MT studies in Western Kazakhstan

In 2007 Nord-West Ltd. carried out a 3D MT survey in the southern part of Precaspian depression (Western Kazakhstan). MT observation have been performed in order to image a single salt-dome structure, map the sediments, overburden in salt and obtain any information on the properties of subsalt strata. Geological settings of the area are typical for the Precaspian region.

Observation grid includes 6 profiles (about 200 MT-sites); step along profile is 1 km. Data interpretation was performed using 2D-inversion; horizons morphology known from seismics was taken into account.

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Main results are represented in the form of geoelectric cross-sections (10 km in depth). An example of the cross-section along one of the profiles is given in figure above. Except of that a 3D-model of resistivity distribution was constructed.

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Conductive suprasalt horizon, resistive salt and conductive subsalt are main but already familiar elements of the geoelectric structure.

Meso-kainozoic suprasalt strata is of terrigenous nature and has resistivity of 0.1-10 Ohm·m. It's thickness varies from 200 m (above the salt-dome) to 3000 m (in a deep mold). There are vertical contrasts of revistivity within the suprasalt strata. It's upper part is formed by clays and has extremely low resistivity (about 0.1 Ohm·m). The lower part (P2-J) is less conductive (1-10 Ohm·m). It is important to mention that suprasalt sediments are the oil reservoirВ  due to oil migration from subsalt strata.

Kungurian salt is highly resistive horizon with resistivity of 100-1000 Ohm*m and thickness of up to 6 km. Top of the salt-dome is located at the depth of 200-300

MT imaged a complex structure of the salt strata. It revealed conductive (5-15 Ohm·m) anomalies, located beneath salt traps, probably related to terrigenous rocks of Upper Permian-Triassic age. The bottoms of these anomalies are determined rather inaccurate.В  However, their height is estimated to be up to 3.5-4 km. It should be mentioned, that peripherical traps, localized within the peripherical parts of the salt-dome structure are of prior interest for oil and gas. In the considered area the reservoirs could be related to sandy layers of Permian and Triassic age.

Subsalt strata is formed by terrigenous sequences (Artinskian) and carbonate rocks (Upper Visean-Sakmarskian). MT allowed to infer generalized characteristics of subsalt strata; vertical differentiation of resistivity within the strata is severely complicated. In general, subsalt rocks are rather conductive (3-80 Ohm·m)В  with thickness of 1.6-2 km.

The pronounced lateral zoning of subsalt conductivity is observed. Southern, south-western and south-eastern parts of the area is characterized by relatively high values of resistivity (20 Ohm·m and more), while northern part is more conductive (3-20 Ohm·m). Resistive subsalt strata in the southern part could be represented by carbonate sequences or effusive rocks.

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