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Basement-Related Strike-Slip Faulting and Its Implication for the Structural Evolution and Fault and Fracture Characterization of the Al Shaheen Field, Offshore Qatar

Abstract

This paper presents an integrated workflow which resulted in a structural model for the Al Shaheen giant oil field located offshore Qatar. The field is characterized by 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. Structural analysis performed using 3D seismic, well data, regional tectonic models and analogues indicates that the observed deformation at reservoir levels can be related to an incipient stage of strike-slip dominated regime characterised by a WNW-ESE and NNW-SSE trending conjugate fault system. In addition, faults and fractures as interpreted from LWD data, image logs and mud losses, are generally localised and tend to cluster in the interpreted fault zones. These fault zones are expressed as elongated depressions and are formed by discontinuous arrays of en-echelon segments resulting in a spatial variability of fault and fracture distribution. The interpreted lineaments can be traced and related, across several mechanical breaks, to well-developed discontinuities at a deeper stratigraphic level and they show consistent orientations with proposed regional basement grains (Zampetti et al., 2009). The interpreted deformation at Cretaceous reservoirs is therefore inferred to be related to the development of shear zones associated with the reactivation of pre-existing deep-seated Infra-Cambrian basement anisotropies during the late Cretaceous Oman Alpine 1 tectonic event and a subsequent reactivation during the Tertiary Zagros Alpine 2 event. The proposed structural model of the Al Shaheen Field provides a key calibration area to constrain the tectonic evolution of Qatar and of the entire Arabian plate. The understanding of the structural style, the tectonic evolution and the timing of deformation also has implications for maturation/charge history as well as reservoir property development in the Al Shaheen Field.