Submarine Gas Hydrate Estimation: Theoretical and Empirical Approaches

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Abstract
The published submarine gas hydrate resource estimates are based on the concepts of their continuous extent over large areas and depth intervals and/or the regionally high hydrate concentrations in sediments. The observational data are in conflict with these concepts. Atpresent such estimates cannot be made to an accuracy etter than an order of magnitude. The amount of methane in shallow subbottom (seepage associated) gas-hydrate accumulations is estimated at 1014m3 STP, and in deep-seated hydrates at 1015 m2, according to observational data. From the genetic standpoint forthe time being gas hydrate potential could be only assessed as far less than 1017m3 because rates of related hydrogeological and geochemical processes have not been adequately studied. INTRODUCTION Natural hydrates frequently occur in outer continental margins and deep-water inland seas and lakes covering bout 10% of the total water area. However the estimates of their abundance that have been made are speculative. The published estimates derive from the concepts of their continuous extent over largeareas and depth intervals and/or of the regionally high hydrate concentrations in sediments. Meanwhile, the analysis of the world wide data suggests that submarine hydrates locally occur, largely in accumulations. The aims of this paper are to substantiate the local nature of the submarine gas hydrate distribution and to try to estimate their abundance from the observational data and the geological notions of their formation. 2.REVIEW OF PREVIOUS ESTIMATES Two recent papers3,4 have presented the overviews of submarine gas hydrate methane content. For brevity we shall consider only the published estimates which are based on the adequately justified underlying data. These estimates4,6 ranging within the limits (1.8-13.91) × 1016 m3 STP have resulted from themultiplication Q = S × h × K × Z × E and relies on three different concepts. One such concept4,6 assumes gas hydrates to occur within the HZ through the sediments relatively enriched with organic, matter (in the papers5,6, Corg > 1%). The range of the obtained values (2.0-13.9) × 10"6 m3 depends mainly on the adopted values of S (1 × 107 to 3.17 × 107km2v) and Z (5 to 10%). Another concept5 deals with seismic records. It was inferred from seismics that in the Beaufort Sea gas hydrates occupy about 75% of the band-like area of 7 × 105 km2 extending along the continental slope at water depths from 400 to 2800 m. They were supposed to completely fill pores of the lowermost 40 m of the HZ where porosity was 30%. As the length of the studied area is 20 times less than the total length of continental margins, Qg was calculated as 20-fold Qr and constituted 1.8 × 1016 m3 The third concept" starts from the fluid migration odel" in which hydrates are precipitated from fluids ascending from under the HZ. According to this model, S (2.3 × 107 km2) includes the area of accretionary prisms and regions of high Cenozoic sedimentation rate, and Z is assumed to be equal 50%.
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