**3.1 Assakeaudan depression**

It is identified by large positive anomalies in the northwest-orientated geomagnetic field, as well as high values in the transformants of magnetic anisotropy and the

derivative of the angle of inclination of the geomagnetic field vector. Conversely, the minimum values are observed in the transformants of the analytical signal, the vertical derivative, and the autotracing of the axes of the ΔTa anomaly (**Table 1**).

The author's materials on study of gravitational, thermal, and radiation fields were involved in order to increase the reliability and geological information content of the research results.

Within the Assakeaudan depression, minimum or reduced values of the modules of the horizontal and vertical gradients of the Bouguer gravity anomalies, local negative gravity anomalies (recalculated to the upper half-space at a height of 2.5 km) are distinguished based on the results of a quantitative interpretation of the gravity field [9].

The upper edge of the causative magnetic masses drops to a depth of 8–10 km, which corresponds to the depths of the basement rocks (according to seismic CDP-2D) and reflects the level of maximum propagation of basic and ultrabasic rocks into the basement [14].

The spatial location of special Euler points in the gravity field calculated in the Geosoft Oasis Moptaj software [8] indicates the presence of rock density heterogeneity up to a depth of 8.0 km. Deeper, the differences in the depths of the Euler points in terms of density inhomogeneities are significantly leveled.

The upper edge of gravity-disturbing masses at the Assakeaudan depression stands out at depths of 1000–2000 m and is confined to the III reflecting horizon (according to seismic CDP-2D) and reflects the transition of unconsolidated and weakly consolidated Lower Cretaceous rocks to Upper Jurassic sediments [11].

Within the Assakeaudan trough based on the interpretation of geothermal fields [15] to a depth of 5 km, a vast area of low geothermal anomalies with small local maxima in the upper part of the geological section to a depth of 2.0 km is distinguished.

The evidence in favor of the completion of this depression by rocks with low thermophysical properties and a relatively isotropic geological structure is supported by the following: minimal values of the Bouguer anomaly horizontal and vertical gradient module transforms, local negative gravity anomalies (recalculated in the upper half-space at a height of 2.5 km), autotrace of the axes of the ΔТа magnetic field anomalies, and reduced values of TDR transformant [4, 14].

At the Assakeaudan depression according to the airborne gamma spectrometry survey data identified 4 anomalous zones with a relatively reduced background of total radioactivity, low isoconcentrations of radioactive potassium isotope; reduced uranium content in relation to the background values (**Table 1**). The formation of these zones is associated with the effect of hydrocarbon microseepage [16, 17] along the regmatic faults network and macrofracture systems, which indirectly indicate the presence of gas or oil fields [12, 18].

#### **3.2 Shakhpakhty tectonic step**

It is distinguished by low values of the transformants of the analytical signal, autotracing of the anomaly axes ΔТа, magnetic anisotropy, minimal values of the TDR transformant, high values of the intensity of negative anomalies ΔТа. Here, increased values of the transformants of the horizontal and vertical derivatives of the geomagnetic field are observed [5].

In general, these characteristics testify to the continuity of tracing the rock assemblages along their strike.

At the same time, in contrast to the Assakeaudan depression, at the Shakhpakhty step, increased values of the vertical gradient of gravity anomalies *The Geomagnetic Field Transformants and Their Complexing with Data of Gravitational… DOI: http://dx.doi.org/10.5772/intechopen.111560*

were revealed (**Table 1**), which indirectly may indicate an increased vertical variability of rocks.

Other characteristics of geophysical potential fields can be attributed to weakly and moderately intense local positive and negative Bouguer anomalies (recalculated to a height of 2.5 km) and increased values of intensity of negative anomalies, such as ΔTa (**Table 1**).

Within the Shakhpakhty step, the upper edge of the magnetically disturbing masses is submerged to depths of up to 8–12 km, while the gravity-disturbing bodies are deepened to 6.0–7.0 km. Apparently, the difference in their distribution demonstrates the difference in the occurrence depth of the tops of basement and the quasi-platform cover [11, 14].

Consequently, we can say that at the Shakhpakhty step, the upper edges of the gravity- and magnetically disturbing masses are distinguished by the maximum depth at the South Ustyurt region.

In turn, this fact is evidence in favor of the high total thickness of the sedimentary cover and the quasi-platform cover on the Shakhpakhty step, which unambiguously puts it in the category of prospective for hydrocarbon accumulation discoveries.

Shakhpakhty step is characterized by relatively large positive anomalies in the thermic fields, which indicates the predominance of rocks with high values of thermophysical properties (**Table 1**).

On the geological section that intersects the Shakhpakhty Step, where the gas field in the Uzbek part of Ustyurt has been explored, a relatively strong negative geothermic anomaly can be observed up to depths of 3000 m. This negative anomaly is further complicated in the upper part by two positive anomalies that reach maximum depths of 2200–2500 m.

However, it is known from literary sources [19, 20] that large gas field is marked by relatively negative thermal field anomalies, and positive anomalies in the upper part of the section, probably, are associated with compaction, providing a good "seal." However, this interpretation of thermic anomalies requires additional study in the process of integrated analysis, including data of high-precision gravity survey, seismic survey, and drilling.

On this step, according to airborne gamma spectrometry survey on the ground surface, the maximum number of zones (9 out of 15 in the South and Central Ustyurt regions) with anomalously low of radioactive potassium isotope and uranium concentration in relation to the background values was detected, which may indicate increased prospects for oil and gas content in local structures [12].

#### **3.3 Central Ustyurt system of dislocations**

Within this large tectonic element, the maximum values of the intensity of positive anomalies ΔТа, the transformants of the vertical derivative, and TDR vector inclination angle of the magnetic field, autotracing of the axes of the anomalies of this field, as well as increased values of the transformants of the analytical signal, magnetic anisotropy, and the horizontal derivative of the anomalies ΔТа were recorded [14].

In the surveyed area, there is a uniform characteristic of increased values of local gravity anomalies (recalculated in the upper half-space at a height of 2.5 km). Here, intermediate values of the transformants of the modules of the horizontal and vertical gradients of the Bouguer gravity anomalies are observed (**Table 1**).

Generally, the values of the transformants of the geomagnetic and gravity fields [9] may indicate an increased lateral and vertical heterogeneity of the rocks assemblage forming the Central Ustyurt dislocation system.

The maximum number of Euler points on the surface of gravity-disturbing bodies is concentrated at depths of 4000-5000 m.

The Central Ustyurt dislocation system displays gravity- and magnetically disturbing masses that coincide in depth and are confined to the basement surface, as identified by seismic CDP-2D [11].

An airborne gamma spectrometric survey conducted in this geostructure identified two zones with a relatively low background of total radioactivity. These zones exhibited low isoconcentrations of radioactive potassium isotope and reduced uranium content when compared to the background [12].

Thus, based on the foregoing, it can be argued that the integration of the magnetic field transformants with the data of aero gamma spectrometry and gravimetry indicates favorable prospects for the oil and gas potential of the Shakhpakhty step.

An indirect factor that testifies in favor of this is the large depth of immersion of the magnetically active layer associated with the basement rocks [10].

There was taken an attempt to tie-up the depth and character of distribution of magnetically active layers with areas potentially prospective for HC (hydrocarbon) accumulations to be detected.

In fact, the scientific novelty and practical significance of the obtained research results are the use of magnetic survey data for prospecting and exploration of hydrocarbon fields at the South Ustyurt.

Thus, the transformants of the initial geomagnetic field presumably increase reliability in detection of anomalous objects, and may be considered as an extra exploration criterion in prospecting and exploration of HC fields.
