The Zagros Folded Belt: Along Strike Structural Variations, Evolution and Petroleum Plays

Ringenbach, Jean-Claude ¹; Wrobel-Daveau, Jean-Christophe ²; Sherkati, Shahram ³; Jahani, Salman ³; Letouzey, Jean ⁴; Frizon de Lamotte, Dominique ¹
(1) Total SA, Paris, France. (2) University Cergy-Pontoise, Dept. Geoscience & Environnement, Cergy-Pontoise, France. (3) National Iranian Oil Company, Exploration Directorate, Tehran, Iran, Islamic Republic of. (4) Univ. Pierre et Marie Curie, Paris, France.

The Zagros Chain is by far the most HC prolific folded belt in the world because it involves a mildly deformed thick sequence of Proterozoic to Cenozoic sediments including many petroleum systems that vary along strike together with the mechanical stratigraphy and a variable thick-skinned component of the deformation. A set of balanced-structural and play cross-sections from the back-stop to the foreland illustrate the structural changes from SE to NW. The first order variations are obvious at the scale of the belt with from SE to NW: the Fars Arc, the Dezful Embayment, the Lurestan Arc and the Kirkuk Embayment (Kurdistan, Iraq).

The Fars structural section exhibits a simple detachment fold style with a single Late Proterozoic Hormuz salt detachment. Spectacular diapirs that evolved since the Lower Proterozoic were squeezed by the Neogene compression. The shape and elevation of the Arc are both related to the paleogeography of the Hormuz salt and to basement thrusting. HC accumulations are mostly gas fields in the Permo-Triassic carbonates (Kangan and Dalan Fms, Iranian equivalent of the Khuff) in front of the Qatar Arch and in Tertiary sediments in the Bandar Abbas area.

In the Dezful section, the a narrow fold belt is uplifted along the Izeh zone, and in front, the Dezful Embayment is characterized by buried anticlines detached on a less active Hormuz detachment and complicated by secondary detachments in the Triassic (Dashtak) and Miocene (Gascharan) evaporites. Most discoveries are in the Oligo-Miocene (Asmari) and, Middle-Late Cretaceous carbonates, in traps which axes are offset from the surface below the Gasharan Evaporite.

In the Lurestan section, deformation from the suture zone ramps up to the Late Cretaceous shaly (Gurpi) SW-ward, with a correlative decrease of the folds wavelength. The whole Lurestan Arc is uplifted by recent basement thrusting and the large folds (e.g. Kabir Kuh) at the edge of the foreland are interpreted in relation with the inversion of normal faults. Some non commercial gas discoveries have been made in the Kangan and Dalan Formations of these frontal anticlines and some oil discoveries in Middle Cretaceous carbonates in SE part of the Lurestan.

The Kurdistan section is characterized by the absence of décollement in the Upper Proterozoic strata (eq. Hormuz). North of the mountain front, the section has been built using a décollement in Silurian shales combined with a late development of basement faults. South of the mountain front, the deformation ramps up into the Triassic evaporites (Kurrachine) and then into the Cretaceous shales (eq. Gurpi) and Miocene (eq. Gascharan). HC accumulations are mostly oil fields in the Oligocene Palani (eq. Asmari) and the Mid-Cretaceous Sarmord-Qamchuqa limestone.

Structural balancing together with new thermochronological data along the Dezful and Lurestan sections allow better constraining the deformation sequence, burials and uplifts and their relation to the prolific petroleum systems.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.