The Role of Regional Basement Fabric on Cretaceous Structural Deformation; a Case Study from Al Shaheen Field, Offshore Qatar

The giant Al Shaheen Field in Block 5 and Block 5 Extension, offshore Qatar, contains a stacked sequence of thin Lower Cretaceous reservoirs associated with a complex array of subtle faults which influence dynamic reservoir behaviour in certain areas of the field. A multidisciplinary analysis that integrates 3D seismic, well data and a regionally developed structural model indicates that at reservoir levels, deformation occurred in an incipient low-displacement (lateral and vertical) strikeslip dominated regime, characterized by a complex pattern of predominately steep to sub-vertical branching fault zones. Seismic interpretation and well calibration indicates two major fault zones trending WNW-ESE and NNWSSE, showing a dextral and sinistral sense of shear respectively. These fault zones are expressed as elongated depressions on structure maps and are formed by discontinuous arrays of en-echelon segments resulting in a spatial variability of fault distribution and behaviour. A secondary set of faults trending NW-SE and bounded between major WNW-ESE fault zones can be consistently mapped and is interpreted to represent either synthetic Riedel structures or normal faults. At deeper Permo-Triassic levels, continuous, well-developed fault lineaments are observed below the weaker Cretaceous lineaments. The interpreted deformation in the Al Shaheen Field can therefore be related to the development of shear zones associated with the reactivation of pre-existing deepseated Infra-Cambrian basement anisotropies during the Cretaceous Oman Alpine 1 and Tertiary Zagros Alpine 2 tectonic events. The proposed structural model of the Al Shaheen Field provides a key input and a further calibration area to constrain the tectonic evolution of Qatar and of the entire Arabian plate with regards to basin evolution, timing of structuration, structural style and faults and fractures distribution on both regional and field scales.