Ann E. Cook

Electrical anisotropy due to gas hydrate-filled fractures

In 2006, the Indian National Gas Hydrate Program Expedition01,orNGHP-01,discoveredgashydrateasfillinnearvertical fractures in unconsolidated sediments at several drillingsitesontheIndiancontinentalmargins.Thesegashydrate-filled fractures were identified on logging-while-drilling resistivity images. The gas hydrate-filled fracture intervals coincide with high measured resistivity at the NGHP-01 sites. High measured resistivity translates into high hydrate saturationsviaArchie’sequation;however,thesehighsaturations contradict lower gas hydrate saturations determined from pressure core and chlorinity measurements.Also, in intervalswithnear-verticalgashydrate-filledfractures,thereis considerable separation between phase shift and attenuation resistivity logs, with 2-MHz resistivity measurements being significantly higher than 400-kHz resistivity measurements. We modeled the sensitivity of the propagation resistivity measurements in the gas hydrate-filled fracture intervals at NGHP-01 Sites 5 and 10. Near-vertical hydrate-filled fracturescancausetheabnormallyhighresistivitymeasurements in vertical holes due to electrical anisotropy. The model suggests the gas hydrate saturations in situ are usually significantlylowerthanthosecalculatedfromArchie’sequation.In addition, these modeled gas hydrate saturations generally agree with the lower gas hydrate saturations obtained from pressure core and chlorinity measurements at NGHP-01 Sites5and10.

Short‐range, overpressure‐driven methane migration in coarse‐grained gas hydrate reservoirs

Two methane migration mechanisms have been proposed for coarse-grained gas hydrate reservoirs: short-range diffusive gas migration and long-range advective fluid transport from depth. Herein we demonstrate that short-range fluid flow due to overpressure in marine sediments is a significant additional methane transport mechanism that allows hydrate to precipitate in large quantities in thick, coarse-grained hydrate reservoirs. Two-dimensional simulations demonstrate that this migration mechanism, short-range advective transport, can supply significant amounts of dissolved gas and is unencumbered by limitations of the other two end-member mechanisms. Here, short-range advective migration can increase the amount of methane delivered to sands as compared to the slow process of diffusion, yet it is not necessarily limited by effective porosity reduction as is typical of updip advection from a deep source.

The connection between natural gas hydrate and bottom‐simulating reflectors

Bottom-simulating reflectors (BSRs) on marine seismic data are commonly used to identify the presence of natural gas hydrate in marine sediments, although the exact relationship between gas hydrate and BSRs is undefined. To clarify this relationship we compile a data set of probable gas hydrate occurrence as appraised from well logs of 788 industry wells in the northern Gulf of Mexico. We combine the well log data set with a data set of BSR distribution in the same area identified from 3-D seismic data. We find that a BSR increases the chances of finding gas hydrate by 2.6 times as opposed to drilling outside a BSR and that the wells within a BSR also contain thicker and higher resistivity hydrate accumulations. Even so, over half of the wells drilled through BSRs have no detectable gas hydrate accumulations and gas hydrate occurrences and BSRs do not coincide in most cases.

Geophysical signatures for low porosity can mimic natural gas hydrate: An example from Alaminos Canyon, Gulf of Mexico

Natural gas hydrate in sand sediments can increase both the measured compressional velocity and resistivity. The same geophysical signatures occur, however, in low-porosity sand. We investigate the possible occurrence of natural gas hydrate in a sand interval in Alaminos Canyon Block 21 (AC 21) in the Gulf of Mexico, drilled by the U.S. Gas Hydrate Joint Industry Project. The sand interval has an increase in resistivity (~2.2 Ω m) and a strong peak and trough at the top and bottom of the sand on exploration seismic, which has been interpreted as natural gas hydrate. We reexamine the logging data and construct a new reservoir model that matches the measured resistivity, the high-density sublayers in the sand, and the surface seismic trace. Our model shows that the sand interval in AC 21 is most likely water saturated; and the slight increase in resistivity, higher-measured density, and the seismic amplitudes are caused by a reduction in porosity to ~30% in the sand interval relative to a porosity of ~42% in the surrounding marine muds. More broadly, we show that the average depth where the porosity of marine muds becomes lower than sand sediment is 900 mbsf, though it could be as shallow as 600 mbsf for high-porosity sands. In any case, the similar geophysical signatures for water-saturated sand and low saturations of natural gas hydrate in sand probably occur throughout the gas hydrate stability zone at most sites worldwide.

VELOCITY ANALYSIS OF LWD AND WIRELINE SONIC DATA IN HYDRATE-BEARING SEDIMENTS ON THE CASCADIA MARGIN

Downhole acoustic data were acquired in very low-velocity, hydrate-bearing formations at five sites drilled on the Cascadia Margin during the Integrated Ocean Drilling Program (IODP) Expedition 311. P-wave velocity in marine sediments typically increases with depth as porosity decreases because of compaction. In general, Vp increases from ~1.6 at the seafloor to ~2.0 km/s ~300 m below seafloor at these sites. Gas hydrate-bearing intervals appear as high-velocity anomalies over this trend because solid hydrates stiffen the sediment. Logging-while-drilling (LWD) sonic technology, however, is challenged to recover accurate P-wave velocity in shallow sediments where velocities are low and approach the fluid velocity. Low formation Vp make the analysis of LWD sonic data difficult because of the strong effects of leaky-P wave modes, which typically have high amplitudes and are dispersive. We examine the frequency dispersion of borehole leaky-P modes and establish a minimum depth (approx 50-100 m) below the seafloor at each site where Vp can be accurately estimated using LWD data. Below this depth, Vp estimates from LWD sonic data compare well with wireline sonic logs and VSP interval velocities in nearby holes, but differ in detail due to local heterogeneity. We derive hydrate saturation using published models and the best estimate of Vp at these sites and compare results with independent resistivity-derived saturations.

Archie's Saturation Exponent for Natural Gas Hydrate in Coarse‐Grained Reservoirs

Accurately quantifying the amount of naturally occurring gas hydrate in marine and permafrost environments is important for assessing its resource potential and understanding the role of gas hydrate in the global carbon cycle. Electrical resistivity well logs are often used to calculate gas hydrate saturations, Sh, using Archie’s equation. Archie’s equation, in turn, relies on an empirical saturation parameter, n. Though n = 1.9 has beenmeasured for ice-bearing sands and is widely used within the hydrate community, it is highly questionable if this n value is appropriate for hydrate-bearing sands. In this work, we calibrate n for hydrate-bearing sands from the Canadian permafrost gas hydrate research well, Mallik 5L-38, by establishing an independent downhole Sh profile based on compressional-wave velocity log data. Using the independently determined Sh profile and colocated electrical resistivity and bulk density logs, Archie’s saturation equation is solved for n, and uncertainty is tracked throughout the iterative process. In addition to the Mallik 5L-38 well, we also apply this method to two marine, coarse-grained reservoirs from the northern Gulf of Mexico Gas Hydrate Joint Industry Project: Walker Ridge 313-H and Green Canyon 955-H. All locations yield similar results, each suggesting n ≈ 2.5 ± 0.5. Thus, for the coarse-grained hydrate bearing (Sh > 0.4) of greatest interest as potential energy resources, we suggest that n = 2.5 ± 0.5 should be applied in Archie’s equation for either marine or permafrost gas hydrate settings if independent estimates of n are not available.

Linking basin‐scale and pore‐scale gas hydrate distribution patterns in diffusion‐dominated marine hydrate systems

The goal of this study is to computationally determine the potential distribution patterns of diffusion-driven methane hydrate accumulations in coarse-grained marine sediments. Diffusion of dissolved methane in marine gas hydrate systems has been proposed as a potential transport mechanism through which large concentrations of hydrate can preferentially accumulate in coarse-grained sediments over geologic time. Using one-dimensional compositional reservoir simulations, we examine hydrate distribution patterns at the scale of individual sand layers (1-20 m thick) that are deposited between microbially active fine-grained material buried through the gas hydrate stability zone (GHSZ). We then extrapolate to two-dimensional and basin-scale three-dimensional simulations, where we model dipping sands and multilayered systems. We find that properties of a sand layer including pore size distribution, layer thickness, dip, and proximity to other layers in multilayered systems all exert control on diffusive methane fluxes toward and within a sand, which in turn impact the distribution of hydrate throughout a sand unit. In all of these simulations, we incorporate data on physical properties and sand layer geometries from the Terrebonne Basin gas hydrate system in the Gulf of Mexico. We demonstrate that diffusion can generate high hydrate saturations (upward of 90%) at the edges of thin sands at shallow depths within the GHSZ, but that it is ineffective at producing high hydrate saturations throughout thick (greater than 10 m) sands buried deep within the GHSZ. Furthermore, we find that hydrate in fine-grained material can preserve high hydrate saturations in nearby thin sands with burial.

Cambrian (Guzhangian Stage) trilobites from Ohio, USA, and modification of the Cedaria Zone as used in Laurentia

Two Cambrian trilobites, Olenoides? sp. from the Mount Simon Sandstone and “Cedaria” woosteri from the Eau Claire Formation, are described from the subsurface of western Ohio, USA. The definition of the Cedaria Zone is modified to reflect differing interpretations of the zone as used historically in restricted-shelf and open-shelf lithofacies of the Laurentian palaeocontinent. The Cedaria prolifica Zone is proposed for use primarily in open-shelf lithofacies, and the “C.” woosteri Zone is proposed for use in more restricted, inner-shelf lithofacies. Olenoides? sp. from the Mount Simon Formation is interpreted as representing the C. prolifica Zone, and “C.” woosteri is the eponymous species of the “C.” woosteri Zone. Both polymerid trilobite biozones are in the Guzhangian Stage (Cambrian, provisional Series 3). Based on trilobite zonation, the Mount Simon Sandstone and most of the Eau Claire Formation of western Ohio are interpreted as being Guzhangian in age.

Crack extension induced by dissociation of fracture-hosted methane gas hydrate: CRACKING-METHANE HYDRATE DISSOCIATION

We investigate extension of a vertical crack filled by water and methane gas induced by dissociation of methane hydrate in low-permeability muddy sediment. We model the sediment as an impermeable elastic medium and consider a thin sheet configuration of hydrate that initially occupies a vertical fracture. The crack development and the amount of dissociated hydrate are determined using sediment elasticity, fracture mechanics, kinetics of hydrate dissociation, and the equation of state for methane gas. Youngs modulus and fracture toughness of the sediment greatly affect the mass of hydrate required for crack extension; lower Youngs modulus and higher fracture toughness require greater hydrate mass to induce unstable, buoyancy-driven crack propagation. Our numerical results suggest that propagation of gas/water-filled cracks occurs quickly in a matter of seconds and may be an important mechanism for methane transport in low-permeability sediments when hydrate-filled fractures are under conditions of gas hydrate dissociation.