Many times, reservoir studies use legacy or former petrophysical cutoffs already computed. However, you may want to validate those VSH, PHIE and SW cut-offs, or even estimate or compute suitable cutoffs for a new formation or members under study.
There are several techniques or criteria to define cutoffs from a production point of view. This section illustrates and provides a free GLS script program to compute or estimate the cutoffs from interpreted LAS petrophysical logs (that is, with VSH, PHIE and SW curves), using the Cumulative Hydrocarbon Column technique.
The concept of the hydrocarbon column in a formation is simple: HCOL = PHIE * (1-SW) * Delta_H, is the equivalent height of pure hydrocarbon column contained in a zone of thickness Delta_H > HCOL, when poured into a recipient.
Clean rocks with low Volume of Shale VSH usually have few problems or capability to store hydrocarbons. As a rock becomes more shaly, it will be more difficult either to store hydrocarbons, or the hydrocarbon to migrate from the source rock to be trapped into the reservoir. There is an elbow point of VSH beyond which there are no more significant contributions to store hydrocarbons. That point could be taken as a VSH cutoff for pay rocks. The same concept applies for effective porosity PHIE (and also permeability). There is a threshold point of tight porosity with a low capability to store hydrocarbons.
The sequential algorithm can be summarized as follow: