Journal of the Japanese Association for Petroleum Technology

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Porosity of rocks comprises intergranular and fracture porosities. But the latter has been ignored in most measurements.
Fracture porosity may be estimated for a fracture set by using fracture density or the number of fracture planes in a unit length in the direction that is perpendicular to the planes, if the set is distinguished with the study of fracture analysis. If, for the fracture set that has dip Δ and the angle θ to the measured line, n fractures are measured in the length of D, fracture density c and average interval of fractures a are given in the formula below.
_??_ ……(1)
Errors in the estimations of fracture density are produced at two procedures, statistical process on the Schmidt Net and computation using formula (1). In the former, errors are given in formula (2) and Table 1: in the latter, those which are mainly produced by θ and Δ are given in formula (3) and Table 2. Total errors are estimated to be less than 20%, when the results using θ or Δ of less than 30° are excluded from consideration.
Fracture porosity PØ is equal to a total spaces made by fracturing in a unit volume and is given in the formula below. (See Fig. 7)
_??_ ……(6)
where, h is the width of fractures. Formula (6) must be applied, when the value of c is proved not to be varied through an extent as the circular area whose diameter is more than 2 r, where r equals 15 a and 15/c.
The calculation was examined in the Hiki Hills. The density was computed at 11 localites and for four kinds of fracture set and three kinds of fracture, with the results shown in Tables 4, 5, 6 and 7. In summary, the most probable values of the density may be given in Table 8, in which the values are to be doubled on the most deformed rocks.
By using the data of density of Table 8 and presuming the average width of fractures as in the top of chapter 5, fracture porosity in the studied area was computed with formula (6). In the results, the porosity ranges from 0.1 to 1.5% for most rocks, and it is hardly more than 4% at the most when the rocks are composed of alteration of shale and sandstone and are affected by the strongest deformation.

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The writer examined micropaleontologically cores and cuttings from the test wells drilled in the Hodogaya area, Yokohama City. The stratigraphic sequences of each wells are shown in the figure 1. On the other hand, the stratigraphic sequence of the Miura peninsula has been divided microbiostratigraphically as showing in the table 2. From the aid of Foraminifera, the following conclusions was obtained.
1. The Miocene sediments (Zushi, Ikego and Urago formations) of the both area are represented by Nonion zonule and their environmental conditions seem to be nearly equal.
2. The Pliocene sediments of the Hodogaya area and Miura peninsula are represented respectively by Bulimina zonule and Elphidium zonule.
3. The pelagic forms, which are biostratigraphically more valuable than the benthonic forms, show the same vertical distribution in both area.

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