Geology topics

John C. Pinkston

Publications and source records attributed to John C. Pinkston.

5 recordsLinked to original sources

USGS2019

The effect of brine on the electrical properties of methane hydrate

Gas hydrates possess lower electrical conductivity (inverse of resistivity) than either seawater or ice, but higher than clastic silts and sands, such that electromagnetic methods can be employed to help identify their natural formation in marine and permafrost environments. Controlled laboratory studies offer a means to isolate and quantify the effects of changing individual components within gas‐hydrate‐bearing systems, in turn yielding insight into the behavior of natural systems. Here we investigate the electrical properties of polycrystalline methane hydrate with ≥25% gas‐filled porosity and in mixture with brine. Initially, pure methane hydrate was synthesized from H 2 O ice and CH 4 gas while undergoing electrical impedance measurement, then partially dissociated to assess the effects of pure pore water accumulation on electrical conductivity. Methane hydrate + brine mixtures were then formed by either adding NaCl (0.25–2.5 wt %) to high‐purity ice or by using frozen seawater as a reactant. Conductivity was obtained from impedance measurements made in situ throughout synthesis while temperature cycled between +15 °C and −25 °C. Several possible conduction mechanisms were subsequently determined using equivalent circuit modeling. Samples with low NaCl concentration show a doping/impurity effect and a log linear conductivity response as a function of temperature. For higher salt content samples, conductivity increases exponentially with temperature and the log linear relationship no longer holds; instead, we observe phase changes within the samples that follow NaCl–H 2 O–CH 4 phase equilibrium predictions. Final samples were quenched in liquid nitrogen and imaged by cryogenic scanning electron microscopy (cryo‐SEM) to assess grain‐scale characteristics.

Journal of Geophysical Research↗

USGS2012

Basal slip and mechanical anisotropy of biotite

The basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on (001). Samples loaded at 45° to (001) exhibit gentle external rotations associated with dislocation glide. High‐angle kink bands in these samples, unlike those developed in micas loaded parallel to (001), are limited to sample corners. Samples shortened perpendicular to (001) show no evidence of nonbasal slip and fail by fracture over all conditions tested. The mechanical response of biotite shortened at 45° to (001) is nearly perfectly elastic‐plastic; stress‐strain curves are characterized by a steep elastic slope, a sharply defined yield point, and continued deformation at low (mostly 1%. Stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 MPa and exhibit weak dependencies upon strain rate and temperature. Assuming an exponential relationship between differential stress σ and strain rate of the form , the data collected over strain rates and temperatures of 10−7 to 10−4 s−1 and 20° to 400°C, respectively, are best fit by an exponential constant α of 0.41±0.08 MPa‐1 and an activation energy of 82±13 kJ/mol. A power law fits the data equally well with = 18±4 and = 51±9 kJ/mol. Samples oriented favorably for slip in directions [100] and [110] are measurably weaker than those shortened at 45° to [010] and [310], consistent with the reported Burgers vectors 〈100〉, 1/2 〈110〉, and 1/2 〈110〉. The anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to (001). Previous studies have shown that biotite loaded in the (001) plane is strong prior to the nucleation of kink bands. The strength of biotite shortened perpendicular to (001) exceeds that measured parallel to (001) and is pressure dependent. Application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions. Its presence within foliated rocks and shear zones may limit locally the stresses that can be supported.

Journal of Geophysical Research B: Solid Earth↗

USGS2011

Gas hydrate characterization and grain-scale imaging of recovered cores from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

Using cryogenic scanning electron microscopy (CSEM), powder X-ray diffraction, and gas chromatography methods, we investigated the physical states, grain characteristics, gas composition, and methane isotopic composition of two gas-hydrate-bearing sections of core recovered from the BPXA–DOE–USGS Mount Elbert Gas Hydrate Stratigraphic Test Well situated on the Alaska North Slope. The well was continuously cored from 606.5 m to 760.1 m depth, and sections investigated here were retrieved from 619.9 m and 661.0 m depth. X-ray analysis and imaging of the sediment phase in both sections shows it consists of a predominantly fine-grained and well-sorted quartz sand with lesser amounts of feldspar, muscovite, and minor clays. Cryogenic SEM shows the gas-hydrate phase forming primarily as a pore-filling material between the sediment grains at approximately 70–75% saturation, and more sporadically as thin veins typically several tens of microns in diameter. Pore throat diameters vary, but commonly range 20–120 microns. Gas chromatography analyses of the hydrate-forming gas show that it is comprised of mainly methane (>99.9%), indicating that the gas hydrate is structure I. Here we report on the distribution and articulation of the gas-hydrate phase within the cores, the grain morphology of the hydrate, the composition of the sediment host, and the composition of the hydrate-forming gas.

Alaska↗

USGS2011

Observations of mass fractionation of noble gases in synthetic methane hydrate

As a consequence of contemporary or longer term (since 15 ka) climate warming, gas hydrates in some settings are presently dissociating and releasing methane and other gases to the oceanatmosphere system. A key challenge in assessing the susceptibility of gas hydrates to warming climate is the lack of a technique able to distinguish between methane recently released from gas hydrates and methane emitted from leaky thermogenic reservoirs, shallow sublake and subseafloor sediments, coalbeds, and other sources. Carbon and deuterium stable isotopic data provide only a first-order characterization of methane sources, while gas hydrate can sequester any type of methane. Here, we investigate the possibility of exploiting the pattern of noble gas fractionation within the gas hydrate lattice to fingerprint methane released from gas hydrates. Starting with synthetic gas hydrate formed under careful laboratory conditions, we document complex noble gas fractionation patterns in the gases liberated during dissociation and explore the effects of aging and storage (e.g., in liquid nitrogen), as well as sampling and preservation procedures. The laboratory results confirm a unique noble gas fractionation pattern for gas hydrates, one that shows promise in evaluating modern natural gas seeps for a signature associated with gas hydrate dissociation.

Conference Paper↗

USGS2006

Methane Hydrate Dissociation Rates at 0.1 MPa and Temperatures above 272 K

We performed rapid depressurization experiments on methane hydrate under isothermal conditions above 272 K to determine the amount and rate of methane evolution. Sample temperatures rapidly drop below 273 K and stabilize near 272.5 K during dissociation. This thermal anomaly and the persistence of methane hydrate are consistent with the reported recovery of partially dissociated methane hydrate from ocean drilling cores.

Annals of the New York Academy of Sciences↗