INTERNATIONAL FEATURE

Continued h r n September, 1993 Bulletin

Offshore Oil and Gas Fields In Azerbaijan: H i m and Description, Part I1

by Prof. h n i d A. Buryakovsky Indepdmt Petroleum Geologist Houston, Texas

ZONE 11: !30UTH APSHERON OFF- SHORE AREA

Anticlinal belts of the Apsheron Peninsula stretch southward. The main portion of these anticlinal belts is located onshore and the oldest major oil and gas fields (interns of development history) are confined to these belts. Offshore structures dewloped on the shelf are the southern c o n t i n d o n of these anticlinal belts forming the South Apsheron offshore zone. These s t ruc tu res a r e : Peschany-more, Bakhar and Shakhovomore, lying on the continua- tion of the Central Apsheron anticlinori- um. Individual structures are generally steepsided, have high relief, and are of diapiric nature with mud piercement to the surfice. Oil and gas seeps and mud volcano eruptions have been known from antiquity.

OIL AND GASCONDENSATE FIEU) l.luHAu

Locatior and history. The Bakhar structure is located in the southern off- shore continuation of the FatmaiBakhar anticlinal belt, which crosses the central part of the Apsheron Peninsula, and was discovered in 195657 by seismic and elechic marine survey. In 1955, the first deep well was located on the southeast- e m edge of the Banka Makarova mud volcano crater. This well did not encounter oil- and gas-bearing forma- tions and was abandoned (well depth was 3384 m). Exploration drilling started once again in 1961. Commercial hydru- carbon reserves were discovered in' 1967 by well #3 at the crestal area of the struc- ture. Gas flow has been obtained from depths of 3897-5907 rn with a daily gas production rate of 300,000 m3 and con-

densate of 40 tons; daily gas production rates of 300,000 m3 and condensate of 100 tons were obtained from depths of 3782-3764 m. Wellhead pressure was up to 26 MPa.

Explorato~y drilling of the Bakhar Keld was conducted in three stages. The first stage consisted of initial completions in the base horizon and recompletion in the upper horizons. The second stage consisted of well drilling and testing NKP and PK suitea The third stage consisted of drilling the KaS suite. of January, 1984, there were 121 completed wells including 83 exploratory wells and 38 producing wells. Well inventory according to exploration stage and productive for- mation is shown in Table 5.

Productive intervals within the Productive Unit extend from the top of Balakhanskaya Suite through the PK Suite. The thickness here of this interval is about 2000 m and the following pro- ductive horizons are identified here: V, VI, VII, VIII, M, X (all in the Balakhanskaya Suite), the "Pereryvn Suite, the NKP Suite, and the PK Suite. These productive horizons are separated by shale beds and differ from one another by reservoir properties and petrophysical data.

Geology. Geophysical study and drilling of

deep exploratory wells demonstrated that at its northern margin, the Bakhar structure is separated from the Peschany more structure by a distinct saddle. The southern margin, according to the seis- mic survey, is separated from Shakhovo- more structure by an elongated gentle saddle.

What is known about the geological structure of the Bakhar Field is based on data from more than 120 exploratory and development wells. The structure is

Deep thinking. Top results.

2950 North Loop West, Suite 300 Houston, Texas 77092 (7 13) 688-628 1

Productive

v VI VII wI IX x-upper x-lower " ~ " NKP PK KaS

completed

7 1 71 68 68 66 58 50 45 27 22

Table 5: Formations drilled during the three exploration - stages, Bakhar Field.

Fig. 14. Geologic cross section of the Bakhar Field: 1 - sand; 2 - shale; 3 - alternating silt and sand; 4 - alternating sandstone and shale; 5 - gas accumulation;

Fig. 13. Structural map of the Bakhar oil and gas-condensate Field: 1 - major faults; 2 - crowsection direction; 3 - isolines on the top of IX horizon; 4 - wells; 5 - mud volcanic breccia; 6 - breccia outcrops on sea bottom

6 - oil accumulation

a brachyanticline striking about north- south, with a length of 9.5 km and width of 3.5 km in the limits of contour line of 4500 m on the top of IX horizon The height of the fold is up to 400-450 m. The fold is slightly asymmetrical, with dip angles of the west limb at 1618', and 1 4 20". The fold is faulted into eight blocks (Figs. 13 and 14).

The west limb is complicated by a bent longitudinal fault that intersects the entire fold and is tmced through all p r e ductive horizons. Amplitude of the Eault is about 250 m at the southern pericline near the mud volcano, and gradually decreases to 100 m in the central part of west limb and to 50 m in the northwest. In addition to the longitudinal Eault, four transverse faults are identified on the west limb. These faults separate the indi- vidual tectonic blocks [IV to VIII. Apparent offbet of beds between most of the adjacent tectonic blocks.] are up to 250-300 m but apparent slip between blocks W and V is only 25m.

The main portion of the structure includes the central part, east limb and pericliial areas and is divided into three tectonic blocks (I, 11, 111) by two trans- verse dislocations. The transverse fault that separates the northern pericline (block I) from the central part of the fold (block 11) is distinctly traced. Offset along this fault ranges from 100 m in the central part of the structure to 25 m . The southern transverse fault separates blocks II and III with offbets of no more than 20-25 m.

The transverse fractures between blocks I, I1 and III are characterized by small offsets, which is why these fractures

had trapping quality only before the beginning of development. The differ- ent location of original gas and oil pool outlines may be used as evidence of this statement. These fractures became per- meable during gas and oil withdrawal.

The Southern end of the structure is complicated by the Banka Makarova mud volcano (Fig.13). The edge of the crater, composed of mud volcanic breccia, can be easily identified on the sea floor.

The gas, oil and gascondensate reser- voirs of the Bakhar Field are character- ized by their occurrence at the great depths (3650-5350 m), the presence of many tectonic dislocations, and by their heterogeneity. These factors caused the complex geologic and thermobaric con- ditions of these oil and gas accumula- tions. The characteristics of these accu- mulations are given in Table 6.

Reld development. Regularities of oil and gas distribution within the strati- graphic section testifies that the multiple pay zones are united by a single hydrody- namic system. Gas accumulations are characterized by significant condensate content which increases from 150 to 210 g/m3 with depth increasing (Table 6). One of the characteristic features of ga.+ condensate accumulations in the Bakhar Field is the presence of residual oil satu- ration in the gas zones of productive for- mations. This particular feature is of great importance to reserve estimation and field development.

Sandy-silty reservoir rocks are of good quality. Average porosity varies between 14 and 22%; average permeability varies between 12 and 166 mD, and water satu- ration ranges from 8 to 56%.

Reservoir Average reservoir

depth, m

Initial feservoir pressure,

MPa

38 39 41 43 45 46 47 48 5 1

Table 6: Selected reservoir characteristics. Bakhar Field

The pilot field development of gas- condensate pools with oil rims (X Horizon) was carried out without reser- voir pressure maintenance. The signifi- cant gas withdrawal and decrease of ini- tial reservoir pressure caused noticeable retrograde condensation. This phenom- enon was more visible in the wells locat- ed in the gascondensate zone of the X horizon. Average initial output of stable condensate in the wells 7. 10 and 26 was 205 g/m3, and gas-condensate ratio was 4880 m3/t. During the pilot develop ment of the X horizon, a reservoir p r e sure decrease of more than 6 MPa changed the above-mentioned parame- ters up to 175 g/m3 and 5676 m3/t, respectively. The noticeable decrease of stable condensate yield was caused by ret- rograde losses G c i a t e d with liberation of light hydrocarbons from reservoir gas system, their solution in liquid phase, and precipitation on the surfaces of pore spaces.

During further development of the X horizon, the producing wells were drilled on the GOC. These wells can simultane- ously drain both the gascondensate zone and the oil rim. During this phase of development, water flooding for reser- voir pressure maintenance was not car- ried out The advantage of this method of development is connected with reten- tion of GOC immobility and with preven- tion of oil movement in the gas zone and losses of liquid hydrocarbons. For recov- ery of remaining oil reserves the addi- tional producers should be drilled and water flooding can be initiated.

~ e v e l o ~ m e n t of gas/gascondensate reservoir without oil rims (V, VI, VII, VIII, and IX horizons) is carried out with depletion drive. Condensate recovery factor is assumed to be 50%. The rate of gas withdrawal from each reservoir is

Type of accumulation

Gas-condensate -"- -"- -"- -"-

Gasandensate, oil Oil, gas-condensate

Oil Gasandensate

proportional to gas in-place reserves. Many non-productive wells drilled on the lower horizons can be recompleted in the upper horizons and used for gas prcl duction.

ZONE III: BAKU ARCHIPELAGO The Baku Archipelago zone is situat-

ed on the marine continuation of the Lower Kura and Dzheirankechmes troughs (Figs 2 and 3). where six narrow anticlinal belts have been identified. They are complicated by longitudinal (axial) and transverse fractures, and large mud volcanoes, many of which are active. The Sangachaly-more, Duvanny- more, Bulla Island oil fields, and Bulla- more gas/condensate field are located within this zone.

SANGACHALY-MORE, DUVANNY- MORE AND BULLA ISLAND

Location and histoy. The Sangachaly. more. Duvanny-more, and Bulla Island oil fields are located on the southeastern submerged flank of the Utalgi- Kyanizadag anticlinal belt The structure was discovered in May, 1963 when an oil influx of 250 tons per day was obtained from the VII Horizon in well 24.

Geological study of the Baku ~ r c h i ~ e l a g o area was initiated in the thir- ties when the area was covered by a g e e physical survey. A seismic survqr was car- ried out in the following exploration stages: the first stage focused on the VII Horizon (suite 'Pereryv" according to the stratigraphic column of the Apsheron area), the second stage focused on the VIII Horizon (NKP Suite in the Apsheron area section), and the third stage on the PK Suite. In crestal areas of structures, where drilling depths were relatively shallow (about 2600 m),

Condensate conteat in

reservoir gas, g/m3

100 150 160 180 190 200 210

the second and the third stages were combined into one stage. On the limbs, where drilling depths to the top of the VIII ~ o r i z o n was up to 5800 m, the PK Suite was designated as the third exploration stage.

Geology. The structure containing the Sangachaly-more, Duvanny-more, and Bulla Island oil fields includes three indi- vidual uplifts (folds) which a;e separated from one another by

narrow and shallow saddles. Total length of the structure is 23 km. The structure is slightly asymmetrical with a steep northeastern limb (dip angles are about 28") and relatively gently sloping south- western limb (dip angles are about 15"). The structure is complicated by two axial (longitudinal) disjunctive dislocations and &era1 transvek faults. These disle cations divide the whole structure into nine tectonic blocks (Fig. 15). The cre- stal area, located between two longitudi- nal dislocations, is raised above-both limbs and has a horst character. The amplitude of fault NO. 1 is 200-500 m, and that of fault No. 2,80-90 m.

The Sangachaly-more uplift is located to the south from Cape Sangachal. The uplift is 8 km long and 7.5 km wide. Two transverse faults (Nos. 3 and 4) are found at the northeastern limb. Fault throws near the longitudinal dislocation are about 50 m and decrease to 30 m toward the limbs. The Duvanny-more fold is located to the southeast from Sangachaly-more uplift. The length of the fold in 10 km, and the width is 7.5 krn. Two transverse faults (Nos. 5 and 6) are found at the northeastern limb. The throw of fault No. 5 near the longitudi- nal dislocation is about 50 m and decreases to the edge of structure. Fault No. 6 is one of the largest transverse faults and its throw is 170 m. This fault separates the Duvanny-more and Bulla Island folds.

Commercial oil saturation is found in the V Horizon (VIII Horizon in the Apsheron area) and in the VII Horizon ('Pereryv" Suite in the Apsheron area). Commercial oil and gas-condensate accu- mulations are found in the VIII Horizon (NKP Suite in Apsheron area). All accu- mulations are located on the northeast- ern limb. Accumulations in the VIII

pressure decrease did not stop complete- ly.

Pilot reservoir development of the VII

Fig. 15. Structural map of the Sangachaly-more, Duvanny-more, and Bulla Island Fields: 1 - faults; 2 - crosssection direction; 3 - contours on the top of VII horizon; 4 - initial owc

Horizon are slightly shifted toward the limb relative to the VII Horizon. That is why wells drilled in 'the crestal area p r e duced commercial quantities of oil from the VII Horizon and only water from the VIII Horizon.

Oil saturation of the MI Horizon with- in the Sangachaly-more uplift is encoun- tered at depths of 3600.5240 m; within the limits of Duvanny-more fold, at depths of 2200-4200 m, and within the limits of Bulla Island area, at depths of 3700-4500 m.

Resewoir rocks include sandstones and siltstones, which are characterized by lat- eral and vertical heterogeneity. Porosity varies from 17.7% to 23.6% and perme abilihr from 5 to 540 md. Reservoir rock properties improve outward from the c re stal area toward the limbs and from north- west to southeast. The bottom member of the V I I Horizon has the best reservoir rock properties.

The primary oil-bearing reservoir is the VII Horizon. Crude oil in the V I I Horizon is of paraffin-naphthene base, wax-bearing, and nonsulfurous. Oil den- sity at the surface is 0.865 g/cm3 and crude dynamic viscosity is in the reservoir is 1-2 cp. A gas cap is found only in the crest of the Sangachaly-more Uplift. Dissolved gas is dry, with methane content up to 95%.

Initial reservoir pressure reduced to OWC level exceeds hydrostatic pressure by 7 MPa. The bubble-point pressure is

below the initial reservoir pressure by 5 7 MPa and rises with increasing depth. Initially, the reservoir had an elastic water drive. Because of pressure drop in the zones of extensive oil withdrawal and wells produced with very high drawdowns (up to 20 rnpa), a dissolved gas drive was induced in some areas of reservoir. Average initial GOR in accordance with analyses of downhole samples and surface measurements during the initial phase of development varies from 10f2-125 ms/t.

Field deveIopment. Pilot reservoir development of the VII Horizon in the Sangachaly-more area was started in July 1965. During the initial period of devel- opment the oil withdrawal was very high, the pressure decrease was rather signifi- cant (O.Bl.0 MPa per month), and the reservoir pressure fell

Horizon at the ~uvanny-more area (tec- tonic blocks 11,111, and N) was started in May 1963. Oil production rates per well were up to 350 t/d under significantly lower drawdowns (M mpa) compared to those in the Sangachaly-more area. Nevertheless, reservoir pressure decrease, daily oil production decrease, and GOR increase took place during reservoir development without stimulation of the productive formation. Water flooding was initiated in July 1971 for reservoir pressure maintenance. The results of water injection were successful, especially in the producing wells located in vicinity of injection wells.

The efficiency of reservoir stimulation in the Duvanny-more area was higher than that at the Sangachaly-more area due to better reservoir rock properties in the former. The results of water-injection included a GOR decrease and oil produc- tion rate increase (Table 7). During a two-year period the reservoir pressure increased from 18 to 22 mpa, and some wells began flowing in response to reser- voir Dressure maintenance.

Another productive formation is the VIII Horizon. This gascondensate accu- mulation was discovered in 1968 after completion of well 58. Gas and conden- sate influx was obtained from depths of 45694589 m. Daily gas inflow was 500,000 rn.' under wellhead pressure of 28 mpa. Subsequent wells produced gas and con- densate from depths of 3762-5040 m under wellhead pressures of 10-20 MPa. Gas flow rates varied between 300,000 and 600,000 myd and the condensate produc- tion rate varied from 50 to 150 t/d.

At present, the gas-condensate reser- voir is developed with nine producing wells, which is adequate for reservoir depletion due to th; absence of an oil rim.

below bub61epoint pres- ) sure. Production rates per well decreased by three times (from 150 to 45 t/d), and gasoil ratios in some wells located in the crestal area increased from 120 to 1000 m y t due to expansion of the gas cap. Repeating-pat- tern water flooding for reservoir pressure main- tenance was initiated in August, 1971. Its imple mentation was efficient although the reservoir

Midland Valley Associates Inc. Announces toll free sales and

technical sup rt F In USA 48 states 1 8 482-2001

Balancing A~IC -3 - D

Paleotectonic Restoration Software - Consultin with BSP

or Tectonics CA & Tectonics CAM

Put geology back into seismic Interpretation

Bulletin Houston Geological Society. Nowember 1993

Table 7: Response of Duvanny-more wells to water injection.

No.

115 117 119 227 .

References Alikhanov, E. N., 1977. Neftegazonosnost' Kaspiyskogo

morya [Oil and gas content in the Caspian Sea area], Moscow. Nedra.

Alizade, A. A.. Salayev, S. G., and Aliyev, A. I., 1985. Nauchnaya ochenka perspektiv neftegazonosnosti Azerbaijana i Yuzhnogo Kaspiya i napravlenie poisko\,o-razvedochnykh rabot [Theoretical estima- tion of oil and gas content in Azerbaijan and the South Caspian Basin and the trend of exploratory works], Baku. Elm.

Bagir-zadc. F. M.. and Buryakovsky, L. A., 1974. Osnovy morskoi neftegazovoi geologii [Principles of marine petroleum geology], Baku, Elm.

Bagir-zade, F. M., Buryakovsky, L. A,, and Babayev, F. R., 1974. Ckochimiya neftei i gazov Apsheronsko-Pribalk- hanskogo poroga [Geochemistry of oil and gas content with the Apslieronsko.Prebalkhansky Threshold], Mosmr\.. Xedm.

Buryakovsky, L. A,, 1974. Distribution patterns of oil and gas deposits within the Apsheron Archipelago, Inter- national Geology Review, v. 16, No. 7, AGI, Washing- ton, D.C.. USA, pp. 749-758.

Buryakovsky, L. A., Dzhafarov, I. %,and Kerimov. V. Yu., 1991. Poiski i razvedka morskikh mestorozhdeniy hefti i gaza [Prospecting and exploration of the marine oil and gas fields], Moscow, Nedra.

Buryakovsky, L. A., and Dzhevanshir, R. D., 1992. Mathe- matical models of interconnections between composi- tion and properties of oils in the Apsheron oil- and gas-bearing region of Azerbaijan, Energy Sources, v. 14, pp. 51-71.

11 I1 III N

Buryakovsky, L. A., 1993. Outline of general and petro- leum geology in Azerbaijan and the South Caspian Basin. Houston Geological Society Bulletin, vol. 35, No. 6, February, pp, 16-23; 43-47.

Mekhtiev, Sh. F., and Pashaly, N. V., 1987. Lithology of Middle Pliocene deposits of the Southern Caspian and their correlation, Lithology and Mineral Resources, v. 22, No. 2, Plenum Publishing Corporation, LISA, pp. 56-72.

Samedov. F. I., Buryakovsky. L. A,, and Dzhalilov. D. G., 1960. Gryazevaya Sopka -new oil field in the Caspian Sea, Petroleum Geology, v. 4, No. 3-B, McLean, VA, USA. pp. 173-178.

Samedov, F. I., and Buryakovsky, L. A,, 1961. Gas possi- bilities of the productive unit of the southeast part of the Apsheron Archipelago, Petroleum Geology, v. 5, No. 4, McLean, VA, USA, pp. 186-189.

Samedov, F. 1.. and Buryakovsky, L. A,, 1966. Neftyanaya gidrogeologiya Apsheronskogo Arkhipelaga [Oil field hydrogeology within the Apsheron archipelaga], Baku, Azerneshr.

Yusufzade, Kh. B., 1079. Razrabotka i razvedka morskikh neftyanykh i gazovykh mestorozhdeniy [Development and exploration of offshore and gas fields], Baku. Azerneshr.

Well pamneters before water flooding implementation

Well parameters after water flooding implementation

choke diameter,

mm

16 28 25 29

choke diameter,

mm

15 14 15 14

oil prod. rate, t/d 63 90 60

110

wellhead pressure,

MPa

27 38 52 92

GOR, m31t

623 489

2071 1582

wellhead pressure,

MPa

30 17 17 16

oil prod. rate, tld 160 204 161 312

GOR, m31t

200 194 360 158