Reservoir

Wiki Write-Off Entry
Student Chapter	University of Brawijaya
Competition	June 2015

Reservoir rocks are rocks that have the ability to store fluids inside their pores, so that the fluids (water, oil, and gas) can be accumulated. In petroleum geology, reservoir is one of the elements of petroleum system that can accumulate hydrocarbons (oil or gas). Reservoir rock must have good porosity and permeability to accumulate and drain oil in economical quantities.

Reservoir Porosity

Porosity is the void space in a rock that can store the fluids. It is measured as either a volume percentage or a fraction (expressed as a decimal). In the subsurface this volume may be filled with petroleum (oil and gas), water, a range of non hydrocarbon gasses (CO2, H2S, N2), or some combination of these.

Porosity is symbolized in phi (ϕ) and its value is expressed in percentage. Porosity value calculation:

$\phi ={\frac {\text{pore volume}}{\text{bulk volume}}}\times \ 100\%$

Porosity divided into two types, absolute porosity and effective porosity. Absolute porosity is the ratio of the total pore volume in the rock to bulk volume, obtained by the calculation:

$\phi _{\text{a}}={\frac {\text{total pore volume}}{\text{bulk volume}}}\times \ 100\%$

Effective porosity is the ratio of interconnected pore volume to bulk volume, obtained by calculation:

$\phi _{\text{e}}={\frac {\text{total interconnected pore volume}}{\text{bulk volume}}}\times \ 100\%$

Based on the way of its formation, there are two types of porosity:

1. Primary porosity is porosity that formed while sedimentary rock deposited.
2. Secondary porosity is porosity that formed after the sedimentary rock deposited.

Reservoir Permeability

Figure 1 Average permeability for various producing fields on the UK and Norwegian continental shelves.

Permeability is an intrinsic property of a material that determines how easily a fluid can pass through it. In the petroleum industry, the Darcy (D) is the standard unit of permeability, but milidarcies (1 mD = 10-3 D) are more commonly used. A Darcy is defined as a flow rate of 10-2 ms-1 for a fluid of 1 cp (centipoise) under a pressure of 10-4atm m-2. Permeability in reservoir rocks may range from 0.1 mD to more than 10 D.

Type of Reservoir Rocks

All types of rock (igneous, sedimentary, metamorphic) can act as reservoir rocks if it can accommodate and drain hydrocarbons. Reservoir rocks around the world is dominated by sedimentary rocks because generally it has primary porosity. Igneous and metamorphic rocks can be reservoir if there are in fracturing state (secondary porosity).

Figure 2 Scheme of classification of reservoir rocks. (Adapted from Nichols, from lecture handout by Alamsyah )

Siliciclastic Reservoir

Siliciclastic sedimentary rocks are the most abundant of the sedimentary rock. They are formed from the detritus left over from the weathering of igneous, metamorphic, and older sedimentary rocks.

Figure 3  Depositional environment and grain size.

Shallow and Deep Marine Reservoir

Depending on the type of sedimentary rock, hydrocarbon reservoir can be divided into two groups. There are clastic sedimentary rock and non clastic sedimentary rock. On the clastic sedimentary, contained some precipitation area, as one in marine area. Rock type can be formed in deposition marine areas such as shelf sandstone and turbidity sandstone.

Shelf sandstones are formed from precipitating in the shallow marine area. Sands transported by water current from river to shallow marine area. Because of that process, has been formed sands body around the grow delta and maybe form the fan like in delta.

Turbidity sandstones: rock formed in deep marine area with rotation force existing rotation deep current so as formed the coarse layer which has interaction with shale layer in the deep marine. Shape from deposition of turbidity sandstones can be like a lens, duct, or fans.

Lacustrine Reservoir

This type of reservoir formed in basin containing water surrounded by land and initially formed by tectonic processes, volcanic, rifting, soil movement, the erosion by the wind on the coast or in land. The texture of sedimentary rocks in the environments usually granules grained and the size between 2 mm–4 mm.

Eolian Reservoir

Formed in large areas with the accumulation of sand deposition. The sediment resulting from wind-blown sand. The clastic texture of the environment is granules (2–4 mm) and coarse (over 2 mm; Table 1).

Table 1 Modern dunes classification (Adapted from McKee, 1979).

Name	Morphology	No. of Slipfaces	Wind Direction	Sand Supply (relative)	Relative Size
Sheet	Sheetlike with broad flat surface	None	Multidirectional	Abundant	Large, thick
Stringer	Thin, elongate strip	None	Unidirectional	Sparse	Extensive, thin
Dome	Circular or elliptical mound	None	Bidirectional	Sparse	Small, thin
Barchan	Crescent	1	Unidirectional	Sparse	Small, thin
Barchanoid ridge	Row of connected crescents	1	Unidirectional	Abundant	Large, thick
Transverse ridge	Asymmetrical ridge	1	Unidirectional	Very abundant	Large, thick
Parabolic	U-shape	1 or more	Unidirectional	Abundant	Large, thick
Linear (seif)	Symmetrical ridge	2	Bidirectional	Sparse	Extensive, thin
Reversing	Asymmetrical ridge	2	Bidirectional		Small, thin
Star	Central peak with 3 or more arms	3 or more	Multidirectional	Sparse	Small, thin
Blowout	Circular rim of depression	1 or more	Multidirectional	Variable	Small

Fluvial Reservoir

Figure 4 Change in grain size of bed load in Mississippi River from Cairo, Illinois, to the mouth. (Form U.S. Army Corps of Engineers 1935 in Berg 1985).

Type reservoir generated by the flow of the river where the process is formed by the erosion, transport and deposition of forming depositional formation.

Deltaic Reservoir

On this type of reservoir, it was formed by the accumulation of lacustrine sediments. Very fine grain sediment rocks.

Figure 5 Diagrammatic cross sections of depositional units within deltas. (A) Delta Concept of Gilbert showing topset, foreset, and bottomset beds; (B) Deltaic and neritic facies from Frazier; (C) Sediment types and depositional units of an idealized delta.

Carbonate Reservoir

Figure 6 Depositional environment of marine carbonate. Adapted from Alamsyah

Carbonate rocks is a sedimentary rocks with carbonate fraction more than 50%. Carbonate rocks can be used as a reservoir because of its porosity and permeability. Carbonate rocks can be classified to clastic and non clastic sediment. Its environmental formation is tropic ocean. The porosity concept of a carbonate rock might be a little bit more complex than other rocks, because of its vary secondary porous, from carbonate dissolution made from skeletal remain and microbe with cement. Almost all of carbonate reservoir type accumulated as a shallow marine sedimentary, except on a pelagic chalk and deep marine re-sediment reservoir carbonate in Mexico seas. Carbonate reservoir rocks can be found as a clastic limestone, carbonate framework (reef), and dolomite.

Reef Reservoir

Reef is a framework made of sea organism containing skeletal, grow in a shallow clean water where sunlight can reach as a nutrition. Reef distribution is varies, some on the edge of the shelf and become a barrier, some scattered called patch reef. Patch reef can reach a few kilometer size while barrier can be elongated along the edge and limit the exposure with basin. Meanwhile the shape of a reef can be a pole(pinnacle) or lengthened (fringing). Both can become a good reservoir. Reef is a non-clastic carbonate rocks without transportation process on its formation.

Figure 7 Reef habitat in the shallow sea. Adapted from Alamsyah

Clastic Limestone

Clastic limestone usually associated with oolit and become a pretty good reservoir. Limestone associated with oolite often referred to as calcarenite. The deposition is in shallow marine environments along the coast with high energy (strong wave currents). Porosity may be extremely high because of the dissolution, but permeability is not far from 5 mD. It is called clastic because oolite associated with limestone is present through the transport process before finally being deposited.

Dolomite

Dolomite formed by processes of calcite dolomitization from other carbonates (e.g. limestone). Dolomitization occurred not long after the process of sedimentation. Dolomitization process can be reviewed as the secretion of magnesium contained in sea water into the compounds associated with carbonate.Dolomite production begins from the Pre-Cambrium, continued from Paleozoic to Mesozoic, up to Tertiary time. Calcite will be replaced with the dolomite compound that has smaller volume, so that the space between the pores of the rock grew wide. This resulted in the cavities. Calcite that has not been dolomitized will dissolve quickly and arising secondary pores, sometimes with very large size. Dolomite has its price of porosity as high as limestone porosity, which is about 20-30%. However, permeability is very high, reaching up to 5 mill Darcy to 2 Darcy. Hence, dolomite carbonate reservoirs have higher production of hydrocarbon than limestone. According Zenger (1980) estimates that 80% of carbonate reservoirs in the USA contains dolomite, while the 20% rest of it contains limestone.

Afanitic Limestone

Afanitic limestone formed in shallow ocean areas with low levels of energy, such as for example in the lagoon. This limestone has a very fine grain size and tight. However, the porosity and permeability was appeared by the events of dissolution and cracking that causing this type of limestone have secondary porosity. Thus, it can be a reservoir of oil.

Another Type of Reservoir

Although the porosity and permeability are poor, shale, silt stone, limestone can even act as reservoir due to fractures in the rock body (secondary porosity – secondary permeability). For example, an oil field in Florence, Colorado which is having shale (Lower–Upper Cretaceous) as reservoir rock.

Then, it shows that for other than sedimentary rocks (igneous – metamorphic) could be reservoir rock if there are in fracturing state. For example, in Cuba, the oil is obtained from ultra-base igneous rock or volcanic rock that has fractured. There are eight oil fields in Cuba in 1964 that produce 710 barrels oil per day. Reservoir from this type has a very small percentage compared to the reservoir from sedimentary rock (about 1% of the overall reservoir in the world).

Figure 8 Comparison of reservoir rock types around the world in 1956 (based on Knebel & Rodriguez)

Volcanic rocks are igneous rocks that formed on the earth surface (extrusive igneous rock). Volcanic rock can be formed from mafic minerals such as olivine, pyroxene, amphibole, and biotite, or felsic minerals such as feldspar, muscovite, and quartz. For example, oil obtained in Jatibarang (West Java, Indonesia), produced from fractures that occur in volcanic rock (tuff). Oil production from volcanic rock reservoir shows higher production in initially, and then shows a rapid decline in production.

Other sources

• Baganz, O. W., B. Yuval, B. Kevin, and N. Dag, 2012, Lacustrine Sandstone Reservoirs and Hydrocarbon Systems: AAPG Memoir 95.
• Ehrenberg, S. N., and P. H. Nadeau, 2005, Sandstone vs. Carbonate Petroleum Reservoirs: A Global Perspective on Porosity-depth and Porosity-Permeability Relationships: AAPG Bulletin vol. 89, pp. 435-445.