Geoscience Reference
In-Depth Information
Clay minerals may also in
uence conductivity, a fact
first recognised by the failure of Archie
5.3.1.4
Geological controls on conductivity
The degree of electrical interconnectivity between the
conductive constituents is the major influence on electrical
conductivity. The effects of common geological processes
on the electrical conductivity/resistivity of rocks can be
understood in terms of their effect on conduction paths.
If porosity and permeability are increased, conductivity is
expected to increase. Processes that reduce these param-
eters, such as compaction, diagenesis and metamorphism,
have the opposite effect. In general, weathering increases
porosity and permeability, but the creation of clay minerals
may have the opposite effect.
Geological processes that affect connectivity through
conductive mineral grains include recrystallisation during
metamorphism, which may enhance conductivity through
changes in the rock
sequationin
rocks that the petroleum industry refers to as shaly sands.
In these rocks the formation factor, being the slope of the
curves shown in
Fig. 5.18
, changes with the degree of
conduction through the matrix minerals. The substitution
of ions within the sheet-like structure of clay minerals
causes substantial negative charge to accumulate on the
grain surfaces. To balance this charge, cations are
adsorbed onto the grain surfaces. The surface ions are
not bound to the crystal structure and may be exchanged
with other ions held in solutions that come in contact
with the grain. This alters the type and concentration of
ions in the pore fluid. All minerals exhibit this surface
conduction effect to some extent, notably zeolites and
organic material, but clay minerals are the most import-
The in
'
'
s texture which increase the number of
connected conductive grains. Also, sulphides deform com-
paratively easily during metamorphism, allowing them to
form interconnecting conductive networks. Silicification,
even if not intensive, can reduce the conductivity of mas-
sive sulphide mineralisation to effectively zero despite the
presence of conductive mineral species: see, for example,
Rocks tend to be layered, so most rocks are electrically
anisotropic. The effect occurs at a range of scales, from
individual grains through to that due to fabrics such as
bedding and schistocity, and at the scale of discontinuities
such as joints. The largest conductivity is usually parallel
to the fabric or discontinuities, since grains are more
elongated in this direction and there is more continuous
pore space.
Clearly there are no guarantees about the electrical
properties of rocks and minerals in the natural environ-
ment. Even massive sulphide mineralisation may be highly
resistive, particularly if sphalerite is part of the assemblage.
The responses of conductive graphite-bearing rock types
are likely candidates for being mistaken as the responses of
conductive mineralisation. Rocks with significant pore
space, especially when filled with saline water, will also
appear as conductivity anomalies.
Like most other physical properties, conductivity/
resistivity is not diagnostic of lithology, and a geophysical
interpretation and a geological interpretation from the
same area may be in con
ict. The geophysical response is
primarily re
ecting the response of the pore space or
perhaps a minor mineral phase, whereas it is the rock-
forming minerals that are the primary basis for the geo-
logical description.
uence of clay minerals depends on their species,
grain size, volume and distribution through the rock mass,
and the pore surface area. Their large surface area relative
to their weight means even quite small amounts of clay
minerals can signi
cantly increase bulk conductivity, par-
ticularly when their grain size is small. The importance of
clay minerals as conduction paths increases as the con-
ductance through the pores diminishes.
10
1
Non-conductive grain
Electrolyte
10
0
Interface layer
Clay mineral
Clay
10
-1
Archie
region
10
-2
Non-conductive grain
10
-3
Electrolyte
Sand/gravel
Non-conductive grain
10
-4
10
-4
10
-3
10
-2
10
-1
10
0
10
1
Pore-water conductivity (S/m)
Figure 5.18
Effect of matrix conduction on the formation
factor shown on a logarithmic plot. The linear relationship
between bulk conductivity and pore fluid conductivity breaks
down where matrix conduction is significant. Where clay minerals
are present, conduction occurs through an interface layer on the
margins of the clay grains. Redrawn, with permission, from
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