Base Of Hydrate Stability Zone Research Articles

Widespread subseafloor gas hydrate in the Barents Sea and Norwegian Margin

The distribution and concentration of subseafloor natural gas hydrate across margins is not well understood, because these systems are challenging to image and quantify remotely. Furthermore, it is unknown if shallow hydrate systems are linked to deeper oil and gas reservoirs. Herein, we analyze petroleum industry well logs with data in the gas hydrate stability zone and find that low concentrations of hydrate commonly occur below the seafloor in the Barents Sea and the Norwegian Margin. We observe hydrate in half of analyzed industry wells using a set of conservative criteria that requires a resistivity increase of at least 0.5 Ωm above background resistivity. Hydrate accumulations occur significantly above the base of the hydrate stability zone, in layers with thicknesses ranging from tens of centimeters to tens of meters. Moreover, we find that there is no relationship between wells with hydrate accumulations and deeper oil and gas reservoirs; hydrate is just as likely to occur above identified oil and gas reservoirs as in areas with dry holes (i.e., no oil or gas reservoir). We argue the low concentration of hydrate, the occurrence of hydrate significantly above the base of the gas hydrate stability zone, and the lack of association between hydrate occurrence and deeper oil and gas reservoirs implies that the gas in these hydrate systems is likely transported via diffusion and is primarily microbial in origin.

Three‐Dimensional Free Gas Flow Focuses Basin‐Wide Microbial Methane to Concentrated Methane Hydrate Reservoirs in Geological System

AbstractWe present a systematic model that links the generation, migration, phase partitioning, and accumulation of methane into a closed loop as the sediment is deposited from the seafloor and buried through the base of hydrate stability zone (BHSZ). In our model, methane is generated by biodegradation of organic carbon in muds. Hydrate does not form and methane is not trapped until a coarse‐grained layer is deposited, because the small pores prevent hydrate formation in muds. Instead, methane diffuses into sands/silts where methane solidifies into hydrate. As hydrate‐bearing sands/silts pass through the BHSZ during sediment burial, methane hydrate dissociates, and releases free gas. The released and the newly generated free gas below the BHSZ concentrates into a vertical/dipping zone with low capillary entry pressure and high permeability and flows upward driven its buoyancy. When free gas reaches the hydrate stability zone (HSZ), capillary forces drive free gas to flow laterally, preferentially enter sands/silts, feed hydrate growth, and elevate hydrate saturation. With three‐dimensional focused free gas flow, microbial methane that is generated from a much larger fetch area of the entire basin, both above and below the BHSZ, is concentrated into coarse‐grained layers at structural closures for hydrate formation. Our model illustrates how geological evolution, microbial methane generation, and gas flow by buoyancy couple to generate concentrated hydrate deposits in geological system. These insights can be used to explore for high‐concentration methane hydrate and are important for understanding the methane budget and carbon cycle under the seafloor.

Quantitative interpretations and assessments of a fractured gas hydrate reservoir using three-dimensional seismic and LWD data in Kutei basin, East Kalimantan, offshore Indonesia

Concentrated fractured gas hydrate reservoirs in fine-grained sediments have been considered to have limited potential for commercially-viable production of natural gas. They may also pose significant geohazard where they overly conventional oil and gas production targets. The genesis of such accumulations is not well known and the characterization of the reservoirs is difficult. Thus, integrate characterization using 3D seismic interpretation, well logs, and geotechnical analyses are vital. The present study is the first detailed description of a fractured gas hydrate reservoir in the Kutei basin, offshore Indonesia.In this study, vertical fluid flow features (such as mud volcanoes and chimney structures) and other indicators of gas hydrate occurrence (including Bottom Simulating Reflectors (BSRs) and high-resistivity zones) are interpreted over a region of 500 km2. Seismic amplitude anomalies provide evidence of gas migration that has reached the seafloor. An interpreted double BSR below a mud volcano is inferred to indicate thermogenic gas present in the shallow sections. Geological and geotechnical analyses show overpressure associated with gas migration across the base of gas hydrate stability zone (BGHSZ) results in the formation of hydraulic and pneumatic fracturing. Seismic and well logs data indicate that the vertical migration of ascending free gas results in large-size macro-fractures whereas free gas accumulation below less permeable hydrated sediments allows for the generation of local small micro-fractures. Consequently, the fractures play roles as conduits and reservoirs for gas hydrate accumulations.Hydrate saturation was estimated from seismic and well log data by using seismic inversion and Archie equation, respectively. Gas hydrate is inferred to uniformly occur above the base of hydrate stability zone and its average thickness is estimated to be approximately 30 m. The gas hydrate is estimated to occupy generally 5–10% by volume in micro-fracture reservoirs, with as high as 30–35% near the top of macro-fracture reservoirs.

Evaluation of gas production potential from gas hydrate deposits in National Petroleum Reserve Alaska using numerical simulations

An evaluation of the gas production potential of Sunlight Peak gas hydrate accumulation in the eastern portion of the National Petroleum Reserve Alaska (NPRA) of Alaska North Slope (ANS) is conducted using numerical simulations, as part of the U.S. Geological Survey (USGS) gas hydrate Life Cycle Assessment program. A field scale reservoir model for Sunlight Peak is developed using Advanced Processes & Thermal Reservoir Simulator (STARS) that approximates the production design and response of this gas hydrate field. The reservoir characterization is based on available structural maps and the seismic-derived hydrate saturation map of the study region. A 3D reservoir model, with heterogeneous distribution of the reservoir properties (such as porosity, permeability and vertical hydrate saturation), is developed by correlating the data from the Mount Elbert well logs. Production simulations showed that the Sunlight Peak prospect has the potential of producing 1.53 × 109 ST m3 of gas in 30 years by depressurization with a peak production rate of around 19.4 × 104 ST m3/day through a single horizontal well. To determine the effect of uncertainty in reservoir properties on the gas production, an uncertainty analysis is carried out. It is observed that for the range of data considered, the overall cumulative production from the Sunlight Peak will always be within the range of ±4.6% error from the overall mean value of 1.43 × 109 ST m3. A sensitivity analysis study showed that the proximity of the reservoir from the base of permafrost and the base of hydrate stability zone (BHSZ) has significant effect on gas production rates. The gas production rates decrease with the increase in the depth of the permafrost and the depth of BHSZ. From the overall analysis of the results it is concluded that Sunlight Peak gas hydrate accumulation behaves differently than other Class III reservoirs (Class III reservoirs are composed of a single layer of hydrate with no underlying zone of mobile fluids) due to its smaller thickness and high angle of dip.

Pore-water chemistry of sediment cores off Mahanadi Basin, Bay of Bengal: Possible link to deep seated methane hydrate deposit

Organoclastic degradation and anaerobic oxidation of methane (AOM) are known to influence sulfate, methane and bicarbonate concentrations of sediment pore-waters. In this work we have analyzed the pore-fluid composition of three piston cores (36–39 m long) collected from water depths ranging from 1429 to 1691 m. We attribute the observed linear sulfate concentration profiles and (ΔCa + ΔAlk)/ΔSO42− ratios dominantly to anaerobic oxidation of methane (AOM) induced sulfate consumption. The gas rich layers just below the base of hydrate stability zone (BGHSZ) is the possible source of the enhanced diffusive flux of biogenic methane (δ13CCH4 ranging from −99.7 to −106.3‰ VPDB). Occurrences of disseminated gas hydrate in the Mahanadi Basin within the hydrate stability zone (210–220 m thick) supports the link between linear sulfate profile, high rate of AOM and hydrate occurrence.

A kinetic model for the methane hydrate precipitated from venting gas at cold seep sites at Hydrate Ridge, Cascadia margin, Oregon

We develop a kinetic model for hydrate crystallization from methane gas venting through shallow sediments at Hydrate Ridge on the Cascadia margin of Oregon that predicts how pore water chlorinity, temperature, and crystallized hydrate evolve after the onset of steady venting. Predictions are compared to observations at Ocean Drilling Program Site 1249. In the preferred model, calculated gas hydrate saturation and chloride concentrations reach those observed at depths less than 20m below seafloor (bsf) under the southern summit of Hydrate Ridge in similar to 650years, and the vertical water flux must be less than 50kg/m(2)/yr. Hydrate accumulates more slowly between 20m bsf and the base of the hydrate stability zone where there is no free gas, accumulating to observed levels of a few volume percent of hydrate in 10(5) to 10(6)years, depending on the water flux that is assumed through this zone. This dichotomy means that the presently observed gas venting must have been diverted to this area similar to 650years ago, or be episodic and infrequent. If the gas venting for the last 650years has been as observed today, the latent heat of hydrate precipitation in the upper 20m of sediments would have caused the temperature to increase similar to 0.8 degrees C at similar to 20m bsf and similar to 0.2 degrees C at similar to 100m bsf. This would have caused a similar to 5m rise in the elevation of the base of hydrate stability zone, and decreased the rate of hydrate crystallization from 1.5kg CH4/m(2)/yr 650years ago to 0.7kg CH4/m(2)/yr today.

Heat flow estimated from BSR and IODP borehole data: Implication of recent uplift and erosion of the imbricate thrust zone in the Nankai Trough off Kumano

Heat flow values were estimated in the Nankai Trough fore-arc slope region off Kii Peninsula from the Bottom-Simulating Reflector (BSR) and other data obtained during IODP Expeditions 315 and 316. The heat flow has an uncertainty of ∼20% mainly due to ambiguities in the temperature estimate at the BSR and uncertainties on the thermal conductivity. BSRs occur intermittently in the Imbricate Thrust Zone (ITZ). They are significantly shallower below anticlines, designated “Anticlinal High Zone” (AHZ), than in the adjacent synclines (“Basal Low Zone” (BLZ)). The heat flow in the BLZ (55–65 mW/m2) is consistent with the regional heat flow trend. The shallow BSR in the AHZ produces an apparent high heat flow anomaly of 70–90 mW/m2, and discontinuities are observed across thrust faults. The most likely cause is the transient effect of thrust faulting followed by erosion on the hanging wall side. A one-dimensional time-dependent numerical model, with latent heat for hydrate and gas transition considered, indicates that the relaxation time for the BSR to reequilibrate after such a disturbance is ∼10 kyr. Although the possibility that the shallow BSR is a remnant base of hydrate stability zone cannot be ruled out, if the shortening has occurred in the recent 10 kyr the convergent rate at the ITZ would be ∼30 m/kyr, which is much larger than the recent horizontal shortening across the megasplay fault, and could even exceed the portion of the plate convergence than that accommodated at the frontal thrust.

Velocity and AVO analysis for the investigation of gas hydrate along a profile in the western continental margin of India

The occurrence of gas hydrate has been inferred from the presence of Bottom-Simulating Reflectors (BSRs) along the western continental margin of India. In this paper, we assess the spatial and vertical distribution of gas hydrates by analyzing the interval velocities and Amplitude Versus Offset (AVO) responses obtained from multi-channel seismics (MCSs). The hydrate cements the grains of the host sediment, thereby increasing its velocity, whereas the free gas below the base of hydrate stability zone decreases the interval velocity. Conventionally, velocities are obtained from the semblance analysis on the Common Mid-Point (CMP) gathers. Here, we used wave-equation datuming to remove the effect of the water column before the velocity analysis. We show that the interval velocities obtained in this fashion are more stable than those computed from the conventional semblance analysis. The initial velocity model thus obtained is updated using the tomographic velocity analysis to account for lateral heterogeneity. The resultant interval velocity model shows large lateral velocity variations in the hydrate layer and some low velocity zones associated with free gas at the location of structural traps. The reflection from the base of the gas layer is also visible in the stacked seismic data. Vertical variation in hydrate distribution is assessed by analyzing the AVO response at selected locations. AVO analysis is carried out after applying true amplitude processing. The average amplitudes of BSRs are almost constant with offset, suggesting a fluid expulsion model for hydrate formation. In such a model, the hydrate concentrations are gradational with maxima occurring at the base of hydrate stability zone.

Dynamic multiphase flow model of hydrate formation in marine sediments

We developed a multicomponent, multiphase, fluid and heat flow model to describe hydrate formation in marine sediments; the one‐ and two‐dimensional model accounts for the dynamic effects of hydrate formation on salinity, temperature, pressure, and hydraulic properties. Free gas supplied from depth forms hydrate, depletes water, and elevates salinity until pore water is too saline for further hydrate formation: Salinity and hydrate concentration increase upward from the base of the regional hydrate stability zone (RHSZ) to the seafloor, and the base of the hydrate stability zone has significant topography. In fine‐grained sediments, hydrate formation leads to rapid permeability reduction and capillary sealing to free gas. This traps gas and causes gas pressure to build up until it exceeds the overburden stress and drives gas through the RHSZ. Gas chimneys couple the free gas zone to the seafloor through high‐salinity conduits that are maintained at the three‐phase boundary by gas flow. As a result, significant amounts of gaseous methane can bypass the RHSZ, which implies a significantly smaller hydrate reservoir than previously envisioned. Hydrate within gas chimneys lies at the three‐phase boundary, and thus small increases in temperature or decreases in pressure can immediately transport methane into the ocean. This type of hydrate deposit may be the most economical for producing energy because it has very high methane concentrations (Sh > 70%), located near the seafloor, which lie on the three‐phase boundary.

Seismic velocities from the Yaquina forearc basin off Peru: evidence for free gas within the gas hydrate stability zone

Multichannel seismic (MCS) data from the Yaquina forearc basin off Peru reveal a complex distribution of gas and gas hydrate related reflections. Lateral variations of the reflection pattern at the assumed base of the gas hydrate stability zone in terms of continuity, amplitude, and signal attenuation underneath are observed, as well as the possible occurrence of paleo-bottom simulating reflectors (BSRs). Phase reversed reflections above the bottom simulating reflector point to free gas within the gas hydrate stability zone (GHSZ). To constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the MCS line from high-resolution ocean bottom hydrophone recordings with two independent methods. Heat flux values estimated on the basis of the velocity-depth functions increase with decreasing amplitude of the BSR and peak near chemoherms. These results suggest a model of the Yaquina Basin where free gas is trapped under parts of the BSR, and within the GHSZ, particularly under the seafloor and under an erosional unconformity. The hypothesis of a paleo-BSR that reflects the uplift of the base of the hydrate stability zone caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values.

Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness.

Below water depths of about 300 metres, pressure and temperature conditions cause methane to form ice-like crystals of methane hydrate. Marine deposits of methane hydrate are estimated to be large, amassing about 10,000 gigatonnes of carbon, and are thought to be important to global change and seafloor stability, as well as representing a potentially exploitable energy resource. The extent of these deposits can usually be inferred from seismic imaging, in which the base of the methane hydrate stability zone is frequently identifiable as a smooth reflector that runs parallel to the sea floor. Here, using high-resolution seismic sections of seafloor sediments in the Cascadia margin off the coast of Vancouver Island, Canada, we observe lateral variations in the base of the hydrate stability zone, including gas-rich vertical intrusions into the hydrate stability zone. We suggest that these vertical intrusions are associated with upward flow of warmer fluids. Therefore, where seafloor fluid expulsion and methane hydrate deposits coincide, the base of the hydrate stability zone might exhibit significant roughness and increased surface area. Increased area implies that significantly more methane hydrate lies close to being unstable and hence closer to dissociation in the event of a lowering of pressure due to sea-level fall.

Evaluation of normative EEG spectral parameters in normal children. A five-year follow-up study : K. Sadowsky, E. Mullner, S.J. Pöppl, W.S. Tirsch and H.M. Weinmann (Munich, W. Germany)

Concentrated fractured gas hydrate reservoirs in fine-grained sediments have been considered to have limited potential for commercially-viable production of natural gas. They may also pose significant geohazard where they overly conventional oil and gas production targets. The genesis of such accumulations is not well known and the characterization of the reservoirs is difficult. Thus, integrate characterization using 3D seismic interpretation, well logs, and geotechnical analyses are vital. The present study is the first detailed description of a fractured gas hydrate reservoir in the Kutei basin, offshore Indonesia.In this study, vertical fluid flow features (such as mud volcanoes and chimney structures) and other indicators of gas hydrate occurrence (including Bottom Simulating Reflectors (BSRs) and high-resistivity zones) are interpreted over a region of 500 km2. Seismic amplitude anomalies provide evidence of gas migration that has reached the seafloor. An interpreted double BSR below a mud volcano is inferred to indicate thermogenic gas present in the shallow sections. Geological and geotechnical analyses show overpressure associated with gas migration across the base of gas hydrate stability zone (BGHSZ) results in the formation of hydraulic and pneumatic fracturing. Seismic and well logs data indicate that the vertical migration of ascending free gas results in large-size macro-fractures whereas free gas accumulation below less permeable hydrated sediments allows for the generation of local small micro-fractures. Consequently, the fractures play roles as conduits and reservoirs for gas hydrate accumulations.Hydrate saturation was estimated from seismic and well log data by using seismic inversion and Archie equation, respectively. Gas hydrate is inferred to uniformly occur above the base of hydrate stability zone and its average thickness is estimated to be approximately 30 m. The gas hydrate is estimated to occupy generally 5–10% by volume in micro-fracture reservoirs, with as high as 30–35% near the top of macro-fracture reservoirs.