Page 133 - Geochemical Remote Sensing of The Sub-Surface

110 S.M. Hamilton

balance in all parts of the groundwater environment. The migration of redox-inert ions
(such as Na + and CI) may play a role in maintaining charge balance in some part of the
cell.
The model just described was proposed by Hamilton (1998) to account for selective
leach geochemical anomalies over conductive mineralisation in high water-table
Quaternary glaciated terrain. It should, however, be applicable in most overburden
environments that meet the main criterion for the development of such a cell, i.e.,
overlying a redox anisotropy on the bedrock surface. The presence of any geological unit
that is strongly reducing relative to surrounding rock should result in an overburden cell.
Indeed, the association of reduced bodies with field data that suggest electrochemical
activity has been made in the past (Clark, 1996, 1997). Probable reduced sources of such
cells are discussed later in this chapter.
Since at least the late 1970s, very large-scale, low-voltage SP current fluxes have
been thought to occur in close association with petroleum reservoirs. Pirson (1981)
attributed these to the reducing action of hydrocarbons in seepage areas above reservoirs.
All petroleum reservoirs have overlying seepage areas of varying size, as evidenced by
drill cuttings. These areas are attributed to the movement of fluids (mainly water)
carrying trace amounts of hydrocarbons upward through microfractures in the shale as
part of the early compaction process and later due to lithostatic pressure. Core logging
technology and experimentation (Tomkins, 1990) have revealed that these areas are
strongly reducing due to the cracking of long-chain hydrocarbons by natural zeolites in
the shale. This creates around most reservoirs an envelope or bag of negative charge that
extends over a hundred metres above and a few metres below the petroleum-bearing
horizons (Tomkins, 1990).
Pirson (1981) provided core-log evidence that large-scale electrical gradients are
established between the reduced zone above the reservoir and the more oxidised zone in
the shallow subsurface and referred to the process as a "redox cell". Tomkins (1990)
further developed this model by reviewing many of the commonly-associated physical
features of petroleum reservoirs and unifying them into a single model based on a
central redox-cell overlying the reservoir. Such physical features include areas of darker
surface discoloration relative to the more oxidised plain surrounding the hydrocarbon
zone, metal anomalies in a halo around the zone, magnetite and magnetic sulphide
mineralisation above the zone, calcareous cementation above the reservoir and uranium
deposits flanking it. He also attributed leakage of gases such as CO2, He and Rn to redox
processes but did not clearly indicate how these related to the cell. Tompkins considered
current in these reservoir-based cells to be maintained by ongoing seepage and cracking
processes. He cites other sources as having determined that depletion of the reservoir
results in a reversal of hydrostatic gradients above the reservoir which cuts off the
upward seepage of hydrocarbons and effectively "shuts off" current flow.
This redox cell of Pirson (1981) and Tomkins (1990) has sources of oxidising and
reducing agents that are separated in space and also has a zone of apparently-elevated
current between the two sources, although, to the author's knowledge, this current