Modern methods wrest more gas, oil from Ukraine's historic producing basins

Newly constructed separation plant serves Rudovsko-Chervonozavodskoye gas/condensate field in Ukraine's Dnieper-Donets basin as Carpatsky-Poltavanaftogaz Well 104 (background), TD 19,080 ft, awaits casing from Romania.

• The field's first western wellhead, a 15,000 psi model manufactured by ABB Vetco, tops Well 102 awaiting tests. Photos by L.C. Texas. [16,392 bytes]

The basin connects on the northwest with the Pripyat depression of Belarus and on the southeast with the Donetz coal basin (Fig. 1 [198,753 bytes]).

Dnieper-Donets is an assymmetric, Paleozoic rift basin, 500 by 100-125 miles in size, formed between the Ukrainian and Voronezs crystalline massifs in three phases of development:

1. The prerift period, between the Proterozoic and Middle Devonian;
2. The rifting phase in the Upper Devonian-Early Visean era; and
3. The post rifting phase in late Visean.

The pattern of morphology of salt diapirism, halogenesis, and the resulting formation of traps for hydrocarbon accumulation is very similar to the Gulf of Mexico province, but the basin fill is much older. The oldest sediments are thought to be Middle Devonian age. The basin fill is composed of mainly Devonian, Carboniferous, and to a lesser extent Mesozoic and Cenozoic rocks. The salt deposits are Devonian and Permian origin.

The main structural element of the basin is the central graben, divided by transverse faults into series of blocks, with stair-step like dipping toward the southeast. Tectonics associated with the movements of salt and crystalline basement rocks played a significant role in the formation of various traps for oil and gas, and to a much lesser degree Devonian volcanism also contributed to trap formation.

The sedimentary cover increases from the NW toward the SE, reaching a thickness of over 12.5 miles (average 7.5-10 miles). The lower part of the Paleozoic is represented by the least studied Devonian strata with a thickness of around 13,000 ft consisting of mainly carbonates and various terrigenous and halogenic deposits. In the central part of the basin various volcanic rocks of Devonian origin also have been discovered.

The Carboniferous strata can be divided into three sections with a combined maximum thickness of 13,000-16,400 ft in the central part of the basin where the rocks are mainly Tournasian and Lower Visean age carbonates. Argillites, aleurites, and sandstones were also deposited with terrigenous sediments, characterized by intercalated limestone markers and coal deposits in the remaining parts of the Carboniferous, mostly in the shelf areas. In the Lower Permian mostly salt was deposited, reaching a thickness of 4,900 ft with layers of gypsum and dolomite which are covered by angular and stratigraphic unconformities of multicolored shale, sandstone, and a terrigenous Upper Permian-Triassic complex that is 650-980 ft thick.

The Jurassic, Cretaceous, Paleogene, and Neogene sediments are mostly of terrigenous origin with partial chalk and carbonate formations.

Oil, gas accumulations, potential

Most of the potential of the deeper lying strata (14,760 ft and 18,000 ft) and the stratigraphic complex deposited between the Lower Permian-Upper Carboniferous and the Visean-Tournasian is not yet explored. There is a well defined distribution of the hydrocarbons in the basin. In the NW part only oil and oil-gas fields occur. From there toward the SE a transition to gas condensate and finally to dry gas accumulations takes place. Within individual fields with depth the lighter oil changes to gas with condensate to dry gas at deeper depths.

Among the six basic oil and gas bearing complexes of the region the most significant hydrocarbon accumulations occur in the Lower Permian-Upper Carboniferous strata, such as Shebelinka field with 4.7 billion bbl of oil equivalent, West-Krestishinskoe with 700 million BOE, Glinsko-Rozbushevskoe with 1.25 billion BOE, and Efremovskoe with 800 million BOE recoverable reserves. It must be noted that most of the already discovered fields are depleted, some up to 97%.

In order to find additional reserves future exploration must involve 3D seismic prospecting in the interdomal areas, under salt and salt overhangs, etc. The oil fields of this complex are connected with salt domes associated with numerous radial, antithetic, and listric faults of different ages and also with angular unconformities. Grabens often formed over these features and, as a rule, with depth the degree of their subsidence decreases.

The very thick Devonian strata are the least explored and studied. Presently the only commercial Devonian oil accumulations lie beneath the salt bearing strata in the Sagaydag area on the southern flank of the basin. Here, under and between the different salt layers, hydrocarbon accumulation potential should not be overlooked.

Most of the discovered oil and gas reserves and the potential for future discoveries lies in the central part of the D-D basin associated with combination traps formed in the Lower Carboniferous strata. These accumulations can be divided into 7-20 productive horizons with sand thicknesses ranging between 3-66 ft and carbonate thicknesses 164-984 ft. The Upper Serpuhovsky-Middle Carboniferous strata also have a substantial number of fields, most of which are depleted up to 80%, but the size of these accumulations is not significant.

For future exploration the most prospective place is the central part of the basin within the Visean-Tournasian sandstone and carbonate deposits at 14,760-18,000 ft. Among the terrigenous rocks the nearshore bar and river channel formations deposited in valley-like morphologies and depressions should be mentioned. Within the carbonate rocks possible bioherm reef formations analogous to the already discovered Ignatovskoe and Machehskoe fields could be exploration targets.

Among the largest discoveries of 1987-98 the most significant are the Mehedovsko-Rudovsko group of fields (Mehedovskoe, Sviridolvskoe, Licenkovskoe, Chervonozavodskoe, and Rudovskoe). These fields represent over 720 million BOE of proven and probable gas and condensate reserves. They consist of lithologic-tectonic combination traps incidental to the axial part of the basin in the Glinsko-Rozbushensky ridge area.

Gas found in the D-D basin is free of H2S. It contains 94-96% carbon dioxide and 1-4% nitrogen. The condensate specific gravity varies from 0.636 g/cu cm to 0.7806 g/cu cm.

The crude is light (specific gravity 0.785-0.80 g/cu cm) and middle (0.81-0.86 g/cu cm) gravity, with 0.15-0.5% sulfur and 7-10% paraffin content.

Future outlook

Field activity

Both areas are further divided by several basement, high angle faults. Since its discovery, a total of 27 wells have been drilled in the field, of which 15 are on production. In addition, four wells are currently being drilled and/or are in various stages of completion.

R-Ch field has two major gas bearing intervals with a total thickness of 4,920 ft. The older, Tournasian-Lower Visean section has one gas reservoir (T-3 horizon), and the rest of the Visean section has eight gas condensate reservoirs (V-15L, V-20, V-21U, V-21L, V-22U, V-22M, V-22L, and V-23 horizons) and one oil bearing horizon V-18U (U = Upper, M = Middle, L = Lower). All hydrocarbon accumulations occur in sandstones with 6.5-20% (average 9-12%) porosities and 10 to 815 md permeabilities (Fig. 5 [324,367 bytes]).

Most of the reserves developed to date are in nearshore bars, conical shaped sand bodies formed by underwater currents. To a much lesser degree production is obtained from river channel deposits.

Daily gas production of an individual well ranges from 2.4 MMcfd (V-20 horizon) to 28 MMcfd (T-3 horizon) on 8-16 mm chokes. Most of the gas reserves are in the V-22 and V-23 horizons, but the V-22 horizons (L, M, U) have the largest areal extent.

Original gas reserves in R-Ch field are calculated to be 2 to 2.25 tcf of gas, 49.5 million bbl of condensate, and 2.2 million bbl of oil. The expected recovery factor is 80% for gas, 60% for condensate, and 20% for oil. Since 1987 the field has been produced under a pilot production license, which will be the case until the northwest, west, and south flanks have been explored.

As of July 1, 1998, 53 bcf of gas and 1.8 million bbl of condensate had been produced.

In order to increase condensate recovery from the V-20 horizon (condensate content: 937 g/cu m), the feasibility of cross-flowing the higher pressured dry gas from the underlying T-3 horizon in Well No. 102 (now awaiting perforation) is being studied. Despite the great depth of the T-3 formation in this well, 18,000-18,380 ft, the porosities are between 11-17% and the permeabilities are up to 340 md. Based on the reservoir pressure of 12,400 psi, and the above, substantial gas production rates are expected.

The V-22 and V-23 horizons are associated with elevated carbonate content mineral waters, which in high producing rate wells cause some carbonate scaling in the tubing.

R-Ch field is being developed jointly by Poltavanaftogaz, a subsidiary of the open joint stock company Ukrnafta (Ukrnafta), and Carpatsky Petroleum Corp., a U.S. company listed on the Alberta Stock Exchange as Carpatsky Petroleum Inc. Ukrnafta, as a result of the most recent reorganization program of the Ukrainian government aiming to privatize its oil and gas industry, became a subsidiary of the newly created National Shareholding Company "Naftogaz Ukraine" (NAK). NAK also owns the not yet privatized Ukrgazprom enterprise, Ukrtransgaz Co., the "Druzhba" and the Pridneprovsk municipal oil pipelines, the Odessa oil terminal (under construction), GDP Chernomorneftegaz, and the government owned stocks in various open stock privatized companies.

At this writing in late October 1998, Poltavanaftogaz and Carpatsky Petroleum are running six rotary rigs in the field (OGJ, Sept. 28, 1998, p. 111). Based on recent experience, a completed 17,000 ft well costs $2.8-3.3 million, while a 19,000 ft completed well costs $4.3-4.4 million. These figures represent the utilization of Ukrainian drilling rigs (Uralmash 3-D) manned by Ukrainian crews.

Most of the materials used are Ukrainian. In some cases wellheads (15,000 psi) and perforating charges and equipment have been accessed out of the U.S. Romanian manufactured API standard casing and tubing have also been used.

The full field development, which is projected to require the drilling of at least 20 additional wells (unless horizontal or inclined drilling methods prove more effective and require the drilling of a lesser number of wells), will necessitate the construction of additional field separation facilities. A new gas separation facility capable of processing the production of 10 or more wells in the field was recently completed.

The final processing facilities for pipeline quality gas are located at the nearby (10 km) gas plant in Glinsk. This plant is tied in to the 2,400 mile, international Urengoi (Western Siberia)-Uzhgorod (Western Ukraine) pipeline system. This 56 in. pipeline carries all of the Russian gas deliveries to Western Europe (Fig. 6 [98,518 bytes]). It is owned by Ukrgazprom and has an annual transit capacity of some 5.6 tcf. In 1996, according to Ukrgazprom, the transit volumes reached 3.5 tcf.

Precarpathian region

The first oil well was drilled in 1887 in Dolina field, and that field still produces today. Another large field, Borislavskoe, was opened in 1893. In the following years numerous other fields were brought on production despite several wars and the territory's turbulent history.

Between 1924 and 1958 the gas fields of the area accounted for almost all of the natural gas produced in the Soviet Union, but due to the production decline of the old fields, coinciding with the rapid decrease of funds, today the region contributes little to Ukraine's oil and gas production.

Geologically, the region can be divided into three distinct zones: on the west the folded Carpathian Belt, in the center the Internal Zone of the Precarpathian foredeep, and on the east the External Zone of the foredeep.

The Precarpathian foredeep was formed on the SW edge of the Russian platform and during its prolonged subsidence has accumulated over 30,000 ft of sedimentary cover. It is divided into an internal and external zone (Fig. 7 [118,609 bytes]). Both the internal and external zones are similar in their geological formations of Upper Cretaceous and Paleogene flysch and Miocene molasse and also in their structural development. The external zone is entirely gas bearing whereas the internal zone is mostly oil bearing with a few gas accumulations.

The thickness of the internal zone increases from 1,200 m on the SE to 2,000-4,000 m on the NW and is overthrusted on the external zone. It is separated from the Precambrian basement rocks by a system of stair-step faults displacing the Paleozoic and Mesozoic rocks some 1,500-2,000 m. All rocks of the internal zone were subjected to intensive dislocations into anticlinal folds which are overturned and divided by faulting. The NE crest of these folds is steep, whereas the southwest is gently dipping. The most studied of these is the Internal zone, which is related to the main oil fields of western Ukraine: Dolina, Bitkov, Borislav, and others.

West of the foredeep is the folded Carpathian belt, which consists of four major tectonic zones pushed over one another, but oil fields have been discovered only in the Skibov tectonic zone located in the extreme NE. The rock formations there consist of Upper Cretaceous and Paleogene flysch, crumpled and crushed into numerous scale-like anticlinal folds, in many ways analogous to the folds of the external zone of the foredeep.

Oil, gas accumulation, potential

Almost all known oil fields have similar geologic structures associated with asymmetrical, overturned anticlinal folds in the inner zone and the Skibov zone of the Carpathians. The gas fields of the external zone are connected with uplifting, thrusting, and folding related anticlinal structures. From 1948 to 1992 over 700 million bbl of oil were produced in this region from the Paleogene and Neogene sediments of the Precarpathian foredeep, not to mention the unrecorded volumes of production prior to 1948.

Oil and gas specifications

The produced natural gas is almost entirely methane (95.5-99.5%) without any H2S. CO2 content is only 0.1-0.5%. A few heavy oil deposits exist, such as in Kohanovskoe field (specific gravity 0.986-1.01 g/cu cm with high sulfur content of 5.5-7.4%).

Future outlook

The theory became fact with the discovery of Lopushnyanskoe (LP) field in the Nadvirn oil and gas producing area in 1983. This find is not only a significant field discovery, but its real importance may be that it confirmed the existence of an entirely new, unexplored oil province. It could potentially reactivate interest toward exploratory drilling in the area. Seismic surveys have indicated LP-analogous anticlinal structures on trend.

Judging by their size, these structures should be capable of holding substantially larger reserves than LP's, which are calculated to be 50-70 million bbl at a depth of 13,780-14,436 ft.

Field activity

Only 8.3% recovery has been achieved to date. This is due to a variety of reasons such as complex lithology which varies horizontally and vertically, high paraffin content of the oil, the mineral composition of the reservoir rocks, characteristics of the depletion drive reservoir, and the 1960s production techniques used in field development.

Aiming to increase the recovery rate by redeveloping the field, Nadvirnanaftogaz, the local subsidiary of Ukrnafta, has formed a joint venture company Ukrcarpatoil Ltd. (UKO) with Carpatsky Petroleum. The present approximately 1,000 b/d oil plus associated gas production is called "base line" and belongs to Ukrnafta. All production above this belongs to UKO and Carpatsky.

The B-B field complex produces from multilayered, separate reservoirs of the Middle and Lower Menilitic (Oligocene) formation located between 6,200-7,600 ft with a thickness of 100-350 m. Eight principal zones are recognized; most have lenticular development with small areal extent.

About 95% of all wells were completed using slotted 53/4 in. casing. This causes insurmountable difficulties in trying to assess both the past and present performance of any of the sandstone reservoirs independently.

There are five folds in the field complex. The Glubinnaya fold, with a reversed limb, is by far the largest. It has produced some 15.8 million bbl of oil and 35.6 bcf of associated gas as of 1963. All of the other four folds, part of the B-B field complex, Molodkovskaya, Babchenskaya, Staraya Kopalnaya (including another smaller structural unit), and Gazovaya, have produced only smaller quantities of oil. The Glubinnaya fold is divided into five blocks by high angle faults with a throw of 328-492 ft (Fig. 8 [89,668 bytes] and Fig. 9 [90,889 bytes]). Among the blocks, Bitkov and Pasechnyanskoe proved to be the most productive, while Pasechnyanskoe, Lubizhnyanskoe, and Delyatinsky have produced less than 14% of the field's total output.

Some 1,000-1,600 ft below the Middle and Lower Menilitic reservoirs a large Eocene gas pool is nearing depletion after the delivery of over 1.2 tcf from 68 wells. Some 200 bcf is still expected to be produced from the remaining 32 wells. This operation is owned and managed by the not-yet-privatized Ukrgazprom enterprise, which is responsible for 90% of all Ukrainian domestic natural gas production. The Eocene sandstone reservoir qualities are considerably better than the overlying Oligocene oil reservoirs, which have 7-17% porosity and 8-56 md permeability.

According to a preliminary field redevelopment plan developed by Ukrcarpatoil Ltd., the present initial field work calls for the selective re-entry of depleted gas wells and perforation of the oil zones. Four of these wells are already on production delivering some 76 b/d of oil and 350 Mcfd of associated gas. Production from these wells continues to be restricted only because of the locally available lifting equipment inefficiency and lack of paraffin control.

Planned for early 1999 is the drilling of new infill wells in parts of the Glubinnaya fold where the reservoirs were not drained or were only partially drained by existing wells. Detailed geologic and engineering studies have defined 14 such locations.

After the results of this drilling program a final field redevelopment plan will be worked out. This might include the implementation of secondary recovery technology and-or the drilling of inclined and horizontal wells.

Acknowledgment

Bibliography

Machuzhak, M.I., and Bodnar, A.V., Actual geologic problems with the commercial oil and gas presence in the Rudovsko-Chervonozavodsky oil and gas condensate field, Poltava, 1998 Collection of Scientific Works for the 5th International Conference UNGA, Vol. 2, pp. 70-73.

Garshin V.K., Abyssal fractures, geotectonic exploration and oil saturation of rift basins, Naukova Dumka, Kiev, 1974.

Abrikosov, I.H., and Gutman, I.S., General oil and oil industry geology, Nhedra, Moskva, 1974.

Zyuzkevich I.P., Krivosheya, V.A., Machuzsak, M.I., Characteristics of the geologic structure and hydrocarbon exploration problems in the carbonate sediments of the lower Carboniferous in the central part of the Dniepr-Donetz basin, Geologic Journal, 1994, No. 4-6, pp. 27-35.

Chirvinskaya, M.V., Deep structure of the Dniepr-Donetz aulacogen according to geophysical data, Kiev, Naukova Dumka, 1980.

UKRNIGRI atlas of the geological structure and oil and gas presence in the Dniepr-Donetz basin, Kiev, 1984, 192 p.

The Authors

Les Texas is president and founder of Carpatsky Petroleum and is an explorationist and certified petroleum geologist. He started his carrier in 1965 with the Hungarian Oil and Gas Trust, now MOL. After working for Signal Oil Co. and Getty Oil Co. he became an independent in 1972. In the last years of Angola's colonial era he prematurely tried pioneering exploration south of the Congo delta. Later he was instrumental in the discovery of Mestena Grande and Berry R. Cox gas fields in South Texas. Since 1992 he has been active in Ukraine with his company, Carpatsky Petroleum. He holds an MS degree in geology from Eotvos Lorand University, Budapest. E-mail: [email protected]

Mihail I. Machuzhak is chief geologist of Poltavanaftogaz company and a corresponding member of the Ukrainian Oil and Gas Academy. He wrote his candidatorial dissertation on Characteristics of the geologic structure and oil and gas perspectives of the lower Visean-Tournesian complex of the southern zone of the Dnieper-Donets basin and authored 23 scientific and geologic publications. He spent his entire professional carrier working in the Dnieper-Donets basin, initially with the Poltavaneftegazgeologia exploration company. He has a PhD in geologic sciences and a degree in geology from the Ivano-Frankivsk Institute of Oil and Gas. E-mail: [email protected]

Pyotr M. Chepily is head of the geologic department of the Chernigovnaftogazgelogia enterprise in charge of exploration. His department's work resulted in the discovery of Rudovsko-Chervonozavodsky field among many others. In 1991 he defended his dissertation, Characteristics of the geological structure and hydrocarbon occurrences of the central part of the Dnieper-Donets basin. He has a PhD degree in geology-mineralogy with a degree from the Ivano-Frankivsk Institute of Oil and Gas and is a correspondent member of the Ukrainian Oil and Gas Academy.

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