H. Tomaru¹, R. Matsumoto¹, T. Uchida², and H. Lu³

¹University of Tokyo, Tokyo, Japan
²JAPEX Research Center, Chiba, Japan
³JNOC-TRC, Chiba, Japan

ABSTRACT: Hydrate Formation System from the Pore Water Analysis in Nankai Trough

Interstitial waters squeezed from the hydrate bearing sediments in Nankai Trough were analyzed to measure chloride concentration, and oxygen and hydrogen isotopic compositions. Dissolved anions, chloride concentrations were anomalously low while oxygen and hydrogen are enriched in heavy isotopes, reflecting that the hydrate-forming waters exclude Cl^- and I^-, and concentrate ¹⁸O and ²H (D) during hydrate formation. We calculated the amount of hydrate in pore space (hydrate saturation) using chloride and oxygen isotopic anomalies, and observed that the saturation values are significantly higher than those estimated from resistivity data (Ro / Rt) of well-logging. We also identified that oxygen isotopic anomalies are not consistent with chloride anomaly; that is, isotopically heavy oxygen is not always associated with low chloride anomaly, and even in the hydrate free depth by chloride, oxygen isotope shows small positive shifts (1‰). The discrepancy seems to suggest a different behavior between chloride and oxygen during hydrate formation.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado