A Petrophysical Investigation of Low Resistivity Hydrocarbon Zones in the Moran Field, Papua New Guinea EA
Crabb, Ivan P.A.
Honours Degree, 2001
University of Adelaide
The petrophysical characterization of low resistivity pay zones has been the topic of extensive research due to the difficulty of obtaining reliable water saturation estimates in such zones, and indeed the potential for overlooking these hydrocarbon-bearing zones on log data. Water saturations determined using resistivity-based, capillary pressure-based and nuclear magnetic resonance (NMR)-based methods in the Toro and Digimu sandstone reservoirs in the Moran 2X sidetrack 2 (2XST2) and Moran 4 exploration wells, Papua New Guinea, are inconsistent, and each method is subject to parameter uncertainty. The oil-bearing Toro and Digimu reservoirs in Moran 4 exhibit lower resistivities than equivalent oil-bearing reservoirs within the Moran Field.
Three working hypotheses were investigated to resolve the origin of low resistivity in Moran 4, and to evaluate its effect on each of the methods of determining water saturation. The first hypothesis was that uncertainty in the depth of the oil water contact may render capillary pressure-based determinations of water saturation unreliable, and that the low resistivity may indeed reflect high water saturations. This hypothesis is considered unlikely because it would require unrealistic variations in the oil water contacts in both Moran 2XST2 and Moran 4, thus enabling a high degree of confidence to be placed in water saturations determined using the capillary pressure-based method. The second hypothesis was that elevated water salinity may be the cause of low resistivity hence discrepancies in water saturation. This is also considered unlikely in the Digimu in Moran 4 because geological constraints and SP data indicate present day, low salinity waters are likely to be present. Elevated water salinities cannot be precluded from occurring in the Toro in Moran 4. The third hypothesis was that variable mineralogy and rock properties are the cause of low resistivity and water saturation discrepancies. Clay mineralogy is variable and may account for the variable clay resistivities required in order for the dual water resistivity-based method to yield water saturations which match those determined using the capillary pressure-based method. Laminated bedding also occurs in Moran 4 and this is considered another likely cause of low resistivity. Variable proportions of iron-rich minerals may be responsible for discrepancies between NMR-based and capillary pressure-based water saturations due to their likely influence on transverse relaxation time (T2) distributions, a critical parameter required for determining NMR-based water saturations.
Water saturations determined using the capillary pressure-based method for the Toro and Digimu in Moran 2XST2 and Moran 4 are likely to be representative of true water saturations within the reservoirs and should be used for reserve estimates. ‘Quick-look’ water saturation estimates using resistivity-based methods in low resistivity, oil-bearing reservoirs in the Moran Field are likely to be more reliable if the clay resistivity parameters used in this investigation are applied. However, mineralogy within the Toro and Digimu reservoirs in the Moran Field, and possibly the Papuan Foldbelt, is likely to be complex and variable and increased sampling of cation exchange capacity, capillary pressure and T2 distributions within the reservoirs may be necessary in order to obtain reliable and consistent water saturations from resistivity, capillary pressure or NMR data.