Abstract
Eocene Sokor-1 reservoir is intrinsically heterogeneous and characterized by low-contrast low-resistivity log responses in parts of the Termit subbasin. Discriminating lithology and fluid properties using petrophysics alone is complicated and undermines reservoir characterization. Petrophysics and rock physics were integrated through rock physics diagnostics (RPDs) modeling for detailed description of the reservoir microstructure and quality in the subbasin. Petrophysical evaluation shows that Sokor-1 sand_5 interval has good petrophysical properties across wells and prolific in hydrocarbons. RPD analysis revealed that this sand interval could be best described by the constant cement sand model in wells_2, _3, _5 and _9 and friable sand model in well_4. The matrix structure varied mostly from clean and well-sorted unconsolidated sands as well as consolidated and cemented sandstones to deteriorating and poorly sorted shaly sands and shales/mudstones. The rock physics template built based on the constant cement sand model for representative well_2 diagnosed hydrocarbon bearing sands with low Vp/Vs and medium-to-high impedance signatures. Brine shaly sands and shales/mudstones were diagnosed with moderate Vp/Vs and medium-to-high impedance and high Vp/Vs and medium impedance, respectively. These results reveal that hydrocarbon sands and brine shaly sands cannot be distinctively discriminated by the impedance property, since they exhibit similar impedance characteristics. However, hydrocarbon sands, brine shaly sands and shales/mudstones were completely discriminated by characteristic Vp/Vs property. These results demonstrate the robust application of rock physics diagnostic modeling in quantitative reservoir characterization and may be quite useful in undrilled locations in the subbasin and fields with similar geologic settings.
Highlights
Sedimentary rocks are considered as mechanical systems of mineral grains, pores and pore fluids formed by single or combinations of geologic processes
Brine shaly sands and shales/mudstones were diagnosed with medium-to-high impedances and moderate Vp/Vs ratio and medium impedance and high Vp/Vs ratio, respectively
Petrophysics and rock physics diagnostic modeling have been adopted in an integrated scheme to evaluate reservoir microstructure and quality for quantitative reservoir characterization in Termit subbasin
Summary
Sedimentary rocks are considered as mechanical systems of mineral grains, pores and pore fluids formed by single or combinations of geologic processes. The environment of deposition and burial history controls textural and mineralogical characteristics of the rock and determines their geologic properties. Rock physics is an indispensable tool for quantitative interpretation in reservoir studies, providing better understanding of the relationships between geologic properties and seismic elastic parameters. It represents the bridge that links geologic properties of porosity, clay content, saturation, lithology and texture and seismic elastic parameters such as P-wave velocity (Vp), S-wave velocity (Vs) and their ratio (Vp/Vs), acoustic impedance (P-Imp), bulk density and elastic moduli (Avseth et al 2000, 2010; Simm and Bacon 2014). Termit subbasin situated within the Chad Basin in Niger is filled with lower Cretaceous to Neogene sedimentary rocks (Fig. 1). Tectonic activities affected sedimentary processes such that structural, lithological and textural characteristics of the sediments are grossly impacted (Warren 2009). The reservoir sandstone intervals in some parts of the subbasin are radioactive with high clay content (Chang and Zung 2017), typical of low-contrast low-resistivity reservoirs (LCLRs) (Zhiqiang et al 2007)
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