新潟県の天然ガス鉱床の研究 II

地層圧と累層境界の関係

抄録

In Part I of this paper, the writer discussed the characteristics of natural gas and water which have close relationships with formations.
Part II deals with the relationships between the formations and pressures.
(1) Static pressure of reservoirs (Ps)
The writer considers the value; [Ps (ksc)/Depth (m)]×10. It ranges from 1.00 to 1.20 in the case of normal pressure, and it amounts to more than 1.20 in the case of abnormal pressure.
In Higashi-Niigata Gas Field, the values of gas reservoirs in Nishiyama formtion are 1.00-1.14, but the values in Shiiya formation are about 1.30. In Mitsuke Oil Field, whereas the values of oil reservoirs in Shiiya formation (shallow oil zone) are 1.05-1.15, the values in Nanatani formation (deep oil zone) are 1.35-1.50.
(2) Δt sh^* by Sonic logging
From Sonic logging data (Δt sh), porosities of mudstones can be calculated, and they are the indices of compaction or pressure. Usually crossplots of Δt sh versus depth are made and the tendencies of formation pressure increases not only for reservoirs but also for sandstones and mudstones in uncompleted wells can be obtained.
The writer makes such crossplots for Hirakida, Higashi-Niigata Gas Field and Mitsuke Oil Field. As a result, it is observed that the tendencies of formation pressure increases are nearly constant in one formation, and often change near the formation boundaries.
(3) The formation boundary is usally a geological idea based on the rock facies and fossils, but from the standpoint of petroleum geology, it clearly contains physical and chemical contents, that is, the characteristics of gas and water and the tendencies of formation pressure increases are often different in each formation.
^* Δt sh: velocity of sound wave to traverse mudstones.
(microseconds/foot)