CARBONATE RESERVOIR ROCKS AT GIANT OIL AND GAS FIELDS IN SW IRAN AND THE ADJACENT OFFSHORE: A REVIEW OF STRATIGRAPHIC OCCURRENCE AND PORO-PERM CHARACTERISTICS

B. Esrafili-Dizaji^1*, H. Rahimpour-Bonab¹

¹ School of Geology, University of Tehran, P.O. Box 14176- 14411, Tehran, Iran.

^* corresponding author, esrafilidizaji@ut.ac.ir

Key words: SW Iran, Zagros foldbelt, Persian Gulf, hydrocarbons, carbonate reservoirs, oil field, gas field, Dehram Group, Khami Group, Bangestan Group, Asmari Formation, grainstone, rudist facies.

SW Iran and the adjacent offshore are prolific petroleum-producing areas with very large proven oil and gas reserves and the potential for significant new discoveries. Most of the oil and gas so far discovered is present in carbonate reservoir rocks in the Dehram, Khami and Bangestan Groups and the Asmari Formation, with smaller volumes in the Dashtak, Neyriz, Najmeh, Gurpi, Pabdeh, Jahrum, Shahbazan, Razak and Mishan (Guri Member) Formations. The Permo-Triassic Dehram Group carbonates produce non-associated gas and condensate in Fars Province and the nearby offshore. The Jurassic – Lower Cretaceous Khami Group carbonates are an important producing reservoir at a number of offshore fields and in the southern Dezful Embayment, and are prospective for future exploration. Much of Iran’s crude oil is produced from the Oligo-Miocene Asmari Formation and the mid-Cretaceous Sarvak Formation of the Bangestan Group in the Dezful Embayment.

This review paper is based on data from 115 reservoir units at 60 oil- and gasfields in SW Iran and the adjacent offshore. It demonstrates that the main carbonate reservoir units vary from one-another significantly, depending on the particular sedimentary and diagenetic history. Ooidal-grainstones and rudist- and Lithocodium-bearing carbonate facies form the most important reservoir facies, and producing units are commonly dolomitised, karstified and fractured. In general, reservoir rocks in the study area can be classified into six major types: grainstones; reefal carbonates; karstified, dolomitised and fractured carbonates; and sandstones. The stratigraphic distribution of these reservoir rocks was principally controlled by the palaeoclimatic conditions existing at the time of deposition. A comparative reservoir analysis based on core data shows that dolomitised and/or fractured, grain-dominated carbonates in the Dehram Group, Lower Khami Group and Asmari Formation typically have better reservoir qualities than the Cretaceous limestones in the Upper Khami and Bangestan Groups.

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