The formations are mostly unconsolidated sands and shales, in which it is not possible to take cores or to make Drill Stem tests.

Accordingly formation evaluation is based mostly on logs, with the help of sidewall samples and wire line formation tests.

Hole Conditions

Nominal hole diameters are often 12 1/4", or even larger. Moreover the sands tend to cave when excessive pumping rates are used. These hole conditions may affect the quality of the information given by the logs.

Mud Resistivity

Drilling muds are never very salty Rm BHT usually ranges from .1 to 1.0 ohm.


It seems to be a rather commonly accepted idea that in sands with good porosities, invasion tends to be shallow. In Nigeria invasion is often shallow in clean hydrocarbon bearing sands, but it can be fairly deep in other conditions. Shallow invasion appears to be mostly the consequence of vertical segregation of the mud filtrate in the invaded zone (reference 1); for this segregation to take place two conditions are required :

  1. The vertical permeability of the formation has to be good,

  2. The densities of the formation fluid and of the mud filtrate have to be substantially different.

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