Page 150 - Petroleum Geology

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mud based on sea water also lead to SP reversal over at least the upper part
of the hole, and where the salinity of the mud and its filtrate are equal to
that of the formation water, no deflexion of the SP occurs.
The total electromotive force corresponding to the electrochemical po-
tential can be expressed:

E,, = -K log a, (6.10)
--
amf
where a, and amf are the chemical activities of the formation water and the
mud filtrate, and K is a coefficient proportional to the absolute temperature.
However, if the concentrations of the two NaCl solutions are less than about
80,000 ppm, their chemical activities are approximately inversely proportional
to their resistivities, so that eq. 6.10 can be approximated by:
RInf
E,, = -K log -. (6.10a)
Rw
Of these parameters, E,, can be estimated and R,, measured.
The electrokinetic (or streaming) potential (Ek) in a borehole is consid-
ered by some to be very small, and by others to be non-existent from a
practical point of view. This potential is attributable to the flow of mud
filtrate across the mud cake into the permeable rock unit. It is probably
large, but approximately equal to that opposite the mudstones. The contri-
bution of Ek is the difference between the e.m.f. across the mud cake and
that into the mudstone (for few mudstones are so impermeable that no flow
takes place into them). There is no argument that whatever its value, the
electrokinetic potential makes at most a small contribution to the total de-
flexion of the SP on most logs.
In theory, many parameters influence the shape and amplitude of the de-
flexion of the SP opposite the more permeable beds. They include the thick-
ness and true resistivity of the permeable bed, and the resistivity of the ad-
jacent beds; the resistivity of the mud and its filtrate; the resistivity of the
invaded zone and its diameter; and the diameter of the borehole. It is not
strictly a permeability log because a small fraction of a millidarcy (mD) is
sufficient for an e.m.f. to be generated. Nevertheless, it is an invaluable log
from which mudstones and shales can be distinguished from more permeable
lithologies, fresh water can be distinguished from brine (by its reversed SP
and high resistivity). Quantitative analysis leads to the determination of
formation water salinities without the need of a sample at the surface. It
is a log from which lithological boundaries and their depths are recorded,
and from which the thicknesses of rock units are obtained.

Radioactivity logs

Some rock types, particularly mudstones, emit gamma rays spontaneously