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Recent theoretical studies have enabled speculative predictions to be made of likely natural (as opposed to induced) electrical potentials above reservoirs. Due to improved instrumentation, these potentials can now be tested in the field. Most oil and gas traps are somewhat leaky, and the buoyant and highly reducing hydrocarbons create a redox chimney. This inevitably develops a potential field, because subsurface formations are rarely dry, and deeper formation waters may be rich in salts and thus have low resistivities. However, subsurface rock packages are also inhomogeneous and highly anisotropic with respect to electrical and fluid properties. Dissolved salts, rates of fluid flow, charged scale membrane effects, chemical reactions, pH, Eh, etc, affect the electrical c rrent pattern and polarity. Consequently, natural cells can be very complex.
The most important geobatteries to be expected around hydrocarbon traps will be affected by interacting ionic changes, cells of various types, pressure, temperature, concentration changes, and electrokinetic phenomena.
Tectonic, sedimentational, and diagenetic influences will affect electrical potentials and local conductances, both of which are amenable to measurement in the field.
The models that we propose are simplistic, because only these are readily amenable to theoretical analysis; chemically "dirty" natural systems are much more complicated. However, it is hoped that this approach will stimulate dialogue and criticism that will benefit this too-long-ignored subject.
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