Hydrate Ridge

Chapter 7 discusses in situ hydrates in the oceans and permafrost. Seven key concepts are presented for hydrates in nature. These concepts are illustrated in four field case studies for hydrate assessment (Blake Bahama Ridge, Hydrate Ridge) and production (Messoyakha and Mallik, 2002). 

In the above hydrate evidence, some locations are double-listed because they had more than one piece of evidence. It is interesting that 63 BSR locations provide the most evidence for hydrates—a factor of almost 3 larger than the 23 sample locations with the most irrefutable evidence. As will be demonstrated later in the Blake-Bahama Ridge and Hydrate Ridge Case Studies and Section 7.4.2, BSR evidence is not totally reliable, but provides a first approximation of hydrated subsurface depth and area extent. 

Such a summary overview for the Gulf of Mexico, also finds a clear example in the case study of Hydrate Ridge. While Roberts provided a qualitative summary of the three ranges of gas fluxes, his conceptual picture evolved from a career of field experiments, serves as a basis for quantification, such as in the following models. 

In the following southern Hydrate Ridge Leg 204 case study, Trehu et al. (2004) compared estimates of hydrate fractions by the three methods, relative to the hydrate fractions estimated by resistivity at bit (RAB) logs. All four results consistently indicate that, at depths greater than the crest of the ridge, the average gas hydrate content is generally <2% of the pore space. Such numbers are typical of oceanic hydrates. 

It can be argued that, if it is not possible to recover oceanic hydrates at high concentrations, it will be impossible to recover hydrates at lower concentrations in the ocean. Thus, success in the Gulf of Mexico, at Hydrate Ridge, or in a similar setting is vital to the energy recovery from more dispersed, deeper hydrates. 

Case Study 2 Hydrate Ridge (Hydrate Assessment)... 

Scripps Institute of Oceanography and others laid the groundwork for Leg 204 through hydrate work in Sites 889 and 892 on ODP Leg 146 (Kastner et al., 1995, ODP Leg 146 Scientific Party). Controlled source electromagnetics at Hydrate Ridge was done by a different Scripps group (Weitemeyer et al., 2006). 

The MBARI (Brewer, Pauli, Peltzer, Ussier, and colleagues) have pioneered subsea science using remote operated vehicles (ROVs)— particularly the use of spectroscopic equipment to quantify hydrate measurements, especially with Raman subsea deployment at southern Hydrate Ridge (Hester et al., 2005), to supplement the NMR spectroscopy with MBARI (Kleinberg et al., 2003) in Monterey Canyon waters. 

From the above studies has come a composite picture of hydrate formation at Hydrate Ridge, with particular emphasis on the southern summit. While a composite picture is attempted in the following few pages, the interested reader will turn to the above review articles for a detailed expansion. 

Hydrate Ridge is located 80 km west of Newport, Oregon on the second accretionary ridge of the Cascadia subduction of the Juan de Fuca Plate and the North American Plate. The northern summit is in 600 m of water depth with an area of... 

Figure 7.23 The location of the Leg 204 drilling sites on southern Hydrate Ridge. Note that sites 1249 and 1250 are at the summit. (From Trehu, A.M., Long, P.E., Torres, M.E., Bohrmann, G., Rack, F.R., Collett, T.S., Goldberg, D.S., etal., Earth Planet. Sci. Lett., 222, 845 (2004b). With permission from Elsevier.)...

At the southern Hydrate Ridge summit, hydrate is deposited in two regions (1) the first region with hydrates in 30—40% of the pore volume, extends from the surface to approximately 25 m below the seafloor and (2) a deeper region with much less hydrate (averaging less than 2 1% of pore volume) beginning approximately 25 mbsf to the BSR at a depth of approximately 115 mbsf. 

The biologically active surface (to 10 cm depth) has a methane flux that varies between 1 and 100 mmC/m2 per day. The hydrate results from free gas and gas dissolved in water. Two types of hydrate fabric result (1) porous hydrates, from accumulation of bubbles of free gas and (2) massive hydrates, with twice the density of porous hydrates (0.9 g/L versus 0.4 g/L). In the recent Raman spectroscopy, southern Hydrate Ridge experiments by the MBARI (Hester et al., 2005), the near-surface hydrate Raman specta contained significant amounts of free gas as well as hydrates, with only a trace of hydrogen sulfide in the methane gas. 

Another way of considering the hydrates at Hydrate Ridge relates to the SMI concept of Pauli, indicated in Section 7.2.2. At Hydrate Ridge, hydrates near the surface (up to a few meters) are related to rapid methane flux from below, perhaps with contributions from dissociated hydrates. While these hydrates relate to the SMI, as indicated in reaction (7.5), the methane comes from below, and the... 

Deeper hydrates at Southern Hydrate Ridge characterization and assessment... 

FIGURE 7.26 (See color insert following page 390.) Site 1245A southern Hydrate Ridge Flank logs (gammaray, density, Resistivity at Bit, and Archie water saturation). (T.S. Collett, Personal Communication, November 18, 2005, Leg 204, Scientific Party, 2005)... 

Sediment stratigraphy controls the hydrate distribution at Hydrate Ridge. The methane-rich migration pathway of Horizon A provides enriched hydrate formation relative to other sediments. In Figure 7.26, the remote sensing logs (gamma ray, density, RAB, and Archie water saturation) are most sensitive to hydrates. 

Well logs from Hydrate Ridge indicated acceptably consistent estimates of hydrate occurrence, particularly water saturation, and RAB. Hydrate concentration data from logging tools have been confirmed and quantified by more direct core methods of IR sensing of temperature, gas evolution, and chlorinity decrease. 

The cold seeps at Hydrate Ridge, cause this site to be labeled a sweet spot of high hydrate concentration, relative to others such as the Blake-Bahama Ridge (ODP Leg 164) example that may be more representative of hydrate deposits in the world. Trehu et al. (2004a) estimate 1.5-2 x 108 m3(STP) of methane for the summit deposit, on the basis of the drilling and seismic data, which also define the limits of the deposit. Trehu (Personal Communication, January 8, 2006) indicated that this amount is comparable to a small gas field, not economical unless facilities are already in place. 

New case studies summarizing Hydrate Ridge and Mallik 2002 test drillings... 

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