Bahman Soleimany

Identification of inversion tectonics and inverted structures is very important in petroleum exploration and production due to the positive or negative impacts they can exert on the hydrocarbon traps. In order to characterize the occurrence of multi-phase inversion tectonics and its effect in the Rag-e-Sefid Oilfield, located in the southern Dezful Embayment (SW Iran), this research describes the structural style and deformation history through structural and tectono-sedimentary analyses. The results obtained from the interpretation of seismic profiles and the investigation of the geometry of the sedimentary layers across the growth structures indicate that the Rag-e-Sefid anticline is created from multiple fault-propagation folds. The final shape and geometry of the Rag-e-Sefid anticline are affected by the Rag-e-Sefid and Nourooz-Hendijan-Izeh Faults activity. The deformation history in the north-western part of the Rag-e-Sefid anticline is older than the south-eastern part of it. The Rag-e-Sefid Fault set in the north-western and south-eastern parts of the Rag-e-Sefid Oilfield experienced activity at least during seven and four tectonic phases, respectively. Multiple fault-propagation folding and the fold axis rotation in the north-western part of the Rag-e-Sefid anticline controlled by the presence of several décollement surfaces and the occurrence of inversion tectonics along the pre-existing basement structures. Three-phase inversion tectonics along the Nourooz-Hendijan-Izeh Fault and the Rag-e-Sefid Fault set in the north-western part of the Rag-e-Sefid Oilfield occurred at the Late Devonian-Carboniferous (positive inversion), Permian-Early Triassic (negative inversion), and Late Cenomanian-Early Turonian (positive inversion) boundaries and modified the evolutionary history of the sedimentary basin. The inversion affected hydrocarbon trap development at the Late Cretaceous and controlled their current conditions in the southern Dezful Embayment. Based on the hydrocarbon migration in these regions from the Late Miocene to the present day, the N-S trending (Pan Africa trend) north-western part of the Rag-e-Sefid anticline has a larger share of oil and gas compared to the NW-SE trending (Zagros trend) south-eastern part. The results of this study could add data to worldwide examples of the positive impact of tectonic inversion on the hydrocarbon traps development in the foreland of a collisional orogen.

Thrust-related anticlines in the Zagros Simply Folded Belt provide excellent exposed analogue structures for fractured reservoirs located in the more external sectors of the belt. In these structures it is possible to study the fracture network attributes and understand their relationships to the folding process, thus gathering fundamental information for fracture modelling in reservoirs. In this work we analyse the mesoscopic deformation pattern of the NW–SE-trending Bangestan anticline (SW Zagros, Iran) and discuss its relationship to the kinematic evolution of the hosting structure. The deformation pattern mostly includes extensional structures and pressure solution cleavages striking parallel to the fold axial trend (i.e. longitudinal), transversal extensional structures, and N–S- and E–W-striking extensional structures (oriented oblique to the fold axis). With the aid of deep wells and a transversal reflection seismic profile, we constructed a balanced cross-section of the anticline and propose a kinematic evolution pathway constrained by the mesoscopic deformation pattern. Longitudinal and transversal deformation structures developed before and/or in the very early stages of fold growth. During this stage, the Bangestan anticline grew as a set of unconnected décollement anticlines involving the Cambrian to Pliocenic sedimentary cover. In a later stage, inherited basement faults were reactivated with a right-lateral strike-slip component and the previously developed anticlines propagated laterally up to their complete linkage and thrust breakthrough. This produced the right-lateral strike-slip reactivation of longitudinal joints and the development of N–S- and E–W-striking extensional structures, which were also frequently reworked as strike-slip faults.

The origin and kinematic evolution of the Dorood growth anticline, a kilometre-scale fold located in the Iranian NW part of the Persian Gulf on the Zagros front, is discussed based on the geological interpretation of a seismic section and subsequent application of a number of techniques such as depth to detachment estimations, sequential cross-section restoration, estimations of crestal structural relief, shortening, wavelength and fold core area in various stages of fold amplification, and comparison between functions derived from the anticline analysis and functions for theoretical folds. Long-term fold growth rates indicate a slow amplification for c . 88.6 Ma punctuated by two periods of faster growth during the Late Cretaceous and from the Late Miocene to the present day. The kinematic evolution proposed involves vertical push related to reactivation of a basement fault during the oldest amplification event and fold tightening owing to buckling as a consequence of horizontal compression in the youngest event, both causing evaporite motion. Fold amplification took place by a combination of hinge migration and limb rotation.