Differentially-normalized Method of Electroinvestigation (DNME) - An Efficient Instrument for HC Exploration Off- and Onshore

Abstract

Considerable costs of onshore and offshore wells drilling require effective geophysical methods of HC deposits exploration and study. One of such methods is DNME dealing with conductive and polarization medium properties. In some cases electrical resistivity can indicate HC presence in reservoir that have increased resistivity or water having reduced resistivity. Polarizable medium properties alter in presence of HC. Such alterations are observed to the first regional confining layer, which typically occurs at depth to 1000m. Hence studying medium polarization properties HC occurrence can be detected even at relatively small depth. DNME exploration is conducted both off and onshore. Horizontal electric lines are used as the source and the receiver of the electromagnetic signal. At marine exploration electric field potential differences of the first and second order are registered at several spans. Offshore measurements are carried out on the move of a vessel. At that inductive sounding based on skin-effect is combined with induced polarization (IP) based on the geometric principle. Combined inversion of measurements on several spans allows studying distribution along with conductivity depth and polarization properties of geological environment. In order to obtain quality electromagnetic data the series of measuring techniques was developed. Digital filtration also significantly increases S/N ratio. Onshore and offshore DNME explorations are carried out all over the world: shelves of Sea of Okhotsk and Barents Sea, Caspian Sea, Sea of Azov, Baltic and Caribbean Seas, Gulf of Ob and Tar Bay, Volga-Ural Timan-Pechora, West Siberian, East Siberian, Peri-Caspian, Baltic, North-Caucasian oil-and-gas provinces, China, Cuba, Peru. One of the examples of successful DNME usage offshore is the exploration in Barents Sea (Severo-Gulyaevskoye field). Technique of induction (EM) and IP fields' separation in the whole measuring range allowed to map deposit’s contour occurring in IP field. Geoelectric modeling showed polarizability coefficient increase in deposit's contour at depth complying to regional confining layer. According to well testing data this depth interval (500-600 m) contains high concentration of epigenetic pyrite. The same is observed directly above the oil-saturated reservoir at depth around 2500.