GEOCHEMICAL PROXIES AND TEMPERATURE REGIMES FOR THE DISTRIBUTION OF GAS HYDRATES IN NANKAI TROUGH

Ryo Matsumoto
University of Tokyo, Japan

Gas Hydrate Drilling in Nankai Trough:

Nankai Trough area, an active convergent margin between the Philippine Sea Plate and Eurasian Plate, is characterized by a wide distribution (32,000 km²) of high amplitude bottom simulating reflectors (BSRs). High resolution 2D and 3D seismic surveys for the last decade identified high amplitude but discontinuous reflectors with reverse polarity 30 to 40 meters below the strong BSRs off Tokai in the eastern Nankai Trough area (e.g., Foucher et al., 2002). In 1997 and 1999-2000, the Japan National Oil Corporation (JNOC) (later restructured to form the Japan Oil, Gas and Metals National Corporation or JOGMEC) drilled the exploratory wells “MITI-Nankai Trough” at 34°12’56.07”N and 137°45’02.63”E ~50km off Omaezaki Peninsula in the forearc basin of the eastern Nankai Trough. Cored interval from the sea-floor to 320 mbsf is divided into Unit 1(0~70 mbsf, predominated by mud), Unit 2 (70~150 mbsf, mud with thin ash beds), Unit 3 (150-270 mbsf, mud with thin to moderate sand beds), and Unit 4 (270-320 mbsf, predominated by mud).

Distribution and Amount of Gas Hydrates:

Interstitial waters extracted from the sediment cores from the exploration wells were analyzed for the chloride and sulfate concentrations and oxygen and hydrogen isotopic compositions to examine the distribution and amounts of subsurface gas hydrates. The base-line level of Cl - concentration is 540 mM, whereas the spiky low chloride anomalies were identified in sandy horizons at around 207 mbsf (horizon A), 234-240 mbsf (horizon B), and 258-265 mbsf (horizon C) in Units 3. Gas hydrate saturation (Sh%) in pore space was calculated to be 60% (zone A) to 80% (zones B, C) in sands whereas only a few % in clay and silt. Amount of gas hydrates is calculated to be 26 to 34 vol.% of sands and 2 to 4 vol.% of clay and silt from hydrate saturation (60 to 80%Sh) and sediment porosity (40 % to %). Given the total thickness of 30 m for the ‘hydrated sands’, the total amount of gas hydrates in sands is estimated to be 8 to 10 m in thickness. This gives a rough estimation of gas hydrate deposits to be 1.48 km³ CH₄ (0.056 TCF) per km².

Double BSRs and BGHS:

Wire-line logs of resistivity and velocity are generally consistent with the chloride anomalies and distribution of gas hydrates (Fig. 1): high resistivity and high velocity zones occur at horizons A, B, and C. To the contrary, low velocity anomalies at 260-265 mbsf and 265-273 mbsf correspond to the upper BSRs (BSR-1) and lower BSRs(BSR-2), respectively. Direct measurements of the formation temperature for the top 170 m interval give the geothermal gradient of ~4.3°C/100m (Fig. 2). Extrapolation of the gradient gives the theoretical base of gas hydrate stability (BGHS) at ~230 mbsf, surprisingly ~35 m shallower than the base of gas hydrate bearing sands (horizon C) and BSR-1.

Discussion and Conclusions:

Integrated geochemical and geophysical evaluations of gas hydrates have revealed discrepancy between the BGHS and observed base of gas hydrate zones and confirmed a regional distribution of double BSRs. These findings raise questions concerning the stability and dynamics of the subsurface gas hydrates in an active margin. Discrepancy between the theoretical BGHS and BSRs was documented in the Blake Ridge during ODP Leg 164 (Paull and Matsumoto, 2000) and Leg 204 (Shipboard Scientific Party, 2003), in which BSRs occur 10 to 80 m shallower than BGHS. The discrepancy may reflect thermal conductivity anomaly and/or lower concentration of dissolved methane (Blake Ridge) or upward migration of warmer deep-seated fluids and dissociation of gas hydrates. The discrepancy in the eastern Nankai Trough is different from these sites, perhaps reflecting unique tectonic setting, and probably related with the regional occurrence of double BSRs. Base of gas hydrate stability could occur at different depths for gas hydrates with different gas composition. However, gases from the sites are nearly 100% methane. The double BSRs in Nankai Trough are thought to reflect the shoaling of the BGHS caused by relative sea level drop, probably by tectonic uplifting (Foucher et al., 2002). Likewise, the discrepancy between BGHS and BSR-1 may suggest a recent rapid uplifting of the fore-arc basin and shoaling of BGHS. Shoaling of the BGHS would cause massive dissociation and re-deposition of gas hydrates at the basal part of the stability zone (recycling of hydrate methane). Rapid tectonic uplifting is thought to be the major process to accumulate gas hydrates, perhaps forming economically important hydrate deposits.

References:

Foucher, J. P., Nouze, H., and Henry, P., 2002, Observation and tentative interpretation of a double BSR on the Nankai Trough. Marine Geology, 187, 161-175.

Matsumoto, R., Tomaru, H., and Lu, H., 2004, Detection and Evaluation of Gas hydrates in the Eastern Nankai Trough by Geochemical and Geophysical Methods. Res. Geol., 54, 1, 53-69.

Paull, C. and Matsumoto, R., 2000, Leg 164 gas hydrate drilling: An Overview. In Paull, Matsumoto, Wallace, et al., Sci. Results, Volume 164, College Station, TX (Ocean Drilling Program), 3-12.

Shipboard Scientific Party, 2003, Leg 204 summary, In Trehu, a. M., Bohrmann, G., Rack, F. R., Torres, M. E., et al., proc. ODP, Init. Repts., 204: College Station TX (Ocean Drilling Program), 1-75.