AZERBAIJAN - Geology:.

A landlocked country, Azerbaijan has an area of 86,600 sq km and diverse resources. It is the biggest and easternmost country in Transcaucasia, which occupies the southern part of the isthmus between the Black and Caspian seas. The country has three main petroleum provinces:

The onshore Lower Kura, Shamakhy-Gobustan and Absheron areas, and the offshore archipelagoes of Baku and Absheron. Their thick sandstone sequences of the Middle Pliocene - known as Productive Series (PS) - are the main reservoirs. They have produced since the late 19th century.

The Middle Kura depression, between the Greater and Lesser Caucasus mountain ranges. Their main oil reservoirs are in the Middle Pliocene PS. There are poor reservoir properties in some of the Upper Cretaceous volcanogenic rocks and the Paleogene, Miocene and Upper Pliocene formations. There are fairly large oil and gas reservoirs in the Yevlakh-Aghjabedi trough, with oil pools having been developed in a wide stratigraphic range - such as the Akchagylian, Sarmatian, Chokrakian, Maikopian, Eocenian and Upper Cretaceous formations.

The Pre-Caspian Guba region in the north of Azerbaijan. Hydrocarbons in this province are associated with Jurassic, Cretaceous, Paleogene, Miocene and Pliocene sediments.

The Middle Jurassic's thickness in Azerbaijan is up to 4,300 metres. Deposits from the Mid-Jurassic are distributed widely in the watershed section of the Major Caucasian Ridge and in its frame. These are represented by black slates interbedded with sandstones and aleurolites. Deposits occur within the Tengi-Beshbarmag anticlinorium and the superimposed trough of Gusar-Devechi.

There are deposits from Lusitanian, Kimmeridgian and Tithonian stages in the watershed section in the lateral ridge of Tengi, composed of carbonaceous and sandy-argillaceous rocks and are represented by two lithofacies. Callovian and Oxfordian deposits of the south-eastern Caucasus seem to be missing.

Upper Jurassic carbonaceous lithofacies, mainly of the Tithonian stage, predominate in the Shahdag region. They have been traced from the massifs of Shahdag and Gyzulgaya up to the Tengi gorge. They consist of dense and highly fractured limestones and dolomites which are zoogenic and pink. There are abundant sandy-argillaceous lithofacies in the central and south-eastern Caucasus and within the lateral ridge. They consist of clayey slates, alternated with aleurolites, sandstones, marls and partially limestones. The lithofacies subside towards the Shamakhy-Gobustan region.

There are no Upper Jurassic deposits in the south-eastern tip of the Tengi-Beshbarmag anticlinorium and the Shahdag-Khyzy synclinorium. Lower Cretaceous deposits are transgressively overlain by those of the Upper Jurassic. Upper Jurassic's thickness in these areas is up to 1,500 metres. Cretaceous deposits are spread widely in the Greater Caucasus mountains.

Jurassic sediments appear to be post-mature for oil and gas generation. But generation may have been late enough to enable migration and accumulation into younger, shallower reservoirs. Hydrocarbons remaining in situ may have been displaced by gas generated at high temperature.

The Cretaeous: In the south-eastern subsidence of the Greater Caucasus, the Lower Cretaceous (LC) is most common. After numerous surveys and structural drilling, the superimposed trough of Gusar-Devechi and the Tengi-Beshbarmag anticlinorium were found in Northern Gobustan. There, LC deposits consist of alternating sandy, aleurolite, carbonaceous and argillaceous rocks. There are Albian limestones in the upper interval. Separate blocks of LC sandstones, clays and limestones were found among the solid outbursts from mud volcanoes in Gobustan's central and southern sections. It confirmed their distribution in the depression of the south-eastern Caucasus. The thickness of the LC deposits is up to 3,300 metres.

LC deposits are common in the Shamakhy-Gobustan and Pre-Caspian-Guba regions and in parts of the Shahdag-Khyzy area, represented by flysch-type alternation of sandstones, aleurolites, limestones and clays. The number and thickness of carbonaceous and sandy beds are markedly increased in the Upper Cretaceous section in the south and south-east and in the Gusar-Devechi trough.

Solid outbursts from mud volcanoes in the Southern Gobustan and the Lower Kura Basin have large blocks of Upper Cretaceous (UC) limestones and sandstones. The thickness of UC deposits is up to 1,800 metres in the Shamakhy-Gobustan area and up to 600 metres in the Pre-Caspian-Guba zone.

Generally the Cretaceous is characterised by poor source rocks. A very rich Late Cretaceous oil source facies was obtained from a flow of debris. The local authorities say the facies was a potentially significant source, but sediments are unlikely to be a major source of hydrocarbons.

The Eocene: There are Paleogenic deposits in a big part of Azerbaijan, including the south-eastern Caucasus where deposits in Koun rocks are divided into Lower, Middle and Upper Eocene. The thickness of the Eocene deposits in the Shamakhy-Gobustan and the Pre-Caspian-Guba regions is up to 1,500 metres, with a marked rise towards the south-east. In the Absheron peninsula, it is over 1,700 metres. In terms of both TOC content and generative potential, the Eocene is the poorest complex in the sedimentary pile.

The Lower Eocene (LE) consists of light-gray/white marls and clays with inter-layers of sandstones, siltstones and bentonites. The Middle Eocene (ME) is built up from dark-brown clays, bituminiferous marls and slates with sandstone inter-layers. The Upper Eocene (UE) consists of greenish marlaceous clays with silicon sandstone inter-layers, foraminifers, shales of ostracordes and the teeth of fish. In the Koun series, the claystones of the brown (LE) and green (ME) depositions are characterised by inertinitic and woody organic matter.

These Upper Eocene (UE), ME and LE depositions are poor source rocks for oil, as in the case of the ME black claystones representing the dark bituminous lithology recorded in that region. The hydrocarbons accumulated in the ME volcano-clastic formations may have been generated by underlying LE argillaceous beds.

The Oligocene & Lower Miocene: In the early stages of the Oligocene, there was a turning point in the geotectonic development of the Caucasus. Many local troughs ceased to exist in the Greater and Lesser Caucasus. The elevation within their boundaries intensified sharply.

Conversely, in the Kura depression there was a sharp strengthening of submersions. Some interior elevations have disappeared.

The explored parts of the Shamakhy-Gobustan Basin and the nearby Absheron peninsula in the Lower Oligocene were surrounded by relatively low land. The south-eastern end of Caucasus island, bordering from the north, had low relief and was marked only by a low elevation. As a result, clays were deposited in the basin and inter-layers of sands were rare and of limited thickness.

The depth of the sea at that period was about 200 metres. Some extension of the area occurred from the beginning of the Middle Oligocene. This and the northern wing of the Shamakhy-Gobustan trough were occupied by sea.

The Oligocene/Lower Miocene, or Maykop series, is found in the south-eastern Caucasus and in the interfluve of the Kura and Iori rivers. It is lithologically represented by brownish, brownish-gray clays with jarosite tarnish and inter-layers of gray, fine-grained sandstones, and by marlaceous and sideritic concretions.

Most Maykopian outcrops are tied with phenomena of diapirism, which have caused the deposits to be in the surface or down along the kernels of numerous diapiric folds.

In Southern Gobustan and the Pre-Caspian region, the commercial flows of oil and gas of the Umbaki and Siazan monoclines are related to the Maykop series, with thickness of up to 1,800 metres. The thickness of the Maykop in the Shamakhy-Gobustan trough is less than that in the basic part of the Kura depression, and its axial zone is slightly thicker than 1,500 metres.

The Maykop series, by lithological composition, consists of dark-gray and chocolate-laminated shaly clays with numerous imprints of fish, scales of melletas and residues of stems of fossilised Cedroxylon trees. Numerous amphisyles were found in inter-layers of fish clays, exposed in ravines on the right bank of the Sumgait-chay river. The flatness of stratification marks out the high content of jarozite. There are hardly any sand strata. The Maykop deposits' cover is well separated by dark clays with numerous imprints of spririalis. Peculiar in some sections is the richness of fauna of vertebrates, with large fish and cateceans, and a skeleton of Archalocety was found there.

There are good source rocks in the Early, Middle and Upper Maykop. They are best developed and most abundant in the Upper Maykop, mainly in the east towards the Caspian Sea where they are the richest and most oil-prone. Offshore, these sediments are likely to be richer and more uniform. Onshore, there are big variations both vertically and laterally, recording short- and long-term differences in the environment of deposition. The Upper and Middle Maykop have the same geochemical composition.

The Middle-Upper Miocene: The Middle Miocene, of Helvetian and Tortonian stages, can be traced in the Gobustan and Pre-Caspian-Guba regions and in the interfluve of the Kura and Iori rivers, where the Tortonian state is spread and is found in the Absheron region. Tortonian deposits are clays with inter-layers of marls, quartz sands and sandstones. The sands and sandstones in the Pre-Caspian-Guba and Gobustan regions and, to some extent, on the Absheron peninsula, are abundant in gas and oil. The thickness of these deposits is up to 8,000 metres.

The Upper Miocene of the Sarmatian and Meotis stages is widely developed in the south-eastern subsidence of the Greater Caucasus and in the inter-fluve of Kura and Iori. The deposits are clays, brecciated dolomites, organogenous limetones, with inter-lays of conglomerates. There are Sarmatian limestones among the solid outbursts from the Akhtarma-Pashaly mud volcano. The Sarmatian sandstones and sands in the interfluve of Kura and Iori are stained with oil.

There are weakly petroliferous Sarmatian sands and sandstones in the Pre-Caspian-Guba, Absheron and Shamakhy-Gobustan regions. The thickness of the Upper Miocene deposits ranges from 1,200 to 2,000 metres in the south-eastern Caucasus and from 2,200 to 2,300 metres in the interfluve of Kura and Iori.

There are Diatom sections consisting mainly of Upper Miocene sediments and containing an amount of Middle Miocene sediments. They are common in the west of the Absheron peninsula and in Gobustan. Their thickest part is 220 metres and is in the south-west of Gobustan.

The shallow Karagan basin was separated from the Mediterranean Sea. During the Kounian age, but for a short period, it communicated with the ocean. As a result, its salinity increased and the oceanic forms of duraminifers crossed into the Karagan basin. During the latter part of the Sarmatian period, the basin became shallow and fresh-water fauna appeared. A considerable regression occurred at the end of the Sarmatian period and throughout the Meotisian period. The sea was only preserved in the Shamakhy-Gobustan trough, where maximum depth was 500 metres.

Most of the claystones from the Diatom series are poor source facies for gas. But in the same section, there are several intervals of good and very good source rocks for oil, with each interval being a few metres thick. On biomarkers, the Diatom series cannot be distinguished from the Upper Maykop, but its carbon isotope composition is distinctively different.

Middle Pliocene - Productive Series: The Middle Pliocene, or PS, is well spread by alternation of sandy-argillaceous rocks in the oil and gas bearing regions of Shamakhy-Gobustan, Pre-Caspian-Guba, Absheron and Pre-Kura, as well as in the Caspian Sea. The conglomerates and coarse-grained sandstones predominate in the section of the Pre-Caspian-Guba region and in the interfluve of Kura and Iori. The most abundant oil and gas pools of Eastern Azerbaijan are related with the Middle Pliocene sandy-silty deposits. Thickness is up to 3,500 metres.

The PS is said to be a poor quality source rock with kerogen types II/III, consisting of inertinite, algal, herbaceous and woody material. The TOC content in the PS is quite low, indicating that these sediments are generally immature. The PS's organic matter was accumulated in deltaic and near-shore/marine environments. The oil accumulations in the PS may be partly syngenetic, with the bulk of the oil generated in the Diatomaceous series and then migrated upwards.