FROZEN HEAT | Volume 2

2.5.2 GRAIN-DISPLACING, FRACTURE- DOMINATED OCCURRENCES IN MUDDY SEDIMENTS Gas hydrates in the form of grain-displacing veins and nod- ules in fine-grained sediments have been reported from cor- ing programs offshore Japan (Fujii et al. 2009) and Malaysia (Hadley et al. 2008) and are likely very common worldwide. Perhaps the best-studied sites are the particularly thick and rich occurrences discovered offshore India in 2006 (Collett et al. 2006) and offshore Korea in 2007 (Park et al. 2008). Drilling at Site 10 in the Krishna-Godovari Basin, a part of In- dia’s 2006 NGHP Expedition 01, showed gas hydrates occur- ring as a pervasive network of fracture-filling veins and lenses in mud-rich sediments (Figure 2.10). The 150-metre-thick unit lay below roughly 20 metres of gas-hydrate-free mud-rich sed- iments and had no clear sea-floor expression. Core samples revealed the fossilized remains of an earlier sea-floor chem- osynthetic community at the top of the gas hydrate deposit (Mazumdar et al. 2009), suggesting that a relatively recent sea-floor slump had buried a once-active cold seep, promoting the accumulation of sub-sea-floor gas hydrates at the site. The gas hydrates are not evident in standard analyses of geophysi- cal data, but advanced techniques have delineated a 1.5-square- kilometre area inferred to represent the zone of increased gas hydrate occurrence (Riedel et al. 2010a, b). The site also provided an opportunity to cross-calibrate core-based and log- based analyses, enabling scientists to refine significantly the models used to estimate gas hydrate saturation from log data in fracture-dominated systems (Lee and Collett 2009; Cook et al. 2010). Core data confirmed that gas hydrate concentrations are about 25 per cent of the pore space, on average, throughout the gas hydrate deposit. Prior to the drilling at Site 10, it was widely believed that gas hydrates could not accumulate to val- ues much in excess of 10 to 15 per cent in muddy sediments, and that whatever gas hydrates occurred in such settings would generally be dispersed within the sediment pore space. The surprising findings at Site 10, therefore, fundamentally changed the view of fine-grained gas hydrate systems. Confirmation of the potential global abundance of rich gas- hydrate occurrences as fracture-fill in muddy sediments was obtained in the Ulleung Basin, offshore Korea, in 2007 (Ex-

pedition UBGH1). Among other targets, this program tested several chimney structures (Figure 2.11), anomalous verti- cal features of reduced seismic amplitude that are observed worldwide in areas of significant gas seepage (Riedel et al. 2002; Wood et al. 2000; Westbrook et al. 2008). UBGH1 provided both well-log and core data through two chimneys (Park et al. 2008), confirming significant fracture-filling gas-hydrate occurrence. A second expedition (UBGH2), con- ducted in 2010, tested several more chimney structures with similar results. Abundant chimney structures, perhaps more than 1 000, have been identified in the Ulleung Basin alone (Horozal et al. 2009; Kang et al. 2011), and it now appears likely that virtually all these structures represent significant occurrences of grain-displacing gas hydrates. Preliminary analyses of logging-while-drilling and core data show that concentrations are quite variable, but likely similar to those seen offshore India (about 25 per cent of pore space). While fracture-filling gas hydrate deposits probably represent significant global in-place resources, no promising produc- tion strategies have yet been proposed. Challenges include the production difficulties (many of which are related to the geomechanical stability of the formation and of the wellbore assembly) and the potential environmental impact associated with extraction from such shallow, highly unconsolidated, and low-permeability sediments. 2.5.3 PORE-FILLING GAS HYDRATES IN MUDDY SEDIMENTS Perhaps the bulk of global gas hydrate in-place resources occurs in low concentrations, dispersed within the pores and grains of clay-rich sediments. Such accumulations exist broadly across the globe, their presence commonly betrayed by conspicuous geophysical responses such as bottom-simulating reflectors (BSRs) and blanking zones (Tucholke et al. 1977; Text Box 2.3). The investigation of such features and their potential links to gas hydrates turned the attention of the first dedicated gas-hydrate scientific field program (IODP Leg 164) to the Blake Ridge, off- shore eastern North America, in 1995 (Paull et al. 1996). At the Blake Ridge, drilling confirmed the widespread occur- rence of gas hydrates throughout a thick (approximately 200 metres) and very fine-grained sediment section. The concen-

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