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    Shallow gas charged sediments off the Indian west coast: Genesis and distribution

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    Mar_Geol_267_71a.pdf (699.2Kb)
    Date
    2009
    Author
    Mazumdar, A.
    Peketi, A.
    Dewangan, P.
    Badesab, F.K.
    Ramprasad, T
    Ramana, M.V.
    Patil, D.J.
    Dayal, A.M.
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    Abstract
    Geophysical and geochemical surveys were carried out off Goa, central west coast of India, to understand the genesis and distribution of shallow gases in marine sediments. Shallow gas charged sediments within the water depths of approx. 15 to 40 m are reported all along the west coast and are characterized by gas masking, high amplitude and reverse polarity reflections in high resolution seismic (sparker) data, attributed to the presence of free gas. These high amplitude reflections (gas front) are observed within a Holocene fine grained, wedge shaped sediment package overlying the maximum flooding surface. The gas front lies between 1.2 and 5 m below the seabed and demarcates the transition from gas in the dissolved phase to bubble phase when the gas concentration exceeds the saturation level. The observed and extrapolated sulfate methane transition zone (SMTZ) lies between 0.7 and 2.25 m below the seabed and it is related to the depth of the gas front. Methane concentration reaches saturation below the SMTZ, and produces a bubble phase which lowers the p-wave velocity and produces high amplitude reflection observed in the seismic data. Depleted carbon isotope ratios as low as -88.9 ppt suggest a microbial origin for the methane in the study area. High sulfate reduction rate, high concentrations of pore water methane, phosphate and ammonium in the shallowest site (Sasu123/1 at 14.7 m water depth) suggest an enhanced availability of reactive organic matter required for microbially mediated biodegradation processes. High gas flux in the shallow waters is also indicated by the shallowing of the gas front and the widening of the gas masked zone towards shore. The present investigations do not testify the origin of Holocene shallow gases either from buried channel deposits or the Pleistocene sediments
    URI
    http://drs.nio.org/drs/handle/2264/3423
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