OS21A-01
Introduction of the 2007-2008 JOGMEC/NRCan/Aurora Mallik Gas Hydrate Production Research Program, NWT, Canada
Japan Oil, Gas and Metals National Corporation (JOGMEC) and Natural Resource Canada (NRCan) have embarked on a new research program to study the production potential of gas hydrates. The program is being carried out at the Mallik gas hydrate field in the Mackenzie Delta, a location where two previous scientific investigations have been carried in 1998 and 2002. In the 2002 program that was undertaken by seven partners from five countries, 468m3 of gas flow was measured during 124 hours of thermal stimulation using hot warm fluid. Small-scale pressure drawdown tests were also carried out using Schlumberger's Modular Dynamics Tester (MDT) wireline tool, gas flow was observed and the inferred formation permeabilities suggested the possible effectiveness of the simple depressurization method. While the testing undertaken in 2002 can be cited as the first well constrained gas production from a gas hydrate deposit, the results fell short of that required to fully calibrate reservoir simulation models or indeed establish the technical viability of long term production from gas hydrates. The objectives of the current JOGMEC/NRCan/Aurora Mallik production research program are to undertake longer term production testing to further constrain the scientific unknowns and to demonstrate the technical feasibility of sustained gas hydrate production using the depressurization method. A key priority is to accurately measure water and gas production using state-of-art production technologies. The primary production test well was established during the 2007 field season with the re-entry and deepening of JAPEX/JNOC/GSC Mallik 2L-38 well, originally drilled in 1998. Production testing was carried out in April of 2007 under a relatively low drawdown pressure condition. Flow of methane gas was measured from a 12m perforated interval of gas-hydrate-saturated sands from 1093 to 1105m. The results establish the potential of the depressurization method and provide a basis for future prolonged testing planned in the near future. The authors acknowledge the Research Consortium for Methane Hydrate Resources in Japan (MH21), the Ministry of Economy, Trade and Industry (METI) and NRCan for the support and funding. The Mallik 2002 program was undertaken jointly by JNOC, NRCan, GeoForschungsZentrum Potsdam (GFZ), the United State Geological Survey (USGS), the United States Department of Energy (USDOE), the India Ministry of Petroleum and Natural Gas (MOPNG)-Gas Authority of India (GAIL), and the BP-Chevron Texaco Mackenzie Delta Joint Venture.
OS21A-02
Gas Hydrate Investigations at the 2007 Mount Elbert Test Site, Milne Point Area, Alaska North Slope
As part of an ongoing effort to advance our understanding of the future resource potential of permafrost- associated gas hydrates, a team of scientists from the U.S. Department of Energy, BP Exploration Alaska, the U.S. Geological Survey, Oregon State University, and RPS Energy conducted an extensive data collection program in February, 2007, at the "Mount Elbert" gas hydrate stratigraphic test well within the Milne Point Unit of the Alaska North Slope (ANS). In the first application of wireline retrievable coring on the ANS, the well was continuously cored from a depth of 1995 ft to a depth of 2494 with core recovery of roughly 85%. A full suite of advanced well- logs were collected in the well, including magnetic resonance and borehole resistivity imaging. Log quality was outstanding due primarily to excellent hole conditions that resulted from the use of chilled, oil-based drilling fluids that prevented gas hydrate dissociation during coring and data collection. In close agreement with pre-drill geological and geophysical estimates, the core and log data confirmed the existence of two sands containing a total of roughly 100 feet of gas hydrate bearing reservoir. Gas hydrate saturations varied with reservoir quality and ranged from 60 to 75%. The "Mount Elbert" data collection program culminated with four, 6-to-12 hour duration open-hole tests of reservoir flow and pressure response, as well as gas and water sample collection using Schlumberger's Modular Formation Dynamics Tester wireline tool.
OS21A-03
Gas Hydrate in the Arctic Ocean: A Submarine Terrestrial Reservoir
Gas hydrate is found in marine sediments on many continental slopes and on land beneath permafrost. An unusual deposit occurs below shallow water on continental shelves in the Arctic Ocean. These shelves are too shallow to create a permanent stability zone, given the present-day water depth and bottom-water temperature. Instead, this hydrate is a relic of the previous glacial periods, where the shelves were exposed to the much lower atmospheric temperature. As permafrost develops on the shelves, a shallow zone of hydrate stability is created. The subsequent rise in sea level at the end of the glacial period makes the hydrate deposit intrinsically unstable due to the encroachment of warm temperature from below and the relatively warmer seawater from above. Because these deposits have been warming for the last 10 kyrs, they provide a natural laboratory for studying the long-term response of gas hydrate to climate change. We attempt to model this transient feature of the Arctic with the aim of reproducing observations from sediment cores and quantifying the transfer of methane through the sediments and ocean. We model the volume fraction of gas hydrate, gas bubbles and ice (as well as salinity and methane concentration) as a function of depth over several glacial cycles. We account for both global changes in sea level and regional isostatic adjustments. The surface temperature of the seafloor switches between marine and terrestrial values, according to local sea level. Low air temperature at the sediment surface during glacial periods allows gas hydrate and free gas to accumulate within and below the permafrost layer. Our results show that hydrate and gas bubbles occupy a few percent of the pore volume within the permafrost. Small volumes of hydrate and bubble are also found below the permafrost layer. It appears that several glacial cycles are required to accumulate the hydrate observed today. The volume fraction of hydrate can also be increased by allowing a deep source of methane. A comparison with observations provides a constraint on this deep methane source.
OS21A-04
Estimates of Vertical Methane Fluxes in Porangahau Ridge Sediment on the Hikurangi Margin, New Zealand
Potential gas hydrate deposits were outlined with bottom simulating reflections in seismic data on the Porangahau Ridge, landward of the Hikurangi Channel, along the northeastern coast New Zealand. This expedition, CHARMNZ (CH4 Hydrates on the AccRetionary Margins of New Zealand; R/V Tangaroa voyage TAN0607), in 2006 was the first survey dedicated to studying gas hydrates on the Hikurangi Margin east of New Zealand. Geochemical data from shallow sediment porewater profiles and vertical fluid migration measured with a heatflow probe were compared with seismic profiles over potential gas hydrate deposits. Spatial orientation of piston cores and heatflow probing was organized to compare vertical methane fluxes along the seismic lines, with focus on predicted areas of concentrated gas seepage or hydrate accumulation. Core and heatflow transects were set on the landward and seaward side of the ridge in order to determine variations in the vertical fluid and gas fluxes. Heatflow data suggested vertical fluid advection on the landward side of the ridge exceeds vertical fluid fluxes on the seaward side. Porewater sulfate and methane profiles showed a range for the depth of the sulfate-methane interface (SMI) between 12.9 m upslope on the landward side of the ridge and 1.84 m near the ridge. Vertical sulfate fluxes in the study area appear to be dominated by diffusion with a range of -4.2 mM m-2 a-1 upslope, away from the ridge, and up to -208.6 mM m-2 a-1 on the ridge. Porewater sulfide profiles suggest elevated sulfate reduction on the landward side of the mound where elevated vertical fluid flux was measured. Stable carbon isotope analysis (δ13C) and gas composition indicated that porewater methane originate from microbial production with δ13 below the SMI in the range -60 ppt and -110 ppt VPDB and methane the dominant gas. At this depth a large variation in the methane δ13C likely results from variations in the vertical flux rates along the ridge, near surface methanogenesis and variable rates of anaerobic methane oxidation (AOM). Above the SMI the methane δ13C was elevated up to -45 ppt and corresponded to low methane concentrations likely resulting from AOM at the SMI. Vertical methane fluxes in this area are within the range observed in other seep areas such as Atwater Valley in the Gulf of Mexico and mid Chilean margin (-9 to -362 mM m-2 a-1). Because methane flux estimates can not be attributed to near surface methanogenesis alone, these data suggest deep sediment methane pools are present along the Porangahau Ridge.
OS21A-05
Near Sea Floor Gas Hydrate Formation and Influence on Pore Water Chemistry and Authigenic Carbonate at the Formosa Ridge, South China Sea
Multiple flares, unusual chemosynthetic community, and authigenic carbonate were found on top of the Formosa Ridge in the passive margin of the Northeastern South China Sea. Very densely populated mussel ( Bathymodiolus platifrons) together with a type of white colored crab ( Shinkai cronieri) usually found in the hydrothermal vent indicated that methane and hydrogen sulfide maybe dominating this environment. The purpose of this research is to study the geochemical property of the environment and mechanism driving this unusual chemosynthetic community. A set of piston cores and ROV short core were collected and analyzed for pore water dissolved sulfide, chloride, sulfate and sediment carbonate content, organic carbon, AVS, pyrite concentrations, stable carbon and oxygen isotopes variations in carbonate. Near sea floor gas hydrate formation control pore water compositions adjacent to the plume. Both pore water chloride and sulfate showed unusually higher values near the gas plume, and radially outward until reaching normal seawater compositions away from the plume. At some locations, chloride concentrations also increase with increasing depth in cores, probably a result of gas hydrate dissolution after sampling. In addition, very high concentrations of pore water dissolved sulfide were also observed. Up to ~20 mM of dissolved sulfide were found in cores near the plume area. Methane provides extra carbon source for the observed rapid sulfate reduction and pyrite formation. Depending on the proximity to the seep, degree of sulfate depletion and pyrite formation in cores varies significantly. Sulfate depleted rapidly near seep; however, there was almost no depletion away from the seep. Very high concentrations of pyrite were also found near sediment/water interface adjacent to seep. The depth of maximum pyrite occur also varies, depending on the distance away from the seep. Authigenic carbonates (up to 70 wt percent) with very light stable carbon isotope, as low as -55 per mil, were found at depth. In addition, partially cemented mud and vent tube-like mudstone with lighter carbon isotopic signature were also found in some overlying sediment. Partially cemented vent-tubes are acting as gas conduit for gas migration. The abundance of B. platifrons and S. Cronieri, pore water chloride and sulfate, authigenic carbonate and its isotopic signatures indicated that seep methane and hydrogen sulfide are the primary driving mechanism for the observed chemistry variations and biological community.
OS21A-06
Seismic Reflection Blank Zones in Ulleung Basin, Offshore Korea, Associated With High Concentrations of Gas Hydrate
It has recently been recognized that high concentrations of gas hydrate occur in localized zones of upwelling fluids. We report a study of multi-channel seismic reflection data across such structures in the Ulleung Basin, East Sea backarc offshore Korea, an area with few bottom-simulating reflectors. The structures are commonly up to several km across and a few hundred metres tall, and are characterized by seismic blanking (reduction in reflectivity) and upbowing (in time sections) of the sediment reflectors. We interpret the seismic pull-up to be mainly a velocity effect, consistent with velocities derived from stacking analysis, although physical deformation due to faults imaged on time-migrated sections, is not ruled out. Structures associated with geological indicators of a high upward flux of gases show the largest amount of pull-up for reflectors close to the seafloor, and a decrease in the rate of change of seismic pull-up with increasing depth. This implies the presence of concentrated gas hydrate in the near-surface layers. Other structures, associated with a moderate upward flux of gases, show the largest increase of seismic pull-up close to the base of the gas hydrate stability zone. The spectral character of the rays travelling through the body of the blanked zones is not significantly different to rays travelling outside these zones, which indicates that the blanking results from loss of reflectivity of the hydrate-filled sediments, rather than an intrinsic attenuation caused by hydrate. The correlation between low velocities and reduced reflection amplitudes point to the accumulation of free gas several kilometres beneath the base of gas hydrate stability zone. This gas migrates upwards to feed the vents through near-vertical conduits, in addition to the regional, upward fluid flow caused by tectonic compression of the basin.
OS21A-07
Accumulation and Migration of Gas Hydrates in Sediments of the Peru Margin Revealed by Rock Magnetism
Gas hydrate disseminated in marine sediments encourages microbial activity, including bacterial metabolism that results in production of magnetic iron sulphides. Previous studies of sediment cores from continental margins where gas hydrate occurs have shown that the rock magnetic parameter DJH, an index based on hysteresis properties representing position on a Day plot, is useful as a proxy for the modern and glacial distribution of gas hydrate. Ocean Drilling Program sites 688 (Leg 112) and 1230 (Leg 201) provided samples through hydrate-bearing accretionary margin sediments on the Peru margin. Perhaps surprisingly, results from samples from Site 688, which have been curated for 13 years, are broadly similar to those from Site 1230, sampled immediately after core recovery, indicating that protracted storage does not entirely destroy the rock magnetic signal. Profiles of DJH at both sites suggest a preferential concentration of gas hydrate within the porous upper lithological unit, with the highest values of DJH occurring near the base of the unit, where hydrates would be expected to accumulate, and in other intervals where the high concentrations of hydrate have been observed or inferred. Viable bacterial counts parallel the trend in DJH, suggesting that the principal control on bacterial numbers in this profile is hydrate concentration. Site 688, which penetrated below the modern base of gas hydrate stability (BGHS), displays a feature on the DJH profile similar to that interpreted at the Cascadia margin as the marker of the BGHS at the last glacial maximum. This feature lies about 65 m below the bottom-simulating reflector that marks the modern BGHS. Thermal modelling based on this determination of the glacial BGHS gives a bottom-water temperature at the Peru margin during the last glacial maximum of - 1.5°C, and serves to explain the anomalously low temperature predicted for the modern BGHS at the Peru margin on the basis of geothermal gradient measurements in the upper 50 m of the sediment column.
OS21A-08
High-Resolution 3D-Seismic Investigations Indicate Focused Fluid Flow Systems in Hydrated Sediments at the Vestnesa Ridge off the W-Svalbard Margin.
High-resolution seismic data were acquired using the 3D seismic P-Cable system of the University of Tromsoe to investigate how the fluid flow penetrates gas hydrate systems of the Vestnesa Ridge. The ridge represents a current-controlled sediment drift on the continental margin offshore western Svalbard. The survey area is located at the northwestern part of the Vestnesa Ridge and centered at the ridge crest that resembles an anticline in a water depth of 1250-1320 m. The seafloor morphology at the crest is characterized by an abundance of pockmarks with a diameter between 50-500 m indicating recent fluid flow activity. Since the area is within the gas hydrate stability zone (GHSZ), it is an ideal site to understand where and how fluids escape through a hydrated sediment drift. 35 reflection seismic profiles with a spacing of about 40-60 m were shot resulting in a seismic cube covering an area of approximately 22 km2. In addition, regional single channel streamer (SCS) seismic lines were acquired across the ridge perpendicular to the crest to connect the 3D area with the regional structural setting. The seismic data provide images of the subsurface to about 500 ms TWT (two-way time) below the seafloor (bsf), where gas accumulations cause acoustic attenuations that hinder deeper acoustic signal penetration. The well-stratified sediments exhibit a bottom simulating reflector (BSR) at about 200 ms TWT bsf at the base of the GHSZ. The BSR is difficult to identify due to the stratification, but it is accompanied by the onset of an ubiquitous band of strong reflectivity indicating free gas accumulation zones beneath the GHSZ. Fluid flow activity is evident from a link between gas accumulations (bright spots), gas wipeouts and disturbed reflectivity in the seismic data. These features are observed not only beneath the pockmark structures, but also in the sediment without seafloor expressions of fluid venting. The fluid source might be related to deep tectonic processes at the sedimented ocean ridge. Regional faults seen in the bathymetry data running across the Vestnesa Ridge are located beneath the pockmark fields. They may provide major fluid migration pathways, and in addition, heat flow driven hydrate dissociation is likely to play a significant role in fluid flow dynamics.