A rapid, accurate, and continuous method for measuring porosity or bulk density of cores from boreholes has been developed and is described by H. B. Evans in a companion paper. The method is based on gamma-ray scattering. The principal technical advantage of this porosity measuring method is that variations in porosity encountered by a moving, pencil-sized gamma-ray beam are recorded continuously, whereas other methods yield only an average porosity for a given core piece or plug. Excellent agreement exists between porosities of common sedimentary rocks obtained by this method and other conventional methods. This agreement reflects the preponderance of Compton scattering for gamma-rays with energies of about one Mev and the close relationship of number of electrons per unit volume to density for most minerals common in sedimentary rocks. Porous rocks contain two or more "density phases". A "density phase" as defined here consists of one or more non-porous minerals or fluids having a characteristic number of electrons per unit volume. The density phases common in sedimentary rock are:
average silicates (quartz, plagioclase feldspar, unhydrated clay),
light silicates (sodium and potassium feldspars, some micas, and slightly hydrated clays),
calcite,
dolomite,
anhydrite,
gypsum and halite,
gas-filled pores (air or hydrocarbon), and
liquid-filled pores (water or oil).
If a rock contains two density phases, for example silicates and air-filled pores, the volumetric proportions of each phase can be directly determined if the densities are known. If a rock contains three density phases, the proportions of two phases must be independently measured or estimated and assigned an average density before the volumetric proportion of the third phase can be derived. This can commonly be done with enough precision to yield accurate porosities. Porosity or bulk density values are useful for evaluating rock as a fluid reservoir, for understanding depositional, compactional and diagenetic history of sediments, for estimating acoustic wave velocities or average density for geophysical purposes, for compositional analysis, for detecting density anisotropies, and as a sampling guide. Profiles measured by the gamma-ray device for a variety of rock types illustrate these applications.
