AZERBAIJAN - 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 is 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.