Benefits of the induced polarization geoelectric method to hydrocarbon exploration

Abstract

Delineation of hydrocarbon prospective areas is an important issue in petroleum exploration. The geoelectric method helps to identify attractive areas and reduces the overall drilling risk. For this purpose, induced polarization (IP) effects are mapped caused by the presence of epigenetic pyrite microcrystals in sedimentary rocks. These crystals occur in a shallow halo-shaped mineralogical alteration zone, often overlying a deeper-seated hydrocarbon accumulation. Local enrichment in pyrite results from reducing geochemical conditions below an impermeable layer. The imperfect top seal of the accumulation permits minor amounts of hydrocarbons to escape and migrate through the overlying rocks to shallower levels. During migration, hydro-carbons encounter an impermeable barrier, forming an altera-tion zone. Induced polarization logging and coring in wells confirm this working model. Geoelectric surveying visual-izes anomalies in electric potential difference measured be-tween receiver electrodes. The differentially normalized method (DNME) inverts the registered decay in potential differences, establishing a depth model constrained by seismic and petro-physical data. Diagnostic geoelectric attributes are proposed, giving a better grip on chargeability and resistivity distribution. Acquisition and processing parameters are adjusted to the target depth. Encouraging results are obtained in deeper [Formula: see text] as well as in very shallow water. Onshore, a grounded current transmitter is used. Geoelectric surveys cover different geologic settings with varying target depths. The success ratio for predicting hydrocarbon occurrences is high. So far, 40 successful wells have been drilled in Russia on mapped geoelectric anomalies. Out of 126 wells, the method produced satisfactory results in all but two cases. The technique reduces the risk attached to new hydrocarbon prospects and allows better ranking at a reasonable cost.