Extensive occurrence and genesis of authigenic carbonates from Krishna-Godavari offshore basin (Bay of Bengal): Possible influence of methane hydrates occurrences.

Abstract

We report here the extensive occurrences of authigenic carbonate nodules/concretions from gas hydrate bearing sediments. Bulk mineralogical compositions by X-Ray diffractometry and EDS (Energy Dispersive Spectrum) analysis revealed that the carbonate nodules are composed dominantly of Siderites and High-Mg calcites (HMC) and occasionally Low-Mg calcites (LMC). Strontianite (SrCO3), witherite (BaCO3) and cerussite (PbCO3) are present as inclusions in nodules. The observations indicate that the carbon source for the formation of authigenic carbonates were gas hydrates below, and the carbonates were formed from the flow of methane-enriched fluids through fracture network formed because of shale diapirism. The detailed genetic framework is under investigation for comprehensive understanding of the process involved. The cores were collected as part of our gas hydrate exploration program from KG Basin, Bay of Bengal. One of the objectives of the Gas-hydrate research program at National Institute of Oceanography Goa is to establish the diagenetic finger prints/proxies in the sediments that are associated with the gas-hydrate formation and dissociation. Since occurrence of authigenic carbonates can help to decipher the source of gas seepages in an area we undertook a study of authigenic carbonates from the shallow sediments of KG offshore basin characterized by shallow gas hydrate deposits. In the present study we report on the extensive occurrence of methane-derived authigenic carbonate nodules/concretions in gas hydrate bearing sediments of K-G basin Eastern continental margin of India and discuss their origin based on their mineralogy, morphology, geochemical and stable isotopic compositions