Manhal Sirat

Chloritization in Proterozoic granite from the Äspö Laboratory, southeastern Sweden: record of hydrothermal alterations and implications for nuclear waste storage

Abstract Hydrothermal alteration of Proterozoic granitic rocks in the Äspö underground laboratory, southestern Sweden, resulted in the formation of chlorite with large variations in textural and chemical characteristics, which reflect differences in formation temperatures, fluid composition, and reaction mechanisms. The mineral assemblage associated with chlorite, including Ca-Al silicates (prehnite, pumpellyite, epidote, and titanite), Fe-oxides, calcite, albite and K-feldspar, suggests that chloritization occurred at temperatures of between 200−350°C during various hydrothermal events primarily linked to magmatism and rock deformation. Petrographic and electron microprobe analyses revealed that chlorite replaced biotite, amphibole and magnetite, and hydrothermal chlorite phases filled fractures and vugs in the granitic rocks. While fracture-filling chlorite reduces fracture permeability, chloritization reactions in the host granite resulted in the formation of new localized microporosity that should thus be taken into consideration when evaluating the safety of the granitic basement rocks as a repository for nuclear waste. It is also important to take into account that similar alteration reactions may occur at the site of stored nuclear waste where temperatures in excess of 100ºC might be encountered.

Analogue Modelling of Shah Structure in Abu Dhabi; Mode and Structural Evolution

Three-dimensional seismic data have been used to construct a series of scaled analogue models simulating the structural evolution of the Shah hydrocarbon Field. Models consisted of a set of layers of loose sand on two basal plates whose contact simulated a basement fault. Depositional history of the Shah structure was simulated in the model by producing syn-kinematic deposition and erosion. Deformation of the model was achieved by moving one of the basement plates in a way that it initiated an oblique slip along the basement fault. This oblique movement induced both a strike slip movement in and shortening of the cover sand layers, which resulted in formation of an open anticline (box fold) along the strike of the fault similar to the Shah structure.The 3D seismic mapping of the Shah hydrocarbon Field reveals carbonates dominating lithological units from the Pre Khuff Fm (Permian) to the Dammam Fm (Eocene). A major unconformity in the stratigraphy indicates uplift, exhumation and erosion at Simsima level (U. Cretaceous) with onlapping of the younger units (Eocene-Miocene Formations). The Shah structure is an open SW-NE striking anticline, which changes symmetry along strike; asymmetric in the SW, more symmetric in the middle, and asymmetric open anticline in the NE. Model results indicate that the structure is a basement related cover anticline formed due to the oblique movement along a basement fault, which was reactivated during the obduction of Semail Ophiolite and later collision of Arabia with Iranian plate. These results show that this anticline formed without the need for a shortening component perpendicular to the strike of the structure. An oblique slip along a basement fault, with a relatively small degree of shortening could equally well result in the formation of such open and low-amplitude anticline.The modelling results presented here provides an alternative scenario for the formation of Shah structure (low-amplitude, and open) and addresses some of the key questions about its structural constraints (e.g. geometry, timing and evolution history) by linking it with basement structures. Our modelling results are of high significance for understanding entrapment mechanism, migration pathways, and fracture development. This newly proposed mechanism impacts the exploration and development of the Onshore Fields in Abu Dhabi, at the vicinity of Shah Structure.