The Central Basin in the Iranian Plateau lies between the geologically better known regions of the Zagros and Alborz mountains. Exploration in the Central Basin of Iran by PTTEP and NIOC has revealed the details of the Late Oligocene-Recent evolution of the basin, based on seismic reflection data, geological fieldwork and satellite interpretation. The basin has a multi-stage history commencing with a broad sag-type subsidence with isolated normal faults, in the Oligocene-Early Miocene. It evolved to a ranstensional basin in the early-middle Miocene, with up to 8 km of Upper Red Formation section being deposited in some parts of the basin. The upper part of the Upper Red Formation is associated with a change to transpressional deformation, with the development of thrusts and folds, which form the traps for the Alborz and Sarajeh fields. The latest (probably Late Miocene-Pliocene) deformation was transpressional, with a mixture of basement-involved strike-slip and thrust faults and thin-skinned folding and thrusting<br>detached on Oligocene evaporites. Higher local detachment levels in the stratigraphy also exist. NW-SE to NNW-SSE dextral to compressional faults dominate the area with subordinate E-W (restraining bend) and N-S (strike-slip) orientations also present. Structure plays an important role in many aspects of the petroleum system: locally influencing the thickness and facies of the Qom Formation (reservoir/source unit) due to underlying salt movement; trap timing-migration is complex, fracture systems are important for reservoir permeability, whilst the combination of modern stress state, fractures and Upper Red Formation evaporite seal have resulted in high overpressures in the Alborz and Sarajeh fields (and the very large Alborz-5 blow-out).


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