UNCONVENTIONAL OIL AND GAS TRAPS OF FERGANA VALLEY (UZBEKISTAN)

The Fergana Valley is located in West – Central Asia near the western Chinese border, at the western end of the Southern Tian-Shan orogenic system. A central graben is bounded by high-angle reverse faults and the entire post-Paleozoic sedimentary section from the Jurassic through the Neogene is productive. It is generally believed that the Fergana basin’s main source rocks are of Jurassic and Paleogene (Paleocene- Eocene-Oligocene) age. The basin’s subsurface reservoir seals typically consist of shales. An overpressure condition appears to begin at depths of about 3 658 meters. Wells drilled below such depths are primarily in the central basin graben, which is filled with a thick molasse of Neogene and Quaternary clastic sediments. The valley floor slopes from an elevation of about 1 000 meters on the east to 320 meters on the west, where it is drained by the Syr-Dar’ya River. The valley is about 300 kilometers long and 175 kilometers wide; its area is about 38 000 square kilometers.

The Fergana intermontane basin formed during the Alpine orogeny as a result of horizontal compression and thrust Paleozoic formations at the Meso-Cenozoic rocks. In the on-board parts of the basin formed by powerful structure such as overthrust, which may be potential traps for petroleum accumulations.

The geological process involved three stratigraphic complexes. Lower represented strongly pressed Paleozoic formations. The second includes the Mesozoic-Paleogene sediments, which with the erosion and an angular unconformity lies on Paleozoic formations. The third complex consists of thick layer of Cenozoic molasses and the overlying Quaternary sediments.

It made a significant amount of geophysical work, such as refraction, seismic, gravimetric, magnetic, electrical and etc. We have reinterpreted seismic materials.

The results of reinterpretation were presented in a structural map of the reflecting boundary (P2) of Paleogene sediments.

Structural map of the surface of the Paleozoic basement is based on reinterpreting seismic materials conducted over the past 20 years.

Almost on all seismic profiles, crossing the zone of the North-Fergana deep fault, clearly indicates a tectonic failure zone with an amplitude of 1,0-1,5 km. In this zone northern blocks are thrusted to the southern and the magnitude of the horizontal displacement reaches 4-5 km.

The search area by the nature of tectonic structures and the depth of their occurrence are separated from north to south by three major tectonic blocks: I – the upper thrust block is bounded on the south by the North-Fergana fault, characterized by weak development of anticlinal structures (North Hanabad and North Chadak); II – subthrust block is located in the zone of the North-Fergana fault, the width of the block of 4-5 km. Here, the subthrust structures as Sarvak, Khanabad, Chadak, East Chadak and Yangi Charkesar, closed upper thrust block; III – near subthrust block includes a front Chustpap anticlinal and rear synclinal zone. Within this block are clearly distinguished anticlinal Chustpap and North Chustpap local structures.

Such a discordant structure of III block indicates that during post platform orogenesis (N2-Q), i.e. during the formation of modern structures, on the studied area passed a process of horizontal pressure from the Kuramin ridge. These tectonic movements such as drawing resulted in a crumpled layers in the system of folds, whose axes are shifted horizontally. Thus, in the formation of modern nappe-folded structures of the northern edge of the Fergana Basin took a part as collision, so postcollision processes.

Selected structural features of this zone of the northern edge of the Fergana depression can evaluate it as a prospective area for oil and gas production operations. On the basis of geodynamic model can expect the presence of the following types of oil and gas traps: anticlinal, hemianticlinal tectonically screened traps within the first block; zones of fracturing, or "pockets" of the Meso-Cenozoic productive horizons, covered with igneous formations of the Paleozoic, subthrust anticlinal structures, shielded folds, etc. – in the second tectonic block; in the third southern block most likely the presence of productive folds, whose axes are shifted horizontally. Located discordantly on top of each other, they form oil and gas traps, as in the Cretaceous-Paleogene, so in the Neogene sediments.