Geo-electric Analysis Based on Quantitative Separation between Electro-magnetic and Induced Polarisation Field Response

Abstract

The geo-electric method allows mapping of resistivity and induced polarization effects in the subsurface. It permits detection of diagenetic alteration zones with micro-pyrite crystals, situated some distance above hydrocarbon accumulations due to leakage from a non-perfect top seal. Geo-electric surveying (DNME) uses special parameters for monitoring the electric earth response in time. Inversion of relaxation curves is done to establish a depth model, whereby non-uniqueness of the solution is a well known problem. Additional constraints are needed for a plausible scenario. Two basic components are measured: the electromagnetic and induced polarisation response. A synthetic model illustrates their contributions in spite of the non additive character. The inversion is done in a 1D mode with a Cole-Cole simulation. The modelled response at a recording station has been compared with that of the known 3D model. The discrepancy is better than 0.1 %, illustrating the validity of the inversion modelling. A considerable amount of successful study cases has been performed in Russia, like the Severo Guljaevskaya study in the Barents Sea. The geo-electric technique reduces risks attached to drilling of new hydrocarbon prospects and provides a better ranking at a reasonable cost.