Science practice 126

Oil and natural gas stores are formed naturally in reservoirs. However, these reservoirs are not like the giant man-made lakes that might come to mind; they are underground. In fact, a reservoir occurs in a rock that looks on the surface like any other. The oil and gas pool in millions of tiny pores in the rock. Porous rocks include sandstone, limestone, shale, and pumice. There are several factors affecting porosity (the ability to store fluid in the open spaces located between rock particles). Among these factors are the nature and characteristics of the sand and other sediment that accumulate to form rocks. The sand particle size is not critical to porosity, but the uniformity or sorting of the sand grains greatly affects porosity. Uniform or well-sorted sand will tend to be more porous than poorly sorted sand.

Experiment 1

In a laboratory, a scientist wanted to determine the porosity of four samples of different rocks. The scientist recorded the mass of the rock samples prior to the experiment. To measure porosity, the samples were soaked in water and the mass of the samples was recorded periodically. A highly porous rock would have a significant increase in mass the longer it soaked in water, eventually reaching the point at which it could no longer absorb any more water (saturation point). The data was recorded in Table 1.

Experiment 2

Geologists in Arizona were interested in the porosities of different varieties of sandstone. They documented the relationship between porosity, density, sand particle size, and the uniformity or sorting of the sand grains. The geologists found that, in general, neither density nor particle size affected the porosity if the particles were uniform. However, they did find that in samples with a wide variety of particle sizes, the porosity was generally lower. Also, the presence of contaminants such as clay or silt negatively affected the porosity. The geologists recorded their results in the following diagrams.