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Two versions of the table of physical properties are given below; the second shows the comparison of the electron density seen by the density log versus the true density, as well as the chemical formulae of the minerals.
This is followed by a table of the elastic properties of minerals. Igneous rocks are covered separately at the bottom of this page..
Metamorphic Rock Properties
The quality of the rock is based on the amount of heat and pressure applied to it during the metamorphic processes. Changes That Occur during metamorphism are:
Specific sedimentary rocks become specific metamorphic rocks, as shown below:
Igneous rocks are classified in several ways – by composition, texture, and method of emplacement. The composition (mineral mixture) determines the log response. The texture determines the name used for the mineral mixture, and the method of emplacement determines the texture and internal porosity structure (if any).
Intrusive igneous rocks are formed inside the earth. This type of igneous rock cools very slowly and is produced by magma from the interior of the earth. They have large grains, may contain gas pockets, and usually have a high fraction of silicate minerals. Intrusions are called sills when lying roughly horizontal and dikes when near vertical.
Extrusive igneous rocks form on the surface of the earth from lava flows. These cool quickly. They have small grains and contain little to no gas.
Both intrusive and extrusive rocks may contain natural fractures from contraction while cooling, and may have carried non-igneous rocks with them, called xenoliths.
Intrusive rocks may alter the rocks above and below them by metamorphosing (baking) the rock near the intrusion. Extrusives only heat the rock below them, and may not cause much alteration due to rapid cooling. Extrusives can be buried by later sedimentation, and are difficult to distinguish from intrusives, except by their chemical composition and grain size.
The mineral composition of an igneous rock depends on where and how the rock was formed. Magmas around the world have different mineral make up.
Felsic igneous rocks are light in color and are mostly made up of feldspars and silicates. Common minerals found in felsic rock include quartz, plagioclase feldspar, potassium feldspar (orthoclase), and muscovite. They may contain up to 15% mafic mineral crystals and have a low density.
Mafic igneous rocks are dark colored and consist mainly of magnesium and iron. Common minerals found in mafic rocks include olivine, pyroxene, amphibole, and biotite. They contain about 46-85% mafic mineral crystals and have a high density.
Ultramafic igneous rocks are very dark colored and contain higher amounts of the same common minerals as mafic rocks. They contain about 86-100% mafic mineral crystals.
Intermediate igneous rocks are between light and dark colored. They share minerals with both felsic and mafic rocks. They contain 15 to 45% mafic minerals.
Plutonic and volcanic rocks generally have very low porosity and permeability. Natural fractures may enhance porosity by allowing solution of feldspar grains. Some examples with average porosity as high as 17% are known.
Tuffs and tuffaceous rocks have high total porosity because of vugs or vesicles in a glassy matrix. This is most common in pyroclastic deposits. Interparticle porosity may also exist. Some effort has to be made to separate ineffective microporosity from the total porosity. Pumice (a form of tuff) has enough ineffective porosity to allow the rock to float! When other minerals fill the vesicles by precipitation, the tuff is called a zeolite.
The numerical data below has worked well in igneous reservoirs using standard lithology models given earlier in this chapter (Mlith-Nlith, DENSma-Uma, etc).
The table is in English units. If you work in Metric units, divide neutron values by 100, multiply density by 1000, and multiply sonic by 3.281.
All these values have a moderate range (+/- 10%) and some tuning may be necessary. Don’t forget to metricate the numbers if needed. Use these matrix values in the matrix density or PE crossplots shown earlier.
Since a typical log suite can solve for 3 or 4 minerals at best, you need to chose the dominant minerals and zone your work carefully. If you have additional useful log curves, you might try for more minerals or set up several 4 mineral models in a probabilistic solution. A good core or sample description will help you choose a reasonable mineral suite.
Sometimes lithology is determined by triggers. For example, where basalt beds are interspersed between conventional granites or quartzites, it is easy to use the PE or density logs to trigger basalt, leaving the remaining minerals to be defined by a two or three mineral model. This approach is widely used in sedimentary sequences to trigger anhydrite, coal, or salt.
Two crossplots are useful for rock identification in metamorphic rocks, as shown below. The math for running two and three mineral models was shown earlier in this Chapter.