Page 139 - Petroleum Geology

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Fig. 6-3. Oil or gas occupies the central parts of the pores in a water-wet reservoir rock.
The “connate” water is concentrated in pendular rings around the grain contacts.

of an oil or gas reservoir rock because there are cogent physical arguments
that the water is largely confined to what are called pendular rings (cf. pendu-
lar cement) around the grain contacts, and that these pendular rings are
separated from each other by a thin film of adsorbed water no thicker than
1 nm (3 or 4 water molecules). We shall consider this in more detail in Chap-
ter 8, and must accept meanwhile that eIectrica1 current can flow through
pores that are largely saturated with petroleum, and that there are reason-
ably satisfactory methods of estimating the water saturation (and so the
petroleum saturation) of reservoirs (see Morrow, 1971a, b, for a discussion of
these points). This is the principal quantitative use to which resistivity logs
are put.
Saturation is expressed as a fraction or percentage of the pore space oc-
cupied by water, oil or gas. It is of central importance to determine the oil
or gas saturation of a reservoir both for reserve estimation and for produci-
bility assessments, the latter because water saturation above some critical
value means that water only will be produced.
By definition:
s, + so = 1.
The resistivity of an oil-bearing reservoir rock is a function of the water
saturation s, and it has been found experimentally that:
s, = (R,/R,)lln (6.7)
where Rt is the true resistivity of the rock containing some oil, and R, is the