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We investigated the sediment properties and gas hydrate saturations in pressure-cores recovered during the National Gas Hydrate Program Expedition 02 (NGHP-02) in Areas B, C, and E of the Krishna–Godavari Basin (K–G Basin) off the eastern margin of India. These investigations included the analyses of grain size, mineralogy, and grain density and employed scanning electron microscopy (SEM). Sediments with median grain sizes below 90 μm were generally observed within most of the pressure-cores, whereas sediments with median grain sizes exceeding 250 μm were only observed in a few pressure-cores. Higher concentrations of gas hydrate were found in sediment layers with median grain sizes of 10–90 μm; the occurrence of gas hydrates was lower in sediment layers with median grain sizes below 10 μm. Three types of gas hydrate-bearing sediments containing above 50 vol.% gas hydrate saturations were observed: (1) sediments with silt grain size ratios above 70 vol.%, (2) sediments with sand grain size ratios above 35 vol.%, and (3) coarser sandy sediments with gravel. Sandy sediments with high gas hydrate saturations (above 50 vol.%) were very minor constituents in the pressure-core sediments. The gas hydrate saturations were lower in sediments with a clay fraction above 30 vol.%. SEM images revealed microfossil debris, fragments of sedimentary rock, and silt/sand-sized-agglomerated grains throughout the pressure-cores, which indicate that the sediments were deposited at and/or transported to the foot of the continental slope through reworking of upper-slope deposits. The geological setting and SEM observations indicate that the pressure-core sediments are mainly a terrigenous source, with a small component from pelagic or hemipelagic sedimentation. The results of the pressure-core analyses confirm that natural gas hydrates can occur in silt-rich sediments. The occurrence of gas hydrates in the pressure-core sediments recovered during the NGHP-02 is controlled by the lithology of the sediments, especially their clay content. • Gas hydrate-bearing sediments are found to deposit mostly in silty layers. • Gas hydrates also locally occur in sandy sediments with gravels. • Gas hydrates tend not to occur in sediments with clay fraction above 30 vol.%. • Pore and apparent grain sizes increase by sedimentation of grain agglomerates. [ABSTRACT FROM AUTHOR]

The India National Gas Hydrate Program is being steered by the government of India's Ministry of Petroleum and Natural Gas (MoPNG) with participation of Directorate General of Hydrocarbons (DGH), Oil and Natural Gas Corporation Limited (ONGC), and the National Oil Companies and Research Institutes of India. The India National Gas Hydrate Program Expedition 01 (NGHP-01) established the presence of gas hydrate in the Krishna Godavari (KG) and Mahanadi Basins and in the offshore area of the Andaman Sea Basin. However, the gas hydrates discovered during NGHP-01 were mainly distributed as fracture-filling material in fine-grained clay-rich sediments. The India National Gas Hydrate Program Expedition 02 (NGHP-02) was carried out with an objective to discover gas hydrate in sand-rich sediment along the eastern offshore margin of India. ONGC planned and executed NGHP-02 on the behalf of the MoPNG. NGHP-02 started on March 3, 2015 and was completed on July 28, 2015 (total 147 days) using the Japanese scientific Drilling Vessel Chikyu (D/V Chikyu). During NGHP-02, 42 holes at 25 sites were drilled, cored, and/or surveyed with downhole logging tools. These sites were located in four areas along the eastern margin of India and formally named Area A (Mahanadi Basin, three sites), Area B (northern part of the KG-Basin, twelve sites), Area C (central part of the KG-Basin, six sites), and Area E (southern part to the KG-Basin, four sites). All 25 sites established during NGHP-02 were first drilled and logged with logging-while-drilling (LWD) tools and an additional 17 holes were then drilled and/or cored with conventional coring tools (HPCS/ESCS) or pressure coring tools (PCTB). Wireline logging was conducted in 10 holes and formation tests using a dual packer Modular Formation Dynamics Tester (MDT) tool were carried out in two holes. The onboard science team used the laboratory facilities on the D/V Chikyu to examine and analyse the physical properties, geochemistry, and sedimentology of all the cores collected during the expedition. Core samples were also analysed in additional post-expedition shore-based studies conducted in numerous domestic and international gas hydrate research laboratories. The NGHP-02 sediment cores were archived at the National Gas Hydrate Core Repository in Mumbai (India), which is associated with the ONGC Gas Hydrate Research and Technology Centre (GHRTC). The necessary data for characterizing the occurrence of gas hydrate, such as interstitial water chlorinities, core-derived gas chemistry, physical and sedimentological properties, thermal images of the recovered cores, pressure core and downhole measured logging data (LWD and/or conventional wireline log data), were obtained from most of the drill sites established during NGHP-02. Almost all the drill sites yielded evidence for the occurrence of gas hydrate; however, the inferred in situ concentration of gas hydrate varied substantially from site to site. For the most part, the interpretation of downhole logging data, core thermal images, interstitial water analyses, and pressure core images from the sites established during NGHP-02 indicate that the occurrence of concentrated gas hydrate is mostly associated with coarser grained (sand-rich) sediments. This paper presents the operational and technical summary of NGHP-02. • This paper presents the operational & technical summary of the Indian National Gas Hydrate Program Expedition 02 in Krishna Godavari and Mahanadi Offshore Basins offshore India for gas hydrate exploration during March 3, 2015 to July 28, 2015. • Total 42 wells were completed in 147 days (25 LWD holes and 17 holes for conventional/pressure cores/Wireline logging/MDT. • Vertical seismic profiles (VSP) were acquired in 4 holes and APCT-3 formation temperature measurement was conducted in 9 holes. • Infrared (IR) imaging was conducted on all recovered conventional core samples for quick identification of gas hydrate in the core. Onboard analyses for organic & inorganic geochemistry, sedimentology, x-Ray CT Scan and pressure core analysis was carried out. [ABSTRACT FROM AUTHOR]

Over the last decade global natural gas consumption has steadily increased since many industrialized countries are substituting natural gas for coal to generate electricity. There is also significant industrialization and economic growth of the heavily populated Asian countries of India and China. The general consensus is that there are vast quantities of natural gas trapped in hydrate deposits in geological systems, and this has resulted in the emerging importance of hydrates as a potential energy resource and an accompanying proliferation of recent studies on the technical and economic feasibility of gas production from hydrates. There are then the associated environmental concerns. This study reviews the state of knowledge with respect to natural gas hydrates and outlines remaining challenges and knowledge gaps. [ABSTRACT FROM AUTHOR]