Complex lithologies, low-salinity formation water and conductive clays in the Jurassic reservoirs of the Cooper Basin in Australia provide a challenging environment for reliable petrophysical interpretations. Traditional interpretations often underestimate hydrocarbon saturation and fail to identify productive intervals. To provide more reliable interpretations three wireline tools were employed that offer new technology in an attempt to resolve these uncertainties. The additional tools consisted of a pulsed neutron geochemical log to derive mineralogy and hydrocarbon content, nuclear magnetic resonance to provide an accurate pore distribution and a multi-component induction tool to assess differences in the horizontal and vertical resistivities and to better understand the structural dips of the inter-bedded strata comprising the producing intervals. While some of the individual results were unexpected, integrating the results from all measurements provided significant improvements in understanding the reservoirs and their production performance. The trial of all these technologies involved measurements close to and sometimes outside their normal operating envelope. Data acquisition proved to be challenging and involved modifications to mud programs to achieve the optimum results. This paper demonstrates how the integration of these measurements increased the ability to better assess hydrocarbons in place in these reservoirs beyond what traditional methods had derived.
The mineralogy of the Jurassic reservoirs in the Cooper Basin of central Australia consists of feldspathic litharenites with volcanic lithic components. The elevated GR from the feldspathic minerals and thorium associated with the lithic components makes deriving a reliable shale/clay volume difficult. These oil-bearing formations were deposited under fluviatile-lacustrine conditions with fluviatile, low-energy mixed load as the predominant facies type. Deriving a reliable saturation model is exacerbated by the low-salinity formation water (3 – 4 kppm NaCl equivalent), which leads to subtle and sometimes non-existent resistivity contrast between the hydrocarbon and water-bearing zones.