The Blake Ridge represents a large area of well-studied gas hydrate on a passive continental margin off the coast of S. Carolina that was drilled during ODP Leg 164 [Paull, et al., 1996]. Most other hydrate areas that have been studied using ODP logs are on accretionary prisms in subduction zone settings. On this passive margin, the hydrate concentrations are relatively low but occur over a considerable depth interval. Sites 994, 995, and 997 comprise a transect of holes that penetrate through a gas hydrate zone within the same stratigraphic interval over a relatively short lateral distance (9.6 km). Prominent BSRs are apparent in seismic profiles across most, but not all, of the length of this transect. The presence of hydrate at Sites 994 and 997 was documented by direct sampling—several pieces of hydrate were recovered from about 260 mbsf at Site 994 and disseminated hydrate was observed at almost the same depth at Site 994. One large, solid piece of hydrate about 15 cm long was recovered from about 331 mbsf at Site 997. No hydrate samples were conclusively identified at Site 995. Instead, based on wireline logging data and geochemical core analyses, disseminated hydrate was inferred to occur from a depth of about 190 to 450 mbsf in all three holes drilled across the Blake Ridge.
Figure 3 shows the wireline logs recorded at Site 995 on the Blake Ridge. Similar to the early observations on the Guatemala margin, disseminated hydrate occurrences on the Blake Ridge are also characterized by increases in log-measured velocity and resistivity. These wireline logs reveal that the hydrate-bearing interval corresponds with an increase in electrical resistivity of 0.1-0.3 ohm-m above background reference levels and with an increase of 0.1-0.2 km/s in compressional velocity. The deep-reading resistivity log reveals several high resistivity intervals of 1.4-1.5 ohm-m near 220 mbsf. Below these anomalies, the resistivity and velocity logs increase with depth to 450 mbsf. Examination of the natural gamma ray and bulk density logs from this hole reveal no apparent lithologic causes for the observed velocity and resistivity increases through the interval. These observations are consistent with the presence of a material having increased resistivity and velocity, but similar density to the pore water
GAMMA RAY BULK-DENSITY VELOCITY RESISTIVITY
n 30 API 801.1 g/cm3 1.9 1.5 km/sec 1.90.7 ohm-m 1.6
Figure 3. Wireline log data at ODP Site 995 located on the Blake Ridge.
that it partially replaces in open sediment pores. The depth of the BSR and the lower boundary of hydrate occurrence on the Blake Ridge is slightly higher than the predicted base of the methane hydrate stability zone [Ruppel, 1997]. Increases in velocity and resistivity logs above the BSR have been conclusively associated with the presence of hydrate, in overall agreement with sampling and other geochemical estimates [Paull, et al, 1996; Dickens, et al., 1997]. The decrease in VSP and sonic log velocity below the BSR is attributed to the accumulation of free gas stratigraphically trapped at this depth [.Holbrook, et al., 1996], Within the interval below the BSR to the depth of the theoretical base of gas hydrate stability, evidence from the sonic logs suggests that hydrate and small amounts of free gas may co-exist at this site [Guerin et al, 1999]. The integrated interpretation of in situ logs and seismic data from the Blake Ridge indicates that the velocity contrast across the BSR, though marking a presence of hydrate and free gas over the interval, clearly is not a simple gas-water contact.
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