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Behrooz Esrafili-Dizaji (ESRAFILY@Khayam.ut.ac.ir)
Farkhondeh Kiani Harchegani (bitaa1979@yahoo.com)

Introduction
The Asmari succession is the youngest and most prolific reservoir horizon in Iran. This succession produces more than 80­% of total Iranian crude oil (85% in some estimation and references). In addition, supergiant and giant oil and gas resource are stored in the formation equivalents in the Kirkuk province of Iraq (for example Bai Hassan field). Also, the Asmari Formation hosts associated gas (gas cap or solution gas) and gas not associated with oil pools. The Asmari reservoirs have gas caps with associated oil pools in more than 22 fields of the Dezful Embayment area. In this area, large amounts of gas from these fields is injected for effective production from adjacent older oil fields. The main sandstone reservoir layers in Iranian fields are present in the Asmari Formation (with “sand production problems”).

The Asmari is well-known as typical fractured carbonates reservoirs. Exploration and production from this most famous oil-producing formation goes back to the last century. In 26 May 1908, D’Arcy and his associates struck oil from the Asmari Formation in southwest Iran (Masjid Sulaiman-1 well in Fig. 1). It was the first commercial oil discovery in the Middle East. So, some of the Asmari fields have been in production for over a century.

Fig. 1) First hydrocarbon well in Middle East (Well No. 1 of Masjid Sulaiman) after one century; in 1908. (upper photo from Anglo-Persian Oil Company) and 2011 (lower photo from Mehrnews agency).

Geology
The Oligo-Miocene Asmari Formation is mainly carbonate (limestone and dolomite) but it also has a mixed siliciclastic/carbonate (Ahwaz member; fluvio-deltaic siliciclastic deposition in Khuzestan) portion, and evaporitic successions in Lurestan. The Ahwaz Member and its equivalent (Ghar Sandstone) are major oil plays in the west Persian Gulf sub-basin (such as the Forouzan, Soroush, Abuzar, Nowruz, and Ahwaz Fields). Many of giant and supergiant fields in Iran have multiple reservoir horizons, commonly involving the Asmari carbonate (such as Agha Jari, Gachsaran, Ahwaz, Mansuri, Lali, Karanj, Masjide- Suleiman, Marun, Par-e-siah, Dehluran, Cheshmeh Khosh, Hendijan, Bibi-Hakimeh and Paydar Fields).

The Asmari Formation is very interesting for following characteristics and reasons (Fig. 2):
- The presence of extensive fracturing system, with other pore types (high poroperm values, 12 % average porosity).
- Shallow depth of burial for low-priced drilling (ranges from 300 to 3300 meters).
- High reservoir pressure (from the gas-cap).
- High quality crude oil with low sulfur content (mean 1.6 %) and light gravity oil (mean API >30°).
- Numerous and giant anticlinal structures that act as traps (with economic closure).
- The presence of oil seeps, good indicators for exploration and survey.
- The presence of thick and efficient seal rock (Gachsaran evaporite).
- The occurrence of extensive organic-rich source rocks.
- Good position in stratigraphic framework for lateral migration and trapping of hydrocarbon.

Fig. 2) Fracturing and presence of oil seep are two main characteristics of the Asmari outcrops in Zagros mountains. Fractured system in Kavar, Iran (Left; photo from M. Mirkazemian) and oil seeps (right; photo from M.T. Barin) in Asmari outcrops.

Reservoirs
Some 59 hydrocarbon fields (51 oil and 8 gas reservoirs) are explored from the succession in the Zagros and Persian Gulf Basins (Fig. 3). A majority of the Asmari oil reservoirs are clustered in Dezful Embayment and western Fars and Persian Gulf provinces. Some Asmari gas fields are located in Bandar Abbas Hinterland (Sothern-Gashu, Western Salt, Gavarzin gas fields) and in Persian Gulf (Bahregansar, Thundar, Farur A) area. The Milatun and Kuh-e-Asmari are two other Asmari gas fields in the Dezful Embayment. Except of Gavarzin Field, other gas fields are not developed yet. The dominant trend of the fields is parallel to NW-SE Zagros structural elements. Apparently, the absense of Asmari reservoirs north of Balarud and Mountain Front Faults in the Lurestan and Izeh provinces may explain the lack of Asmari Field in these areas.

Fig. 3) A map showing distribution of Asmari oil and gas reservoirs in the Zagros and Persian Gulf Basins. In general, most of oil reservoirs are clustered in Dezful Embayment and western Fars and Persian Gulf provinces. Three Asmari gas reservoirs are situated in Bandar Abbas Hinterland.

One-third of the total proven Asmari reservoirs (7 gas and 10 oil fields in offshore and onshore) are not developed (Table.1). The Naft Shahr, Dehloran and West Paydar Fields are three common Asmari fields that are shared between Iran and Iraq. The Aghajri, Pazanan, Bibi Hakime, Parsi, Labe Sefid, Rage Sefid, Nafte Sefid are examples of Asmari associated gas reservoirs.

(note: Table 1 may be arranged or inserted as appropriate for the layout)
Table.1) Asmari reservoirs data.

Although the Asmari Formation contains very important reservoir intervals, there remain many questions in geological age, depositional environment history (particularly source of the Ahwaz sands), regional controls on reservoir properties, environmental problems involving oil seepage and unconsolidated sand production, for continuing investigations.


From: AAPG-Middle East-Newsletter-Autumn 2011