- Copyright © 2004 Society of Exploration Geophysicists
Often it is difficult to map subtle faults and other trace-to-trace discontinuities hidden in 3D seismic data. They may appear as minor changes in the seismic waveform that are not easily correlated using conventional interpretation of seismic cross-sections. To map these changes we have investigated and computed several different seismic attributes sensitive to the existence of small faults and fractures in our target reservoir, the Late Permian Unayzah sandstone of central Saudi Arabia. In addition to the traditional geometric horizon attributes such as dip and azimuth, we found that both coherence and spectral decomposition were useful in imaging seismic discontinuities.
In this article we demonstrate the use of seismic attributes for mapping small faults and fractures in the Unayzah sandstone—a deep, tight, low-porosity (1–6%) and low-permeability (0.1–1.0 mD) clastic reservoir. Our main goal was to gain a better understanding of and predict fault trends and traps in our study area, where the Unayzah reservoir is gas-bearing and occurs at a depth of approximately 4000 m, immediately below the Permian Khuff carbonates. A 3D seismic survey (Figure 1) was acquired over the study area in order to define structural closure, major faults, and any fracture zones that might enhance permeability in the Unayzah reservoir. Only one well has been drilled in the area, and only 3D seismic data can provide high-resolution information about faulting and fracturing away from the well control.
Faults were interpreted both conventionally (using cross-sections) and using seismic attributes. Major faults are easily identified at the base Khuff reflector due to the large contrast in acoustic impedance between the Khuff carbonates and …