Magnetic Mineralogical Approach for the Exploration of Gas Hydrates in the Bay of Bengal

Abstract

We evaluate the environmental magnetic, geochemical, and sedimentological records from three sediment cores from potential methane-hydrate bearing sites to unravel linkages between sedimentation, shale tectonics, magnetite enrichment, diagenesis, and gas hydrate formation in the Krishna-Godavari basin. Based on downcore rock magnetic variations, four sedimentary magnetic property zones (I-IV) are demarcated. A uniform band of enhanced magnetic susceptibility (zone III) appears to reflect a period of high-sedimentation events in the Krishna-Godavari basin. Highly pressurized sedimentary strata developed as a result of increased sedimentation that triggered the development of a fault system that provided conduits for upward methane migration to enter the gas hydrate stability zone, leading to the formation of gas hydrate deposits that potentially seal the fault system. Magnetic susceptibility fluctuations and the presence of iron sulfides in a magnetically enhanced zone suggest that fault system growth facilitated episodic methane venting from deeper sources that led to multiple methane seepage events. Pyrite formation along sediment fractures resulted in diagenetic depletion of magnetic signals and potentially indicates paleo sulfate-methane transition zone positions. We demonstrate that a close correlation between magnetic susceptibility and chromium reducible sulfur concentration can be used as a proxy to constrain paleomethane seepage events. Our findings suggest that the interplay between higher sedimentation events and shale tectonism facilitated fluid/gas migration and trapping and the development of the gas hydrate system in the Krishna-Godavari basin. The proposed magnetic mineralogical approach has wider scope to constrain the understanding of gas hydrate systems in marine sediments.

Magnetic Mineral Diagenesis in a Newly Discovered Active Cold Seep Site in the Bay of Bengal

“…Trapping of H 2 S within the hydrate deposits hindered the pyritization processes (Housen and Musgrave, 1996), and lead to the preservation of detrital magnetite particles in MZ-3 (Riedinger et al, 2005) (Figures 2A, 4I, 5E, 6K,L, 7N,O). Rapid burial of magnetic particles is another important factor which can explain the existence of high χ lf in gas hydrate bearing sediments (Badesab et al, 2019). We believe that both these processes hindered the diagenetic reactions and favored high χ lf values in the gas hydrate bearing sediments at site Stn-9.…”

“…A systematic down-core increase in χ lf and SIRM (from 7 to 17 cmbsf) manifested by the presence of magnetic particles below the proposed present-day SMTZ (3-5 cmbsf) indicate the period of weak AOM activity and minimum dissolution of the magnetic minerals (Figures 2A,C, 6B, 7A). Rapid burial and preservation of the detrital magnetic particles due to increased sedimentation specifically during this period is another possibility (Badesab et al, 2019). A marked depletion in χ lf between 25-29 cmbsf and 43-52 cmbsf reflects the periods of increased magnetite dissolution and pyrite formation linked with at least two intense methane seepage events at the studied site ( Figures 2A, 6C,D, 7C,E).…”

“…A good correlation between χ lf and SIRM is observed in all the samples, but exhibits differences in their slope suggesting the presence of ferri-and paramagnetic mineral assemblages in the samples ( Figure 10A). A cross plot of SIRM/χ lf vs. χ lf has been successfully utilized as a proxy for characterizing the magnetic mineral assemblages in the gas hydrate bearing sediments (Larrasoaña et al, 2007;Kars and Kodama, 2015;Badesab et al, 2019). We broadly classify the samples into three subgroups ( Figure 10B).…”

“…All data falls on a 1:1 line. Samples exhibiting low values δ ZFC and δ FC indicate the presence of diagenetically formed ferrimagnetic iron sulfides, while higher values of δ ZFC and δ FC indicates the presence of mixed-type (magnetite, pyrite) magnetic mineral assemblages ( Figure 10D; Kars and Kodama, 2015;Badesab et al, 2019). Variations in magnetic mineralogies (titanomagnetite, pyrite, greigite) and concentration governed the down-core changes in the magnetic properties.…”

“…High methane flux and favourable P-T conditions supported the accumulation of gas hydrates (Mazumdar et al, 2019). A recent rock magnetic study by Badesab et al (2019) on the long drilled sediment cores that overlie the deep-seated gas hydrate deposit provided the longest and most detailed sediment magnetic record in the K-G basin, Bay of Bengal. They successfully delineated the control of higher sedimentation events and shale tectonics on the development of fracturefilled gas hydrate deposit and magnetic mineral diagenesis in K-G basin.…”