Tertiary sequence of deformation in a thin-skinned/thick-skinned collision belt: The Zagros Folded Belt (Fars, Iran)

International audienceWe describe how thin-skinned/thick-skinned deformation in the Zagros Folded Belt interacted in time and space. Homogeneous fold wavelengths (15.8 ± 5.3 km), tectono-sedimentary evidence for simultaneous fold growth in the past 5.5 ± 2.5 Ma, drainage network organization, and homogeneous peak differential stresses (40 ± 15 MPa) together point to buckling as the dominant process responsible for cover folding. Basin analysis reveals that basement inversion occurred ∼20 Ma ago as the Arabia/Eurasian plate convergence reduced and accumulation of Neogene siliciclastics in foreland basin started. By 10 Ma, ongoing contraction occurred by underplating of Arabian crustal units beneath the Iranian plate. This process represents 75% of the total shortening. It is not before 5 Ma that the Zagros foreland was incorporated into the southward propagating basement thrust wedge. Folds rejuvenated by 3–2 Ma because of uplift driven by basement shortening and erosion. Since then, folds grew at 0.3—0.6 mm/yr and forced the rivers to flow axially. A total shortening of 65–78 km (16–19%) is estimated across the Zagros. This corresponds to shortening rates of 6.5–8 km/Ma consistent with current geodetic surveys. We point out that although thin-skinned deformation in the sedimentary cover may be important, basement-involved shortening should not be neglected as it requires far less shortening. Moreover, for such foreland folded belts involving basement shortening, underplating may be an efficient process accommodating a significant part of the plate convergence

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

Structural evolution of the Kopeh Dagh fold-and-thrust belt (NE Iran) and interactions with the South Caspian Sea Basin and Amu Darya Basin

We present a detailed stratigraphic and structural study of the Kopeh Dagh fold-and-thrust belt in NE Iran, which is an investigation of the complex polyphased tectonic history of this belt and its links with the adjacent South Caspian Sea and Amu Darya basins. Based on numerous field surveys, a large amount of 2D and 3D seismic data, borehole data and more than 150 new biostratigaphic datings, a new detailed biostratigraphic chart and 4 main regional cross-sections illustrate the importance of lateral facies variations and structural inheritance in the present-day structure of the belt. After the Cimmerian orogeny corresponding to the closure of the Paleotethys Ocean in Late Triassic/Early Jurassic times, a Middle Jurassic post-collisional rifting event was associated with the deposition of one of the main source rocks of the Kopeh Dagh and the Amu Darya Basin (Kashafrud Formation). Following this rifting event, over 7km of sediments were accumulated until the Tertiary above a regional post-Triassic unconformity. The occurrence of local uplifts during the Late Cretaceous-Early Paleocene is interpreted as a consequence of regional-scale modification of plate-slab coupling in the Neotethys subduction zone. The main inversion of the Kopeh Dagh occurred at Late Eocene times, when the far-field deformation developed in Eurasia as a consequence of the locking of the Neo-Tethys subduction. This folding phase is sealed in the western part of the belt by a major Eocene-Oligocene unconformity atthe base of the thick sedimentary series belonging to the South Caspian Sea Basin. The bulk of sedimentary infill in the South Caspian Sea Basin is Oligocene and younger, and it is probably related to syn-compressional downward flexure of the resistant basement basin at the onset of the Alpine phase. In the eastern part of the Kopeh Dagh, this deformation is characterized by Middle Jurassic graben inversion with evidenceof forced folding, short-cuts and as well by larger scale basement uplifts. In contrast, the northwestern part of the belt shows thrust faults involving basement and fault-propagation folds within the sedimentary sequence. The Kopeh Dagh presents tectonic structures that are parallel to the Paleotethys suture zone, which emphasizes the importance of the structural inheritance and inversion processes during the structural evolution of the belt. Finally, a change from a mostly dip-slip to a mostly strike-slip tectonics occurred during the Pliocene within the Kopeh Dagh as a consequence of a major tectonic reorganization in North-East Iran. © 2014 Elsevier Ltd.This study was funded by the Darius Programme and thanks to the strong support and collaboration with the National Iranian Oil Company (NIOC), and in particular the Exploration directorate. Additional funding was provided by the projects ATIZA (CGL2009-09662-BTE) and TECLA (CGL2011-26670).Peer Reviewe

Subsurface fracture analysis and determination of in-situ stress direction using FMI logs: An example from the Santonian carbonates (Ilam Formation) in the Abadan Plain, Iran

The relationship between the present-day stress field and natural fractures can have significant implications for subsurface fluid flow. In particular, fractures that are aligned in orientations favourable for reactivation by either shear or tensile failure in the in-situ stress field often exhibit higher hydraulic conductivities. The Ilam Formation of southwestern Iran is an important hydrocarbon reservoir containing numerous natural fractures. However, little is known about the state of stress in this region, or any of Iran's petroleum provinces. We conducted analysis of the present-day maximum horizontal stress orientation and the density, orientation and hydraulic conductivity of natural fractures in the Ilam carbonates using high resolution Formation Micro Imager resistivity logs in two wells. A total of 51 breakouts with an overall length of 215 m were observed in the two wells, indicating a maximum horizontal stress orientation of 68°N (± 7.6°) in well A and 58°N (± 6.3°) in well B. Furthermore, the wellbore-derived stress orientations determined herein are consistent with those inferred from nearby earthquake focal mechanism solutions, indicating that stresses in the sedimentary cover are linked to the resistance forces generated by Arabia–Eurasia collision. Furthermore, the correlation between stress orientations estimated from earthquake focal mechanism solutions and breakouts indicates that focal mechanism solution data, which is often considered to be unreliable for stress field analysis near transform margins, may provide reliable information on the stress orientation near continental collision zones. The image log data also reveals three sets of open, and presumably hydraulically conductive, fractures with strikes of (i) 160–170°N, (ii) 110–140°N and (iii) 070–080°N. Fracture set (iii) is consistent with being formed and open in the present-day stress field. However, fracture sets (i) and (ii) strike at a high angle to the present-day maximum horizontal stress, and are interpreted herein to be the result of either pre- or syn-folding related forces. The observation that different sets of open fractures in the field can be either sensitive or insensitive to the present-day stress is critical for improving hydrocarbon recovery.Mojtaba Rajabi, Shahram Sherkati, Bahman Bohloli and Mark Tinga

Why Folding Style Varies In The Zagros Orogenic Belt?

Hydrocarbon Exploration in the Zagros Mountains of Iraqi Kurdistan and Iran, 23-25 January 2013The Zagros Mountains, geographically elongated from NW of Iran to Strait of Hormuz, are situated within the general context of the convergence between the Arabian and Eurasian plates (Fig.1). The Proterozoic-to-Recent sedimentary cover has been detached from the Pan- African basement and folded. The folds in this belt host giant hydrocarbon accumulations at different geological levels and better understanding of their geometry and kinematic evolution have crucial importance to promote exploration activities in the area. In last decade Zagros fold belt has undergone extensive investigations, from which a general understanding of prominent structural characteristics and involved deformational processes have emerged (Sherkati & Letouzey 2004, Molinaro et al 2004, Sepehr et al 2006, Verges et al 2011. Jahani 2009).The mentioned works mostly concentrated in specific areas of the belt. The aim of this paper is to integrate achievements from different parts of the belt and to present an overview regarding controls on folding style through the belt. Meanwhile additional new available data (Seismic lines and well information) is used to improve interpretation

Controls on fold development in the Zagros Orogenic Belt

Evolution of the Zagros-Makran Fold Belts, Darius Workshop 2012 ,, 14, may 2012 , Spai

Impact of the Late Triassic Dashtak intermediate detachment horizon on anticline geometry in the Central Frontal Fars, SE Zagros fold belt, Iran

Integration of 2-D seismic lines, well data and field studies allow us to determine the geometry variations of anticlines in the highly prolific Central Frontal Fars region in the SE Zagros fold belt. These variations are directly related to changes in thickness of the principal evaporitic intermediate detachment level, located along the Late Triassic Dashtak Formation. Anticlines of short wavelength contain a significant over-thickening of the evaporitic detachment level in their crestal domain that may reach 1900m (from an original thickness of 550-800m). Folds containing thick Dashtak evaporites show decoupling across the detachment level and, thus, a shift of the anticline crest in the underlying Permo-Triassic carbonates of the Dehram Group, which form the major gas reservoir in the Central Frontal Fars. Four main parameters control the extent and distribution of the decoupled anticlines in the study area: (a) original large thickness of the Late Triassic evaporitic basin; (b) coinciding larger amounts of anhydrites with increasing total thickness of formation; (c) parallel occurrences of abnormally high fluid pressures; and (d) shortening variations across, and along, the strike of specific folds. The present work relating the different parameters of the Dashtak evaporites with the anticline geometry allows a better understanding of the fold geometry variations with depth, which is applicable to oil and gas exploration in the SE Zagros and other similar hydrocarbon provinces characterised by intermediate detachment horizonsPeer reviewe