Page 110 - Hydrocarbon

Reservoir Description 97

Connate water is the water which remains in the pore space after the entry of
hydrocarbons.
Quartz (SiO 2 ) is one of the most stable minerals and is therefore the main
constituent of sandstones which have undergone the most severe weathering and
transportation over considerable distance. These sediments are called ‘mature’ and
provide ‘clean’ high-quality reservoir sands. In theory, porosity is not affected by the
size of the grains but is purely a percentage of the bulk rock volume. In nature,
however, sands with large well-sorted components may have higher porosities than
the equivalent sand comprising small components. This is simply the result of the
higher transport energy required to move large components, hence a low probability
of fine (light) particles such as clay being deposited.
Very clean sands are rare and normally variable amounts of clay will be contained
in the reservoir pore system; the clays being the weathering products of rock
constituents such as feldspars. The quantity of clay and its distribution within the
reservoir exerts a major control on permeability and porosity. Figure 6.2 shows
several types of clay distribution.
Laminae of clay and clay drapes act as vertical or horizontal baffles or barriers to
fluid flow and pressure communication. Dispersed clays occupy pore space which in a
clean sand would be available for hydrocarbons. They may also obstruct pore throats,
thus impeding fluid flow. Reservoir evaluation is often complicated by the presence
of clays. This is particularly true for the estimation of hydrocarbon saturation.
Bioturbation, due to the burrowing action of organisms, may connect sand layers
otherwise separated by clay laminae, thus enhancing vertical permeability. On the

Figure 6.2 Types of clay distribution.