Search

Abstract: In Central Iran two adjacent anticlines contain hydrocarbons within the Late Oligocene–Early Miocene Qom Formation. The anticlines formed at approximately similar times (Late Miocene–Pliocene), have similar overall geometries, but the Alborz Anticline is an oil field, while the Sarajeh Anticline is a gas field, and both are underfilled structures. Basin modeling and structural restorations identified why the two different accumulations developed. The Sarajeh and Alborz anticlines are modified detachment anticlines. Asymmetric development of growth strata on the SW margin of the folds was caused by salt withdrawal and normal fault activity on the basin margin, which modified the detachment fold style and tilted the basal detachment to the SW. The detachment dies out to the NE as the basal (Lower Red Formation) evaporites pinch out in the same direction, yet the Alborz Anticline is north vergent, while the Sarajeh Anticline is south vergent. The mini salt-withdrawal basin is best developed on the southern limb of the Sarajeh Anticline and is responsible for the SW vergence. This difference in vergence permitted the Alborz Anticline to attain 4-way dip closure earlier in its structural history than the Sarajeh Anticline, and consequently the trap was at least partially developed as a four-way closure during oil generation and expulsion. For the Sarajeh Anticline oil migrated across the fold when it was still unclosed, continued subsidence of the SW fold limb resulted in gas generation and expulsion during the time when the fold attained closure. [Copyright &y& Elsevier]

Abstract: During continent–continent convergence of the Arabian and Eurasian plates, and after the late Eocene inversion of a back-arc rift, the Iranian Plateau underwent broad subsidence resulting in the formation of the Central Basin (Morley et al., 2009). New 2D seismic data acquired by National Iranian Oil Company (NIOC) in the NW–SW-trending arm of the Central Basin suggest that during the main stage of shortening (middle–late? Miocene to Pliocene), strain concentrations resulted in the development of the thin-skinned Kuh-e-Gachab, Kuh-e-Gugerd, Garmsar and Sorkh-e-Kuh structures. These structures are built of Oligocene–Miocene/Pliocene(?) rocks belonging to the Lower Red, Qom and Upper Red formations. Seismic data suggest that one of these structures comprises the south-verging Kuh-e-Gachab anticline, which is bounded by the N-dipping Kuh-e-Gachab thrust and cored by a complex array of thrust sheets forming a triangle zone. During the deformation process, two salt evaporate levels played a significant role as detachment horizons. The main detachment horizon was rooted within the Lower Red Formation, whereas the second detachment horizon was located along evaporites belonging to the Upper Red Formation. Variations in the thin-skinned style of deformation between the larger triangle zone in the western part of the Kuh-e-Gachab structure contrasts with less shortening in the smaller triangle zone to the east. This suggests that the change resulted from the increase of thickness of the mobile detachment horizon to the east. Contraction deformations are still active south of the Alborz Mountains, which is confirmed by GPS data and present-day seismicity. [Copyright &y& Elsevier]

In the Central Iran Basin, the mixed carbonate–siliciclastic deposits of the C member of the Qom Formation were deposited on a carbonate platform which is dominated by rhodalgal associations occurring in tropical–subtropical environment. The biogenic rhodalgal association is dominated by bryozoa, coralline red algae, bivalves and echinoids together with smaller amounts of photo-dependent biota including large benthic foraminifera and corals. The abundance of heterozoan association and the bloom of suspension-feeding organisms are the result of an increase in nutrient availability which has profound controlling effect on the biotic system. The low occurrence of symbiont-bearing benthic foraminifera and coral, typical of stable, oligotrophic condition, represents their low tolerance to unstable, nutrient-rich environment. In the investigated Oligocene–Miocene shallow marine carbonate succession, 10 different microfacies were distinguished through depositional texture and biotic components. The rock sequences investigated are referred to an open shelf carbonate platform in which the depositional environments range from outer shelf to inner shelf conditions. [ABSTRACT FROM PUBLISHER]