Page 40 - Applied Petroleum Geomechanics

Rock physical and mechanical properties 31

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1.08 g/cm in sedimentary basins. For example, in general cases, r f ¼
1.07 g/cm 3 in the Gulf of Mexico; r f ¼ 1.02 g/cm 3 in the Rocky
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Mountains; r f ¼ 1.08 g/cm in the Niger Delta; and r f ¼ 1.02 g/cm in the
North Sea.
Tables 2.1 and 2.2 list the matrix densities for common minerals (Schön,
1996) and bulk density for various rock types. For most rocks and soils, the
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matrix density is about 2.65 g/cm , roughly the density of quartz and clay
minerals (Table 2.1). Therefore, bulk density of a rock can be calculated
from Eq. (2.1) if the formation porosity is known. Fig. 2.1 shows an
example of bulk density variations with depth below the sea floor in the
Gulf of Mexico. The densities in the figure were calculated from core
porosity data (Ostermeier et al., 2001) using Eq. (2.1). It shows that bulk
density of the formation is very low at the shallow depth (<100 m below
the sea floor), and this mostly is due to that the formation is unconsolidated
with very high porosity. Bulk density increases as the burial depth increases
because formation compaction causes porosity reduction with depth.
Rock bulk density can be measured in the laboratory using core sam-
ples. For subsurface rocks, it is more convenient to obtain the bulk density
from density log in a borehole. However, density log data, particularly the

Table 2.1 Average matrix densities in common minerals and fluids.
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Mineral or fluid Density r m (g/cm )
Quartz 2.65
Calcite 2.71
Dolomite 2.87
Montmorillonite 2.06
Illite 2.64
Kaolinite 2.59
Chlorite 2.88
K-Feldspar 2.56
Biotite 2.90
Halite 2.165
Anhydrite 2.96
Sylvite 1.99
Plagioclase (Na) 2.62
Plagioclase (Ca) 2.76
Barite 4.48
Oil 0.85
Fresh water 1.0