Hydrates Related Traps

Generating a large amount of gas by biogenic means is problematic; there simply is not that much labile organic carbon deposited and preserved for anaerobic consumption in most localities (Paull et al. 1994, Waseda 1998). However, those in search of commercial, hydrate-related hydrocarbon accumulations will probably consider thermogenic sources more rewarding.

Thermogenic and mixed thermogenic/biogenic gas hydrate plays will be associated with gas migration pathways, and therefore will be localised along permeable layers, faults or fractures, or above spillpoints in deeper (conventional reservoirs). Such focused flow will not give rise to a continuous BSR unless the gas should spread out for some reason (pervasive fracturing or rather uniform sediment properties). Rather the gas would be expected to pool and reach high saturations in coarser layers (sands) because the formation of hydrate at and immediately above the depth of three phase stability blocks the pore space in the updip direction. This hydrate-capillary seal concept is at least three decades old (Hunt 1997), but as yet is unproven commerically. However, as exploration and production frontiers advance into deep and superdeep waters (1000- 2000 m), there is a good chance that this situation will change (Grauls et al. 1999). To evaluate such prospects more work is needed to understand how hydrate grows within sediments, and the capillary seal capacity that can be produced by partial hydrate cementation.

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