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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