M.U. UsuvaliyevK.T. Toktonaliyev
Structural exploration drilling began in Kirgizia in 1962. During 1976-82 it was concentrated in the southwest part of the Fergana depressions, where two oil fields were discovered, Beshkent and Togap.
Both the Beshkent and Togap folds flatten out with depth, and on the top of the Paleogene sediments they are expressed as structural noses that plunge to the west at 5-100 at Togap and 15-200 at Beshkent. The axis of the fold on the top of the Paleogene is shifted 600-900 m to the north. Even though these folds have identical structure, the composition and quality of the oil is markedly different.
BESHKENT, TOGAP FIELDS
Beshkent field was discovered by Well No. 5. A flow of oil at 5.6 cu m/day was recovered from Oligocene Stratum III in the 1,809-20 m interval. Formation pressure is 236 atm, and density of the oil is 0.864. Content in percent is sulfur 0.2, asphalt tar 28.0, silica-gel tar 12.9, and paraffin 11.6. Structural exploration wells 8 and 9 yielded oil at 1.5 and 7 cu m/day, respectively.
Stratum III in Beshkent field consists of dense, fine-grained clayey sandstone 1518 m thick. Net pay is 5 m. Porosity is in the 15-25% range.
Togap field was discovered by structural-exploration Well No. 1. A flow of oil at 5.7 cu m/day was recovered from Stratum III in the 1,310-20 m interval.
Formation pressure is 115 atm, and density of the oil is 0.873. Content in percent is sulfur 0.27, asphalt tar 32, silica-gel tar 19.5, and paraffin 17.4.
In Togap field Stratum III consists of sandy siltstone with beds of fine-grained sandstone. Thickness is 1015 m, and net pay is 3.5 m. Porosity is in the 13-21% range.
OTHER DRILLING
A profile of five wells was drilled on the Tamchin and Vtoraya Tamchin structures. With depth these two features flatten out and pass into a single fold on the top of the Paleogene sediments-Tamchin proper.
Five additional wells were drilled to study the flanks of the Tamchin structure; these showed that on the Paleogene the fold has shifted its position somewhat to the west. Well 10 yielded 0.2 cu m/day of oil and water from Paleogene Stratum IV of the Rishtan beds. Density of the oil is 0.867. Content in percent is sulfur 25, asphalt tar 28, silica-gel tar 10.3, and paraffin 10.5. The productive stratum is a sand-silt rock, and porosity is 18.25%. Clay content is 24-60%.
A total of 12 wells were drilled in four areas: Severnyy Togap, Murgun, Shark, and Ayzar.
Structural and parametric wells were drilled in the north border area of the Kara-Tau (southwest part of the Fergana depression) to determine reservoir quality of the rocks and to trace facies changes. Three parametric wells were drilled at distances of 7-10 km from one another. It was found that the Permo-Triassic and Jurassic drop out of the section, and that thin (50 m) Cretaceous sediments rest on eroded metamorphosed Paleozoics-and Paleogene on the east.
There are also discordant breaks in deposition between the Ancient Quaternary and the Neogene Baktriy beds, the Neogene Brick-Red formation, and the Oligocene Sumsar beds, the Sumsar beds and the Eocene Green Clay formation, and Paleogene and Cretaceous, and the Cretaceous and Paleozoics.
Practically no marker horizons are present in the Neogene because of pinchouts due to breaks in deposition. This situation necessitates expenditure of excessive amounts on deep drilling because of discordance between the Neogene and Paleogene.
1990S EXPLORATION
In the coming years structural-exploration drilling will be carried out largely in Southwest and East Fergana and then in the north of Kirgizia.
Several structures on the south flank of the Kara-Tau high (Karagan, Staro-Sulyuktin, Ketmenkul, Sarydzhar, and Dashtakin) are primary drilling targets. A profile of drilling is planned for the Tashravat-Murgun downwarp, which is located between the Kara-Tau and Mayday-Tau highs.
Another target is the Zambarych group of structures of Southwest Fergana, which were found in 1948. Seventeen structures were mapped, nine of which are in Kirgizia. The oil-gas potential of this region is unclear. However, taking into account the proximity of the Karagach and Togap-Beshkent fields, this region may be highly favorable.
V.A. Babakhyan (1948-50) mapped 19 local structures in the Uzgen-Kurshab depression of East Fergana. He described most of these as structural noses. The Paleocene sediments were assessed as without prospects, but the Lower Cretaceous and Jurassic were considered as favorable. Babakhyan thought that the Kurshab group of structures--Digmay, Zerger, Tokubay anticlines, and Dzhilandin, Kondubin, Shurabashat structural noses--were prime targets.
Two exploration wells were drilled in the Dosmet area of the Uzgen downwarp in 1981-82. A flow of water with dissolved gas was recovered from Jurassic Stratum XXIII in the 9841,009 m interval. The gas is largely methane. This indicated favorability for the Jurassic as well as the Permo-Triassic.
FUTURE TARGETS
Targets for drilling over the next 20 years are the Alay, Talas, Vostochno-Chuy, Issuk-Kul, and Naryn-Chatyrkul depressions.
The Paleogene section of the Alay depression appears to be the same as in the Fergana depression. This depression joins the Cis-Kun-Lun oil-gas downwarp of China. Three anticlinal structures are known, which geological studies indicate are drilling targets.
The Talas depression is joined on the northwest by the Chu-Sarysuy depression of Kazakstan. Its oil-gas potential has not yet been studied. Several local structures are present, but the sedimentary section has not vet been studied. It is possible that this depression is similar to the Chu-Sarysuy.
In the Vostochno-Chuy depression there is interest in pinchouts to the north of Upper Paleozoic and Lower Cenozoic deposits.
The west part of the Issuk-Kul Valley has not been drilled, although salt is known in the Neogene section, and thick Upper Paleozoic clastics are present in the mountain frame. Methane is present in formation water of the Carboniferous and Paleogene-Neogene.
The Naryn-Chatyrkul depression was connected with the Fergana depression before uplift of the Fergana Range beginning in the late Jurassic. Drilling has penetrated Jurassic and unmetamorphosed Paleozoics in the section. Their presence beneath thick Cenozoic deposits suggests that they have oil-gas potential.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
