Using instantaneous frequency and colored inversion attributes to distinguish and determine the sandstones facies of the Late Ordovician Mamuniyat reservoir, R-field in Murzuq Basin, Libya

Abstract

The Mamuniyat petroleum reservoir in southwestern Libya is comprised of clean sandstones and intercalated shale and sand facies that are characterized by spatial porosity variations. Seismic reflection data from the field exhibit relatively low vertical seismic resolution, side lobes of reflection wavelets, reflection interference, and low acoustic impedance contrast between the reservoir and the units underneath the reservoir, which make mapping those facies a difficult task. In the absence of broadband seismic data, optimizing frequency bands of bandlimited data can be used to suppress pseudoreflectors resulting from side-lobe effects and help to separate the clean sandstone facies of the reservoir. We have optimized the data based on our investigation of seismic frequency bands and used instantaneous frequency analysis to reveal the reflection discontinuity that is mainly associated with the reservoir boundary of the sandstone facies of the clean Mamuniyat reservoir. We also preformed rock-physics diagnostic modeling and inverted the seismic data using spectral-based colored inversion into relative acoustic impedance. The inverted impedance matches the up-scaled impedance from the well data and the inversion of relative acoustic impedance confirms the conclusion that was drawn from the instantaneous frequency results. The interpretation of facies distributions based on the instantaneous frequency was supported by the inversion results and the rock-physics models.

Application of amplitude spectrum and genetic inversion to determine shaly facies distribution: a case study: Murzuq Basin, Libya

“…11). It also supports the geological observation that the Bir Tlacsin thins or is absent at near the mouth bar and distributary channel (Abushalah and Serpa 2016) and is present or thick at the flank of it. The top Mamuniyat reservoir was approximated by the base of the inverted density of the shaly facies of Bir Tlacsin as indicated by the solid black line in Fig.…”

“…Considering the change of the density and the increases of seismic resolution after inversion (Hampson et al 2001;Hill 2005), one can expect that the density inversion should resolve the thin bed of the Bir Tlacsin. In addition, from our experience of using inverted relative acoustic impedance (RAI) in previous work (Abushalah and Serpa 2016), we noted that the RAI was not able to resolve completely the Bir Tlacsin, which might have been related to the low acoustic impedance contrast between the Bir Tlacsin and the underlying unit of Mamuniyat.…”

“…In this case, it is unclear whether the reflections represent the top of the Mamuniyat reservoir or the Bir Tlacsin using the seismic data alone. There are also side lobe reflections just below the reservoir (Abushalah and Serpa ), as indicated by red arrows in Fig. .…”

“…1). The Clean Mamuniyat is the main petroleum reservoir in the sequence because it has excellent porosity compared with the Dirty Mamuniyat, which has numerous shale layers that reduce the porosity locally (Abushalah and Serpa 2016). The Bir Tlacsin Formation is a mud-prone unit in the Murzuq Basin that was interpreted to be a transitional unit between Mamuniyat Formation and Silurian Shales (Echikh and Sola 2000).…”

“…1). The Clean Mamuniyat is the main petroleum reservoir in the sequence because it has excellent porosity compared with the Dirty Mamuniyat, which has numerous shale layers that reduce the porosity locally (Abushalah and Serpa 2016).…”