Compaction and Migration of Fluids in Miocene Mudstone, Nagaoka Plain, Japan

The main hydrocarbon reservoirs in Nagaoka Plain, Japan, are volcanic and pyroclastic rocks of the Nanatanian-Nishiyaman Stages (Neogene). The migration of hydrocarbons seems to have occurred from adjacent sedimentary source beds to the volcanic reservoirs, facilitated by differential compaction. Therefore, it is possible to determine the directions of the compaction currents by using horizontal porosity distributions of covered mudstone bodies. The differential compaction maps of the mudstone source beds overlying the reservoirs may indicate the directions of hydrocarbon migration, and may suggest the location of hydrocarbon accumulations in Nagaoka Plain. As fluid is expelled from pores of the mudstone, compaction occurs and porosity decreases. However, if fluid expulsion is inhibited because of low permeability, mudstone compaction may not be great and a high porosity-high fluid pressure situation will result. With such a low-permeability mudstone acting as a barrier to fluid expulsion, compaction currents could move downward in the level below the low-permeability zone and upward above that zone. The volumes of the compaction currents are estimated from the vertical porosity distributions and the porosity differences before and after compaction. The resultant figures are discussed in connection with the possible hydrocarbon accumulations. There are some abnormal pressures in the reservoirs of the Fujikawa, Kumoide, and Mitsuke gas and oil fields in the plain, whereas normal hydrostatic pressures are observed in the reservoirs of such fields as Katagai, Nishi-Nagaoka, and Higashi-Sanjo. This suggests the presence of permeability barriers in the volcanic reservoir rocks. The depths of the volcanic reservoirs and the thicknesses of the sealing mudstone beds are greater at Fujikawa, Kumoide, and Mitsuke, where abnormal pressures are found, than at Katagai, Nishi-Nagaoka, and Higashi-Sanjo, where reservoir pressures are hydrostatic. Therefore abnormal pressures in this district may be ascribed to the thick mudstone seals and overburden pressures.