Author: K. Guru Rajesh , C. Mahapatra, Ch. B.M. Mishra, B.N. Pradhan and S.K. Das

Abstract - There was good exploratory success confined to 1200m bathymetry which prompted to embark on more aggressive high risk high reward effort in the ultra-deep water beyond 2000m bathymetry.  Q marine seismic data was acquired in in three campaigns Study area is located nearly 140 Km off South Yanam where bathymetry varies from 2000-3000m during Exploration of biogenic methane has attracted worldwide attention in the light of its ubiquitous occurrence in soils at shallow depths and also due to the low cost of drilling for its exploitation. The on-land part of Mahanadi basin, covered by the Quaternary Mahanadi delta, has evidenced surface gas seepages at several locations which turned out to be biogenic in origin as indicated by the compositional and carbon isotope analysis. A unique attempt has been made to utilize the geological, hydrogeochemical and electrolog in formations of the tube-wells drilled for groundwater by Central Ground Water Board to study the occurrence, depth and aerial distribution of the potential reservoir zones charged with biogenic gas, which led to some important observations and leads. The empirical study of electrologs suggested that normal resistivity values great than 60 ohm.m might indicate presence of biogenic gas in the reservoir zones. Using this criterion, 6 tube-wells have been identified as having positive indication from amongst the 13 considered. The study also further identified that the high resistivity zones are confined to shallow depths between 30m and 200m below the ground level. The scattered nature of the gas seepage sites and tube-wells with positive indications suggests possible presence of localized pools of few square kilometres area in accordance with the reservoir facies of the fluvial depositional environment. The hydrogeochemical study of the groundwater indicated that they are characterized by low sulfate, high bicarbonate and low calcium ion concentrations, which are favourable for biogenic gas generation.

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