Formation properties in the Nubian Sandstone, a braided fluvial reservoir, are very variable. This is clear from porosity and permeability data cross-plots. There is no simple relationship between the two. The Petrotype-based sampling approach, where the routine core data are first plotted on an underlying, pre-determined grid, allows the selection of core plugs with a range of properties in order to carry out special core analysis (SCAL) measurements. With a few SCAL measurements, in this saturation exponent screening study, the underlying trend of the property and its controls (in this case primary depositional texture) becomes clear. This understanding can then be further exploited in the enhanced reservoir description of the Nubian Sandstone.
In this study, the porosity and permeability in the Lower Cretaceous Nubian Sandstone Formation of the Sirt Basin, Libya, as determined by laboratory measurements, are seen to be highly variable across the whole volume of the reservoir, from moderate to good in the some intervals to very poor in other intervals. In a braided fluvial reservoir, the primary texture plays an important role on the distribution of properties (Brayshaw et al., 1996). The continuing challenge for geoscientists and engineers is to improve the reservoir description programs, by including a description at the pore throat scale through rock typing (Amaefule et al., 1993). For this reason Global Hydraulic Elements, an adaptation of traditional rock typing method, have been used in this study to improve the description of Nubian Sandstone Formation. Using this breakdown of petrophysical data – and we see that other rock typing approaches produce similar results – is a process of Petrotyping (Corbett and Potter, 2004) and this is different from more conventional petrophysical rock typing approach (Amaefule et al., 1993) because the boundaries of the petrophysical elements are predetermined prior to analysing any plug data. This allows for a rapid and more systematic approach for varying data sets in various wells. The estimation of hydrocarbon reserve is strongly dependent on electric log data and the saturation exponent has either been assumed or an average value for the whole of reservoir has been used. The reason for that is the petrophysist does not usually have a more detailed description of reservoir. Therefore, the main objective of this study is to use this approach to estimate the distribution of the saturation exponent in the Nubian Sandstone Formation. Fluvial sandstones are often characterised by great variability and the determination of the correct saturation can be critical (O'Sullivan et al., 2009).
Reservoir description has many applications in the geology, petrophysics, reservoir engineering and production. Reservoir description is very important to understanding of the reservoir and it is generally hoped that more consideration of reservoir description may lead to less time spent history matching in reservoir modelling. Petrophysists have long tried to define a hydrocarbon–bearing reservoir as a limited set of elements number with unique characteristics of each one. To address this issue, Amaefule et al. (1993) introduced the first approach of the Hydraulic Flow Units (HFU) concept.
