# Petrophysics

Petrophysics usually refers to the science of geophysical log data collection, processing, and analysis. More broadly, the term might include the specialist subject of rock physics.

## Determination of Vsh from GR

Quantitative evaluation of shale content using gamma ray data assumes that radioactive minerals other than shales and clay are absent. The gamma ray shale index, ${\displaystyle I_{\mathrm {GR} }}$ is defined as

${\displaystyle I_{\mathrm {GR} }={\frac {\mathrm {GR} -\mathrm {GR} _{\mathrm {clean} }}{\mathrm {GR} _{\mathrm {shale} }-\mathrm {GR} _{\mathrm {clean} }}}}$

where ${\displaystyle \mathrm {GR} }$ is the log response in the zone of interest (API units), ${\displaystyle \mathrm {GR} _{\mathrm {clean} }}$ is the log response in a clean, shale free zone (API units), and ${\displaystyle \mathrm {GR} _{\mathrm {shale} }}$ is the log response in a shale bed (API units).

## Determination of Vsh from SP

Equation, pitfalls, and considerations...