Page 158 - Well Logging and Formation Evaluation

148 Well Logging and Formation Evaluation

Channel systems
Deltaic
Distributary channels
Delta plain
Delta front
Glacial
Loess
Tillites
Varves
Deep marine
Turbidite fans
Slumps, slides, debris flows
Turbidity currents
Shoreline
Beaches, bars, barrier islands
Coastal aeolian plains
Cheniers
Swamps, marshes, estuaries

Knowledge of the environment of deposition is crucial to understand-
ing how the good-quality sands, if any, are likely to be distributed over a
prospect. Mineral composition of clastic reservoirs, in order of abundance
are: quartz, clay minerals, rock fragments, feldspars, chert, mica, and car-
bonate fragment.
A “clean” sandstone will comprise mainly quartz grains. This may have
a porosity as high as about 40% and permeability up to 5 darcies. The
presence of clays and minerals forming cement between the grains will
have the effect of reducing the porosity and permeability, as will differ-
ent grain distributions.
The distribution of clay minerals is also related to the environment of
deposition and is of particular significance to the petrophysicist, in view
of the effect that clay has on permeability, conductivity, and water satu-
ration. Clays found in sandstones are classed as either allogenic or authi-
genic. Allogenic clays are those that were present prior to deposition.
These may take the following forms:

• Individual clay particles, dispersed as matrix or as laminae
• Pellets formed from clay flocculation or excreted by organisms
• Clay aggregates derived from preexisting shales outside the deposi-
tional basin