Ingo A. Pecher

Physical properties and rock physics models of sediment containing natural and laboratory-formed methane gas hydrate

Abstract This paper presents results of shear strength and acoustic velocity (p-wave) measurements performed on: (1) samples containing natural gas hydrate from the Mallik 2L-38 well, Mackenzie Delta, Northwest Territories; (2) reconstituted Ottawa sand samples containing methane gas hydrate formed in the laboratory; and (3) ice-bearing sands. These measurements show that hydrate increases shear strength and p-wave velocity in natural and reconstituted samples. The proportion of this increase depends on (1) the amount and distribution of hydrate present, (2) differences in sediment properties, and (3) differences in test conditions. Stress-strain curves from the Mallik samples suggest that natural gas hydrate does not cement sediment grains. However, stress-strain curves from the Ottawa sand (containing laboratory-formed gas hydrate) do imply cementation is present. Acoustically, rock physics modeling shows that gas hydrate does not cement grains of natural Mackenzie Delta sediment. Natural gas hydrates are best modeled as part of the sediment frame. This finding is in contrast with direct observations and results of Ottawa sand containing laboratory-formed hydrate, which was found to cement grains (Waite et al. 2004). It therefore appears that the microscopic distribution of gas hydrates in sediment, and hence the effect of gas hydrate on sediment physical properties, differs between natural deposits and laboratory-formed samples. This difference may possibly be caused by the location of water molecules that are available to form hydrate. Models that use laboratory-derived properties to predict behavior of natural gas hydrate must account for these differences.

Trapping and migration of methane associated with the gas hydrate stability zone at the Blake Ridge Diapir: new insights from seismic data

Abstract The Blake Ridge Diapir is the southernmost of a line of salt diapirs along the Carolina trough. Diapirs cause faulting of the superjacent sediments, creating pathways for migration of fluids and gas to the seafloor. We analyzed reflection seismic data from the Blake Ridge Diapir, which is located in a region with known abundant gas hydrate occurrence. A striking feature in these data is a significant shallowing of the base of gas hydrate stability (BGHS) over the center of the diapir: The seafloor is warped up by about 100xa0m above the diapir, from about 2300xa0m to about 2200xa0m. The BGHS, as indicated by a bottom simulating reflection (BSR), is about 4.5xa0s off the flanks of the diapir, rising to about 4.15xa0s at the center. Above the diapir, a fault system appears to rise vertically from the BGHS to about 0.05xa0s below the seafloor (40–50xa0m); it then diverges into several steeply dipping faults that breach the seafloor and cover an area ∼700xa0m in diameter. Other secondary faults diverge from the main fault or emerge directly from the BGHS near the crest of the diapir. Gas and other fluids may migrate upward through the faults. We performed complex trace analysis to compare the reflection strength and instantaneous frequency along individual reflections. A low-frequency anomaly over the center of the diapir indicates high seismic attenuation. This is interpreted to be caused by migration of fluids (probably methane) along fault pathways. The migration of gas (i.e. probably mainly methane) through the gas hydrate stability zone is not yet understood. We speculate that pore fluids in the faults may be too warm and too salty to form gas hydrate, even at depths where gas hydrate is stable away from the diapir. Alternatively, gas hydrates may seal the fault walls such that water supply is too low to transform all the gas into gas hydrates. The shallowing of the BSR may reflect increased heatflow above the diapir either caused by the high thermal conductivity of the underlying salt or by advective heat transport along with fluids. High pore water salinity shifts the gas hydrate stability to lower temperatures and may also play a significant role in BSR shallowing. We, therefore, investigated the possible effect of pore water salinity on shallowing of the BSR. We found that BSR shallowing may theoretically be entirely caused by increased salinity over the diapir, although geologically this would not be reasonable. This observation demonstrates the potential importance of pore water salinity for lateral variations of BSR depths, in particular, above salt structures.

Migration of methane gas through the hydrate stability zone in a low-flux hydrate province

New high-resolution seismic data show clear evidence for upward injection of methane gas well into the hydrate stability zone at the stable, low-methane-flux Blake Ridge crest. This movement of gaseous methane, through a thermo-dynamic regime where it should be trapped as hydrate, suggests that dynamic migrations of gas play an important role in the interaction of subseafloor methane with the ocean. In the study area, none of the seismic amplitude anomalies that provide evidence for gas migration reaches the seafloor; instead they terminate at the base of a highly reflective, unfaulted capping layer. Seismic inversions of anomalous regions show (1) increased velocities beneath the hydrate stability zone, suggesting less gas, and (2) increased velocities within the hydrate stability zone associated with observed low-amplitude chimneys and bright spots, indicating increased hydrate concentrations. These observations and analyses indicate that methane migrates upward as free gas hundreds of meters into the hydrate stability zone before forming hydrate. The observations strongly imply that given appropriate permeable pathways, free gas can escape into the ocean. Even in a low-flux environment, the hydrate stability zone is not an impermeable barrier to free-gas migration.

Direct seismic detection of methane hydrate on the Blake Ridge

Seismic detection of methane hydrate often relies on indirect or equivocal methods. New multichannel seismic reflection data from the Blake Ridge, located approximately 450 km east of Savannah, Georgia, show three direct seismic indicators of methane hydrate: (1) a paleo bottom‐simulating reflector (BSR) formed when methane gas froze into methane hydrate on the eroding eastern flank of the Blake Ridge, (2) a lens of reduced amplitudes and high P‐wave velocities found between the paleo‐BSR and BSR, and (3) bright spots within the hydrate stability zone that represent discrete layers of concentrated hydrate formed by upward migration of gas. Velocities within the lens (∼1910 m/s) are significantly higher than velocities in immediately adjacent strata (1820 and 1849 m/s). Conservative estimates show that the hydrate lens contains at least 13% bulk methane hydrate within a 2‐km3 volume, yielding 3.2 × 1010kg [1.5 TCF (4.2 × 1010 m3] of methane. Low seismic amplitudes coupled with high interval velocities with...

Escape of methane gas through sediment waves in a large methane hydrate province

Despite paleoceanographic evidence that large quantities of methane have escaped from marine gas hydrates into the oceans, the sites and mechanisms of methane release remain largely speculative. New seismic data from the Blake Ridge, a hydrate-bearing drift deposit in the western Atlantic, show clear evidence for methane release and suggest a new mechanism by which methane gas can escape, without thermal or mechanical disruption of the hydrate-bearing layer. Rapid, post–2.5 Ma formation of large sediment waves and associated seafloor erosion created permeable pathways connecting free gas to the seafloor, allowing methane gas expulsion. The amount of missing methane, 0.6 Gt, is equivalent to ∼12% of total present-day atmospheric methane. Our results imply that significant amounts of methane gas can bypass the hydrate stability zone and escape into the ocean. Mechanisms of tapping methane directly from the free-gas zone, such as widespread seafloor erosion, should be considered when seeking the causes of large negative carbon isotope excursions in the geological record.

The link between bottom-simulating reflections and methane flux into the gas hydrate stability zone-new evidence from Lima Basin, Peru Margin

Bottom-simulating reflections (BSRs) are probably the most commonly used indicators for gas hydrates in marine sediments. It is now widely accepted that BSRs are primarily caused by free gas beneath gas-hydrate-bearing sediments. However, our insight into BSR formation to date is mostly limited to theoretical studies. Two endmember processes have been suggested to supply free gas for BSR formation: (i) dissociation of gas hydrates and (ii) migration of methane from below. During a recent campaign of the German Research Vessel Sonne off the shore of Peru, we detected BSRs at locations undergoing both tectonic subsidence and non-sedimentation or seafloor erosion. Tectonic subsidence (and additionally perhaps seafloor erosion) causes the base of gas hydrate stability to migrate downward with respect to gas-hydrate-bearing sediments. This process rules out dissociation of gas hydrates as a source of free gas for BSRs at these locations. Instead, free gas at BSRs is predicted to be absorbed into the gas hydrate stability zone. BSRs appear to be confined to locations where the subsurface structure suggests focusing of fluid flow. We investigated the seafloor at one of these locations with a TV sled and observed fields of rounded boulders and slab-like rocks, which we interpreted as authigenic carbonates. Authigenic carbonates are precipitations typically found at cold vents with methane expulsion. We retrieved a small carbonate-cemented sediment sample from the seafloor above a BSR about 20 km away. This supported our interpretation that the observed slabs and boulders were carbonates. All these observations suggest that BSRs in Lima Basin are maintained predominantly by gas that is supplied from below, demonstrating that this endmember process for BSR formation exists in nature. Results from Ocean Drilling Program Leg 112 showed that methane for gas hydrate formation on the Peru lower slope and the methane in hydrocarbon gases on the upper slope is mostly of biogenic origin. The δ13C composition of the recovered carbonate cement was consistent with biologic methane production below the seafloor (although possibly above the BSR). We speculate that the gas for BSR formation in Lima Basin also is mainly biogenic methane. This would suggest the biologic productivity beneath the gas hydrate zone in Lima Basin to be relatively high in order to supply enough methane to maintain BSRs.

Methane seepage and its relation to slumping and gas hydrate at the Hikurangi margin, New Zealand

Abstract Dissolved methane and high resolution bathymetry surveys were conducted over the Rock Garden region of Ritchie Ridge, along the Hikurangi margin, eastern New Zealand. Multibeam bathymetry reveals two prominent, northeast trending ridges, parallel to subduction along the margin, that are steep sided and extensively slumped. Elevated concentrations of methane (up to 10 nM, 10× background) within the water column are associated with a slump structure at the southern end of Eastern Rock Garden. The anomalous methane concentrations were detected by a methane sensor (METS) attached to a conductivity‐temperature‐depth‐optical backscatter device (CTDO) and are associated with elevated light scattering and flare‐shaped backscatter signals revealed by the ships echo sounder. Increased particulate matter in the water column, possibly related to the seepage and/or higher rates of erosion near slump structures, is considered to be the cause of the increased light scattering, rather than bubbles in the water column. Methane concentrations calculated from the METS are in good agreement with concentrations measured by gas chromatography in water samples collected at the same time. However, there is a c. 20 min (c. 900 m) delay in the METS signal reaching maximum CH4 concentrations. The maximum methane concentration occurs near the plateau of Eastern Rock Garden close to the edge of a slump, at 610 m below sea level (mbsl). This is close to the depth (c. 630 mbsl) where a bottom simulating reflector (BSR) pinches out at the seafloor. Fluctuating water temperatures observed in previous studies indicate that the stability zone for pure methane hydrate in the ocean varies between 630 and 710 mbsl. However, based on calculations of the geothermal gradients from BSRs, we suggest gas hydrate in the study area to be more stable than hydrate from pure methane in sea water, moving the phase boundary in the ocean upward. Small fractions of additional higher order hydrocarbon gases are the most likely cause for increased hydrate stability. Relatively high methane concentrations have been measured down to c. 1000 mbsl, most likely in response to sediment slumping caused by gas hydrate destabilisation of the sediments and/or marking seepage through the gas hydrate zone.

Seismic detection of marine methane hydrate

As offshore petroleum exploration and development move into deeper water, industry must contend increasingly with gas hydrate, a solid compound that binds water and a low-molecular-weight gas (usually methane). Gas hydrate has been long studied in industry from an engineering viewpoint, due to its tendency to clog gas pipelines.nnHowever, hydrate also occurs naturally wherever there are high pressures, low temperatures, and sufficient concentrations of gas and water. These conditions prevail in two natural environments, both of which are sites of active exploration: permafrost regions and marine sediments on continental slopes. In this article we discuss seismic detection of gas hydrate in marine sediments.nnGas hydrate in deepwater sediments poses both new opportunities and new hazards. An enormous quantity of natural gas, likely far exceeding the global inventory of conventional fossil fuels, is locked up worldwide in hydrates. Ex-traction of this unconventional resource presents unique exploration, engineering, and economic challenges, and several countries, including the United States, Japan, Canada, India, and Korea, have initiated joint industry-academic-governmental programs to begin studying those challenges. Hydrates also constitute a potential drilling hazard. Because hydrates are only stable in a restricted range of pressure and temperature, any activity that sufficiently raises temperature or lowers pressure could destabilize them, releasing potentially large volumes of gas and decreasing the shear strength of the host sediments. Assessment of the opportunities and hazards associated with hydrates requires reliable methods of detecting hydrate and accurate maps of their distribution and concentration.nnHydrate may occur only within the upper few hundred meters of deepwater sediment, at any depth between the seafloor and the base of the stability zone, which is controlled by local pressure and temperature. Hydrate is occasionally exposed at the seafloor, where it can be detected either visually or acoustically by strong seismic reflection amplitudes or high backscatter …

GHASTLI — Determining Physical Properties of Sediment Containing Natural and Laboratory-Formed Gas Hydrate

Gas-hydrate samples have been recovered at about 16 areas worldwide (Booth et al., 1996). However, gas hydrate is known to occur at about 50 locations on continental margins (Kvenvolden, 1993) and is certainly far more widespread so it may represent a potentially enormous energy resource (Kvenvolden, 1988). But adverse effects related to the presence of hydrate do occur. Gas hydrate appears to have caused slope instabilities along continental margins (Booth et al., 1994; Dillon et al., 1998; Mienert et al., 1998; Paull & Dillon, (Chapter 12; Twichell & Cooper, 2000) and it has also been responsible for drilling accidents (Yakushev and Collett, 1992). Uncontrolled release of methane could affect global climate (Chapter 11), because methane is 15–20 times more effective as a “greenhouse gas” than an equivalent concentration of carbon dioxide. Clearly, a knowledge of gas-hydrate properties is necessary to safely explore the possibility of energy recovery and to understand its past and future impact on the geosphere.

Seismic anisotropy at Hydrate Ridge

[1]xa0P-wave velocity increases in the presence of gas hydrates and decreases in the presence of free gas in the sediments, making it an excellent means to investigate gas hydrate systems. However, seismic velocity is typically derived from surface seismic data without consideration of seismic anisotropy. The presence of anisotropy in the hydrate bearing sediments adds an additional complexity in data analysis; however anisotropy can help reveal the distribution of hydrates. Here we report on the evidence of seismic anisotropy at Hydrate Ridge along the Cascadia convergent margin. We find that the south summit is anisotropic, while the basin side (east of south summit) is isotropic. Anisotropy is likely caused by the hydrate veins. We interpret the anisotropy parameters in terms of the distribution and fabric of gas hydrates.