Pore and Pore Throat: Their Impact on Reservoir

This article offers a quick insight into the behavior of pores and pore throats in hydrocarbon reservoirs. With ample graphical representations, it aims to enhance understanding on this topic. I hope readers find the content informative and engaging.

This article has following key sections,

1. Introduction
2. Types of Pore & Pore Throats
3. Common behaviour in Clastic and Carbonate Reservoir
4. How to measure Pore types and Pore throats
5. What affects Pore and pore throat variation and Fluid flow

1. Introduction

Pores are the relatively large voids or spaces within a rock where fluids like oil, gas, and water can be stored. The size and number of pores determine the storage capacity of the rock. Whereas Pore throats are the smaller passages that connect the pores. They can be thought of as the "doors" between rooms (pores). A pore system is an aggregate of pores and pore throats that shares a similar morphology. The size, shape, and number of pore throats control the permeability of the rock, which is its ability to allow fluids to flow through it.

Various key difference are provided in the below table,

Various pictorial representation and SEM photographs shown below provided a clear understanding of Pore and Pore throats.

Below figure shows typical 3-D pore system geometries found in intergranular, intercrystalline, vuggy, or fractured rocks.

2. Types of Pore & Pore Throats

Combining both pore geometry and pore type foloowing classification scheme was proposed for better describing the pore systems.

Choquette and Pray's porosity types include two different groups of pore system shapes: petrophysically simple Archie porosity and petrophysically complex non-Archie porosity.The table below describes pore system shapes and other important characteristics of Archie and non-Archie rocks.

Often different pore types observed in different reservoirs impact differently the flow of the fluids, few common types and their behaviour was described in the following table.

Below diagrams are quick representation of the various pore types that may exists.

3. Common behaviour in Clastic and Carbonate Reservoir

In case of Clastic reservoir Porosity and Pore throats are mostly originated during sediment deposition i.e. Intergranular type, however Intragranular (partial dissolution within grains) or cemented pores due to significant diagenetic history can be observed.

Above is an example of Primary pore and throat types in sandstone samples in thin section and SEM images.

(A) Residual intergranular pores preserved by chlorite

(B) Residual intergranular pores that are triangular and regularly polygonal

(C) Dissolution pores in feldspar

(D) Dissolution pores in debris

(E) Intercrystalline pores in illite

(F) Intercrystalline pores in chlorite

(G) Compacted throats

(H) Dissolution throats

(I) Intercrystalline throats formed by clay-space necking

Carbonate exhibits more heterogeneities, unlike clastics and shows a diverse types of pore behaviour.Below example of carbonate rock demonstrate heterogeneous morphology due to marine biological deposits which shows five pore-throat size networks.

• Isolated Bioclast (Green-label)
• Carbonate Cement (Yellow label)
• Intragranular Connected Vugs and Mini-Fractures (Purple-label)
• Heavy Mineral of Pyrite (Blue label)
• Intergranular Pore (void).

Another Example as below demonstrate different types of carbonates based on depositional environments can also shows drastic variability of porosity and permeability within the reservoir.

In the above figure

(a) Wackestone. Intragranular pores, bioclastics are miliolids, benthic forams and peloids,

(b) Packstone. Moldic pores, bioclastics are benthic forams, bivalves, peloids and skeletal fragments

(c) Wackestone. Residual moldic pores, bioclastics are echinoids, bivalves and peloids,

(d) Grainstone. Intergranular pores, bioclastics are ostracods, echinoids, benthic forams, and bivalves

(e) Mud-lean packstone. Micropores (diameter?<?30?μm), bioclastics are gastropods, echinoids and bivalves

(f) Rudstone. Mixed pores, including intergranular and intercrystal pores, bioclastics are rudists and peloids

(g) Rudstone. Intergranular pores, bioclastics are rudists, bivalves

(h) Floatstone. Mixed pores, including intergranular, moldic and intregranular pores, bioclastics are rudists, echinoids, bivalve and peloids

Below thin section examples shows different diagenetic effects in carbonate and the porosity evalution.

Diagenetic imprints as seen are as follows,

(a) Micritization (M) of benthic forams and ostracods. Conversion of the exterior of bioclastics partly to micrite-sized calcium carbonate, due to microscopic boring algae or fungi on the seafloor environment of syngenetic stage

(b) Cementation (Ce), micritization (M) and dissolution (Di) in bioclastic grainstone. The dissolution enhanced the intergranular pores, few syntaxial overgrowth cement occludes some pores between echinoids.

(c) The part of aragonitic grains were completely leached to form some moldic pores, drusy mosaic cement (Ce) partially filled the dissolution moldic pores in meteoric environment to form residual moldic pores

(d) Neomorphism (N) of bivalve. The shell of bivalve has been converted to coarsely blocky calcite cement.

(e) Mechanical compaction (Co) in a rudist grainstone. The fractured rudist skeletal indicated that it underwent little of cementation prior to burial and overburden loading.

(f) Pressolution formed stylolite (S) in the marginal dolomitization (Do) zone of micrite, some dolomites were cut, which indicated that dolomitization prior to pressolution. Pressolution presents a burial-related pressure-induced zone of dissolution with differential grain interpenetration depending on the relative solubility of each side of the surface.

(g) Dissolution (Di) in echinoids grainstone, dissolution enhanced the intergranular pores. Few euhedral dolomite crystals (Do) formed after the replacement of echinoid fragments, and filled the intergranular pores.

(h) Dissolution (Di) of bivalves in packstone formed moldic pores. Very few euhedral calcites and dolomites filled partially moldic pores reduced the pore space very slightly.

Depending on depositional and diagenetic evolution then carbonates were properly classified to understand their poro-perm relationships and effect of pore throats during fluid mobility.

Another similar example is illustrated in the figure below, which presents a thin-section image of different rock types along with the pore throat size distribution (PTSD). As observed, pore sizes range from the nanoscale, with throat radii smaller than 0.1 micron, to the micro, meso, macro, and mega scales (greater than 10 microns).

SEM data and Thin section analysis commonly provided us enough evidences of pore and pore throat presented in the reservoir and integrating that with other open hole log evaluation we can characterise the heterogenity present with in the reservoir.

Examples of different type of Pores in carbonates as seen in SEM analysis are as follows:

Few diagenetic features can be seen as follows,

4. How to measure Pore types and Pore throats:

Several common Measurement Techniques can be followed are described as below;

1. Scanning Electron Microscopy (SEM): Provides high-resolution images to analyse pore structures and connectivity. (Examples shown above)
2. Mercury Injection Capillary Pressure (MICP): Measures pore throat sizes by injecting mercury into the rock sample under controlled pressure.

3. Computed Tomography (CT) Scanning: Non-destructive 3D imaging technique to visualize internal pore structures.

Above figure represents (a) Intergranular pores; (b) Intergranular dissolution pores and remaining intergranular pores; (c) Intragranular dissolution pores, strong heterogeneity; (d) Intergranular/ particle dissolution pores, strong heterogeneity; (e) Development of intragranular dissolved pores, mold pores, and microfractures.

Above image represents (a) The original 2D grayscale images. (b,c) The reconstruction process of the pore network model. (d) Pore network model. Notice that the balls in different diameters represent the pore, the blue tube bundles of different diameters correspond to the throats.

4. Nuclear Magnetic Resonance (NMR): Measures pore size distribution based on the relaxation times of hydrogen nuclei in fluids within the pores.

5. Thin Section Analysis: Microscopic examination of thin slices of rock to study pore types and pore throats. (Several examples shown as above)

5. What affects Pore and Pore Throat variation and Fluid flow

Fluid flow behavior depends on pore and pore throat variations, where larger, well-connected structures enhance permeability, while smaller, constricted throats restrict movement. In general Carbonates shows complex flow behavior due to variable pore sizes and throat connectivity, where as Clastics shows more predictable flow behavior due to intergranular porosity and better throat connectivity.

Influence of Pore Size on Fluid Storage

• Larger pores increase the rock’s porosity, allowing more fluid storage.
• Smaller pores, although they contribute to porosity, may trap fluids, limiting extraction.

Role of Pore Throats in Permeability

• Permeability is directly controlled by pore throat size, as it governs the ease of fluid flow.
• Wider throats allow higher permeability, while smaller or poorly connected throats restrict flow.

Effect of Pore-Throat Connectivity

• Even with high porosity, poorly connected throats result in low permeability.
• some cases Rock types such as carbonates often exhibit high porosity but poor permeability due to isolated pores.

Impact of Diagenesis

• Processes like cementation reduce pore throat size, decreasing permeability.
• Dissolution enlarges throats, enhancing flow behavior.

Role of Sorting and Grain Size

• Well-sorted rocks with uniform grain size generally exhibit larger and more consistent pore throats, enhancing permeability.
• Poorly sorted rocks have variable throat sizes, leading to irregular fluid flow behavior.

The intricate relationship between pores and pore throats plays a fundamental role in determining reservoir quality. While pores provide fluid storage, pore throats regulate fluid movement and permeability, ultimately influencing hydrocarbon recovery. Variations in pore size, throat connectivity, sorting, grain size, and diagenetic alterations significantly impact the flow dynamics in both clastic and carbonate reservoirs. By integrating pore-throat analysis with petrophysical and geological insights we can better predict fluid flow behavior, permeability trends, and overall reservoir performance, ensuring more efficient hydrocarbon management.

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Please feel free to cite this article as:

Mahapatra, Mahabir Prasad (2025), Pore and Pore Throat: Their Impact on Reservoir

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