the study area (Figure 3). The upper part of the sedimentary layer has transparent seismic facies and low-amplitude parallel seismic facies (Figure 3).
The acoustically turbid characteristic is thought to be caused by the partial charge of gases or gas bubbles, whereas the transparency is thought to be caused by high-pressure accumulated gases-at least above the limit of methane solubility-in sediment pore space [13,17,43]. As shown in Figure 3, the high-resolution seismic profiles include three types of acoustic anomalies in the study area. Acoustic blanking with low amplitude seismic facies penetrates the upper and lower sedimentary layers and forms a chimney shape (Figure 3A). In some study areas, it develops as a chimney shape characterized by strong amplitude at the top (Figure 3B). High-amplitude negative polarity reflectors develop around folds and faults in the study area. The extent of these anomalies varies from up to 1 km, mainly in the upper parts (Figure 3C).
In the water column, stacked hyperbolic backscatter reflections that occur as single to closely spaced groups are observed ubiquitously (Figure 3), as in the southeastern Yellow Sea [20]. These acoustic anomalies are commonly recorded by various high frequencies of acoustic waves, indicative of plumes of seeping gas bubbles [16,21,44]. The plumes extend straightly to heights of several tens of centimeters to ~50 m above the seafloor (Figure 3) and are generally concentrated in the southwestern part of the basin to a distance of more than 50 km (Figure 2). Single plumes and groups of plumes are observed within the tidal range of 0.5–2.5 m and are concentrated in the range of 1.0–1.5 m (Figure 4a). No relationship between the height of the plumes and the tidal range was found (Figure 4b).
As shown on the chirp seismic profiles, a V-shaped crater and a diapiric dome occur in the eastern part of the Gunsan Basin (Figure 2, Figure 5a and Figure 6a). The mosaic of multi-beam images shows that the former is elongated and funnel-shaped with a width of ~250 m and a maximum depth of 7 m on the west side of a sedimentary ridge (5 km in length and 15 m in height), the top of which is rippled with sand waves (Figure 5b). The scale of this crater is larger than the scale of common pockmarks [45]. The sand waves are oriented in the NW–SE direction, oblique to the coast of western Korea, indicating the strong influence of tidal currents [8,35,39]. Internally, the ridge is acoustically transparent in the central part, whereas some acoustically enhanced internal reflectors occur parallel to the seafloor on both sides (Figure 5a). The diapiric dome, which is ~200 m wide and 4 m high, shows no internal sedimentary structures due to turbid reflection in the chirp seismic profile (Figure 6a). On the multi-beam echo sounding images, two distinctive diapirs with irregular but slightly concave-surfaced tops that are conical and elongated in shape, respectively, are apparent (Figure 6b). They are uplifted ~3–4 m above the surrounding seafloor. In particular, they are slightly moated on the west side, like those found in the southeastern Yellow Sea [20].

3.2. Core Sediments and Methane

Sediment cores were recovered from depths of 180 to 270 cm below the seafloor (Table 1 and Figure 7). The sediments are olive-gray in color, slightly burrowed in parts, and consist of various sediment types, namely, sandy mud, mud, sandy clay, silt, and sandy silt to muddy sand, with some amount of shell fragments through the depth range. Core no. EEZ21-8G is characterized by abundant clay (44.0%), more than in the other two cores; core no. EEZ21-9G has considerable sand (27.0%; and core no. EEZ21-17G has abundant silt (57.6%). These comprise a variation in mean grain size of sediments from medium to very fine silt (5.3 to 8.3 Φ); the finer sediments have smaller mean grain size. The sediment type, textural composition, and mean grain size are distributed in an irregular pattern along the core depths, as found previously in areas adjacent to the study area [8,39].
The chemical characteristics of sediments in the cores, including the concentration and carbon isotope ratio of pore water methane, are distinctive depending on the site (Table 1 and Figure 7). Core no. EEZ21-8G contains 0.33–0.97% TOC (average, 0.71%) and 0.03–0.08% TN (average, 0.06%), twice as much as the other two cores (Figure 7a). The TOC generally decreases upward to a minimum value (0.33%) within the top 10 cm of depth, similar to the distribution of TN. TOC and TN distribution characteristics result in an upward increasing pattern of the C/N ratio (8.9 to 16.9; average, 12.6) along the core depth. In core no. EEZ21-9G, TOC decreases to the depth of 40 cm (0.22%), then increases toward the bottom of the core (0.56%), with some irregularity of trend (Figure 7b). However, TN is distributed in a simple pattern with an abrupt increase (0.02% to 0.05%) at the depth of 130 cm. The C/N ratio decreases toward the top (8.3 to 15.5; average, 11.6), similar to the case in core no. EEZ21-8G. Core no. EEZ21-17G contains the lowest TOC content (0.15–0.42%; average, 0.26%), which is relatively constant to the core bottom within its variation (Figure 7c). However, TN tends to increase slightly toward the bottom (0.02% to 0.05%), resulting in an upward increase of C/N ratio (5.9 to 9.6; average, 7.2). The distribution patterns of C/N ratio in the cores indicate that the southeastern part of the basin increasingly received sediments containing marine organic matter, whereas the basin generally received organic matter from terrestrial plants [6,46].
The sediment methane concentration in the two cores from the Gunsan Basin is relatively small (Table 1 and Figure 7), but ~10 times higher than the concentration in the surface waters of the South and North Yellow Sea [47,48]. Core no. EEZ21-8G has the lowest methane concentration (2.1 to 5.7 ppb; average, 4.5 ppb), which increases from the top to the depth of 55 cm but is more or less constant toward the bottom within a variation of ~1 ppb (Figure 7a). The δ13C1 values range from −49.7‰ to −44.7‰ Vienna Pee Dee Belemnite (VPDB) (average, −49.0‰) and are relatively constant throughout the core, excepting an abrupt increase at the depth of 125 cm. In core no. EEZ21-9G, the methane concentration is 5.1 to 9.4 ppb (average, 6.3 ppb) and is distributed in a relatively irregular pattern with depth (Figure 7b). The δ13C1 values are quite constant, from −52.6‰ to −50.7‰ VPDB (average, −52.4‰), throughout the core depth, with a slight decrease in the interval between 60 and 120 cm. The carbon isotope ratios of methane in the two cores are very similar to those (−54.8‰ to −39.0‰) in the Neogene gas reservoirs of the Bohai Bay Basin [10,29]. Notably, the concentrations and δ13C1 of methane in core no. EEZ21-17G is highly variable. The former ranges from 7.1 to 137.2 ppb (average, 44.5 ppb), and the latter ranges from −91.4‰ to −43.8‰ VPDB (average, 79.5‰) (Figure 7c). Their distribution patterns are abnormal, especially in the depth interval between 10 and 50 cm, where the methane concentration is lowest (4.0 to 21.3 ppb) but the δ13C1 value is largest (−58.6‰ to −43.8‰ VPDB).

4. Discussion

In the Gunsan Basin, the eastern part of the NSYSB, gas seepage is very active, as shown by the wide distribution of gas plumes along the survey lines, especially in the southwestern part of the area (Figure 1 and Figure 2). The gas seepage is not characteristically associated with pockmarks in the seafloor, similar to the southeastern Yellow Sea off the west coast of Jeju Island [20].
The absence or rare occurrence of pockmarks may result from their easy burial by redistribution of highly mobile sand-dominant palimpsest sediments in the eastern part of the Yellow Sea [8,35,39], as indicated by the sedimentary ridges covered with sand waves (Figure 3 and Figure 5). It is common for pockmarks to be rare where sediment reworking is active due to strong tidal currents, storm surges, or other water movements [17,21]. In this case, gases seem to escape into the water column through groups of small vents in the reworked sediments, which have been observed visually by a remotely operated underwater vehicle fitted with a camera [13,34,49]. Largely different from the case in the Gunsan Basin, many pockmarks and some sedimentary domes occur in the western part of the NSYSB [50], where fine-grained sediments derived from the Yellow River are deposited at a relatively rapid deposition rate [40]. This difference in pockmark occurrence pattern in the Yellow Sea is comparable to the compilation of [17], in the sense that pockmarks are mostly formed and remain on the seafloor where deposition of fine-grained sediments prevails. The elongated crater occurring along the side wall and the acoustic characteristics of the sedimentary ridge (Figure 5) suggest that the ridge-forming sedimentary processes can trap the gases seeping from depth on the seafloor by sealing them with sediments until the ridge cannot support the pressure of the accumulating gases [20,27,45]. The burial of gas seepages and pockmarks by sediment reworking and deposition is regarded to have continued since the Holocene sea-level rise in the Yellow Sea.
Gas seepage can be episodically triggered by cyclic variations of tide, seismic activities, seasonal changes in seawater temperature, and other causes [21,33,43]. In the Gunsan Basin, the gas seepage appears to be influenced partially by the reduced hydrostatic pressure of low tide (Figure 4a), although the seepage intensity is inconsistent with the tidal range (Figure 4b). The gas expulsions in the shallow nearshore off of California in the United States [23,25] and the Yampi Shelf off of northwest Australia [21] are clearly related to periods of low tide. However, this phenomenon is controversial even at neighboring sites and during similar observation times off of the Hikurangi Margin, New Zealand [34,51,52]. Gas seepage is strongly activated after and often prior to earthquakes, although seepage has been only limitedly observed after events stronger than magnitude 6 [16,24,53]. As suggested by the occurrence of a crater and diapirs (Figure 6 and Figure 7), gas seepage also appears to have occurred episodically in the Gunsan Basin by substantial expulsion of pressurized gases, fluid, and fine-grained sediments through the near-surface sedimentary layer [21,22,54]. This study did not confirm whether earthquakes caused the episodic gas expulsion because observations of their contribution to gas seepage were not made; however, in and around the Gunsan Basin, earthquakes with magnitudes of 1.4 to 3.2 occurred 11 times between January and June 2010, before the basin survey [55]. The gas seepage seems to be quiescent at present, despite a recent expulsion evidenced by the crater being unburied even with active sediment redistribution processes (Figure 5) and the poor concentration of methane (Table 1 and Figure 7), which is only slightly greater (about 10 times) than the values in the surface water layer of the Yellow Sea [47,48].
In the Gunsan Basin, the geochemical characteristics of core sediments, including the concentration and carbon isotope ratio (δ13C1) of methane, are largely different depending on the site (Table 1 and Figure 7). These locally dependent characteristics suggest that hydrocarbons can partially leak vertically via the buoyancy of microbubbles rather than by lateral migration of major flows related to regional seals and key carrier beds such as faults and diapirs [56]. Applying the simple classification of gas origin based on the value of δ13C1 [26,32,57], the methane seeping near the crater and diapirs would be thermogenic (δ13C1 greater than −50‰ VPDB) and mixed methane with a trace amount of biogenic methane (δ13C1 less than −50‰ VPDB), whereas in the southeastern basin, the methane would be biogenic with pockets of thermogenic methane and slightly mixed methane in the near-seafloor sediment layer. The apparent discrepancy between methane origins and TOC concentrations in the cores strongly implies that the methane has migrated upward from a considerable depth, regardless of its origin, because methane is generated exclusively in sediments containing organic carbon at levels greater than 0.4% [46,57]. The two organic carbon-rich sediment cores near the crater and diapirs must contain biogenic methane instead of a thermogenic component (Figure 7a,b), whereas the organic carbon-depleted sediment core of the southeastern basin would have the opposite (Figure 7c). In the latter core, the anomalous distribution of the two kinds of methane is ascribed to sequential episodic upward advection of porewater containing biogenic methane prior to that with thermogenic methane [58,59], rather than to erratic microbial processes [45,60]. In this case, the biogenic methane-containing porewater overlying the rising thermogenic methane-containing pore water pocket would be displaced toward the rear tracks of the pocket at the rapid pace of a few weeks, which is too rapid to cause any fractionation of carbon isotopes or mixing with methane of different origins in the pathways [58,59,61].
These findings pertaining to the gas seeping in the Gunsan Basin indicate that gas seepage continues episodically and that the seepage is related to the internal structures of the underlying sedimentary strata [22,43,49] with partial facilitation from low tide (Figure 5a). The preferential distribution of gas plumes in the southwestern region (Figure 2) is comparable to the distribution of the faults and anticlines in Cretaceous-Cenozoic sedimentary strata that were delineated in multi-channel seismic profiles [8]. This implies that hydrocarbon resources can be reserved in the Gunsan Basin, as is possibly confirmed by the criteria of petroliferous basins, namely, water column and seafloor evidence such as gas plumes, craters, and diapirs, although these are not always restricted to being directly above a reservoir [14,15,49]. Furthermore, the methane seeping near the crater and diapirs is isotopically very similar to the methane in the Neogene gas reservoirs of the Bohai Bay Basin, that is, secondary microbial methane mixed with small amounts of thermogenic methane initially present in the reservoirs [10,29]. A possible reservoir of this type of methane in the Gunsan Basin is postulated from the proven distribution map of secondary biogenic gases in the northern and southern South Yellow Sea Basins [30,62]. As evaluated by [12], the Gunsan Basin seems to satisfy the requirements for generation of hydrocarbon resources in a continental rift basin filled with a thick Mesozoic–Cenozoic succession intercalated with lacustrine and fluvial source rock strata, as in the petroliferous basins of eastern China and the Yellow Sea [3,11,41,57]. To test these implications further, more geochemical analyses of hydrocarbon gases and CO2 are needed to define in detail the origin, maturity, and degradation of gases and source materials in the Gunsan Basin. Besides, additional seismic exploration is required to find structures that facilitate the migration and trap of gas.

5. Conclusions

A composite survey of high-resolution chirp (3.5 kHz) seismic profiling and geochemical analyses of methane and sediments in short cores was carried out in the Gunsan Basin of the central Yellow Sea. On the chirp seismic profiles, gas seepage was recognized acoustically by stacked hyperbolic reflections indicating plumes in the water column extending from the uppermost sedimentary layer, which is either acoustically turbid or transparent with some enhanced internal reflectors due to fully or partially charged gases. Here, hydrocarbon gases are actively seeping through the formation of many plumes, which occur singly or in groups up to a size of more than 50 km, especially in the southwestern part of the basin. The gas seepage seems to be partially favored by the reduced hydrostatic pressure of low tide, and it is often strong enough to create a crater at the sidewall of the sedimentary ridge and two diapirs. Pockmarks are not characteristically observed, probably due to their easy burial by the active sediment redistribution processes under the strong tidal currents and storm surges. These processes appear to seal the gas seeping from depth by forming a sedimentary ridge until the accumulating gases and fluids become overpressurized and can be expelled through a crater.
Near the crater and diapirs, methane is poorly concentrated but rich in the heavy carbon isotope (δ13C1, −52.6‰ to −44.7‰ VPDB) in relatively organic carbon-rich sediments. These characteristics indicate that methane formed through secondary microbial activity on oils at depth migrated upward and remained after the latest gas expulsion. In the southeastern basin, secondary microbial methane is pocketed in the upper narrow interval of the sediment core, which is charged with primary biogenic methane. The core sediments are too depleted in organic carbon to generate methane. This anomalous distribution of the two kinds of methane is interpreted as the result of porewater containing the secondary microbial methane being episodically advected upward from depth sequentially after the primary biogenic injection.
Based on these findings, gas seepage is postulated to have occurred often through catastrophic expulsions largely related to the underlying fault and anticline system of the Mesozoic-Cenozoic sedimentary succession in the southwestern part of the Gunsan Basin. The isotopic character of the methane is very similar to that in the petroliferous basins of eastern China and the Yellow Sea, suggesting that the underlying conditions of the Gunsan Basin are favorable for the generation and preservation of hydrocarbon resources. To confirm this hypothesis, detailed analyses of deep seismic data and additional types of sediment porewater gases and deep drilling are required.

Author Contributions

All authors have contributed extensively to the work presented in this paper. Conceptualization, J.-H.C. and B.-R.L. Methodology, all the authors. Data curation, all the authors. Writing-original draft preparation, N.-D.J., C.-K.L., S.-Y.L., B.-C.K. and S.-K.J. Writing-review and editing, all the authors. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Korea Institute of Marine Science & Technology Promotion (KIMST), funded by the Ministry of Oceans and Fisheries, Korea (20210696).

Data Availability Statement

The original contributions presented in the study are included in the article material, further inquiries can be directed to the corresponding author.

Acknowledgments

We would like to express our deep appreciation to the officers and crew of R/V Onnuri and the research scientists of the Marine Domain & Security Research Department (MSSRC) of KIOST for their help in collecting seismic data and sediment cores during the survey. B.C. Kum is especially appreciated for his help collecting the multi-beam echo-sounding data.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The distribution of sedimentary basins and physiography of Gunsan Basin in the Yellow Sea (modified from [7]).
Figure 1. The distribution of sedimentary basins and physiography of Gunsan Basin in the Yellow Sea (modified from [7]).
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Figure 2. Track lines of composite survey of multi-channel deep and high-resolution (3.5 kHz) Chirp seismic profiling in the Gunsan Basin. Only Chirp seismic profiles were analyzed for this study. The figures cited in the text and sediment core sites are represented by thick black bars and filled circles with numbers, respectively. The occurrence of gas plumes is presented by thick blue bars.
Figure 2. Track lines of composite survey of multi-channel deep and high-resolution (3.5 kHz) Chirp seismic profiling in the Gunsan Basin. Only Chirp seismic profiles were analyzed for this study. The figures cited in the text and sediment core sites are represented by thick black bars and filled circles with numbers, respectively. The occurrence of gas plumes is presented by thick blue bars.
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Figure 3. (A,B) Chirp seismic profile indicates acoustic blacking resulting from shallow gas in the Gunsan Basin. (C) Selected sparker seismic profile across the Gunsan Basin and its shows acoustic anomalies of shallow gas. Seismic profiles showing gas plumes and acoustic blanking in the eastern part of Gunsan Basin. Gas plumes are represented by stacked hyperbolic backscatter reflections that occur as single or closely spaced groups with various heights for a distance of more than 50 km (Figure 2). The uppermost sedimentary layer is acoustically turbid and transparent, probably depending on the degree of gas charge in the sediment pore space.
Figure 3. (A,B) Chirp seismic profile indicates acoustic blacking resulting from shallow gas in the Gunsan Basin. (C) Selected sparker seismic profile across the Gunsan Basin and its shows acoustic anomalies of shallow gas. Seismic profiles showing gas plumes and acoustic blanking in the eastern part of Gunsan Basin. Gas plumes are represented by stacked hyperbolic backscatter reflections that occur as single or closely spaced groups with various heights for a distance of more than 50 km (Figure 2). The uppermost sedimentary layer is acoustically turbid and transparent, probably depending on the degree of gas charge in the sediment pore space.
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Figure 4. Relationships of number (a) and height of plumes to tidal range in the Gunsan Basin (b). The plume height is assumed to represent the seepage intensity.
Figure 4. Relationships of number (a) and height of plumes to tidal range in the Gunsan Basin (b). The plume height is assumed to represent the seepage intensity.
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Figure 5. A crater with about 250 m width and 7 m depth occurring in the west side of a sedimentary ridge ((a), Chirp seismic profile) on which sand waves are dominant with NE-SW direction ((b), multi-beam echosounding image). On Chirp seismic profile, the ridge is acoustically transparent in central part, whereas partially turbid with some enhanced internal reflectors on both sides.
Figure 5. A crater with about 250 m width and 7 m depth occurring in the west side of a sedimentary ridge ((a), Chirp seismic profile) on which sand waves are dominant with NE-SW direction ((b), multi-beam echosounding image). On Chirp seismic profile, the ridge is acoustically transparent in central part, whereas partially turbid with some enhanced internal reflectors on both sides.
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Figure 6. A diapiric dome on the Chirp seismic profile (a), but two domes are observed on the multi-beam echo sounding image (b). On the latter, the domes are about 4 m high, surrounded by a moat-like depression on the west side.
Figure 6. A diapiric dome on the Chirp seismic profile (a), but two domes are observed on the multi-beam echo sounding image (b). On the latter, the domes are about 4 m high, surrounded by a moat-like depression on the west side.
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Figure 7. Distributions of geochemical characteristics, textural composition (%), mean grain size ( ∅ ), TOC (%), C/N ratio, CH4 (C1) concentration (ppb), and δ13C1 (‰) in core nos. EEZ21-08G (a), −09G (b), and −17G (c).
Figure 7. Distributions of geochemical characteristics, textural composition (%), mean grain size ( ∅ ), TOC (%), C/N ratio, CH4 (C1) concentration (ppb), and δ13C1 (‰) in core nos. EEZ21-08G (a), −09G (b), and −17G (c).
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Table 1. Summary of geochemical characters of sediments, concentration, and stable carbon isotope ratio of methane in 3 cores from the Gunsan Basin.
Table 1. Summary of geochemical characters of sediments, concentration, and stable carbon isotope ratio of methane in 3 cores from the Gunsan Basin.
Core
No.		Mz *
(φ) 		TOC **
(%) 		C/N ***
Ratio 		Methane
Conc.
(ppb) 		δ13C
(‰)
EEZ21-08G
(average)		5.6–8.3
(7.5) 		0.33–0.97
(0.71) 		8.9~16.9
(12.6) 		2.1–5.7
(4.5) 		−49.7–44.7
(−49.0)
EEZ21-09G
(average)		5.3–8.0
(6.4) 		0.23–0.56
(0.38) 		8.8~15.5
(11.6) 		5.1–9.4
(6.3) 		−52.6–50.8
(−51.4)
EEZ21–17G
(average)		5.9–7.5
(6.9) 		0.15–0.42
(0.26) 		5.9~9.6
(7.2) 		7.1–137.2
(44.5) 		−91.4–43.8
(−79.5)
* Mz: mean grain size; ** TOC: total organic carbon; *** C/N: carbon to nitrogen ratio.
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Cho, J.-H.; Lee, S.-Y.; Jang, S.; Jang, N.-D.; Lee, C.-K.; Lee, S.-H.; Kum, B.-C.; Lee, B.-R.; Jung, S.-K. The Implications of Seeping Hydrocarbon Gases in the Gunsan Basin, Central Yellow Sea, off the Southwest of Korea. Geosciences 2024, 14, 230. https://doi.org/10.3390/geosciences14090230

AMA Style

Cho J-H, Lee S-Y, Jang S, Jang N-D, Lee C-K, Lee S-H, Kum B-C, Lee B-R, Jung S-K. The Implications of Seeping Hydrocarbon Gases in the Gunsan Basin, Central Yellow Sea, off the Southwest of Korea. Geosciences. 2024; 14(9):230. https://doi.org/10.3390/geosciences14090230

Chicago/Turabian Style

Cho, Jin-Hyung, Seung-Yong Lee, Seok Jang, Nam-Do Jang, Cheol-Ku Lee, Seung-Hun Lee, Byung-Cheol Kum, Bo-Ram Lee, and Seom-Kyu Jung. 2024. "The Implications of Seeping Hydrocarbon Gases in the Gunsan Basin, Central Yellow Sea, off the Southwest of Korea" Geosciences 14, no. 9: 230. https://doi.org/10.3390/geosciences14090230

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