Title:Geological, geophysical, geochemical and microbial studies of the Indian continental margins to decipher gas hydrates

Scientific results:

I. Goa offshore: (Western continental margin of India)

1. The high resolution swath bathymetry map generated from the hydrosweep system reveals detailed topography of the Goa offshore where the depths to the seabed vary between ~2500 and >3200m. A positive ~NW-SE trending topographic feature rising to ~2400 m from the surrounding seafloor of 3000 in the central part of the study area is a noteworthy feature. Two basins like depressions of about 20 km wide are present on either side of this feature roughly align in the N-S direction with an easterly swing. The bathymetry rises to < 2400 m towards northeastern part of the study area.
   
   
2. The seafloor images obtained from the deep tow digital side scan sonar system are devoid of pockmarks or any other gas escaping features that might serve as indirect proxies of the gas hydrates. Similarly, the multifrequency chirp sonar and 3.5 KHz sub-bottom profiler data are also devoid of gas escape features in these water depths. However, at few locations, some micro features resembling the gas escape features such as pockmarks and blanking were observed on the chirp sonar records. Reprocessed multichannel seismic reflection data depicts the presence of BSRs at the subsurface layers in the study area.
   
   
3. Amplitude of magnetic anomalies range from -180 to +180 nT and the magnetic anomaly contour map depict NW-SE trending structural features.
   
   
4. The free-air gravity anomalies range between -56 to -28 mGals and the free air gravity anomaly contour map depict WNW-ESE, NW-SE, NNW-SSE trending deeper source structures in the basement.
   
   
5. Forward modeling of the potential field data suggests that the top of the crystalline basement lies about 3.0 to 5 km below the sea surface. Model studies also depict that the magnetic and gravity anomalies were generated from the basement that is comprised of the two distinct layers.
   
   
6. The analyses of ~5 m long sediment cores reveal that top 30-40 cm is characterised by moderate yellowish brown sediments, whereas the remaining length of the core exhibits greyish olive coloration.
   
   
7. The bulk sand fraction varies both in lateral as well as vertical direction between 4 and 13%. In general, the clays dominate and vary from 54 to 72% in the bottom sediments. Mostly, the observed sediments are of calcareous fraction. The average water content on the wet basis is 44.8%, which perhaps indicates high porous nature of sediments (porosity 55%). The average wet density is 57%. The sediments are of calcareous composition due to which cementation of pore space is possibly by calcareous ooze and mostly of authigenic origin. The porosity varies from 56.2 to 58.6% and <53.6-55.8% in the top and bottom sediments respectively. There is no appreciable change in the wet density either in the top or bottom sediments, and the values vary from 1.46 to 1.57 g/cc.
   
   
8. The vertical distribution pattern of Al shows a higher concentration (~5%) at the bottom of the sediments, which probably indicates high terrigenous input.
   
   
9. The redox sensitive element Mn distribution pattern shows a decrease trend (0.25-0.04%) from top to bottom of the core. This reduction trend may be attributed to the prevalence of reducing environment below the seabed. Whereas, Fe, another redox sensitive element shows only a marginal change both at the top and bottom of the core. The variation is 1.5- 2.2% in the top sediments and 1.6 to 3.2% in the bottom sediments.
   
   
10. The sulfate concentration varies from 28.4 mM at the top of the core to 13.3 mM at the core bottom. Out of 23 core locations, the sulfate shows a decreasing trend at 13 station locations, and less than 23 mM at bottom at 8 locations. At two station locations 7 and 8, the sulfate shows an upward trend in the bottom sediments. These two station locations are at a relatively shallow depth than rest of the locations. On extrapolation towards depth, it was noticed that the sulfate further reduces to minimum. This shows that in ideal conditions, the sulfate minima can be expected between 8 and 48 m below mean seafloor in the Goa offshore.
    
    
11. The Cl- depletion pattern is more or less in resonance with sulfate reduction at several locations. The surface value of Cl- measured is around 560 mM. The lowest concentration measured is 282 mM. Depletion trend in Cl- is detected at more than 10 locations (1,2,6, 10,11,12,14,16,20 and 24) indicting the probable influence of deep saline fluids by breakdown of halite layers on release of methane from the deeper regions (fluid expulsion or lithological variations).
    
    
12. The methane concentration detected in the Goa offshore was in order of nano moles. The methane concentration in most of the sediments depicts enrichment with depth. At 12 stations locations i.e., 3,5,6,10,11,12,13,17,20, 22,23,24 the enrichment trend in the bottom sediments is a significant observation, which probably implies the onset of methanogenesis in the bottom layers.
    
    
13. The methane enrichment, sulfate reduction and Cl- depletion appear to be related to the processes of sulfate methane interface (SMI) in the deeper layers. Most of the offshore areas studied world over for gas hydrates shows the sulfate reduction, methane enrichment and occasionally Cl-depletion in the marine sediments with depth. Since most probable BSRs are identified on the multichannel seismic reflection data in Goa offshore, west coast of India the observed methane sulfate and chloride behavior can be used as proxy to emphasize the presence of subsurface hydrates.
    
    
14. The TOC% varies between 0.01 and 0.7% in the top sediments, while at the intermediate depths it varies between 0.4 and >1.6%. Whereas in the bottom sediments it ranges from 0.3 to 0.8% with an average of 0.6%.
    
    
15. The percentage of CaCO₃ in the surface sediments is high. It varies from 59 to 68%. Though, high percentages of CaCO₃ were observed at 8 locations, the area in general is associated with decrease trend (<42-60%) of CaCO₃ in the bottom sediments with depth. Relatively high CaCO₃ of 69% was found in the bottom sample of the core at the location 8. The sampling location 8 is situated on an elevated topography. Presence of abundant appreciable quantity of CaCO₃ at the core bottom perhaps indicates the prevalence of favorable environment for primary productivity during late Pleistocene (average deposition rate of 5 cm / kilo year).
    
    
16. Microbial studies indicate that the sulfate reducing bacteria (SRB_R) are negligible at station locations 6,10,12,14,23 and 24. Whereas the Fermenters (SRB_F) varies between 0.07x10³ and 4.44 x 10³, and Nitrifiers between 2.8 x 10³ and 20.5 x 10³. High SRB_F count at the bottom.
    
    
17. A comparative study of the geochemical, microbial parameters such as SO₄^-2, Cl^-, CH₄, TOC, CaCO₃, SRB_R, SRB_F, Nitrifiers and BSRs suggests the following in bottom sediments.
    i) Increase TOC trend at station locations 1,2,5,7,9,11, 13,15,19 and 24
    ii) Increase CaCO₃ trend at locations 2,3,5-9, 19 and 23.
    iii) CH₄ enrichment at 3,5,6,10-14,17,20,22-24
    iv) Cl- depletion at 1,2,,6,10-12,14,20,23,24;
    v) SO₄^-2 reduction at 1,2,5,6,9-12,14,23,24; SRB_R minimum count at 2,6-18,19,21-24; SRB_F of appreciable quantity at 3,6,10,11,12,15,18,20,23,24
    vi) Nitrifiers in decreasing trend at stations 2,3,1,5,15,12,9,6,21,11,8,14,13,20,10,16,19,22,7,17,23,24,18
    
    
18. The SO₄^-2 and Cl^- show a decrease trend from core top to bottom at locations 6,10,11,12,14,23 and 24. The methane concentration shows enrichment trend with depth at the same locations.
    
    
19. The qualitative and quantitative analysis of 5 m long gravity core reveal that the station locations 6,10,11,12,14,23 and 24 are suitable either for carrying out further investigations or collection of long cores. The statistical treatment of the data show a gradation for qualifying station locations 11,12,14,6,10,24 and 23 which are associated with strong, geological, geochemical, microbial and geophysical proxies.
    

II. Results from KG Offshore (Eastern continental margin of India)

1. High-resolution swath bathymetry map depicts the macro and micro bathymetric features more conspicuously in KG offshore. The WNW-ESE trending positive topographic feature appears to be a prominent sedimentary ridge in the southwestern part of the study area. The crest of the ridge rises from the surrounding water depth of 1400m to about 900m. Width of this ridge is about 10km. Towards the northeastern part in the study area, a fault controlled fan like geomorphic province is present. The seabed topography in this fan like unit is gentle and the depth to the seabed varies from 1300 to 1600 m. Towards northwest, the bathymetry map shows another tongue like delta front up to 500m water depth. The magnetic and gravity anomaly contour maps suggest the presence of a deep basin like basement structure filled with more than 7 km thick sediments. The crustal structure of the lithosphere appears to be associated with NE-SW trending basement fabric.
   
   
2. Deep tow digital side scan sonar images depict the presence of clusters of pockmarks and other gas escaping features on the seabed. The multifrequency chirp sonar and 3.5 KHz sub-bottom profiler records depict the sub-bottom penetration of up to 40 - 60m below the seabed and several gas escape features unambiguously. Sometimes, the seabed is seen affected by gas up thrust and in other cases, the sub-bottom reflectors are seen affected by gas masking. Some blanking or disappearance of sub-bottom reflectors is a common observation. The chirp sonar records show the sub-bottom layers are affected by gas plumes.
   
   
3. Analysis of ~5m long gravity cores (~76 Nos) indicate that the silt fraction within the bottom sediments varies between <14 and >24%, while the clay fraction varies between 76 and 80 %. The distribution path classifies the KG offshore sediments as silty clays. Water content in the top and bottom sediments varies between < 45 and > 56 %. There is no appreciable change either in the porosity or wet density in top and bottom sediments. The specific gravity varies between 2.2 and 2.56 in top sediments. The specific surface area within the top sediments varies between 1.60 and 2.6 m²/gm whereas the specific surface area ranges from 1.60 to >2.4 m²/gm in the bottom sediments.
   
   
4. SO₄^-2 concentrations range from <4 mM at the bottom to 28.7 mM in the top sediments. The SO₄^-2 concentration in the bottom sediments shows a variation between <4 and 18 mm. Cl- profiles do not depict a complete depletion trend with depth within the 5 m long core, though the values are found to be relatively lower than the value at surface. The general value observed at the surface is around 550 mM, and most of the vertical profiles oscillate around 500 mM at the core bottom.
   
   
5. Methane concentration in the top sediments varies between 0 and 2.27 nM. At station location GC 7, the highest concentration of >158 nM was observed in the bottom sediments. The methane enrichment trend with the core depth was observed at 23 out of 76 locations.
   
   
6. Total Organic Carbon (TOC) concentration (wt %) in the middle level is relatively high (1.6%) when compared with the surface and bottom sediments. In the top sediments, it varies from 0.35 to 1.35, whereas it varies in the bottom sediments between 0.10 and 2.0. A high concentration of > 2.5 was observed at station location GC46.
   
   
7. In general, the carbonate content in the surface sediment is high. The CaCO₃ (wt %) content varies between 5.76 and 18.7 at the top, while it varies from 2.18 to 29.68 in the bottom sediments. Though a general decrease trend with core depth has been noticed at most locations, an increase trend around station GC 11 is a significant observation.
   
   
8. XRD and SEM studies indicate the presence of authigenic carbonates in subsurface layers. These carbonates might have formed in presence of the methane gas rising from the deeper layers. The geophysical investigations amply illustrated the presence of several gas escape features, which are indirect evidences for the occurrence of gas at deeper layers.
   
   
9. Nutrients such as ammonia, nitrate, nitrite and silicon dioxide are found to be at more than detectable levels in the KG offshore, which indicate the favorable conditions for the survival of microbes.
   
   
10. Mn content in the surface sediments is generally high around station locations 59, 50, 29, 30 and 31. The Mn concentrations are relatively low in bottom sediments. However, at station locations 21, 23, 24 and 25, it is high. The low Mn concentrations in bottom sediments perhaps suggest the prevalence of reducing environment. The Ce, another redox sensitive element, does not show much variation with depth.
    
    
11. Increasing Ba concentration from core top to bottom up to 498 ppm is observed at station locations 15, 19, 38, 40, 71, 72 and 73. Sediment cores at these locations show Ba enrichment of nearly 5 to 6 times compared to core top. These elevated Ba concentrations at the core bottom suggests a phenomenon related to Barium Front. Sulfate reduction zone is formed by the anaerobic oxidation, and generally methane concentration increases below the sulfate reduction zone. The observed 'Ba Front' may therefore serve as a precursor to predict the occurrence of methane below the sulfate reduction zone.
    
    
12. Though the methane concentrations measured are in nano moles, the enrichment trend with depth has been noticed at more than 25 locations, indicating about 33% of the total core locations are associated with methane enrichment with core depth. On the other hand, SO₄^-2 reduction trend with core depth was noticed at more than 48 out of 76 core locations. The Cl- profiles do not depict much variation with depth. However, relatively lower concentration of Cl- (~500 mM) is observed at several locations.
    
    
13. Most of the offshore areas studied world over for gas hydrates show sulfate reduction, methane enrichment and occasionally Cl- depletion in the marine sediments with depth. The methane enrichment, sulfate reduction and Cl- depletion appear to be related to the processes widely known as "Sulfate Methane Interface" (SMI). A similar sulfate reduction process has been observed in the KG offshore, and the extropolated SMI depth varies between 4 and 13 meters at several core locations. This relation needs to be further investigated by studying longer cores.
    
    
14. Abundance of nitrifiers in the surface sediments indicates the supply of nutrients and Oxygen. The presence of SRB_r, SRB_f and NRB in appreciable quantities at the bottom layers indicates the continuous supply of methane from the deeper layers.
    
    
15. Hydrographic parameters at the near bottom (close to seabed) indicate that the salinity is about 34.95 psu. The dissolved oxygen varies from 0.30 to 1.8 ml/l and the seabed temperature ranges from 5 to 10°C (deeper to shallow).
    
    
16. Though the Methane enrichment with depth in the KG Offshore is far less (i.e., nano moles, nM) compared to other areas where they are in milli and micromoles (µ M) levels, the occurrence of shallow SMI in the east coast is conducive to suggest the on going process of methane enrichment in the subsurface layers.
    
    
17. Synthesis of geophysical, geological, geochemical and microbial data results indicates that the KG offshore appears to be conducive for gas hydrate occurrence. Some station locations have been selected for further investigations based on the various criteria discussed earlier.
    
    
18. Several combinations of the inferred multidisciplinary proxies facilitated to evaluate the weightages of probable locations in KG Offshore for further focused studies including ground truth by drilling and high resolution seismic investigations. Further, for any given combination of the proxies, the probability of occurrence of gas hydrates in the KG offshore is more than 54 %. This inference is based on i) the vertical distribution of SO₄^-2, Cl-, CH₄, TOC, CaCO₃ and SRB, ii) inferred BSRs from multichannel seismic data, and iii) observed gas escape features on the deep tow digital side scan sonar images, chirp sonar and sub-bottom profiling data sets.
    
    
19. The ratios of lighter hydrocarbons, i.e., C₁/C₂ and C₁/(C₂+C₃) are found to be higher in the KG offshore, which probably suggest a mixed mode of origin of methane present in bottom sediments. However, these inferences require to be ascertained in light of stable isotopic studies.
    
    
20. Proxies inferred from multidisciplinary investigations facilitated in identification of station locations 7, 21, 11, 19, 30, 46, 32, 62, 70, 24, 16, 60, 66, 73, 36, 31, 22, 56, 55, 33, 57, 68, 63 and 39 as per the adopted criteria in order of merit for further evaluation.