Ab

Figure 7-20. Relationship between the montmorillonite content and geothermal gradient (a) and pore-pressure gradient (b) (Modified after Buryakovsky et al., 1995).

Figure 7-20. Relationship between the montmorillonite content and geothermal gradient (a) and pore-pressure gradient (b) (Modified after Buryakovsky et al., 1995).

adjacent offshore areas, which have a higher geothermal gradient (24.0-28.5°C/km), are characterized by lower montmorillonite contents.

Low temperature apparently does not favor the transformation of montmorillonite to illite; this reduces the montmorillonite transformation rate. Under otherwise equal conditions, the transformation increases with depth, which means that some additional factors must be influencing the transformation. One of these factors, discussed by Serebryakov et al. (1995), is the lack of potassium ion in interstitial water.

Inasmuch as the transformation of montmorillonite into illite proceeds with the removal of interstitial water, conditions at which desorbed water leaves the pore space without hindrance will be favorable for the development of this process. Factors opposing the withdrawal of fluids from the interlayer space of clays, therefore, may lead to slowing down or cessation of transformation of montmorillonite into illite or chlorite. The writers believe that such a factor is the abnormally high pore pressure, which occurs virtually throughout the section of the area under study. The hydrostatic pressure gradients in the pores of shales at 1,000-6,000 m are based on more than 2,000 determinations and range from 0.012 to 0.024 MPa/m, with a mean of 0.018 MPa/m (see Figure 7-16 and Table 7-14).

The dependence of the montmorillonite content on the pore pressure gradient in shales is shown in Figure 7-20b. There is a close correlation between these two parameters. In regions of the Baku Archipelago and Lower Kura Depression, characterized by intense development of AHFP (pore pressure gradients in shales of 0.018-0.019 MPa/m), the montmorillonite content in shales reaches an average of 53%. In regions with moderate development of AHFP (Apsheron Archipelago and the South Apsheron Offshore Zone), the montmorillonite content decreases to 17%.

These authors found no adequate discussion in relevant literature on the role of pore pressure in shales on clay-mineral diagenesis and catagenesis. It can be shown theoretically that rising pressures reduce the dehydration rates. The production of illite in shales involves an increase in the free water volume as a result of the release of bound water, which is denser than free water. A factor opposing this increase in volume (such as high pore pressure in shales) will reduce the dehydration rate. This agrees well with the conditions which exist in the above-described sections.

On the other hand, AHFP can lead to transformation of illite to secondary montmorillonite by the absorption of water. At AHFP, smaller grain size of the clay minerals favors transformation of illite, as shown by the relationship between the pore size and depth (Table 7-10 and Figure 7-21; pore sizes were determined from SEM data).

The writers propose the following scheme for the relationship between clay-mineral transformation and the thermobaric conditions:

In a basin where the subsidence rate is equal to the rate of accumulation of sediments, the depth at which catagenetic transformation (desorption of water) begins remains more or less the same and is largely determined by the geothermal gradient. Inasmuch as the desorbed (interlayer) water is added to the interstitial water, abnormally high pore pressures may develop if the water cannot escape. Under some conditions, the rising pore pressure in shales may reduce the mont-morillonite dehydration rate and release of water. The result will be similar to that arising from a low geothermal gradient, i.e., reduction in the rate of illite formation. Under favorable conditions, the illite may be hydrated, which is accompanied by a release of heat and their transformation to secondary montmorillonite. The relative rates of dehydration (illite formation) and the illite hydration (formation of secondary montmorillonite) may determine the pore pressure.

The sedimentation rate and the sediment sources do not remain constant with time. Thus, different zones may differ in the dehydration rate because of changes in the sedimentation rate or type of sedimentary material. Transitions from a zone with normal pressures and normal dehydration rate to an AHFP zone may indicate either the effect of diagenetic and catagenetic processes or a lag in development of these processes. The montmorillonite content may remain the same or even increase with depth. This, however, does not mean that the process of dehydration of montmorillonite to illite is replaced by the illite hydration, although this is possible. Instead, it could mean that dehydration process in the AHFP zones is slow; therefore, these zones may be characterized by higher (or equal) montmorillonite contents than those in the younger zones with normal shale pore pressure.

Effect of Hydrochemical Environment

The hydrochemical environment in a basin of sedimentation has significant influence on the intensity of post-sedimentary transformations. Thus, it is important to ascertain the nature of the hydrochemical regime observed in the Cenozoic complex of the South Caspian Basin,

Figure 7-21. Pore size distribution of argillaceous rocks from the Productive Series of Baku Archipelago. m is relative frequency (Modified after Buryakovsky et al., 1986c). a—Duvanny Deniz, well 529, depth interval of 1,415-1,420 m/ 4,642-4,659 ft and 1,450-1,455 m/4,757-4,774 ft; b—same but depth intervals are 1,700-1,705 m/5,577-5,594 ft and 1,785-1,790 m/5,856-5,873 ft; c—same field but Well 275 and depth interval of 3,323-3,328 m/10,90210,919 ft; d—Sangachal, Well 534, depth interval of 4,295-4,303 m/14,09114,117 ft; e—Bulla Deniz, Well 537, depth interval of 4,993-5,000 m/16,38116,404 ft; f—same field but Well 15 and depth interval of 5,128-5,132 m/ 16,824-16,837 ft.

Figure 7-21. Pore size distribution of argillaceous rocks from the Productive Series of Baku Archipelago. m is relative frequency (Modified after Buryakovsky et al., 1986c). a—Duvanny Deniz, well 529, depth interval of 1,415-1,420 m/ 4,642-4,659 ft and 1,450-1,455 m/4,757-4,774 ft; b—same but depth intervals are 1,700-1,705 m/5,577-5,594 ft and 1,785-1,790 m/5,856-5,873 ft; c—same field but Well 275 and depth interval of 3,323-3,328 m/10,90210,919 ft; d—Sangachal, Well 534, depth interval of 4,295-4,303 m/14,09114,117 ft; e—Bulla Deniz, Well 537, depth interval of 4,993-5,000 m/16,38116,404 ft; f—same field but Well 15 and depth interval of 5,128-5,132 m/ 16,824-16,837 ft.

namely: whether it is a consequence of diagenetic and catagenetic processes in shales and the transformation of clay minerals, or it is formed predominantly as a result of the action of other factors (e.g., compaction; see Rieke and Chilingarian, 1974). In this connection, the problem presenting the greatest interest is the origin of inverted hydrochemical profile in the section of the South Caspian Basin, i.e., with depth, calcium chloride waters are replaced by less saline sodium bicarbonate waters. The writers obtained numerous data from the laboratory analyses and field observations, indicating a decrease in the mineralization of pore waters in sands with depth. Replacement of calcium chloride water by alkaline sodium bicarbonate water is more characteristic for the AHFP zones in the South Caspian Basin areas (Buryakovsky, 1974a). Analogous data on the decrease of formation water salinity with increasing pressure were also noted in the Gulf of Mexico (Fertl, 1976).

The appearance of hydrochemical inversion in the stratigraphic section of the South Caspian Basin may be explained by the genetic relationship between the hydrochemical regime and the development of abnormally-high pore pressures in shales. The water of primarily sodium bicarbonate type characterize the most pronounced AHFP zones in the Baku Archipelago and Lower Kura Depression.

Chemistry of pore waters are determined largely by the compaction processes in argillaceous rocks and squeezing out of pore water (Chilingarian et al., 1994). The hydrochemical environment influences the diagenetic and catagenetic transformation (of clay minerals) processes.

Figure 7-22 shows the dependence of montmorillonite content on the total salinity of formation water for the calcium chloride and sodium bicarbonate types of pore waters in sands, which are characteristic for the South Caspian Basin. As shown, a direct relationship exists for the sodium bicarbonate type of water, i.e., with increasing water salinity, the conditions for preservation of montmorillonite are improved and its content in the clays increases. Increase in the total salinity of water is caused by an increase in the content of carbonate and bicarbonate salts of alkali-earth metals. Sodium bicarbonate type waters are present in the Baku Archipelago and the Lower Kura Depression, as well as in the rocks from the Lower Productive Series of Apsheron Peninsula and adjacent offshore area, i.e., sections in which the argillaceous rocks are characterized by higher montmorillonite

Figure 7-22. Relationship between the montmorillonite content and formation water salinity (Modified after Buryakovsky et al., 1995). 1—Sodium bicarbonate water, 2—calcium chloride water.

content. There is an inverse relationship between the montmorillonite content and the presence of calcium chloride type of waters. The chloride content [in particular sylvite (KCl)] increases with increasing water salinity.

Thus, the alkaline medium is favorable for the formation and preservation of montmorillonite. This was also confirmed by the results of computer geochemical simulation (Buryakovsky et al., 1990c).

The RAMIN program was utilized, which is similar to the geo-chemical model proposed by Kharaka and Barnes (1973). The RAMIN program makes it possible to simulate the equilibrium distribution of the majority of elements present in the pore solutions at temperatures up to 350°C on the basis of data on the chemical composition of formation water, temperature, pH and Eh. For determination of the possibility of dissolution or precipitation of one or another mineral, calculation of AG values of the Gibbs free-energy difference is included in the program.

The results of the chemical analyses of formation water in Wells 96 and 521 of the Unit VII of the Sangachal-Duvanny Deniz-Khara Zyrya Field served as initial data for the computer-based simulation (Table 7-17). Average depth and formation temperature are as follows: Well 96: -3,091 m, +80°C; Well 521: -4,320 m, +97°C. The pH value used averaged 7.0-7.5.

Table 7-18 gives the results of determination of the Gibbs free-energy difference, AG, for various clay minerals. As shown, within the pH interval of 6 to 8, in most cases AG values for minerals of the montmorillonite and kaolinite groups exceed zero. This indicates a possibility that they are of authigenic origin. The values of AG for illite are always less than zero, which indicates the possibility of its precipitation from solution.

Thus, the geochemical environment at great depths in the South Caspian Basin deposits is not only conducive to the preservation of allothigenic montmorillonite, but possibly allows the transformation of illite into secondary montmorillonite.

Secondary Montmorillonite

According to the data cited above, a rather close relation exists between the various clay mineral contents and the thermobaric and

Table 7-17

Chemical Analyses of Formation Water from Wells 96 and 521, Unit VII,

Sangachal-Duvanny Deniz-Khara Zyrya Field

Concentration,

Concentration,

Components

mg/liter

Components

mg/liter

Cl-

709.0

Ca2+

8.02

SO42-

211.2

Mg2+

2.44

HCO3-

195.2

Na++K+

618.7

CO32-

36.0

Al3+

<0.1

RCOO-

17.7

S1O2

70.0

Gibbs' Free-Energy Difference (AG) for Various Clay Minerals at Different pH

Table 7-18

Gibbs' Free-Energy Difference (AG) for Various Clay Minerals at Different pH

Clay Mineral

AG @ pH

6

7

8

Well 96:

7.34

5.46

1.99

Ca-montmorillonite

6.97

5.08

1.60

K-montmorillonite

7.44

5.55

2.08

Na-montmorillonite

7.38

5.49

2.01

Kaolinite

5.91

3.84

0.48

Well 521:

6.14

3.20

-0.52

Ca-montmorillonite

5.61

2.67

-1.07

K-montmorillonite

5.94

3.00

-0.72

Na-montmorillonite

6.21

3.27

-0.47

Kaolinite

4.72

1.37

-1.84

hydrochemical characteristics of the section in the South Caspian Basin and onshore of Azerbaijan. The stability of montmorillonite at great depths depends on many parameters. In the section of the Baku Archipelago at depths greater than 4-5 km, formation of secondary montmorillonite from illite was observed using scanning electron microscope (SEM). This is explained by the relatively low temperatures, abnormally high pore pressures in shales, and the alkaline pore water enriched in Mg2+, Na+, and Ca2+ ions.

Particle (or aggregate) sizes of primary and secondary montmoril-lonite at great depths were established. This was achieved by quantitative analysis of SEM data. The writers used photomicrographs of surfaces cut parallel to bedding of a shale sample from a depth interval of 5,128-5,132 m in the Bulla Deniz gas-condensate oilfield. Differences between the primary and secondary montmorillonites were established clearly using magnifications of x1,000 and x3,000.

Statistical analysis of the data on particle sizes of the primary and secondary montmorillonites is presented in Table 7-19. As shown, the particle sizes for primary montmorillonite are within the 0.5-11.2 ^m limits, whereas the size of secondary montmorillonite ranges from 0.6 to 6.5 ^m. The average particle sizes for the primary and secondary

Table 7-19

Statistical Analysis of Particle Sizes of Primary and Secondary Montmorillonites at Two Different Magnifications (x1,000 and x3,000)

Particle Size, pm __Asymmetry

Standard of Particle-

Deviation, Variance, Size

Magnification Range Median Average pm % Distribution

Primary Montmorillonite x1,000 0.9-11.2 1.9 2.6 3.3 127 0.58

Secondary Montmorillonite x1,000 0.9-6.5 2.0 2.7 2.9 107 0.70

montmorillonites are very close. Some differences were observed using photomicrographs with magnifications of x1,000 and x3,000, because, using a magnification of x3,000, it is possible to observe a greater number of smaller particles. This, naturally, yields a somewhat smaller average value of the clay particle sizes (Figure 7-23).

As shown in Table 7-19 and Figure 7-23, the distributions of particle sizes are right-asymmetric, close to log-normal law. In all cases, the average values of the particle sizes exceed the median by 0.3-0.7 ^m. It is significant that sizes of the montmorillonite particles are close to those of the pores, as established by SEM data.

Utilizing the data obtained, the writers estimated the primary and secondary montmorillonite contents (Table 7-20). On the average, the total montmorillonite content (percent of the total area of photomicrographs) reached 19.2% (13.2% for primary and 6% for secondary montmorillonites). The secondary montmorillonite fraction constituted 31.5% (average) of the total montmorillonite mass. This indicates a rather high rate of secondary montmorillonite formation from illite at great depths.

The post-sedimentary (diagenetic and catagenetic) transformation of Middle Pliocene shales of the South Caspian Basin is characterized

Figure 7-23. Histograms of the distribution of particle sizes for primary (a, c) and secondary (b, d) montmorillonite (Bulla Deniz Field, depth interval of 5,128-5,132 m/16,824-16,837 ft); (a, b) x1000; (c, d) x3,000; a is relative frequency (Modified after Buryakovsky et al., 1995).

Figure 7-23. Histograms of the distribution of particle sizes for primary (a, c) and secondary (b, d) montmorillonite (Bulla Deniz Field, depth interval of 5,128-5,132 m/16,824-16,837 ft); (a, b) x1000; (c, d) x3,000; a is relative frequency (Modified after Buryakovsky et al., 1995).

by the retardation of transformation of montmorillonite into illite or chlorite at great depths, and the replacement of this process by the process of transformation of illite into highly-swelling minerals of the montmorillonite group. These processes are closely related to the low geothermal gradient and increasing pressure at depth. The inverted hydrochemical profile of these deposits is possibly a consequence of the relationship between the transformation of clay minerals and thermobaric conditions at depth. On the basis of compaction experiments, Rieke and Chilingarian (1974) suggested that compaction fluids become saltier as they move upwards.

Table 7-20

Portions of Primary and Secondary Montmorillonites at Two Different Magnifications (x1,000 and x3,000)

Content of Montmorillonite, %

Table 7-20

Portions of Primary and Secondary Montmorillonites at Two Different Magnifications (x1,000 and x3,000)

Content of Montmorillonite, %

Photomicrograph Number

Magnification

Primary

Content,

Montmorillonite,

%

1

x3,000

14.5

7.1

21.6

32.9

2

x1,000

13.5

6.5

20.0

32.5

4

x3,000

10.5

5.4

15.9

34.0

5

x1,000

14.3

5.2

19.5

26.7

0 0

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