Uchida, Takashi¹, Amane Waseda¹ 
(1) JAPEX Research Center, Chiba, Japan

ABSTRACT: Pore Properties of Gas Hydrate-Bearing Sandy Sediments: Geological and Geochemical Implications for Subsurface Occurrence of Natural Gas Hydrate

Plenty of gas hydrate-bearing sand core samples have been successfully retrieved from the two Mallik wells at Mackenzie Delta as well as the Nankai Trough wells, allowing for various laboratory analyses and physical measurements. The anomalies of chloride contents in extracted pore waters, core temperature depression, core observations as well as visible gas hydrates confirm the presence of pore-space hydrates as intergranular pore filling within moderate to thick sand layers, which clarified the characteristics of subsurface natural gas hydrate beneath the deep sea floor and the permafrost zone. It is noted that there are many similarities in appearance and occurrence between Mallik and Nankai Trough with observations of well-interconnected and highly saturated pore-space gas-hydrate within sandy sediments. It is necessary for evaluating subsurface fluid flow behaviors to know both of porosity and water permeability of gas hydrate-bearing sediments. Mercury porosities and pore-size distributions were obtained by the mercury porositmetry, which revealed that porosities of gas hydrate-bearing sandy strata range from 25 to 40 %. Measurements of water permeability were conducted for gas hydrate-bearing sand core samples indicating that highly gas hydrate-saturated sands may have permeability of a few milidarcies. Hydrate saturations are up to 80 % in pore volume throughout most hydrate-dominant sand layers. Muddy sediments such as siltstone and claystone usually contain low concentrations or less. It is suggested that the distribution of a porous and coarser-grained host rock is one of important factors to control the occurrence of gas hydrates, as well as physicochemical conditions.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.