Porous and permeable rocks are required for an oil reservoir to form. Porosity is defined as pore volume per unit gross volume of rock. Effective porosity is the porosity that is available to free fluids, excluding unconnected porosity]] and space occupied by bound water and disseminated shale. A porous rock consists of a solid skeleton and a continuously connected fluid phase.
There are various types of porositiy. Intergranular porosity is common in sandstone. Rocks with vugular porosity contain small cavities and connected channels created by solution and recrystallization in existing rock. Fracture porosity consists of intersecting cracks caused by tectonic stresses applied to an otherwise impermeable rock.
What are primary porosity and secondary porosity? Primary porosity is the porosity that remains after sediments have been compacted but have not been changed by subsequent chemical action or water flow through them. Secondary porosity is the additional porosity created by subsequent changes in sediments — especially such changes as fissures, fractures, solution vugs, and porosity resulting from dolomitization. Many rocks have small amounts of pore space in the form of isolated microcracks. It is useful to look at this as a characteristic of the skeleton and to reserve the term porosity for interconnected space capable of supporting fluid flow.
How is porosity determined? Porosity is determined from cores, from sonic logs, from density logs, from neutron logs, or from resistivity logs. In the study of seismic-wave propagation through rocks, it is not possible to specify each grain, vug, or crack. A means of determining average properties is necessary. Two significant approaches are Gassmann’s theory and Biot’s theory (Gassmann, 1951; Biot, 1956).
- Gassmann, F., 1951, Ueber die elastizitaet poroeser medien: Vierteljahresschrift der Naturforschenden Gesellschaft in Zuerich, 96, 1–23.
- Biot, M. A., 1956, The theory of propagation of elastic waves in a fluid-saturated porous solid, Parts I and II: Journal of the Acoustical Society of America, 28, 168–191.
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Also in this chapter
- The seismic reflection method
- Seismic interpretation
- Absorption loss and transmission loss
- The wave equation
- Wave velocity
- Velocity analysis
- Seismic tomography
- Appendix C: Exercises