I characterized microporosity by performing low pressure nitrogen adsorption measurements on 13 shallow marine mudstone samples from the Nankai Trough offshore Japan. The samples were from two reference Sites on the incoming Philippine Sea Plate, and one Site above the accretionary prism. I determined pore size distributions using the Barrett–Joyner–Hallenda (BJH) model, and merged these with existing mercury injection capillary pressure (MICP) measurements to construct a full distribution covering micro- to macropores. I found that overall pore sizes decrease with consolidation, and that microporosity content (pores < 2 nm in diameter) is influenced mainly by mineralogy, with some influence of diagenetic processes. A small amount of microporosity (~ 0.25% of bulk sediment volume) is present in these sediments at the time of burial, presumably contained mainly in clays. Additional microporosity may develop as a result of alteration of volcanic ash at the reference Sites, and may be related to diagenetic processes that create zones of anomalous high porosity. Comparisons with porewater chemistry (K+, Ca2 +, Sr, Si) show inconsistent relationships with microporosity development and cannot confirm or deny the role of ash alteration in this process. The strongest correlation observed at the three Sites was between microporosity volume and clay mineral fraction. This suggests that microporosity content is determined mainly by detrital clay abundance and development of clay as an ash alteration product, with some contribution from amorphous silica cement precipitated in the zones of anomalous high porosity.
We measured nuclear magnetic resonance (NMR) relaxation times on samples from Integrated Ocean Drilling Program Expedition 333 Sites C0011, C0012, and C0018. We compared our results to permeability, grain size, and specific surface measurements, pore size distributions from mercury injection capillary pressure, and mineralogy from X‐ray fluorescence. We found that permeability could be predicted from NMR measurements by including grain size and specific surface to quantify pore networks and that grain size is the most important factor in relating NMR response to permeability. Samples within zones of anomalously high porosity from Sites C0011 and C0012 were found to have different NMR‐permeability relationships than samples from outside these zones, suggesting that the porosity anomaly is related to a fundamental difference in pore structure. We additionally estimated the size of paramagnetic sites that cause proton relaxation and found that in most of our samples, paramagnetic material is present mainly as discrete, clay‐sized grains. This distribution of paramagnetic material may cause pronounced heterogeneity in NMR properties at the pore scale that is not accounted for in most NMR interpretation techniques. Our results provide important insight into the microstructure of marine sediments in the Nankai Trough.
The base of the methane hydrate stability zone (MHSZ) in the Kumano Basin, offshore Japan, is marked by a bottom‐simulating reflection (BSR) on seismic data. At Integrated Ocean Drilling Program Site C0002, which penetrates this BSR, the in situ temperature profile combined with bulk seawater methane equilibrium conditions suggest that the base of the MHSZ is 428 m below seafloor (bsf), which is 28 m deeper than the observed BSR (400 m bsf). We found that submicron pore sizes determined by mercury injection capillary pressure are sufficiently small to cause 64% of the observed uplift of the base of the MHSZ by the Gibbs‐Thomson effect. This is the most thorough characterization of pore sizes within the MHSZ performed to date and illustrates the extent to which pore size can influence MHSZ thickness. Our results demonstrate the importance of considering lithology and pore structure when assessing methane hydrate stability conditions in marine sediments.
Determining the porosity associated with organic and inorganic components of shales is an important but difficult part of formation evaluation in unconventional resources. Nuclear magnetic resonance (NMR) measurements offer a means of quantifying organic and inorganic porosity by separating the inorganic porosity, where proton relaxation occurs by paramagnetic interactions, from the organic porosity, where proton relaxation occurs by intermolecular dipole interactions. We performed laboratory measurements on preserved Bakken and Eagle Ford samples with a 2.2 MHz nuclear magnetic resonance (NMR) core analysis system. We constructed two-dimensional maps of T1 and T2 with different echo spacings for the T2 measurement and computed distributions of T1/T2 ratio and the secular relaxation rate, which is the difference between the transverse and longitudinal relaxation rates. Based on the distribution of T1/T2 ratios and the change in secular relaxation rate with echo spacing, we were able to differentiate organic porosity, inorganic porosity, and the relaxation signal from the organic material itself. The differentiation is based on theoretical consideration of relaxation times due to paramagnetic and dipole interactions.
The location of the seismogenic zone across the To-nankai margin segment has been widely investigated using geodetic, tsunami, seismologic, and heat-flow data as well as thermal models. Seafloor heat-flow measurements show large scatter, raising questions about the thermal state of the incoming and overriding plates. In this study, new temperature and conductivity measurements recorded in the Philippine Sea Plate (PSP) and in the accretionary prism during IODP Expedition 333 are integrated with recent data on the margin structure and seismicity to improve constraints on the thermal structure of the incoming plate and the subduction zone as well as the thermal conditions within the seismogenic zone.
IODP Expedition 333 measurements provide heat-flow values that are lower than the average of nearby seafloor measurements. Thermal modeling for the PSP suggests that hydrothermal warming is insignificant in the trench. Thus, an observed widespread positive thermal anomaly in the Shikoku Basin that peaks at the Kashinozaki knoll more likely results from vigorous thermal convection in the upper mantle in back-arc context.
The modeled thermal structure of the margin from the trench to the mantle shows a significantly colder interplate contact than in previous studies. The seismogenic zone is likely to be 30–170 km from the deformation front, corresponding to the 100–340 °C temperature range. The 1944 co-seismic slip zone extends farther southward where temperatures are about 60 °C. Slow-slip earthquakes are located in the downdip transition zone between 340 and 440 °C across the intersection of the Moho and the megathrust fault. This improved thermal structure from the trench to the mantle results mainly from a better estimate of oceanic plate age, slab dip and sedimentation rate variations in the trench.
This last parameter cools the interplate contact until a depth of ∼35 km, shifting the thermally defined seismogenic zone landward by ∼20 km and significantly diminishing its influence near the corner flow area.
Two collocated seismic surveys acquired 8 years apart at Hydrate Ridge offshore Oregon, USA, show migration of free gas in a permeable conduit, Horizon A, feeding an active methane hydrate province. They also reveal transients in active gas venting to the water column. We propose that episodic gas migration and pressure fluctuations in the reservoir underlying the regional hydrate stability zone (RHSZ) at southern Hydrate Ridge influence methane supply to the RHSZ and are linked with periodic fracturing and release of methane into the water column by complex feedback processes. We model the effect of pore pressure variations within the deep methane source on fracturing behavior with a 1D model coupling multiphase flow, hydrate accumulation, and pore pressure buildup. Fractures open when the pore pressure exceeds the fracture criterion, which we assume is the vertical effective stress assuming hydrostatic conditions. We define a rate of pressure increase, which determines the time required to reach the fracture criterion, and a maximum pressure based on estimates of the reservoir size. Once fractures open, gas flows through the fractures until the maximum reservoir pressure is reached, after which the gas pressure is depleted quickly because the high gas pressure drives rapid gas flux through the fracture system. This results in gas venting at the seafloor and accumulation of hydrate in the fracture system. If the amplitude of pressure oscillation is near the vertical effective stress in Horizon A (~0.87 MPa) and the time for pressure increase is on the order of years, the gas pressure will meet the fracture criterion on a time scale of months to a few years. The high gas pressure is then depleted over a time scale of a few months. Thus we conclude that gas migration pathways at southern Hydrate Ridge may evolve on a time scale of months to years. This provides important constraints on the time scale of transient effects on the methane hydrate system at southern Hydrate Ridge, and illustrates how pore pressure pulses affect fluid flow and fracturing behavior in active methane hydrate provinces.
We simulate 1‐D, steady, advective flow through a layered porous medium to investigate how capillary controls on solubility including the Gibbs‐Thomson effect in fine‐grained sediments affect methane hydrate distribution in marine sediments. We compute the increase in pore fluid pressure that results from hydrate occluding the pore space and allow fractures to form if the pore fluid pressure exceeds a fracture criterion. We apply this model to Hydrate Ridge and northern Cascadia, two field sites where hydrates have been observed preferentially filling cm‐scale, coarse‐grained layers. We find that at Hydrate Ridge, hydrate forms in the coarse‐grained layers reaching saturation of 90%, creating fractures through intervening fine‐grained layers after 2000 years. At northern Cascadia, hydrate forms preferentially in the coarse‐grained layers but 2 × 105years are required to develop the observed hydrate saturations (∼20%–60%), suggesting that hydrate formation rates may be enhanced by an additional source of methane such as in situ methanogenesis. We develop expressions to determine the combinations of sediment physical properties and methane supply rates that will result in hydrate‐filled coarse‐grained layers separated by hydrate‐filled fine‐grained layers, the conditions necessary to fracture the fine‐grained layers, and the conditions that will lead to complete inhibition of hydrate formation as pore space is constricted. This work illustrates how sediment physical properties control hydrate distribution at the pore scale and how hydrate distribution affects fracturing behavior in marine sediments.
Constant-rate-of-strain consolidation experiments and grain-size analyses are used to characterize the flow and deformation behavior and grain-size distribution of vertically and horizontally oriented specimens from 0 to 100 meters below seafloor at Integrated Ocean Drilling Program Expedition 316 Sites C0004 and C0006–C0008. Interpreted in situ permeability was generally <10–14 m2, but values ranged from 4.6 × 10–14 m2 to 1.1 × 10–16 m2 and do not exhibit any definitive trends with depth or grain size. Compression indexes, defining stress-strain behavior during normal consolidation, ranged from 0.15 to 0.9. The overconsolidation ratio (OCR) of vertically oriented specimens, which relates the in situ effective stress to the hydrostatic effective vertical stress, decreased downhole, and most samples had an OCR >1. Grain-size characterization by settling analysis documented that these shallow sediments are dominated by silt and/or clay, with median grain sizes ranging from 0.003 to 0.026 mm, excluding one sand-rich specimen.
We develop a technique for extending nuclear magnetic resonance (NMR) permeability estimation to clay‐rich sediments. Our technique builds on the Schlumberger‐Doll Research (SDR) equation by using porosity, grain size, specific surface, and magnetic susceptibility data to yield more accurate permeability estimation in mudstones with large pore surface areas and complex mineralogies. Based on measurements of natural sediments as well as resedimented laboratory mixtures of silica, bentonite, and kaolinite powders, we find that our method predicts permeability values that match measured values over four orders of magnitude and among lithologies that vary widely in grain size, mineralogy, and surface area. Our results show that the relationship between NMR data and permeability is a function of mineralogy and grain geometry, and that permeability predictions in clay‐rich sediments can be improved with insights regarding the nature of the pore system made by the Kozeny theory. This technique extends the utility of NMR measurements beyond typical reservoir‐quality rocks to a wide range of lithologies.
We develop a model to describe development of permeability anisotropy and fabric in clay‐rich sediments due to clay grain reorientation during consolidation and shearing. The model considers porosity, grain aspect ratio, and average angle of grains with respect to the horizontal plane. To validate the model, we determined permeability anisotropy ratios (ratio of horizontal permeability to vertical permeability) of porous media composed of flat cylindrical particles by lattice‐Boltzmann simulations. Over representative ranges of grain aspect ratio (diameter/thickness = 1–20) and porosity (44%–82%) the simulation results match the predicted values well. We show that permeability anisotropy ratios up to ∼20 can be attained within highly sheared (shear strain >20), low‐porosity (<40%) materials with aspect ratios >20, and that the maximum anisotropy ratio attainable by grain rotation is limited by grain aspect ratio. We further show that the anisotropy ratio of mixtures of low aspect ratio and high aspect ratio particles, like silty clays, are low (<2). This occurs because the low aspect ratio particles reduce the difference in tortuosity in transverse directions. Our results demonstrate why larger permeability anisotropy ratios are possible only through diagenesis, layering, or development of aligned microcracks.
Episodic seafloor methane venting is associated with focused fluid flow through fracture systems at many sites worldwide. We investigate the relationship between hydraulic fracturing and transient gas pressures at southern Hydrate Ridge, offshore Oregon, USA. Two colocated seismic surveys, acquired 8 years apart, at Hydrate Ridge show seismic amplitude variations interpreted as migration of free gas in a permeable conduit, Horizon A, feeding an active methane hydrate province. The geophysical surveys also reveal transients in gas venting to the water column. We propose that episodic gas migration and pressure fluctuations in the reservoir underlying the regional hydrate stability zone (RHSZ) at southern Hydrate Ridge influence methane supply to the RHSZ and are linked with periodic fracturing and seafloor methane venting. We model the effect of pore pressure variations within the deep methane source on fracturing behavior with a 1D model that couples multiphase flow, hydrate accumulation, and pore pressure buildup. As the reservoir pressure increases, fractures open when the pore pressure exceeds the hydrostatic vertical effective stress. Gas then flows through the fractures and vents at the seafloor while hydrate precipitates in the fracture system. We show that active seafloor gas venting occurs for approximately 30 years, and that the available methane reservoir is exhausted 30 to 55 years after the onset of pressure buildup. This provides important constraints on the time scale of transient fluid flow at southern Hydrate Ridge, and illustrates how pore pressure pulses affect fluid flow and fracturing behavior in active methane hydrate provinces.
We simulate methane hydrate formation with multiphase flow and free gas within the regional hydrate stability zone (RHSZ). We find that hydrate distribution and fracture behavior are largely determined by the phase of the methane supply. We allow free gas to enter the RHSZ when porewater salinity increases to the value required for three‐phase equilibrium. Fractures nucleate when the excess pore pressure exceeds the vertical hydrostatic effective stress. At Hydrate Ridge, where methane supply is dominantly free gas, hydrate saturation increases upwards and fractures nucleate high within the RHSZ, eventually allowing gas to vent to the seafloor. At Blake Ridge, where methane supply is dominantly in the dissolved phase, hydrate saturation is greatest at the base of the RHSZ; fractures nucleate here and in some cases could propagate through the RHSZ, allowing methane‐charged water to vent to the seafloor.
Fracture‐hosted methane hydrate deposits exist at many sites worldwide. These sites often have hydrate present as vein and fracture fill, as well as disseminated through the pore space. We estimate that thousands to millions of years are required to form fracture systems by hydraulic fracturing driven by occlusion of the pore system by hydrate. This time scale is a function of rates of fluid flow and permeability loss. Low‐permeability layers in a sedimentary column can reduce this time if the permeability contrast with respect to the surrounding sediments is of order 10 or greater. Additionally, we find that tensile fracturing produced by hydrate heave around hydrate lenses is a viable fracture mechanism over all but the lowermost part of the hydrate stability zone. With our coupled fluid flow‐hydrate formation model we assess fracture formation at four well‐studied hydrate provinces: Blake Ridge offshore South Carolina, Hydrate Ridge offshore Oregon, Keathley Canyon Block 151 offshore Louisiana, and the Krishna‐Godavari Basin offshore India. We conclude that hydraulic fracturing due to pore pressure buildup is reasonable only at Hydrate Ridge and the Krishna‐Godavari Basin owing to sediment age constraints, and that hydrate‐filled fractures observed at Blake Ridge and Keathley Canyon Block 151 are formed either by hydrate heave or in preexisting fractures. Our findings offer new insight into the processes and time scales associated with fracture‐hosted hydrate deposits, which help further our understanding of hydrate systems.
We combine nuclear magnetic resonance (NMR) transverse relaxation time data and gamma ray data to estimate lithology-dependent permeability in silt- and clay-rich sediments. This approach extends the utility of the Schlumberger-Doll Research (SDR) permeability equation from reservoirs to aquicludes and seals, and thus improves the value and robustness of NMR data. Data from Keathley Canyon, northern Gulf of Mexico show that NMR data can be used to define permeability from 10−18 to 10−14 m2 (0.001–10 millidarcies) as calibrated and tested by direct measurements on core samples. We performed uniaxial, constant rate-of-strain consolidation experiments on sediments from Keathley Canyon to determine core-scale permeability. Permeabilities from these experiments were compared to permeabilities calculated from logging-while-drilling data. A better fit between log-derived permeability and laboratory-measured permeability was obtained using the SDR equation with a variable coefficient A, rather than a constant A as is typically used. We show how A is a function of lithology and can be modeled from gamma ray data. The relationship between A and gamma ray values suggests that variations in A are caused by platy clay minerals and the effect they have on the pore system. Our results provide improved means for permeability estimation for application in basin flow modeling, hydrocarbon migration modeling, and well completion design.