Page 40 - Petrophysics

14 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

is composed of accumulated deposits of skeletal or shell remains
of microscopic animals; and coquina is fossiliferous limestone composed
almost entirely of fossil fragments cemented by a calcareous mud.
Dolomite forms in areas where sea water has been restricted, or
trapped, by land enclosure where the concentration of salts increases
due to evaporation. As the concentration of magnesium increases it reacts
with the calcite that has already been deposited to form dolomite by the
following reaction:

2CaCO3 + Mg2+ += CaMg(C03)~ + Ca2+
In some cases the limestone formations are changed to dolomite by
reaction with magnesium which is dissolved in water percolating through
pores and fractures in the limestone. Porous carbonate rocks derived
from chemical and biogenic precipitation of calcium carbonate form
a large portion of the petroleum reservoirs [ 13 .
J

Evaporites
Evaporites are salts that are deposited in isolated marine basins by
evaporation of the water and subsequent precipitation of salts from the
concentrated solutions. Salt Lake in Utah, United States, and the Dead Sea
in the Middle East, are examples of lakes that are gradually forming beds
of evaporites as the water evaporates. Anhydrite (CaSO4), sodium halite
(NaCl), sylvite (KCl), and other salts are associated with evaporites.
Table 1.5 contains a general description of the rocks that have been
discussed. The principal rock-forming minerals are feldspars, olivine,
pyroxene, amphibole, mica, and quartz. Almost all coarse-grained rocks
contain feldspars. There are three feldspars: calcium-, potassium-, and
sodium-aluminum silicates. Other descriptive names that are used for
them are placed in parentheses.

PROPERTIES SEDIMENTARY PARTICLES
OF

There are a large number of tests that can be made to obtain
quantitative and qualitative data for characterization of sedimentary
rocks. All of the methods listed in Table 1.6 are discussed in various
locations in the book and can be found by reference to the Index. The
loose particle analyses are made on disaggregated rock particles that
are obtained using a crushing apparatus, or by carefully breaking the
rock with a hammer. The other analyses are obtained from core samples
of rock which are oriented parallel to the bedding planes. Tests of the
vertical fluid flow properties can be useful for analyses of gravity drainage
of oil, vertical diffusion of gas released from solution, and transport