Indian National Gas Hydrate Program Research Articles

A late Miocene–Early Pliocene biogenic silica crash in the Andaman Sea and Bay of Bengal

Variations in the mass accumulation rate of biogenic silica (BSi) in continental margin sediments can be used to reconstruct relative changes in productivity through time in these settings. In the northern Bay of Bengal a lack of long sedimentary records has historically precluded this type of reconstruction. The acquisition of 21 new long sedimentary records during the 2006 Indian National Gas Hydrate Program (NGHP) Expedition-01 has made it possible for the first time to reconstruct paleoproductivity in this important region of the world that is dominated by intense changes in the geological and biological fluxes largely driven by tectonic and climate related mechanisms. In the research presented here, fluctuations in the mass accumulation rate of biogenic opal during the past ∼9.4 Myrs are reconstructed using continental margin sediment cores from the Andaman Sea (Site NGHP-01-17A) and the northern Bay of Bengal (Site NGHP-01-19). Within these records, a biogenic silica crash is recorded at ∼6 Ma and is consistent with previous geotectonic, geochemical and paleontological studies of the southern Indian Ocean and Pacific Ocean that suggest connectivity, and thus exchange of nutrient-rich water masses, between the eastern tropical Indian Ocean and western tropical Pacific Ocean was diminished as a result of the tectonic restriction of the northerly sector of the Indonesian Throughflow (ITF). The biogenic silica crash at Sites 17 and 19 is consistent with a decrease in surface water productivity that may have been driven by the reduction of nutrient-rich Pacific waters delivered to the Andaman Sea and Bay of Bengal via the northerly route of the ITF. Following the BSi crash at ∼6 Ma, subsequent recovery of the BSi mass accumulation rates at Sites 17 and 19 occurred and was perhaps renewed by an enhanced supply of nutrient-rich freshwater from the nearby Irrawaddy and Mahanadi Rivers, which could have occurred during a documented increase in the intensity of the Indian monsoon at ∼5 Ma. Although recovery is noted at both core locations, biogenic silica mass accumulation rates did not fully recover in the Andaman Sea. This could be explained by the restricted nature of the Andaman basin and its more distal location from a major source of nutrient-rich freshwater.

Sedimentation rates from calcareous nannofossil and planktonic foraminifera biostratigraphy in the Andaman Sea, northern Bay of Bengal, and eastern Arabian Sea

In this study we determined calcareous nannofossil and planktonic foraminifera events in sediments recovered during the 2006 Indian National Gas Hydrate Program (NGHP) Expedition-01. Selected bioevents permitted the assignment of orbitally calibrated ages, derivative sedimentation rate estimates, and identification of sedimentary discontinuities. In the Andaman Sea at Hole NGHP-01-17A, a late Miocene to recent record was recovered, but with a significant hiatus during the latest Miocene and Pliocene. Sedimentation rates here vary from 50 m/Myr during the Pleistocene to 130 m/Myr for the late Miocene. In the northern Bay of Bengal at Hole NGHP-01-19A the base of the cored record also reaches the late Miocene and extends to recent, but the same hiatus, missing the Pliocene, is present at this hole. a. The Early Pleistocene at Hole NGHP-1-19A shows sensible variation in sedimentation rates ranging between 130 m/Myr and 10 m/Myr during the last 0.5 Ma. For the late Miocene the sedimentation rate was ca. 50 m/Myr. In the western Bay of Bengal at Hole NGHP-01-10B/D, calcareous nannofossils were sparse and only foraminifera datums are available. The base of the section is latest Early Pliocene to Late Pliocene, and the majority of the record is considered younger than Late Pliocene. Due to the scarcity of calcareous nannofossils and the few foraminifera datums, sedimentation rates were not determined here. However, Hole NGHP-01-16A, in the same region, recorded the highest occurrence of Pseudoemiliania lacunosa, an event that confirms a Pleistocene age, allowing us to estimate a sedimentation rate between 110 m/Myr and 450 m/Myr. Along the western peninsular Indian margin at Hole NGHP-01-01A a continuous record from the Holocene to the early Oligocene indicates sedimentation rates of ca. 25 m/Myr until the middle Miocene, and a sensible reduction downward, reaching 4 m/Myr in the early Miocene to the early Oligocene. No hiatuses were observed. The large changes in sedimentation rates observed in these cores reflect long timescale changes in the environment likely induced by changes in the strength of the Indian monsoon system.

Compressional and shear-wave velocities from gas hydrate bearing sediments: Examples from the India and Cascadia margins as well as Arctic permafrost regions

Shear wave velocity data have been acquired at several marine gas hydrate drilling expeditions, including the India National Gas Hydrate Program Expedition 1 (NGHP-01), the Ocean Drilling Program (ODP) Leg 204, and Integrated Ocean Drilling Program (IODP) Expedition 311 (X311). In this study we use data from these marine drilling expeditions to develop an understanding of general grain-size control on the P- and S-wave properties of sediments. A clear difference in the downhole trends of P-wave (Vp) and S-wave (Vs) velocity and the Vp/Vs ratio from all three marine regions was observed: the northern Cascadia margin (IODP X311) shows the highest P-wave and S-wave velocity values overall and those from the India margin (Expedition NGHP-01) are the lowest. The southern Cascadia margin (ODP Leg 204) appears to have similar low P-wave and S-wave velocity values as seen off India. S-wave velocity values increase relative to the sites off India, but they are not as high as those seen on the northern Cascadia margin. Such regional differences can be explained by the amount of silt/sand (or lack thereof) occurring at these sites, with northern Cascadia being the region of the highest silt/sand occurrences. This grain-size control on P-wave and S-wave velocity and associated mineral composition differences is amplified when compared to the Arctic permafrost environments, where gas hydrate predominantly occurs in sand- and silt-dominated formations. Using a cross-plot of gamma ray values versus the Vp/Vs ratio, we compare the marine gas hydrate occurrences in these regions: offshore eastern India margin, offshore Cascadia margin, the Ignik-Sikumi site in Alaska, and the Mallik 5L-38 site in the Mackenzie Delta. The log-data from the Arctic permafrost regions show a strongly linear Vp–Vs relationship, similar to the previously defined empirical relationships by Greenberg and Castagna (1992). P- and S-wave velocity data from the India margin and ODP Leg 204 deviate strongly from these linear trends, whereas data from IODP X311 plot closer to the trend of the Arctic data sets and previously published relationships. Three new linear relationships for different grain size marine sediment hosts are suggested:a)mud-dominated (Mahanadi Basin, ODP Leg 204 & NGHP-01-17): Vs = 1.5854 × Vp − 2.1649b)silty-mud (KG Basin): Vs = 0.8105 × Vp − 1.0223c)silty-sand (IODP X311): Vs = 0.5316 × Vp − 0.4916We investigate the relationship of gas hydrate saturation determined from electrical resistivity on the Vp/Vs ratio and found that the sand-dominated Arctic hosts show a clearly decreasing trend of Vp/Vs ratio with gas hydrate saturation. Though limited due to lower overall GH saturations, a similar trend is seen for sites from IODP X311 and at the ash-dominated NGHP-01-17 sediment in the Andaman Sea. Gas hydrate that occurs predominantly in fractured clay hosts show a different trend where the Vp/Vs ratio is much higher than at sand-dominated sites and remains constant or increases slightly with increasing gas hydrate saturation. This trend may be the result of anisotropy in fracture-dominated systems, where P- and S-wave velocities appear higher and Archie-based saturations of gas hydrate are overestimated. Gas hydrate concentrations were also estimated in these three marine settings and at Arctic sites using an effective medium model, combining P- and S-wave velocities as equally weighted constraints on the calculation. The effective medium approach generally overestimates S-wave velocity in high-porosity, clay-dominated sediments, but can be accurately used in sand-rich formations.

Potential impact of gas hydrate and its dissociation on the strength of host sediment in the Krishna–Godavari Basin

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, conducted between April–August 2006, a number of pressure cores containing gas hydrate-bearing sediments were recovered under in-situ pressures. Two of these cores (NGHP-01-10B-08Y and NGHP-01-21C-02E), after initial characterization, were sub-sectioned. They were then rapidly depressurized and frozen in liquid nitrogen. A number of sub-sections were delivered to the University of Southampton for further analysis. High-resolution 3-D X-ray computer tomography carried out in the first instance revealed that the structure and form of the hydrate within the host sediment had been preserved. However, the exsolution of free gas from the pore water during depressurization, and before the subsequent freezing, had caused the formation of significant voids within each sub-sections, precluding detailed testing of intact cores.Following this initial appraisal detailed soil characterization, such as particle size analysis, water content and salinity, were carried out on sub-samples from the core sections following thawing and dissociation of the hydrate. In addition, specific gravity and strength tests, including liquid and plastic limit and undrained triaxial compression tests, were carried out on the combined sediment from all the core sections. The results of these tests showed the sediment to be a high plasticity clay. The remoulded combined sediment had a relatively high water content, and low shear strength, considering the vertical effective stress to which the in-situ sediment had been subjected, therefore creating significant under-consolidation of the in-situ sediment.In this paper a number of hypotheses are presented in an attempt to explain this under-consolidation, including factors, such as the change in moisture content due to hydrate dissociation, over-pressure, the influence of salinity on sediment strength, as well as the effect of hydrate veins. It is considered likely that hydrate veins, enhancing sediment strength, were the main contributing factor to the apparent under-consolidation. Hydrate dissociation induced by production of gas hydrates within this under-consolidated sediment may potentially lead to seafloor instability and further research is required to assess this likelihood.

Monsoon-influenced variation in productivity and lithogenic sediment flux since 110 ka in the offshore Mahanadi Basin, northern Bay of Bengal

The Indian monsoon drives seasonal changes in precipitation and weathering across India as well as circulation and productivity in the northern Indian Ocean. Variation in paleo-monsoon intensity and its effect on productivity and lithogenic fluxes is poorly constrained in the Bay of Bengal. In this paper, we present analysis of a sediment record from the offshore Mahanadi Basin recovered during the Indian National Gas Hydrate Program Expedition 01 (Site NGHP-01-19B). We reconstruct variation in biogenic and lithogenic components during the last 110 kyr using measurements of total organic carbon (TOC), total nitrogen (TN), TOC/TN, CaCO3, biogenic silica (BSi), δ13TOC, δ15TN, bulk mineralogy from X-ray diffraction, bulk and lithogenic grain size distribution, magnetic susceptibility, bulk density, and Ca, Br, and Zr/Rb from x-ray fluorescence (XRF). The mass-accumulation rate (MAR) of CaCO3, a function of marine productivity, drastically increased between 70 and 10 ka and is correlated to previously-documented elevated Asian dust fluxes and increased Bay of Bengal salinity during a weakened southwest monsoon. Decreased freshwater input over this period likely diminished stratification, allowing for increased mixing and nutrient availability, thus enhancing productivity despite weaker southwest monsoon winds. The MAR of lithogenic material is highest during the Holocene suggesting that sediment supply driven by monsoon intensity is a stronger control on margin sedimentation than sea level at the Mahanadi Basin. Over the entire record, magnetic susceptibility and XRF Zr/Rb are strongly correlated with CaCO3, suggesting higher primary mineral input under a weakened southwest monsoon. TOC/TN and δ13TOC also increase under glacial conditions, suggesting higher relative input of terrestrial C4 organic matter. These results highlight the Mahanadi Basin as a supply-dominated margin where terrigenous sedimentation is strongly influenced by monsoon intensity, and that productivity is limited by variation in monsoon-driven stratification on glacial-interglacial timescales rather than a direct response to monsoon winds.

Assessment of gas hydrate saturation in marine sediments from resistivity and compressional-wave velocity log measurements in the Mahanadi Basin, India

Gas hydrate was recovered in the Mahanadi Basin along the eastern continental margin of India during the India National Gas Hydrate Program (NGHP) Expedition-01 in 2006. Infrared imaging of recovered core confirmed gas hydrate occurs predominantly in discrete layers. Pore-water chemistry, electrical resistivity and acoustic velocity down-hole logs were used to estimate gas hydrate saturations at three of the sites in the Mahanadi Basin: Sites NGHP-01-08, -09, and -19. Gas hydrate saturation estimated from pore-water chloride concentrations shows values up to ∼10% of the pore space at ∼200 m below seafloor just above the base of the gas hydrate stability zone (BGHSZ). Gas hydrate saturations estimated from electrical resistivity and acoustic velocity logs using standard relations and modeling approaches are comparable to each other and saturations are ∼10–15% of the pore space. Seismic reflection data were also analyzed for the evidence of gas hydrate, and a bottom-simulating reflector (BSR) was imaged along the seismic profiles in the study area. The depth of the BSR is varying from ∼200 m to ∼300 m below seafloor depending on water depth in the Mahanadi Basin. The occurrence of gas hydrate was observed to be associated with deep-water channel and levee complexes (especially at Site NGHP-01-19) based on the regional seismic data. But the cored/logged section at each site lacked any significant sand fraction, which does not allow for higher gas hydrate saturations. As identified from seismic time-slice data, all sites drilled in the Mahanadi Basin are within the steeper slope region of the channel system and any sand bypassed this region. Significant sand deposition would occur further down slope where typical fan-type deposits can be inferred from the seismic data and thus higher accumulations of gas hydrate would be expected.

Assessment of gas hydrate stability zone and geothermal modeling of BSR in the Andaman Sea

Wide-spread bottom simulating reflectors (BSRs) are observed along available multichannel seismic profiles covering an area of about 290km2 in the Andaman Sea. The seismic data shows that the BSR occurs at places where water depth exceeds 1000m, and is identified by cross-cutting relationships with the dipping reflectors. The BSR that represents the base of gas hydrate stability field can be used to infer the gas hydrate stability thickness, which ranges between ∼518m to ∼861m depending on water depths. In situ measurement at site 17 during the Indian National Gas Hydrate Program (NGHP) Expedition-01 shows very low geothermal gradient 19±2°C/km. A conductive model was used to determine geothermal gradients from BSRs, which is calculated and varying between 10°C/km to 40°C/km. The low geothermal gradient is responsible for the deepest BSR or gas hydrate stability zone (GHSZ) in the Andaman region and in the world. The geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed BSR depths.

Gas hydrates saturation using geostatistical inversion in a fractured reservoir in the Krishna–Godavari basin, offshore eastern India

A reservoir of gas hydrates filling fractures was discovered in the Krishna–Godavari (KG) Basin during the Indian National Gas Hydrate Program (NGHP) Expedition 01 at site NGHP01-10. The existing methods for estimating gas hydrate saturation from seismic data rely on establishing an empirical relation between acoustic impedance and density porosity from well logs assuming isotropic pore-filling gas hydrate. This method, however, yields a misleading saturation for fractured clay-dominated sediments. Here we present a methodology to estimate gas hydrate saturation from seismic data based on geostatistical inversion. It integrates the verticals detail of well log data with the lateral details from seismic data to produce highly detailed estimates of gas hydrate saturation. First, gas hydrate saturation is calculated from P-wave velocities assuming anisotropic distribution at the well site. Then, probability density functions (PDFs) between acoustic impedance and the calculated gas hydrate saturation at the well site are analyzed. A Markov Chain Monte Carlo method is employed to integrate well logs with the seismic data to produce acoustic impedance. Finally, crossplots and histograms at the well site are used to estimate gas hydrate saturations along the seismic line from inverted acoustic impedance. The spatial distribution of gas hydrate varies both laterally and vertically along the line with an average saturation of 22.5%. The estimate matches reasonably with the value at the wells.

Gas hydrate and free gas saturations using rock physics modelling at site NGHP-01-05 and 07 in the Krishna–Godavari Basin, eastern Indian margin

Several approaches exist for the estimation of gas hydrate and free gas depending on the nature and distribution of gas hydrate into the sediments. Here, we apply various rock physics models to the P-wave sonic velocity logs that were collected during the Indian National Gas Hydrate Program (NGHP) Expedition 01 at sites NGHP-01-05 and NGHP-01-07 for the resource estimate of gas hydrate. Using the weighted equation, it is found that maximum up to13% and 12% volumes of the pores of regional sediments above bottom simulating reflector (BSR) of sites NGHP-01-05 and NGHP-01-07 respectively are occupied by gas hydrates. Alternatively, gas hydrate saturations computed for these sites using effective medium models are found maximum up to 16% and 22%. We have further used the velocity porosity transforms to estimate the free gas saturations below the BSRs for both the sites. The saturation of free gas in the sediments at both the sites is found in the range of 0.5–2.2%.

Heat flow and gas hydrate saturation estimates from Andaman Sea, India

Gas hydrate was recovered in the Andaman Sea along the eastern coast of the Andaman Islands during the India National Gas Hydrate Program (NGHP) Expedition-01 at Site NGHP-01-17. Coring confirmed gas hydrate occurs predominantly in discrete volcanic ash layers. Pore water chemistry, electrical resistivity and P-wave velocity logs are used to estimate gas hydrate saturations at Site NGHP-01-17. Gas hydrate saturation estimated from chloride concentrations shows values up to ∼85% of the pore space for distinct ash layers from ∼270 m below seafloor to the base of gas hydrate stability zone (BGHSZ). Gas hydrate saturations estimated from the electrical resistivity and acoustic velocity logs using standard empirical relations and modeling approaches are comparable to each other, but saturations are only ∼20% of the pore space on average. This much lower gas hydrate saturation estimate from the log data is a result of overall reduced resolution of the logging tools relative to the typically 20–30 cm thick hydrate-bearing ash layers. Available 2D multi-channel seismic data were also analyzed and a bottom-simulating reflector (BSR) was imaged along several seismic profiles. The depth of the BSR is more than 600 m along the seismic line crossing Site NGHP-01-17, which makes this one of the deepest BSRs observed worldwide. To understand the unusual depth of the BSR, we mapped its depth and estimated heat flow from the BSR depth using a simple conductive model. BSR-derived heat flow values range from ∼12 to ∼41.5 mW/m2 from the study area and follow the bathymetry trend of dominant North–South ridges and can be explained with the east-ward trending increase in heat-flow toward the current seafloor spreading center. We also modeled the BGHSZ to analyze the linkage between gas hydrate occurrences in the Andaman Sea and its relation to the tectonic activity. Our analysis suggests an extensively variable BGHSZ in the Andaman Sea controlled mainly by overall low geothermal gradients. Consistent local minor variations were observed with lower heat flow values over prominent topographic highs and higher values in valleys/troughs due to focusing and defocusing effects of the topography.

Geothermal modeling for the base of gas hydrate stability zone and saturation of gas hydrate in the Krishna-Godavari basin, eastern Indian margin

The passive eastern Indian margin is rich in gas hydrates, as inferred from the wide-spread occurrences of bottom-simulating reflectors (BSRs) and recovery of gas hydrate samples from various sites in the Krishna Godavari (KG) and Mahanadi (MN) basins drilled by the Expedition 01 of the Indian National Gas Hydrate Program (NGHP). The BSRs are often interpreted to mark the thermally controlled base of gas hydrate stability zone (BGHSZ). Most of the BSRs exhibit moderate to typically higher amplitudes than those from other seismic reflectors. We estimate the average geothermal gradient of ∼40°C/km and heat flow varying from 23 to 62 mW/m2 in the study area utilizing the BSR’s observed on seismic sections. Further we provide the BGHSZ where the BSR is not continuous or disturbed by local tectonics or hidden by sedimentation patterns parallel to the seafloor with a view to understand the nature of BSR.

The structure of methane gas hydrate bearing sediments from the Krishna–Godavari Basin as seen from Micro-CT scanning

The Indian National Gas Hydrate Program (NGHP) Expedition 1, of 2006, cored through several methane gas hydrate deposits on the continental shelf around the coast of India. The pressure coring techniques utilized during the expedition (HYACINTH and PCS) enabled recovery of gas hydrate bearing, fine-grained, sediment cores to the surface. After initial characterization core sections were rapidly depressurized and submerged in liquid nitrogen, preserving the structure and form of the hydrate within the host sediment. Once on shore, high resolution X-ray CT scanning was employed to obtain detailed three-dimensional images of the internal structure of the gas hydrate. Using a resolution of 80 μm the detailed structure of the hydrate veins present in each core could be observed, and allowed for an in depth analysis of orientation, width and persistence of each vein. Hydrate saturation estimates could also be made and saturations of 20–30% were found to be the average across the core section with some portions showing highs of almost 60% saturation. The majority of hydrate veins in each core section were found to be orientated between 50 and 80° to the horizontal. Analysis of the strikes of the veins suggested a slight preferential orientation in individual sample sections, although correlation between individual sections was not possible due to the initial orientation of the sections being lost during the sampling stage. The preferred vein orientation within sample sections coupled with several geometric features identified in individual veins, suggest that hydraulic fracturing by upward advecting pore fluids is the main formation mechanism for the veined hydrate deposits in the K–G Basin.

Electrical anisotropy due to gas hydrate-filled fractures

In 2006, the Indian National Gas Hydrate Program Expedition 01, or NGHP-01, discovered gas hydrate as fill in near-vertical fractures in unconsolidated sediments at several drilling sites on the Indian continental margins. These gas hydrate-filled fractures were identified on logging-while-drilling resistivity images. The gas hydrate-filled fracture intervals coincide with high measured resistivity at the NGHP-01 sites. High measured resistivity translates into high hydrate saturations via Archie’s equation; however, these high saturations contradict lower gas hydrate saturations determined from pressure core and chlorinity measurements. Also, in intervals with near-vertical gas hydrate-filled fractures, there is considerable separation between phase shift and attenuation resistivity logs, with [Formula: see text] resistivity measurements being significantly higher than [Formula: see text] resistivity measurements. We modeled the sensitivity of the propagation resistivity measurements in the gas hydrate-filled fracture intervals at NGHP-01 Sites 5 and 10. Near-vertical hydrate-filled fractures can cause the abnormally high resistivity measurements in vertical holes due to electrical anisotropy. The model suggests the gas hydrate saturations in situ are usually significantly lower than those calculated from Archie’s equation. In addition, these modeled gas hydrate saturations generally agree with the lower gas hydrate saturations obtained from pressure core and chlorinity measurements at NGHP-01 Sites 5 and 10.

Documenting channel features associated with gas hydrates in the Krishna–Godavari Basin, offshore India

During the India National Gas Hydrate Program (NGHP) Expedition 01 in 2006 significant sand and gas hydrate were recovered at Site NGHP-01-15 within the Krishna–Godavari Basin, East Coast off India. At the drill site NGHP-01-15, a 5–8m thick interval was found that is characterized by higher sand content than anywhere else at the site and within the KG Basin. Gas hydrate concentrations were determined to be 20–40% of the pore volume using wire-line electrical resistivity data as well as core-derived pore-fluid freshening trends. The gas hydrate-bearing interval was linked to a prominent seismic reflection observed in the 3D seismic data. This reflection event, mapped for about 1km2 south of the drill site, is bound by a fault at its northern limit that may act as migration conduit for free gas to enter the gas hydrate stability zone (GHSZ) and subsequently charge the sand-rich layer. On 3D and additional regional 2D seismic data a prominent channel system was imaged mainly by using the seismic instantaneous amplitude attribute. The channel can be clearly identified by changes in the seismic character of the channel fill (sand-rich) and pronounced levees (less sand content than in the fill, but higher than in surrounding mud-dominated sediments). The entire channel sequence (channel fill and levees) has been subsequently covered and back-filled with a more mud-prone sediment sequence. Where the levees intersect the base of the GHSZ, their reflection strengths are significantly increased to 5- to 6-times the surrounding reflection amplitudes. Using the 3D seismic data these high-amplitude reflection edges where linked to the gas hydrate-bearing layer at Site NGHP-01-15. Further south along the channel the same reflection elements representing the levees do not show similarly large reflection amplitudes. However, the channel system is still characterized by several high-amplitude reflection events (a few hundred meters wide and up to ~1km in extent) interpreted as gas hydrate-bearing sand intervals along the length of the channel.

Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient ( a), cementation factor ( m) and saturation exponent ( n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 ( S h = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 ( S h = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km 2. Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.

Seismic imaging of a fractured gas hydrate system in the Krishna–Godavari Basin offshore India

Gas hydrate was discovered in the Krishna–Godavari (KG) Basin during the India National Gas Hydrate Program (NGHP) Expedition 1 at Site NGHP-01-10 within a fractured clay-dominated sedimentary system. Logging-while-drilling (LWD), coring, and wire-line logging confirmed gas hydrate dominantly in fractures at four borehole sites spanning a 500 m transect. Three-dimensional (3D) seismic data were subsequently used to image the fractured system and explain the occurrence of gas hydrate associated with the fractures. A system of two fault-sets was identified, part of a typical passive margin tectonic setting. The LWD-derived fracture network at Hole NGHP-01-10A is to some extent seen in the seismic data and was mapped using seismic coherency attributes. The fractured system around Site NGHP-01-10 extends over a triangular-shaped area of ∼2.5 km 2 defined using seismic attributes of the seafloor reflection, as well as “seismic sweetness” at the base of the gas hydrate occurrence zone. The triangular shaped area is also showing a polygonal (nearly hexagonal) fault pattern, distinct from other more rectangular fault patterns observed in the study area. The occurrence of gas hydrate at Site NGHP-01-10 is the result of a specific combination of tectonic fault orientations and the abundance of free gas migration from a deeper gas source. The triangular-shaped area of enriched gas hydrate occurrence is bound by two faults acting as migration conduits. Additionally, the fault-associated sediment deformation provides a possible migration pathway for the free gas from the deeper gas source into the gas hydrate stability zone. It is proposed that there are additional locations in the KG Basin with possible gas hydrate accumulation of similar tectonic conditions, and one such location was identified from the 3D seismic data ˜6 km NW of Site NGHP-01-10.

Effective medium modeling of gas hydrate‐filled fractures using the sonic log in the Krishna‐Godavari basin, offshore eastern India

Indian National Gas Hydrate Program Expedition 01 has established that clay‐rich marine sediment from the Krishna‐Godavari (KG) basin in the eastern Indian margin hosts one of the richest gas hydrate deposits in the world. Resistivity at‐bit images and pressure cores reveal that the gas hydrate morphology in clay‐rich sediment varies from complex vein structures (grain displacing) to invisible pore filling. Existing rock physics models, which relate acoustic data to in situ gas hydrate concentrations, generally assume isotropic pore‐filling gas hydrate, which yields misleading concentration estimates for fractured fine‐grained sediments. The anisotropic KG basin sediment presents additional complications. Here we apply differential effective medium theory to incorporate grain‐displacing morphologies by which gas hydrate is included as vertical ellipsoids with aspect ratios ranging from those of thin veins up to those of nodules in an elastic anisotropic background. We have estimated gas hydrate concentrations from sonic velocities at hole 10D in the KG basin considering three basic gas hydrate morphologies: (i) pore filling, (ii) grain displacing, and (iii) a combination of grain displacing and pore filling. Average gas hydrate saturations for these three cases are 35–42%, 27–30%, and 33–41% of the total porosity, respectively, in the depth range 60–140 m below seafloor (mbsf). Saturation is highest at ∼67 mbsf for any morphology but the values differ between morphologies. For the pore‐filling morphology, the maximum gas hydrate saturation of 56% is 18–22% higher than the grain‐displacing morphology and 2–9% higher than the combined morphology. Estimates differ by ±6% of the sediment volume with rotations of gas hydrate veins from vertical to horizontal.

Gas hydrate saturations estimated from fractured reservoir at Site NGHP‐01‐10, Krishna‐Godavari Basin, India

During the Indian National Gas Hydrate Program Expedition 01 (NGHP‐01), one of the richest marine gas hydrate accumulations was discovered at Site NGHP‐01‐10 in the Krishna‐Godavari Basin. The occurrence of concentrated gas hydrate at this site is primarily controlled by the presence of fractures. Assuming the resistivity of gas hydrate–bearing sediments is isotropic, the conventional Archie analysis using the logging while drilling resistivity log yields gas hydrate saturations greater than 50% (as high as ∼80%) of the pore space for the depth interval between ∼25 and ∼160 m below seafloor. On the other hand, gas hydrate saturations estimated from pressure cores from nearby wells were less than ∼26% of the pore space. Although intrasite variability may contribute to the difference, the primary cause of the saturation difference is attributed to the anisotropic nature of the reservoir due to gas hydrate in high‐angle fractures. Archie's law can be used to estimate gas hydrate saturations in anisotropic reservoir, with additional information such as elastic velocities to constrain Archie cementation parameters m and the saturation exponent n. Theory indicates that m and n depend on the direction of the measurement relative to fracture orientation, as well as depending on gas hydrate saturation. By using higher values of m and n in the resistivity analysis for fractured reservoirs, the difference between saturation estimates is significantly reduced, although a sizable difference remains. To better understand the nature of fractured reservoirs, wireline P and S wave velocities were also incorporated into the analysis.

Extent of gas hydrate filled fracture planes: Implications for in situ methanogenesis and resource potential

High‐angle gas hydrate filled fracture planes were identified along a 31 m interval in logging while drilling images in two holes located ∼11 m apart drilled during the Indian National Gas Hydrate Program Expedition 01, offshore India. Using Monte Carlo simulations to account for uncertainty in hole location, hole deviation, strike and dip, we assert with 95% confidence that the fracture planes in the two holes are not the same. The gas hydrate filled fracture planes likely only extend a few meters laterally from each borehole and occur in an isolated interval in the middle of the gas hydrate stability zone. This suggests gas generated microbially within in the gas hydrate stability zone may have supplied the gas hydrate‐filled fracture interval. Production of methane from these reservoirs using conventional methods may be quite challenging.

Multidisciplinary investigations exploring indicators of gas hydrate occurrence in the Krishna–Godavari Basin offshore, east coast of India

We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in 2002–2003 in the Krishna–Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW–ESE-trending sedimentary ridge in the south. Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO4 2−) concentrations (28.7 to <4 mM), increasing downcore methane (CH4) concentrations (0–20 nM) and relatively high total organic carbon contents (1–2.5%), and microbial analyses a high abundance of microbes in top core sediments and a low abundance of sulphate-reducing bacteria in bottom core sediments. Methane-derived authigenic carbonates were identified in some cores. Combined with evidence of gas-escape features in association with bottom simulating reflections, the findings strongly suggest that the physicochemical conditions prevailing in the study area are highly conducive to methane generation and gas hydrate occurrence. Deep drilling from aboard the JOIDES Resolution during 2006 has indeed confirmed the presence of gas hydrate in the Krishna–Godavari Basin offshore.