Ugno-Ciganskoe Uplift (Tatarstan republic, Russia).
The works were conducted by “Scientific and Technical Company ZaVeT-GEO” by request of Limited Liability Company “TNG-Kazan’geofizika” in the Tatarstan republic, Russia. The works were aimed at evaluation of the presence of hydrocarbons in sub-surface positive structure detected by seismic prospecting and delineation of an oil accumulation if hydrocarbons are revealed. The works were carried out not far from a well-known oil deposit and we took advantage of this circumstance for comparison of signals above the oil deposit with those registered in a medium over the seismic uplift being surveyed.
The following techniques were used during these works:
1) Time-domain electromagnetic sounding by means of the horizontal line – inductive measurement of component. We considered this stage as preliminary that was necessary for comprehensive processing of results obtained by the methods VECS-M and SCL. The works were conducted along one profile that intersected the uplift being surveyed, as well as the well-known deposit. When working with the help of TDEM method, measurement points coincided with measurement points recorded during the subsequent work with counter curves. For determining layer thicknesses, we made use of the data acquired by electrical prospecting. We needed time-domain electromagnetic sounding for: а) development of a layer-by-layer model of a surrounding medium for 3D modeling; b) development of a layer-by-layer model of a surrounding medium for calculation of a residual CED field. We calculated the residual field in order to determine the reliable time range for VECS-M; c) development of horizontally-layered models of a medium prior starting interpretation of SCL data having regard to IP parameters. In interpreting results of SCL works, models of a medium obtained at this stage of works were used as resistivity and thickness of layers.
2) Method of sounding by counter lines. Measurements points from counter lines coincided with measurement points for horizontal line – inductive measurement of component configuration. Points at which measurements were conducted were situated at different distances from centers of the counter lines. After measurements, 1D interpretation of medium parameters was executed having regard to polarization parameters by means of the Cole-Cole formula. The model was constructed from resistivities based on results of the first stage of works by the horizontal line – inductive measurement of component configuration.
3) VECS-M method. Measurements were carried out with different observation networks. Maps of signals at various measurement times 10 ms to 100 ms were constructed. During performance of these works, we applied the equipment for sounding consisting of eight current stabilizers GTE-10C, line-operated power supply unit, a control unit, and nine modules of ballast resistors.
Results of areal works by VECS-M for the time 32.2 ms and the band of isolines for the polarization parameter constructed along the profile of SCL signal measurements.
In Pic.1, the data of areal works by VECS-M for the time 32.2 ms and the band of isolines for the polarization parameter drawn along the profile of SCL measurements are coincident. From the data of SCL measurements, the coordinates of points are shifted from the measurement point to the center of the counter line – horizontal line receiver configuration by 1/3 of the direction between them. This Figure demonstrates that the oil accumulation outlines obtained by two principally different techniques of electrical prospecting such as VECS-M and SCL coincide well.
Discussion of results and Conclusions.
Integrated works by different electrical prospecting techniques allows surveying of different electrical parameters of a medium and not just a change in resistivity of a medium.
Works by the VECS-M technique provide outlines of oil accumulations. Such outline is directly conditioned not only by the outline of an oil accumulation itself, but also by the outline of aureole changes in a geological medium above the accumulation. This is very valuable information, but its subsequent interpretation (distribution of geophysical parameters over depth) is hindered owing to the fact that the signal nature is ambiguous. Draw attention to the fact that, as compared to other electrical prospecting techniques, VECS-M provides information mainly on a medium beneath the measurement point. It is worthwhile to apply the denser network of measurements in the VECS-M method since a signal provides insignificant information on an averaged medium between the measurement point and CED.
Applying conventional IP parameters increases the interpretation reliability of electrical prospecting data in surveying for hydrocarbons. Because of this, we propose the SCL method in addition to the VECS-M method. For interpretation of SCL data, the Cole-Cole model of frequency dispersion of medium resistivity. We select the SCL technique based on its far greater sensitivity to polarization parameters than it is in conventional IP electrical exploration technique. The SCL techniques provide theoretically justified results understandable to the community of geophysicists. However, signals in SCL, as those in the most electrical prospecting techniques, average the information on a medium between the measurement point and generator and it is not capable to provide such detailed areal information, as it is possible in the VECS-M technique. It is worthwhile to carry out preliminary SCL works along profiles and to set VECS-M method based on the SCL results.
For interpretation process making use of both VECS-M and SCL, the data on the resistivity distribution in a medium are needed. It is necessary to use either data of previously conducted conventional TDEM technique, or to include the TDEM works into the current system of works. We propose different variants of horizontal line – inductive measurement of component configuration to be used together with current CED and SCL equipment.
The very important aspect in conducting these works was the fact that similar geophysical results regarding the oil accumulation were obtained based on two different electrical prospecting methods such as VECS-M and SCL, as well as on the basis of the distribution of different parameters of a medium.
The integrated VECS-SCL-TDEM system is unified because the components forming this system are interchangeable to the extent that the instrumentations for SCL and TDEM can be elements for VECS equipment. Components of receiver-generator circuits are also used in combination.
Articles
V.S. Mogilatov, A.V. Zlobinskiy. The complex prospecting of a variety of electromagnetic parameters of the medium above seismic uplifting for the detection of oil deposits. // Geophysics, 2013, No. 2, p.51-57 1 Mb, pdf-file
About VECS
Electrical prospecting by the method of vertical electric current sounding (VECS) as applied to oil targets. 2 Mb, pdf-file