330 Subsurface Geologic Methods 



part, by tectonic movements. Conglomerates, sandstones, limestones, and 

 most chemical precipitates show very little reduction in rock volume as 

 result of gravitational pressures. However, the fine-grained, argillaceous 

 sediments show maximum volume reduction of more than 80 percent. In- 

 asmuch as a large percentage of the sediments of the earth's crust are 

 composed of clays and shales, their compaction is of special interest to 

 the geologist. Reduction of rock volume by compaction is related to the 

 weight of overburden and resultant reduction of porosity, and possibly, to 

 a certain extent, age of the sediments, and tectonics. Compaction therefore 

 results in density increase and porosity decrease. 



Investigation by several workers, notably Athy ^^ and Hedberg,^^ has 

 shown the relationship of increased density with greater depth of burial. 



Definitions 



Several terms used in shale-density analysis work require clarifica- 

 tion to form a basis for discussion on this subject. Adopting, in general, 

 the nomenclature of Hedberg,^^ these may be described as follows: 



Bulk Density (rock density, lump density) is the density of the 

 thoroughly dry rock, that is, the rock with pore space free of liquids. Hed- 

 berg's samples were weighed in air, coated with paraffin and weighed in 

 water. Corrections were made for temperature of water and water con- 

 tent of the sample. The water content was determined from the difference 

 in weight of the powdered sample before and after thorough drying at a 

 temperature of 110 to 120 degrees C. This is also sometimes called "dry 

 density," particularly when applied to naturally dried core samples. 



Grain Density (mineral density, absolute density) is the density of 

 the constituent particles of a rock, that is, the rock substance free from 

 pore space. 



Natural Density is the density of the rock with all pore space filled 

 with water which is assumed to be the usual condition found in nature. 

 A true natural density of a shale sample is, in most cases, essentially im- 

 possible to obtain with complete accuracy as there is a certain amount of 

 loss of density due to dissipation of fluid or gases when a core sample is 

 brought to the surface in a well. This is particularly true at the greater 

 depths, where subsurface pressures are very high. However, a result 

 approaching natural density can be obtained with fair accuracy if the 

 sample is weighed immediately after extraction from the core barrel. 

 Some geologists refer to this as "wet density" inasmuch as the true 

 "natural density" can be most nearly obtained from wet core samples 

 while they are still saturated with subsurface fluids. 



Apparent Specific Gravity. B. C. Refshauge, in a private report, used 

 the term "apparent specific gravity" as the specific gravity of a porous 



^^ Athy, L. F., Density, Porosity and Compaction of Sedimentary Rocks: Am. Assoc. Petroleum 

 Geologists Bull., vol. 14, pp. 1-24, 1930. 

 3" Hedberg, Hollis D., op.cit., 1936. 

 ^"Hedberg, Hollis D., op. cit., p. 252, 1936. 



