Page 42 - Well Logging and Formation Evaluation

32 Well Logging and Formation Evaluation

2. Define the shale volume, V sh, as (GR - GR sa)/(GR sh - GR sa). By com-
paring V sh with the density/neutron response, determine a value of V sh
to use as a cutoff. Typically 50% is used.

If the GR is not usable as a sand indicator, then for now just treat the
entire gross as being net sand and apply a porosity cutoff at a later stage
(see next section).

2.3 IDENTIFYING THE FLUID TYPE AND CONTACTS

Because the porosity calculation will depend on the formation fluid
type, it is good at this stage to at least have a working assumption regard-
ing the fluids. If regional information is available regarding the positions
of any gas/oil contact (GOC) or oil/water contact (OWC), then convert
these subsea depths into measured depths in the current well and mark
them on the logs. If the formation pressures have already been measured
(this is usually never the case), then any information on possible free water
levels (FWLs) or GOCs can also be marked on the log.
Start by comparing the density and deepest reading resistivity log for
any evidence of hydrocarbons. In the classic response, the resistivity and
density (and also GR) will be seen to “tramline” (i.e., follow each other
to the left or right) in water sands and to “Mae West” (i.e., be a mirror
image of each other) in hydrocarbon sands. However, some hydrocar-
bon/water zones will not exhibit such behavior, the reasons being:

• When the formation-water salinity is very high, the resistivity may also
drop in clean sands.
• In shaly sand zones having a high proportion of conductive dispersed
shales, the resistivity may also fail to rise in reservoir zones.
• If the sands are thinly laminated between shales, the deep resistivity
may not be able to “resolve” the sands, and the resistivity may remain
low.
• If the well has been drilled with very heavy overbalance, invasion may
be such as to completely mask the hydrocarbon response.
• When the formation water is very fresh (high R w), the resistivity may
Mae West even in water-bearing zones.

When either of the first two situations arises, it is very important to look
at the absolute value of the deep resistivity, rather than at only the behav-
ior compared with the density. As long as a known water sand has been