In 50-75% of reservoirs, porosity and lithology can be successfully determined by well log analysis using crossplot techniques. This method is particularly useful when the reservoir is a shaly sand or carbonate sequence. In complex lithologies, with an abundance of accessory minerals, the technique can produce erroneous results if applied without modifications to the standard model. The method is not intrinsically flexible and needs adaptation whenever the formation differs markedly from a sandstone/carbonate/shale model. Use of crossplots and iterative analytical solutions derived from two porosity logs sometimes gives unsatisfactory results in hydrocarbon-bearing complex lithologies.
With modifications, the crossplotting technique can describe 90% of South-East Asia's reservoirs. A more flexible approach and an improved presentation format for results is presented. Two or more shale indicators are chosen and calibrated with the resulting shale volume being a weighted avearge of the indicators. Porosity information from all porosity devices is displayed. A quality curve is shown, demonstrating how well the answers fit the chosen model. Lithology definition is achieved by the study of slowness ratio from the long-spaced sonic device, photoelectric effect from the litho-density tool, apparent grain density from crossplot data and electro facies analysis.
Moveable hydrocarbons and permeability in clastic reservoirs are indicated by the nuclear magnetism log, wireline formation tester, electromagnetic propagation tool, core analysis and the resistivity invasion profile. For carbonate reservoirs, fracture indicators are displayed.