基礎試錐「佐渡南西沖」の椎谷層砂岩への新たな物理検層解析法の適用

抄録

METI Sado Nansei Oki Wells, drilled in the deep water Toyama Trough in the Sea of Japan offshore Niigata Prefecture, Japan, encountered the Pliocene Shiiya Formation that contains 15 meters of oil column. Sandstones of the Shiiya Formation are turbidite origin and with varieties of lithology. Fine grained and poorly sorted lithic arenite is dominated in the upper part, and fine grained and poorly sorted lithic wacke is dominated in the lower part of the formation. Due to fine grained and poorly sorted nature, and tuffaceous or muddy matrix, pore size in the wacke varies substantially.
Combinable Nuclear Magnetic Resonance (CMR) logging, which measures three different sizes of porosities, namely clay bound water porosity (pore occupied by clay-bound water), capillary bound fluid porosity (pore occupied by capillary-bound fluid) and free fluid porosity (pore in the center of medium to large size pores where fluid can flow), is very useful to characterize the sandstone in the Shiiya Formation.
Complicated lithology of the Shiiya sandstone renders the formation evaluation a challenging task at METI Sodo Nansei Oki S Well. The resistivity of the oil reservoir wacke interval is very low, which is almost same with the arenite interval with no oil shows at all.
Formation evaluation using CMR (CMR ELAN) revealed that the oil reservoir wacke contains up to 15% of capillary bound fluid porosity while free fluid porosity is 8∼14% at best. Although oil saturation is not high enough for the total porosity, the effective oil saturation is quite high if all oil is assumed to be confined in the free fluid pore. High oil saturation within free fluid pore with a large amount of clay bound water and capillary bound fluid is a reasonable explanation for the oil bearing but very low resistive wacke interval.