Gas sorption in source rocks - A short discussion

Until now the gas sorption capacity threshold from Pepper and Corvi (1995) of 20 mgGas/gTOC based on coal measurements was used widely to estimate gas expulsion from source rocks in a conventional setting and to provide estimates of sorbed gas quantities retained for unconventional resources. However, more recent gas sorption experiments on oil-prone source rocks show that an average of 35 mgGas/gTOC is more appropriate to use for non-terrigenous influenced source rocks and this value does not include sorption on minerals. The overall range of sorption capacity is very wide for non-terrigenous source rocks and ranges between 15 and 65 mgGas/gTOC (+/- 25 mgGas/gTOC). Hence, in the absence of Langmuir sorption measurements, the rock intrinsic uncertainty is much larger than the Langmuir pressure and temperature effects. Only when the uncertainty of rock sorption capacity is reduced to +/- 10 mgGas/gTOC pressure and temperature effects on source rock sorption capacity should be considered in the form of Langmuir calculations.