OS34B-01 16:00h
{\it In Situ} Raman Analyses of Natural Gas and Gas Hydrates at Hydrate Ridge, Oregon
During a July 2004 cruise to Hydrate Ridge, Oregon, MBARI's sea-going laser Raman spectrometer was used to obtain {\it in situ} Raman spectra of natural gas hydrates and natural gas venting from the seafloor. This was the first {\it in situ} analysis of gas hydrates on the seafloor. The hydrate spectra were compared to laboratory analyses performed at the Center for Hydrate Research, Colorado School of Mines. The natural gas spectra were compared to MBARI gas chromatography (GC) analyses of gas samples collected at the same site. DORISS (Deep Ocean Raman In Situ Spectrometer) is a laboratory model laser Raman spectrometer from Kaiser Optical Systems, Inc modified at MBARI for deployment in the deep ocean. It has been successfully deployed to depths as great as 3600 m. Different sampling optics provide flexibility in adapting the instrument to a particular target of interest. An immersion optic was used to analyze natural gas venting from the seafloor at South Hydrate Ridge ($\sim$780 m depth). An open-bottomed cube was placed over the vent to collect the gas. The immersion optic penetrated the side of the cube as did a small heater used to dissociate any hydrate formed during sample collection. To analyze solid hydrates at both South and North Hydrate Ridge ($\sim$590 m depth), chunks of hydrate were excavated from the seafloor and collected in a glass cylinder with a mesh top. A stand-off optic was used to analyze the hydrate inside the cylinder. Due to the partial opacity of the hydrate and the small focal volume of the sampling optic, a precision underwater positioner (PUP) was used to focus the laser spot onto the hydrate. PUP is a stand-alone system with three degrees-of-freedom, capable of moving the DORISS probe head with a precision of 0.1 mm. {\it In situ} Raman analyses of the gas indicate that it is primarily methane. This is verified by GC analyses of samples collected from the same site. Other minor constituents (such as CO$_{2}$ and higher hydrocarbons) are present but may be in concentrations too low to be detected by the current DORISS instrument. {\it In situ} analyses of the hydrates show them to be structure I hydrates with methane as the primary guest molecule; the data compare well to laboratory data.
http://www.mbari.org/raman
OS34B-02 16:15h
Structural Controls on Hydrate Distribution and Morphology at Hydrate Ridge, Oregon
Analysis of resistivity-at-the-bit (RAB) images from nine sites drilled during Ocean Drilling Program (ODP) leg 204 to southern Hydrate Ridge reveals a complex pattern of fracture orientations that correlate with the structural location of the borehole on the ridge. Sites 1244 and1245 on the eastern and western flanks of the ridge, respectively, have concentrated zones of resistive fractures within the gas hydrate stability zone that exhibit a preferred orientation orthogonal to the regional direction of plate convergence and crustal shortening. The mean orientation of the identified fracture planes at Site 1244 strikes N10W and dips $40\deg$E, and the mean fracture plane at Site 1245 strikes N15W and dips $33\deg$E. Higher on the eastern ridge flank, at site 1246, fracture planes exhibit a transitional behavior from clustering on the flanks to a less well organized pattern at the ridge crest. Poles to structural planes at the ridge crest sites (1247-1250) are evenly distributed, though their typically shallow dips (i.e. steeply dipping planes $>$ $30\deg$) create a girdle pattern in stereographic projections. The steeply dipping planes at the ridge crest occur in zones of chaotic resistivity in the RAB images and correlate to zones of massive hydrate in the cores. Simple planar relationships can be obscured in these zones by hydrate growth patterns and morphology. Bedding plane dips were also measured and are consistently sub horizontal at all sites. Little change in bedding dip occurs with depth at sites on the ridge crest and flanks, though the basin site shows bedding dips increase below $\sim$ 200 mbsf. As expected, few fracture planes were documented in the basin site images where hemipelagic sedimentation dominates. Results of the RAB image analyses indicate that the orientation and morphology of hydrate filled fractures is controlled by structural position on the ridge. At the ridge crest, undergoing extension, migrating aqueous fluid and free gas have the potential to hydrofracture the sediment creating a network of steeply dipping, but randomly oriented fractures with massive hydrate accumulations. Along the ridge flanks where fluid flow is controlled primarily by lithology, hydrate filled fractures maintain a preferred orientation that parallels that of the uplifted sedimentary section and is oriented perpendicular to the regional maximum compressive stress.
OS34B-03 16:30h
Mechanism for free gas migration through South Hydrate Ridge hydrate system
At southern Hydrate Ridge, offshore Oregon, free gas vents through hydrate-bearing sediments where free gas coexists with gas hydrate and brine in shallow subsurface. To establish three-phase (liquid, hydrate and gas) equilibrium within the gas hydrate stability zone (GHSZ), salinity must increase from a baseline value of 3 wt.% at the base of the GHSZ to about 12 wt.% at the seafloor. This predicted salinity profile is similar to in situ pore water salinity calculated at ODP Site 1249. We interpret that the upward increase in salinity is driven by rapid hydrate formation in the case of high methane flux from below. As hydrates form, dissolved ions are excluded and pore water salinity rises. Ultimately, salinity rises to the point where three-phase equilibrium is reached and additional methane exists in free gas phase. By assuming that there is no transport of water or salt, we can estimate the hydrate amount from change in salinity relative to the baseline. Hydrate volume faction would increase from 0 below the GHSZ to 70% at the seafloor. This predicted hydrate distribution is consistent with resistivity log- and PCS- derived hydrate occurrences at Hydrate Ridge. Thus, both observations and calculations of three-phase equilibrium suggest that the formation of marine methane hydrate is a self-equilibrating process at locations where methane supply is abundant. This provides a mechanism whereby free gas migrates through the GHSZ without being converted into hydrate.
OS34B-04 16:45h
Systematic Specification of Thermodynamic State and Phase Transition Processes of Natural Gas Hydrate Systems
Natural gas hydrate systems, either in permafrost areas or under the seafloor, consist of water, salts, gases, and their combinations in various phases, reside in and interact with host sediments, and constantly undergo dynamic evolution and interact with environmental changes. Dynamic processes within these systems are further complicated by numerous feedback mechanisms involved. Modeling and simulation are therefore one of the indispensable approaches in studying natural gas hydrate systems. Specification of thermodynamic state and phase transition processes, and mathematical description of conservation laws are the two essential tasks that need to be done before carrying out any successful numerical simulation work. According to the state postulate in thermodynamics, the total number of components (water, salts, gases and their compounds) of a natural gas hydrate system determines the number of the independent parameters required to specify the thermodynamic state of the system. The maximum number of properties that are homogeneous to all phases of a system in thermodynamic equilibrium, such as pressure, temperature and chemical potentials, that may be used together as the independent thermodynamic parameters is limited by the Gibbs phase rule. Therefore, it is critically important to first carefully choose the number and proper types of independent properties to specify the thermodynamic state of and phase transition processes involved in the system. Pressure and temperature are usually not appropriate to be used together as two of the independent thermodynamic parameters since they are dependent to each other along the gas hydrate stability boundary. Consequently, models using both pressure and temperature as independent thermodynamic parameters are not able to quantify phase transition processes along the gas hydrate stability boundary. A non-homogeneous thermodynamic property should be chosen to replace a non-independent homogeneous property. Examples will be discussed for systems of water-gas and water-gas-salt, and systems containing multiple gases.
OS34B-05 17:00h
Investigations on Mixed Gas Hydrates: Unexpected Experimental Observation Versus Calculated Data
The knowledge of the conditions for gas hydrate formation and their stability fields are necessary for the prediction of their influence on climate changes or slope stability. In the past, the phase equilibria of gas hydrates composed of pure gases like methane, ethane or propane and water have been described in detail in phase diagrams, based on experimental and modelled data. However, natural gas mixtures are composed of methane, ethane and propane and other components. Hence, it is no longer acceptable to assume that the phase behaviour of natural gas hydrates is similar to that of pure methane gas hydrates. Investigations on pseudo-binary systems like methane+ethane+water indicates unusual phase behaviour like structural transitions (Subramanian et al., 2000). Therefore, Ballard and Sloan (2001) published phase diagrams for pseudo-binary and pseudo-ternary hydrocarbon mixture with water, which have been generated from modelled data at 277.6 K using a Gibbs free energy minimization flash routine. Since the phase behaviour of multicomponent gas hydrates are almost unpredictable, we present in this contribution the stability fields and phase transitions of mixed gas hydrates composed of methane, ethane, propane and water based on experimental data. The multicomponent systems have been investigated in a pressure range between 1 MPa and 6 MPa and a temperature range between 270 K and 290 K. Microscopic observations, Raman spectroscopy and X-Ray diffraction data provide information about the phase boundaries and transformation processes, the composition and the structure of the hydrate phase, respectively. The similarities and discrepancies between modelled and experimental data will be discussed. References: A. L. Ballard, E.D. Sloan, Chemical Engineering Science 56 (2001) 6883-6895 S. Subramanian, R.A. Kini, S.F. Dec, E.D. Sloan, Chemical Engineering Science
OS34B-06 17:15h
Interaction Between Methane Hydrates and mud Diapirs in the South Caribbean
The prominent reflector observed in marine seismic data from many continental margins known as `Bottom Simulating Reflector' (BSR) is known to coincide with the base of the methane hydrate stability zone. Because of the great significance of methane hydrates, BSRs have been the focus of many studies worldwide. Few studies, however, have investigated the nature and extent of the BSR in the south Caribbean. Seismic reflection data acquired off the Caribbean coast of Colombia display a prominent BSR beneath much of the seafloor of that area. This BSR has been observed between 13.5\deg N and 11.9\deg N, and between 72.5\deg W and 74.6\deg W, in water depths ranging between 1.6 and 4.1 km. The thickness of the methane hydrate layer represented by the BSR and the seafloor is consistent throughout the region, and is equivalent to some 550 to 600 ms of seismic two-way-time. The BSR is relatively continuous throughout the study area, except when disrupted by local faults and mud diapirs. Two end-members describe the interaction between BSRs and mud diapirs. On one end, the BSR is fully disrupted by the seismically transparent mud column. On the other end, the BSR is continuous across the mud column, and preserves the seismic amplitude as in the unintruded area. Some BSRs appear broken but maintain relicts embedded in the mud column. We hypothesize that the continuity of BSRs may provide an indication of the relative velocity of mud ascent. When the BSR is continuous across the mud column, it may indicate a relatively slow mud ascent, suggesting that the hydrate stability zone keeps pace with mud emplacement. On the other hand, BSRs disrupted by the mud column may indicate a relatively fast mud intrusion where the stability field cannot be restored upon being altered by the thermal and pressure disturbance from rising mud. Therefore, the presence and continuity of a BSR across mud diapirs in the south Caribbean may provide clues on the occurrence and pace of diapirism in the area.
OS34B-07 17:30h
Probable Occurrence of Gas Hydrates Along the Continental Margins of India
The gas hydrates have gained the importance of being potential to be an alternate energy resource and are know to occur world wide in the sediments of the outer continental margins and polar regions associated with permafrost and are stable under certain temperature-pressure conditions. A gas hydrate stability zone (GHSZ) thickness map of Indian offshore has been created from spatial analysis of physical parameters that influence the formation and preservation of gas hydrates by using Arc/Info GIS software. These parameters include bathymetry and heatflow/geothermal gradient data obtained from published literature, digital databases and unpublished sources, and the seabed temperature from extrapolation of the best-fit polynomial of hydrothermal profile. In addition to the measured heatflow/geothermal data, the values were predicted in the regions devoid of data by considering the Indian offshore comprising of three provinces such as: oceanic, stretched continental crust and the crust influenced by the hotspot. Heatflow versus age relationship is used in case of oceanic crust in Arabian Sea, Bay of Bengal, and Andaman Sea in the regions beyond the shelf edge. The stretched crust criterion is applied only in the northern Arabian Sea between shelf and the inferred oceanic crust. A regression line is fitted between the available heatflow data and the corresponding age of the crust to compute heatflow values, in the region where the crust influenced by the Reunion hotspot. Seabed temperature data were also predicted in deficient areas based on the regression analysis of available bathymetry versus seabed temperature relationship. The thickness of gas hydrate stability zone was computed both in TWT (msec) and meters, by solving the gas hydrate phase and pressure-temperature equations at each GIS grid node. The GHSZ thickness map is reliable to the extent it can be used as an important indicator for gas hydrate potential of an area. Large volumes of multi-channel seismic reflection data acquired by oil majors in India have been examined and inferred the probable presence of bottom simulating reflections (BSR) in the offshore areas of India. The depths of the BSRs on several seismic profiles in the offshore regions of Goa and Saurashtra (west coast), Krishna-Godavari and Mahanadi (east coast) of India and the Northern Andaman Sea are within the zone of computed gas hydrate stability. Assuming the depth of BSR observed from seismic data as the base of the gas hydrate stability zone, the geothermal gradient values have been computed. Most of the values are in close agreement with the observed geothermal gradient data. Sediment thickness and Total Organic Carbon (TOC) content maps were prepared from the compiled data, which were used as constraints in assessing the GHSZ thickness and the probable occurrence of gas hydrates in the marine sediments in Indian offshore. The non-geophysical proxies, derived from the analysis of 5m long sediment cores, such as chlorine anomalies, reduction trend of sulphate concentration, enrichment of methane and abundance of sulfate reducing bacteria, fomenters and nitrate reducing bacteria with depth provide indirect clue for the occurrence of gas hydrates at deeper depths. The high-resolution seismic data and sediment cores longer than 20m, perhaps would enhance the understanding of occurrence of gas hydrates in the marine sediments.
OS34B-08 17:45h
Modeling Heat and Fluid Flux of Seafloor Mounds in the Gulf of Mexico
Finite element modeling of fluid and heat flux associated with bathymetric mounds in the Gulf of Mexico suggests that fluid flux is likely constrained to the area of the bathymetric expression of the mound, and that when active, the fluid flux through the mound is at least two orders of magnitude higher than that in the surrounding sediments. The venting of methane is corroborated by sulfate depletion less than 10 cm below the seafloor in mound sediments. The modeling is constrained by high resolution seafloor thermometry readings and seismic data over two mounds (labeled F and D) that are the focus of a Department of Energy sponsored joint industry project to study safety and energy issues associated with gas hydrate in the Gulf of Mexico. The mounds lie on the floor of the Mississippi Canyon in Atwater Valley lease blocks 13 and 14 in about 1300 m water depth, and are similar in morphology to many other mounds in the area. Mound F is about 10 m high and 500 m in diameter, and mound D is about 6 m high and 200 m in diameter, both exhibiting a several meter deep moat. All thermal probe penetrations were complete, suggesting no significant carbonate or gas hydrate surface deposits. Linear gradients in the deepest portion of the 3m thermal profiles suggest that advection has been minimal for at least several decades. Interpolation of the linear portion of the thermal profiles from 3 mbsf (meters below seafloor) down to 500 mbsf results in a base of gas hydrate stability (BGHS) that is consistent with the top of gas estimated from seismic data, i.e. about 60-70 mbsf below the mounds to 200-300 mbsf away from the mounds. Most, if not all of the apparent perturbation to the BGHS can be attributed to elevated temperatures within the sediments, but elevated chloride concentrations (up to twice that of seawater) were also found in the sediments of mound F. This inferred severe perturbation of the BGHS has been seen on the Cascadia margin but has never before been corroborated by such a detailed transect of pore water chemistry and thermometry, with average spacing less than 100 m over each of the two mounds modeled.