Page 38 - Petrophysics

12 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

in the contacting water. The compacted mud forms the siltstones and
beds of shale that are encountered throughout the stratigraphic column,
making up two-thirds of the sedimentary deposits. Where they overlie
hydrocarbon reservoirs, the compacted layers of mud provide seals for
the petroleum traps.
Beds of mud containing organic materials that are deposited in
anaerobic environments, such as swamps, form siltstones and shales
that are gray to black in color. Many of these are the source rocks of
petroleum hydrocarbons. Red deposits of mud were exposed to oxygen
during burial and the organic material was lost to oxidation while
iron compounds formed ferric oxide (Fe2O3) that produced the bright
red coloration. Brown muds underwent partial oxidation with iron
constituents, forming the hydroxide geothite [FeO(OH)] . If the mud does
not contain iron, it will exhibit the coloration of the clays (biotite,
chlorite, illite, etc.) that range in color from beige to green.

Sandstones
The quartz grains and mixed rock fragments resulting from mechanical
and chemical degradation of igneous, metamorphic, and sedimentary
rocks may be transported to other areas and later transformed into
sandstones.
After the loose sediments of sand, clay, carbonates, etc., are accu-
mulated in a basin area they undergo burial by other sediments form-
ing on top. The vertical stress of the overlying sediments causes
compaction of the grains. Transformation into sedimentary rocks occurs
by lithification, or cementation, from minerals deposited between the
grains by interstitial water. The main cementing materials are silica,
calcite, oxides of iron, and clay. The composition of sandstones is
dependent on the source of the minerals (igneous, metamorphic,
sedimentary) and the nature of the depositional environment.
Theodorovich [ll] used the three most general constituents of
sandstones to establish a scheme of classification which is useful in
petroleum engineering because it encompasses the majority of the
clastic petroleum reservoirs (Figure 1.2). Only the three most important
classifications are shown; many other subdivisions of these were devel-
oped by Theodorovich and other investigators, and are summarized by
Chilingarian and Wolf [ 121.
A distinctive feature of sandstones is the bedding planes, which are
visible as dark horizontal lines. The bedding planes are the consequence
of layered deposition occurring during changing environmental
conditions over long periods of deposition in the region. Layering
introduces a considerable difference between the vertical (cross-bedding
plane direction) and horizontal (parallel to the bedding planes) flow