天草および北西九州における岩石の高圧物性とその石油地質学的意義

抄録

Amakusa Well, a kind of exploratory drilling by the governmental base was drilled to depth of 2, 807m in the Upper Amakusa Island in 1972, from which the authors were given 8 kinds of core samples. In addition, 7 more kinds were collected from the surface localities in the Upper and the lower Amakusa lslands. These dry samples consisting of 10 sandstones and 5 shales ranging Eocene and Cretaceous in age were tested under high confining pressure up to 1, 500 bars. The experimental results are shown in Figs 2-10. Most sandstones are very hard, compact, medium alkose ones. Their porosity ranges between 0.4 and 3.6%. Under high pressure, they exhibit brittle behavior. The shales have also small value of porosity, from 1.4 to 2.5%, and they are brittle up to 1, 500 bars confining pressure.
Then, strength coefficient b, an indicator of physical potentiality determined by the following formula (Hoshino and Inami, 1974) was analysed.
n=A_0e^-bσ_s
where, n, σ_s, and A₀ are porosity, strength and a constant respectively. The strength coefficient for Amakusa was studied in comparision with that for northwestern Kyushu and Shikoku. It is indicated that the b value in both Amakusa (later Cretaceous) and northwestern Kyushu (OligoMiocene) are nearly normal, while in Shiratake Sandstone (Eocene) it is higher than normal. The meaning of the strength coefficient was further discussed in relation of geological setting of each sedimentary basin.