GUIDE TO GAS HYDRATE STABILITY IN VARIOUS GEOLOGICAL SETTINGS

The presence of clathrates (gas hydrates) is well established observat ionally in the offshore region of the South Caspian Basin, as well as in many other regions of the world. Hydrates of fixed composition can exist under particular pressure-temperature conditions. However, under the impact of neotectonic processes, those conditions can change. Par t, or all, of the mass of hydrates can then be dissociated and release d as gas and water. The dissociation can take place gradually, or expl osively, depending on how fast the pressure drops or temperature incre ases. Five major variations of hydrate evolution are considered to ill ustrate possible patterns of behavior caused by variation of geologica l conditions: (1) Variations in hydrate existence conditions due to se diment and mud redeposition: earthquakes, volcanic eruptions, and othe r processes, can release part of the sediments from a sloping sea bott om and transport them to be redeposited in other places, thereby chang ing pressure conditions at the top of a hydrate layer. (2) Variations in hydrate existence conditions due to glacial-interglacial conditions : removal of ice cover not only decreases overlying pressure, but als o allows water temperatures overlying the sediments to increase. (3) V ariations in hydrates due to sea-level rise and fall. (4) Enrichment o f ethane in hydrates as a consequence of varying neotectonic condition s. (5) Evaporation and reformation of hydrates in aeolian conditions d ue to winter cooling and summer heating. Hydrates can be a potential e xplosive hazard for exploration rigs, production platforms and pipelin es, especially in deep water conditions. These geological patterns of instability should therefore be considered when potential hazards are assessed. (C) 1998 Elsevier Science Ltd. All rights reserved.