The Albian Kazhdumi Formation of the Dezful Embayment, Iran: One of the Most Efficient Petroleum Generating Systems

Abstract

The Zagros orogenic belt of Iran is one of the most prolific petroleum provinces of the world. The Albian Kazhdumi source rock and two calcareous reservoirs of Early Miocene (Asmari) and the Cenomano-Turonian age (Bangestan) constitute by far the dominant petroleum system of the Dezful Embayment, being responsible for the accumulation of more than 7.3% of the world reserves in a surface area of only 40000 km2. The Kazhdumi source rock accumulated in an intrashelf silled depression communicating with the South Tethys Ocean, under a humid equatorial climate and during a period of sea level rise. The influx of clastics provoked the building of rapidly prograding deltas, while eastwards, argillaceous material was deposited under prodelta conditions. Freshwater from rivers and deep oceanic counter-currents caused water stratification according to a ‘positive water balance’ mechanism, as described by Demaison and Moore (1980). Large amounts of nutrients were brought in by rivers, causing high biological activity in surface waters while euxinic conditions prevailed at depth. These conditions resulted in the deposition of up to 300 m of black low-energy marl and argillaceous limestone in the center of the depression. These sediments contain large amounts of algal type II organic matter, with TOC values as high as 11%, S1 + S2 up to 40 g HC/kg rock and source rock potential indices (SPI) in excess of 20 t/m2. The high productivity of the Kazhdumi source rock resulted from the coincidence of several favorable factors resulting from post-Albian geological history. Some of them, such as the existence of excellent reservoirs, the efficient caprock of the Gachsaran evaporites capping the Asmari, and large whale-back anticlines, have been known since the early days of petroleum exploration. Some were discovered more recently, i.e., the Zagros folds began to form at the end of the Early Miocene, whilst the Kazhdumi reached the oil window over most of the Dezful Embayment as the result of the deposition of the thick flysh-type Agha Jari Formation, in the Late Miocene to Pliocene. Secondary migration of hydrocarbons, essentially vertical, was guided by areas of drainage which were similar to those evidenced by present-day seismic. Moreover, oil/gas migration was enhanced by the fracturing of limestone and marls which resulted from the Zagros folding. Such fracturing is easily observed in outcropping breached anticlines. The Kazhdumi origin of the oil of the Dezful Embayment main fields such as Agha Jari, Ahwaz, Bibi Hakimeh, Gachsaran, Mansuri, Marun, and Rag-e Safid was verified from stable isotope oil-to-oil and oil-to-source rock correlation and bio-marker signatures as described in a former paper (Bordenave and Burwood 1990).