Blake Bahama Ridge

Drilling results in the Blake Bahama Ridge have given promise for recovery of energy from hydrate reserves. Hydrate recovery results from ODP Leg 164 in the Blake Bahama Ridge seem to confirm the large resource estimation (Pauli et al., 1997, 2000 Lorenson and Collett, 2000). 

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. 

Figure 7.6 Samples from ODP Leg 164 Hole 994 at Blake Bahama Ridge, showing the sub-bottom reduction in sulfate until the depth of the Sulfate-Methane Interface (SMI), and the increase in methane concentrations below the SMI. (Pauli, Personal Communication, October 25, 2001.)...

As one of many examples of the relative sulfate and methane concentrations with depth, Pauli et al. (1996) measured samples (Figure 7.6) from the Blake-Bahama Ridge ODP Leg 164 hole 994. 

Layered hydrates are separated by thin layers of sediments, such as cores recovered from the Blake-Bahama Ridge. Such hydrates probably occur both offshore and in permafrost regions. 

Figure 7.12a shows the most famous BSR related to hydrates, in the Blake-Bahama Ridge. Sediments associated with this reflector have been drilled on two ODP voyages, the more recent (Leg 164) in December 1995. Figure 7.12b illustrates velocity of the wave in the sediment (at the arrow in 7.12a) as a function of depth. Table 7.8 shows the areal extent of many of the BSR s cited in the hydrate locations of Table 7.4. 

FIGURE 7.12 (a) Bottom simulating reflector for hydrate deposit in Blake-Bahama Ridge,... 

Case Study 1 Leg 164 in the Blake-Bahama Ridge (Hydrate Assessment)... 

Figure 7.21 b Bottom simulting reflector for the Three Leg 164 Holes at Blake-Bahama Ridge. (Pauli C.K., et al., Leg 164 Scientific Party, in Gas Hydrates—Relevance to World Margin Stability and Climatic Change, The Geological Society, London, Special Publication, 1998. With permission.)... 

FIGURE 7.22a Chloride anomalies and acoustic and resistivity logs for Blake-Bahama Ridge site 994. (Paul et al., 1996. With permission.)... 

Site 997 was drilled at the crest of the Blake-Bahama Ridge (where the strongest BSR occurs) at 450 mbsf. One large solid piece of gas hydrate was recovered from approximately 331 mbsf at a suspected small fault plane. However, the presence of more disseminated hydrates was inferred over a zone from approximately 180 to 450 mbsf. It was indicated that gas hydrate development may be extensive at this location, possibly acting as a means of sealing with permeability and porosity reduction. 

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. 

COLOR FIGURE 7.20 Seafloor slump in the Blake-Bahama Ridge shown in both seismic (top) and cartoon (bottom) relief. (From Dillon, W.P., Nealon, J.W., Taylor, M.H., Lee, M.W., Drury, R.M., Anton, C.H., Natural Gas Hydrates Occurrence, Distribution, and Detection, (Pauli, C.K., Dillon, W.P., eds.) American Geophysical Union Monograph, 124, p. 41, Washington DC (2001). With permission.) Note the bottom simulating reflector parallel to the ocean bottom, except in the middle section where it appears a seafloor eruption has occurred. 

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