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1.
耦合碳、硫同位素及其它地球化学指征研究海底甲烷渗漏   总被引：1，自引：1，他引：0  
Methane seepage is the signal of the deep hydrocarbon reservoir. The determination of seepage is significant to the exploration of petroleum, gas and gas hydrate. The seepage habits microbial and macrofaunal life which is fueled by the hydrocarbons, the metabolic byproducts facilitate the precipitation of authigenic minerals. The study of methane seepage is also important to understand the oceanographic condition and local ecosystem. The seepage could be active or quiescent at different times. The geophysical surveys and the geochemical determinations reveal the existence of seepage. Among these methods, only geochemical determination could expose message of the dormant seepages. The active seepage demonstrates high porewater methane concentration with rapid SO_4~(2–) depleted, low H_2S and dissolved inorganic carbon(DIC), higher rates of sulfate reduction(SR) and anaerobic oxidation of methane(AOM). The quiescent seepage typically develops authigenic carbonates with specific biomarkers, with extremely depleted ~13C in gas, DIC and carbonates and with enriched ~34S sulfate and depleted ~34S pyrite. The origin of methane, minerals precipitation, the scenario of seepage and the possible method of immigration could be determined by the integration of solutes concentration, mineral composition and isotopic fractionation of carbon, sulfur. Numerical models with the integrated results provide useful insight into the nature and intensity of methane seepage occurring in the sediment and paleooceanographic conditions. Unfortunately, the intensive investigation of a specific area with dormant seep is still limit. Most seepage and modeling studies are site-specific and little attempt has been made to extrapolate the results to larger scales. Further research is thus needed to foster our understanding of the methane seepage.  相似文献   

2.
Cold seeps are pervasive along the continental margin worldwide,and are recognized as hotspots for elemental cycling pathway on Earth.In this study,analyses of pore water geochemical compositions of one-400 cm piston core(S3) and the application of a mass balance model are conducted to assess methane-associated biogeochemical reactions and uncover the relationship of methane in shallow sediment with gas hydrate reservoir at the Makran accretionary wedge off Pakistan.The results revealed that approximately 77% of sulfate is consumed by the predominant biogeochemical process of anaerobic oxidation of methane.However,the estimated sulfate-methane interface depth is-400 cm below sea floor with the methane diffusive flux of 0.039 mol/(m~2·a),suggesting the activity of methane seepage.Based on the δ~(13)C_(DIC) mass balance model combined with the contribution proportion of different dissolved inorganic carbon sources,this study calculated the δ~(13)C of the exogenous methane to be-57.9‰,indicating that the exogenous methane may be a mixture source,including thermogenic and biogenic methane.The study of pore water geochemistry at Makran accretionary wedge off Pakistan may have considerable implications for understanding the specific details on the dynamics of methane in cold seeps and provide important evidence for the potential occurrence of subsurface gas hydrate in this area.  相似文献   

3.
The hydrocarbon gases in the L1 gas field of the Lishui-Jiaojiang Sag have been commonly interpreted to be an accumulation of pure sapropelic-type thermogenic gas. In this study, chemical components, stable isotopic compositions, and light hydrocarbons were utilized to shed light on the origins of the hydrocarbon fluids in the L1gas pool. The hydrocarbon fluids in the L1 gas pool are proposed to be a mixture of three unique components:mid-maturity oil from the middle Paleocene coastal marine Lin...  相似文献   

4.
The Taixinan Basin is one of the most potential gas hydrate bearing areas in the South China Sea and abundant gas hydrates have been discovered during expedition in 2013. In this study, geochemical and microbial methods are combinedly used to characterize the sediments from a shallow piston Core DH_CL_11(gas hydrate free) and a gas hydrate-bearing drilling Core GMGS2-16 in this basin. Geochemical analyses indicate that anaerobic oxidation of methane(AOM) which is speculated to be linked to the ongoing gas hydrate dissociation is taking place in Core DH_CL_11 at deep. For Core GMGS2-16, AOM related to past episodes of methane seepage are suggested to dominate during its diagenetic process; while the relatively enriched δ18O bulk-sediment values indicate that methane involved in AOM might be released from the "episodic dissociation" of gas hydrate.Microbial analyses indicate that the predominant phyla in the bacterial communities are Firmicutes and Proteobacteria(Gammaproteobacteria and Epsilonproteobacteria), while the dominant taxa in the archaeal communities are Marine_Benthic_Group_B(MBGB), Halobacteria, Thermoplasmata, Methanobacteria,Methanomicrobia, Group C3 and MCG. Under parallel experimental operations, comparable dominant members(Firmicutes and MBGB) are found in the piston Core DH_CL_11 and the near surface layer of the long drilling Core GMGS2-16. Moreover, these members have been found predominant in other known gas hydrate bearing cores, and the dominant of MBGB has even been found significantly related to gas hydrate occurrence. Therefore,a high possibility for the existing of gas hydrate underlying Core DH_CL_11 is inferred, which is consistent with the geochemical analyses. In all, combined geochemical and microbiological analyses are more informative in characterizing sediments from gas hydrate-associated areas in the South China Sea.  相似文献   

5.
The northern slope of the South China Sea is a gas-hydrate-bearing region related to a high deposition rate of organic-rich sediments co-occurring with intense methanogenesis in subseafloor environments.Anaerobic oxidation of methane(AOM) coupled with bacterial sulfate reduction results in the precipitation of solid phase minerals in seepage sediment,including pyrite and gypsum.Abundant aggregates of pyrites and gypsums are observed between the depth of 667 and 850 cm below the seafloor(cmbsf) in the entire core sediment of HS328 from the northern South China Sea.Most pyrites are tubes consisting of framboidal cores and outer crusts.Gypsum aggregates occur as rosettes and spheroids consisting of plates.Some of them grow over pyrite,indicating that gypsum precipitation postdates pyrite formation.The sulfur isotopic values(δ~(34) S) of pyrite vary greatly(from –46.6‰ to –12.3‰ V-CDT) and increase with depth.Thus,the pyrite in the shallow sediments resulted from organoclastic sulfate reduction(OSR) and is influenced by AOM with depth.The relative high abundance and δ~(34) S values of pyrite in sediments at depths from 580 to 810 cmbsf indicate that this interval is the location of a paleo-sulfate methane transition zone(SMTZ).The sulfur isotopic composition of gypsum(from–25‰ to –20.7‰) is much lower than that of the seawater sulfate,indicating the existence of a 34 S-depletion source of sulfur species that most likely are products of the oxidation of pyrites formed in OSR.Pyrite oxidation is controlled by ambient electron acceptors such as MnO_2,iron(Ⅲ) and oxygen driven by the SMTZ location shift to great depths.The δ~(34) S values of gypsum at greater depth are lower than those of the associated pyrite,revealing downward diffusion of 34 S-depleted sulfate from the mixture of oxidation of pyrite derived by OSR and the seawater sulfate.These sulfates also lead to an increase of calcium ions from the dissolution of calcium carbonate mineral,which will be favor to the formation of gypsum.Overall,the mineralogy and sulfur isotopic composition of the pyrite and gypsum suggest variable redox conditions caused by reduced seepage intensities,and the pyrite and gypsum can be a recorder of the intensity evolution of methane seepage.  相似文献   

6.
The North Yellow Sea Basin ( NYSB ), which was developed on the basement of North China (Huabei) continental block, is a typical continental Mesozoic Cenozoic sedimentary basin in the sea area. Its Mesozoic basin is a residual basin, below which there is probably a larger Paleozoic sedimentary basin. The North Yellow Sea Basin comprises four sags and three uplifts. Of them, the eastern sag is a Mesozoic Cenozoic sedimentary sag in NYSB and has the biggest sediment thickness; the current Korean drilling wells are concentrated in the eastern sag. This sag is comparatively rich in oil and gas resources and thus has a relatively good petroleum prospect in the sea. The central sag has also accommodated thick Mesozoic-Cenozoic sediments. The latest research results show that there are three series of hydrocarbon source rocks in the North Yellow Sea Basin, namely, black shales of the Paleogene, Jurassic and Cretaceous. The principal hydrocarbon source rocks in NYSB are the Mesozoic black shale. According to the drilling data of Korea, the black shales of the Paleogene, Jurassic and Cretaceous have all come up to the standards of good and mature source rocks. The NYSB owns an intact system of oil generation, reservoir and capping rocks that can help hydrocarbon to form in the basin and thus it has the great potential of oil and gas. The vertical distribution of the hydrocarbon resources is mainly considered to be in the Cretaceous and then in the Jurassic.  相似文献   

7.
The distributions of different forms of nitrogen in the surface sediments of the southern Huanghai Sea are different and affected by various factors. The contents of IEF-N, SOEF-N and TN gradually decrease eastward, and those of SAEF-N northward, while those of WAEF-N westward. Around the seaport of the old Huanghe (Yellow) River, the contents of both SOEF-N and TN are the highest. Among all the factors, the content of fine sediment is the predominant factor to affect the distributions of different forms of nitrogen. The contents of IEF-N, SOEF-N, and TN have visibly positive correlation with the content of f‘me sediments, and the correlative coefficient is 0.68, 0.58 and 0.71 respectively, showing that the contents of the three forms of nitrogen increase with those of f-me sediments. The content of WAEF-N is related to that of fine sediments to a certain extent, with a correlative coefficient of 0.35; while the content of SAEF-N is not related to that of fine sediments, showing that the content of SAEF-N is not controlled by fine grain-size fractions of sediments. In addition, the distributions of different forms of nitrogen are also interacted one another, and the contents of IEF-N and SOEF-N are obviously affected by TN, while those of inorganic nitrogen (WAEF-N, SAEF-N and IEF-N) are not affected by SOEF-N and TN obviously, although they are interacted each other.  相似文献   

8.
Previous studies of gas hydrate in the Dongsha area mainly focused on the deep-seated gas hydrates that have a high energy potential, but cared little about the shallow gas hydrates occurrences. Shallow gas hydrates have been confirmed by drill cores at three sites(GMGS2 08, GMGS2 09 and GMGS2 16) during the GMGS2 cruise, which occur as veins, blocky nodules or massive layers, at 8–30 m below the seafloor. Gas chimneys and faults observed on the seismic sections are the two main fluid migration pathways. The deep-seated gas hydrate and the shallow hydrate-bearing sediments are two main seals for the migrating gas. The occurrences of shallow gas hydrates are mainly controlled by the migration of fluid along shallow faults and the presence of deep-seated gas hydrates.Active gas leakage is taking place at a relatively high-flux state through the vent structures identified on the geophysical data at the seafloor, although without resulting in gas plumes easily detectable by acoustic methods.The presence of strong reflections on the high-resolution seismic profiles and dim or chaotic layers in the subbottom profiles are most likely good indicators of shallow gas hydrates in the Dongsha area. Active cold seeps,indicated by either gas plume or seepage vent, can also be used as indicators for neighboring shallow gas hydrates and the gas hydrate system that is highly dynamic in the Dongsha area.  相似文献   

9.
As an active back-arc basin, the Okinawa Trough is located in the southeastern region of the East China Sea shelf and is strongly influenced by the subduction of the Philippine Sea Plate. Major element, trace element and Sr-NdPb isotopic composition data are presented for volcanic rocks from the Iheya Ridge(IR), the middle Okinawa Trough. The IR rocks record large variations in major elements and range from basalts to rhyolites. Similar trace element distribution characteristics together with small variations in ~(87)Sr/~(86)Sr(0.703 862–0.704 884), ~(144)Nd/~(143)Nd(0.512 763–0.512 880) and Pb isotopic ratios, demonstrate that the IR rocks are derived from a similar magma source. The fractional crystallization of olivine, clinopyroxene, plagioclase, and amphibole, as well as accessory minerals, can reasonably explain the compositional variations of these IR rocks. The simulations suggest that approximately 60% and 75% fractionation of an evolved basaltic magma can produce trace element compositions similar to those of the intermediate rocks and acid rocks, respectively. The analysis of their Sr-Nd-Pb isotopic content ratios suggest that the source of the rocks from the IR is close to the depleted mantle(DM) but extends to the enriched mantle(EMII), indicating that the mantle source of these rocks is a mixture between the DM and EMII end members. The simulations show that the source of the IR volcanic rocks can be best interpreted as the result of the mixing of approximately 0.8%–2.0% subduction sediment components and 98.0%–99.2% mantlederived melts.  相似文献   

10.
The natural gas generation process is simulated by heating source rocks of the Yacheng Formation, including the onshore-offshore mudstone and coal with kerogens of Type II_2-III in the Qiongdongnan Basin. The aim is to quantify the natural gas generation from the Yacheng Formation and to evaluate the geological prediction and kinetic parameters using an optimization procedure based on the basin modeling of the shallow-water area. For this, the hydrocarbons produced have been grouped into four classes(C_1, C_2, C_3 and C_(4-6)). The results show that the onset temperature of methane generation is predicted to occur at 110℃ during the thermal history of sediments since 5.3 Ma by using data extrapolation. The hydrocarbon potential for ethane, propane and heavy gaseous hydrocarbons(C_(4-6)) is found to be almost exhausted at geological temperature of 200℃ when the transformation ratio(TR) is over 0.8, but for which methane is determined to be about 0.5 in the shallow-water area. In contrast, the end temperature of the methane generation in the deep-water area was over 300℃ with a TR over 0.8. It plays an important role in the natural gas exploration of the deep-water basin and other basins in the broad ocean areas of China. Therefore, the natural gas exploration for the deep-water area in the Qiongdongnan Basin shall first aim at the structural traps in the Ledong, Lingshui and Beijiao sags, and in the forward direction of the structure around the sags, and then gradually develop toward the non-structural trap in the deep-water area basin of the broad ocean areas of China.  相似文献   

11.
About 120 gas seepage vents were documented along the west and southwest coast of the Hainan Island, South China Sea, in water depths usually less than 50 m. The principal seepage areas include the Lingtou Promontory, the Yinggehai Rivulet Mouth, Yazhou Bay, the Nanshan Promontory and the Tianya Promontory. They occur along three major zones, reflecting the control by faults and lateral conduits within the basement. It is estimated that the total gas emission from these seepage vents is 294–956 m3/year. The seepage gases are characterized by a high CH4 content (76%), heavy δ13C1 values (−38 to −33‰) and high C1/C1–5 ratios (0.95–1.0), resembling the thermogenic gases from the diapiric gas fields of the Yinggehai Basin. Hydrocarbon–source correlation shows that the hydrocarbons in the sediments from seepage areas can be correlated with the deeply buried Miocene source rocks and sandstone reservoirs in the central depression. The 2D basin modeling results based on a section from the source rock center to the gas seepage sites indicate that the gas-bearing fluids migrated from the source rocks upward through faults or weak zones encompassed by shale diapirism or in up-dip direction along the sandstone-rich strata of Huangliu Formation to arrive to seabed and form the nearshore gas seepages. It is suggested that the seepage gases are sourced from the Miocene source rocks in the central depression of the Yinggehai Basin. This migration model implies that the eastern slope zone between the gas source area of the central depression and the seepage zone is also favorable place for gas accumulation.  相似文献   

12.
Significant offshore asphaltic deposits with active seepage occur in the Santa Barbara Channel offshore southern California. The composition and isotopic signatures of gases sampled from the oil and gas seeps reveal that the coexisting oil in the shallow subsurface is anaerobically biodegraded, generating CO2 with secondary CH4 production. Biomineralization can result in the consumption of as much as 60% by weight of the original oil, with 13C enrichment of CO2. Analyses of gas emitted from asphaltic accumulations or seeps on the seafloor indicate up to 11% CO2 with 13C enrichment reaching +24.8‰. Methane concentrations range from less than 30% up to 98% with isotopic compositions of –34.9 to –66.1‰. Higher molecular weight hydrocarbon gases are present in strongly varying concentrations reflecting both oil-associated gas and biodegradation; propane is preferentially biodegraded, resulting in an enriched 13C isotopic composition as enriched as –19.5‰. Assuming the 132 million barrels of asphaltic residues on the seafloor represent ~40% of the original oil volume and mass, the estimated gas generated is 5.0×1010 kg (~76×109 m3) CH4 and/or 1.4×1011 kg CO2 over the lifetime of seepage needed to produce the volume of these deposits. Geologic relationships and oil weathering inferences suggest the deposits are of early Holocene age or even younger. Assuming an age of ~1,000 years, annual fluxes are on the order of 5.0×107 kg (~76×106 m3) and/or 1.4×108 kg for CH4 and CO2, respectively. The daily volumetric emission rate (2.1×105 m3) is comparable to current CH4 emission from Coal Oil Point seeps (1.5×105 m3/day), and may be a significant source of both CH4 and CO2 to the atmosphere provided that the gas can be transported through the water column.  相似文献   

13.
The relationship between surface bubble composition and gas flux to the atmosphere was examined at five large seeps from the Coal Oil Point seep field (Santa Barbara Channel, CA, USA). The field research was conducted using a flux buoy designed to simultaneously measure the surface bubbling gas flux and the buoy’s position with differential GPS, and to collect gas samples. Results show that the flux from the five seeps surveyed a total of 11 times ranged from 800–5,500 m3 day?1. The spatial distribution of flux from the five seeps was well described by two lognormal distributions fitted to two flux ranges. The seafloor and sea surface composition of bubbles differed, with the seafloor bubbles containing significantly more CO2 (3–25%) and less air (N2 and O2). At the sea surface, the mole fraction of N2 correlated directly with O2 (R 2 = 0.95) and inversely with CH4 (R 2 = 0.97); the CO2 content was reduced to the detection limit (<0.1%). These data demonstrate that the bubble composition is modified by gas exchange during ascent: dissolved air enters, and CO2 and hydrocarbon gases leave the bubbles. The mean surface composition at the five seeps varied with water depth and gas flux, with more CH4 and higher CH4/N2 ratios found in shallower seeps with higher flux. It is suggested that the CH4/N2 ratio is a good proxy for total or integrated gas loss from the rising bubbles, although additional study is needed before this ratio can be used quantitatively.  相似文献   

14.
High-resolution sonar surveys, and a detailed subsurface model constructed from 3D seismic and well data allowed investigation of the relationship between the subsurface geology and gas-phase (methane) seepage for the Coal Oil Point (COP) seep field, one of the world’s largest and best-studied marine oil and gas seep fields, located over a producing hydrocarbon reservoir near Santa Barbara, California. In general, the relationship between terrestrial gas seepage, migration pathways, and hydrocarbon reservoirs has been difficult to assess, in part because the detection and mapping of gas seepage is problematic. For marine seepage, sonar surveys are an effective tool for mapping seep gas bubbles, and thus spatial distributions. Seepage in the COP seep field occurs in an east–west-trending zone about 3–4 km offshore, and in another zone about 1–2 km from shore. The farthest offshore seeps are mostly located near the crest of a major fold, and also along the trend of major faults. Significantly, because faults observed to cut the fold do not account for all the observed seepage, seepage must occur through fracture and joint systems that are difficult to detect, including intersecting faults and fault damage zones. Inshore seeps are concentrated within the hanging wall of a major reverse fault. The subsurface model lacks the resolution to identify specific structural sources in that area. Although to first order the spatial distribution of seeps generally is related to the major structures, other factors must also control their distribution. The region is known to be critically stressed, which would enhance hydraulic conductivity of favorably oriented faults, joints, and bedding planes. We propose that this process explains much of the remaining spatial distribution.  相似文献   

15.
The Coal Oil Point seep field located offshore Santa Barbara, CA, consists of dozens of named seeps, including a peripheral ～200 m2 area known as Brian Seep, located in 10 m water depth. A single comprehensive survey of gas flux at Brian Seep yielded a methane release rate of ～450 moles of CH4 per day, originating from 68 persistent gas vents and 23 intermittent vents, with gas flux among persistent vents displaying a log normal frequency distribution. A subsequent series of 33 repeat surveys conducted over a period of 6 months tracked eight persistent vents, and revealed substantial temporal variability in gas venting, with flux from each individual vent varying by more than a factor of 4. During wintertime surveys sediment was largely absent from the site, and carbonate concretions were exposed at the seafloor. The presence of the carbonates was unexpected, as the thermogenic seep gas contains 6.7% CO2, which should act to dissolve carbonates. The average δ13C of the carbonates was ?29.2?±?2.8‰ VPDB, compared to a range of ?1.0 to +7.8‰ for CO2 in the seep gas, indicating that CO2 from the seep gas is quantitatively not as important as 13C-depleted bicarbonate derived from methane oxidation. Methane, with a δ13C of approximately ?43‰, is oxidized and the resulting inorganic carbon precipitates as high-magnesium calcite and other carbonate minerals. This finding is supported by 13C-depleted biomarkers typically associated with anaerobic methanotrophic archaea and their bacterial syntrophic partners in the carbonates (lipid biomarker δ13C ranged from ?84 to ?25‰). The inconsistency in δ13C between the carbonates and the seeping CO2 was resolved by discovering pockets of gas trapped near the base of the sediment column with δ13C-CO2 values ranging from ?26.9 to ?11.6‰. A mechanism of carbonate formation is proposed in which carbonates form near the sediment–bedrock interface during times of sufficient sediment coverage, in which anaerobic oxidation of methane is favored. Precipitation occurs at a sufficient distance from active venting for the molecular and isotopic composition of seep gas to be masked by the generation of carbonate alkalinity from anaerobic methane oxidation.
Figure
Processes modulating carbonate formation at Brian Seep (California) during times of high and low sediment burden  相似文献   

16.
Concentrations of dissolved methane in seawater and bottom sediments, as well as of methane emanating from gas seeps were measured at 18 stations including several small bays in the Sevastopol coastal area (Black Sea) during 2007–2008. Methane concentrations in surface waters ranged from 10 to 2,970 nmol l?1, and correlated well with values recorded for sediments. Methane concentrations in the water column were influenced by water depth, as well as by air and water temperatures. In the spring and summer of 2008, in situ CH4 saturation relative to air was in the range of 970–71,900%. Maximum saturation was in summer. CH4 fluxes to the atmosphere from the Sevastopol coastal area were estimated to vary from 190 to 1,550 μmol m?2 day?1. Gas bubbles escaping from the seepages contained about 57 vol% methane. Radiocarbon dating of the methane revealed an age not exceeding 150 years, implying a biogenic origin.  相似文献   

17.
Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C1/C2+ from all the seepage sites were in the range of 1,500–50,000, while δ13C and δD values of methane ranged from ?66.0 to ?63.2‰ VPDB and ?204.6 to ?196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO2. δ13C values of ethane and propane (i.e., ?40.8 to ?27.4‰ VPDB and ?41.3 to ?30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3–16.6‰, while there were no differences in δ13C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.  相似文献   

18.
Gas occurrences consisting of carbon dioxide (CO2), hydrogen sulfide (H2S), and hydrocarbon (HC) gases and oil within the Dodan Field in southeastern Turkey are located in Cretaceous carbonate reservoir rocks in the Garzan and Mardin Formations. The aim of this study was to determine gas composition and to define the origin of gases in Dodan Field. For this purpose, gas samples were analyzed for their molecular and isotopic composition. The isotopic composition of CO2, with values of −1.5‰ and −2.8‰, suggested abiogenic origin from limestone. δ34S values of H2S ranged from +11.9 to +13.4‰. H2S is most likely formed from thermochemical sulfate reduction (TSR) and bacterial sulfate reduction (BSR) within the Bakuk Formation. The Bakuk Formation is composed of a dolomite dominated carbonate succession also containing anhydrite. TSR may occur within an evaporitic environment at temperatures of approximately 120–145 °C. Basin modeling revealed that these temperatures were reached within the Bakuk Formation at 10 Ma. Furthermore, sulfate reducing bacteria were found in oil–water phase samples from Dodan Field. As a result, the H2S in Dodan Field can be considered to have formed by BSR and TSR.As indicated by their isotopic composition, HC gases are of thermogenic origin and were generated within the Upper Permian Kas and Gomaniibrik Formations. As indicated by the heavier isotopic composition of methane and ethane, HC gases were later altered by TSR. Based on our results, the Dodan gas field may have formed as a result of the interaction of the following processes during the last 7–8 Ma: 1) thermogenic gas generation in Permian source rocks, 2) the formation of thrust faults, 3) the lateral-up dip migration of HC-gases due to thrust faults from the Kas Formation into the Bakuk Formation, 4) the formation of H2S and CO2 by TSR within the Bakuk Formation, 5) the vertical migration of gases into reservoirs through the thrust fault, and 6) lateral-up dip migration within reservoir rocks toward the Dodan structure.  相似文献   

19.
Characteristics of two natural gas seepages in the North Sea   总被引：1，自引：0，他引：1  
Two occurrences of active gas seepages are described from the North Sea. The southernmost one, situated above a salt diapir in Norwegian block , has been studied and sampled by use of a remotely operated vehicle (ROV). This seepage consists of about 120 single seeps located within a diameter of 100 m. It is estimated to produce 24 m3 of methane gas per day (at ambient pressure, 75 m water depth). Isotope values of the methane gas and higher hydrocarbon gases in the surrounding seafloor sediments, show that their origin is from a deep seated, thermogenic source. No typical gas-induced erosion features are found on the seafloor at this location, probably due to the lack of very fine grained material.The second occurrence is located in U.K. block (Geoteam, 1984), where the seepage is associated with a very large pockmark depression, measuring 17 m in depth and 700×450 m in width. This depression represents an eroded fine grained sediment volume of 7.105 cubic metres. No detailed inspection or sampling of the gas has been performed here. However seismic reflection anomalies are seen on airgun seismic records at various levels down to a depth of at least 1100 m below seafloor. The seeping gas, possibly mixed with liquids, at this location is therefore also expected to be of a thermogenic origin.  相似文献   

20.
Large quantities of methane (C1), ethane (C2), and propane (C3) emanate from shallow marine seeps near Coal Oil Point (COP), California. Concentrations of these gases were analyzed in the surface water down-current of the seep field over a 15-month period. The variable proportions of C1, C2, and C3 analyzed in gas bubbles emitted from 16 distinct seeps in the COP field encompass much of the variability found in the surface waters down-current. However, waters with disproportionate levels of C1 suggest the presence of additional C1 sources. Based on three spatial surveys, covering areas up to 280 km2, C2 and C3 air-sea fluxes were estimated to be in the order of 3.7 and 1.4 μmol day?1 m?2, respectively. Only 0.6% of C2 and 0.5% of C3 in the dissolved plume originating from the COP seep field are transferred to the atmosphere in the study area, with the fate of the remainder uncertain.  相似文献   

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