Changes

====Density-compensated neutron crossplot====

====Density-compensated neutron crossplot====

This type of crossplot is used for binary mixtures of sandstone and [[limestone]], limestone and [[dolomite]], dolomite and [[anhydrite]], and sandstone and shale.

This type of crossplot is used for binary mixtures of [[sandstone]] and [[limestone]], limestone and [[dolomite]], dolomite and [[anhydrite]], and sandstone and shale.

====Sonic-compensated neutron crossplot====

====Sonic-compensated neutron crossplot====

===Shaly sandstones===

===Shaly sandstones===

The interpretation method best suited for shaly sandstones is dependent upon the distribution of shale, the clay type, the mineralogy of the silt fraction, and the resistivity of water within the sandstones. The classic approach is the sand-silt-shale method introduced by Poupon et al.<ref name=pt04r10>Poupon, A., W. R. Hoyle, and A. W. Schmidt, A. W., 1971, [https://www.onepetro.org/journal-paper/SPE-2925-PA Log analysis in formations with complex lithologies]: Journal of Petroleum Technology.</ref> An approximate correction for a single heavy mineral was provided for in this approach. Silt is considered to be primarily [[quartz]]. Volume of clay, volume of silt, and porosity are determined from interpolation of the density-neutron crossplot. Matrix response points are defined for sand and silt, water, and dry clay minerals. A wet clay point is defined on the dry clay minerals-100% water line. A shale point was defined on the quartz-wet clay line. The model can then determine porosity, shale volume, and silt index from interpolation in this framework. Water saturation can be determined using an appropriate shaly sandstone resistivity equation. This method does not adequately address the more complex case of shaly sandstones with variable volumes of feldspar, mica, or carbonate material. This model can be solved using the graphical, linear matrix, or least squares minimization method.

The interpretation method best suited for shaly [[sandstone]]s is dependent upon the distribution of shale, the clay type, the mineralogy of the silt fraction, and the resistivity of water within the sandstones. The classic approach is the sand-silt-shale method introduced by Poupon et al.<ref name=pt04r10>Poupon, A., W. R. Hoyle, and A. W. Schmidt, A. W., 1971, [https://www.onepetro.org/journal-paper/SPE-2925-PA Log analysis in formations with complex lithologies]: Journal of Petroleum Technology.</ref> An approximate correction for a single heavy mineral was provided for in this approach. Silt is considered to be primarily [[quartz]]. Volume of clay, volume of silt, and porosity are determined from interpolation of the density-neutron crossplot. Matrix response points are defined for sand and silt, water, and dry clay minerals. A wet clay point is defined on the dry clay minerals-100% water line. A shale point was defined on the quartz-wet clay line. The model can then determine porosity, shale volume, and silt index from interpolation in this framework. Water saturation can be determined using an appropriate shaly sandstone resistivity equation. This method does not adequately address the more complex case of shaly sandstones with variable volumes of feldspar, mica, or carbonate material. This model can be solved using the graphical, linear matrix, or least squares minimization method.

The solution for the complex case of sandstones with feldspar, mica, and carbonate material was resolved after log analysts became comfortable with the new spectral gamma ray (K, Th, and U) and photoelectric (Pe) measurements. The spectral gamma ray log is helpful in sandstones containing potassium feldspars or thorium-bearing clays. The natural gamma ray spectra, Pe, density, and neutron expanded response equations can be combined to solve for porosity and to estimate volumes of calcite, [[quartz]], dolomite, clay, feldspar, anhydrite, and salt. Once porosity is determined, saturation can be estimated from the appropriate shaly sandstone resistivity equation. This model is too complex to address using graphical methods and must be done using the linear matrix or least squares minimization method.

The solution for the complex case of sandstones with feldspar, mica, and carbonate material was resolved after log analysts became comfortable with the new spectral gamma ray (K, Th, and U) and photoelectric (Pe) measurements. The spectral gamma ray log is helpful in sandstones containing potassium feldspars or thorium-bearing clays. The natural gamma ray spectra, Pe, density, and neutron expanded response equations can be combined to solve for porosity and to estimate volumes of calcite, [[quartz]], dolomite, clay, feldspar, anhydrite, and salt. Once porosity is determined, saturation can be estimated from the appropriate shaly sandstone resistivity equation. This model is too complex to address using graphical methods and must be done using the linear matrix or least squares minimization method.

Carbonates and [[evaporite]]s can complicate the interpretation process through the occurrence of fractures and vugs. The three primary porosity devices—the density, compensated neutron, and sonic—respond differently to the presence of fractures and vugs. The density investigates one side of the borehole, while the compensated neutron averages the formation properties around the borehole. The sonic generally does not respond to the secondary porosity associated with either fractures or vugs. This unequal response to these properties can lead to significant problems in determining lithology from any of the methods, as they generally presume that all the porosity devices see a common porosity and lithology.

Carbonates and [[evaporite]]s can complicate the interpretation process through the occurrence of fractures and vugs. The three primary porosity devices—the density, compensated neutron, and sonic—respond differently to the presence of fractures and vugs. The density investigates one side of the borehole, while the compensated neutron averages the formation properties around the borehole. The sonic generally does not respond to the secondary porosity associated with either fractures or vugs. This unequal response to these properties can lead to significant problems in determining lithology from any of the methods, as they generally presume that all the porosity devices see a common porosity and lithology.

Binary mixtures of carbonates or evaporites can be evaluated using graphical crossplot approaches. The density-neutron is generally the most used approach. This method will solve for sandstone-limestone, limestone-dolomite, or dolomite-anhydrite mixtures.

Binary mixtures of carbonates or evaporites can be evaluated using graphical crossplot approaches. The density-neutron is generally the most used approach. This method will solve for [[sandstone]]-[[limestone]], limestone-dolomite, or dolomite-anhydrite mixtures.

More complicated mixtures are best handled using the linear matrix or preferably the weighted least squares minimization technique. Additional minerals can be identified by incorporating the photoelectric, spectral gamma ray, and sonic logs. Some log measurements do not provide any relevant information in a given section. If the K, Th, and U values are uniformly low in the evaporite minerals, these logs have no resolving ability and are of little help in determining the rock constituents. If dolomite contains uranium, the uranium log is diagnostic.

More complicated mixtures are best handled using the linear matrix or preferably the weighted least squares minimization technique. Additional minerals can be identified by incorporating the photoelectric, spectral gamma ray, and sonic logs. Some log measurements do not provide any relevant information in a given section. If the K, Th, and U values are uniformly low in the evaporite minerals, these logs have no resolving ability and are of little help in determining the rock constituents. If dolomite contains uranium, the uranium log is diagnostic.