Style tectonique et cinématique du plissement dans le Zagros iranien (zone d'Iseh) (conséquences pétrolières)

Quatre coupes structurales régionales ont été construites à travers le Zagros Central Iranien, depuis l'avant-pays jusqu'à la zone interne. Elles sont fondées sur des observations de terrain, sur les cartes géologiques et sur les données de sub-surface. Elles montrent que la géométrie des plis change de façon significative dans les directions horizontale et verticale. Le style structural est étroitement lié aux variations du comportement mécanique des unités lithostratigraphiques. Une nouvelle subdivision structurale du Zagros Central est proposée, qui illustre cette variation du comportement mécanique de la colonne sédimentaire. Elle est basée sur la présence de plusieurs niveaux de décollement, activés durant la compression. Pour étudier l'influence d'un niveau de décollement intermédiaire sur le style de plissement et de chevauchement, nous avons réalisé des expériences analogiques en changeant la profondeur de ce niveau de décollement. En sus de cette reconstruction cinématique expérimentale, l'évolution cinématique d'un pli est reconstruite à partir d'observations de terrain : les évolutions de différents plis, de plus en plus internes et de plus en plus déformées, sont interprétées comme un indicateur de la cinématique de déformation. Dans la zone d'étude, le principal style de déformation est représenté par les plis de détachement, au moins pour les phases initiales de déformation, lesquelles sont suivies par la formation de plis de rampe. Nous discutons aussi d'autres aspects du pli de détachement, liés à la mobilité du sel, à des niveaux de décollement multiples et au contrôle tardif par implication du socle. L'évolution de la couverture sédimentaire, depuis l'épisode obductif du Crétacé Supérieur jusqu'à la collision du Miocène Inférieur, est décrite grâce à des cartes isopaques et à des courbes de subsidence. Cette analyse montre une compression continue et des mouvements le long de failles de socle de direction N-S et NW-SE. Les mouvements, qui précèdent le plissement Néogène du Zagros, ont influencé l'histoire de la sédimentation. Les mouvement des failles de socle, reliés aux épisodes tectoniques régionaux, ont déclenché la croissance discontinue de diapirs de sels. Dans le Zagros Central le rifting permo-triasique de la Néo-Téthys, le long de la faille du High Zagros ainsi que l'obduction crétacée-paléocene et ses événement compressifs associés, avec la réactivation des directions arabiques N-S, pourraient avoir activé des épisode de diapirisme salifère. L'analyse des déformations par modèle analogique suggère que l'initiation des chevauchement et des décrochements est influencée par les dômes de sel préexistants. Le moteur de l'halocinése et de l'extrusion du sel d'Hormuz a été l'écrasement des dômes de sel préexistants ainsi que les mouvement de pull-apart sur les failles décrochantes. La modélisation du système pétrolier dans la zone d'Izeh est basée sur la présence hypothétique de roches-mères dans les formations de l'Albien, du Jurassique et du Paléozoïque Inférieur. Cette modélisation montre que l'interaction entre sédimentation et déformation, qui varie suivant deux orientations, aurait eu un impact direct sur la maturation et l'expulsion des hydrocarbures des roches mères de l'Albien et du Jurassique. Cependant, les roches-mères du Paléozoïque Inférieur auraient expulsé leurs hydrocarbures avant le plissement du Zagros. Ceci suggère que les principaux facteurs favorables pour l'exploration du gaz dans les réservoirs permiens du Zagros sont la présence de paléo-structures et d'une couverture triasique efficace. Ainsi, les carbonates permiens pourraient piéger les hydrocarbures expulsés hors des roches mère du Paléozoïque Inférieur, avant leur re-migration et leur piégeage dans les structures du Zagros.CERGY PONTOISE-BU Neuville (951272102) / SudocSudocFranceF

An Investigation of Abnormal Fluid Pressure within an Evaporitic Cap Rock in the Gavbandi Area of Iran and its Impact on the Planning of Gas Exploration Wells

A synthesis of well logs was carried out and drilling mud weight data were analyzed to figure out anomalous high fluid pressure within the Triassic evaporitic cap rock (the Dashtak formation) and study its impact on the geometry of anticlinal traps in the gas rich Gavbandi province located in the southeast part of the Zagros Mountains. The results indicated that the location of anticlinal traps at the depth in which the Permian Dehram Group reservoir unit exists is horizontally displaced with respect to surficial crest of many anticlines within the Gavbandi area. This crestal shift may be induced by abnormally high fluid pressure in the “A evaporate” member of the Dashtak formation, detected in many exploration wells across the area. When fluid pressure increases due to compaction during shortening, the higher shaliness could probably cap more fluids and consequently increase the fluid pressure within the Dashtak formation. Anomalous high fluid pressure decreases internal friction and shear strength of rock units and facilitates fracturing and faulting within the Dashtak formation, which consequently causes crestal shift of anticlinal traps. This should be taken into account when planning a new exploration well in Gavbandi area in order to prevent trap drilling

Control of folding and faulting on fracturing in the Zagros (Iran): The Kuh-e-Sarbalesh anticline

The Kuh-e-Sarbalesh anticline (Fars region), shows a strong and poorly studied curvature in the vicinity of the Kazerun transfer zone. Here we investigated this structure by analyzing the orientation and distribution of fractures in the Oligocene-Early Miocene Asmari Formation (a major reservoir rock of the Zagros petroleum system) and paleo-burial conditions. Far away from the Kazerun Fault, the orientation of fractures is similar to that observed in other folds in the Zagros: two tensional fracture sets (F1 and F2) develop respectively parallel and perpendicular to the fold axis, whereas two shear fracture sets (dextral F3 and sinistral F4) develop at 30° from F2. Closer to the Kazerun fault, F1 and F2 become less abundant, whereas F3 fractures do prevail. F3 dextral shear fractures are associated with dextral shear zones (SZ3). Within SZ3, sets of tensional fractures (F5) occur. F4 shear planes are less abundant than F3 sets and are sometimes associated with sinistral shear zones (SZ4). Within SZ4, sets of tensional fractures (F6) occur. The frequency of F3 and SZ3 structures increases from south to north, suggesting a control by dextral shear along the Kazerun Fault. Average fracture spacing is around 20. cm and fracturing saturation values is larger than 0.7, at 20. km distance from the Kazerun fault whilst spacing is smaller than 1. cm and saturation is as low as 0.32 (typical of well-developed fracture systems) close to the fault highlighting a strong strain gradient from south to north. The orientation of F1 and F2 fracture sets changes from south to north according to the clockwise rotation of the northern part of the fold with respect to the southern part. Inorganic thermal parameters derived from X-ray diffraction analysis of clay minerals indicate that the Asmari formation was deformed at burial depths of 3. km (assuming a geothermal gradient of 20. °C/km). We suggest that the progressive differential advancement of the thrust front produced the curved shape of the Kuh-e-Sarbalesh anticline with associated rotation of the pre-/early-folding F1 and F2 fracture sets observed in the field, consistently with a dextral shear associated with the Kazerun transfer fault. © 2013 Elsevier Ltd

Control of Cambrian evaporites on fracturing in fault-related anticlines in the Zagros fold-and-thrust belt

Orientation and distribution of fractures in the Oligocene-Early Miocene Asmari Formation (a major reservoir rock of the Zagros petroleum system) were investigated in two anticlines of the Zagros fold-and-thrust belt. The Sim and Kuh-e-Asmari anticlines developed in the areas of the Zagros characterized by the occurrence and absence of Cambrian evaporites at the bottom of the stratigraphic pile, respectively. The aim was to outline major differences in terms of fracture spacing and saturation. Organic matter maturity and clay minerals-based geothermometers suggest that the depth of deformation for the top of the Asmari Formation in the Kuh-e-Asmari anticline was in the range of 1.5-2.7 km assuming a geothermal gradient of 22.5 A degrees C/km. The Asmari Formation in the Sim anticline probably experienced a slightly deeper sedimentary burial (maximum 3 km) with a geothermal gradient of 20 A degrees C/km. The spacing of fractures is generally 2-3 times larger (i.e., strain accommodated by fracturing is smaller) in the Sim anticline than in the Kuh-e-Asmari anticline. This is consistent with regional geological studies, analogue, and numerical models that suggest that thrust faults geometry and related folds are markedly different in the absence or presence of a weak decllement (evaporites). The larger spacing in the Sim anticline is also consistent with higher temperature predicted for the Asmari Formation in this area. By contrast, the orientation of fractures with respect to the fold axes is the same in both anticlines. The fracture systems are rather immature in both anticlines. The amount and density of fractures in the twofolds are controlled by regional (occurrence/absence of salt and probably different burial), rather than local features (fold geometry)

Structural Style and Timing of Nw-Se Trending Zagros Folds in Sw Iran: Interaction with North-South Trending Arabian Folds and Implications for Petroleum Geology

72 pages, 53 figuresThe Zagros foldbelt – foreland system in SW Iran is a prolific hydrocarbon province with known reserves of more than 90 billion brl of oil and 800 TCF of natural gas. Establishing the structural style of folding in the Zagros area presents a major challenge due both to the geographical extent of the foldbelt, which is some 1600 km long in total, and the presence of marked lateral variations in fold style related to the complex regional tectonic history. In addition, while numerous high-quality structural studies of the Zagros have been completed over the last 20 years, they support a variety of different interpretations and are therefore $$difficult to synthesize. In this paper, we review the general structural style of the Zagros fold-and-thrust belt in SW Iran, and in particular the style of folding in the Lurestan arc, Dezful embayment, Izeh Zone and Fars arc. We summarise relationships between folding in these areas and fracture development, and investigate the timing of folding and the interaction between NW-SE oriented “Zagros” folds and north-south oriented “Arabian” folds. Finally, we briefly assess the implications of fold style for petroleum systems in the Zagros area. Although no new data are presented in this paper, a series of unpublished maps are used to illustrate the main results and include: a map showing the extent of the main detachment levels across the Lurestan, Dezful and Fars structural domains; two palaeotectonic maps (for Late Cretaceous – Paleocene and Miocene – Pliocene times, respectively), showing the position of the deformation fronts of the Zagros and the North Oman thrust systems and their potential spatial and temporal relationship with folding; and a set of four maps showing the distribution of reservoir rocks which are grouped by age into the Permian – Triassic Dehram Group, the Late Jurassic – Early Cretaceous Khami Group, the Late Cretaceous Bangestan Group, and the Oligocene – Miocene Asmari Formation. In addition, for the Lurestan, Dezful and Fars structural domains, a series of regional cross-sections at the same scale are presented and discussed. Most of the data in this review paper were acquired in order to gain an improved understanding of the petroleum systems in the Zagros area; however the data are used here to investigate a range of interacting processes including tectonics, sediment deposition and subsurface fluid flow in the development of the fold-and-thrust belt and its associated foreland basins. The resulting synthesis is intended to provice a starting point for future tectonostratigraphic and hydrocarbon-related studies which will make use of both existing and new multidisciplinary techniques to constrain the results. The knowledge acquired and the techniques used will be of benefit in future challenges including the identification of subsurface reservoirs suitable for the permanent storage of CO2 to mitigate the effects of climate changeThis research was funded partly by the DARIUS Programme and by the Group of Dynamics of the Lithosphere, Geosciences Barcelona, Geo3Bcn-CSIC through the Spanish Ministry of Economy and Competitiveness Projects SUBTETIS (PIE-CSIC-201830E039) and ALORBE (PIE-CSIC-1063 202030E310)With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)Peer reviewe