UZBEK LICENSING ROUND BRINGS GEOLOGY, POTENTIAL INTO FOCUS
Adrian P. Heafford
Jebco Seismic Ltd.
London
Grant S. Lichtman
Jebco Seismic Inc.
Houston
Uzbekistan is a Central Asian Republic that declared independence from the former Soviet Union in 1991 (Fig. 1). It covers 447,400 sq km, about the size of California.
Uzbekistan produces about 18 million bbl/year of oil and 40 bcf/year of gas. It is the third largest gas producer in the Commonwealth of Independent States and imports oil.
The Uzbek government and oil and gas industry are offering exploration acreage for foreign participation via competitive bid. Acreage on offer includes fields for development and unproven-underexplored areas.
Terms awaiting approval by the Cabinet of Ministers provide financial incentives for rapid development of existing reserves, creation of required infrastructure, and long term investment growth.
License areas concentrate on acreage where western equipment and technology can bring new reserves economically on line in the near future. National oil company Uzbekneftegaz was created in 1992 to oversee the extraction, transport, and refining of hydrocarbons in Uzbekistan.
FIELDS, INFRASTRUCTURE
Some of the larger fields in Uzbekistan (Fig. 1) have been prepared for development, but production has not vet begun. Prospects for finding new fields remain good. It is also possible that fields may have been overlooked due to use of over-weight drilling mud plugging the reservoirs.
Oil was discovered at Shorsu in the Fergana basin in 1880, and significant discoveries have continued to be made through the present (Fig. 2).
A widely reported blowout in Mingbulak field in the Fergana basin was an ecological disaster with an estimated 150,000 b/d gushing to the surface before it was capped in May 1992. The field, located in the unlicensed area in the middle of the Fergana basin, is under negotiation with a western oil company.
Minimum reserve estimate for Mingbulak field is 350 million bbl of oil. The reserves are in Paleogene and Neogene reservoirs at as deep as 6 km and at pressures exceeding 1.8 times normal.
Uzbekistan lies between the Syrdarya and Amudarya rivers, which drain into the receding Aral Sea. The country is a land of deserts and high mountains, home to a multicultural mix of ethnic groups.
The capital, Tashkent, contains 2.2 million of the 20 million population of the republic. Gas pipelines link the capital to the Fergana basin and the Bukhara-Chardzhou gas fields and to the other C.I.S. republics. Roads, railways, and airways link the capital with other parts of the C.I.S. and Middle East.
STRUCTURAL GEOLOGY
The easternmost part of Uzbekistan is formed by two basins sandwiched between the Tien-Shan and Pamir mountains; the Fergana basin and Surkhandarya (Afghan-Tadzhik) basin (Fig. 3).
This area has undergone a complex history of Alpine (Late Cenozoic) tectonism. The Fergana intermontane basin is bounded by major faults that separate it from mountains to the north and south. The Surkhandarya basin has been extensively deformed into a series of overturned folds, thrusts, and back-thrusts with a north-northeast strike.
These basins contain as much as 10 km of Mesozoic-Cenozoic sediments.
The rest of Uzbekistan is part of the Turan platform. The depth to basement in the Central Kyzylkum uplift is generally less than 1 km. To the southwest the basement drops to depths of 3 km or more across two normal faults that bound the Bukhara and Chardzhou steps. These two steps are part of the much larger Amu-Dan,a basin, most of which lies outside Uzbekistan.
To the west of the Central Kyzylkum uplift lies the northerly striking Aral Sea rift, which continues to the south as the Murgab and Zaunguz grabens with a sinistral offset of 200 km.
These early Mesozoic grabens forme(.i on the site of the older Ural-Oman lineament. The latter has an important bearing on hydrocarbon prospectivity as it marks the western limit of the Tithonian (Upper Jurassic) salt seal. To the south of the Zaunguz graben lies the largest gas field in the region, Dauletabad-Donmez field (Fig. 3).
To the west of Uzbekistan lies the Ustyurt plateau. Here the basement lies as deep as 10 km, but the base of Jurassic is only 2-4 km deep. The Ustyurt plateau has been affected in parts by the "Kimmerian" (Late Triassic) diastrophism, which substantially deformed the Buzachi-Mangyshlak area.
The general west-northeast/east-southeast trend of Uzbekistan was formed during the "Hercynian" or Tien-Shanian (Permian) orogeny. This is shown by the strike of the outcrops of basement in the Central Kyzylkum uplift (Fig. 3).
The Central Ustyurt System of Dislocation s follows this trend, dividing Uzbekistan into two parts, and has been repeatedly reactivated. To the west it downthrows to the north, while to the east it downthrows to the south.
It sinistrally offsets the Aral Sea and Zaunguz grabens and dips nearly vertically, suggesting it's a strike slip fault. South of the Central Ustyurt System of Dislocations lies the South Mangyshlak-Ustyurt basin, which contains the 11 billion bbl Zhetybai field in the Kazakhstani part of the basin.
There is some thinning of Early Cretaceous sediments over the crests of structures in the Bukhara and Chardzhou steps, but the most important structure forming episode for petroleum exploration was the "Alpine" (Cenozoic) deformation', which mainly affected the eastern part of Uzbekistan. Cross-sections show that structures in the Bukhara and Chardzhou steps were enhanced at this time.
The Surkhandarya basin is particularly intensely deformed with the Late Jurassic acting as a decollement between more competent Mesozoic rocks and the thrusted and folded younger strata.
STRATIGRAPHY
Late Paleozoic-Triassic sediments, termed the intermediate sequence, have not been studied in detail. They are inferred to exist beneath the Bukhara and Chardzhou steps, and in the Ustyurt plateau wells have penetrated (?)Carboniferous carbonates in,,i reefs, conglomerates, volcanics, and black laminated mudstones.
Permo-Triassic redbeds were shed off the Ural Mountains area, but in the western areas their thickness has been reduced by Kimmerian movements. Triassic sediments have not been penetrated in the Aral Sea rift, but seismic data suggest that they thicken markedly within it.
Coal bearing clastic sediments began to be deposited in the Surkhandarya and Fergana basins, and there was minor extension in the Surkhandarya basin at this time.
Mainly continental clastic sediments were deposited until Callovian-Kimmeridgian time, when they were replaced by reef-bearing marine carbonates in the Bukhara and Chardzhou steps, Surkhandarya, and S.W. Gissar (Fig. 4).
The Fergana basin was uplifted in late Jurassic time. In the Ustyurt plateau there was a Kimmeridgian-Tithonian marine transgression, but only a thin carbonate band was deposited at the top of the sequence. A Tithonian salt seal was deposited in the Bukhara and Chardzhou steps and Surkhandarya basin at this time.
Clastic sediments with some carbonates were deposited during Early Cretaceous and Cenomanian time. As the Late Cretaceous marine transgression advanced, an increasing quantity of carbonates and fine-grained clastic sediments was deposited, and deposition continued through early Paleogene time.
The Alpine orogeny resulted in their replacement by coarser clastics (molasse) to the east, where several kilometers of Cenozoic sediment accumulated in the Fergana and Surkhandarya basins. In western Uzbekistan there was a widespread hiatus.
PETROLEUM DISTRIBUTION
The large hydrocarbon fields are distributed in a large west-northwest trending belt parallel to the Central Ustyurt System of Dislocations (Fig. 3).
Consideration of the smaller fields reinforces this but shows that there is also a trend from Dauletabad-Donmez field northwards through the Murgab graben to the Zaunguz graben. This latter trend suggests high potential for the little explored Aral Sea rift, where the Berdakh well flowed 150,000 cu m/day of gas from Jurassic deposits in 1990.
To the west of the rift on the Ustyurt plateau, Kuanysh an(f Akchalak gas fields and West Barsakelmes oil field await development of Jurassic reservoirs but are not included in the license area. HoA,ever, the Karachalak reef has been tested at 500,000 cu m/day from Paleozoic limestones at 3,521 m - a gas pipeline passes through this field-and oil has been tested from the Karakuduk reef, suggesting significant potential in Paleozoic strata. Shakhpakhty field contains 1.5 tcf of gas in Jurassic clastics.
In the Surkhandarya basin Gadzhak field contains about 500 bcf of gas, and Kognysai field contains 200 bcf, mostly in Jurassic carbonates. The S.W. Gissar offering includes acreage around South Tandyrcha (1.5 tcf) gas field.
The oil fields in the Fergana basin are mostly distributed along the northern and southern margins of the basin. This is largely because the reservoirs in the deeper, central part of the basin have not been adequately explored and few deep wells have been drilled.
The Fergana basin contains the largest number of pools, mostly oil in Paleogene strata. Although the deeper part of the basin is the main prospective area, the large thickness of Neogene sediment means that hydrocarbons will continue to be found in Cenozoic sediments.
The Bukhara and Chardzhou steps are also prolific and contain mostly gas, predominantly below the Late Jurassic salt cap. This distribution is based on discoveries to date. Lack of deep drilling capability has skewed this distribution to the shallower horizons. Several small fields and shows have been found in the Devonian-Triassic section.
SOURCE ROCKS
Little data have been published on the source rocks of Uzbekistan.
In western Uzbekistan Lower-Middle Jurassic carbonaceous and coal-bearing units are the most likely source rock for the gas fields in the Bukhara and Chardzhou steps, and total organic carbon content reaches 4.3%. The organic matter is largely humic.
Callovian-Oxfordian dark grey to black bituminous clays alternating with limestones are also known. It is possible that Paleozoic black laminated shales could have sourced the fields of the Ustyurt plateau. Source rocks have also been found in the Early Triassic succession.
in the Surkhandarya basin more than 20 source rock horizons are known, ranging in age from Jurassic to Paleogene. In the Fergana basin, Jurassic, Turonian, and Paleogene source rocks have been identified. The Paleogene rocks are oil prone.
The Fergana basin's low geothermal gradient of 2530 C./km means that the oil window lies over a wide depth range, as confirmed by the discovery of oil at 6 km depth at Mingbulak.
RESERVOIR ROCKS
More than 30 pay zones, mostly sandstones, have been discovered in the Fergana basin, though many are not regional in extent. The porosity of the Jurassic reservoir XXIII in S. Alamyshik field is 18%. Cretaceous porosities range between 1020%. Paleogene porosities are typically 20% and reach 30%.
In the Surkhandarya basin there are three Jurassic reservoir horizons with porosities of about 1017, in the carbonates. Permeabilities exceed 200 md. Flow rates from the Jurassic carbonates of Gadzhak gas field reach 3 million cu m/day.
Porosities in Cretaceous sandstones vary between 10-24%. The best reservoirs are in Paleocene-Eocene carbonates, where porosities are commonly about 20%.
Porosities in the Bukhara and Chardzhou steps are typically 5-15% in the Jurassic limestones, but permeabilities may reach more than 100 md in some of the reefs. The Cretaceous sandstones have much better reservoir qualities with porosities of 15-25% and permeabilities of hundreds to thousands of millidarcies.
Early-Middle Jurassic fluvial sandstones of the Ustyurt plateau have open porosities of 4-16%. The Paleozoic limestone samples have porosities of only 3% but some are cavernous, and it is likely that the cores taken to date do not represent the permeable intervals. In Shakhpakhty field the Callovian reservoir has 12-16% porosity and 100-1,500 md permeability. Gas flow rates of as much as 1.5 million cu m/day have been recorded.
CAP ROCKS
The best regional seal is the Kimmeridgian-Tithonian Gaurdak suite of the AmuDarya basin. It is limited to the west by the Murgab-Zaunguz rift and pinches out to the northeast on the Bukhara step. This seal reaches 1 km thickness on the Chardzhou step, explaining the more limited stratigraphic distribution of hydrocarbons as compared with the Bukhara step.
Kimmeridgian-Tithonian salt bearing horizons 3001,200 m thick are also present in the Surkhandarya basin, where they form a structural decollement. Jurassic seals in the Fergana basin are generally of moderate quality, but very good Cretaceous and Upper Eocene-Oligocene seals exist with an average thickness of 150 m.
TRAPS
The majority of known traps are anticlinal or anticlinal-lithological. In the Bukhara and Chardzhou steps and Southwest Gissar, Jurassic reef traps are also important. The trap heights are often large (the Kultak reef trap is 240 m high). Anticlines began to form early in the Bukhara and Chardzhou steps and were enhanced in Neogene time.
In the Fergana and Surkhandarya basins many of the anticlines are folds related to faults (Fig. 5) and the faults may play a part in confining oil pools. In the Fergana and Surkhandarya basins, traps with closures of 26-300 m were formed in Neogene times. Lithologic traps are important in Early-Middle Jurassic reservoirs, particularly in Ustyurt.
HC COMPOSITION
The gases of the Bukhara and Chardzhou steps generally contain 80-90% methane, and condensate content is 25-30 g/cu m.
Hydrogen sulfide levels reach 6.5% on the Chardzhou step but fall to below .5% on the Bukhara step-in the Fergana basin the oil density varies between .77-.85 g/cu in Jurassic rocks, increasing to .83-.93 in the Paleocene.
Sulfur content in Mingbulak field is less than .5%. Gadzhak field has not been developed due to its high H,S content, but the methane rich gases of S.W. Gissar contain less than .5% H2S.
THE FUTURE
FERGANA BASIN.
Five license blocks cover all but the area immediately around Mingbulak oil field (Fig. 6). Of primary interest will be further delineation and development of new oil reserves at a depth of about 5,000 m.
Due to poor deep drilling capability, only a handful of wells have penetrated the deep Cenozoic section, proving up about 350 million bbl of oil tested in the Mingbulak structure.
More deep drilling is required, and the Fergana basin, once thought to be a mature region, may well become a rejuvenates oil province.
SURKHANDARYA.
Gadzhak and Kognysai fields require development with advanced technology that can cope with the high H,S contents of the gas. Some 15 targets have been identified in Late Jurassic carbonates within the license block.
Reefs as thick as 1,000 m may exist. The complex faulting requires the application of advanced seismic techniques.
S. W. GISSAR.
S. Tandyrcha gas field requires development but has been excluded from the license block. The gas occurs in a Late Jurassic barrier reef sealed by evaporates. Two other wells have been drilled on the block.
The prospecting effort in this region is directed towards identifying the continuation of the barrier reef trend, determining the hydrocarbon potential of the pre-Late Jurassic section, and exploring the subthrust traps.
USTYURT.
Three blocks are to be licensed. While the region is known for developed and undeveloped gas reserves, recent drilling in Paleozoic has tested oil and gas. The gas fields are awaiting the development of local infrastructure, although they are close to the gas pipeline connecting with the Russian grid.
Acreage to the east of the Kuanysh-Koskala block, where gas has been discovered in Jurassic clastics, and Paleozoic reefs have been identified, is currently under negotiation.
The aim of the Uzbek government is to achieve self sufficiency in oil and gas production and the generation of hard currency revenues from export. Given adequate terms for exploration and development concessions, these goals are obtainable in the future.
TERMS, CONDITIONS
Data, terms, and conditions for the first international Uzbekistan license round will be presented Aug. 25 in Houston and Sept. 16 in Tashkent.
The Uzbeki authorities have compiled a book translated into English summarizing Uzbek laws. A summary of parts relevant to hydrocarbon exploration and extraction will be available at the presentation.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.