Abstract

Geoelectrical methods measure resistivity and induced polarization (IP) effects in the subsurface. The differentially normalized electromagnetic method (DNME) detects geochemical alteration zones due to anomalous electrical responses which are often located above a hydrocarbon accumulation. Leakage above a non-perfect top seal is postulated to change the pH in the overlying sediments where a shallower mobility barrier is encountered. Epigenetic pyrite mineral growth is stimulated when iron and sulfur ions are available. Leakage and mineral growth are modelled by mathematical formulae assuming diffusion migration through the overburden. Effects of time and tectonic faulting are examined. Pyrite is highly polarizable and easily detected by an electrical IP survey in the field, whereby a current is introduced into the ground and subsequently turned off. Decay of the potential difference over the receiver electrodes is monitored in time. Special parameters facilitate detection of IP anomalies related to the presence of hydrocarbons. An inverted electrical depth model is computed using the Cole–Cole formula. The impact of geochemical modelling and predicted epigenetic mineral growth is demonstrated for the Severo-Gulyaevskaya dataset (Siberia). The overall DNME track record shows a reduced risk attached to hydrocarbon prospects, with a more reliable ranking at reasonable costs.

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