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Geol 463
Supplementary Notes
463-RWR-6a
GEOL 463.3—RWR-6a
SILICICLASTIC RESERVOIRS
(See also the figures provided on handouts)
Reservoir Rocks
The essential requirements for good reservoir rocks high porosity and permeability, and a sufficient
thickness and volume to enable it to hold large quantities of petroleum. Three basic conditions must
be met:
•
The reservoir rock must have retained much of its primary porosity (more than 10–20%) or
acquired secondary porosity before oil migration.
•
The reservoir rock must be overlain by a cap rock with low permeability that forms a
structure with upward closure.
•
The reservoir must be within the range of hydrocarbon migration from mature source rocks.
The most important properties of a reservoir rock are porosity and permeability. Porosity is a basic
feature of the sediment, whereas permeability depends on:
1. Effective porosity — which is a function of the shape and size of the pores and their
interconnections (Pe = interconnected pore volume/bulk volume X 100)
2. Fluid properties — viscosity, pressure gradients, capillary force, etc.
The two main types of porosity in siliciclastic rocks are primary (intra- and interparticle porosity)
and secondary porosity. Secondary porosity in sandstones forms during diagenesis mainly by
dissolution of grains and cements that are unstable in the prevailing pore fluids. Carbonates,
feldspars, and mafic minerals are commonly dissolved or partially dissolved, but authigenic silicate
minerals (clays, zeolites, quartz, feldspars) may form in their place. Some secondary porosity
forms by fracturing; although this may enhance permeability, the increase in pore volume may be
small. The processes that decrease porosity in sandstones are compaction (including pressure
solution) and cementation (quartz, calcite, feldspar, and clay minerals).
Most reservoir sandstones have porosities of about 10–30% and permeabilities of about 100–500
millidarcies. The highest primary porosities and best quality reservoirs are preserved in coarse,
texturally and compositionally mature sandstones, composed mainly of quartz. Greywackes and
quartz wackes lose porosity rapidly with burial by compaction and alteration of the matrix, whereas
arkoses and lithic sandstones (litharenites) may be good or poor reservoir rocks depending on
whether the feldspars survive or alter to clay minerals.
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Geol 463
Supplementary Notes
463-RWR-6a
Depositional Environments of Reservoir Rocks
During exploration, it is very important to know the distribution of potential reservoir rocks and
their properties. Mapping the sedimentary facies is therefore necessary for delimiting potential
prospecting targets. Those facies, in turn, allow prediction and determination of the geometry of the
reservoir rocks, and the planning of production (e.g., location of injection wells in secondary
recovery).
The depositional environment is also a major factor in controlling the initial vertical and horizontal
variations in porosity and permeability of the reservoir rock, and whether suitable source rocks and
caprocks are present in the same depositional system.
Some depositional environments generate better sandstone reservoirs than others do… see the
handouts for the geometry of the following types:
Fluvial systems
The reservoirs are generally located in channel fill and bars sands and crevasse-splays, if present.
Most fluvial sand bodies are oriented parallel to depositional dip, but the geometry of sandstones
will vary greatly according to the type of depositional system. Reservoir continuity is typically
good to excellent. Internally, however, the sand bodies are typically heterogenous. Variations are
present according to the type of fluvial system:
Braided (bedload) streams commonly have excellent lateral and vertical continuity and high
primary porosity, but because of the paucity of mud, they have poor bounding facies (i.e. seals).
Stacked sandbodies can produce thick reservoirs.
Meandering (mixed-load) streams produce complex, compartmentalized reservoirs. Sand bodies
commonly fine upward leading to decreasing quality. Lateral continuity may be broken by muds
(overbank deposits, oxbow plugs, etc.), vertical continuity by thin muddy lenses and sheets within
the sand bodies.
Sandy suspended load streams (some anastomosing systems) can produce good quality reservoir
rocks (stratigraphic traps) providing the mud component is not too high. They commonly have
complex geometries. Sand bodies commonly become muddy upward.
The organic matter in fluvial systems is predominantly herbaceous and woody, so is not oil-prone.
Fluvial sandstones, therefore, normally rely on other depositional systems to provide an oil source
of oil-prone organic matter.
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Geol 463
Supplementary Notes
463-RWR-6a
Delta Systems
Delta systems have been termed the “ultimate hydrocarbon generators”. They have
large potential reservoirs, and good sealing and source facies.
The main reservoirs are:
•
•
•
distributary channels, which are usually continuous dip-oriented
delta-front sands and mouth bars, which are finer, but more extensive, better sorted than the
latter
crevasse-splay sands, which form small but often well selaed (by mudstone) reservoirs
Fluvially dominated deltas comprise many distributary channels, commonly with local crevasse
splays. Sand that is deposited in the channel will characteristically fine upward and may have a
high clastic clay content. With overbank interdistributary fills the sealing potential is good, but
reservoirs may be compartmentalized or have discontinuous vertical permeability due to muddy
layers.
Wave-dominated deltas produce laterally extensive, well-sorted coarse sand bodies that may have
good lateral and vertical continuity. They may have poor sealing quality unless buried by
transgressive marine shales. They form high quality structural traps.
Tidal deltas and estuaries are generally of poorer quality. Tidal flats experience strong currents
alternating with still-standing water, that together produce thin sand lenses in a muddy matrix
formed through deposition from suspension at high tide. These sediments will have low porosity
and permeability. Tidal channels, however, may be filled by well-sorted sand that fines upward.
Subtidal sand ridges deposited by tidal currents show increasing sorting upward, and are another
potential reservoir facies.
Delta systems are typically organic rich. Type III kerogen is produced in the terrestrial settings
from herbaceous plants. In paralic and fully marine offshore settings, oil- and gas-prone organic
matter can be abundant, giving a local potential source rocks. Transgressive marine clays generate
good caprocks. Syndepositional fault traps (growth faults) are very common.
Eolian systems
Eolian sandstones are coarse-grained and extremely well sorted, so form good reservoir rocks.
They need an external source of hydrocarbons and caprocks to become a reservoir. Evaporites
form caprocks to some North Sea petroleum fields.
Shore zone sands
Sands on beaches and shorefaces in shallow marine environments have good to excellent sorting
due to winnowing of fine material through wave activity. This produces very high porosity and
permeability, which in turn can result in excellent reservoirs. There are several important types of
reservoir rock.
•
•
•
Beach sand
Barrier island sand
Washover fans deposited landward of any barrier
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Geol 463
Supplementary Notes
463-RWR-6a
Most sand bodies are parallel to depositional strike (unlike fluvial sands which are parallel to dip).
Many of the sand bodies become progressively thinner and pinch out landward giving good
potential for stratigraphic traps. Transgressive barriers and prograding strandplains provide good
quality, often clean reservoirs. With increasing tidal influences (muddy tidal channels and estuarine
complexes), the sand bodies become highly compartmentalized and reservoir quality and continuity
decline. Offshore sands may become degraded by mud and bioturbation. With proximity to the
coast, marine shoreline sediments have a potential source rocks nearby. Marine shales again can
provide favourable caprocks.
Shelf sands
Sands deposited on continental shelves generally are minor reservoir rocks, but when conditions are
favourable, they can be rich in oil and gas. The sand bodies are typically lensoid and encased in
finer sediments, and they thicken parallel to the shoreline. They both coarsen and fine upward so
permeability stratification is common. Many are prone to diagenetic degradation, with formation of
clay minerals, including glauconite. The environment commonly has abundant organic productivity,
so local and intebedded shales may become source rocks with Types I and II kerogen.
Marine slope and basins
Below wave base, much of the sedimentation is from suspension, so the deposits are fine-grained,
muddy, internally complex, and more likely to generate source rocks than reservoir rocks. Along
the continental slopes, however, strong currents may transport coarse sediment to form sand bodies.
This includes submarine canyons.
There are, however, three main reservoirs in this setting:
Sandy turbidite fans with both channel and suprafan facies, can form lenticular, multistorey, sand
bodies oriented downdip. Turbidites generally have low primary porosity, but proximal turbidites
may be sufficiently thick and extensive to form large reservoirs.
Submarine slope and basin floor sands produced from non-turbid density flows can generate good
reservoirs.
Canyon fill sands form narrow dip-oriented lenses that pinch-out upchannel to form stratigraphic
traps.
These sand bodies are commonly associated with good quality source rocks.
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