R&D PROJECTS
Project: HYDRAMED
Gas Hydrate Stability and Prospectivity in the Mediterranean Sea
by Daniel Praeg , Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy (dpraeg@ogs.trieste.it); Vikram Unnithan, School of Engineering and Science, International University Bremen, Germany; and Angelo Camerlenghi, ICREA, c/o Universitat de Barcelona, Spain
Gas hydrates have been proven at one location in the Mediterranean Sea (in mud volcanoes of the Anaximander Mountains), but their wider occurrence remains unknown. The EC-funded HYDRAMED project was designed to examine the potential gas hydrate system in the Mediterranean Sea as a whole, over the timescales of glacial-interglacial change. The hydrate stability zone (HSZ) was modelled using gridded parameters for pressure (water depth), bottom water temperature and geothermal gradient, assuming a range of pore water salinites and gas compositions. For pure methane in seawater, the HSZ is present throughout the Mediterranean in depths >1 km (due to bottom waters of 12-14°C), but in thicknesses >200 m only in the east (which is geothermally cooler). Gas compositions including higher hydrocarbons yield a thicker and shallower HSZ; however, higher pore water salinites have an opposite and stronger effect, such that the HSZ is not present in the Mediterranean for salinites≥20%. Pore waters at deep-sea drill sites vary from .5% to >30% salinity above the Messinian salt, reflecting processes of diffusion and advection that represent a first-order control on Mediterranean hydrate stability. Hydrate stability has also been strongly influenced by changes in temperatures since the last glacial stage, when the effects of lowered sea level (-125 m) were outweighed by bottom waters estimated to have been at least 4°C cooler, corresponding to a HSZ up to 50% thicker and hundreds of metres shallower.
A dramatic reduction in hydrate stability is implied for the glacial to interglacial transition, with implications both for slope stability along basin margins and for the functioning of cold seep systems associated with hydrates (e.g. Anaximander Mountains).
Correlation of areas where the HSZ is thicker (>100 m) with areas of known or potential flux of gas to seabed (e.g. biogenic gas from Plio-Quaternary depocentres, thermogenic gas from accretionary prisms or areas beyond the Messinian salt seal) allows the identification of several areas of interest for possible hydrate occurrence. Within these areas, the modeled HSZ has been used to guide a search for indicators of hydrates in existing datasets, both geochemical (deep-sea drill sites) and geophysical (BSRs, using a regional multichannel seismic dataset held in archive by OGS). The most prospective areas lie in the eastern Mediterranean and include the Nile Fan and the inner basins of the Calabrian Arc accretionary prism. The Calabrian Arc was the target of an OGS acquisition campaign in summer 2005, in collaboration with the HERMES project, that resulted in the discovery of a new province of cold seep features and the acquisition of 2D and 3D seismic data that are being used to examine the relation of mud volcanoes to shallow fluid flow, including possible hydrates. Given the presence of the HSZ and the prevalence of deep-water seeps in the eastern Mediterranean Sea, it seems unlikely that known gas hydrates will remain restricted to the mud volcanoes of the Anaximander Mountains.