Conclusions

(Chapters 1 to 7)

The distribution patterns of oil and gas presence in the onshore and offshore areas, and the principal reservoir and sealing properties of the oil- and gas-bearing formations in the South Caspian Basin can be summarized as follows:

1. The main oil- and gas-bearing formation in the Eastern Azerbaijan and the offshore areas of Apsheron and Baku archipelagoes is the Middle Pliocene Productive Series. In the Western Turkmenistan and the adjacent shelf areas this formation is named as Red-Bed Series. The section is composed of sands, sandstones, siltstones, loams, shales and unsorted rocks. The Productive/Red-Bed Series is separated into two divisions, lower and upper, and into several suites according to lithology, i.e., prevalence of sandstones or shales.

2. Core data, paleontological studies and log response suggest that sediments of the Productive Series were deposited in a shallow-water, fluvial-deltaic environment. The large volume of clastics present in the Productive Series indicate a proximal source of sediments. The Russian Platform and islands, which existed north of the Apsheron Peninsula and Apsheron Archipelago, as well as the southeastern slope of the Greater and Lesser Caucasus, served as the primary sources for clastic material for the Apsheron Peninsula and adjacent areas of the Caspian Sea. The clastics were deposited by the Paleo-Volga, Paleo-Ural, Paleo-Kura, and other rivers. The Productive Series is divided into seven sedimentary sequences according to the transgression/ regression cycles during the basin development.

3. Mineral composition of the Middle Pliocene section is characterized by several associations of both light and heavy minerals. Light minerals include quartz, feldspar, glauconite, and fragments of various rocks. To the south, quartz content decreases, whereas the content of rock fragments increases due to remoteness from quartz-feldspar sources within the Russian Platform to the north. Rock fragments here owe their origin to the sedimentary and volcanic rocks of the Greater and Lesser Caucasus. Heavy minerals include five groups: ore minerals, stable minerals, kyanite or disthene group, micas, and glauconite. A classification of clastic rocks is presented here based on the petro-graphic attributes and reservoir-rock properties.

4. Reservoir-rock properties of the Productive Series, which are very good, vary both within the section and over the area. In the Apsheron Peninsula and Apsheron Archipelago, porosity varies from 15 to 30%, with permeability ranging from 10 to 1,000 mD. The cement is usually clayey-carbonate with a predominance of clay minerals. Several statistical correlations were obtained, including relationships among porosity, permeability and depth of occurrence. These correlations can be used for estimation of reservoir-rock properties in undrilled areas and at great depths.

5. Paleogene to Neogene argillaceous rocks (shales, mudstones, etc.), which are widespread in the geologic section of the Azerbaijan and the South Caspian Basin, make up 50 to 95% of the section and play a key role in establishing lithologic, mineralogic, geochemical, and thermobaric characteristics of the basin. Most clay minerals in the Productive Series belong to the smectite (montmorillonite), mixed-layered and illite groups. Presence of smectite results in low permeability of argillaceous rocks and ensures their good sealing properties (caprocks overlying reservoir rocks).

Onshore, the Oligocene to Miocene argillaceous rocks (shales, mudstones, etc.) are higher in volcanic ash content, owing to their proximity to the Lesser Caucasus, than the Pliocene argillaceous rocks of the South Caspian Basin. The most characteristic feature of the Tertiary argillaceous rocks in Azerbaijan and the South Caspian Basin is their undercompaction and the presence of numerous pores of various sizes (measured by SEM). Their effective porosity (as used in USA) ranges from 3 to 20%.

6. The montmorillonite content of the Baku Archipelago shales is constant down to depths of around 6.5 km, because the formation of secondary montmorillonite from illite predominates over the transformation of primary montmorillonite to illite. Abnormally-high pore pressures in shales hinder the dehydration of montmorillonite and favor the transformation of illite to secondary montmorillonite, which produces heat. The produced heat, in turn, causes an increase in temperature, which results in the transformation of montmorillonite to illite. All these transformations are characteristic of young basins with rapidly accumulated thick argillaceous sediments.

7. Undercompaction of argillaceous rocks, even at depths down to 6.5 km, is explained by the comparatively young age, a high sedimentation rate (up to 1 km per million years), their great thickness, and incomplete squeezing-out of pore water. As a result, such argillaceous rocks have abnormally high pore pressures, often higher by a factor of 1.5 (and more) than hydrostatic pressure. The sealing properties of the udercompacted shales in the South Caspian Basin are determined mainly by their AHFP and still continuing squeezing-out of pore water. The sealing properties of argillaceous rocks (caprocks) are very effective and are determined by the progressively rising capillary pressures as the pore channel (throat) diameters decrease.

8. The sealing properties of argillaceous rocks (caprocks) at depths greater than 6.5 km still persist, because of the presence of large amounts of montmorillonite clay. If accompanied by (1) good reservoir rocks, (2) abnormally-high pore pressures in shales and sandstones, and (3) relatively low formation temperatures (which allow hydrocarbons to persist), the South Caspian Basin may contain commercial oil and gas accumulations at depths of 9 km and deeper.

9. The oil and gas potential increases considerably in a southeasterly direction. The principal types of traps are structural (anticlines and faults).

A decrease in density and increase in gas saturation of crude oils occur in the direction of regional sinking of the anticlinal trends from northwest to southeast. In the reservoirs, the crude oil density increases downward along the dip toward the oil-water contact. In the exposed accumulations, on the other hand, density increases upward along the bedding. With time, there is a trend of a certain increase in oil density upon the field development.

10. The formation waters of the Productive Series (South Caspian Basin formation waters) are characterized by a regional, very slow movement of the waters from the more depressed portions of the basin toward the higher parts having discharge zones.

The Lower Productive Series is characterized by the low-salinity formation water of the sodium bicarbonate type, whereas the Upper Productive Series includes more saline water, as the primary alkalinity is replaced by a secondary salinity. The total water salinity decreases with depth and with increasing alkalinity of the water.

The effect of lithology is superimposed on the above trends of formation-water chemistry.

In the offshore area, water salinity increases from the more depressed to the more uplifted structures, and also with transition from the southwestern flanks of the local uplifts toward the northeastern flanks. This may indicate movement of the water from southwest to northeast and stagnation of water in the uplifted structures.

11. The above described patterns of changes in the properties of crude oils, gases, and formation waters vertically and laterally suggest the migration of hydrocarbons along the strata and their accumulation in the structural traps, with differential entrapment according to the physical and chemical properties of migrating fluids. Migration within the individual structures led to the distribution of crude oil properties in the vertical section.

12. Development of abnormally-high pore pressures may lead to the lateral variation of rock density and, under certain geologic conditions, to folding, clay diapirism, mud volcanism, and earthquakes.

CHAPTER 9

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