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

17 Pages·1.205 MB·English

by  Gregory F.

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