Porosity is the measure of the void spaces (pores) within a rock or sediment. It is expressed as a percentage of the total volume of the material and determines the capacity of the material to store fluids like water, oil, or gas.

Types of Porosity

1. Primary Porosity:

   • Original porosity formed during the rock's formation.
   • Example: Spaces between grains in sedimentary rocks or vesicles in volcanic rocks like basalt.
   • Typically higher in unconsolidated materials (e.g., sands) and vesicular rocks.

2. Secondary Porosity:

   • Develops after the rock is formed due to processes like fracturing, dissolution, or weathering.
   • Example: Fractures and joints in granite or solution cavities in limestone.

Factors Affecting Porosity

1. Grain Size:

   • Smaller grains (e.g., clay) pack more tightly, reducing porosity. Larger grains (e.g., sand) often have higher porosity.

2. Sorting:

   • Well-sorted materials (uniform grain size) have higher porosity than poorly sorted ones (mix of sizes).

3. Packing Arrangement:

   • Cubic packing has higher porosity than rhombohedral packing.

4. Cementation:

   • Cementation fills voids, reducing porosity, common in sedimentary rocks.

5. Fracturing:

   • Fractures increase secondary porosity, particularly in crystalline rocks like granite.

6. Weathering:

   • Enhances porosity by breaking down minerals and creating voids, especially in basalt and limestone.

Porosity in Different Rock Types

• Sedimentary Rocks: High porosity in sands and gravels; lower porosity in shales due to compaction.
• Igneous Rocks: Low primary porosity; fractures and vesicles may provide secondary porosity (e.g., basalt, granite).
• Metamorphic Rocks: Generally low porosity due to recrystallization, though fractures can increase it.

Mathematical Expression

• High porosity: ≥20% (e.g., unconsolidated sands).
• Moderate porosity: 5–20% (e.g., sandstones).
• Low porosity: <5% (e.g., granite, unfractured basalt).

Porosity is a crucial parameter in hydrogeology, petroleum geology, and soil science, as it governs a material's ability to store fluids. However, porosity alone does not determine fluid flow; permeability is equally important.