Seismic reflection characteristics of the Precambrian and Upper Cretaceous reservoirs in Nafoora-Augila oil field, Sirte Basin, Libya
The research project aimed to establish the seismic characteristics of the Upper Cretaceous and Precambrian basement reservoirs in the area of the Nafoora-Augila field. The characteristics studied include acoustic impedance, amplitude, phase, and reflection strength, derived from seismic profiles closely tied to well data. The project demonstrates to a certain degree the successful use of combined techniques to assist in the interpretation of the seismic data by improving its quality. The techniques included: one-dimensional and two-dimensional modelling, seismic attributes and seismic inversion techniques. The generated synthetic seismograms show a reasonably good match with observed seismic data at the well locations. The mismatch between reflection interfaces at some intervals indicated that it could be the result of inaccurate sonic measurements. Investigation on the sonic readings was conducted in the laboratory by measuring the transit time for core samples obtained from these intervals. The laboratory measurements generally supported the sonic. Two-dimensional modelling was carried out in order to compare synthetic sections with the observed seismic sections. The comparison indicated that model sections can be used for the recognition and confirmation of the reflection events of interest in the observed seismic sections and especially the seismic expression of the thin layers such as the Rachmat shale and the Bahi sandstone. The presence of multiples on the final seismic sections has obscured the primary reflection events of interest. Velocity analysis using the velocity spectra technique is conducted to improve the stack of the seismic data and minimize the effect of multiples. The newly picked stacking velocities show better results than those picked by the contractor by using the velocity function technique. The post-stack deconvolution was applied to attenutate the existing multiples by testing different deconvolution parameters. The tested parameters gave very encouraging results by bringing out distinctive reflectors and suppressing multiples. Seismic attributes and inverse modelling were used to enhance the interpretation of the seismic data. The use of the attributes on the observed seismic data are generally of little use due to the poor quality of the data. However, the amplitude and the phase displays show a reasonably good response even with the effect of noise and multiples. Comparison of pseudo impedance logs and sections with well impedance logs and seismic sections shows fair to reasonably good comparison.