C

ni ni S

Figure 7-13. Contents of clay minerals in the Productive Series of Baku Archipelago, a—Montmorillonite, 2—illite, c— kaolinite, d—chlorite, and e—mixed-layer minerals.

Table 7-11

Clay-Mineral Composition of Argillaceous Rocks in the Azerbaijan and the South Caspian Fields (Average Values are Shown in the Denominator)

Clay-Mineral Composition, %

Field Montmorillonite Illite

Kaolinite

Chlorite

Mixed-layered

Bibieibat

tr.-30

40-65

10-30

tr.-5

tr.-5

17

53

26

3

1

Palchygh Pilpilasi

10-35

45-60

20

tr.-5

tr.

24

51

3.8

Gyuneshli

40

40

15-20

tr.

tr.-5

17.5

2.5

Bakhar

10-55

30-55

15-25

tr.-10

tr.-5

27.7

46.1

20.4

4.2

0.8

Duvanny-Khara Zyrya

5-60

5-60

tr.-20

tr.-15

tr.-5

41

39

13

6

1

Bulla Deniz

5-70

5-70

tr.-35

tr.-10

tr.-10

39

37

16

4

4

Alyat Deniz

45-50

25

15

5

5-10

47

8

Khamamdag Deniz

30-75

10-45

10-15

tr.-10

tr.

49

28

12.5

5

Kyurovdag & Karabagly

40-75

10-35

tr.-20

tr.-15

53

23

12

10

Series are characterized by the highest content of smectite (31 and 35%, respectively), whereas in the Balakhany Suite, smectite content

decreases to 21.1%. The NKG and KaS suites of the Lower Productive Series have the highest smectite content (30%).

Figure 7-14 shows the smectite and illite contents in rocks of the (1) Apsheron Archipelago (Oguz, Palchygh Pilpilasi, Dzhanub-2, and Gyuneshli offshore areas), (2) South Apsheron Offshore Zone (Bakhar Field), (3) Baku Archipelago (Sangachal—Duvanny Deniz—Khara

Figure 7-14. Montmorillonite (1) and illite (2) contents in argillaceous fraction (<0.01 mm) of Middle Pliocene clays. a—Apsheron Archipelago and South Apsheron Offshore Zone; and b—Baku Archipelago and Lower Kura Depression.

Zyrya, Bulla Deniz, Alyaty Deniz, Khamamdag Deniz, Garasu, Sangi-Mugan, and Aran Deniz offshore areas); and (4) Lower Kura Region (Kyurovdag and Karabagly onshore areas).

In the Baku Archipelago and Lower Kura Region, the smectite content (38%) is approximately equal to the illite content (39%) as shown in Table 7-11. In contrast to the region of the Apsheron Peninsula and adjacent offshore area, this section is, in general, characterized by a more stable mineral composition, preserving a significant amount of swelling clay minerals (smectites). This results in the low permeability of argillaceous rocks, and ensures good sealing properties of shales (caprocks) that overlie reservoir rocks.

Table 7-12

Clay-Mineral Content in the Productive Series, Apsheron Peninsula and Adjacent Offshore Areas (average values are shown in the denominator)

Suite

Smectite,

Illite,

Suite

Smectite,

Illite,

Upper Productive Series

Lower Productive Series

Surakhany

Sabunchi

Balakhany

10-45 31.0

25-45

35.0

10-40

21.1

35-60 46.3

35-40

37.5

40-55

50.6

0.67 NKG

0.93 KS

0.42 KaS

25-35 30.0

10-30 20.0

10-40 30.0

30-50 41.3

40-60 50.0

40-60 47.0

0.73

0.40

0.67

Oligocene to Miocene shales have been studied onshore (Muradkhanly Oil Field in the Middle Kura trough). The cores of Chokrak rocks, which were studied from a depth of 2,825-2,830 m, contain mont-morillonite and mixed-layered clays with chlorite, illite, and ash. Organic matter is represented by the skeletons of coccoliths (marine microorganisms). The Chokrak rocks are fairly loose and unconsolidated.

The Maikop rocks have been studied from cores taken from depths of 3,080-3,085 m and 3,287-3,292 m. These rocks of marine origin contain montmorillonite clay with some volcanic ash. The ash (glass) is often altered to montmorillonite. Montmorillonite, chlorite and mixed-layered clays are widespread. Broken grains of pyroxenes and amphiboles with a typical cleavage are locally present.

The distribution of clay minerals is due to (1) different sources of clastic material brought to the different parts of the sedimentary basin, (2) predominantly allothigenic origin of clay minerals, and (3) variations in the rate of sedimentation. The Russian Platform, the Kilyazi-Krasnovodsk Zone of Uplift, the islands existing north of the Apsheron Peninsula and Archipelago, and the southeastern slope of the Greater Caucasus Mountains served as the primary source of clastic material for the Apsheron Peninsula and the adjacent Caspian Sea area. The more ancient (Mesozoic—Paleogene) magmatic and sedimentary rocks of the Greater and Lesser Caucasus and Talysh mountains served as the primary source of sediments for the Lower Kura region and the Baku Archipelago.

Pores exceeding 5 ^m in size were studied in thin sections under the optical microscope, whereas those less than 5 ^m in size were studied with the scanning electron microscope (SEM) on freshly broken surfaces of rocks. Magnification of x100, x300, x1,000, x3,000, and x10,000 were used in the SEM. A total of 102 electron microscope photographs have been taken. The photographs showed the geometry of pore space and the shape of clay particles. They also showed that shale is incompletely consolidated and poorly sorted, with fairly high interaggregate and intergranular porosity.

Photomicrographs were taken in planes parallel, oblique, and perpendicular to the bedding. Groundmass of argillaceous rocks (and ash) is fairly homogeneous at magnifications of x100 and x300. Details of pore space geometry appear only at magnifications of x1,000 and x3,000. Individual grains, pores, and fractures are clearly visible. The pores and fractures are identified by the appearance of grain separation and by a considerably lower intensity of illumination, which allows measurement of pore sizes and fracture lengths and widths. Pore cross-sections were measured directly in the photographs taken at magnification of x3,000.

With increasing depth, the pore size of shales progressively decreases

(Figure 7-15) according to the following equation:

where dpMe is the median pore size, in ^m; and D is the depth, in km. Pressure and Temperature

During the initial period of penetration of the reservoir rocks, the formation fluids had high pressures and wells were flowing. The petro-physical and geological characteristics of the rocks (permeability, fracture width and density, porosity, clay content, etc.) exerted the main influence on the amount and rate of fluid flow. The initial reservoir pressure was always higher than the hydrostatic pressure (Figure 7-16).

Temperature measurements in deep wells in the areas of the South Caspian Basin and onshore of Azerbaijan show that the average geothermal gradient is approximately 16°C/km. The temperature at a depth of about 6 km does not exceed 110°C.

Depth, km

Figure 7-15. Relationship between the pore size in argillaceous rocks and depth in the Baku Archipelago area.

Table 7-13 shows the initial pressure and temperature in some offshore fields. Tables 7-14 and 7-15 show variation of pore pressure gradient and geothermal gradient with depth in some oil and gas fields of onshore Azerbaijan and the South Caspian Basin.

Numerous measurements of the initial formation pressure in reservoir rocks and wireline logging determination of pore pressure in argillaceous rocks reveal the pattern of the abnormally high formation pressure (AHFP) distribution throughout the section at the northwestern flank of the South Caspian Basin (Table 7-16). The average gradients of the initial formation pressure in the reservoir rocks, nres, and of the pore pressure in shales, nsh, are (in MPa/m): 0.0106 and 0.0120 for the Apsheron Archipelago; 0.0119 and 0.0145 for the South Apsheron Offshore Zone; and 0.0138 and 0.0182 for the Baku Archipelago and Lower Kura region. A significant difference between the initial formation pressures in reservoir rocks and pore pressures in shales (by a factor of over 1.5) exists in the Baku Archipelago, where the average thickness of shales, hsh, is particularly higher than in other regions of Azerbaijan. Generally, AHFP rises with the relative content of shales, xsh, throughout the section (Table 7-16) and within

Figure 7-16. Changes with depth in pore pressure in argillaceous rocks (solid circles) and of initial formation pressure in reservoir rocks (open circles) in the Baku Archipelago area (n is the pressure gradient, MPa/m). (Source: Buryakovsky et al., 1986c.)

the reservoir. The highest pore pressures in shales are associated with shale sequences in the Baku Archipelago and Lower Kura region with their extraordinarily high porosity, §sh, owing to rapid sedimentation and slow compaction.

0 0

Post a comment