**1.1 Geological setting**

The South Ustyurt region stands out on the southeastern margin of the South Mangyshlak sedimentary basin and is the eastern margin of the North Caucasian-Mangyshlak oil and gas province. This region is isolated as part of the South Mangyshlak oil and gas region, located within the Turan epihercynian platform.

In tectonic terms, the South Mangyshlak oil and gas region is located within a large area of Mesozoic intraplatform subsidence of the earth's crust, which is called the South Mangyshlak-Assakeaudan system of depression. This system is bounded in the north by the Mangyshlak-Central Ustyurt uplift zone, in the south by the northern periclines of the Karabogaz arch and the Tuarkyr uplift zone, and in the west, it opens toward the Caspian Sea.

This system of depression has the shape of a triangle in plan, with the apex to the west and the base to the east. The axial part of this zone is complicated by a linearly elongated chain of bath-like depressions separated by structural saddles.

These depressions are distinguished by different areas and irregular, most often isometric shape in plan.

Along the axis of the South Mangyshlak-Assakaudan system of depressions, the Zhazgurli, Uchkuduk, and Assakaudan troughs are differentiated from west to east. Within each of these depressions, smaller depressions are of particular importance, complicating their structure.

A characteristic feature of the tectonics of the South Mangyshlak-Assakaudan system depression is the stepwise subsidence of rocks from the boards of this system to its axial part. On the northern boards, the steps are located parallel or echelon to each other and generally subordinate the trend of the Mangyshlak-Central Ustyurt uplift zone.

Within the steps, chains of local uplifts of the anticlinal type are isolated, serving as traps for oil and gas. Local anticlinal folds are characterized by an asymmetric structure with a steep southern and more gently sloping northern wings and are slightly disturbed by faults.

The most ancient deposits, represented by thick strata of Upper Permian and Triassic rocks, are exposed in the areas of Mountain Mangyshlak. These rocks are drastically deformed and partially metamorphosed.

*The Geomagnetic Field Transformants and Their Complexing with Data of Gravitational… DOI: http://dx.doi.org/10.5772/intechopen.111560*

In the central and most submerged part of the South Mangyshlak oil and gas region, these occurrences are found at depths of more than 4 km. They are overlain by thick Jurassic sediments represented by all three sections, showing a drastic unconformity.

The stratigraphic completeness and thickness of these sediments within local tectonic elements are different. The Jurassic sediments in this area can reach a maximum thickness of 750 m. Traditionally, these sediments are associated with the main prospects for the exploration of hydrocarbon fields within the South Ustyurt. According to the conditions of sedimentation and the type of organic matter, the presence of main gas deposits is predicted here.

The rocks of the Jurassic complex with erosion and angular unconformity are overlain by deposits of the Cretaceous system, the eroded surface of which is overlain by Paleogene sediments.

Cretaceous deposits are widespread and are represented by the lower and upper sections.

Lower Cretaceous: The Neocomian complex includes sediments of the Barremian, Valanginian, and Hauterivian stages. At the bottom of the Neocomian stage, there is a layer of bluish-gray clay with a greenish tint from the Valanginian stage. This layer contains pyritized charred plant remains and thin interlayers of fine-grained sandstone. Above this layer, there is a sequence of interbedding sandstones, siltstones, and, greenish-gray clays with the inclusion of Hauterivian fauna.

The Barremian is the thickest in the Neocomian. The lower part of the Barremian deposits is composed of red-colored clays with subordinate interlayers of siltstones and sandstones; the upper one is predominantly sandstone.

Below the red-colored sediments of the Barremian stage lies a layer of gray-colored rocks, which in the upper part transition to variegated Aptian rocks. The complex is composed of dark gray, silty, finely elutriated clays with charred plant remains.

The Albian stage is represented by dark gray clays with a greenish tint, thinly bedded, silty, with charred plant remains and pyrite. Sandstones are gray and greenish-gray, mostly weakly cemented, often turning into loose sands, quartzfeldspar, glauconite.

The Upper Cretaceous is represented by the Cenomanian, Turonian, Senonian, and Danish stages. The Lower Cretaceous sediments are lithologically subdivided into two complexes: terrigenous rock assemblages (Cenomanian, Turonian) and carbonate rock assemblages (Senonian, Danish).

In general, the above sediments are represented by terrigenous varieties, with the exception of the Upper Turonian-Danish carbonate deposits and layers of limestoneshell rocks in the sandy-clay section of the Neogene.

Paleogene deposits with erosion lie on the Cretaceous deposits and are represented by three sections. The thickness of the Paleogene sediments ranges from 0 to 632 m.

Paleocene sediments are represented in the lower part by greenish-gray clayey limestones, and in the upper part by brownish marls.

Eocene sediments consist of greenish-gray marls, and in the upper part of the section, they transition to dense, strong brownish-gray marls. These sediments contain remains of fauna, plant detritus, and pyrite.

The section of the Oligocene age is composed of clays of marine origin, which conformably lie on Eocene deposits. The clays in this section are greenish-gray with a bluish tint, and are interbedded with siltstone and sands. They are calcareous, and contain faunal remains as well as pyrite.

Neogene sediments are widespread and commonly overlie Oligocene rocks, characterized by erosion and angular unconformity. The Neogene section is divided into

two stages: the Tortonian and Sarmatian, which correspond in age to the Middle and Upper Miocene, respectively. Lithologically, the sediments are composed of marls and limestones, white, chalk-like, with inclusions of pyrite and gypsum. Sarmatian limestones make up the shell of the Ustyurt plateau. The thickness of the Neogene sediments ranges from 0 to 90 m.

The rocks of the Quaternary age lie on sediment deposits of different ages of the Cretaceous, Paleogene, and Neogene. The sediments are represented by brownish-yellow, quartz-feldspar sands and loose sandstones, siltstones, and gray loams. The thickness of the Quaternary deposits varies in the range of 0–15 m.
