4/7/2023 0 Comments Density equation![]() It also indicates the pressure dependence of rock properties.įig. ![]() This demonstrates why local calibration is needed. However, at about 3500 m, pore pressure rises and porosity actually increases with depth. ![]() Porosities for both shales and sands show the expected porosity loss with increasing depth in the shallow portions. This can retard or even reverse the normal compaction trends. The high pore pressure results in an abnormally low differential of effective pressure. Abnormally high pore fluid pressures ("overpressure") can occur because of: 2 = Po river valley mudstone (Storer ), 3 = average coastal Gulf of Mexico shales from geophysical measurements (Dickinson ), 4 = average coastal Gulf of Mexico shales from density logs (Eaton ), 5 = Marcaibo basin well (Dallmus ), 6 = Hungary calculated wet densities (Skeels ), 7 = Pennsylvanian and Permian dry shales (Dallmus ), 8 = Eastern Venezuela (Dallmus ).ĭifferential or effective pressures do not always increase with increasing depth. 1 = Gas saturated clastics: probable minimum density (McCulloh ). 2 – Shale density as a function of depth from several sedimentary basins (after Castagna et al. These kinds of curves are often fit with exponential functions in depth to define the local compaction trend.įig. The shapes and overall behaviors for these curves are similar, even though they come from a wide variety of locations with different geologic histories. 2, generalized densities as a function of depth for shales are plotted. Some of the textures resulting from these processes were seen in the photomicrographs of Rock types. Material may be dissolved at point contacts or along styolites and then transported to fill pores. In addition, diagenetic processes such as cementation work to fill the pore space. Crushing and fracturing is a common result. More force will be imposed on the grain contacts. As pressure increases, grains will shift and rotate to reach a more dense packing. This is expected, because differential pressures usually increase with depth. In general, density increases and porosity decreases monotonically with depth. Pore fluid densities are covered in detail in Pore fluid properties. These will typically have light densities similar in magnitude to those of coals. Reservoir rocks often contain significant amounts of semisolid organic material such as bitumen. This is a particular problem, because clays are among the most common minerals in sedimentary rocks. In this case, densities can vary 40% or more. The most problematic minerals are clays, particularly expanding clays (montmorillonite or smectite) capable of containing large and variable amounts of water. For example, in the plagioclase series, the density increases as sodium (albite, ρ = 2.61 g/cm 3) is replaced by calcium (anorthite, ρ = 2.75 g/cm 3). The density will change systematically as composition varies. Note in Table 2 that there are several densities reported for the same mineral group, such as feldspar or clay. Table 2 - Grain densities for common rock-forming minerals Grain densities for common rock-forming minerals are shown in Table 2. Given a porosity and specific fluid, density can be easily calculated if the mineral or grain density is known. 8 is a fundamental relation used throughout the earth sciences to calculate rock density. 4, 5, and 6 for a rock made up of two minerals, m 1 and m 2, and two fluids, f 1 and f 2, we find Where ρ mix is the density of the mixture ρ A is the density of Component A ρ B is the density of B A and B are the volume fractions of A and B respectively (and so B = 1− A).Įxpanding this into a general system with n components,įor example, exploiting Eqs. The density of a composite such as rocks (or drilling muds) can be calculated from the densities and volume fraction of each component. Where M s, M d, M sat, M b, and M fl are the mass of the solid, dry rock, saturated rock, buoyant rock, and fluid, respectively. Similarly, the definitions of the standard densities associated with rocks then follows: porosity for sandstone, limestone, and dolomite.
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