SOURCE ROCK DISTRIBUTION AND MATURATION IN THE ZAGROS OROGENIC BELT - PROVENANCE OF THE ASMARI AND BANGESTAN RESERVOIR OIL ACCUMULATIONS

The Zagros Orogenic Belt of Iran is one of the most prolific petroliferous areas accounting for approx. 12% of proven global reserves. Correlation techniques reveal that a majority of the oils, variously reservoired in Asmari (Oligo-Miocene) and Bangestan Group (Upper Cretaceous) anticlinal structures, share a common provenance. Oils of the N.E. Khuzestan fields are an exception, showing a conspicuously different origin. Candidate source potential for the crucial Central Zagros Embayment and contiguous areas was assessed on field survey and from well materials collected during the decade from 1968 onwards. Although varying locally in vertical and lateral distribution, attractive sources occur throughout the section. Included were Silurian Shales, the Sargelu (Middle Jurassic), Garau-Gadvan (Lower Cretaceous), Kazhdumi (Albian), Gurpi (Upper Cretaceous) and Pabdeh (Paleogene) Fms. Maturation of these source units was essentially controlled by the foredeep subsidence related to the Zagros orogeny. Being a Neogene event, hydrocarbon generation and migration are very recent and essentially synorogenic. Consolidation of this information with the areal distribution of candidate sources, application of carbon isotope based source-oil assignments and biomarker results, successfully differentiated the provenance of the generic oil groupings. Over much of Khuzestan, the Kazhdumi was recognised as the major contributor. Elsewhere, the Pabdeh Fm. yields isotopically distinctive oils in N.E. Khuzestan. Evaluation of this basin allowed identification of prolific source rocks in low energy, euxinic, argillaceous lime mudstones. Similarly, it provided an early application of pyrolysis techniques, carbon isotopic correlations and subsidence history-maturation-migration modelling approaches now routinely employed in basin evaluation.