Paleogene dinoflagellate cysts and thermal maturity from Pabdeh Formation

: Palynological investigation on 132 samples from the 428m thick outcrop section of Late Paleocene to Early Oligocene of the Pabdeh Formation in southwestern Iran yielded 55 species of dinoflagellate cysts and allowed establishment of seven biozones. Quantity of marine palynomorph elements indicate an open marine environment at that time interval but, a slight increase in number of spore and pollen grains in some samples indicate suitable conditions for forest development as a consequence of increase in climate humidity. The species are common in various latitudes and most of them are cosmopolitan. Thermal maturity index measurement indicates oil prone nature for majority of the samples. ©JASEM


Introduction
Fifty-five dinoflagellate cyst species were identified from Late Paleocene to Early Oligoce of Pabdeh Formation in west of Iran.The Formation is known as the source rock for the eminent reservoir, Asmari Formation, which is extended through Zagros basin from southwest to southeast of Iran.The Pabdeh Formation beginning from Paleogene, shows different age ranges up to Oligocene in different parts of Zagros basin.In Fars and Khuzestan provinces the Formation has been dated as Paleocene to Oligocene while it is dated as Paleocene to Early Miocene in Lurestan province.
There was a trough in Paleogene and Neogene covering today's highlands to the Arabian shield in which strata were countinuously deposited.This sea was separated by Fars platform in Southern Jahrom city.With some lithological differences such as the Rus anhydrite Formation in UAE, Qatar, Eastern Iraq, Kuwait and Hejaz, this trough extends to Arabia and Iraq.Equivallent strata to the Pabdeh Formation consists of Dammam Formation on top, Rus anhydrite in the middle and Ommolradhome Formation at the base in countries to the south of Persian Gulf.The present study is focused on marine palynomorphs and palynostratigraphy of a sections of the Pabdeh Formation in Zagros basin in southwest of Iran.

Previous Research:
No study has been done on dinoflagellate cysts from the Pabdeh Formation though vast and precise studies have been carried out on foraminifera contents of the formation by the National Iranian Oil Company (NIOC)which led to differentiation of 14 foraminifeal biozones dating in age from Paleocene to Late Oligocene (Motiei, 2003).A Recent investigation on calcareous nannofossils extracted from subsurface samples indicated a Danian to Chattian age (Gholami,1386).General researches in Zagros basin suggest a Paleocene to Miocene age for the Pabdeh Formation but, this age range varies in different areas (Aghanabati, 2004).This Formation is a known source rock for the Asmari Formation, a huge reservoir rock in the Middle East and also the olargest Carbonate reservoirs rocks discovered in the world (Ashkan, 1983).For this reason, most studies on Pabdeh Formation are associated to petroleum industries.

Geological Setting:
The area selected for the present study is located in Southwest Iran, Zagros basin, Kohgiliyeh va Boyeramad province.One of the three sections studied for this article is located between Gachsaran and Yasuj city in this province as shown in Figure1.

MATERIAL AND METHOD
A total of 132 samples were processed with mineral acids in several steps taken according to palynological standard methods (Traverse 1988).The processed amount depends on the lithology: for shale, mudstone and siltstone 20-25 grams; for limestone and calcareous shale30 -35grams, and for siltstone and mudstones 25-30 grams are common.For this study approximately 20-30 grams of rock samples were crushed and soaked in 30% Hydrochloric acid (HCl) for at least a day to remove calcareous cements ALI AKBAR, JAFAR NEZHAD 1 *, EBRAHIM, GHASEMI-NEJAD 2 ,TAYEBEH MOHTAT 3 , ALI AGHANABATI 4 and particles.The residue was then washed with distilled water for 10 times to neutralize the acid.Then it was placed in Hydrofloric acid (30%) for 48 hours to remove Silicate compounds.Then, they were boiled for 20 minutes with Hydrochloric acid (10%) to remove the gel formed.To remove palynomorphs from heavy minerals and other wastes based on their specific weight, heavy zinc chloride solution (Zncl 2 ) was used and the residue was centrifuged.When the samples were passed through the 20 micron sieve, palynological slides were mounted from the residues remaining on the sieve.To analyze the palynological elements, the prepared slides were investigated with a binocular transmitted light microscope equipped with objectives of 10X and 40X and the index dinocysts were photographed and presented (Plates 1 and 2).
Fig1.Location map of the studied section.
Biostratigraphy: Biostratigraphic data is used to answer many stratigraphic queries.Biostratigraphic investigation of the Tertiary outcrop, located in the main oil basin in Iran are mostly based on foraminifera and calcareous nanoplanktons.In this study, the authors try to focuse on dinoflagellate cysts to establish a biozonation.Microscope studies of the prepared slides let to identification of 55 species which are used for this purpose (Figure 2.).Dinoflagellate cysts are an important group of microfossils in petroleum exploration due to their potential to resolve questions on age dating and correlation.The remarkable morphological diversity displayed by dinoflagellate cysts facilitates their identification.Most of them are short-lived and, geographically widespread in marine sediments due to their planktonic nature.Several zonation schemes based on dinocysts have been proposed for Eocene age, but almost all of them have been established in high and middle latitudes, whereas data from tropical areas are scarce or even lacking.Zonal schemes established in the Northern Hemisphere (Northwestern Europe) can't be applied to the Middle East without modifications.This is probably due to the following reasons: Water temperature and oceanic circulation patterns seem to constitute the two principal factors in the determination of provincialism in dinoflagellate cysts (Ramirez 1990).Today, some species of dinoflagellate cysts are confined to the tropical/subtropical regions, while others are found exclusively in high latitudes.During the Middle to Late Early Eocene, the oceans and seas are characterized by a rather uniform warm global climate andwarm marine conditions are indicated by oxygen isotope ratios of planktonic and benthic foraminifera recovered worldwide from many locations.Within the Middle Eocene, the climate started to deteriorate, leading to a steeper temperature gradient from lower to higher latitudes.This climatic change was perhaps more pronounced in tropical regions (Sloan e.g.& Rea 1995) and lead to an increase in the differences between low latitude and middle to high-latitude assemblages.The taxa companions whose events concord to this biozone are Polysphaeridiunzohary, Selenopemphix armaeddonsis , Stoverocysta sp.,Homotrybliumpallidum.(Fig.3,4) Thermal Maturity: Pabdeh Formation is a source rock in Zagros basin and the abundance of palynological elements in this Formation provided a suitable condition to study thermal maturity.About 21 samples containing spores and pollen were acquired.Color range was from pale yellow to dark brown.Thus, the yellow color indicated the immature organic matter, and the brown color range indicated petroleum prone stages.Most of the samples are in brown color range.Therefore, the thermal maturity condition is in petroleum generation range.(  (Eaton, 1971) Stover and Williams, 1995, 3-Homotryblium pallidum Davey and Williams, 1966, 4-Lejeunecysta communis Biffi and Grignani, 1983

Table 1 )
Table1.Thermal maturity index, based upon spore and pollen colors that indicate oil prone for most of the Selenopephix spp., Deflandrea heterophlycta, Polysphaeridium zohary, Areoligera gippingensis.All these species, are index-fossils of Paleogene of Zagros basin, but, they are absent in some basins in the world.Conclusion: A new Paleocene -Oligocene palynostratigraphy has been established based on the study of 132 samples.Stratigraphical studies based on palynological events resulted in definition of seven interval zones.These palynological interval zones have been correlated with the foraminifera biozones of West Zagros basin in Iran.The dinoflagellate cyst correlation shows the Late Paleocene to Early Oligocene age that also occurs in the Darisk section of Pabdeh Formation.