Nickolie N. Lisovsky
Ministry of Oil & Gas Industry
Moscow
G.N. Gogonenkov
Central Geophysical Expedition
Moscow
Yuri A. Petzoukha
Institute of Geology and Exploitation of Combustible Fuels
Moscow
The Soviet Union is a major producer of hydrocarbons. Before World War II most Soviet production was concentrated in the Baku region (Fig. 1).
Later, the center of exploration and production moved to the Volga-Urals province. After the large oil and gas basin was discovered in western Siberia, Soviet production reached 4.2 billion bbl/year (600 million metric tons/year) of oil and 28 tcf/ year (800 million cu m/year) of gas.
The Pre-Caspian depression long ago attracted the attention of petroleum geologists. Exploration was conducted here at relatively shallow depths within reach of what was then state of the art technology.
It was only in the last 15 years that improved technology allowed exploration of the deeper structural zones. As a result, a number of major fields were discovered along the margins of the basin, including Tengiz. Tengiz is the world's largest oil field discovered in the 1980s.
PRE-CASPIAN DEPRESSION
The Pre-Caspian depression is one of the largest in the world, with an area of more than 500,000 sq km.
Tengiz field is in desert terrain on the northeast edge of the Caspian Sea, where apart from dramatic seasonal temperature variations, other severe environmental conditions also exist.
The depression is bounded on the northwest by the Russian platform and on the east and south by the Hercynide fold belt, which is the southern extension of the Ural Mountains (Fig. 2).
The Astrakhan-Aktyubinsk system of highs divides the whole depression into two unequal parts. The sedimentary cover is extraordinarily thick in the depression and varies from 20-24 km in the central part to 5 km in the marginal zones. Tengiz field lies along the southern margin of the depression in the southeastern basin.
SALT TECTONICS
Intense salt tectonics occur throughout the central part of the depression (Fig. 3).
Tengiz field is on an upper Paleozoic carbonate platform on the depression's southern margin.
The depression's sedimentary cover is divided into two megasequences by the Permian Kungurian salt formation (Fig. 4).
The post salt megasequence is dominantly composed of terrigenous rocks of upper Permian, Mesozoic, and Cenozoic ages. It is considerably deformed by salt tectonics. Shallow oil fields are present in clastic reservoirs of Jurassic and Cretaceous age.
The pre-salt sequence is of Paleozoic and upper Proterozoic age. It contains both terrigenous rocks and thick carbonates. The major oil fields on the depression's margins are associated with this presalt sequence. The major reservoirs of the pre-salt fields are carbonates of predominantly Carboniferous age.
Permian salt provides an excellent regional seal. Oil source rocks are found in the carbonates themselves and their deepwater marine equivalents.
The Paleozoic interval thins to 2-3 km within the Astrakhan-Aktyubinsk system of highs. It thickens again towards the south to as many as 6-8 km. It is in this area that Tengiz field was discovered.
EARLY EXPLORATION
Tengiz field lies on an upper Paleozoic carbonate platform (Fig. 5).
The first wildcats in this area with Paleozoic objectives were drilled in the Karaton area. When tested, these wildcats produced large flows of water with hydrocarbon gases.
Carbonates with good reservoir properties were found. The exploration strategy was aimed at identifying isolated traps with good caprocks.
The base of salt reflecting horizon P, was first mapped using common depth point data in 1975. The quality of seismic data processed by very basic computer programs was rather low. Nevertheless, geophysicists managed to map a large high at Tengiz.
A structure map of Tengiz constructed on the base of salt reflecting horizon Pi shows that relief of the structure is almost 1,000 m (Fig. 6). Its area at a depth of 4,000 m is more than 400 sq km.
TENGIZ DISCOVERY
The first wildcat on the crest of this structure, drilled in 1979, tested a large flow of oil.
Immediately, four more wells were drilled. Simultaneously, seismic surveys began with high multiplicity and advanced data processing. As a result the reliability of interpretation increased dramatically.
A 1986 seismic depth section, Line B-B', clearly shows the almost flat top of the structure (Fig. 7). Visible on the margins are small ridges, 30-50 m in relief, with relatively steep slopes. The undulating surface of the salt and faulting in the post salt section caused by salt tectonics were able to be mapped much more accurately.
Seismic surveys made it possible to predict a massive pre-salt carbonate buildup to a depth of 7.5 km.
Deep reflections are apparent and show a gentle structural arch at a depth of 7.59.0 km. The interval velocity in the underlying sequence is 5.25 km/sec.
Judging by that velocity the sequence is most probably composed of terrigenous rocks or terrigenous rocks and carbonates. By analogy with other regions of the eastern Russian platform, this underlying sequence is assigned to Middle Devonian.
FOLLOWUP DRILLING
In the early 1980s three of the four outpost wells produced large flows of water free oil, including one well that tested a zone 700 m structurally lower than the crestal wells.
It became clear that a very large field had been discovered.
The third stage of exploration began in 1983 with the simultaneous drilling of 20 wells and the recording of three dimensional seismic surveys over the entire oil field.
The drilling is complicated. Lost circulation is experienced in many intervals, high hydrogen sulfide content is present in the associated gas, and formation pressure is twice as high as hydrostatic pressure. All this called for complex well designs.
Due to these reasons the entire carbonate buildup has not yet been penetrated. The maximum penetrated thickness is 600 m. However, since some of the wells were drilled around the periphery of the structure, it has been possible to study the reservoir to a depth of 5,400 m.
All wells produced waterfree oil. The oil-water contact has not yet been reached.
An oil column in excess of 1,500 m has been established.
STRATIGRAPHIC SECTION
From upper Devonian to middle Carboniferous, the stratigraphic section is composed of biogenic limestones with rare intercalations of thin dolomites, calcarenites and grainstones, thin beds of volcanic ash, and marls (Fig. 8).
A number of unconformities were identified.
The base of the lower Permian Artinskian rocks represents a significant nondepositional and erosional unconformity and overlies rocks ranging in age from Upper Devonian to Middle Carboniferous. Predominantly they are argillaceous rocks composed of erosional material from the carbonate body.
Their thickness, 10-80 m over the top of the structure, increases rapidly down the flanks. The top of the Artinskian rocks is represented by the key seismic reflection horizon P, and is shown on the structure map. The top of the carbonate reservoir is represented by seismic reflection horizon R.
In the pre-salt carbonate buildup, no clear signs of faulting have been identified except for some well data indicating extensive rock fracturing. The available seismic data do not allow reliable mapping of faults with large displacements in the pre-salt section.
TIME SLICES
Knowledge of the structure of Tengiz field is based largely on the seismic data.
Time slices at three levels were derived from three dimensional seismic data (Fig. 9).
One is a time slice equivalent to an average depth of 4,150 m. An outline describes the configuration of the P1 base of salt reflector.
The time slices show the locations of the wells that have penetrated that particular depth. It is easy to map the increase in area with depth as well as the outlines of the salt troughs on the margins of the buildup.
Another time slice is equivalent to an average depth of 4,600 m, and a third is equivalent to an average depth of 4,950 m. These slices give an idea of the size of the structure and its form.
The location of the wells around the periphery of the structure that penetrated the reservoir has been used to predict the reservoir characteristics below the drilled depth of the crestal wells.
A structural map at reflection horizon R represents the top of the carbonate reservoir (Fig. 10). It is a map on the base of the Artinskian unconformity and represents rocks ranging in age from middle Carboniferous on the crest to upper Devonian on the flanks.
The white area only shows the extent of the oil zone penetrated by drilling to date.
SEISMIC SECTIONS
Vertical seismic sections show other features of the Tengiz structure.
One two dimensional line not reproduced with this article shows the flat central part of the carbonate body and its crest. There are no continuous reflectors within the body, which suggests the absence of internal seals. The base of the carbonate section is clearly described by the gently arched P3 mid-Devonian reflection band.
A zone of divergent reflections can be observed on the flanks of the structure. This zone is especially distinct on another line (Fig. 11).
These divergent reflections are confined to the lower Permian Artinskian section. The base of this zone of divergent reflections represents the top of the carbonate reservoir R.
The spill point of the presalt structure at the P1 horizon does not limit the oil accumulation: the oil-water contact is located much lower. It follows that this zone of divergent reflections also acts as a sealing facies.
A geological model represents Tengiz field as a biohermal structure of an atoll type (Fig. 12). It was formed during upper Devonian and Carboniferous time on top of a gentle high.
Bioherm reefs ring the structure on three sides, and high quality grainstone reservoir rock occupies the center of the atoll. The bioherm reefs are expressed in the seismic data as the ridges previously noted.
TENGIZ LITHOFACIES
The lithofacies map gives a general idea of the structure and lithology of pre-salt rocks in the southeastern part of the Pre-Caspian region (Fig. 5).
Tengiz field is an element of a large carbonate platform on which several massive biohermal structures were formed. The carbonate shelf is replaced by deeper water terrigenous facies east and north of the carbonate platform.
The regional seismic lines A-A' and B-B' show the relationship of the structural and stratigraphic elements quite well.
Line A-A' intersects from south to north the biohermal structures Yuzhnaya, Tengiz, and Karaton, which can be clearly observed in the relief of the base of salt horizon P1 (Fig. 13).
The base of the carbonate platform represented by horizon P3 rises gently from south to north. It forms the southern flank of the Guryev Arch, which belongs to the Astrakhan-Aktyubinsk system of highs noted previously on the tectonic map.
The second regional line B-B' spans part of a large ring of structures whose western portion is located in the shallow waters of the Caspian Sea (Fig. 14).
This line crosses a chain of carbonate buildups of different relief: Tengiz, Korolev, and Karaton. Commercial oil has been found in the Tengiz and Korolev structures.
RESERVOIR CHARACTERISTICS
The primary microstructure of the carbonate reservoirs of Tengiz field was formed during sedimentation, but subsequent secondary processes caused rock alteration.
These processes finally formed the present day reservoir rocks, which are characterized by variations in porosity, permeability, and other reservoir properties. Such processes as tectonic fracturing, dissolution, recrystallization, dolomitization, and silicification played a major role.
According to well and core data, rock porosity varies widely from 1-2% to 15-25% (Fig. 15). Permeability varies considerably and is enhanced by intense fracturing of the entire reservoir. A systematic variation of these reservoir properties with depth has not been established.
THIN SECTIONS
A few examples of thin sections illustrate the variability of reservoir properties (Fig. 16).
One thin section enlargement shows an algal-biolithite matrix with high porosity caused by solution enlargement.
A second thin section is an example of a low permeability matrix with a large fracture partially healed by calcite. In such rocks, oil flows through fractures and the matrix as well. The flow rate can be as high as 8,000 b/d (1,000 metric tons/day).
A third thin section is an example of reservoir rock with average porosity of 37%. Clustered solution enlarged pores separated by impermeable matrix are visible. The permeability is only due to fractures.
A fourth thin section is an example of the low porosity reservoir. There is practically no dissolution. The matrix is impermeable. Oil is contained only in fractures.
One of the distinguishing features of the field is a large amount of solid bitumen contained in the pore space of the reservoir that tends to reduce overall porosity (Fig. 17).
The Tengiz reservoir is 1,500 m thick, has 11,110 psi formation pressure, and has a great difference between formation and saturation pressure (see tables).
The high formation pressure is due to the long hydrocarbon column and the very effective salt seal. A gas cap is not present, and the oil is highly undersaturated. The average porosity is 6.3%, and average permeability is 10 md and is enhanced by abundant fracturing.
TENGIZ OIL, GAS
Tengiz oil is of very high quality with a gravity of 46 API.
Sulfur content of the oil is low at 0.7%, but hydrogen sulfide content of the associated gas is very high at 18%. The composition of oil and gas is practically the same in all samples, which suggests fluid communication throughout the reservoir.
Organic matter present in the pre-salt carbonates was deposited in a littoral environment. The geochemical environment was reducing or slightly reducing, which facilitated preservation and further alteration of organic matter.
The organic carbon content in the Paleozoic carbonates is high, 1.2-1.4%, and sometimes reaches 4%. The hydrocarbon generation potential of such rocks is very high. These are Type I and 11 kerogens with a high hydrogen content.
The correlation of oil to source rocks based on chromatographic data shows that, in most characteristics, the oil composition reflects the genetic type of the original organic matter of the carbonates.
Solid bitumen encountered in the reservoir rocks in Tengiz field is classified as transitional between kerites and impsonites and may be regarded as residual material produced during alteration of an earlier oil accumulation in the presence of sulfur that accelerated the process of carbonization.
THREE PAY SUBZONES
The total original oil and gas in place are estimated at 25 billion bbl and 46 tcf of associated gas.
Based on the classification of reserves, the pay zone of the carbonate section can be subdivided into three zones.
The proven category C1 reserves are located to a depth of 4,700 m. The probable reserves of C2 category occur at 4,700-5,400 m. The possible reserves of C3 category are associated with Upper Devonian carbonates and can be expected between 5,400 m and the oil-water contact.
CONCLUSION
Discovery of Tengiz field-one of the world's 10 largest oil fields and the largest field discovered during the last decade-marked a new stage in oil exploration. The field is the deepest of the supergiants.
Tengiz field is important scientifically because it proves that enormous hydrocarbon reserves may in fact be located at such great depths. This discovery lowers the risk for the area and can serve as a basis for new exploration projects whose cost effectiveness would have previously been considered doubtful.
The unique composition of the formation fluid, the complicated geology, and the difficult geographic conditions of field development pose a number of technological problems. Solution of these problems calls for utilization of the best technology available in the industry.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
