砂岩の孔隙の性質

その2. 孔口径分布曲線解析の天然砂岩への応用

抄録

Pore-size distribution of natural sandstones was analyzed by mercury poro-simetry. Cumulative pore-size distribution curves drawn on the log-probability diagram can be regarded as consisting of two to five linear segments which reflect the porosity and petrographic types of Sandstones. Effects of grain size, sorting, roundness, matrix and porous mudrock fragments on the cumulative curves were experimentally elucidated by UCHIDA and TADA (1991) with investigating five series of artificially packed sands as follows: (1) ES series composed of equigranular glass beads, (2) VS series composed of variably sorted glass beads, (3) BS series bimodally composed of two mixtures of sand-size glass beads and either 4-phi glass beads or normally distributed matrix-size glass beads, (4) VR series composed of mixtures of equigranular sphere glass beads and angular quartz grains, and (5) PRF series composed of equigranular glass beads and porous mudrock fragments.
Natures of pores in various petrographic types of natural sandstones, such as Mississippian and Tertiary quartz arenites, Miocene lithic arenites and Miocene lithic wacke, were estimated from the cumulative pore-size distribution curves based on the experimental results of UCHIDA and TADA (1991). As a matter of fact, cumulative distribution curves of these sandstones are regarded as consisting of two to five linear segments, which designate the framework distribution and the toroidal void distribution of intergranular porosities, the framework distribution and the toroidal void distribution of matrix and the intragrain distribution of lithic fragments of mudrocks including in sandstones, respectively.
The gradient and position of the segments are influenced by grain size, sorting and porous mudrock fragments as well as by porosity types. Consequently, shapes of the cumulative curves should correspond to rock types of sandstones. Thus, porosity properties, such as the intergranular porosities, matrix porosities and intragrain porosities of mudrock fragments can be quantitatively estimated from the pore-size distribution analyses.