The cut-off for porosity comes from whether the rock is permeable or not.
Conventional Wireline logs can measure porosity from neutron tools and permeability can be inferred from variability in shallow and deep resistivity measurements.
The way to put them together is to cut a core and measure the porosity and permeability directly. These measurement can then be plotted and the porosity that corresponds to no permeability becomes the cut-off. Core can also have other parameters measures such as GR to aid correlation.
As there can be some changes to the core sample when retrieving it to surface, it is ideal to also conduct a MDT to measure the near wellbore permeability and correlate it against core sample.
All these things take time, and cost money, so Operators would use adjacent field data or analogues to provide an initial cut-off. But as these cutoffs affect reserves and ultimately market cap it is often worth the extra effort to have field specific data.
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