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Petrophysical techniques provide most of the subsurface data available to an exploration geologist. Besides their importance in completion decisions, they are also invaluable methods for mapping and identifying lithologies. Presented here are nine techniques that can assist geologists with lithologic determination and mapping.

These techniques are especially important to a geologist when lithologic data from core or samples are unavailable.

Figures 8.1 and 8.2 are logs through the Silurian Fusselman Formation in West Texas. The log suite for this well consists of a dual induction-SFL, SP, gamma ray, and neutron-lithodensity log (Figure 8.1), and a sonic log with gamma ray (Figure 8.2). Using data from these logs, we will illustrate how lithologies are determined using the first six methods listed above. It is important to mention that the porosity zone (9088 to 9126 ft) in the Fusselman has a combination of vuggy and intercrystalline porosity. How the presence of vuggy porosity affects the different lithology plots will be discussed in the text below. The software used to generate the results from the original log data is called CBA (Carbonate Advisor; The Logic Group, 1994). The various lithology plots and crossplots were rendered in PetroWorks, from Landmark Graphics, a Halliburton company.

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