SALT PLUGGED POROSITY
Reservoirs below salt beds are often plugged with salt in
the pore space, at least over the top of the interval. The
porosity logs will show some porosity and the resistivity
log will read very high values, because pure salt is non
conductive. This combination of curve responses usually
indicates hydrocarbons, but a test will quickly prove
otherwise. The resistivity is just "too good to be true",
and so is the apparent water saturation.
There
may be oil or gas or water in porosity below the plugged porosity.
Oil or gas will usually show lower resistivity than the salt plugged
porosity, due to the irreducible water in the reservoir, which is
usually saturated salt water. Water zones are usually saturated salt
water and very low resistivity. Tight zones will have resistivity
like the salt plugged interval, but the porosity logs will show
little porosity. So with the skill of a CSI detective, you can find
the evidence, assess the evidence with logical deduction, and nail
the guilty party.
The pulsed neutron log is strongly
influenced by chlorine, so this tool ought to be useful in
calculating the amount of plugging. Some reservoirs are only
partially plugged and the conventional logs don't have enough
resolution to distinguish partial plugging.
If the porosity is completely
plugged and the matrix lithology is known, you can calculate the
plugged porosity from the density log equation:
1: PHIplugged = (DENS - DENSMA) / (DENSsalt -
DENSMA)
Where DENSsalt = about 2.05 gm/cc (as seen by a density log).
Salt plugged reservoirs are found
in the Winnipegosis in Saskatchewan and Alberta, and the Niagaran
reef plays in Michigan and Ontario. When depleted, they make
excellent gas storage sites.
Raw data (left for salt plugged example. PE shows nearly 3.7 in
plugged interval, higher than straight dolomite. Red shading shows
density neutron "crossover" in pure salt. Caliper shading shows
washout from salt solution. Answer plot (right) is coded to show
ineffective porosity in salt plugged interval. Perfs are shown at right
edge of Track 4; top perfs in salt plugged interval were dry, lower
perfs made oil with some water.
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