Page 117 - Petrophysics

90 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

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Figure 3.1. Collection of (a) different sized and shaped sand grains and (b) spheres
illustrating a cubic packing of three grain sizes.

lithification and during rock alteration by circulating groundwater.
The process is essentially that of filling void spaces with mineral material,
which reduce porosity. Cementing materials include: calcium carbonate,
magnesium carbonate, iron carbonate, iron sulfides, limonite, hematite,
dolomite calcium sulphate, clays, and many other materials including any
combination of these materials.

(c) Amount of compaction during and after deposition: Compaction tends
to lose voids and squeeze fluid out to bring the mineral particles
close together, especially the finer-grained sedimentary rocks. This
expulsion of fluids by compaction at an increased temperature is the
basic mechanism for primary migration of petroleum from the source to
reservoir rocks. Whereas compaction is an important lithifying process
in claystones, shales, and fine-grained carbonate rocks, it is negligible
in closely packed sandstones or conglomerates. Generally, porosity
is lower in deeper, older rocks, but exceptions to this basic trend
are common. Many carbonate rocks show little evidence of physical
compaction.

(d) Methods of packing: With increasing overburden pressure, poorly
sorted angular sand grains show a progressive change from random
packing to a closer packing. Some crushing and plastic deformation of
the sand particles occur.