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Ulmishek, Uzen Field-U.S.S.R. Middle Caspian Basin, South Mangyshlak Region PDF

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Uzen Field-U.S.S.R. Middle Caspian Basin, South Mangyshlak Region GREGORY F. ULMISHEK U. S. Geological Survey Denver, Colorado FIELD CLASSIFICATION BASIN: Middle Caspian RESERVOIR AGE: Triassic to Cretaceous BASIN TYPE: Foredeep PETROLEUM TYPE: Oil and Gas RESERVOIR ROCK TYPE: Sandstone TRAP TYPE: Anticline RESERVOIR ENVIRONMENT OF DEPOSITION: Alluvial, Fluvial, and Lacustrine LOCATION HISTORY Uzen field is located east of the central Caspian Pre-Discovery Sea in the Shevchenko administrative region, Kazakh S.S.R.; it is the largest oil field in the Middle Caspian The first oil flow in the Middle Caspian basin was basin (Figure 1). The field is situated in the South obtained in 1893 from middle Miocene rocks in the Mangyshlak region (subbasin) on a structural terrace Starogroznenskoye field (Figure 1). Shallow oil pools between strongly folded Triassic rocks exposed north in folds of the Caucasus foredeep remained the main of the terrace and the Mangyshlak trough, a deep exploration target until the late 1940s. A number of Mesozoic-Cenozoic trough to the south (Figure 2). such pools were found in the Terek-Sunzha and Another giant field located about 48 km (30 mi) west South Dagestan regions (Figure 1). Deeper drilling on the same structure is the Zhetybay oil-gas field. began soon after World War II and resulted in the Besides these two giants, several oil, gas, and gas- discovery of major oil reserves in the Mesozoic section condensate fields of much smaller size have been of the Terek-Sunzha region, and large gas reserves discovered in the South Mangyshlak region. in the lower Tertiary strata of the Stavropol region. Total areal extent of the Uzen field is 252 km2 At the same time, multiple oil and gas-condensate (97 mi2). The field, along with all other fields of the discoveries in the Arzgir-Prikumsk region demon region, is operated by a local management unit strated that the productivity is not confined (Ob'yedinenie) located in the Shevchenko city on the exclusively to the foredeep, as many geologists had shore of the Caspian Sea. believed, but extends into the basin foreland. These Oil reserves are considered a state secret in the discoveries paved the way for initiating exploration Soviet Union; thus, no data on reserves of the field in remote semidesert areas east of the Caspian Sea. have been published in the Soviet literature. Reserves in place have been assessed by Ulmishek and Harrison (1981a, 1981b) at 7.5 billion bbl, with an Discovery expected recovery factor of 25 to 26%. Carmalt and St. John (1986) ranked the Uzen field as the 129th The Uzen anticlinal structure was inferred from largest field in the world, with recoverable reserves surface geologic mapping of exposed Upper Creta of 1.875 billion bbl of oil. ceous and Tertiary rocks by S. N. Alekseichik in 1941. 281 . North Stavropol Field Buzachi Ustyurt / Basin , <& Field S South Mangyshlak^ Region • Boundaries of basins - — — — Boundaries of producing regions «••> oil fields 4ZZ> Gas fields Kilometers 40 0 40 80 Figure 1. Middle Caspian basin and its producing regions. Geologic studies of the region were interrupted by well was perforated from 1248 to 1261 m (4094 to World War II and were resumed in 1950. Detailed 4137 ft) and tested 2295 bbl/day of oil in stratum mapping of the structure, with shallow (18 to 91 m XVI (of Bathonian age). The next three wells (nos. or 60 to 300 ft) core drilling, was undertaken from 2, 5, and 22) were tested and confirmed the presence 1950 to 1954. Core drilling to depths of several of oil in other pays, i.e., in strata XIII, XIV, XV, and hundred meters on the Zhetybay and Uzen structures XVII. It became apparent that the new discovery was (Figure 2) began in 1957, contemporaneously with a giant field. The discovery of the Uzen field occurred areal geophysical investigations. Some 395 km (247 at a time when the addition of oil reserves in the mi) of reflection seismic data were run, with the Volga-Ural province (then the main producing region distances between transverse profiles ranging from of the U.S.S.R.) had sharply decreased. The Uzen 2.5 to 4 km (1.6 to 2.5 mi). Longitudinal profiles were discovery stimulated rapid development of explora run along the axis and flanks of the structure. Forty- tory activities in this new region. six core wells, with a total depth of 12,000 m (39,000 ft) and a maximum depth of 985 m (3230 ft), were drilled. Core samples were taken from marker beds. Post-Discovery These activities detailed the structure on Cretaceous horizons and in 1960 revealed gas pools in Cretaceous Exploratory drilling in the Uzen field began in 1962, rocks of the Uzen field. on the basis of data obtained from the first four Judging from gas flows in core wells, the Uzen field wildcats. The first exploration stage involved drilling was originally considered to be mainly a gas prospect. of 26 wells to depths of 1450 to 2500 m (4760 to 8200 Deep drilling in the field began in 1961. The first ft). Most of the wells were located in rows across wildcat (well no. 1), located near the crest of the the highest part of the field, called the Main cupola closure, was drilled to a depth of 1772 m (5814 ft). (Russian geologists use the term cupola to refer to The well penetrated a thick clastic section of Middle local highs). The distance between rows varied from Jurassic age overlain by Upper Jurassic carbonates 3 to 5 km (1.9 to 3.1 mi). By early 1964, this stage and shales. The section contained several tens of of exploration had delineated the part of the field interbedded oil-saturated potential reservoirs. The in the highest area of structural closure. 282 first phase of development varied from 84 acres (34 ha)/well for strata XIII and XIV to 69 acres (28 ha)/ well for strata XV and XVI and to 113 acres (46 ha)/ well for strata XVII and XVIII. Many wells were perforated in two adjoining strata for commingled production. Casing practices include setting surface casing to 180 m (590 ft) with 273 mm casing, and casing to about 1375 m (4500 ft) with 168 mm casing. Several types of completions were tried. Hydraulic fracturing using hydrofluoric and hydrochloric acid additives proved to be most effective. The second phase of field development began in 1972 after the inefficiency of the existing production system had become obvious. Additional rows of injection wells sectioned the field further into bands 2 km (1.25 mi) wide. Infill drilling of more production wells began at about the same time and continues to the present. Many additional injection wells have also been drilled. The strong inhomogeneity of reservoir rocks (permeability varies from a few md to 1000 to 1200 md) resulted in low effectiveness of waterflooding. Injected water channeled into most permeable beds. The zone of flooding greatly expanded, with concomitant ineffectiveness of sweep. Water break through to producing wells took place along very narrow intervals. The total watered-out thickness — — —• Boundaries of tectonic units of pays was extremely low and varied from 2 to 5 ^ Oil fields m (Ilyaev et al., 1975). Most of the produced oil came ?3> Gas fields from reservoir beds with permeabilities greater than Figure 2. Index map of the South Mangyshlak subbasin. 300 md, while less permeable reservoirs barely responded. An extensive program of drilling produc tion and injection wells with completions in low permeability intervals has been undertaken during During the next exploration stage (1964-1965), 33 recent years (Aitkulov et al., 1982; Batyurbayev, more wells were drilled between previously com 1982). The maximum rate of production was achieved pleted rows of wells and on the flanks and plunges in 1975 when the Uzen field produced 120 million of the structure. This drilling phase identified new bbl of oil. But by 1976, a sharp decline of production pools in strata XX and XXI, and discovered an oil had already begun. By 1980, production had field (Karamandybas field) just west of Uzen along decreased by 37% to 75 million bbl, and to 66 million the same structural trend. Gas pools in the Creta bbl by 1984. More recent data are not available, but ceous section were explored by 14 wells. The drilling production has continued to decrease. The rate of indicated that these pools are confined only to the decline may be somewhat less, partly because of Main cupola and gas reserves do not exceed 275 bcf increased production from small pools in the lower (Chakabayev et al., 1977). In 1965, the Uzen field part of the Jurassic that were found during devel was turned over to the Ministry of Oil Industry for opment drilling. The cumulative output from the development and production. Uzen field is probably close to 1400-1450 million bbl Uzen oil has a very high paraffin content (up to of oil. 28%). The paraffin tends to precipitate and plug the permeability in less permeable beds, as a result of either a pressure decrease or cooling of the reservoir. Therefore, a hot waterflood was planned from the REGIONAL GEOLOGY outset of production. The waterflood, however, was delayed for several years because of water supply problems, and full-scale heating of injection water Tectonic History was implemented in 1983. About 800 producing wells and 260 injection wells The Middle Caspian basin formed over a Hercynian were drilled during original development of the field. accreted terrane that is known as the Scythian (west Injection wells were drilled in rows across the axis of the Caspian Sea) and Turanian (east of the sea) of the structure; they sectioned the field into nine plates. The South Mangyshlak subbasin is located productive bands 4 km (2.5 mi) wide. Spacing on the in the western part of the Turanian plate. The 283 basement of the South Mangyshlak subbasin has rifting, but the rift itself was folded and uplifted. been drilled in only a few locations on the margins In the present-day structure, the rift lies outside the of the trough. The basement comprises strongly subbasin. Basins of this type do not belong in existing deformed and metamorphosed Paleozoic elastics and basin classification schemes. carbonates cut by granite intrusions. The deforma tion and metamorphism probably occurred in the late Paleozoic (certainly before the Late Permian). The exact nature of the basement in the central part of STRUCTURE the South Mangyshlak trough remains unknown. It may be formed by the same folded and metamor phosed rocks (Letavin, 1980), or, according to other Regional Structure interpretations (Akramkhodzhayev and Yuldashev, In the present-day structure, the South Mangysh 1984), it may be composed of undeformed Paleozoic lak trough consists of two large depressions separated rocks covering a median massif (microcontinent). by a structural saddle (Figures 3 and 4). The depth The second, taphrogenic stage of development of to the top of the Triassic in the Zhazgurly depression the South Mangyshlak subbasin (as well as all the exceeds 5 km (3.1 mi). The onshore part of the Middle Caspian basin) took place during Late Segendyk depression is slightly shallower; its Permian and Triassic time. A large graben-rift formed offshore continuation is poorly known. The Kara in the region in an area presently occupied by the bogaz arch to the south of the trough is a highly Mangyshlak system of uplifts (Figure 3). The uplifted structure on which Cretaceous rocks overlie Zhetybay step, where the Uzen field is situated, was the pre-Triassic basement at a depth of a few hundred located on the southern shoulder of the rift. The rift meters. The Peschanomys uplift is separated from is filled by continental elastics of Late Permian-Early the Karabogaz arch by the deep and poorly studied Triassic (Induan) age, and late Early Triassic Kazakh depression, which is offshore. On the top (Olenekian) through Late Triassic marine elastics, of the uplift, Jurassic rocks overlie a relatively thin carbonates, and, in places, volcanics. Total thickness Triassic section at a depth of slightly more than 3 of this sequence in the central part of the rift exceeds km (1.9 mi). The uplift was considered a prime 9 km (5.6 mi). Outside the rift, the thickness decreases exploration target for a number of years, but only abruptly. At the end of Triassic time, the rift was one small discovery in the Triassic section has been inverted, sectioned into relatively narrow horsts and made onshore. Offshore exploration appears to have grabens, strongly folded, and eroded. been unsuccessful and failed to establish commercial The third stage of geologic history began in Early fields. The Mangyshlak system of uplifts consists Jurassic time and continues to the present. The area of two chains of swells that are separated by the of the inverted Triassic rift remained highly uplifted, narrow and shallow Chakyrgan trough. Strongly with little or no Jurassic-Tertiary sediments deformed Upper Permian-Triassic rocks are exposed deposited over it. The deep South Mangyshlak trough on the north chain of swells and occur at a very formed between this uplift and the Karabogaz arch shallow depth on the Bekebashkuduk swell. to the south. The structural Zhetybay step developed The Zhetybay step is a structural terrace that between the inverted rift and the trough. The step controls major oil and gas fields of the region. On is separated by deep faults from both of these the north and south, the terrace is bounded by steep structures. The trough and the step are covered by flexures dipping southward (Figure 5). Structural thick, gently deformed sedimentary rocks of Early surfaces in the Jurassic-Cretaceous section are gently Jurassic through Tertiary age. Two unconformities, inclined to the south. The step contains a number pre-Cretaceous and pre-middle Miocene, were of local structures that are predominantly assembled important for formation of structural traps. into two zones parallel to the strike of the step. The The Middle Caspian basin as a whole is classified largest structures, the Uzen and Zhetybay anticlines, as 221 under the modified scheme of Bally and are located in the northern and southern zones, Snelson (1980) (i.e., perisutural basins on rigid respectively. All other structures are smaller. Local lithosphere associated with formation of compres- structures of the Zhetybay step contain about a dozen sional megasuture; foredeep and underlying platform discovered oil and gas fields, but the main reserves sediments, or moat on continental crust adjacent to are controlled by the Zhetybay and Uzen anticlines. a-subduction margin; ramp with buried grabens, but No hydrocarbons have been found in the South with little or no blockfaulting). The basin is classified Mangyshlak trough. Pools of heavy, biodegraded oil as HCa using the scheme of Klemme (1971) (i.e., are known north of the Zhetybay step (Tuybedzhik continental multicycle basins; crustal collision zone- structure). convergent plate margin; closed). The South Man gyshlak subbasin (which also can be considered a separate basin) does not fit well into either classi Local Structure fication. The Alpine foredeep of the western Middle Caspian basin does not extend into the South The Uzen field is associated with a large anticlinal Mangyshlak subbasin. Formation of the subbasin is fold that is about 45 km (28 mi) long and 9 km (5.6 mi) clearly connected with Late Permian-Triassic wide (Figure 6). The fold is significantly asymmet- 284 Boundaries of major tectonic units Boundaries of other structures Figure 3. Main structural units of the South Mangyshlak region. See index map, Figure 2, for location. (After Krylov, 1971.) rical, with the crest (Main cupola) located in the commonly reach 50 to 70°. The presence of faults eastern part of the structure. The northern flank with small displacements is also interpreted to occur dips gently at 1.5 to 2°; whereas, the southern flank in the Jurassic section, especially in its lower part. is steeper, with dip angles reaching 6 to 8°. The This interpretation is based primarily on well log western plunge of the fold is complicated by two correlation and production data. Yuferov et al. (1974) subsidiary closures, the Khumuryn and Parsymuryn concluded that the distribution of small oil pools in cupolas. The Karamandybas structure is separated the lower part of the Jurassic section is controlled from the Uzen anticline by a fault. Hydrocarbon pools by small faults (Figures 6 and 8). Most, if not all, of the former have no hydrodynamic connection with of these faults die out in the upper part of the Jurassic the Uzen pools, and therefore are considered a section. separate field. This general structural configuration The Uzen fold began to form in Jurassic (Gribkov is conformable through all the Jurassic-Cretaceous and Lazarev, 1968) or Early Cretaceous (Dmitriyev, section. Underlying Triassic rocks have a different 1985) time and continued through the Tertiary. structural configuration that is poorly known but However, the two principal phases of deformations is believed to have northern to northwestern trend were connected with the main unconformities in pre- (Klychnikov, 1982). Cretaceous and pre-middle Miocene times. These two Faulting is probably rather intense in significantly phases account for the major part of the structural deformed Triassic rocks. Dip angles in Triassic cores closure. 285 -2.C^~ Structure contours, contour interval 0.5 km, locally 0.1 km, below m.s.l. 20 0 20 40 60 I 1 1 I I 1 1 I I I 10 10 20 30 40 Faults 0 Miles Figure 4. Contour map of the South Mangyshlak region. Contours on the base of the Hauterivian (reflector lll-g). (After Sorotskaya, 1968.) STRATIGRAPHY The presence of Upper Permian-Induan continen tal rocks known in exposures of the Mangyshlak Oil and gas pools on the Zhetybay step have been uplifts has not been proved on the Zhetybay step discovered over a large stratigraphic interval from (Orudzheva et al., 1985). Deformed, dense, gray and the Triassic to the Turonian. Well over 90% of variegated shales and siltstones of Early Triassic age discovered hydrocarbons, however, are concentrated occur at the bottom of the drilled section. Their in the Jurassic section beneath the upper Callovian- penetrated thickness exceeds 1500 m (5000 ft) in the Kimmeridgian regional seal (Dikenstein et al., 1983). Uzen field. This unit does not contain reservoir rocks All known pools in the Jurassic and Cretaceous and only noncommercial flows of hydrocarbons have sections are controlled by structural traps. Explor been tested from it. Overlying rocks are carbonates atory drilling into the Triassic section has also and shales of the upper Olenekian-Middle Triassic. concentrated on local structures. It seems, however, They are found in the southern part of the step, and that hydrocarbon pools in this section are chiefly pinch out under the pre-Jurassic unconformity in its controlled by zones of fracturing (Timurziyev, 1984). northern part. Fractured and cavernous carbonate 286 3000- LOCAL ANTICLINES 1-Karamanata 7-Kariman 13-Aktas 19-Uzen 2-Beke 8-North-Western Zhetybay 14-Tasbulat 20-Enorta 3-Karasyaz-Taspas 9-Tarly 15-Easlern Zhetybay 21-We8tern Tenge 4-Sokko 10-Western Zhetybay 16-A8ar 22-Tenge 5-Shalum 11-Zhetybay 17-Zhalganoy 23-Chukuroy 6-Shalabay 12-Southern Zhetybay 18-Turkmenoy 24-Burmasha V -1400- Structural contours (m), below m.s.l. Kilometers 10 5 0 5 10 15 20 J I I l_ Faults T 5 10 15 Miles Anticlines (Scale is approximate) Figure 5. Contour map of the Zhetybay step. See Figure 3 for location. Contours on the Oxfordian limestone (reflector III). (After Yuferov et al., 1977.) beds in this unit contain relatively small hydrocarbon The overlying Cretaceous section is predominantly pools in a few fields. Upper Triassic elastics and marine clastic rocks (terrestrial in the Barremian), volcanics are present south of the step, in the South with carbonate intervals in the Valanginian- Mangyshlak trough. Hauterivian and the upper part of the Upper The main productive Lower Jurassic through lower Cretaceous. A dozen reservoir strata are identified Callovian section of the Zhetybay step is up to 1000 in this section. Most of them contain gas pools in m (3300 ft) thick. The section consists of an irregular the Uzen field, but are essentially nonproductive in alternation of sandstones, siltstones, and shales. other fields of the step. Rocks of terrestrial origin (mainly alluvial facies) predominate in the section, but marine interbeds occur in its upper part. The section includes 13 sandstone packages that contain oil and gas pools TRAP in different fields (Figure 7). The upper Callovian through Kimmeridgian The Uzen field is controlled by a typical structural interval consists chiefly of shales with beds of marls trap. The closure of the trap on the top of stratum and limestones. These rocks form the main regional XIII is 290 m (951 ft); it increases downward in the seal for hydrocarbon fields in underlying elastics. The section. Upper Jurassic shales and carbonates form thickness varies from over 300 m (1000 ft) on the the seal. Oil in strata XIII-XVIII fill the trap to the southern part of the step to less than 100 m (330 spillpoint. ft) in the Uzen field. Probably because of inadequate The main oil reserves of the Uzen field are found thickness, the Upper Jurassic seal in the Uzen field in strata XIII-XVIII occurring in the Middle Jurassic- leaks gas into Cretaceous reservoirs. lower Callovian section. Pools in these strata have 287 Fault — '1200 —• Contours on top of stratum XIII, contour interv • Wells Figure 6. Contour map of the Uzen field. Contours on top of stratum XIII. (After Makhambetov, 1968.) a common oil-water contact at a depth varying in from leakage of gas from Jurassic rocks. The leakage different parts of the field from 1124 to 1150 m (3688 was related to the significant thinning of the Upper to 3773 ft) below sea level (Figures 8 and 9). Jurassic seal over the Uzen fold, compared with other Accordingly, the areas of the pools decrease in structures of the Zhetybay step. Gas in these strata successively deeper reservoirs. Collectively, all of the consists of 70 to 98% methane, 1.8 to 7.6% nitrogen, pools actually form a single pool with an oil column 0.12 to 2.0% carbon dioxide, and 0.024 to 0.016% 311 m (1020 ft) high. The common oil-water contact helium. indicates a hydrodynamic connection between the strata. The pools in strata XVI and XVII contain small gas caps, but the oil in these pools is undersaturated by gas. This implies the absence of gas/oil equili RESERVOIRS brium in the field and suggests the recent redistri bution of oil and gas. Twenty-five reservoir strata are identified in the Initial reservoir pressure in strata XIII-XVIII Uzen field, and 18 of them contain hydrocarbon pools. ranged from 9610 to 12,061 kPa (1394 to 1749 psi). The upper 12 reservoir strata occur in the Cretaceous Original saturation pressure is estimated at 7355 to section; they contain relatively small gas pools only 10,885 kPa (1067-1579 psi). Initial solution gas-oil on the Main cupola of the field (Figure 9). The other ratio was from 291 to 347 standard cubic feet/stock 13 strata occur in the Jurassic part of the section tank barrel. The oil formation volume factor is and are mainly oil productive. The reservoir rocks estimated at about 1.2 reservoir barrel/stock tank are primarily polymictic sandstones and siltstones barrel. with a high content of clayey material. Thicker shale Oil and gas pools in the underlying part of the beds separate reservoir strata from each other; Jurassic section are controlled by local highs (cupolas) thinner shales are present within the pays. within the larger anticline, and presumably by small Strata I-XII (Figure 7) occur at depths ranging from faults (Figure 8). The pools do not contain significant 190 to 900 m (620 to 2950 ft). They consist of friable oil reserves. Oils of these pools are lighter (40.5° API) sandstones and siltstones with porosities of 26 to and contain smaller amounts of resins and paraffin. 34% and permeabilities of 200 to 600 md. The The oil flow rates from Triassic rocks are considered individual pays range in thickness from 10 to 50 m noncommercial. Gas pools in the Jurassic section are (33 to 160 ft). Each pay includes from one (stratum small and could scarcely be of economic interest in V) to four or five (strata III and VIII) sandstone beds. this remote region. Stratum XIII in the lower Callovian section and Main gas pools in the Cretaceous section are found stratum XIV in the upper Bathonian contain more in strata VIII, X, XI, and XII on the Main cupola than 60% of the oil reserves of the field. The average of the field (Figure 9). These pools probably result thickness of stratum XIII is 35 m (115 ft), and the 288 RESERVOIR STRATA calcareous cement. The clay content may reach 40% Regional Uzen and more. Reservoir rocks are characterized by Stratlgraphlc Column Nomenclature Nomenclature sharply varying permeabilities that range from a few Danlan millidarcys to 1000 to 1200 md. Permeability strongly Maeatrlchtian depends on the amount of clayey cement, but does CampaniBn u) not correlate with porosity. The latter is essentially s Santonian more uniform for different sandstones; porosity can 3 Coniacian Turonlan M-l 1 vary from 18 to 23%, but commonly, it is 21 to 22%. Cenomanian M- II II Porosity is intergranular; fracturing is minor or s M-lll III absent. Deposition of all Lower to Middle Jurassic ou M-IV IV ace M-V V rocks occurred in various facies zones of alluvial Cret Alblan M-VI VI plains, such as in braided rivers, shallow lakes, and M-VII VII swamps, under generally humid climatic conditions wer M-VIII VIII (Aktanova, 1968). o M-IX IX L M-X X The underlying strata (XIX-XXV) contain rela Aptian tively small reserves. Very few data on their reservoir Barremlan M-XI XI properties have been published. Generally, they are Hauterlvian rather similar to rocks of the main pays, but Valanglnian M-X II XII reportedly are even more variable in reservoir er m) f| properties (Aleksin et al.,1969). UppMal f Yu-I XIII ( Bathonjan Yu-lll XV Vu-IV XVI er) Yu-V XVII SOURCE g Yu-VI XVIII g Jurassic Middle (Do Bajocian VVYuuu--V-VIIXIIII XXXIXXX I MSanoguyrcseh larko ckssu bfboars inh ydhraovcea rnboont sb eienn thpoes iStiovuetlhy Yu-X XXII identified by geochemical methods. The most Vu- XI XXIII Aaienian probable source seems to be Lower to Middle Jurassic Yu-XII XXIV c) terrestrial elastics with dominant type III and ssi subordinate type II kerogens. The content of organic a (Li Yu-XIII XXV carbon in these rocks generally varies from 1 to 1.5%. wer Most of the section is located within the oil window o L on the Zhetybay step, but the lower part of the source- I rock interval is supermature in the South Mangy Not Identified shlak trough (Geodekyan et al., 1978). The high content of paraffin in all Mangyshlak oils suggests Figure 7. Stratigraphic column and reservoir strata of a terrestrial source. the South Mangyshlak subbasin. (The Callovian is An opposing point of view is that Triassic marine regarded to be Upper Jurassic in the U.S.S.R.) shales were the source for both Triassic and Jurassic hydrocarbons (Timurziyev, 1986). On the basis of geochemical data, Triassic and Jurassic oils have average net pay is 11.6 m (38 ft). Stratum XIII is common characteristics as well as differences. These characterized by rapid variability in reservoir differences can be attributed either to different properties (Figure 10). From one to twelve sandstone sources for the oils (Kordus et al., 1973) or to the layers constitute the overall pay zone in different higher maturity of Triassic rocks (Timurziyev, 1986). wells. Many individual sandstones rapidly pinch out Modern oil-source rock correlation techniques are laterally. The thickest sandstones have the best needed to solve this problem. permeability; they form elongated bodies (Figure 11) Oil produced from the Uzen field is of the paraffinic that are interpreted to be river channel deposits type (Tissot and Welte, 1984). Gravity of degassed (Kalugin et al., 1975; Yuferov et al., 1977). oil is 36.5 to 33° API; under reservoir conditions, it Strata XIV-XVIII have characteristics in common is 54 to 50.5° API. The oil has high resin (9.7 to 21.1%) with stratum XIII. All sandstones are very nonper- and paraffin (up to 28%) contents and a low sulfur sistent laterally. Channel sandstones have been content (0.1 to 0.24%). Viscosity of the oil in the positively identified in stratum XIV, but probably reservoir is 3.4 to 4.2 cp. Degassed oil congeals at are also present in the other pays. Thicknesses of a temperature of 25 to 30°C (77 to 86°F). Saturation the strata vary from 40 to 66 m (130 to 215 ft) and of connate water in productive reservoir rocks varies net pays range from 17.8 to 31.5 m (58 to 103 ft). from 30 to 38% of pore space. The average geothermal All of the reservoir rocks of strata XIII-XVIII are gradient in the Uzen field is 2.1°F/100 ft (38°C/km). poorly to moderately sorted; they are fine- to medium- In the top pay (stratum XIII), the temperature ranges grained sandstones and siltstones with clayey and from 53.3 to 68°C (128 to 154°F). 289 Parsymuryn cupola Khumuryn Cupola APPROX. SCALE Figure 8. Cross section along the long axis of the Uzen field. (After Yuferov et al., 1974.) ssw NNE + 200 Oil Pools ft i ITT> Gas Pools APPROX. SCALE Figure 9. Tranverse cross section through the Main cupola of the Uzen field. (After Makhambetov, 1968.) EXPLORATION AND gas reserves of the South Mangyshlak subbasin. DEVELOPMENT CONCEPTS Probably, the geologic conditions that resulted in formation of these fields have not been well Discovery of the Uzen and Zhetybay fields understood. Almost all presently discovered oil and demonstrates that the largest fields in a frontier gas fields occur on the Zhetybay structural step. region that are connected with simple structural Numerous attempts to explore for oil and gas in the traps are found very early in the exploration process. South Mangyshlak trough and its southern flank These two giant fields contain the bulk of oil and have failed to result in a single discovery, despite 290

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