The Capillary Concentration Of Gas And 0il

FORMER studies of sedimentary strata have been based upon the mineralogical and mechanical characters of the solid components, rather than upon the open spaces between them. For present purposes let us take another point of view, and consider not the rock material, but rather its cavities, the pores and the fissures or cleaved joints which connect with them. Let us think of the geologic column not as a series of rocks, but as a mass of irregular connecting capillaries. Different parts of the column may be distinguished by the absolute size and by the relative amount of its open spaces. Porosity Each stratum of the column is characterized by a certain porosity, which would be independent of the size of the rock grains if all were of the same shape. Slichter has shown that the percentage of open space in a pile of equal spheres is practically constant regardless of the diameter of the spheres. This is approximately true of clean, assorted sand or gravel, but it is far from being the case with most natural sediments which have a widely variable porosity, the magnitude of which is determined mainly by the amount of fine and of secondary material between the larger grains. The shape of the grains materially affects porosity, round grains leaving more open space than angular or flat grains. The thin plates of kaolin and the fine plastic matter of clay fit quite perfectly when pressed together, and it is probably this close fit, determined by shape of grain, rather than the small size of a grain, that causes the low porosity of ordinary clay shale.