Page 72 - Geology and Geochemistry of Oil and Gas

48 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

where DZ ¼ the free energy of reactions; n ¼ the number of electrons taking part in
the reaction; F ¼ the Faraday constant (96,520 C/g-eq.); and y ¼ the constant for a
given reaction.
The above equation shows that as alkalinity of the environment declines, a less
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reducing environment is needed for the reduction of the same components. It
appears that a strongly alkaline environment is not necessary and may even be
detrimental for the transformation of organic matter into hydrocarbons. Minerals
participating in the processes have significantly different pH and Eh stability limits.
For instance, at a stage of syngenesis and early diagenesis (po10 MPa, To60–701C),
the pH stability limits for various minerals are (after Minskiy, 1975, pp. 147–150):

anhydrite 8.4 calcite 7.8–8.7
halite 7.8 kaolinite 3.5–5.6
hematite 7.4 quartz 7.8
illite 7.8 montmorillonite 7.8–8.2
clay 8.7 pyrite 5.0–6.2
dolomite 8.4 siderite 6.2–7.2

Besides the pressure and temperature, postdepositional processes are affected by a
number of factors, which include the appearance and redistribution of the cement,
changes in the structural (textural) orientation of rock-forming minerals, and
dissolution of minerals unstable in a given environment. The intensity of these
processes is affected not only by the aforementioned factors, but also by the
permeability at a given time. As an example, Fig. 3.8 shows relationship between
the permeability of quartz sands and clay mineral content (Klubova, 1984, p. 151).
The most drastic changes in the permeability occur when the clay content increases
within just a few percentage points. Further increase in clay content between 10%
and 80% does not significantly affect the sand’s permeability.
As indicated previously, rock properties are significantly affected by the
formation of authigenic minerals. Newly-formed authigenic minerals reduce the
porosity and permeability due to fineness of grains and swelling of some minerals.
A significant role in the postdepositional alterations of carbonate rocks belongs to re-
crystallization and dissolution. These processes usually result in increased porosity. In
some cases, however, the emerging pores are easily filled-up with secondary (authigenic)
minerals, mostly calcite and dolomite. In 1954, Chilingar and Terry have shown that
dolomitization of limestones creates additional porosity (up to the theoretical value
of 12.1%) (also see Chilingarian et al., 1992, 1996). Some researchers (Minskiy, 1975,
p. 27) question this because dolomitization is occurring along the grain contacts. Actual
field data, however, supports the findings of Chilingar and Terry (1954).
Reservoir–rock properties of carbonates are substantially affected by the presence
of organic matter, which sometimes gives rise to laminated texture. Organic matter

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Oxidation is a reaction in which the electrons are lost, whereas reduction is a reaction where electrons are
gained.