Page 39 - Petrophysics

MINERAL CONSTITUENTS OF ROCKS-A REVIEW 13

100% (Quartz)

\
Arkose Litharonite
sandstones sandstones
(Graywacke)

100% (Clastic feldspars and mica) 100% (Rock fragments)
Figure 1.2. l”he major classijkations of sandstones, based on composition [ll].

of fluids. The vertical permeability can be 50-75% less than the horizontal
permeability; therefore, any fluid flow experiments, or numerical
simulations, must account for the directional permeability.
Sandstones that originate from the cementation of wind-blown
sand dunes have bedding planes that are oriented at various angles
(cross-bedding). Cross-bedding also can be produced by ripples and
swirling currents in water while it is transporting the grains.
Clastic sediments transported to continental shelves by rivers are
subjected to wave action and currents that sort and transport the grains
over large distances. The sediments tend to form rocks that are quite
uniform in properties and texture over large regions. The deposits can
be several kilometers in thickness due to contemporaneous subsidence
of the zone during the period of deposition.

Carbonates
Carbonate rocks form in shallow marine environments. Many small
lime (CaO) secreting animals, plants, and bacterid live in the shallow
water. Their secretions and shells form many of the carbonate rocks. In
addition, calcite can precipitate chemically: calcite is soluble in water
containing carbon dioxide; however, if the amount of dissolved carbon
dioxide is decreased by changes of environmental conditions, or uplift,
the dissolved calcite will precipitate because it is only slightly soluble in
water free of carbon dioxide.
There are three major classifications of limestone (which is generally
biogenic in origin): oolitic limestone is composed of small spherical
grains of calcite (encapsulated fossils and shell fragments); chalk