# Igneous reservoirs

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Igneous reservoirs are unconventional petroleum reservoirs found within the Earth. These petroleum reservoirs are made up of reservoir rocks that store fluids within the pores of the rocks, which accumulate water, gas and oil. Igneous reservoirs are considered as secondary reservoir targets for oil and gas exploration compared to the more common sandstone and carbonate reservoirs. Igneous reservoirs make up a small percentage of reservoirs explored but they can still be a viable option for oil production. There are prominent igneous reservoirs found in Vietnam, Indonesia, Japan, Venezuela, Argentina, Russia, California and in China. [1]

Figure 1  Various igneous reservoirs worldwide [1]

Igneous rocks form reservoirs in many parts of the world. Reservoir quality varies and most benefit from natural fractures since the majority of these reservoirs can be shown to contain hydrocarbons that migrated from conventional sedimentary sources. [2]

## History and features of igneous reservoirs

Figure 2  Schematic of a typical petroleum reservoir (from Petropedia, How Hydrocarbon Reservoirs Are Formed: The Rock Cycle) [3]

To form a reservoir there must a system where a source rock, which produces the hydrocarbons, is trapped and sealed with a good porosity and permeability to accumulate the petroleum for production in economical quantities. Viable igneous reservoirs need to have a similar attributes as the sedimentary reservoirs, i.e. good porosity and permeability with the ability of migration, see Figure 2. Since igneous rocks are much more resistant to weathering than sedimentary or carbonate rocks, the most common way to have a good porosity in these igneous reservoirs will be fractured porosity. There are three main reservoirs found in the Earth including sandstone, carbonate, and igneous/metamorphic reservoirs. Sandstone and carbonate reservoirs make up the majority of the reservoirs, while igneous reservoirs make up roughly 5 percent. [4]

Figure 3  Main characteristics of volcanic reservoirs worldwide. (translated and modified from Zhang and Wu, 1994) [5]

## Reservoir porosity and permeability

Porosity is the void space/pore space in a rock that can store fluids. In the subsurface this volume of the pores may be filled with petroleum, either oil or gas, and water. [3]

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

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

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:

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

This equation gives the total amount possible of porosity, but most times its not possible for the pores to all be interconnected so we want to know the effective porosity of the reservoir, which gives the economic amount of the pores available for production. Effective porosity is the ratio of interconnected pore volume to bulk volume, obtained by calculation:

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

Primary porosity is the amount of empty pores available during the deposition and creation of the rock, while secondary porosity is the amount of pore space created after the rock was formed, deposited and buried, such as a crack in the rock. Igneous reservoirs mostly have secondary porosity where the igneous rocks where fractured and filled with hydrocarbons then subsequently trapped in the reservoirs, through compaction of sedimentary rocks which forces the hydrocarbons into the igneous rocks. [2]

The main difference between the more explored sedimentary reservoir and igneous reservoirs is that sedimentary rocks are less resistant and have a greater permeability than that of igneous rocks. Permeability is way to describe the ability to flow through rocks. 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. Most reservoir rocks range in permeability from 0.1 mD to more than 10 D. [3]

## Geology of igneous reservoirs

Figure 4  An example of emplacement of igneous rocks. (from Oilfield Review, Spring 2009: 21, no 1) [1]

Igneous rocks are classified by composition, texture, and method of emplacement. Intrusive igneous rocks are formed inside the earth, which cool slowly. They are formed from magma within the earth, have large grains and include silicate minerals. Intrusions are called sills when oriented roughly horizontal and dikes when near vertical. [2] Extrusive igneous rocks form on the surface of the earth due to lava flow, they cool quickly leading to small grain sizes. They typically contain little to no gas from the initial formation. Both intrusive and extrusive serve as the basis of igneous reservoirs that can contain fractures by either contraction while cooling, or metamorphic compactions during diagenesis. [4] Intrusive rocks may alter the rocks above and below them by metamorphosing the rock near the intrusion, while extrusive only heat the rock below them, and may not cause much alteration due to rapid cooling. [2] The mineral composition of an igneous rock depends on where and how the rock was formed. [2]

Type Description [2]
Mafic igneous rocks Dark colored and consist mainly of magnesium and iron. Common minerals found in mafic rocks include olivine, pyroxene, amphibole, and biotite. They contain about 46-85% mafic mineral crystals and have a high density.
Ultramafic igneous rocks Very dark colored and contain higher amounts of the same common minerals as mafic rocks, but with about 86-100% mafic mineral crystals
Intermediate igneous rocks In between light and dark colored. They share minerals with both felsic and mafic rocks. They contain 15 to 45% mafic minerals
Plutonic and volcanic rocks They generally have very low porosity and permeability. Natural fractures may enhance porosity by allowing solution of feldspar grains. Some examples with average porosity as high as 17% are known.

Tuffs have high total porosity because of vugs or vesicles in a glassy matrix compared to pumice has almost no effective porosity. [2]

## Characteristics of igneous reservoirs

Igneous reservoirs are generally small and mainly of Tertiary and Late Cretaceous age, with burial depth in the range of 400–2000 m. [4] These sediments typically undergo rapid early diagenesis at shallow depths and low temperatures which ruins primary porosity by compaction and cementation, but later diagenesis creates secondary porosity through dissolution. [1] Thus, the ability of volcaniclastic deposits to serve as hydrocarbon traps depends on the coincidence of porosity preservation and generation processes with the time of hydrocarbon migration.

Figure 5  Traps caused by igneous intrusions [1]

Igneous reservoirs can be overburdened which help trap the hydrocarbons. The intrusion process can create a migration pathway due to the igneous rocks fracturing. [4] Intrusive igneous rocks can become a reservoir or trap as hot hydrothermal fluid convective cells can generate hydrocarbons that return to the intrusive rock. [2] The possibility of hydrocarbon generation is interrelated with the relative degree of thermal maturity of the potential source rock at the time of intrusion. [4]

Figure 6  Types of igneous petroleum systems related to sills and laccoliths [4]