Geography and Geoecology

. On the basis of the morphology of the folds, their three-dimensional position, the types of faults and the development of mud volcanism, the features of the spread of compressive stresses and their influence on the oil and gas bearing of the Kura and Gabirri interfluve, the Yevlakh-Agjabedi depression and the Godin massif were studied. For this purpose, maps of isomorphs and morphological schemes, positions of local folds on the plan of each of these structural elements were compiled. It has been established that within the depressions, the process of folding took place due to the mechanisms of longitudinal and latitudinal bending, which led to the formation of mostly linear and brachiform folds within them. The high intensity of compressive stresses with - in the Kura and Gabirri interfluve contributed to the development of mud volcanism here


Introduction
The Kura and Gabirri interfluve and Yevlakh-Agjabedi depression, as structural elements of the Middle Kura depression, are located between the collision zones of the Lesser, Greater Caucasus and Talysh. Due to this, the compressive stresses that arise within them have a direct effect on the formation of folds within the mentioned structural elements. To determine the role of compressive stresses in the formation of local folds within these structural elements and the Godin massif (Turkmen shelf), the nature of the areal distribution of compressive stresses was analyzed. The Kura and Gabirri interfluve depression, as well as the Yevlakh-Agjabedi depression, are located in the space of the highest intensity of compressive stresses of the South-Caspian mega-depression (SCMD), connected with the proximity of the collision zones of the Lesser and Greater Caucasus (Kopp, 1985). Due to this, they are exposed to the influence of compressive stresses, the degree of which depends on the proximity of the studied objects to the collision zones, and on the complexity of their disjunctives, the thickness of the earth's crust, magmatism, the lithological composition of the sedimentary layer, etc. Depending on the structural-tectonic structure of the Middle Kura depression and the above-mentioned factors, compressive stresses will be unevenly distributed within its territory, and their influence will be relatively plastic. Compressive stresses contribute to the formation, usually, of linear folds, reverse faults, strike-slip faults, thrusts and nappes.
It is known that the formation of folds and disjunctives in the sedimentary cover is mainly connected with the mechanisms of longitudinal and latitudinal bending. According to some researchers, fold formation in the sedimentary cover can often be connected with the manifestation of creep. The emergence of the latter in rocks is connected with the continuous impact on them of stresses below the limit of their strength.
Mainly linear and elongated brachiform folds are developed within the Kura and Gabirri interfluve, while mainly shortened brachiform and isometric folds are developed in the Yevlakh-Agjabedi depression. It should be noted that within the Lower Kura depression and the Baku archipelago, mainly elongated and shortened brachi-folds are developed, while on the Turkmen shelf, the folding process is represented by shortened brachi-and isometric forms. As follows from the morphological expression of local folds within the above-listed structural elements of the SCMD, the influence of longitudinal bending mechanisms on their formation decreases from the northwest to the southeast.
The presence of numerous brachiform folds in the sedimentary cover within the Kura and Gabirri interfluve indicates the activity of the latitudinal bend-ing mechanism in their formation. The main part of the local folds, consisting of uncompacted clays in the Oligocene-Quaternary complex of the Kura and Gabirri interfluve, are brachiform diapirs formed as a result of the activity of longitudinal and latitudinal bending mechanisms.
Folds formed under the influence of the longitudinal bending mechanism usually extend perpendicular to the direction of compressive stresses. Taking this into account, as well as the morphology of local folds within the objects under consideration, the extent of anticlinal zones, types of faults, and the presence of mud volcanism, it is possible to qualitatively assess the role of compressive stresses in their formation.

Methods
For this purpose, the morphological characteristics of local folds in the Kura and Gabirri interfluve, the Yevlakh-Aghjabedi depression and the Godin massif were studied. Local uplifts of each structural element were plotted on the plan in accordance with their morphological parameters. For a qualitative assessment and determination of the direction of compressive stresses, we also compiled isomorph maps. So, on the map compiled for the Kura and Gabirri interfluve the isolines in general in the WNW-ESE direction, their relative density and high values (7.0-3.4) show a relatively high intensity of compressive stresses within this structural element. This is also confirmed by the morphological scheme of local folds of the latter (Fig. 1, 2). As can be seen from the diagram, elongated brachifolds constitute the absolute majority within the Kura-Gabirri interfluve, and their orderly strike in the WNW-ESE direction indicates that local folds in the Oligocene-Quaternary complex of the depression were formed mainly due to the longitudinal bending mechanism. Here shortened brachifolds are in the minority and are mostly closer to elongated brachiforms. The complexity of linearly elongated local folds , Alachig, Chobandag, Eldaroyug, Tulkutapa, etc. numerous mud volcanos, gryphons and salses, their orientation in a strictly general Caucasian direction, in particular their connection with thrusts, indicates the main role of compressive stresses in their formation ( Fig. 1, a).
The development of numerous salses, gryphons, mud hills and mud volcanoes within the territory of the depression indicates, in this case, the negative role of compressive stresses on the oil and gas potential of the territory. In this regard, part of the fluids through discontinuities, fracture networks and other ways comes to the surface, and the other part is dis-placed into space under the lower forces of compressive stresses.
In the Kura and Gabirri interfluve, the morphology of local folds along the Cretaceous-Eocene sedimentary section differs significantly from the morphology of the folds of the Oligocene-Quaternary complex. For example, the Sajdag, Tarsdallar, and Gurzundag structures by the Cretaceous-Eocene complex are mainly isometric and, in rare cases, shortened brachifolds are close to isometric. The morphology of the folds on the surface of the Oligocene-Quaternary complex, represented as elongated brachiforms and linear folds, indicates the formation of folds of the Cretaceous-Eocene complex only by the mechanism of latitudinal bending and the effect of compressive stresses mainly on the strata of Oligocene-Quaternary rocks, as well as on their activity in this interval of geological time. This fact is also confirmed by the development of overthrusts in the Oligocene-Quaternary sediments above normal faults, which complicate the Cretaceous-Eocene complex. As an example, one can also point to the structural and tectonic features of the Tarsdallar, Gushguna, Sajdag, Gurzundag, Udabno, Boyuk Palantokan, etc. folds (Gadzhi-zade, 2003). Within the structures of Tarsdallar, Udabno, Gurzundag, normal faults, which complicate the Cretaceous-Eocene part of the section, pass into overthrusts and reverse faults in the Oligocene-Miocene rock strata. As follows from the above, the physical and mechanical features of the rocks of the Cretaceous-Eocene and Oligocene-Quaternary complexes, as well as differences in tectonic processes, contributed to the formation of different plicative and disjunctive dislocations, which contributed to the appearance of disharmony between these complexes. The designed 3D model of the Kura and Gabirri interfluve depression is a visual reflection of the effect of compressive stresses on the morphology of the folds on the Earth's surface formed in this depression. Therefore, as can be seen from the model, the Keyrukkeylan, Eldaroyug, Akhtakhtatapa, Armudlu, Alachic acquired elongated brachifold form and are mostly affected by compression stress. The ratio of longitudinal to latitudinal of these folds is greater than the others.
Due to the fact that the Yevlakh-Aghjabedi depression is located at a greater distance from the collision zones, the relatively high carbonate content of its sedimentary section and the development of effusive formations limit the activity of compressive stresses here, and therefore, the sedimentary complex of this territory is less affected. This fact manifests itself on the isomorph map of this depression by thinness and small values (1.5-3.2) of isolines.  On the isomorph map compiled by the surface of the Maikop series (Fig. 3b), the values of the isolines vary between 1.5 -3.2, and on the isomorph map by the surface of the Mesozoic (Fig. 3c), their values range from 1.2 to 2 .4, however, on the southwestern flank of the depression in the Aghjabedi and Sovetlar areas, this index reaches 4.0 and 3.3, respectively. Here, the relatively high values correspond to the relatively steep shape of the effusive ledge covered by the Mesozoic deposits. As seen from the morphological scheme (Fig. 3a) and the isomorph map of folds in the Cretaceous (Fig. 3c), compressive stresses almost did not take part in the development of folds, there is no order in their three dimensional position, while in the southwestern parts of the depression, folds by the Maikop surface (Fig. 3b) are oriented in the NW-SE direction. In other words, the folds along the Cretaceous deposits were formed as a result of covering of effusive massifs with sedimentary formations (Gadzhi-zade,2003). In addition, if the Paleocene-Eocene sediments wrap up effusive massifs, in this case, wrapped folds are formed, the Muradkhanli, Beylagan, Jarli, Sovetlar, and Gulluja folds can be an example. In the formation of the mentioned folds, there is no influence of the latitudinal bending mechanism; therefore, they are mainly developed at a low speed, as well as be-ing submerged folds of wrapping origin. The faults that complicate the Shirvanli and Duzdag folds are normal faults with a small amplitude. As a result of numerous studies, it has been determined that the Barda, Pirazi, Gulluja, Shirvanli structures located in the Yevlakh-Aghjabedi depression had an uneven development history (Fig.4).

Fig.4. Seismological profile
The Shirvanli fold, located in the southwestern zone of the region, shows itself as a fold with a height of 50-70 m by the top of late Cretaceous, mainly on the southwestern slope of depression. Based on the analysis, it developed as a local fold up to 150 m according to the top of Paleocene. The lower part of the Eocene section consists carbonate layer. According to the top of this layer, the height of the fold developed up to 300 m. Due to the top of the Eocene, a terrace was formed on a regional scale in the area. This is due to pressing of the south-western slope of the fold formed by the clays and carbonate layer of the Eocene to the peripheral part of the fold. As a result, on the top of Eocene its southwestern part was completely opened and became horizontal. The reason for the creation of a large terrace-like structure on a regional scale between the Gulluja and Shirvanli areas is that the northeast limb kept its previous lying form. The northeastern slope of the Shirvanli fold has been compounded by a low-amplitude fault lying smoothly in the Paleocene-Eocene as well as the Cretaceous, and the fault surface lies at a large angle to the northeast. The noted fault expressed plastic deformation in the Maikop clays, but in its top part, the fault is traced along the Sarmatian and Chokrak sediments more clearly, and on its surface, in the northeastern zone of the fold, time-equivalent layers meet with the Maikop sediments. As is known, the complete closure of the amplitude of the fault in the Cretaceous along the stratigraphic depth confirms its youth.
The reason for the more intensive development of the Shirvanli fold in Maykop sediments, which began to form in the Eocene, is the high mobility of clays and the activation of compressive stress. Due to the very rapid rise of the arch part of the fold surrounding the western block of the fault, the Sarmatian and Chokrak sediments were severely eroded, while the Maikop, on the contrary, was impacted by less erosion. It is clear from the profile that because of the geostatic pressure created by the Sarmatian and Chokrak sediments on the northeastern slope of the fold, Maikop clays flowed towards its core. The development of the Shirvanli structure in the Cretaceous proves its hereditary origin.
Although the Barda fold, located 10-11 km northeast of the Shirvanli field, is a structural terrace that is clearly defined by top of the Eocene, it does not express either the Paleocene base nor the Cretaceous outwashed surface itself. Numerous studies show that, according to the top of the Maikop sedimentary complex, it is a local fold higher than 300 m. As a result of the analysis, it was determined that the thickness of the Maikop sediments in the core of the Barda fold confirms that Maikop clays play a key role in the development of the fold. Besides, the significant decrease in the thickness of the Sarmatian and Chokrak sediments in the direction of the arch part of the mentioned fold is proof of its syn-depositional development in the time-equivalent interval. It should be said that, unlike the Shirvanli fold, the Barda fold is not hereditary (Narimanov, 2005).
As is clear from the morphological and developmental features of both mentioned folds, they are of diapiric origin, but due to the relatively weak development of compressive stresses, the low thickness of the sedimentary layer lying on the Maikop sediments, these folds did not have the necessary energy potential to fully realize the diaprism. As a result of numer-ous studies, it was discovered that the mechanism of longitudinal and latitudinal bending is active in the development of mud volcanism and clay diaprism in the mobile areas. The mechanism of latitudinal bending in the formation of the mentioned fold was relatively weak, but the registration of effusive massifs in the Yevlakh-Aghjabedi depression significantly limits the action of compressive stress in this area.
According to the conducted analyses, it was determined that the local structures of the Yevlakh-Aghjabedi depression have different morphologies depending on their origin. In this regard, since the Chokrak, Sarmatian, Maikop rock complexes in the Barda fold are clearly expressed as local uplift, their natural reservoirs can perform the function of a structural trap. The Shirvanli area shows itself as a structure with a height of 100-150 m in Maikop sediments, as well as according to the top of the Paleocene. It should be noted that the prospectivity of this structure is related to Eocene, Maikop, Sarmatian sandy reservoirs, and in the wells drilled in its south-eastern plunge, relatively weak gas manifestations from Sarmatian stage sediments were found. From what has been said, it can be concluded that in order to evaluate the oil and gas prospects of the Barda fold, it is considered appropriate to continue the exploration drilling in its arch part up to the bottom of the Maikop.
Compressive stresses have had a certain influence on the structures formed in the Oligocene-Mio-cene sediments on the northwestern slope of the Yevlakh-Aghjabedi depression, but they were weakly expressed in the Neogene sediments that remained between the unconformity surfaces, as is clear from the profile, due to the regional erosion that began in the second half of the Sarmatian period. The aforementioned can be seen more clearly on the eroded surfaces in the Shirvanli structure. An example of what has been said is the folds of Shirvanli and Barda. It should be noted that the Shirvanli structure began its development period in the Cretaceous and the impact of compressive stresses on it noted in Maikop.
The Godin massif is a hypsometrically significantly elevated block of the crystalline basement, and the sedimentary cover covering its surface is represented by deposits of predominantly Cenozoic age up to 12-13 km thick (Narimanov, 2003). The crystalline basement around the Godin massif is complicated by a dense network of disjunctives of different scales, and some of them cross the Moho surface (Fig. 5). Such «compartmentalization» of the crystalline basement of the South Caspian depression indicates its relatively high sensitivity to both compressive stresses and geostatic loading. This can be seen from the relatively high deformation of the sedimentary cover in its Central Sector and in the Baku Archipelago, while the sedimentary cover of the Godin massif is very slightly deformed relative to the rest of the depression.  Mamedov, 1995) In the crystalline base of the depression, as a result of its complexity with disjunctives of different scales and under the influence of compressive stresses, as well as geostatic pressure, a complex graben structure was formed. The reason for the very weak deformation of the sedimentary cover of the Godin massif is its screening of compressive stresses arising in the sedimentary cover of the depression .
Due to the fact that the Godin massif screens the compressive stresses arising in the South Caspian de-pression, the local folds formed on the Turkmen shelf have a disorderly development, are weakly expressed, and represent shortened brachiform and isometric folds (Fig. 6).

Conclusion
The analysis of the effect of compressive stresses on folding allows us to come to the following conclusions: 1. Under the dissections' structural elements, the Kura and Gabirri interfluve is subjected to the most intense impact of compressive stresses; these stresses manifest themselves more weakly within the Yevlakh-Aghjabedi depression and are almost completely absent within the Turkmen shelf due to the screening role of the Godin massif. 2. Despite the relatively low thickness of the sedimentary strata of the Kura and Gabirri interfluve depression, the development of mud volcanism here indicates the manifestation of compressive stresses of high intensity. In turn, the reason for the absence of mud volcanism within the Yevlakh-Aghjabedi trough is both the relatively high carbonate content of the section and the presence of effusive formations, weakening the effectiveness of their impact on the sedimentary strata. 3. Since the Chokrak, Sarmatian, Maikop rock complexes in the Barda and Shirvanli folds are clearly expressed as local fold, their natural reservoirs can perform the function of a structural trap, and their prospects can be related to the Eocene, Maikop, Sarmatian sandy reservoirs. 4. Due to the difference in the mechanisms of folding in the Cretaceous-Eocene and Oligocene-Quaternary sediment complexes within the Kura and Gabirri interfluve depression, disharmony arose between folding, i.e. structural plans of these rock complexes.