秋田油田における異常高圧層の発達

抄録

In diagenesis of mud deposits, abnormal high pressures are expected at certain depths and stages of sedimentation. The high pressures are considered to be due to excess free water released from rigid ice layers of clay and zeolitic minerals in alterative temperature and overburden pressures. In addition to the process which schemed by W. D. KELLER as montmorillonite plus potassium to illite and glauconite mica, the alteration of vitric tuff to clinoptilolite and clinoptilolite to analcime were very common in the Funakawa Formation of the Akita oil-field. High pressure mudstones were investigated by core analysis, well-log analysis, X-ray measurements and bottom-hole evidences. A reservoir is built up with fluid migrated into porous media along innumerable of fractures and pore spaces opened by excess high pressures and extensional stresses in the muddy rocks, of which pre- and post-alteration mudstone close and hold the containing pressure. Calculations for several models of high pressure domain show that reservoir maintenance was continued for a period of million years for an ordinary impermeable cap rock.