Page 37 - Petrophysics

MINERAL CONSTITUENTS OF ROCKS-A REVIEW 11

reactions that are responsible for rock degradation. One of the processes
that takes place is leaching, which is the transfer of chemical constituents
from the rock to the water solution. Some minerals react directly with the
water molecules to form hydrates. Carbonic acid, formed from biogenic
and atmospheric carbon dioxide dissolved in water, plays an important
role in the chemical weathering process by reacting with the minerals to
form carbonates and other minerals such as clays. The feldspars react with
carbonic acid and water forming various clays, silica, and carbonates, as
illustrated in the reaction below for potassium feldspar:

2KAlSijO8 + H2COj + H2O + AlzSi05(0H)4 + 4SiO2 + K2CO3

The sedimentary deposits that make up the large variety of rocks are
continually altered by tectonic activity, resulting in deep burial of
sediments in zones that are undergoing subsidence. Uplift of other
areas forms mountains. The continual movement and collisions of conti-
nental plates cause folding and faulting of large blocks of sedimentary
deposits. This activity forms natural traps that in many cases have
accumulated hydrocarbons migrating from the source rocks in which
they were formed. The geologic processes of sedimentation, subsidence,
compaction, cementation, uplift, and other structural changes occur
continuously on a gradual scale and are intrinsically associated with
the physical properties of the rocks as well as the migration and
accumulation of hydrocarbon reserves. The physical properties of rocks,
such as density, rate of sound transmission, compressibility, and the
wetting properties of fluids, are the consequence of the mineral compo-
sition of the rocks. Thus the basic materials that make up the rocks
and their chemistry are associated with the petrophysical characteristics
of rocks.

Siltstones (mud-rocks)
Quartz grains (originating from weathering of igneous and meta-
morphic rocks) are very hard; they resist further breakdown, but are
winnowed by currents of winds and water and distributed according to
size. Larger grains accumulate as sandstones, and grains having an average
size of 15 pm mix with clays and organic materials in turbulent aqueous
suspensions that are transported and later deposited in quiet, low energy,
valleys from flooding rivers, lakes, and the continental shelves. Tidal
currents on the continental shelves effectively sort the grains of sand,
silt, and clay once more until they settle in quiet regions, forming very
uniform thick beds. Bottom-dwelling organisms burrow through the
mud, kneading and mixing it until the depth of burial is too great for
this to happen. The material then undergoes compaction and diagenesis,
with the clay minerals changing composition as they react with chemicals