砂岩の孔隙の性質 その１． 実験的検討

Abstract

Cumulative pore-size distribution curves of natural sandstones analyzed by mercury porosimetry generally consist of two to five linear segments on the log-probability diagram, which may reflect the nature of porosity and petrographic types of sandstones. In order to clarify the significance of these segmentations in pore-size distribution curves of natural sandstones, effects of grain size, sorting, roundness, presence of matrix and porous mudrock fragments were analyzed for artificially packed sands as follows: (1) equigranular glass beads (ES series), (2) variably sorted glass beads (VS series), (3) bimodal mixtures of sand-size glass beads and either 4-phi glass beads or normally distributed matrix-size glass beads (BS series), (4) mixtures of equigranular sphere glass beads and angular quartz grains (VR series), and (5) equigranular glass beads and porous mudrock fragments (PRF series).Cumulative curves of ES, VS and VR series consist of two to three segments corresponding to the framework distribution attributed to the main part of intergranular pores built up by grain packing and the toroidal void distribution attributed to the subordinate part of intergranular pores around grain contacts. Gradients of the framework distribution segments decrease as sorting index increases. Effect of roundness on shapes of the cumulative curves is negligible. Two more segments appear in the smaller pore-size range of the cumulative curves of BS series, and are attributed to the framework and toroidal void distributions of matrix, respectively. In PRF series, porous mudrock fragments also yield an additional segment in the smaller pore-size range of the curve which is attributed to the intragrain pore-size distribution.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 correspond to rock types of sandstones. Thus, porosity properties can be semiquantitatively estimated from the pore-size distribution analysis.