Resultado final
Introduction:
The deposition of organic material on the sea floor and its burial below the sulfate zone is the basis for the microbiological production of vast amounts of methane. Methane is an aggressive greenhouse gas when emitted into the atmosphere. The microbiological key process of sub-surface methane oxidation accounts for perhaps 90% of the entire methane flux in the sea floor and therefore plays a critical role as a barrier against methane emission. In order to understand the efficiency of this methane oxidation and its environmental regulation it is important to understand how the thickness of the Holocene layer covering the Pleistocene sediment affects the production and consumption of methane and sulfate. As part of the METROL project research cruises were made to the Western Baltic to study sedimentary systems with diffusive methane flux in the holocene deposition environments of the Bornholm and Arkona basins.
Key results:
The combined analyses of field data and reactive transport modelling showed that methane production in these sediments depends strongly on the quantity of labile organic matter deposited during the Holocene at the sediment-water interface and on the Holocene thickness. In areas of large organic matter inputs, high methanogenesis rates result in supersaturation of methane in the porewater and the formation of methane gas bubbles.
Geophysical seismic surveys of the Western Baltic Sea revealed a wide range of
-Holocene sediment thickness,
-The depths of the sulfate methane transition zone (SMTZ) in the sediment,
-The upper limit of free gas.
Calibration of the Reaction-Transport Models allowed for the identification of the most important factors controlling the depth of free gas and methane turnover. The approach of combining improved high resolution hydroacoustic technology with spatially dense sampling for methane profiles in a selected key area of Aarhus Bay closed a gap between interpretations of seismic imaging of free gas and calculations of methane flux dynamics. The data showed linear correlation between the saturation depth of methane and the upward methane flux, which equals the net methane oxidation. The data further allowed calibration of the upper limit of free gas seen in the seismic images with methane saturation depth in the pore water.
Benefits:
These results enabled us to map net oxidation of methane in Aarhus Bay on the basis of hydroacoustic information alone and thus mark another milestone for the project. This work illustrates how geophysical, biogeochemical and modelling techniques can be synthesized to efficiently determine rates of methane turnover in shelf sediments.
Potential users
-Marine geologists and microbiologists may benefit from the overview of gas occurrences in the area
-Environmental agencies in charge of monitoring coastal areas.
The purpose of this work was to investigate if active seeps of the Central and Northern North Sea host communities of methanotrophs utilising methane and thus partly controlling the methane efflux. The hypothesis was that anaerobic methane-oxidizing communities associated with methane seepage in the North Sea are similar to those known from deep-water seeps. We analyzed two types of seep systems in the North Sea, the Gullfaks seep field, and the Tommeliten area.
The Gullfaks seep field, located at about 61°10 N, 2°14 E lies above one of the three giant oil fields of the North Sea Plateau in the western limb of Viking Graben at a water depth of about 150 meters. The commercially exploited reservoirs are located in two to three kilometres depth in Jurassic sedimentary rocks and are charged by deeper, organic rich sediments. From these shallow reservoirs, methane gas constantly leaks into the overlying sand layers. This sand was deposited during the last glacial maximum when the sea level reached a low with about 120m less than today. Because of the relatively high permeability of the sediments no pockmarks are formed upon gas release. As part of the METROL project, we were able to visit the gas seeps of Gullfaks on the RV Heincke cruise 169 and 208.
The first exploratory visit during He169 allowed to map the field and to detect many small gas seeps in an area of about 100x200m covered by mats of giant sulfide oxidizing bacteria. Gas flares in the water column were detected via a high resolution sediment echo sounder system (SES) and observations of the sea ground. Small streams of rising gas bubbles were observed with a towed camera approximately every 5 square metres.
The main objective of the cruise He208 in May 2004 was to investigate the microbial community, which is fuelled by the rising gas. The bacterial mats were identified to be composed of Beggiatoa filaments. First experiments with radioactive tracer indicated that a high proportion of the methane is oxidized anaerobically with sulphate at Gullfaks, producing high fluxes of sulfide. The sulfide is then oxidized by the giant sulphur oxidizing bacteria belonging to the Beggiatoa group. Molecular analyses of the sediments by FISH and 16S rDNA clone libraries showed that members of the ANME 2a and 2c group in consortium with sulfate reducing bacteria of the Desulfosarcina/Desulfococcus clade as responsible methane-oxidizing microorganisms. The most intriguing question is as to how an anoxic setting is maintained in the upper 20 cm of the sand, a prerequisite as most methanotrophs are highly sensitive to oxygen.
The Tommeliten field in the Central North Sea has also been a site of active exploitation, and its natural gas seeps are known since more than 20 years. Here, cracks in a buried marl horizon allow methane to migrate into overlying sediments. Our observations from vibrocorer sampling show that the marl sediments represent a barrier to the advective methane flow from a deeper reservoir. A shallower depth of the marl boundary in the vicinity of plumes indicates that subsurface gas pressure lifts sediments. As a consequence, methane is advected into the overlying, comparably soft, clayish silt. At several spots coinciding with the apex of the marl domes, methane is released into the water column as shown by hydroacoustic sediment echosounding. In the vicinity of the gas seeps, sea floor observations show small mats of giant sulfide-oxidizing bacteria above patches of black sediments. In the gassy subsurface sediments, the presence of ANME1 communities and their associated partner sulfate reducing bacteria was shown. Anaerobic ethane oxidation was low, but higher in this layer compared to sediments above and below the gassy zone. Furthermore, pieces of carbonates were found in association with the gassy subsurface sediments. Close by to the seepage structures, carbonate crusts are exposed up to 20-50 cm above seafloor forming small reef systems. The subsurface and surface carbonates contain 13C-depleted, archaeal lipids indicating previously high gas seepage at Tommeliten. High amounts of sn2-hydroxyarchaeol relative to archaeol in the crusts and low abundances of biphytanes give evidence that ANME2 archaea were the potential mediators of AOM at the time the carbonates formed.
Introduction of the result:
Methane - a green house gas of major importance is often present in marine sediments in concentrations exceeding saturation. In these environments methane exists as free gas, which occupies the sediment as gas bubbles. Gas bubbles are impenetrable to sound waves, which are therefore, reflected when they are aimed into the sediment and hit methane gas. Therefore, a seismic survey (i.e. sound waves penetrating into the sediment) may reveal the depth of methane saturation if methane is present.
Result description:
Seismic surveys show the methane saturation depth with a resolution of about 25cm or better for approx. each 60 cm across the seafloor given a shooting range of 4 s-1 and a speed of 4.5 knots. Thus the methane saturation depth in large areas of the sea floor may be monitored by seismic surveys not only within a relatively short period of time and but also with a very detailed depth and area resolution compared to sediment sampling which is both time consuming and costly. Furthermore, due to the pressure release during sediment retrieval, methane is lost from the sediment and true in situ concentrations of methane saturation are therefore difficult to determine.
Key innovative features/ key findings:
There is a linear correlation (R2=0.881) between the depth of methane saturation measured by seismic survey and sediment sampling. At 20-36 m water depth, methane net oxidation is linearly correlated (R2=0.743) with methane saturation depth. Hence, with these prerequisites it is possible to estimate methane net oxidation by seismic survey, alone.
Current status & use:
The seismic survey method is under development and statistical analysis of the seismic signals obtained during the project are currently performed.
Dissemination & use potential:
The seismic survey will be useful to uncover areas of the sea floor with elevated concentrations of methane gas. The depth of methane saturation may reveal hot spots in the marine environment if methane oxidation predominates close to the sea floor, which implies high production of toxic hydrogen sulfide and thus oxygen consumption.
Thus ultimately seismic survey may also be used to spot areas of the sea floor where oxygen depletion may initiate. The method therefore appeals to environmental management strategies but will also be useful in scientific projects dealing with methane and reduced sediments (i.e. within the ERA-NET BONUS).
Description of the result
A contour map indicating the depth of shallow gas below seafloor was constructed based on a dense grid of acoustic lines. The map is stored digitally as an ESRI shape file, allowing shallow gas occurrences to be compared with other relevant data rather easily. In most areas of the region the depth intervals shown are: (1) 2-4 m; (2) > 4 m; (3) gas plumes. Better resolution was, however, obtained for the Aarhus Bay area: (1) < 0.5 m; (2) 0.5-2 m; (3) 2-4 m; (4) >4 m.
The contour map is available on CD-ROM including GIS software allowing the user to see map details at any given scale. Data on: 1) sediment type; 2) gas seeps; 3) authigenic carbonates and 4) tracks of acoustic profiles, are also stored on the CD-ROM. Selected acoustic profiles (sparker and chirp) are included to allow the user to look up details using the internet browser. The shallow gas maps including gas seep occurrences are being used to test the presumptions made earlier on methane escape to the atmosphere from the area.
Key innovations:
Seismic mapping of shallow gas in sediments is a new tool, that can be used in connection with environmental monitoring. Until now few cores have been taken and methane contents have been measured directly in the sample, but the seismic studies will make it possible to give qualified estimates of methane volumes and distribution.
Current status and use:
Attempts are being made to combine shallow gas information with pore water chemistry to test applicability of acoustic surveys for monitoring the state of condition in coastal areas vulnerable to oxygen deficit. The Western Bornholm Basin area will be studied by GEUS under the umbrella of Geocenter Copenhagen, which includes the Univ. of Copenhagens Dept for Geology and Geography within the follow-up project Bathy-Sed http://www.geus.dk/departments/quaternary-marine-geol/r esearch-themes/bathy-sed-dk.htm. The project is running from August 2005 until July 2006 and plans to obtain detailed bathymetry and sedimentology data, using multi-beam technology. A cruise with MS Line was done already in 2005 in the same areas covered by a METROL cruise, but for the first time taking Rumohr lot samples. Now, further lab work is planned until August 2006.
Users interested in the result
Marine geologists, microbiologists may benefit from the overview of gas occurrences in the area.
Environmental agencies in charge of monitoring the condition in coastal areas, e.g. NERI for Denmark.
Expected benefits:
Acoustic surveys may become useful for monitoring the state of condition in coastal areas vulnerable to oxygen deficit.
Anaerobic oxidation of methane (AOM) consumes around 80% of methane produced in marine sediments. Although the process was first documented three decades ago, significant progress in the mechanistic understanding of AOM has only occurred in the last few years. Given the vast methane reservoirs below the sea floor, the potential role of AOM in the regulation of atmospheric carbon stocks is widely recognized. Accordingly, one of the main aims of METROL is to characterize and model methane dynamics in ocean margin sediments.
The main goal of the modelling component was to integrate experimental and field data into tractable research and predictive modelling tools.
Reactive transport models (RTM) provide a mathematical means of coupling physical and biogeochemical processes within a single interpretative framework. RTMs are the most powerful tool presently available to advance our scientific understanding of the dynamic interplay between fluid flow, constituent transport and biogeochemical transformations. By integrating experimental, observational and theoretical knowledge about geochemical, biological and transport processes into mathematical formulations, RTMs provide the ground for prognosis.
Models of variable complexity are developed to:
- Quantitatively interpret observed methane and sulfate profiles in marine sediments;
- Determine the sensitivity of methane turnover in marine sediments on bioenergetic and kinetic limitations of the resident microbial communities;
- Predict the response of methane fluxes in ocean margin sediments to changes in environmental forcing.
Current status
A microbial growth model for AOM in coastal marine sediments has been developed. The model includes a comprehensive reaction network describing the production and consumption of methane, it explicitly represents the microbial biomasses and accounts for the thermodynamic limitations on the microbial reaction pathways.
Semi-theoretical methods are used to constrain the microbial growth parameters. The model and the results of a global parameter sensitivity analysis are available to the scientific community (Dale et al., in press).
One-dimensional reactive transport models have been developed within the BRNS (Biogeochemical Reaction Network Simulator) environment. The BRNS is a flexible, web-based modelling platform that consists of a MAPLE pre-processor, containing an automated procedure for generation of executable model (Automatic Code Generator, ACG) and a numerical engine of standard routines for solving non-linear partial differential equations. The ACG translates user-specified information (physical dimensions and parameters, state variables, reaction stoichiometries, rate equations, equilibrium constraints and boundary conditions), and can be linked to an AOM-specific, web-based Knowledge Base (KB). The BRNS-KB modelling tools can be accessed on-line via www.geo.uu.nl/~rtm.
A baseline BRNS model incorporating a simple reaction description of AOM is available via the METROL web page. This model allows for a rapid analysis of methane and sulfate pore water profiles in sediments, and an estimation of the rate of AOM. A series of increasingly more complex reaction network models have also been coupled to transport equations in the BRNS, and are the subject of forthcoming publications. Via the above web-link, potential users can built their own reaction network, or modify an existing one.
Key results
The most sensitive growth-related and kinetic parameters influencing the annual rate of AOM in nearshore marine sediments are those corresponding to hydrogenotrophic sulfate reduction and acetotrophic methanogenesis. Reactive transport calculations indicate that thermodynamic limitations play a major role in controlling the distributions of AOM rates in sediments.
Integrated AOM rates in the Kattegat, Skagarrrak and Aarhus Bay are independent of water depth. As shallow waters usually have higher rates of organic carbon deposition this is an unexpected finding. In all three areas, the free gas depth appears to be the main factor determining the depth of the sulfate-methane transition zone (SMTZ) and the integrated AOM rate. In a general sense, the BRNS models predict that the closer the SMTZ is located to the sediment-water interface, the higher the integrated rate of AOM.
Preliminary model exploration in the Bornholm Basin implies that he physical transport of methane and sulfate is the controlling factor in AOM. Ongoing model simulations will show whether this relationship is universally valid for gas-charged sediments.
Key innovations
Implementation of the BRNS to unravel the relative roles of environmental forcings (esp. organic matter input and temperature), microbial growth, thermodynamic driving forces and transport processes, in controlling AOM in ocean margins.
Application of a global sensitivity analysis to the complex geomicrobial AOM reaction system.
Overview of the result:
Six different water samples, taken from above gas seeps and above a pockmark grope at Tommeliten, Kvitebjørn and Holene, have been analysed for microbial diversity. Primers against both bacteria and Archaea have been used. Some samples have been drawn at varying water depth at the same location.
Corresponding sediment samples have been taken close to the seep and at the pockmark grope.
PCR, DGGE, cloning and sequencing has been done. Restriction enzyme pattern (RFLP) has been run with HeaIII and RsaI.
BLAST analyses have been performed and closest match and phylogenetic dendrogram constructed.
Description of the result:
A direct measurement on seep water samples by flow cytometer reveals a particle distribution of very small size particles, e.g. 200 600 nm. Different staining techniques show that most of the organisms are dead when received on shore.
The DGGE run gives different pattern for the different locations and also a variation for the depth of sampling. There is no resemblance to traditional sea water run as a control.
The amplified ribosomal DNA restriction analysis of the sediments samples show a distinct differentiation between the various sites. Only ANDRA-type 10 is found in all three locations.
Corresponding results were found for the seep water samples. ANDRA-types, 8 and 11 are common for Kvitebjørn and Western Slope. The others are distinct and also different from what was found in the sediment samples.
Grouping of clones of Archaea within RFLP types based on restriction enzyme mapping of M13PCR products with HaeIII and RsaI, showed RFLP-type 1 to be common for Tommeliten, Kvitebjørn and Holene. Three other types were distinct to each location.
Based on the RFLP/ANDRA mapping and sequence analysis, the distribution of bacterial groups within the community from the different seep samples could be evaluated.
A phylogenetic dendrogram is generated and closest match to gene bank affiliations looked upon for better understanding of possible origin and function in the actual habitat.
Current status and use:
Sequence analyses of specific clones are used for DNA probe synthesis. These will be used as specific markers in analysis of samples from other locations and other type of samples associated with oil exploration.
Key innovations:
- Unknown gene sequences indicated presence of new species.
- Differentiation between different locations.
- Differentiation between water versus sediment samples.
Users interested in the result:
- Oil exploration industry.
- Scientists interested in constructing new DNA probes/markers.
Expected benefits:
- Use in oil exploration and extended understanding of oil related/ oil associated microorganisms.
- Publication planned together with MPI.
Result description:
A comprehensive database was established integrating biogeochemical key parameters to determine the methane flux and retention in marine sediments based on existing data in literature. The aim of this extensive data collection and information acquisition is to provide easy access to these data for further investigations, such as the compilation of data on methane fluxes in marine sediments for a flux budget and to estimate the amount of methane.
For this purpose literature was acquired to gain useful data for extraction and analysis. Published data of specific parameters e.g. methane, sulfate, rates of anaerobic methane oxidation (AOM) or sulfate reduction rates were extracted from the literature and entered into the new database. The user friendly data query of the new database will give an easy and fast access to a huge amount of data and thus provides a useful tool for further investigation of various biogeochemical processes directly or indirectly related to methane and sulfate in marine sediments. Additionally, if not available, fluxes were calculated or modelled and also entered into the database. One important finding of this work is the rather low amount of coherent published datasets.
Finally, a comparison with the carbon cycle in the sediment could give information on the amount of organic carbon necessary to produce the measured concentrations of methane below the sulfate/methane transition zone (SMTZ).
Current status and use:
So far, all available data from sediment cores from the North Sea, Baltic Sea, Skagerrak, Kattegat and the Black Sea were edited and analysed. This resulted in approximately 3000 extracted and calculated data points, which were formatted and entered into the established database.
An expansion of the geochemical database established in this project is in progress (planned to be continued until approx. end of 2006). All relevant data gained by the METROL project will be integrated, which focused on the locations named above. Also global methane and sulfate data will be entered into the database. To elevate the density of data supplementary datasets from existent databases will be integrated e.g. data gained by the Ocean Drilling Project (ODP).
In this way the rising amount of data scattered all over the globe is channelled and categorized for a more efficient scientific output and a big step ahead towards reliable budgeting of e. g. methane fluxes on at least regional scales.
Key innovative features:
This extensive compilation of specific data edited into a newly established database provides an easy and fast access to a huge amount of information for further investigation of various biogeochemical processes. The aim of this extensive data collection and their processing will provide for the first time easily accessible data e.g. to calculate global budgets of methane in marine sediments.
Dissemination & use potential:
Scientific community (especially working in fields of biogeochemistry and geochemistry)
Expected benefits:
As a next step after incorporating data collected worldwide we will compare the calculated and modelled methane fluxes with environmental settings and water depths. Furthermore, AOM rates will be compared with sulfate reduction rates, and a global methane flux budget will be drawn. The results as well as visualizations of the data will be published in international journals. The data compiled in this project will also provide a good basis for additional investigations regarding the sulfur cycle and the global sulfur budget. In conjunction with new knowledge on shallow gas turnover gained from METROL (TIP result on the GIS database on methane in surface sediments and related seafloor features) we intend to discover interrelationships between geological, geophysical and biogeochemical parameters which will enable an explanation of the distribution of free methane gas in the upper sediments of the North Sea. Combining these research areas represents a new approach, as integrative analyses of such data have not been done before.
The purpose of this work was to investigate if active seeps of the Central (Tommeliten) and Northern North Sea (Gullfaks), the Skagarrak, the Baltic Sea (Aarhus Bay) and the Black Sea host communities of methanotrophs controlling the methane efflux. The hypothesis was that anaerobic methane-oxidizing communities associated with methane seepage in the North Sea are similar to those known from deep-water seeps.
Researchers have not yet been able to isolate microorganisms capable of oxidizing methane under anoxic conditions. Thus, no pure cultures are currently available for physiological and biochemical characterization and the present knowledge about microbes mediating the anaerobic oxidation of methane (AOM) is therefore mostly derived from cultivation-independent techniques. To assess the microbial diversity and community structure at METROL sites, we mainly worked according to the rRNA approach (Amann et al., 1995). DNA is extracted directly from the environmental sample. The sequences of interest are amplified by general archaeal or bacterial specific primers from the bulk DNA, followed by cloning and sequencing. By comparative sequence analysis with existing 16S rRNA gene databases it is possible to identify whether the sequences found in this environment belong to known or to not yet described species. All sequences which have been published in scientific literatrue, were made available to the public by submitting the sequence data to one of the three public databases (GenBank, EBI, DDBJ) maintained by the National Centre for Biotechnology Information (NCBI, www.ncbi.nlm.nih.gov), the European Molecular Biological Laboratory EMBL, www.embl-heidelberg.de), and the DNA Data Bank of Japan (DDJB, www.ddbj.nig.ac.jp). The databases are produced in an international collaboration and each of the three groups collects a portion of the total sequence data reported worldwide, and all new and updated database entries are exchanged between the groups on a daily basis. Currently, there are more than 250000 16S rRNA gene sequences available.
Thus the potential users, which include scientists working in the field of molecular biology or bioinformatics but also scientists studying sulphate reducing bacteria (SRB) and anaerobic methanotrophic archaea (ANME) have an direct access to sequencing data from the METROL project.
Sequences were aligned and phylogenetically analysed using the ARB software package (http://www.arb-home.de). Prior to phylogenetic analysis environmental sequences were checked for chimeras.
Current status:
To study the microbial diversity at different METROL seep sites six 16S rRNA gene libraries (three libraries for archaea and three for bacteria) have been constructed for three sites: Tommeliten seep areas (He180- 1904, 160 cm), Gullfaks field (He208, MUC766, 0-10cm) and Aarhus Bay (HeIV_04, 173 cm). The work on Skagarrak and Aarhus Bay Sediments and the Black Sea sediments and mats is in progress. All samples were taken from communities associated with the sulfate-methane transition zone (SMTZ) of methane seeps. For the evaluation of sequences retrieved during METROL, existing information of the BMBF/DFG project MUMM were utilized (Knittel et al. 2003 and 2005).
The most abundant bacterial clone group consisted of different sulfate-reducing bacteria within the Deltaproteobacteria. Several different SRB groups were retrieved from the samples: The four seep-specific clades SEEP-SRB1-4, which are Desulfobacterium anilini relatives, other Desulfobacterium relatives and two unaffiliated groups. Furthermore, one group affiliated with Desulfuromonas, which are sulfur-reducing organisms, was detected. Organisms of group SEEP-SRB1 were shown to be the sulfate-reducing partners for ANME-2 cells in AOM mediating consortia (Boetius et al., 2000).
Archaeal diversity in SMTZ sediments was as high as for bacteria indicated by the varying presence of six different phylogenetic groups of archaea (ANME-1, Marine benthic group B, C, D, and E, Marine Group 1, and one unaffiliated group). Furthermore, the intra group diversity was high among the archaea with often less than 95% sequence similarity.
The results from gene libraries generally matched the biomarker studies of the SMTZ of all investigated sediments. The development of CARD FISH protocols as a method to detect single cells with low RNA content in subsurface sediments has resulted in the first images and quantification of coastal methanotrophs both in North Sea and Baltic Sea sediments, confirming the clone library results.
Very similar groups as those known from deep water gas hydrate sites and cold seeps have been found, which supports the idea of a cosmopolitan presence of AOM-mediating consortia in SMTZs. However, their biomass is highly variable, spanning a difference of 5 orders of magnitude from low methane flux, subsurface sediments to highly advective deep water seeps.
Description of the result
Collection of METROL relevant data from a compilation of public and confidential sources were performed using GIS-technology (Geographic Information System). Data are digitised as GIS-files and stored on a server platform at the AWI Bremerhaven. Non-confidential data and results are displayed via the internet. From the METROL homepage (GIS site) an Internet Map Service (IMS) can be launched to explore maps interactively.
Data related to the occurrences of methane in the marine environment have been compiled digitally with the main focus on the North Sea and Western Baltic Sea, to a lesser intensity on the Black Sea. Beyond this the global distribution of methane features were compiled, too. For a map-based result portal the IMS was established on the internet homepage of METROL. There, data can be selected and queries can be carried out to a certain extent in order to create interactive maps. Until 2008, when the restrictions for confidential data will expire, information displayed via the IMS can not be downloaded. A help page for the IMS is provided as well as an introduction to the parameter code of the Global Point Data for methane features. The core data are stored at the AWI and cannot be accessed from outside unless agreement by data owners is obtained.
Current status and use:
METROL data integrated into the IMS so far are cruise tracks, ROV tracks, seismic tracks, occurrences of free gas, occurrences of pockmarks and other methane related features.
A compilation of the main data sources used (mainly from literature) can be found on the METROL homepage. For non-METROL users only metadata (i.e. a literature list) are accessible, METROL users are able to enter the Members Area for download of, e.g., the full articles. This compilation was last updated in February 2005.
A further use of digitised data is planned for further analysis of spatial relationships between methane and proxy data sets (in the scope of a Ph.D. thesis). The focus of this thesis reaches partly beyond the METROL key areas.
Key innovations:
High resolution data from microbathymetrical maps exhibiting seafloor features have been digitised and visualised are now available in 3D-format. For the first time an area-wide inspection and analysis of features in connection with depth of features is made possible.
Users interested in the result
Scientists, working in geology, geophysics and biogeochemistry of the sea floor
General public, in the context of press information on methane and hydrates as a source of energy respectively a threat to seafloor stability (e.g. best selling book Der Schwarm, by Frank Schaetzing).
Environmental and geological authorities and survey agencies
Expected benefits:
Combining METROL knowledge on shallow gas turnover with TIP result on Geochemical data on methane flux and retention in sediments extracted from literature and analysed we intend to discover interrelationships between geological, geophysical and biogeochemical parameters which will enable an explanation of the distribution of free methane gas in the upper sediments of the North Sea. Combining these research areas represents a new approach, as integrative analyses of such data have not been done before.
The following further benefits are expected:
Concepts for management of marine geodata by GIS application.
Visualisation of data in 3D will lead to further insights of methane distribution and open new research questions.
GIS as modelling and analysis tools to interpolate data and extrapolate causal relations, deduce reaction of third parameter (calculation of fluxes if base parameters are at hand or can be deduced).
Result description:
Since the early 1980's Statoil has conducted research on seepage of hydrocarbons in northern waters. The current METROL initiative (2002-2005) was incorporated as part of this endeavour.
Main results from the Statoil/METROL work show that:
- Both micro- and macro-seeps are self-sealing, where the formation of bacterial mats and carbonate represent major steps in the sealing process,
- Focussed hydrocarbon seeps are long-lived and reach a steady-state flux situation,
- Bacterial mats are used as nutrients to macro organisms (hermite crabs),
- Large amounts of carbonate can form sub-surface,
- Hydrate-pingoes modify the seabed topography.
'Hydrate pingoes' form part of this landscape. They are up 1 m high mounds and have four main characteristics:
- They are sediment-covered
- They have a 'carpet' of tubeworms
- They are partly covered by bacterial mats
- They have 'corrosion pits' where soil has become fluidised.
Hydrate pingoes only form at locations where there is a continuous supply of 'guest molecules' from below (i.e., where there is seepage of light hydrocarbons). They are a direct result of local formation and disintegration of sub-surface gas hydrates.
ROV-operations:
During METROL methane bubbling through the seafloor at Tommeliten and Gullfaks was detected by the use of hull-mounted single beam echosounder. ROVs were used to document these 'macro' seeps visually on the seafloor. In addition to visible macroseeps, there also occurs 'invisible' microseepage of fluids through the seafloor. These are very difficult to document without the use of ROVs. At Nyegga there are only microseeps. At microseep locations which occur at all sites Tommeliten, Gullfaks and Nyegga, bacterial mats form on the seafloor due to anoxic water flowing up to the seafloor.
ROV-operations conducted by Statoil at the three sites resulted in the visual documentation of the exact location of bacterial mats and microseepage. These ROV-operations have been conducted using dynamically positioned (DP) operated surface vessels and high accuracy underwater position keeping.
Key innovations:
Key innovations are that hydrate pingoes and bacterial mats show the exact locations where active micro-seepage occurs.
At Nyegga, we have for the first time documented the existence of a gas hydrates submarine 'tundra' landscape on the seafloor. The very irregular landscape is suspectedly caused by the formation and disintegration of sub-seafloor gas hydrates, much in the same fashion as sub-surface ice in a true tundra landscape on land.
Current applications:
The results help Statoil and other users of the seafloor to improve our field development strategy and designs, and to improve our understanding of the natural chemical environments and their effects on biology.
Users:
All the results are utilized by Statoil to improve the knowledge on the hydraulics of the seafloor in association with field development and pipeline construction.
Further users interested in these results are: engineers, exploration geophysicists, biologists, environmental and drilling safety specialists, marine geologists.
Expected benefit:
Statoil scientist working within METROL is an advisory member of the IOPD committee since 1986, funded by Texas A&M University, and esp responsible for safety problems in drilling. METROL results are expected to feed into increased safety and improved environmental issues (less impact) in drilling, as it becomes more and more know, which pockets can be drilled into and which are too dangerous and should be left alone.
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