Characteristics of Pore-Size Distributions of Sandstone Reservoir Rocks and Their Relationships to Permeability

Takashi Uchida

Cumulative pore-size distribution curves of sandstones determined by mercury porosimetry always consist of a few segments of normal Gaussian distribution when they are drawn on a log-probability diagram. Numerous mechanically packed assemblages were prepared to elucidate characteristics of segmentations from nonporous glass beads and porous shale fragments which represent constituent mineral grains and rock fragments of sandstones, respectively. These assemblages are classified into three groups. (1) Assemblages composed of sand-size glass beads. In addition to equal-size assemblages (1, 1.5, 3, 5 phi) and unequal-size assemblages (sorting is 0.42, 0.64, 0.89), assemblages of angular quartz grains are also examined to know the effect of angularity of grains. (2) Assemblag s of sand-size and matrix-size beads. Matrix-size beads are arranged in conformity with normal distribution in size. (3) Assemblages of sand-size beads and shale fragments. Proportions of shale fragments to beads are varied to define the effect of intragranular porosities of shale to total porosities.

According to these sphere-packed models, segments of normal distribution may be explicitly explained by (1) a framework distribution of intergranular pores built up by grain packing followed by a subordinate distribution of toroidal voids around grain contacts, (2) smaller size intergranular pores by matrix-size grains, and (3) intragranular pores contained in rock fragments. The same conclusions are obtained in Tertiary sandstones from the back arc of Japan.

Permeability of sandstones may significantly suffer from a framework distribution of intergranular pores built up by grain packing, and specific surface area may also prescribe permeability in addition to porosity and pore size.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.