What is a Reservoir?
A petroleum reservoir is a subsurface formation that stores hydrocarbons like oil and gas. These rocks have porosity (spaces to store fluids) and permeability (the ability for fluids to flow). Common reservoir rocks include Clastic and Carbonate.
1. Why Carbonate Reservoirs Matter?
Carbonate reservoirs are one of the most important sources of petroleum production. Most Oil reserves in the world are in the Carbonate reservoir. More than 60% of the world's proven oil reserves and 40% of the world's gas reserves. Carbonate reservoirs are as important as sandstones, but their development and production present a different set of problems.
Limestone :
https://southerncrest.in/wp-content/uploads/limestone.jpg
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Dolostone :
https://geologyistheway.com/wp-content/uploads/2022/02/49740554426_070191cc59_k-1536x1152.jpg
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2. How Carbonates Form
The carbonate rocks include limestone, composed largely of calcite [CaCO3], and the dolostone, composed largely of calcite, the same name but different chemical compositions [CaMg(CO3)2].
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1. Biogenic Formation
Forming from marine organisms (corals, algae, mollusks, foraminifera). After death, these skeletal fragments accumulate on the seafloor and lithify into limestone. |
2. Chemical Precipitation
In warm, shallow, tropical seas, calcium carbonate can precipitate directly from seawater when it becomes oversaturated. Often occurs in lagoonal, tidal flat, or restricted platform settings. |
https://www.researchgate.net/publication/273834133/figure/fig1/AS:402314654437376@1472930696779/Mollusks-ostracods-foraminifera-diatoms-and-plants-of-the-paleo-poza-Caracol-Valley.png
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https://upload.wikimedia.org/wikipedia/commons/0/03/Shallow_water.jpg
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3. Porosity type in carbonate rocks
- Primary porosity is the pore space that existed in the sediment just after deposition. During burial the primary porosity is reduced by compaction and cementation.
- Secondary porosity is pore space produced by dissolution of grains after deposition. This requires a net transport of carbonate in solution out of the rock.
4. Key Challenges
Carbonate reservoir rocks are highly heterogeneous and reveal complex pore geometry at various scales due to diagenesis and depositional processes.
When first deposited, carbonate sediments are highly porous and permeable and are inherently unstable in subsurface environments. After it is dissolved and reprecipitated into carbonate rock, porosity and permeability distribution are largely secondary in origin and often unrelated to primary porosity.
When first deposited, carbonate sediments are highly porous and permeable and are inherently unstable in subsurface environments. After it is dissolved and reprecipitated into carbonate rock, porosity and permeability distribution are largely secondary in origin and often unrelated to primary porosity.
5. Types of Carbonate Reservoir Systems
classification of carbonate rocks that is based on depositional texture (DUNHAM’S classification)
https://www.beg.utexas.edu/lmod/_IOL-CM01/graphics/Dunham.jpg
Mudstone: Muddy carbonate rock containing less than 10 percent grains. Indicates low-energy depositional setting.
Wackestone: Mud-supported carbonate rock containing more than 10 percent grains. Still indicate a low-energy depositional setting.
Packstone: Grain-supported muddy carbonate rock divided packstones into 1. mud-dominated (pore spaces totally filled with mud) and 2. grain-dominated (some intergrain pore space is free of mud) packstones.
Grainstone: Mud-free carbonate rocks, which are grain supported. They generally are deposited in moderate- to high-energy environments
Boundstone (reefal): A carbonate rock where organisms (corals, stromatoporoids, sponges, algae, microbial mats) bound the sediment during deposition.
(1) framestone: the organisms build a rigid framework,
(2) bindstone: the organisms encrust and bind loose sediment together
(3) bafflestone: the organisms do not form a framework or bind the sediments together but provide protected areas for the sediment to accumulate by baffling the currents.
Boundstones generally are deposited in higher energy environments, where currents can provide nutrients to the organisms that form the boundstone, as well as carry away waste products.
Crystalline carbonates(Dolostone): Formed when limestone is altered to dolomite. Recrystallisation has made the original depositional fabric of a carbonate rock unidentifiable
Wackestone: Mud-supported carbonate rock containing more than 10 percent grains. Still indicate a low-energy depositional setting.
Packstone: Grain-supported muddy carbonate rock divided packstones into 1. mud-dominated (pore spaces totally filled with mud) and 2. grain-dominated (some intergrain pore space is free of mud) packstones.
Grainstone: Mud-free carbonate rocks, which are grain supported. They generally are deposited in moderate- to high-energy environments
Boundstone (reefal): A carbonate rock where organisms (corals, stromatoporoids, sponges, algae, microbial mats) bound the sediment during deposition.
(1) framestone: the organisms build a rigid framework,
(2) bindstone: the organisms encrust and bind loose sediment together
(3) bafflestone: the organisms do not form a framework or bind the sediments together but provide protected areas for the sediment to accumulate by baffling the currents.
Boundstones generally are deposited in higher energy environments, where currents can provide nutrients to the organisms that form the boundstone, as well as carry away waste products.
Crystalline carbonates(Dolostone): Formed when limestone is altered to dolomite. Recrystallisation has made the original depositional fabric of a carbonate rock unidentifiable
6. Famous Carbonate Reservoir Examples
Ghawar Field (Saudi Arabia) : The largest conventional oil field in the world
https://www.nsenergybusiness.com/wp-content/uploads/sites/4/2019/10/Image-1-Ghawar-field.jpg
https://upload.wikimedia.org/wikipedia/commons/thumb/3/37/Map_of_approximate_size%2C_shape%2C_and_location_of_Ghawar_Field.png/500px-Map_of_approximate_size%2C_shape%2C_and_location_of_Ghawar_Field.png
References
https://www.researchgate.net/publication/278141046_Sandstone_vs_carbonate_petrole
um_reservoirs_A_global_perspective_on_porosity-depth_and_porosity-permeability_relationships
https://www.lyellcollection.org/doi/full/10.1144/SP548-2024-117
https://wiki.aapg.org/Carbonate_reservoir
https://www.researchgate.net/publication/314650106_Elements_of_conventional_and_unconventional_petroleum_reservoirs
https://www.sciencedirect.com/topics/engineering/carbonate-reservoir
https://onepetro.org/spe/general-information/1302/Carbonate-reservoir-geology
https://geologyistheway.com/sedimentary/carbonate-rocks/
https://www.beg.utexas.edu/lmod/_IOL-CM01/cm01-step03.htm
https://link.springer.com/book/10.1007/978-3-642-34132-8
Elements of Petroleum Geology Textbook by Richard Selley
um_reservoirs_A_global_perspective_on_porosity-depth_and_porosity-permeability_relationships
https://www.lyellcollection.org/doi/full/10.1144/SP548-2024-117
https://wiki.aapg.org/Carbonate_reservoir
https://www.researchgate.net/publication/314650106_Elements_of_conventional_and_unconventional_petroleum_reservoirs
https://www.sciencedirect.com/topics/engineering/carbonate-reservoir
https://onepetro.org/spe/general-information/1302/Carbonate-reservoir-geology
https://geologyistheway.com/sedimentary/carbonate-rocks/
https://www.beg.utexas.edu/lmod/_IOL-CM01/cm01-step03.htm
https://link.springer.com/book/10.1007/978-3-642-34132-8
Elements of Petroleum Geology Textbook by Richard Selley
Written by
Pitchapa Junnatasna
Pitchapa Junnatasna
