Changes

===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., Hoyle, W. R., Schmidt, A. W., 1971, 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 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., Hoyle, W. R., 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.

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For a more complete explanation of water saturation equations and their terms, refer to Worthington<ref name=pt04r22>Worthington, P., 1985, The evolution of shaly-sand concepts in reservoir evaluation: The Log Analyst.</ref> or Patchett and Herrick.<ref name=pt04r9>Patchett, J. G., Herrick, D. C., 1982, A review of saturation models: SPWLA Reprint Volume Shaly Sands, SPWLA.</ref> The Simandoux and Indonesia equations were designed mainly for relatively salty formation waters and moderate amounts of dispersed clay. The dual water and Waxman and Smits equations were designed for all water salinities and moderate amounts of dispersed clays.

For a more complete explanation of water saturation equations and their terms, refer to Worthington<ref name=pt04r22>Worthington, P., 1985, [https://www.onepetro.org/journal-paper/SPWLA-1985-vXXVIn1a2 The evolution of shaly-sand concepts in reservoir evaluation]: The Log Analyst.</ref> or Patchett and Herrick.<ref name=pt04r9>Patchett, J. G., Herrick, D. C., 1982, A review of saturation models: SPWLA Reprint Volume Shaly Sands, SPWLA.</ref> The Simandoux and Indonesia equations were designed mainly for relatively salty formation waters and moderate amounts of dispersed clay. The dual water and Waxman and Smits equations were designed for all water salinities and moderate amounts of dispersed clays.

Recommended logs to use for interpreting shaly sandstones are

Recommended logs to use for interpreting shaly sandstones are