# Reservoir Geology

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When it comes to a reservoir, there are some rocks that can be found in it, some permeable, semipermeable and others not permeable. Do you think you have a good understanding on these types of rocks or are you in search of a refresher on what we covered in geology class on them? Why don’t you take up the quiz below and see what you can recall?

• 1.

### Which of the following statements is true?

• A.

A rock can have porosity without having permeability

• B.

A rock's porosity is a measure of the ease with which fluid flows through a reservoir

• C.

A rock's permeability is the ratio of empty space to the total volume of rock

A. A rock can have porosity without having permeability
Explanation
Porosity refers to the percentage of empty space or voids in a rock, while permeability refers to the ability of fluid to flow through the rock. It is possible for a rock to have porosity, meaning it has empty spaces, but not have permeability, meaning the fluid cannot flow through it due to the lack of interconnected pathways. Therefore, the statement "A rock can have porosity without having permeability" is true.

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• 2.

### A rock can have permeability without having porosity?

• A.

True

• B.

False

B. False
Explanation
False. Permeability is a measure of how easily fluids can flow through a material, while porosity refers to the amount of empty space within a material. In order for a rock to have permeability, it must also have porosity to allow fluids to flow through it. Therefore, it is not possible for a rock to have permeability without having porosity.

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• 3.

### Pore throat diameters _________ in reservoirs with very fine clay mineral.

• A.

Increase

• B.

Decrease

• C.

Stay the same

B. Decrease
Explanation
In reservoirs with very fine clay mineral, the pore throat diameters tend to decrease. This is because the presence of fine clay particles can clog and fill the pores, reducing their size and restricting the flow of fluids through the reservoir. As a result, the diameters of the pore throats decrease, leading to decreased permeability and fluid flow within the reservoir.

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• 4.

### The spatial arrangement of grains within a reservoir refers to:

• A.

Packing

• B.

Sorting

• C.

Compaction

• D.

Cementation

A. Packing
Explanation
The spatial arrangement of grains within a reservoir refers to packing. Packing refers to how closely the grains are arranged and the spaces between them. In a well-packed reservoir, the grains are tightly arranged with minimal gaps, allowing for better fluid flow and storage capacity. This can affect the porosity and permeability of the reservoir, which are important factors in determining its productivity. Sorting refers to the uniformity of grain size, compaction refers to the compression of grains over time, and cementation refers to the binding of grains by minerals.

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• 5.

### Primary porosity is defined as:

• A.

Pore space created by dissolution of fracturing of rock

• B.

Pore space that is interconnected

• C.

Pore space that is isolated, or not interconnected

• D.

Pore space created during deposition of sediments

D. Pore space created during deposition of sediments
Explanation
Primary porosity refers to the pore space that is initially created during the deposition of sediments. As sediments settle and accumulate, they can create gaps and voids between them, forming the primary porosity. This porosity is important as it allows for the storage and movement of fluids, such as water or hydrocarbons, within the sedimentary rocks. It is different from secondary porosity, which is created later through processes like fracturing or dissolution.

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• 6.

### Secondary porosity is defined as:

• A.

Interconnected pore space created by dissolution

• B.

Pore space created after deposition by some diagenetic process

• C.

Isolated pore space created during deposition of sediments

• D.

Pore space that does not contribute to fluid flow

B. Pore space created after deposition by some diagenetic process
Explanation
Secondary porosity refers to pore space that is created after the deposition of sediments by some diagenetic process. Diagenesis refers to the physical and chemical changes that occur in sedimentary rocks as they are buried and undergo compaction and cementation. During diagenesis, certain processes such as dissolution, fracturing, or mineral replacement can create additional pore space within the rock. This secondary porosity allows for the storage and movement of fluids, such as water or hydrocarbons, within the rock. Therefore, the correct answer is "Pore space created after deposition by some diagenetic process."

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• 7.

### Effective porosity is defined as:

• A.

Isolated pore space created during deposition of sediments

• B.

Isolated pore space created after deposition by some diagenetic process

• C.

Interconnected pore space that contributes to fluid flow

• D.

Interconnected pore space that is completely filled or obstructed by cements

C. Interconnected pore space that contributes to fluid flow
Explanation
Effective porosity refers to the interconnected pore space within a sediment or rock that allows fluids, such as water or oil, to flow through it. This porosity is important for fluid movement and can impact the permeability of the material. It is different from isolated pore space, which is not connected and does not contribute to fluid flow. Effective porosity can be created during deposition or through diagenetic processes, but the key characteristic is that it allows for fluid flow.

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• 8.

### Non-effective porosity is defined as:

• A.

Pore space that is isolated, or not interconnected

• B.

Interconnected pore space that contributes to fluid flow

• C.

Pore space created during deposition of sediments

• D.

Pore space created by dissolution or fracturing of rocks

A. Pore space that is isolated, or not interconnected
Explanation
Non-effective porosity refers to the pore space in a rock or sediment that is isolated or not interconnected. This means that the pores do not allow fluid flow or communication between each other. This type of porosity does not contribute to the movement or storage of fluids within the rock or sediment. It can occur due to various factors such as the presence of clay or fine particles that block the pores, or due to the presence of fractures or faults that disrupt the connectivity of the pore network.

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• 9.

### The distribution of grain sizes within a reservoir refers to:

• A.

Compaction

• B.

Sorting

• C.

Packing

• D.

Cementation

B. Sorting
Explanation
The distribution of grain sizes within a reservoir refers to the arrangement and organization of grains based on their size. Sorting is the process of separating and arranging grains based on their size, with well-sorted reservoirs having grains of similar sizes grouped together and poorly-sorted reservoirs having a wide range of grain sizes mixed together. Compaction, packing, and cementation are related processes but not specifically related to the distribution of grain sizes within a reservoir.

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• 10.

### As the grain size of sediments decreases, permeability of the rock will ________.

• A.

Increase

• B.

Decrease

• C.

Stay the same

B. Decrease
Explanation
As the grain size of sediments decreases, the spaces between the grains become smaller, resulting in a decrease in permeability. Smaller grains pack more tightly together, reducing the ability of fluids to flow through the rock. This leads to a decrease in the rock's permeability, making it more difficult for fluids to pass through.

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• 11.

### Morphology is the external structure of rocks in relation to the development of erosional forms or topographic features.

• A.

True

• B.

False

A. True
Explanation
The statement is true because morphology refers to the study of the external structure of rocks. It focuses on how rocks have been shaped by erosion and other geological processes, which in turn can influence the formation of various landforms and topographic features. Therefore, understanding the morphology of rocks is crucial in comprehending the development of erosional forms and topography.

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• 12.

### Which of the following are part of primary porosity?

• A.

Interparticle

• B.

Intraparticle

• C.

Intercystalline

• D.

Shelter

• E.

Fracture

• F.

Channel

• G.

Vug

A. Interparticle
B. Intraparticle
C. Intercystalline
D. Shelter
Explanation
Primary porosity refers to the original or inherent porosity of a rock, which is formed during its deposition or early diagenesis. Interparticle porosity refers to the spaces between individual grains or particles in a rock. Intraparticle porosity refers to the pores within individual grains or particles. Intercystalline porosity refers to the pores between adjacent crystal grains in a rock. Shelter porosity refers to the pores formed by the dissolution or removal of unstable minerals or organic matter from a rock. Therefore, Interparticle, Intraparticle, Intercystalline, and Shelter are all part of primary porosity.

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• 13.

### Which of the following are part of secondary porosity?

• A.

Interparticle

• B.

Intraparticle

• C.

Intercystalline

• D.

Shelter

• E.

Fracture

• F.

Channel

• G.

Vug

E. Fracture
F. Channel
G. Vug
Explanation
Secondary porosity refers to the additional pore space that is created after the rock formation has been lithified. Fractures, channels, and vugs are all examples of secondary porosity. Fractures are cracks or fractures in the rock that allow for the movement of fluids, while channels are larger conduits that have been eroded or dissolved within the rock. Vugs are small cavities or holes that have formed within the rock. These features enhance the permeability of the rock, allowing for the flow of fluids such as oil, gas, or water.

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• 14.

### It is possible for a reservoir with large amounts of pore space to be completely impermeable.

• A.

True

• B.

False

A. True
Explanation
This statement is true because even though a reservoir may have a large amount of pore space, if the pores are not interconnected or if there are barriers such as clay or shale layers blocking the flow of fluids, the reservoir can still be impermeable. Pore space refers to the empty spaces between grains or rocks where fluids can flow, but if these spaces are not connected or if there are barriers preventing fluid movement, the reservoir will not be permeable.

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• 15.

### Which are the following are true statements about grain size?

• A.

Has little impact on porosity but may greatly affect permeability

• B.

Bigger the grain size the better the permeability

• C.

Has a great impact on porosity but may not affect permeability

• D.

Bigger the grain size the lowers the permeability

A. Has little impact on porosity but may greatly affect permeability
B. Bigger the grain size the better the permeability
Explanation
Grain size refers to the size of individual particles in a sediment or rock. The given answer states that grain size has little impact on porosity, which is the amount of empty space in a rock or sediment. This means that the size of the grains does not significantly affect the amount of empty space present. However, the answer also states that grain size may greatly affect permeability, which is the ability of a rock or sediment to allow fluids to pass through it. This suggests that larger grain sizes can result in better permeability, as there is more space between the grains for fluids to flow through. Therefore, the answer accurately describes the relationship between grain size, porosity, and permeability.

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• 16.

### What are the different types of sorting?

• A.

Well Sorted

• B.

Poorly Sorted

• C.

Half Sorted

• D.

Fully Sorted

• E.

Average Sorted

A. Well Sorted
B. Poorly Sorted
Explanation
The given answer is incorrect. The types of sorting mentioned in the options are not accurate. Sorting refers to the process of arranging elements in a particular order, such as ascending or descending. The correct types of sorting include bubble sort, insertion sort, selection sort, merge sort, quick sort, and heap sort, among others.

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• 17.

### Reservoirs with grains that have the same size are called

• A.

Well Sorted

• B.

Poorly Sorted

• C.

Half Sorted

• D.

Fully Sorted

A. Well Sorted
Explanation
Reservoirs with grains that have the same size are called "Well Sorted" because when the grains are uniform in size, they can be easily arranged and sorted in a consistent manner. This leads to a well-organized and evenly distributed arrangement of grains within the reservoir. In contrast, poorly sorted reservoirs have grains of varying sizes, which results in a more random and disorganized distribution. Half sorted and fully sorted are not commonly used terms in the context of reservoirs and grain size.

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• 18.

### Reservoirs with grains that are different sizes are called

• A.

Well Sorted

• B.

Poorly Sorted

• C.

Half Sorted

• D.

Fully Sorted

B. Poorly Sorted
Explanation
Reservoirs with grains that are different sizes are called "poorly sorted" because the grains are not uniformly distributed. In a poorly sorted reservoir, there is a wide range of grain sizes, with larger and smaller grains mixed together. This lack of sorting indicates that the sediment was deposited in a chaotic or turbulent environment, where the grains were not able to settle according to their size.

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• 19.

### Well sorted reservoirs results in poor porosity and permeability.

• A.

True

• B.

False

B. False
Explanation
The statement is contradicting the general understanding of reservoirs. Well sorted reservoirs actually have good porosity and permeability. Sorting refers to the uniformity of grain sizes in a sedimentary rock, and well-sorted reservoirs have grains that are similar in size, allowing for better connectivity and flow of fluids. This results in higher porosity and permeability, making them more favorable for oil and gas extraction. Therefore, the correct answer is False.

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• 20.

### Poorly sorted reservoirs results in poor porosity and permeability.

• A.

True

• B.

False

A. True
Explanation
Poorly sorted reservoirs refer to reservoirs where the particles or grains are not well sorted or arranged in a uniform manner. This can lead to poor porosity, which is the measure of the amount of empty space within the reservoir rocks, and poor permeability, which is the ability of fluids to flow through the reservoir. When the particles are poorly sorted, the empty spaces between them may be filled with smaller particles, reducing the overall porosity and permeability of the reservoir. Therefore, poorly sorted reservoirs indeed result in poor porosity and permeability.

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• 21.

### As grain shape become less spherical the porosity and permeability is lowered.

• A.

True

• B.

False

A. True
Explanation
When grain shape becomes less spherical, the grains become more irregularly shaped and have more contact points with each other. This leads to a decrease in the amount of space between the grains, resulting in lower porosity. As a consequence, the ability of fluids to flow through the rock, known as permeability, is also reduced. Therefore, the statement is true.

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• 22.

### As overburden pressure increases the porosity and permeability is increased.

• A.

True

• B.

False

B. False
Explanation
As overburden pressure increases, the porosity and permeability of a rock formation generally decrease. This is because the weight of the overlying layers of sediment compresses the rock, reducing the space between the grains and decreasing the ability of fluids to flow through the rock. Therefore, the correct answer is false.

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• 23.

### As grains become more closely packed, porosity and permeability is reduced.

• A.

True

• B.

False

A. True
Explanation
As grains become more closely packed, the spaces between them decrease, resulting in a reduction in porosity and permeability. This is because the tighter packing restricts the flow of fluids through the sediment or rock. Therefore, the statement is true.

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• 24.

### Cementation causes a(n) ________ in porosity and permeability.

• A.

Increase

• B.

Decrease

• C.

No change

B. Decrease
Explanation
Cementation refers to the process of filling the pore spaces in a sediment or rock with cementing materials, such as minerals or chemical precipitates. This process leads to a decrease in both porosity and permeability. As the pore spaces get filled with cement, there is less room for fluids to flow through the rock, resulting in a decrease in permeability. Additionally, the filling of pore spaces reduces the overall volume of open spaces in the rock, leading to a decrease in porosity. Therefore, cementation causes a decrease in both porosity and permeability.

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• 25.

### Most sandstones consist of grains that are cemented together by either _____ or ______.

• A.

Quartz

• B.

Calcite

• C.

Halite

• D.

Gypsum

A. Quartz
B. Calcite
Explanation
Sandstones are sedimentary rocks composed primarily of sand-sized grains. The grains in sandstones are typically cemented together by minerals that act as a binding agent. Quartz and calcite are two common minerals that can serve as cementing materials in sandstones. Quartz is a hard and durable mineral that can form strong bonds between the sand grains, while calcite is a softer mineral that can fill in the spaces between the grains and create a cementing matrix. Therefore, sandstones often consist of grains that are cemented together by either quartz or calcite.

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• 26.

### To achieve total porosity, effective porosity and non-effective porosity are added together.

• A.

True

• B.

False

A. True
Explanation
The statement is true because total porosity is the sum of effective porosity and non-effective porosity. Effective porosity refers to the portion of the rock or soil that can hold and transmit fluids, while non-effective porosity refers to the portion that cannot hold or transmit fluids. By adding these two values together, we can determine the total porosity of the material. Therefore, the statement is correct.

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• Current Version
• Mar 21, 2023
Quiz Edited by
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• Jul 30, 2014
Quiz Created by
Taylor

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