Culture based investigations of key microbial functional groups in gas hydrate bearing sediments of the Krishna-Godavari Basin in offshore India

Abstract

Abstract Results of National Gas Hydrate Program-02 (NGHP-02) expedition have confirmed the resource potential of sand-hosted gas hydrate accumulations in the deep offshore of Krishna-Godavari Basin (KG Basin). NGHP-02 has identified two types of gas hydrate accumulations: one is thick, layer-type units developed in sand-rich facies at depths of a few 100\u202fm below sea floor (mbsf), and the other, fracture type unit of variable thickness at shallow depths. Fracture-type and Layer-type gas hydrates were observed in Areas B, C, and E in the KG Basin as defined under NGHP-02. The importance of each type of gas hydrate occurrence as discovered during NGHP-02 has led to the need for microbial profiling. Different classes of microbes, anaerobic fermentative microbes (AFM), methanotrophs and sulphate reducing bacteria (SRB) were studied to understand the microbial system for biogenic gas generation in sedimentary sequences encountered in eight core holes (4 in Area B, 3 in Area C and 1 in Area E) as established during NGHP-02. This study reveals the availability of highest concentration of methane generating microbes that are closely associated with high total organic carbon (TOC) content in the identified gas hydrate bearing sedimentary units cored in each borehole. The presence of microbial consortia for methane generation through both processes, of reduction of CO2 and fermentation of formate/acetate has been confirmed. Stratigraphic correlation between drill sites have also been generated to better understand the spatial relationship of various microbial processes. An observation from the study, is that the entire late Miocene age sediments has potential to generate biogenic gas in Holes NGHP-02-05B, −17B, −22B and −23B. Late to early Pleistocene age sediments are showing methane generating activity in Holes NGHP-02-01C, −08B and −09B, while early Pleistocene to late Pliocene age sediments contains methane generating microbes in Holes NGHP-02-05B, −17B, −19B, −22B and −23B. The existence of methane generating biogenic microbial system in Areas B, C and E of KG Basin has been confirmed.