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BORDENAVE, M. L., TOTAL, Cedex, France, and A. Y. HUC, Institut Francais du Petrole, Rueil Malmaison, France

ABSTRACT: The Cretaceous Source Rocks in the Zagros Foothills of Iran: an Example of a Large Size Intracratonic Basin

The Zagros orogenic belt of Iran is one of the world most prolific petroleum producing area. However, most of the oil production is originated from a relatively small area, the 60 000 sq km wide Dezful Embayment which contains approximately 12% of the proven oil global reserves.

The distribution of the oil and gas fields results from the areal extent of six identified source rock layers, their thermal history and obviously reservoir, cap rock and trap availability.

In this paper, the emphasis is put on 3 layers of Cretaceous sources rocks, namely: the Garau facies deposited during the Neocomian to Albian interval over Lurestan, Northeast Khuzestan and which extends over the extreme northeast part of Fars, the Kazhdumi source rock which deposited over the Dezful Embayment, and eventually the Senonian Gurpi Formation which has marginal source rock characteristics in limited areas of Khuzestan and Northern Fars. Another source rock layer, the Shilaif Formation of Cenomanian-Turonian age, which is prolific in Abu Dhabi, have a limited impact in the Southern part of the offshore Fars, and perhaps in the extreme southeast part of the onshore Fars.

The environment of deposition of these source rock layers corresponds to semipermanent depressions, included in an overall shallow water intracratonic basin communicating with the South Tethys Ocean. These depressions became anoxic when climatic oceanographical and geological conditions were adequate, i.e., humid climate, high stand water, influxes of fine grained clastics and the existence of sills separating the depression from the open sea. Distribution maps of these source rock layers resulting from extensive field work and well control are also given. The maturation history of source rocks is reconstructed from a set of isopachs.

It was found that the main contributor to the oil reserves is the Kazhdumi source rock which is associated with excellent calcareous reservoirs: Asmari of Lower Miocene and Bangestan (Cenomano-Turonian). The importance of the Kazdumi as the source rock for all the large size fields of the Dezful Embayment was confirmed by stable isotopes (13C and 34S) and by biomarkers. Conversely, the contribution of the rich Garau source rock was limited to a few fields in Lurestan, as only few poor quality reservoirs are available in most of the area, and perhaps in the northeast Khusestand and the northeast Fars. The Gurpi anoxic facies has a limited areal extent and its geochemical characters put it into a marginal source rock, therefore, its contribution is assumed to be negligible.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.