Iranian Journal of Geophysics

Iranian Journal of Geophysics

Providing new factors for estimating lithology and investigating porosity changes using a combination of compressional and shear wave velocities

Document Type : Research Article

Authors
Behzad Nasrnia 1
Reza Falahat 2
1 1 Ph.D., Petroleum Engineering, Exploration, Faculty of Petroleum and Gas Engineering, Tabriz University of Technology, Tabriz, Iran
2 Assistant Professor, Exploration, Faculty of Petroleum and Gas Engineering, Tabriz University of Technology, Tabriz, Iran
10.30499/ijg.2025.505998.1673
Abstract
Electrical resistivity tomography is a non-invasive, near-surface geophysical technique commonly used In recent decades, lithological and porosity factors have been used to a limited extent using seismic data as a criterion that shows lithology type, porosity changes, and saturation in hydrocarbon reservoirs. Most lithological factors have been introduced based on analyses of amplitude changes with offset from the source and determination of the ratio of compressional and shear wave velocities. Thus, by using the relationships related to amplitude changes with offset period as well as the approximating relationships of the Zeoppritz equations, various lithology and fluid factors can be extracted. For example, estimating and calculating saturation and pore pressure changes resulting from production and injection operations in hydrocarbon reservoirs, is also considered one of the applications of Seismography. Lithological separation and investigation of porosity changes play an important role in the realization and advancement of exploration operations. Constructing an appropriate lithological and porosity factor using elastic parameters and their combination can be of great help in estimating lithology and separating porous zones. In addition to conventional factors, cross-plotting compressional wave velocity changes versus shear wave velocity changes, provides valuable information about lithology and their characteristics. The use of standard rock physics templates in many hydrocarbons, reservoirs has led to the separation of gas-bearing from water-bearing reservoirs. In this study, using standard rock physics templates and combinations of elastic parameters involved in them (such as compressional-to-shear wave velocity ratio, lambda, Rho, and elastic impedance) through mathematical routine operations, the trend of porosity and lithology changes was investigated and analyzed with the aim of increasing the resolution. These new combinations were presented with the aim of directly using the results of seismic inversion. In this study, by combining elastic parameters used in standard rock physics templates with four main mathematical operations, appropriate factors for lithological separation and investigation of porosity trends were extracted. In order to better distinguish the lithology in the study area, among the 8 standard patterns made by elastic parameters, the pattern whose horizontal and vertical axes represent the result of subtracting the values of the quantities on the horizontal axes (lambda-rho and acoustic impedance) and the vertical axes (mu-rho and velocities ratio), respectively, has the highest separation among the other patterns. In examining the trend of porosity changes, the standard pattern whose horizontal and vertical axes represent the difference between the values of the quantities on the horizontal axes (lambda-rho - acoustic impedance) and the vertical axes (mu-rho - velocities ratio), respectively, has the highest sensitivity to porosity and the lowest sensitivity to other parameters. In the study area, the results showed that the appropriate combination of elastic parameters (Lambda-Mu-Rho with acoustic impedance quantities and velocity ratios) led to an improvement in the lithology resolution by 7% and the separation of porous zones by 6% compared to the initial standard templates in the study area.
Keywords
lithological and porosity factors
compressional to shear wave velocity ratio
standard rock physics templates
elastic parameters

Subjects

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Iranian Journal of Geophysics
Volume 19, Issue 5
November and December 2025
Pages 183-193

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