岩石磁気パラメータの石油探鉱への応用 : 北海道勇払地域, あけぼのSK-4D坑井定方位コアを例として

抄録

Rock magnetic property is utilized for oil exploration on the basis of three parameters : bulk magnetic susceptibility, remanent magnetization and anisotropy of magnetic susceptibility (AMS). Initial magnetic susceptibility obtained from non-oriented borehole samples is a crucial parameter for reliable modeling of geomagnetic anomaly. Information of remanent magnetization and AMS derived from oriented core samples provides us with insights into tectonic movement and microscopic fabric of reservoir, respectively.
As a case study, rock magnetic experiments are presented, using oriented granite cores obtained from the Akebono SK-4D borehole in the Yufutsu area, Hokkaido, Japan. The analyzed samples at the top of the Cretaceous basement are characterized by relatively low magnetic susceptibility and stable, easterly-deflected thermoremanent magnetization that implies tectonic rotation. Maximum axis (K₁) of AMS coincides with the intersection of two fracture systems within the granitic basement rocks, which implies the direction of maximum permeability. AMS parameters, reflecting microscopic alignment of minerals precipitated on fracture surface, may be useful to assess properties of fractured reservoir.