Why Do You Think Wireline Logs Will Recognize Your Carbonate Facies; Here Is How It Works
F. Jerry Lucia, Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX
Wireline logs can identify only a limited number of facies, and geologic studies often require many more facies than is possible it identify. Wireline logs measure physical properties, not geologic descriptions, so only facies, as defined by core descriptions, having unique physical properties can be identified with logs. Facies defined by lithology can normally be identified using a combination of neutron, density, PE and sonic logs. Facies not defined by lithology are more difficult. Fundamentally, only facies that have unique gamma-ray, porosity, or pore-size values can be uniquely identified with wireline logs. Acoustic-porosity relationships can sometime be used to define highly moldic facies and infer a moldic grainstone, but these relationships must be used with extreme caution. Saturation-porosity relationships describe pore-size distributions that can be linked with basic rock-fabric facies under ideal conditions. These ideal conditions, however, often to not exist and we are left with porosity and gamma-ray logs. Gamma-ray values tend to be lower in grain-dominated than in mud-dominated fabrics, but the presence of diagenetic uranium complicates this basic tenet. Porosity in grain-dominated fabrics tends to be higher than in mud-dominated fabrics, even after early dolomitization. Therefore, vertical porosity and gamma-ray profiles can often be used to identify vertical sequences of mud-to-grain-dominated fabrics. Grain types must be inferred from stratigraphy. Because of the limited ability of wireline logs to identify carbonate facies, the selection of facies used to construct a geologic model is critical.