FROZEN HEAT | Volume 2

Figure 2.10: Gas hydrates in clay-rich sediments. Gas hydrates have been discovered in rich deposits characterized by dense arrays of nodules and hydrate-filled fractures in clay-rich sediments. Left: CT image of a core samples collected offshore India in 2006. The hydrate-filled veins appear as white (from a paper by Rees et al. 2011, published by JMPG, permission would be needed). Above pictures, gas hydrate samples (courtesy NGHP Expedition-01 science party).

tration of gas hydrates was generally low – typically 5 per cent or less of pore space, with local increases up to 10 per cent or more correlated with fine, vertical-scale increases in sediment grain size (Ginsburg et al. 2000). Although the resource con- centration is low, the large area and significant thickness of the gas hydrate occurrence at the Blake Ridge results in very large cumulative in-place resources of gas (Dickens et al. 1997), po- tentially exceeding 28 trillion cubic metres. A recent gas hydrate drilling expedition within the Shenhu region of the South China Sea (Expedition GMGS-1) provid- ed additional insight into gas hydrate occurrences in mud- rich systems (Yang et al. 2008; Figure 2.11). Analysis of log data suggested potential gas hydrates at the base of the gas hydrate stability zone (GHSZ) at five of eight sites drilled.

Cores acquired at these five locations confirmed significant gas hydrates near the base of the GHSZ at three locations. At each well, degassing of pressurized core samples confirmed gas hydrate at saturations routinely of 20 per cent, with local increases to more than 40 per cent in thin zones (Wu et al. 2010). Notably, analysis of X-ray radiographs from Shenhu showed the gas hydrates were primarily in disseminated, pore-filling mode. Only minor macroscopic lenses, nodules, or fracture-fills typically seen in rich, fine-grained occurrences were observed. Such high saturations are unique, so far, for fine-grained gas-hydrate systems lacking macro-scale fractures and may reflect locally high concentrations of silt-sized parti- cles, in particular biologic fragments (foraminifera tests) that might enhance permeability above normally expected levels in predominantly fine-grained sediments (Wu et al. 2010).

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 49