The Property Model - Reservoir Model Design - page 93

Fig. 3.28 Plot of V p /V s versus P-wave Impedance illustrating the separation of facies categories from logs (From Nair
et al. 2012 ) (Redrawn from Nair et al. 2012 , # EAGE reproduced with kind permission of EAGE Publications B.V.,
The Netherlands)

Table 3.4 Definition of terms used to describe the Net-to-Gross ratio
Term Definition Comment

Net sand A lithologically-clean sedimentary rock Can only be proved in core but inferred

from log data

Net
reservoir

Net sand intervals with useful reservoir properties Usually defined by a log-derived porosity

cut-off

Net pay Net reservoir intervals containing hydrocarbons Usually defined by a log-derived

saturation cut-off

Net-to-gross A ratio defined explicitly with reference to one of the above,
e.g. N/G reservoir

N/G sand 6 ¼ N/G reservoir

logical expression [IF Gamma < X AND Poro disciplines - petrophysics, geoscience and reser-

voir engineering - assume different definitions.
Another common piece of folklore, often
propagated within different parts of the petro-
leum industry is that oil and gas reservoirs have
specific values for permeability cut-off that
should be applied: for example the assumption
that the cut-off value for an oil reservoir should
be 1 mD but 0.1 mD for gas reservoirs. This
concept is based in Darcy's law and is best
understood in terms of a dynamic cut-off

> Y THEN NET]. In such a case, 'net sand' has

a rather weak and arbitrary association with the
geological rock type 'sandstone.' Worthington
and Cosentino ( 2005 ) discuss this problem at
length and show how widely the assumptions
vary in definition of net sand or net reservoir.
To avoid any further confusion with terminology
we adopt their definitions (Table 3.4 ). Many
problems arise from misunderstanding of these
basic concepts especially when different