OS11C-01
Occurrence of Marine Gas Hydrates in the Indian Continental Margin: Results of the Indian National Gas Hydrate Program (NGHP) Expedition 01
Studies of geologic and geophysical data from the offshore of India have revealed two geologically distinct areas with seismically inferred gas hydrate occurrences: the passive continental margins of the Indian Peninsula and along the Andaman convergent margin. The Indian National Gas Hydrate Program (NGHP) Expedition 01 was designed to study the gas hydrate occurrences both spatially and temporally off the Indian Peninsula and along the Andaman convergent margin with special emphasis on understand the geologic and geochemical controls on the occurrence of gas hydrate in these two diverse settings. During NGHP Expedition 01, dedicated gas hydrate coring, drilling, and logging operations were conducted from the 28th April, 2006 to the 19th August, 2006. NGHP's Expedition 01 was planned and managed through a collaboration between the Indian Directorate General of Hydrocarbons (DGH), the U.S. Geological Survey (USGS), and the Consortium for Scientific Methane Hydrate Investigations (CSMHI) led by Overseas Drilling Limited (ODL) and FUGRO McClelland Marine Geosciences. Other key participants included the members of Integrated Ocean Drilling Program, including the Joint Oceanographic Institutes, Texas A&M University, and the Lamont-Doherty Earth Observatory of Columbia University. During its 113.5-day voyage, the JOIDES Resolution cored or drilled 39 holes at 21 sites (1 site in Kerala-Konkan, 15 sites in Krishna-Godavari, 4 sites in Mahanadi and one site in Andaman deep offshore areas), penetrated more than 9,250 meters of section and recovered nearly 2,850 meters of core with ~78% recovery. Twelve holes were logged with logging-while-drilling tools and an additional 13 holes were wireline logged. NGHP Expedition 01 established the presence of gas hydrates in Krishna-Godavari, Mahanadi and Andaman basins. The expedition discovered and closely examined one of the richest gas hydrate accumulations yet documented (Site 10 in the Krishna-Godavari basin), documented the thickest and deepest gas hydrate stability zone yet known (Site 17 in Andaman Sea), and established the existence of a fully-developed gas hydrate system in the Mahanadi basin (Site 19). In addition to the work accomplished on board, the science team also collected an unprecedented number of samples and data for analysis by an international team of experts in microbiology, sedimentology, geochemistry, and numerous other disciplines. A final synthesis of the project technical findings is expected to be published in 2008. It is perceived that the NGHP effort will likely include future drilling, coring, and field production testing. It has been concluded that Site 10 represents a world class shale dominated fracture gas hydrate reservoir. NGHP Expedition 01 also discovered significant sand and silt dominated gas hydrate reservoirs. It has been proposed that in a 2009-2010 time-frame, a drill ship of opportunity could be used to drill and log several of the most promising gas hydrate sand dominated prospects.
OS11C-02
Stratigraphy, Sedimentology, and Depositional History of Gas Hydrate Bearing Sediments Along the Eastern Continental Margin of India and in the Andaman Accretionary Wedge: Results from NGHP Expedition 01
An international ocean drilling effort led by the Indian National Gas Hydrate Research Program (NGHP) and the U.S. Geological Survey to study gas hydrates along the Indian continental margins was completed during the summer of 2006. Drill and core sites documented the presence of gas hydrate along the eastern continental margin of India in the Krishna-Godavari (K-G) and Mahanadi basins, and in the Andaman wedge near Little Andaman Island. 2-D and 3-D seismic data provide the high-resolution structural and stratigraphic context for each of the drill sites. These data combined with detailed lithologic descriptions, multi-sensor core logger (MSCL) measurements, and logging while drilling (LWD) data characterize the sedimentology of the recovered cores and of the in situ stratigraphy at each site. In the K-G basin slope environment, sedimentological description and interpretation of the cores collected from seven sites (up to 300 mbsf) reveals a Quaternary history of sedimentation dominated by dark grey to black colored nannofossil bearing to rich clay and silty clay deposition, likely sourced from the nearby Krishna and Godavari Rivers. Silt to fine sand turbidites (1-5 cm thick) also occur throughout the section as well as visible terrestrial organic material. Foraminifera and coccoliths occur throughout the records but are highly diluted by the terrigenous constituents. Authigenic carbonates, present in fine grained bands and as micronodules (less than 1 cm) to large nodules (greater than 5 cm) are particularly abundant throughout most of recovered stratigraphy and are consistent with the recovery and presence of gas hydrate and other indicators of gas hydrate throughout this region. Sediments recovered from the two sites drilled in the Mahanadi basin to the north are biogenic-rich (foraminifera, coccoliths, diatoms, and radiolaria) clays to oozes and void of turbidites. A volcanic ash bed near the top of the section and volcanic glass bearing intervals deeper in the section are also present. Based on initial biostratigraphy, the age of the recovered sediments spans Late Miocene to recent at these sites. Authigenic carbonates are rare throughout the section and there was less gas hydrate recovered at these sites. At the single site drilled in the Andaman wedge, a long record (nearly 700 m) of carbonate and biosiliceous clays and oozes, punctuated by abundant volcanic ashes, was recovered. Initial biostratigraphy here also suggests a late Miocene to recent record. An extremely deep BSR (610 mbsf) indicates a thick zone of potential gas hydrate stability at this site; gas hydrate was recovered throughout this interval and often associated with volcanic ash beds. Post-cruise XRD, XRF, organic and inorganic carbon, and other geochemical measurements will help to further characterize the stratigraphy at all of these sites.
OS11C-03
Geochemical Evidence for Gas Hydrates in the Indian Ocean
Geochemical analyses of pore fluids collected from both non-pressurized and pressurized cores provide important constraints on the presence and distribution of gas hydrates in the Indian Ocean. Cores were recovered from two deepwater (900-1170 meter) basins offshore southeast India, the Krishna-Godavari (KG) (10 sites) and Mahanadi Basins (2 sites), and from one site in the Andaman Sea (1600 meters water depth). A bottom simulating reflector (BSR) (i.e., possible base of methane hydrate stability) was present at most of the sites cored and evidence for methane hydrate was obtained at each of the sites. The gas hydrates in most of the conventional cores decomposed prior to sampling. The former presence and distribution of methane hydrate in some of the conventional cores was inferred by indirect methods: from infra-red imaging of cold core temperatures that correlated with pore fluids having lower salinity and chloride concentrations and occasionally mousse-like textures. Chloride concentrations and salinity anomalies, assuming dilution by water released from decomposed methane hydrate, suggest pore volume occupancies on the order of, <1% to a maximum of ~61% at two of the sites in the KG Basin, and <1% to a maximum of ~76% at the site in the Andaman Sea. In the KG Basin, the highest methane hydrate concentrations were associated with fracture zones in clay/silt sediments or in some coarser grained horizons, although gas hydrate was not present in all coarse- grained sediment horizons. Overall, the % occupancies based on pressure core methane concentrations and the chloride concentrations in conventional cores were similar. The pressure core samples used for these determinations were 1 m long, and the conventional core samples were 10-30 cm whole round core samples, chosen via IR imaging. Variations in sulfate gradients were observed; the steepest gradient with the sulfate/methane interface (SMI) at 8 mbsf was observed in the KG Basin. The deepest SMI obtained, at 25 mbsf, was documented at the Andaman Sea site. The shallow depths of the SMI at all sites indicate that methane advection is intense. The extreme negative δ 13C values of the dissolved inorganic carbon, ranging from -38‰ to -46‰ at the SMI, coincide with the depths of maximum alkalinity values. These δ 13C-DIC values indicate that anaerobic oxidation of methane (AOM) is the dominant reaction responsible for sulfate reduction. The percent of sulfate reduction by organic matter oxidation relative to sulfate reduction by methane oxidation is slightly higher at the KG Basin sites than at the Mahanadi Basin or Andaman Sea sites. Authigenic carbonate formation is widespread, in particular at the Andaman Sea site. Bromide concentrations provide insight on both organic matter diagenesis and the type of organic matter. The Br/Cl ratios are distinct in the three regions cored: they are highest at the Andaman Sea Site, intermediate in the KG Basin and lowest in the Mahanadi Basin.
OS11C-04
Gas hydrate-filled fracture distribution, eastern Indian continental margin
Indian National Gas Hydrate Program (NGHP) Expedition 01 was designed to study the occurrence of gas hydrate along the east and west coast of India and near the Andaman Islands. The expedition discovered gas hydrates in sand, silt, and clay dominated sediments. In this research, we study high resistivity fractures found in unconsolidated clay sediments on logging-while drilling (LWD) borehole resistivity images from four holes drilled at Site 5, Site 7 and Site 10 on the eastern Indian continental margin during NGHP Expedition 01. These fractures are likely filled with natural gas hydrate. Pressure cores from NGHP sites confirm gas hydrate commonly occurs in fracture veins in clay dominated lithology. From the LWD images, we establish the relationship between gas hydrates, fracturing, and the regional or local stress regime on the eastern Indian continental margin. Gas hydrate is identified on the borehole logs and images as high resistivity responses without associated density increases or indications of free gas. Gas hydrate saturations are calculated using Archie's equation. For gas-hydrate bearing intervals, fracture strike and dip are determined and plotted on an Equal Area steronet. The local state of stress at the time of fracturing is determined by these stereonet orientations. Fractures outside the gas hydrate bearing zones are analyzed and compared to the gas-hydrate bearing fractures. Indicators of the regional and local stress orientation based on seismic, bathymetry and other holes from NGHP are related to the fracture-derived stress orientations. Preliminary results from three of four the logging-while-drilling holes show the gas hydrate-filled fractures have an aligned, preferred orientation likely associated with the stress regime. In one hole at Site 10, where 130 m of gas hydrate-filled fractures were observed, preliminary analysis suggests the gas hydrate-filled fractures have no preferred orientation, which may result from local tectonics or diapirism.
OS11C-05 INVITED
Gas Hydrate Occurrence Across the Cascadia Margin From Pore Water Chlorinity and Downhole Geophysical Logs (IODP Exp. 311)
IODP Exp. 311 drilled a transect of four sites crossing the northern Cascadia convergent margin in a slow, diffuse fluid flow environment. These transect sites are an ideal data set to compare variation in gas hydrate (GH) occurrence with distance from the deformation front. In this study, we quantify the variation with depth of GH saturation (fraction of pore space occupied by GH) from pore water chlorinity and downhole logs of porosity and electrical resistivity. At each site we determine a GH occurrence zone (GHOZ), defined as the depth interval where gas hydrates are actually observed. The GHOZ is not necessarily the same as the GH stability zone (GHSZ), which is the depth interval where gas hydrates are thermodynamically stable. In three of the Exp. 311 transect sites gas hydrates occur down to the base of GH stability, but the top GHOZ is not at the seafloor (the top of the GHSZ). The top GHOZ deepens moving away from the deformation front, from 47 mbsf at Site U1326 (5 km from the deformation front), to 73 mbsf at Site U1325 (11 km), and 111 mbsf at Site U1327 (21 km). Site U1329 (38 km from the deformation front) shows no clear evidence of GH. Fluid expulsion from the accretionary wedge has been proposed as the main process to advect methane and form GH at the Cascadia margin. Published studies have quantified fluid expulsion rates from the compaction of sediments that are scraped off the subducting plate and are incorporated into the accretionary wedge. These compaction models generally show fluid expulsion rates that decrease sharply moving inland from the deformation front. The Exp. 311 transect sites show a GHOZ that thins moving away from the deformation front and correlates with the decrease in fluid expulsion rates predicted by the compaction models. Our results are consistent with fluid expulsion being a primary control on the formation of GH in the Cascadia convergent margin.
OS11C-06
The iodine release during organic matter degradation at Northern Cascadia Margin: a numerical approach
Iodine is dominated by organic material decomposition and transports with fluids in reducing environments such as deep marine sediments. It is often strongly concentrated in the pore waters associated with gas hydrates, with enrichment factors up to a few thousands comparing to seawater. The organic source of iodine very likely is also responsible for the large amount of methane in the hydrates. Once iodine is released from organic matter, it migrates with fluids and is rarely involved in diagenetic processes. We apply a Mathematica-based, one dimensional model to the drilling sites of IODP 311 at northern Cascadia Margin, in order to simulate the transport of deep-sourced old iodine and the in-situ release of younger iodine by microbial activity. In the model, iodine in the sediments is assumed to be associated only with organic materials and is released by particulate organic carbon (POC) degradation into the ambient pore waters. Depth profiles of dissolved iodide, bromide concentrations, and sediment-bounded iodine concentrations are calculated by the model to fit the analytical data. Modeling results suggest that the rates of POC degradation in these sediment cores can only account for a small amount of iodine, much lower than the level observed. The dominant organic source for iodine must be the deeper sediment layers. This is consistent with the old source age (~30 Ma) indicated by iodine isotope results. For comparison, we also applied the model to Site 1230, ODP 201 at Peru Margin and obtained similar results. The results demonstrate that iodine in gas hydrate locations is predominantly derived from deep, old sources with only small contributions from local organic material. Because iodine and methane commonly are transported together, the results suggest that a major part of methane in gas hydrate occurrences is also derived from distant sources.
OS11C-07
Fine scale control of microbial communities in deep marine sediments that contain hydrates and high concentrations of methane
Deep subseafloor sediments with high concentrations of organic carbon and microbially-generated methane contain microbial communities that play an important role in the biogeochemical cycling of carbon. However, we still have a limited understanding of the fine (centimeter) scale sediment properties (e.g., grain size, presence/absence of hydrates) that determine key microbial attributes in deep marine sediments. Our objective is to determine the quantity, diversity, and distribution of microbial communities in the context of abiotic properties in gas-rich marine sediments. DNA was extracted from deep marine sediments cored from various continental shelf locations including offshore India and the Cascadia Margin. Abiotic characterization of the same sediments included grain size analysis, chloride concentrations in sediment pore waters, and presence of hydrates in the sediments as determined by thermal anomalies. As in past studies of such systems, most of the samples yielded low levels of DNA (0.3-1.5 ng/g of sediment). Bacterial DNA appeared to be more easily amplified than archaeal DNA. Initial attempts to amplify DNA using primers specific for the methanogen functional gene, methyl- CoM-reductase, were unsuccessful. Infrequently, cores from relatively shallow sediments (e.g., 0.5 mbsf Leg 204, 1251B-1H) from central (Hydrate Ridge), and northern Cascadia (offshore Vancouver Island), and from India's eastern margin contained macroscopically visible, pigmented biofilms. One of these biofilms was composed of high concentrations of cell clusters when viewed microscopically. The predominant cells in the Hydrate Ridge biofilm were large (ca. 10 um) cocci and preliminary characterization of the 16S rDNA amplified and sequenced from this biofilm suggests the prevalence of a microbe with 97% similarity to mycobacteria. These discrete biofilm communities appear to be distinctive relative to the normally sparse distribution of cells in the sediments. By determining how the abiotic properties of deep marine sediments control the numbers and distribution of microbial communities that process organic matter we hope to provide better parameters for computational models that describe carbon cycling in these systems.
OS11C-08
Geochemical Conditions of Magnetic Iron Sulphide Formation in Southern Hydrate Ridge Sediments (ODP Leg 204): Preliminary Results
Southern Hydrate Ridge sediments are composed by hemipelagic silty-clays interbedded with numerous turbidite and debris flow deposits. The magnetic mineralogy of these gas hydrate-rich sediments is dominated by magnetite and the magnetic iron sulphides greigite and pyrrhotite. Magnetite is more abundant in coarse-grained sediments and it's therefore interpreted to be detrital in origin. Greigite and pyrrhotite have been recognized as being diagenetically produced during microbial reduction of sulphate in the sulphate, the anaerobic oxidation of methane, and the methanic zones. In order to study the chemical conditions in which magnetic iron sulphides are produced and preserved, we have measured the total Fe (TFe), reactive-Fe, TOC and total S (TS) of a set of ca. 100 samples with distinctive magnetic assemblages (magnetite+greigite, greigite-, and pyrrhotite-dominated). TOC and TS were analyzed by an elemental analyzer and, TFe and reactive-Fe were measured by atomic absorption spectrometry. TOC contents range between 0.38 and 1.22%, having the greigite-dominated samples the highest relative TOC contents (>0.77%). TS contents vary between 0.2 and 0.9%. TFe values range between 4.3 and 7.3%, in which the reactive-Fe represents the 23-39%. TS/TOC ratio values are near the oxic and normal marine seawater conditions (TS/TOC = 0.36), except for two magnetite+greigite and greigite-dominated samples that have higher TS contents. All the samples show TS/reactive Fe ratios considerably lower that of saturated pyrite (1.15), which implies a low degree of pyritization in the environment. Only 2 samples dominated by greigite+magnetite show TS/reactive Fe contents higher than the saturated pyrrhotite ratio. The reactive Fe/TFe values are low, under reactive Fe/TFe =1/3, which implies that there is not enough reactive-Fe in the system. These preliminary results reinforce the view that formation and preservation of greigite and pyrrhotite in southern Hydrate Ridge sediments is favoured by low reactive-Fe contents, so that pyritization reactions at the different diagenetic zones are not driven to completion.