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  Geology of Iran
Geology of Caspian sea
Author : Seyed Mohammad Mousavi Rohbakhshhsh
Publisher date : August 2001
Chapter title : Fifth chapter - An abstract from Pliocene- quaternary in Caspian Sea basin stratigraphy
Data title : 1-5- Pliocene

Some deposits which have been described in this section of the writing are rocks and Pliocene – quaternary formations in border of Caspian Sea and most of reservoir rocks for oil have been formed in them. Age of these units is Pliocene and younger that. So because of their importance, some extensive studies have been done on them in Russia and they are explained in details here.

1-3-       Pliocene

Pliocene deposits are found in Gorgan, Mazandaran, Gillan and Moghan plains, composed of deposits in two Chelkanian of lower- middle Pliocene age and Agchagylian from upper Pliocene .

1-1-5- Chelekanian deposits

Because of importance in this epoch for oil and gas presence in republics by side of Caspian Sea and this probability that it may be as a row as bed rock and reservoir rock. So it is tried that lithologic and geologic characteristics of oil in sediments of this epoch in outcrops in outside of then our country and also drilled wells in above plains is better and more surveyed and discussed and concluded.

Name of this epoch has been derived from Chelkan in Turkmenistan and in western side of Caspian Sea center and it was studied and suggested by V. Ogner in 1938 .

1-1-1-5- Chelkan in external areas

A-   Chelkan in turkmanistan

In type section of this section and epoch in Chelkan bay, in western side of Caspian sea, in soil of republic of Turkmenistan, some thick deposits (approximately 4000 m) which have gas and in economical amount of oil which is composed of sandstone and red clay stone with continental – semi marine characteristics. They are located on upper Miocene sediments (Ponesion) and it is under Epoch of agchagylian and upper Pliocene deposits.

Big oil and gas fields of Fressman, Okarem, Chisel Arvat, Chikishlar, in land and big fields of Barinova and Darvin in continental plateau in this country of Caspian in Caspian Sea (15). Ostracod and foraminifera about special deposits of sediments in this epoch is in the region besides it has some types of Pulmonata, special for brackish environment which is surveyed in its place.

B-   Chelkan in republic of Azerbaijan

These deposits are due to abundant and economical production in light oil and gas in Apsheron Island and behind of Ghafghaz and especially land and Baku city continental plateau is famous in name of Bahreh Deh. In this region, it has a thickness of 5000 m and sedimentary layers of Bahreh Deh are in some names like Balakhani, Sourakhani, Sabunchi Kermaki and … are famous too and they are mainly from consequent and thick layers of sandy clay and sandstone with weak cement in brown and also grey color .

In oil trap of Nizhne – Kurinsk, located in Baku island, thickness of deposits in this epoch is up to 4000 m and it is located in nearest point to north Iran Astara (in republic of Azerbaijan waters), condancite – famous gas field , Chizil Aghaj- Lankaran which is located in explorative well in top of deposits in this epoch in depth of 1250 m is distinct and it is composed of a sequence from sandy clay and red sandstone with loose cement is from 9 sandy horizon in total thickness of 140m is composed and it has a production in economical amount.

Azerbaijan (before soviet union disruption) is about 50 years that middle Caspian Sea and it is about 2000 explorative wells, descriptive and diurnal developmental with approximate amount of 1.5 million barrels from crude oil was extracted from Chelkan reservoir rock and it has a contract with America – Europe oil companies in last contracts of this republic that it updates its production rate from Chelkan Epoch deposits as 2 million barrels in a day.

2-2-2-5- Chelkan in Iran

For first time in our country, Chelkan sequence is recognized in Gorgan plain explorative wells by B. Mostofi, V. peren and it was studied in 1958 .

In this area, deposits include a relatively high thickness of clay, marl, sandstone and brown to red conglomerate which are mainly with reworked sediments pertaining to Miocene and cretaceous and also some of fresh water ostracods which are transitionally located under sediments pertaining to upper Pliocene.

In well No. 3, in Gorgan, about 800m from Chelkan was drilled but it does not reach to its basement. This 800m has economical gas (9). In wells no 4*5 of Gorgan, with a characterized unconformity, these sediments are located in related sediments (fig 31) and they are mainly from eastern to west in Caspian Sea beaches and thickness of Chelkan deposits are increased and this thickness increase is from east to west of Caspian sea, in marine wells of Caspian Sea too as in marine well (1) in Caspian Sea (Shahrivar, 1370), top of deposits in this epoch is in depth of 2850 m but it is drilled in end part of Caspian Sea (1371) and top of these deposits are gained around 1850m.

According to Z. Faridi studies, in 1964, Chelkan deposits are equivalent to lower zone of ostracod with ostracods in fresh water like Ilyocypris and Stenocypris .

In Iran, these deposits are as famous continental series that some parts of that is equivalent to productive beds in republic of Azerbaijan and it seems that lowest part of these deposits have some layers from deposits related to Ponesian epoch (upper Miocene) which are located on Sarmatian. Fossils in this epoch are limited to fresh water mollusks except for several very rare ostracods such as Planorbis Thiolleri and Limnea Bouilleti (Vatan and yassini, 1969) and sometimes some of lithics in mammal bones (H. G. Stehlin, 1931) (Pelite – 3).

In out crops of Alamdeh section, they are mainly with Chelkan deposits with unconformity on yellow Sarmatian sediments  but usually these deposits are located with a characterized unconformity on cretaceous sediments

In marine well 2, in Caspian Sea, it is located in 15km from west of Hosseingholi bay and these deposits are composed of a sequence from clay and narrow layers of red sandstone they are without any type of fossil which is located with an unconformity on cretaceous deposits.

Chelkanian continental beds have isochronological accordance to Kual – Nejikian sediments from black sea basin in Georgia and or Balkhanskaya layers from Bahreh Deh beds in republic of Azerbaijan .

Important and basic difference between these two items that it has remained Balkhanskaya in lacustrine environment with a considerable depth from water because continental deposits have been emplaced in maximum regression of Caspian sea.

In wells of Gorgan region, usually Chelkan deposits are characterized by gap of Miocene sedimentation which is located with a characterized unconformity on cretaceous sediments (mainly lower ones) as it is with a thickness of 1200m and Gorgan wells in no 4 and 5 with less thickness on sediments related to cretaceous .

In east of Mazandaran (Mazandaran well. No. 3) is located in Farah Abad Pelage (former) with red deposits of Chelkan with little thickness (about 75m) on cretaceous sediments.

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