Sedimentology and Sequence Stratigraphy of the Bangestan Group, Lurestan Province, Zagros Mountains, Iran -- A Working Model

By

Ian Sharp¹, Nils Bang², Davood Morsalnezhad³, Mahmood Bargrizan⁴

(1) Norsk Hydro Research Center, Bergen, Norway (2) Norsk Hydro International, Oslo, Norway (3) NIOC -- National Iranian Oil Company, Tehran, Iran (4) IOOC -- Iranian Offshore Oil Company, Tehran, Iran

 The Albian-Campanian aged Bangestan Group contains some of the most prolific reservoirs of the Zagros hydrocarbon province of Iran, predominantly within neritic carbonates of the Sarvak Formation. These units form extensive outcrops in the frontal fold belt of the Northern Zagros mountains, thus affording a unique opportunity for pseudo-3D reservoir characterisation as an aid to understanding producing fields in the subsurface.

The Lower Sarvak Formation comprises a thick (500 m +) predominantly aggradational section of ramp interior to ramp margin facies. Reservoir facies are predominantly within “grainy” facies, including rudist shoals, but are also developed within supra to intra-tidal facies. Reservoir heterogeneity is marked, related both to primary facies variations and to late-stage fracture controlled dolomitisation. The top of the Lower Sarvak Formation is marked by a regionally extensive transgression leading to deposition of ammonite-bearing deep ramp/basinal facies.

The Upper Sarvak Formation is up to 270 m thick and can be divided into 2 large-scale strongly progradational ramp sequences separated by a regionally extensive karst surface of Upper Cenomanian-Early Turonian age. Reservoir facies are again predominantly developed within the more “grainy” facies, although locally developed dolomitised mud-mounds on the deep ramp can represent excellent reservoirs. The Top of the Sarvak Formation is represented by a regionally extensive flooding followed by deposition of mudstones of the Surgah Formation. Neritic carbonates of the Ilam Formation abruptly overlie the Surgah Formation and pass retrogradationally into hemipelagic carbonates of the Gurpi Formation related to final drowning of the ramp system in Lurestan.