if mudstone overlaps with sand, and the porosities that favour production overlap the vertical boundaries, in this case the reservoir will be the sand, and will also include the mudstone, assuming it has moveable hydrocarbons? if yes, what does this mean for the GRV and Net-To-Gross (NTG)?
Yes. You should have a look at Pickett Plots and the Archie Equation at http://wiki.aapg.org/Pickett_plot_construction
The way these plots relate to GRV and NTG depends on the reservoir (and geological) characteristics of your oil and gas field.
Best of luck, Tiago
Petrophysical properties are a reflection of geological properties and how they affect fluid flow, so they cant be treated in isolation. Also delineation of your pay interval is based on some pre-defined cut off of Porosity, Permeability and fluid saturation.
Cheers!
In mudstones you should have practically zero effective porosity and no movable hydrocarbons; such deposits are not reservoirs, only the sands / sandstones are. If you get "porosities that favour production" also in mudstones, it is most likely due to errors in geophysical / well log data interpretation.
The definition of "Gross reservoir", "Net reservoir", "Gross pay" and "Net pay" is usually done by using cut-off values for the shale volume fraction (Vsh), effective porosity (Phi), water saturation (Sw) and, sometimes, bulk volume of irreducible water ("critical bulk volume of water") - BVWirr. For a certain field or zone, these cut-off values may be derived by correlations between permeabilities measured on cores and petrophysical parameters obtained from log interpretation, i.e. k = f(Vsh, Phi, Sw) relationships.
All the best.
Thanks so much, I really appreciate your answers. How can the drive mechanism in an unconventional reservoir (shale) be classified best?
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