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Geophysical methods make up an integral part of a set of geological methods applied in prospecting and exploration of oil and gas fields. Due to fairly low costs, easiness and convenience of electric survey technologies the geological assignments of oil-and-gas prospecting were previously fulfilled by electric methods, primarily by direct current sounding. This can be exemplified by the operations of Schlumberger Firm executed in East Europe, transfer of experience of such works into the Soviet Union in pre-war years.
The present-day prospecting of hydrocarbon resource commodity is distinguished by the larger complexity than previously. A considerable part of large fields of structural type has been already discovered, and many of them have been worked off. In some regions the hydrocarbon traps are due to tectonics and lithology factors rather than structural ones. Much attention has been given to analyzing the factors, which accompany occurrences, e.g. zones of medium recovery and zones of pyritization lying above, and marginal effects. As compared to the second half of the past century the electromagnetic methods experienced the revolutionary jump of development comparable with CDP introduction into seismic survey. Application of high-rank telemetric systems of electromagnetic data acquisition let the accuracy of geoelectric parameters assessment to be raised multiply. Favorable petrophysical properties supplement an additional positive factor enhancing the efficiency of electromagnetic survey. Thus, for clay-devoid rocks the classes non-reservoir – HC-saturated reservoir – water-saturated reservoir differ manifold, while if the elastic properties are concerned, they differ only by some percent.
By the present time the non-stationary (TEM) and magneto telluric (MT) sounding represent the leading methods for oil-and-gas prospecting. Their application is justified by the ability to solve the following geological assignments:
1. Structural mapping of the basement and horizons of sedimentary cover. This is primarily relevant in the zones, where traceability of reflecting horizons is low, accuracy of their mapping by CMP is diminished due to the contrast of speed properties of the subsurface, or application of seismic survey is limited because of the rugged relief. These regions are referred to as Pre-Carpathians on the territory of Romania, Bulgaria, Pre-Patom trough of the Siberian platform, western part of the Sakhalin Island, etc.
2. The thorough gauging of electric resistance of sedimentary cover for assessing the reservoir properties of horizons. The electric survey procedure is particularly effective for solving such geological assignments in the regions where the sedimentary cover is composed of ancient lithified formations. With presence of highly mineralized formation waters their resistance is very low and constitutes the hundredth fractions Ohm·m. This gives the possibility to recognize the reservoir zones in cross-section and plan, to assess their capacity and to forecast the mode of saturation, if additional geological information is available. Formation reservoirs may be contoured by TEM sounding with a high precision. As regards the Siberian platform, statistic information has been accumulated for all horizons of sedimentary cover, and the boundary levels of conductivity of rock classes «non-reservoir – reservoir with HCs – water-saturated reservoir» have been defined. Such works can be vividly exemplified by TEM utilization within the Siberian platform, Fore-Uralian trough of the Russian plate and other regions.
3. Exploration of the upper stage of the geological section, where the zones of secondary alterations are derived due to ascending movement of hydrocarbon fluids, primarily its gaseous part. Experience of applying electric survey in different regions showed that «breathing» of hydrocarbon pool causes increasing resistance of rocks overlying the pool, forms reduction zone with heightened induced polarization. The fields fitting such a physical-geological model require application of the techniques measuringboth environmental resistance and IP value, e.g. TEM + IIP,etc.
4. Forecasting the conditions for drilling deep wells both at the stage of selecting well placement site, and in the process of drilling. Correlation of reservoir and electric properties of sedimentary cover horizons is helpful in analyzing the results of electromagnetic sounding in case of recognizing occurrence of reservoirs with the zones of absorption or zones of anomalously high formation pressures (AHFP). The experience of a successful selection of the sites of well placement was gained at the Kovykta gas-condensate field and adjacent territories. Considering fairly high costs of drilling availability of a priori information оn the probability to encounter complexities in drilling might essentially diminish the risks and expenditures to remove consequences.
The Irkutsk Electroprospecting Company has accumulated a significant experience and its employees are highly-qualified specialists capable to cope with complicated geological assignments in oil-and-gas prospecting and exploration.
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