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Spectral Decomposition and Colored Inversion as Tools to Extract Stratigraphic Features and Predict Lithology of the Late Ordovician Mamuniyat Reservoir, Murzuq Basin, Libya

Abstract

Late Ordovician glacial deposits of the Lower Mamuniyat Formation represent the oil reservoir for the R-Field oil field in the Murzuq Basin in Libya. The deposits are comprised of two units; the Clean Mamuniyat and the Dirty Mamuniyat. Multiple advances and retreats of ice sheets during the Ordovician resulted in thickness variability and heterogeneity of these units. The Dirty Mamuniyat is observed in the same seismic reflection zone as the clean Mamuniyat representing a facies change from clean sand to intercalation of sand and shale. The thickest part of the Clean Mamuniyat is subdivided into a sand subunit (A) and shaly sand subunit (B) in the upper and lower parts, respectively, of the Clean Mamuniyat. Our study focuses on using seismic attributes to distinguish and map the facies changes that are associated with those subunits within the complex and with the heterogeneous facies distribution. Spectral decomposition and colored inversion techniques were found to be useful to determine reservoir unit distribution and lithological predictions. We decomposed the seismic data using a Continuous Wavelet Transform approach with a Morlet wavelet and extracted frequencies along the reservoir interval. Three frequencies 24, 34 and 64Hz represent the main stratigraphic features and variations in thicknesses of the reservoir. We also inverted the seismic data into relative acoustic impedance and used geobody extraction to extract variation in acoustic impedance associated with facies distributions. Both inversion and spectral decomposition results led us to divide the Clean Mamuniyat into three facies; A1 B1, and B2, to distinguish those facies from non-reservoir facies of Bir Tlacsin and to determine their distributions within the oil field. These newly constructed facies distribution and captured stratigraphic features are critical findings for further studies of reservoir characterization and modeling.