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Geology and geochemistry of cold seepage and venting-related carbonates
... These hydrocarbon seepages are present in seafloor surface sediments as both free and adsorbed forms, altering the geochemical properties of the seafloor sediments. Substantial evidence has demonstrated that the migration of deep geological fluids is a widespread natural phenomenon [10][11][12][13]. Various processes drive fluid migration in the sedimentary basins, allowing these fluids to reach the surface. ...
... Submarine hydrocarbon seepage is classified into macro-seepage and microseepage [10,25,26]. Macro-seepage involves visible or detectable methane bubble leakage on the seabed, with a significant methane flux forming geological structures like pockmarks and mud volcanoes and promoting chemoautotrophic biological communities and authigenic carbonate rocks. ...
Widespread submarine hydrocarbon seepage can form complex fluid seepage characteristics, with submarine sediment geochemistry effectively recording seepage activities and fluid component changes due to hydrocarbon seepage. This is crucial for offshore oil and gas exploration and understanding global climate change. Therefore, using the geochemical information of submarine sediments to trace hydrocarbon seepage activities is of great significance. In order to identify the geochemical anomaly characteristics and genetic types of acid-hydrolyzed hydrocarbons in submarine sediments in the Yantai Depression of the South Yellow Sea Basin, Eastern China, and to explore the relationship between these anomalies and deep oil and gas, geochemical columnar samples were taken at 100 stations in the study area. A total of 100 sets of acid-hydrolyzed hydrocarbon data and 26 sets of carbon isotope data were analyzed. The results show that the content of acid-hydrolyzed hydrocarbons at each station is in the following order: methane (AC1) > ethane (AC2) > propane (AC3) > butane (AC4) > pentane (AC5). The determination coefficient between the saturated hydrocarbon indicators exceeds 0.9, indicating that these components have the same source. Data analysis reveals that the genetic type of hydrocarbon gases in the study area is generally thermogenic, with limited microbial contribution to saturated hydrocarbons, indicating deep oil and gas characteristics. The coincidence between the anomalous areas and geological structures indicates that the distribution of these anomalies is closely related to fault distribution.
... Cold seep fluids have a temperature that closely match that of seawater and are mainly controlled by the pressure gradient. These fluids flow or leak into the seawater column, thereby undergoing a series of physical, chemical and biological changes that lead to the formation of magnificent submarine cold seep systems (Chen et al., 2002;Chen et al., 2007;Judd and Hovland, 2007). Cold seep systems occur in all seas and oceans. ...
... Some of these fluids may seep into the seafloor, producing microtopography related to seabed fluid flow (e.g., pockmarks, mounds, domes and mud volcanoes) (Judd and Hovland, 2007;Shang et al., 2013). Cold seep carbonates, a small amount of sulfide and sulfate form on the seafloor because of biochemical sedimentation (Chen et al., 2002), and it is easier for gas, oil droplets, and water (including some re-suspended fine-grained sediments) to escape, resulting in plumes on the seafloor; thus, plumes are direct evidence of active cold seeps. ...
Submarine cold seeps are widely distributed in the continental margin seas around the world. As opposed to the earlier detections of cold seep plumes using a high frequency acoustic method, a conventional multichannel seismic reflection (Seismic Oceanography) method is applied in this study to image the water column near the seafloor. Along with ananalys of the fluid escape structures, the development position, seismic reflection characteristics, and features of the cold seeps have been described and discussed here. Seismic reflection from the water column is very weak; therefore, seismic sections above and below the seafloor are processed in two different sequences as follows: (1) geometry definition, direct wave attenuation and amplitude recovery, highpass filter, common midpoint sorting, constant velocity (seawater sound velocity) stack, and post-stack FK filter in some sections; and (2) data quality control, amplitude recovery, 6~100 Hz bandpass filter, multiple attenuation, deconvolution, velocity analysis, normal moveout correlation, common midpoint stack, post-stack noise attenuation, 4~70 Hz bandpass filter, and FX migration. The processed sections are then assembled together along the seafloor after the color scale has been carefully adjusted. The analysis shows that active cold seeps primarily present plume, broom, or irregular geometric shapes in the water column, with weak and chaotic seismic reflections. Sometimes the seismic reflection amplitude is enhanced, likely because of the suspended mud or particles in the water column. Cold seepage activities are usually associated with the fluid escape structures of mud diapirs, pipes, faults, fissures, gas chimneys, seabed pockmarks, and mud volcanoes, reflecting fluid migration from deep to shallow strata, seep or escape at the seafloor, and formation of wide-spread cold seep activities. All the results are derived from a comprehensive interpretation of the seismic sections of the water columns and strata; however, field studies, theoretical simulations, and experiments are required for further confirmation.
... Cold seep fluids have a temperature that closely match that of seawater and are mainly controlled by the pressure gradient. These fluids flow or leak into the seawater column, thereby undergoing a series of physical, chemical and biological changes that lead to the formation of magnificent submarine cold seep systems (Chen et al., 2002;Chen et al., 2007;Judd and Hovland, 2007). Cold seep systems occur in all seas and oceans. ...
... Some of these fluids may seep into the seafloor, producing microtopography related to seabed fluid flow (e.g., pockmarks, mounds, domes and mud volcanoes) (Judd and Hovland, 2007;Shang et al., 2013). Cold seep carbonates, a small amount of sulfide and sulfate form on the seafloor because of biochemical sedimentation (Chen et al., 2002), and it is easier for gas, oil droplets, and water (including some re-suspended fine-grained sediments) to escape, resulting in plumes on the seafloor; thus, plumes are direct evidence of active cold seeps. ...
Submarine cold seeps are widely distributed in the continental margin seas around the world. In this study, we apply a conventional multichannel seismic reflection (seismic oceanography) method to image the water column near the seafloor in order to detect cold seeps. In addition to analyzing the fluid escape structures, we also describe and discuss the development positions, seismic reflection characteristics as well as features of the cold seeps. The seismic reflection from the water column is very weak; therefore, the seismic sections above and below the seafloor are processed in two different sequences as follows: (1) geometry definition, direct wave attenuation and amplitude recovery, high-pass filter, common midpoint sorting, constant velocity (seawater sound velocity) stack, and post-stack FK filter in some sections; and (2) data quality control, amplitude recovery, 6∼100 Hz bandpass filter, multiple attenuation, deconvolution, velocity analysis, normal move-out correction, common midpoint stack, post-stack noise attenuation, 4∼70 Hz bandpass filter, and FX migration. The processed sections are then assembled together along the seafloor after carefully adjusting the color scale. The analysis shows that active cold seeps primarily present plume, broom, and/or irregular shapes that have weak and chaotic seismic reflections in the water column. The seismic reflection amplitude is enhanced at times; this could be attributed to the suspension of mud or particles in the water column. Cold seepage activities are typically associated with fluid escape structures, including mud diapirs, pipes, faults, fractures, gas chimneys, seabed pockmarks, and mud volcanoes. This indicates fluid migration from deep to shallow strata, fluid seepage or escape at the seafloor, as well as the formation of widespread cold seep activities. All the results herein are derived from a comprehensive interpretation of the seismic sections of the water columns and strata; however, further field studies, theoretical simulations, and experiments are required to confirm these conclusions.
... Cold seeps are frequently observed in marine settings of continental margins worldwide, and the last 30 years' investigations show that they are known from the tropics to the poles, in shallow shelf to hadal depths [1][2][3][4][5][6][7][8][9][10][11]. The seep fluids are commonly composed of water and methane, and occasionally heavy hydrocarbons, such as crude oil. ...
... When migrating towards the seafloor, seeping methane would set off anaerobic oxidation (AOM: CH 4 +SO 4 2 → HCO 3 +HS +H 2 O) mediated by the methane-oxidizing ar-chaea and the sulfate-reducing bacteria at the sulfate-methane transition zone (SMT) [12][13][14][15]. This reaction increases the water saturation state with respect to the bicarbonate (HCO 3 ), which favors precipitation of carbonate minerals, forming so-called seep carbonates [1, 2,14]. Therefore, seep carbonates are fingerprint files of past cold seepage activities. ...
Hydrocarbon seeps, widely occurring in continental margins, have become increasingly focused owing to their close relationships with gas hydrates, strong greenhouse gas methane, and biological resources in extreme environments. Ancient hydrocarbon seeps have already been recognized from Devonian to Quaternary strata worldwide based on seep carbonates or seep-related fossil chemosynthetic assemblages. However, seep-related deposits are rarely found from ancient strata in the mainland China. Here, we report the first discovery of an ancient seep deposit, specifically late Cretaceous seep carbonates from Xigaze in Tibet, China. Xigaze seep carbonates, occurring as nodules, are enclosed in upper Cretaceous turbidite strata in Xigaze forearc basin. These carbonates are composed of authigenic carbonate (56.2% on average), clastic quartz and feldspar (27.3% on average), and clay minerals (chlorite, illite and smectite, 16.5% on average). Clotted micrites, peloids and framboid pyrites are frequently observed, all of which are common in modern seep carbonates. The carbonates have negative δ
13C values varying from −27.7‰ to −4.0‰(V-PDB), suggesting that thermogenic methane is the primary carbon source. Ce/Ce* values revised by eliminating La effects show no real Ce anomaly, indicating the carbonates were primarily precipitated in a weak reducing environment. Overall, these features provide unequivocal evidences that the seafloor of Xigaze forearc basin developed hydrocarbon seeps in late Cretaceous.
... Petroleum seepage is also recorded in ancient deposit. Peckmann et al. [2] reported the early Carboniferous seep deposits in Harz Mountains, Germany. The most depleted 13 C value is 32‰, impsonite is also observed and the possible carbon sources are considered to be from a petroleum reservoir. ...
... The Sr concentration of different component of the seep deposits shows large variation (Table 2). The Sr concentration of seep carbonate is related to its mineralogy , the content of Sr in aragonite is higher than that of in calcite [2] . Because all analyzed components of the seep deposits here are of similar mineral compositions that are mainly composed of aragonite, thus the influence of mineralogy to Sr concentration can be ruled out. ...
Seep carbonates were collected from the Alaminos Canyon lease area, Gulf of Mexico. The carbonates are present as slabs and
blocks. Bivalve shell and foraminifer are the dominant bioclasts in carbonate. Pores are common and usually filled with acicular
aragonite crystals. XRD investigation shows that aragonite is the dominate mineral (98%). Peloids, clotted microfabirc and
botryoidal aragonite are developed in carbonate and suggest a genesis linked with bacterial degradation of the hydrocarbons.
The δ13C value of bioclasts in carbonate is from −4.9% to −0.6%, indicating that the carbon source is mainly from sea water as well
as the small portion incorporation of the seep hydrocarbon. The microcrystalline and sparite aragonite shows the δ13C value from −31.3% to −23.4%, suggesting that their carbon is derived mainly from microbial degradation of crude oil. 14C analyses give the radiocarbon age of about 10 ka. Rare earth elements (REE) analyses of the 5% HNO3-treated solution of the carbonates show that the total REE content of the carbonates is low, that is from 0.752 to 12.725
μg·g−1. The shale-normalized REE patterns show significantly negative Ce anomalies. This suggests that cold seep carbonate is most
likely formed in a relatively aerobic environment.
... The chemical properties and gas sources of hydrates are reflected by the gas composition and carbon isotopic characteristics of the gas hydrates (Chen et al., 2002;Fu et al., 2011;Fang et al., 2019) . The methane contents of the gas samplesare greater than 98.5%, while ethane and propane are about 1 and 0.5%, respectively, but high Frontiers in Earth Science | www.frontiersin.org ...
Gas hydrate production testing was conducted in 2017 in the Shenhu Area in the northern part of the South China Sea, and unprecedented success was achieved. In order to obtain gas production and physical properties of gas hydrate reservoirs in the study area and determine the location of test production wells, the seismic and logging data and drilling cores were analyzed in detail, the physical characteristics of the sediments, faults, gas components, and reservoir were studied. The results show that 1) the gas hydrates are diffusion type, with reservoirs dominated by clayey silt sediments, and the gas hydrate-bearing layers are characterized by soup-like, porridge-like, cavity, and vein structures; 2) the resistivity and acoustic velocity of gas hydrate formation are significantly higher than those of the surrounding sediments, while the neutron porosity, density, and natural gamma are slightly lower; the Bottom Simulating Reflectors (BSRs) in seismic profiles exhibit the exist of gas hydrates; 3) gas chimneys and faults are well-developed beneath the BSRs, and hydrocarbon gases can easily migrate into the gas hydrate reservoirs in areas with stable temperature and pressure conditions; 4) the gas hydrate saturation is high, the highest saturation in site W17 was up to 76%, with an average of 33%; while the highest saturation in site W19 was up to 68%, with an average of 31%. The gas source is considered as mixed gas of thermogenic gas and microbial gas. By comparing the core samples and geophysical characteristics of sites W17 and W19 in the study area and calculating the thickness, distribution area, and saturation of the hydrate deposition layer, it was found that site W17 is characterized by a thick layer, large area, high saturation, and good sealing, and thus, site W17 was established as the test production site. The development of gas chimney and faults provides pathways for the upward migration of deep gas, and the gas migrates to gas hydrate stable zone in forms of diffusion, water soluble and free state, forming high saturation of diffusion gas hydrates.
... Qiongdongnan Basin (Wang et al. 2018;Wei et al. 2019;Liu et al. 2020). In the first decade of the 21st century, many evidences (bottom simulating reflectors, blanking zone, geochemical evidence for pore water, etc) show that the Qiongdongnan Basin is rich in hydrate resources (Zhu et al. 2001;Wu et al. 2003;Chen et al., 2002Chen et al., , 2004Wang et al. 2008). The large range of bottom simulating reflectors (BSR), mud volcanoes, mud diapirs and gas chimneys in the basin were related to the accumulation of NGH (Wu et al. 2003;Chen et al. 2004;Wang et al. 2008； Hui et al. 2016). ...
The seepage activity of paleo-methane has been proven to exist in geological history and play an important role in the evolution of the Earth's environment. This study reports grain size, major/trace elements, total nitrogen (TN), total carbon (TC), total organic carbon (TOC), total sulfur (TS), magnetic susceptibility and AMS-¹⁴C measurements for sediment samples in a drill core (~282 cm long; Site Q6 at ~1400 m water depth) from the ‘Haima seep’ sedimentary area in the Qiongdongnan Basin, along the northern slope of the South China Sea (SCS). The alternative index system for paleo-methane seepage identification was established through testing and analysis, and three methane release events (MREs) were successfully identified: 254–282 cm (MRE1: 19.3–17.8 ka), 144–162 cm (MRE2: 11. 9–10.8 ka) and 90–124 cm (MRE3: 10.0–8.5 ka). When methane seepage occurred, the enrichment of authigenic carbonate minerals (high Mg-calcite or aragonite) led to the increase in the content of CaO and ‘dilution’ in the total rare earth element content (∑REE). However, the occurrence of authigenic pyrite resulted in a significant increase in TS and the total sulfur/total organic carbon ratio (S/C) in the sediments, and a synchronous decrease in the magnetic susceptibility. Meanwhile, the change of redox environment also induced the enrichment of Mo, U, and the positive anomaly of δCe. Based on the records of methane release events, the control mechanisms for the different time scales and different periods, such as sea level change, sea floor temperature change, and turbidity current activity were preliminarily discussed. Finally, the history of methane seepage activity was reconstructed, and conceptual diagrams were drawn of the study area encompassing the last 20 ka.
... In the appropriate submarine environment (temperature and pressure), part of the leaked natural gas will be precipitated into hydrate. Cold spring carbonate rock, chemical auto trophic organism, natural gas hydrate, undersea natural gas overflow, etc. mark the development of undersea natural gas leakage system [38][39][40]. ...
The northern waters of the South China Sea are rich in natural gas hydrate resources. The source of gas is a crucial factor controlling the formation of natural gas hydrates. Gas sources that form natural gas hydrates can be related to three categories: pyrolysis genesis, biological genesis, and inorganic genesis. A series of large- and medium-sized Mesozoic and Cenozoic sedimentary basins have developed in the northern South China Sea. These basins are rich in organic matter and there are multiple sets of source rocks buried deeply, which are the main gas sources for hydrate accumulation. The Chaoshan Depression is the largest Mesozoic residual depression in the northern South China Sea. Analysis of geochemical and geological samples show that there is a significant oil and gas leakage activity on the seabed. Seismic data and drilling confirmed the existence of natural gas hydrates in Chaoshan Depression. The natural gas hydrate accumulation pattern of Chaoshan Depression is a leak- type accumulation, which has the characteristics of unique structure, abundant Mesozoic gas source, and development of faulting system.
... Previous research has shown that part of leaky natural gas can be transformed into carbon dioxide by bacterial action in the seabed, meanwhile, sulfate in the seawater is deoxidized to sulfureted hydrogen, then integrate with calcium and iron in the water and deposit on the seabed. These depositions which are called cold seep carbonates slowly solidify and finally generate carbonate crust near the leakage point [17,18]. The reflectors of ligule have been distinguished clearly in the sub-bottom profile nearby the survey line Gh1, which indicates that the natural gas is leaking from the seabed in the research area [19]. ...
Submarine seep plumes are a natural phenomenon in which different types of gases migrate through deep or shallow subsurface sediments and leak into seawater in pressure gradient. When detected using acoustic data, the leaked gases frequently exhibit a flame-like structure. We numerically modelled the relationship between the seismic response characteristic and bubble volume fraction to establish the bubble volume fraction in the submarine seep plume. Results show that our models are able to invert and predict the bubble volume fraction from field seismic oceanography data, by which synthetic seismic sections in different dominant frequencies could be numerically simulated, seismic attribute sections (e.g., instantaneous amplitude, instantaneous frequency, and instantaneous phase) extracted, and the correlation between the seismic attributes and bubble volume fraction be quantitatively determined with functional equations. The instantaneous amplitude is positively correlated with bubble volume fraction, while the instantaneous frequency and bubble volume fraction are negatively correlated. In addition, information entropy is introduced as a proxy to quantify the relationship between the instantaneous phase and bubble volume fraction. As the bubble volume fraction increases, the information entropy of the instantaneous phase increases rapidly at the beginning, followed by a slight upward trend, and finally stabilizes. Therefore, under optimal noise conditions, the bubble volume fraction of submarine seep plumes can be inverted and predicted based on seismic response characteristics in terms of seismic attributes.
Breakthroughs in shale gas exploration in the Upper Ordovician-Lower Silurian strata of the Upper Yangtze Platform have attracted interest in its sedimentary-tectonic evolution, but the tectonic background of the northern margin of the Upper Yangtze Platform remains unclear. In this paper, the Wufeng-Longmaxi formations on the northern margin of the Upper Yangtze Platform were investigated. Based on geochemical and mineralogical analyses of the tuffs/K-bentonites of the Wufeng Formation and the barite in the Longmaxi Formation, as well as previous research results, it was concluded that the northern margin of the Upper Yangtze Platform was in an extensional tectonic background during the Late Ordovician-Early Silurian. Detailed analysis revealed that, (1) the U–Pb zircon age of the tuff in the Bajiaokou section in South Qinling is 443.91 ± 0.92 Ma. The Zr/TiO2–Nb/Y diagram of the tuffs/K-bentonites indicates that their protoliths were alkaline-subalkaline basalt and andesite series rock. Based on the Th–Hf/3-Ta, Th–Tb*3-Ta*2, and TiO2–Nb/3-Th diagrams, there are undiscovered intraplate tension calc-alkaline basalts in the northern Yangtze Platform or the southern Qinling region, which provided volcanic clastic materials to the Ziyang, Lan'gao, Chengkou, Yichang and other regions. (2) Scanning electron microscopy revealed that the barite crystals in the Longmaxi Formation exhibit dissolution features and have a large particle size. Energy spectrum analysis of these barite crystals revealed that they have C, O, S, and Ba contents of 8.48 wt%, 22.98 wt%, 13.09 wt% and 55.44 wt%, so they are speculated to have been formed via cold methane seep genesis in a weak extensional tectonic setting. The ⁸⁷Sr/⁸⁶Sr ratios of the barite revealed that different types of barite were simultaneously formed in this area under the influences of hydrothermal and cold methane seeps. (3) The analysis of the heavy minerals in the Lower Silurian strata in the Bajiaokou section revealed that the provenance in the South Qinling area changed significantly during the late Early Silurian. Based on the above analyses, the northern margin of the Upper Yangtze Platform was in an extensional tectonic setting during the Late Ordovician-Early Silurian. The distribution of the total organic carbon content indicated that the extensional tectonic background provided good conditions for the enrichment and preservation of organic matter. The results of this study provide an understanding of the regional sedimentary-tectonic pattern and evolution of the Yangtze Platform during this period, as well as a reference for future shale gas exploration in this region.
In this paper, the QS-1 sediments (3.64 m long; at 1500 m water depth) collected from the Qiongdongnan Basin were used to analyze the relationship between sedimentary geochemical anomalies and methane seepage. Mo-U covariation, Ce anomaly, and Mg/Ca-Sr/Ca end-member model were analyzed to identify the variations of redox environment and authigenic carbonate precipitation, finally constructing the methane release events in this study area. These observations suggest that: 1) Significant Mo/U enrichments and Ba front indicate the presence of high-flux active methane seepage with SMI of 260 cmbsf in this study area. The depositional environment evolved as oxic-anoxic-reducing-sulfide environment from bottom to top, as evidenced by Mo-U covariation and Ce anomaly; 2) Based on Sr/Ca and Mg/Ca end-member model proposed by Bayon et al. (2007), the high-Mg calcite was the dominant authigenic carbonate currently which usually precipitated behind the methane seep events; and 3) The active cold seep can be divided into three methane release events (MREs): MRE I, the most intense methane seepage occurred at 22.1 ka BP; MRE II, three low-intensity methane seepage occurred at 19.3, 16.5, and 11.3 ka BP, respectively; and MRE III, the continuous methane seep began at 9.3 ka BP until now. The synthesis of geochemical indicators has significant implications for tracing methane seepage of varying intensity and duration.
Sinian Dongpo Formation overlying tillite of Luoquan Formation consists mainly of shales, silty shales and siltstones. Lenticular dolomites, dolomitic siltstones and sandstones are unexpectedly found in Dongpo Formation. Soft sediment reformation occurs in dolomitic siltstones which are inconsistent with the shales of Dongpo Formation. Negative anomaly of δ13C is found in lenticular dolomites and dolomitic siltstones. δ13C of lenticular dolomites is -4.19‰ to -6.18‰ and δ13C of dolomitic siltstones mostly between -2‰ to -4‰. So it is considered that lenticular dolomites, dolomitic siltstones and sandstones in Dongpo Formation, similar to cap dolomite of Sinian and seep carbonates overlying tillite of Sturtian ice-age in South China, have been formed by interaction between CO2 seeped from gas hydrate and Mg2+ in sea water, that is, lenticular dolomites, dolomitic siltstones and sandstones in Dongpo Formation are the cause of formation of cold spring.
In the Northeast Guizhou and its surrounding, a series of black shales, covering the till sheet of Tiesiao Formation which is thought to be formed during the Sturtian, distributes at the bottom of Datangpo Formation, lower Nanhua Series, Neoproterozoic. Generally, the lower part of the black shales is mainly composed of carbonate such as dialogite, dolomite and so on, near the center of a succession of syngenetic fault basins. Through the studies of the carbonate outcrop in this area in terms of the field structure, mineral components and the characteristics of carbonate, oxygen and sulfur isotopes, it is shown that sulfur isotope presents distinct positive excursion in contrast with carbonate isotope that indicates an intense negative excursion. In addition, a mount of stomatal dialogite and significant structures like mud volcano, diapers and so on have been found. Based on the above features, the studied carbonate is supposed to have come into being under the same mechanisms as modern cold venting carbonate on the sea floor via the seep and release of gas hydrate and the environmental mutation. Therefore a simple conclusion is obtained that the carbonate for research is possible to be generated on the ancient ocean floor in the Proterozoic as the deposition of cold venting carbonate, because of the gas hydrate seep. This study widens the research areas for the ocean floor gas hydrate and its spatial and temporal distribution, supplying a new idea for the analysis of the diagenesis, mineralization and environmental assessment.
There develop a number of carbonate concretes in the black mud shale sediments on the deep outer continental shelf of Upper Permian Linxi Formation in the sum area of Harigentai village, Xi Ujimqin Qi, Inner Mongolia. These carbonate concretes show lentoid, concretionary and mound structures in the field, similar to the cold spring carbonate rocks formed on the ancient and modern ocean floor. The micro fabric, matter composition and carbon isotopic characteristics are researched in detail. Microscopic observations show that there are many vesicular structures, organisms and pyrite to aggregate in the carbonate concretes. The stable carbon-oxygen isotopic characteristics show that in the carbon isotope exist negative values, indicating the origin of the organic carbon. These features are similar to that of the cold spring carbonate rocks formed on the modern ocean floor because of natural gas seepage. Therefore, the paper infers that the carbonate concretes in study area are the cold spring carbonate rocks formed on the ancient ocean floor because of natural gas seepage. This suggests that during the deposition of Linxi Formation, there were gas hydrates on the ocean floor, which might be relative to the methane release from the deep grey-black mud shale deposited earlier. A potential analysis about hydrocarbon source rocks also show that the deep grey-black mud shale wide developed in study area have a certain hydrocarbon-generating potential, and the Linxi Formation has a relatively good potential for petroleum exploration. But because of the relatively old strata, and the influences of diagenesis and tectogenesis, the petroleum exploration is rather difficult in present.
Gas Hydrate reservoir in the northeastern continental slope of South China Sea(SCS) have been confirmed by investigations and well drillings. Also, large amounts of methane-derived authigenic carbonates (MADC) were discovered in 2004 among this area. Based on sub-bottom profiles and seismic profiles, attributes and features associated with cold seepage system in the northeastern continental slope of SCS were analyzed. A seismic line cross the northeastern continental slope of SCS was used to show local sedimentary environment. Sub-bottom data were pieced together and denoised. Shallow gas, fluid migration and morphology features associated with cold seepage in processed sub-bottom profile were identified and discussed, with assistance of corresponding seismic lines and bathymetric charts. From observation of sub-bottom profiles and comparison with seismic and bathymetry data, cold seepage related features were listed as follow: (1) Acoustic plume (suspected of being cold seep) was found in a sub-bottom profile, with height of about 30 meters and width of 50 meters. (2) Mud volcanoes were found in this area, and caused discontinuity of Bottom Simulating Reflectors in seismic profiles. (3) Acoustic voids, the most frequent features in this region, had two types: “narrow” acoustic void and “broad” acoustic void. “narrow” acoustic void had width of 80~400 m and no layers information, “broad” acoustic void had width over 1000m and weak layers information. (4) The area of “narrow” acoustic void overlaid with mud volcanoes concentrated area, and the acoustic plume was located around “broad” acoustic voids. Cold seepage activities exist on the northeastern continental slope of SCS, both in history and in present. The results suggest that “narrow” acoustic voids on sub-bottom profiler correspond to fluid migration path, while “broad” acoustic voids possibly related with shallow gas accumulation along rock layers. The relationship between cold seepage system and gas hydrate in the northeastern continental slope of SCS worth further investigation.
A cold vent is an area where methane-rich fluid seepage occurs. This seepage may alter the local temperature, salinity, and subsequent accumulation of the gas hydrate. Using a kinetic gas hydrate formation model and in situ measurement of temperature, salinity and fluid flux at the southern summit of Hydrate Ridge, we simulate the gas hydrate accumulation at three distinct fluid sites: clam, bacterial mat, and gas discharge sites. At the clam sites (pore water flux < 20 kg m−2 yr−1), pore water advection has little influence on temperature and salinity. However, the salinity and temperature are increased (peak salinity > 0.8 mol kg−1) by the formation of gas hydrate causing the base of the hydrate stability zone to move gradually from ∼115 to ∼70 meters below seafloor (mbsf). The gas hydrate saturation at the clam sites is relatively high. The water flux at the bacterial mat sites ranges from 100 to 2500 kg m−2 yr−1. The water flow suppresses the increase in salinity resulting in a salinity close to or slightly higher than that of seawater (< 0.65 mol kg−1). Heat advection by water flow increases temperature significantly, shifting the base of the hydrate stability zone to above 50 or even 3 mbsf. The gas hydrate saturation is relatively low at the bacterial mat site. At the gas discharge sites, the pore water flux could reach 1010 kg m−2 yr−1, and the temperature could reach that of the source area in 9 min. There is no gas hydrate formation at the gas discharge sites. Our simulative analysis therefore reveals that a lower pore water flux would result in lower salinity, higher temperature, and a shallower base of the hydrate stability zone. This in turn induces a lower gas hydrate formation rate, lower hydrate saturation, and eventually less gas hydrate resources.
Here we reported the fatty-acids and their δ
13C values in seep carbonates collected from Green Canyon lease block 185 (GC 185; Sample GC-F) at upper continental slope (water
depth: ∼540 m), and Alaminos Canyon lease block 645 (GC 645; Sample AC-E) at lower continental slope (water depth: ∼2200 m)
of the Gulf of Mexico. More than thirty kinds of fatty acids were detected in both samples. These fatty acids are maximized
at C16. There is a clear even-over-odd carbon number predominance in carbon number range. The fatty acids are mainly composed of
n-fatty acids, iso-/anteiso-fatty acids and terminally branched odd-numbered fatty acids (iso/anteiso). The low δ
13C values (−39.99‰ to.32.36‰) of n-C12:0, n-C13:0, i-C14:0 and n-C14:0 suggest that they may relate to the chemosynthetic communities at seep sites. The unsaturated fatty acids n-C18:2 and C18:1Δ9 have the same δ
13C values, they may originate from the Beggiatoa/Thioploca. Unlike other fatty acids, the terminally branched fatty acids (iso/anteiso) show lower δ
13C values (as low as −63.95‰) suggesting a possible relationship to sulfate reducing bacteria, which is common during anaerobic
oxidation of methane at seep sites.
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