James S. Booth

Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned by host sediment properties

The stability of submarine gas hydrates is largely dictated by pressure and temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of deep marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our conceptual model presumes that gas hydrate behaves in a way analogous to ice in a freezing soil. Hydrate growth is inhibited within fine-grained sediments by a combination of reduced pore water activity in the vicinity of hydrophilic mineral surfaces, and the excess internal energy of small crystals confined in pores. The excess energy can be thought of as a “capillary pressure” in the hydrate crystal, related to the pore size distribution and the state of stress in the sediment framework. The base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature (nearer to the seabed) than would be calculated from bulk thermodynamic equilibrium. Capillary effects or a build up of salt in the system can expand the phase boundary between hydrate and free gas into a divariant field extending over a finite depth range dictated by total methane content and pore-size distribution. Hysteresis between the temperatures of crystallization and dissociation of the clathrate is also predicted. Growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, and lenses in muds; cements in sands) can largely be explained by capillary effects, but kinetics of nucleation and growth are also important. The formation of concentrated gas hydrates in a partially closed system with respect to material transport, or where gas can flush through the system, may lead to water depletion in the host sediment. This “freeze-drying” may be detectable through physical changes to the sediment (low water content and overconsolidation) and/or chemical anomalies in the pore waters and metastable presence of free gas within the normal zone of hydrate stability.

Evidence for faulting related to dissociation of gas hydrate and release of methane off the southeastern United States

Abstract An irregular, faulted, collapse depression about 38 × 18 km in extent is located on the crest of the Blake Ridge offshore from the south-eastern United States. Faults disrupt the sea floor and terminate or sole out about 40–500 m below the sea floor at the base of the gas hydrate stable zone, which is identified from the location of the bottom simulating reflection (BSR). Normal faults are common but reverse faults and folds also are widespread. Folds commonly convert upward into faults. Sediment diapirs and deposits of sediments that were erupted onto the sea floor are also present. Sea-floor depressions at faults may represent locations of liquid/gas vents. The collapse was probably caused by overpressures and by the decoupling of the overlying sediments by gassy muds that existed just beneath the zone of gas hydrate stability.

A note on the relationships between organic matter and some geotechnical properties of a marine sediment

Abstract An analysis of the relationship between organic matter and liquid and plastic limits, and grain‐specific gravity of a marine sediment was accomplished by making a stepwise adjustment in the organic content of that sediment. The sample used was from Santa Barbara Basin (off southern California) and is typical of fine‐grained marine sediments: it is a clayey silt with a common suite of minerals and other constituents. During the experiment, texture and composition (except organic content) were constant; only the quantity of natural, indigenous organic matter was changed. A strong linear relationship exists between the independent variable, the amount of organic carbon present in the sample, and the dependent variables. Liquid limit, plastic limit, and plasticity index all increased with increasing organic content over the range studied (0.57–3.20% organic carbon). Grain‐specific gravity decreased. All had linear correlation coefficients (r) greater than |0.90| and r 2 values greater than 90%, excep...

Major occurrences and reservoir concepts of marine clathrate hydrates: implications of field evidence

Abstract Questions concerning clathrate hydrate as an energy resource, as a factor in modifying global climate and as a triggering mechanism for mass movements invite consideration of what factors promote hydrate concentration, and what the quintessential hydrate-rich sediment may be. Gas hydrate field data, although limited, provide a starting point for identifying the environments and processes that lead to more massive concentrations. Gas hydrate zones are up to 30 m thick and the vertical range of occurrence at a site may exceed 200 m. Zones typically occur more than 100 m above the phase boundary. Thicker zones are overwhelmingly associated with structural features and tectonism, and often contain sand. It is unclear whether an apparent association between zone thickness and porosity represents a cause-and-effect relationship. The primary control on the thickness of a potential gas hydrate reservoir is the geological setting. Deep water and low geothermal gradients foster thick gas hydrate stability zones (GHSZs). The presence of faults, fractures, etc., can favour migration of gas-rich fluids. Geological processes, such as eustacy or subsidence, may alter the thickness of the GHSZ or affect hydrate concentration. Tectonic forces may promote injection of gas into the GHSZ. More porous and permeable sediment, as host sediment properties, increase storage capacity and fluid conductivity, and thus also enhance reservoir potential.

Wave Processes and Geologic Responses on the Floor of the Yellow Sea

From Shoreline to Abyss: Contributions in Marine Geology in Honor of Francis Parker Shepard - Francis P. Shepard left a rich scientific legacy including more than 230 published papers and books primarily addressed to the study of submarine canyons and turbidity currents, continental shelves and associated sediments, coastal processes and sediments and marine physiography and tectonics. He is best remembered for his work on submarine canyons; however, his broad range of scientific interests and his remarkable ability to break new ground in each of these disciplines have served as a model for at least four generations of ?Shepard? students. This new work from these Shepard students addresses problems in marine geology from the global scale to the local outcrop scale. Relationships among tectonics, eustacy and both siliciclastic and carbonate sedimentation create a unifying theme. Special topics include coastal processes, shelf and slope evolution, and submarine canyon and fan systems.

Formation of Natural Gas Hydrates in Marine Sediments: Gas Hydrate Growth and Stability Conditioned by Host Sediment Properties

Abstract: The stability conditions of submarine gas hydrates (methane clathrates) are largely dictated by pressure, temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of the host sediments also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our model presumes that gas hydrate behaves in a way analogous to ice in the pores of a freezing soil, where capillary forces influence the energy balance. Hydrate growth is inhibited within fine‐grained sediments because of the excess internal phase pressure of small crystals with high surface curvature that coexist with liquid water in small pores. Therefore, the base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature, and so nearer to the seabed than would be calculated from bulk thermodynamic equilibrium. The growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, sheets, and lenses in muds; cements in sand and ash layers) can be explained by a requirement to minimize the excess of mechanical and surface energy in the system.

A statistical overview of mass movement characteristics on the North American atlantic outer continental margin

Abstract An analysis of 179 mass movements on the North American Atlantic continental slope and upper rise shows that slope failures have occurred throughout the geographic extent of the outer margin. Although the slope failures show no striking affinity for a particular depth as an origination level, there is a broad, primary mode centered at about 900 m. The resulting slides terminate at almost all depths and have a primary mode at 1100 m, but the slope/rise boundary (at 2200 m) also is an important mode. Slope failures have occurred at declivities ranging from 1° to 30° (typically, 4°); the resultant mass movement deposits vary in width from 0.2 to 50 km (typically, 1–2 km) and in length from 0.3 to 380 km (typically, 2–4 km), and they have been reported to be as thick as 650 m. On a numeric basis, mass movements are slightly more prevalent on open slopes than in other physiographic settings, and both translational and rotational failure surfaces are common. The typical mass movement is disintegrative ...

Slope-stability Analysis and Creep Susceptibility of Quaternary Sediments on the Northeastern United States Continental Slope

The continental slope off the northeastern United States is a relatively steep, morphologically complex surface which shows abundant evidence of submarine slides and related processes. Because this area may be developed by the petroleum industry, questions arise concerning the potential for further slope failures or unacceptable deformations and the conditions necessary to cause such instabilities. Accordingly, a generalized analysis of slope stability and the stress—strain—time-dependent behavior of the sediments is being conducted.

Creep behavior of submarine sediments

A series of experiments on drained creep of marine sediment indicates that strength degradation results from the creep process, which implies an associated reduction in slope stability. Furthermore, the highest creep potential of a sediment may be at its preconsolidation stress. Results from the experiments on samples from Georges Bank continental slope were also used in conjunction with a preliminary theoretical model to predict creep displacements. For the case illustrated in this report, steep slopes (>20°) and thick sections (>30 m) give rise to substantial creep and probable creep rupture; as angles or thicknesses decrease, displacements rapidly become negligible.Creep may be a significant geologic process on many marine slopes. Not only can it cause major displacements of surface sediment, but it may also be the precursor to numerous slope failures.

Geotechnical, geological, and selected radionuclide retention characteristics of the radioactive waste disposal site near the Farallon Islands

Abstract A geotechnical and geological investigation of the Farallon Islands low‐level radioactive waste (LLW) disposal area was conducted to qualitatively assess the host sediments’ relative effectiveness as a barrier to radionuclide migration, to estimate the portion of the barrier that is in contact with the waste packages at the three primary disposal sites, and to provide a basic physical description of the sediments. Box cores recovered from within the general disposal area at depths of 500, 1,000, and 1,500 m were subcored to provide samples (≍30 cm in length) for detailed descriptions, textural and mineralogical analyses, and a suite of geotechnical tests (index property, CRS consolidation, and CIU triaxial compression). The sediments are coarse, consisting chiefly of sands and silts, yet are highly plastic and compressible at the two deeper stations. Although they are very soft (vane shear strengths typically ≤ 1 kPa in the top 20 cm), all sediments exhibit overconsolidated behavior. The very low...