sedi lecture 2
Porosity & Permeability
Definitions and Concepts
Porosity is defined as the fraction of a rock that is occupied by voids (pores). It signifies the fluid storage capacity of the rock and is an intensive property.
There are two main types of porosity: Primary (original) and Secondary (induced).
Primary Porosity: Developed at the time of deposition, typified by intergranular pores in clastic or carbonate rocks.
Secondary Porosity: Developed by geological processes after deposition, such as grain dissolution or fracture development.
Types of Porosity
Total Porosity: Represents the total volume of pore spaces within a rock. Can be mathematically defined as:
Total Porosity, ft = (Volume of Pores / Total Volume of Rock)
Effective Porosity: Represents the portion of the total porosity that contributes to fluid flow, essential for reservoir quality.
Effective Porosity, fe = (Volume of Connected Pores / Total Volume of Rock)
Comparison of Porosities
In very clean sandstones, effective porosity (fe) is approximately equal to total porosity (ft).
In poorly to moderately well-cemented intergranular materials, total porosity is greater than effective porosity (ft > fe).
In highly cemented materials and many carbonates, effective porosity is typically less than total porosity (fe < ft).
Rock Matrix and Pore Space
/
/
/
Components
Rock Matrix: Composed of solid materials forming the bulk of the rock.
Pore Space: Refers to the voids present in the rock that can contain fluids such as water, oil, or gas.
In sandstones, the major components include framework material (such as quartz or feldspar), cement that binds the grains, and the accompanying pore space.
Factors Affecting Porosity
Porosity is influenced by various factors:
Particle Sphericity and Angularity: More spherical and rounded grains can pack more closely, potentially reducing porosity.
Packing: Cubic packing leads to higher porosity compared to rhombohedral packing.
Sorting: Different grain sizes can enhance or reduce porosity depending on how they fit together.
Cementing Materials: The presence of cement can fill in pores, decreasing effective porosity.
Overburden Stress: Compaction from overlying material alters porosity.
Vugs, Dissolution, and Fractures: These can form additional pore spaces in the rock.
Porosity Types in Sandstone
Intergranular (Primary) Porosity: Space between the grains of sand in the framework.
Dissolution Micropores: Formed due to the dissolution of framework grains or cement, creating smaller void spaces.
Fractures: Formed due to mechanical stresses, contributing to additional porosity.
Diagenesis and Its Effects
Diagenesis refers to the post-depositional changes that occur in sedimentary rocks, involving chemical and mechanical transformations.
Processes include compaction, precipitation of cements, and dissolution of the framework, which can either enhance or degrade reservoir quality.
Example of Carbonate Porosity Types
Interparticle, Intraparticle, Intercrystal: Referring to pore types between and within grains and crystals, which can greatly affect the porosity of a carbonate rock.
Moldic Pores: Form typically through the dissolution of individual grains.
Fenestral, Fracture, and Vug Pores: Larger voids contributing to porosity through different geological processes.
Importance of Effective Porosity
Effective porosity directly relates to how fluid can move through and be stored in the rock, impacting resource extraction in petroleum and water reservoirs.
Conclusion
Porosity and permeability are vital properties in geology and resource extraction, interconnected through rock types, geological history, and the physical characteristics of the sedimentary materials involved. Understanding these parameters is essential for evaluating the capacity of a rock formation to store and transmit fluids.