Fault linkage and relay structures in extensional settings—A review

We thank the following projects for their additional support: DARIUS Programme and its sponsors, TopoMed CGL2008– 03474-E/BTE, ESF-Eurocores 07-TOPOEUROPE-FP006, TopoAtlas (CGL2006–05493/BTE), ATIZA (CGL2009– 09662-BTE) and Consolider-Ingenio 2010 Topo-Iberia (CSD2006–00041).

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Crustal-scale cross-sections across the NW Zagros belt: implications for the Arabian margin reconstruction

This work has been carried out as part of a joint study between Norsk Hydro (now Statoil) and NIOC (National Iranian Oil Company).

Stratigraphic architecture and fracture-controlled dolomitization of the Cretaceous Khami and Bangestan groups: an outcrop case study, Zagros Mountains, Iran

The carbonate-dominated Albian to Turonian succession is one of the key petroleum systems of the Arabian Plate. It is dominated by shallow water platform carbonates that were deposited in a shallow epeiric sea on the margins of Neotethys. In general, the reservoirs in this succession have high porosities but exhibit heterogeneous permeabilities. This study reviews published data for the region and attempts to unravel the key diagenetic controls on the porosity and permeability of the reservoirs. The results demonstrate that a spectrum of diagenetic processes created highly heterogeneous multimodal pore networks. Intense boring and micritization of skeletal allochems, differential cementation of a pervasive burrow network and preferential dissolution of aragonitic skeletal allochems are ubiquitous. Locally, particularly on the northern and eastern Arabian Plate, deep-penetrating epikarst can be tied to a differential response to global sea level fluctuation and local tectonism. The development of a peripheral bulge in late Cenomanian–Turonian times, halokinesis, localized influx of channelized clastic material and sub-regional climatic variability contributed to a heterogeneous pattern of meteoric diagenesis across the Arabian Plate. The succession was then buried to up to 10 km during the Late Cretaceous–Tertiary. Where deep-penetrating fault systems were reactivated by Alpine tectonism, flushing by hydrothermal brines resulted in highly localized patterns of hydrothermal dolomitization and leaching, associated with hydrocarbon emplacement.

Diagenetic controls on reservoir properties of carbonate successions within the Albian–Turonian of the Arabian Plate

The Mountain Frontal Flexure shows a single step along the front of the Pusht-e Kuh Arc with about 3 km of structural relief. This front has been interpreted as being formed by a basement monocline above a blind crustal-scale and low-angle thrust with a ramp­flat geometry (the ramp dips 12­158 towards the inner part of the orogen and cuts the entire crust). The Anaran anticline on top of the Mountain Frontal Flexure shows an irregular geometry in map view and consists of four segments with diverse directions of which the SE Anaran, the Central Anaran and the NW Dome are culminations. The North­South Anaran segment may form a linking zone developed during the rise and amplification of single culminations, the NW Dome and the Central Anaran, above the Mountain Frontal Flexure. The asymmetric Anaran anticline is characterized by the existence of multiple normal faults, some of them with significant dip-slip displacements of up to 1000 m. These faults limit grabens located along the crests of the anticline segments. Crosscutting relationships show that the normal faults along the Central Anaran are older than along the North­South Anaran, reinforcing the temporal constraints on the later growth of this segment of the anticline. The geometry of the Anaran anticline is asymmetric with the subvertical forelimb very little exposed. This forelimb is cut above and below by a thrust system that seems to develop along the fold hinges. The lower thrust, with a ramp­flat geometry, carries the entire anticline towards the foreland on top of slightly deformed rocks in the footwall. The thrust flattens in the Gachsaran evaporitic level forming a typical triangular zone filled with evaporites, which produce a strong fold disharmony between the overburden (Passive Group) and the underlying rocks (Competent Group). The growth of the Anaran anticline lasted for about 6 Ma and was the consequence of detachment folding that was subsequently thrust, rotated and uplifted above the Mountain Frontal Flexure with coeval reactivation of earlier crestal layer-parallel extension normal faults to accommodate the large increase of structural relief between the foreland and the tectonic arc. Three main results from analogue modelling have been combined with field data to resolve the geometry of the Anaran anticline as well as its evolution: (1) a thickening of intermediate evaporites (Gachsaran Formation) is produced above the flat segment of the thrust carrying the anticline on top of foreland strata; (2) growth strata deposited in the adjacent syncline modify the geometry of the anticline by increasing the dip and the length of its forelimb; (3) coeval erosion to anticline growth, as well as thick growth strata deposition, increases fold amplification rather than foreland propagation of deformation. The proposed fold model may be applied to other anticlines on top of this major basement-related thrust, such as the Siah Kuh and Khaviz anticlines in the Pusht-e Kuh Arc and Dezful Embayment domains.

Structure of the Mountain Front Flexure along the Anaran anticline in the Pusht-e Kuh Arc (NW Zagros, Iran): insights from sand box models

This paper provides an updated understanding of the reservoir stratigraphy, sedimentology, palaeogeography and diagenesis of the Upper Permian Hauptdolomit Formation of the Zechstein Supergroup (“Hauptdolomit”) in a study area on the southern margin of the Mid North Sea High. The paper is based on the examination and description of core and cuttings data from 25 wells which were integrated with observations based on existing and new 3D seismic.

Sedimentology, palaeogeography and diagenesis of the upper permian (z2) hauptdolomit formation on the southern margin of the mid north sea high and implications for reservoir prospectivity

A multidisciplinary approach combining geological mapping based on seismic and well data with petrographic analyses of core and cuttings samples was used to gain a better understanding of the distribution of Upper Permian (Zechstein, Z2) Hauptdolomit platforms and their depositional facies around the Elbow Spit High in the northern Dutch offshore. A detailed understanding of the Hauptdolomit's lateral facies variability is of great importance for assessing its reservoir potential, since both the thickness and reservoir properties of these carbonate platforms greatly depend on local accommodation within different palaeo‐depositional environments. The platforms generally contain the thickest Hauptdolomit sequences and are largely characterised by a mix of oolitic and coated grainstones, as well as by some dolomicrites. Porosities of around 15% are reached at well E02‐02 within the grainstone intervals, and interconnectivity between the pores is generally present. Seismic mapping has indicated a rim of isolated Hauptdolomit platforms, which are up to 10 km wide, around the southern and NW margins of the Elbow Spit High. No Hauptdolomit platforms are present on the NE margin of the High, likely because the palaeo‐ basin margin was too steep and hence lacked accommodation for carbonate growth. Discoveries made in recent years in the UK sector of the southern North Sea have highlighted the importance of the Hauptdolomit hydrocarbon play, and the results of the current study provide a solid base for assessing the reservoir potential of this play in the relatively underexplored northern part of the Dutch offshore.

Seismic and petrographic characterisation of the zechstein hauptdolomit platforms around the elbow spit high, dutch offshore

Late burial dolomite of hydrothermal origin replaces Cretaceous carbonate shelf sediments of Albian to Turonian age (Sarvak Formation) in the NW closure of the Anaran Anticline, Zagros Mountains, Simply Folded Belt, Iran. The outcrops, spectacularly exposed along deep river canyons, offer the possibility for a 3D reconstruction of the geobodies, combining both field data (sedimentary logs, cross sections and samples for diagenetic and petrophysical studies) and LIDAR derived photorealistic model of the most representative dolomite bodies. The aim is to quantify the impact of hydrothermal dolomitisation on the reservoir quality. The hydrothermal origin of the dolomitising fluids will be discussed integrating different geochemical data (fluid inclusions, stable and radiogenic isotopes, minor and trace elements). Dolomite replaces carbonate rocks characterised by different facies, showing different geometries: 1) massive plus stratabound (Lower Sarvak); 2) pipes (Upper Sarvak). Dolomite distribution decreases vertically with at least three discrete breaks corresponding to two main aquitard intervals: 1) Ahmadi Shales, separating massive dolomite form from plume-like bodies; 2) Turonian Mudstones; 3) Surgah Fm., above, which no dolomite has been observed. Fractures and faults play a major role in controlling the distribution of the dolomite bodies, in particular the ENE-WSW and WNW-ESE conjugate fault systems, which are often associated with dolomitic halos. A geological conceptual model is built taking into account different fracture and matrix porosity models for limestone and dolomite.

J. Garland

Papers

Stratigraphic architecture and fracture-controlled dolomitization of the Cretaceous Khami and Bangestan groups: an outcrop case study, Zagros Mountains, Iran

Sedimentology, palaeogeography and diagenesis of the upper permian (z2) hauptdolomit formation on the southern margin of the mid north sea high and implications for reservoir prospectivity

Seismic and petrographic characterisation of the zechstein hauptdolomit platforms around the elbow spit high, dutch offshore

Fracture-controlled Dolomite Reservoirs in Late Cretaceous Carbonates of the Sarvak Formation, Anaran Anticline, Islamic Republic of Iran