Download Hydrocarbon Potential of the Shemshak Formation

Hydrocarbon Potential of the Shemshak Formation
Abstract
Very few, if any, outside the Zagros and the Kopet Dagh Basins are more favourable
hydrocarbon source rock potential than the Upper Triassic- Lower Jurassic Nayband–Shemshak
together with older Paleozoic source rocks. The Shotori, Jamal; and sandstones within the
source rocks are favourable reservoirs. Thick shale horizons and anhydrites in the younger
formations are favourable caprocks. It is postulated that the Tabas Basin and the Tabas Block
were oriented E–W during the Jurassic. However, the fact is that they possess E–W trend
currently. Since the Tabas area is situated below the curvature of the earth, with respect to the
surrounding mountain highlands, it is therefore, possible hydrocarbon potential for exploration.
The juxtaposition N–S trends of Kuh-e Shotori and Kamar (Kuh) Mahdi where easy fault imprints
to structural trend present less resistance by paralleling to N-S bedding planes with the E-W
Tabas Block trend and possibly to the Neogene covered subsurface Tabas Basin E–W trend.
However, faults are seldom observed to cut across structural trends except in cases of
basement fault reactivations. Therefore, the depressed, wide Tabas Basin Area which extends
from S of Kuh-e Derenjal to Chashmeh-e Razam (Gholgholab Fault) with about 100 km long and
circa 60 km wide possibly possesses E–W structural trend similar to the Parvadeh Anticline and
is favourable for hydrocarbon exploration. The Parvadeh Coal Mine possesses two types of coal:
Antracite Coal to the western part proximal to Kamar (Kuh) Mahdi and Coking Coal eastward.
The high temperature Antracite Coal is attributed to the granite piercement, and to the Hormuz
Salt intrusion to just below the surface with N–S trend in the Kamar (Kuh) Mehdi. The Coking
Coal is mainly from the Shemshak Formation and partly from the Nayband Formation. The coal
is very rich in gas content and analyses have shown the following constituents with percentages
by the Parvadeh Coal Mine affiliated to the Esfahan Steel Company:
C1 + C 2
70% to 80%
C3+ C4+C5+C6
30% to 20%
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In order to confirm the validity of the above gas constituents, new samples should be collected
and tested by NIOC Research Centre.
It is suggested that the downthrown N Flank of the Parvadeh Anticline is laterally feeding the
Jamal and the Shotori reservoirs southward into the Parvadeh Anticline. More important would
be the possible subsurface structures N of the Parvadeh Structure buried under the Neogene
sediments. Finally, the rich Organic Matter could be obtained as hydrocarbon extracts from the
Shemshak Source Rocks in the areas where no structural traps are present other than the Tabas
area.
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Overview
At the close of the Palaeozoic, large blocks from the Gondwana-land (named after a tribe in
central India) broke off and drifted northwards to close the Paleotethys and created the
Newtethys Ocean during the Triassic. The rifting stage opened the South Caspian Basin during
Late Triassic (Brunet et al)–Liassic (Furish et al). Compression, faulting and later extension
resulted in sedimentation of clastic and the Karaj Formation from the Cretaceous to Late
Eocene and lasted until the Early Miocene respectively between Iran and Turan. This was
followed by subduction and culminated in collision of Iran–Eurasia in the Cenozoic. Thereafter,
continent to continent sedimentation has continued to the present day.
The Upper Triassic Nayband marine to poludal occasional coal-bearing and the Lower Jurassic
deltaic to fluvial coal-bearing Shemshak Formation total to 4000 m. They are widely developed
in the Alborz Binalud Tabas Basin, Tabas Block, N of Kashan and around Esfahan. The
sedimentary sequences are the result of collision between the northern parts of the
Gondwana-land with the Turan Block at the end of the Triassic.
The East Central Iranian Ranges reach altitudes of 3000 m in Kuh-e Nayband and 2600 m in Kuhe Shotori. Whether the difference of 400 m is due to the down-throw of the Tabas Basin with
respect to the Tabas Block or otherwise, remains to be proved through geophysical
investigations.
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Previous Work
Geological reconnaissance of the Tabas Basin and Tabas Block was conducted by Stocklin,
Huber, Mohajer and under the supervision of Ganssar in the early 1950s. In 1961–1962 a
further reconnaissance by Stocklin and Moshtaghian from Parvadeh to Bahabad was carried out
on camelback. Stocklin resigned from NIOC and joined the Geological Survey of Iran (GSI) in
1964. Thereafter, both the mentioned institutions performed geological survey independent of
each other in the Tabas Area; Huber in NIOC and Stocklin in GSI. Later on, a detailed
stratigraphic Type Section of the Nayband Formation was measured and studied by Huber and
Dashti in 1965. This Nayband Section was further studied and reviewed and published in a
Journal by Bronnoman. Two papers—one by Dashti and another by Hasanzadeh—were
presented on the Shemshak Source Rock at the First International Conference on Petroleum
Geochemistry and Exploration in the Afro-Asian Region held in Dehra Dun, India in Nov. 1985.
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