Volume of natural gas within gas-hydrate and free-gas occurrences in ODP Leg 164 sites

Leg 164 of the Ocean Drilling Program was designed to investigate the occurrence of gas hydrate in the sedimentary section beneath the Blake Ridge on the southeastern continental margin of North America. Sites 994, 995, and 997 were drilled on the Blake Ridge to refine our understanding of the in situ characteristics of natural gas hydrate. Because gas hydrate is unstable at surface pressure and temperature conditions, a major emphasis was placed on the downhole logging program to determine the in situ physical properties of the gas hydrate-bearing sediments. Downhole logging tool strings deployed on Leg 164 included the Schlumberger quad-combination tool (NGT, LSS/SDT, DIT, CNT-G, HLDT), the Formation MicroScanner (FMS), and the Geochemical Combination Tool (GST). Electrical resistivity (DIT) and acoustic transit-time (LSS/SDT) downhole logs from Sites 994, 995, and 997 indicate the presence of gas hydrate in the depth interval between 185 and 450 mbsf on the Blake Ridge. Electrical resistivity log calculations suggest that the gas hydrate-bearing sedimentary section on the Blake Ridge may contain between 2 and 11 percent bulk volume (vol%) gas hydrate. We have determined that the log-inferred gas hydrates and underlying free-gas accumulations on the Blake Ridge may contain as much as 57 trillion m**3 of gas.

Carbon/oxygen ratios, clay composition and gas-hydrate saturation of ODP Leg 164 sites

The analyses of downhole log data from Ocean Drilling Program (ODP) boreholes on the Blake Ridge at Sites 994, 995, and 997 indicate that the Schlumberger geochemical logging tool (GLT) may yield useful gas hydrate reservoir data. In neutron spectroscopy downhole logging, each element has a characteristic gamma ray that is emitted from a given neutron-element interaction. Specific elements can be identified by their characteristic gamma-ray signature, with the intensity of emission related to the atomic elemental concentration. By combining elemental yields from neutron spectroscopy logs, reservoir parameters including porosities, lithologies, formation fluid salinities, and hydrocarbon saturations (including gas hydrate) can be calculated. Carbon and oxygen elemental data from the GLT was used to determine gas hydrate saturations at all three sites (Sites 994, 995, and 997) drilled on the Blake Ridge during Leg 164. Detailed analyses of the carbon and oxygen content of various sediments and formation fluids were used to construct specialized carbon/oxygen ratio (COR) fan charts for a series of hypothetical gas hydrate accumulations. For more complex geologic systems, a modified version of the standard three-component COR hydrocarbon saturation equation was developed and used to calculate gas hydrate saturations on the Blake Ridge. The COR-calculated gas hydrate saturations (ranging from about 2% to 14% bulk volume gas hydrate) from the Blake Ridge compare favorably to the gas hydrate saturations derived from electrical resistivity log measurements.

Mineralogy and stable isotopic composition of carbonates and sulfide minerals from ODP Leg 164 sites

During Ocean Drilling Program Leg 164, gas hydrates were recovered in the Blake Ridge where the top of the gas hydrate zone lies at about 200 meters below seafloor (mbsf) and the bottom-simulating reflector (BSR) is located at about 450 mbsf. There is no sedimentological discontinuity crossing the BSR. The BSR is disrupted by the salt piercement of the Cape Fear Diapir. The authigenic carbonates (dolomite and siderite) are always present in small amounts (a few weight percent) in the sediments; they are also concentrated in millimeter- to centimeter-sized nodules and layers composed of dolomite above the top of the gas hydrate reservoir, and of siderite below the BSR. In the Blake Ridge, the dolomite/siderite boundary is located near 140 mbsf. The distribution with depth of the d18O values of dolomite and siderite shows a sharp decrease from high values (maximum 7.5 per mil) in the topmost 50 m, to very low values (minimum -2.7 per mil) at 140 mbsf, and at greater depth increase to positive values within the range of 1.8 per mil to 5.0 per mil. The d13C distribution is marked by the rapid increase with greater depth from low values (-31.3 per mil to -11.4 per mil) near 50 mbsf to positive values at 110 mbsf, which remain in the range of 1.7 to 5.4 down to 700 mbsf. Diagenetic carbonates were precipitated in pore waters in which d18O and d13C values were highly modified by strong fractionation effects, both in the water and in the CO2-CH4 systems associated with the formation and dissociation of gas hydrates.

Grain size composition of sediments from ODP Leg 164 sites

This research was designed to check the assumption of the grain-size control on a gas hydrate presence in the Blake Ridge sediments; the assumption had originated from the data gained at Deep Sea Drilling Project (DSDP) Site 533. Granulometric analysis (the combined pipette-sieve method) of the 345 sediment samples obtained after pore-water squeezing from Ocean Drilling Program (ODP) Sites 994, 995, and 997 has provided support for this assumption. The zone of negative anomalies of pore-water chlorinity, which is generally recognized to be gas hydrate bearing, is confined, as a whole, to the interval of comparatively coarse-grained sediments in each of the three site columns because content of the fine fractions <0.05, <0.01, <0.005, and <0.001 mm is lower there (although the character of this control changes from site to site). The individual chlorinity anomalies also coincide, for the most part, with relatively coarse-grained sediments.