Akhtubinsk–Astrakhan (1-EV line across Precaspian depression)

Regional geophysical investigations along the 1-EV line were carried out in the framework of government project initiated by the Ministry of natural resources of the Russian Federation and included 2D-seismics, MT-soundings and some other techniques. The southern edge of the 1-EV line goes within the Precaspian depression, known as an area of salt-dome tectonics. The step of MT observations is about 1 km.

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Figure below shows seismo-geological cross-section together with geoelectric cross-section, interpreted from the MT-data aquired within Akhtubinsk-Astrakhan segment of the 1-EV line. Geoelectric model (15 km in depth) has been obtained by utilizing of 2D-inversion code by R. Mackie (Rodi and Mackie, 1997).

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In general, obtained model is 4-layered and includes conductive suprasalt horizon (P2-Q), resistive salt (P1k), conductive subsalt horizon (PC3-P1), and resistive basement (Archean).

Subsalt strata is of key interest in the context of the prospects for hydrocarbons. Complicated geoelectric structure doesnпїЅt allow to image subsalt strata in detail, because the latter is overburden by salt and suprasalt sequencies. Main features ofпїЅ the subsalt structure are given below.

The upper part of the subsalt horizonпїЅ(from the bottom of Paleozoic and above) is less resistive in comparison with itпїЅs lower part.

Within the Sarpin trough and Between-rivers step subsalt layer is characterized by relatively low resistivity and it increases significantly in Astrakhan arch and Karakul-Smushkov thrust zone. However, in this area there are several conductive (4 OhmпїЅm or less) anomalies. Regional trend, reflecting this increase in resistivity, is probably due change in subsalt lithology. In Sarpin trough the lower-middle Devonian is primarily terrigenous, while in Astrakhan arch area it has carbonate composition. Another mechanism leading to low resistivity of the subsalt horizon in Sarpin trough is an increase of ground water salinity. ItпїЅs well known that water mineralization in given horizon is lower when the horizon is located closer to the ground surface and higher when itпїЅs layed deeper.

Local conductive inhomogeneities, mapped in the subsalt strata, seem to be related to the zones of high porosity or cracks concentration. So, these anomalies are promising of hydrocarbons.


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