The Role Of Regional Basement Fabric On Cretaceous Structural Deformation; A Case Study From Al Shaheen Field, Offshore Qatar
Authors: Zampetti V. , Madsen L. , Cromie H. , Durance G. , Emang M. , Bounoua N. , Gagigi T. 
Published: March 2015

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) strike-slip 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 NNW-SSE, 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 deep-seated 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.

Introduction

The Al Shaheen Field is structurally located within the Qatar Arch (Fig.1), a broad N to NNE trending Infra-Cambrian (610-520 Ma) structure belonging to the Central Arabian Plate tectonic mega province. The Qatar Arch developed as a "basement block" flanked by thick deposits of the "Hormuz series" and separating the Northern & Southern Gulf Hormuz Salt basins. The kinematics and the geometry of the early Qatar Arch are still speculative (Bell, 2004; Edgell, 1992; Harvey et al., 2012; Husseini, 1992; 2004; Johnson, 2008; Konert et al., 2001; Stampfli et al., 2001; 2002; Ziegler, 2001).

In the region, basement-related anisotropies are interpreted to have formed as old as Proterozoic to Ediacarian time and to follow three main trends (Fig. 2): N-S (Nabitah),NW-SE to WNW-ESE (Najd) andNNW-SSE to NW-SE (Mesopotamian; Zampetti et al., 2009 and references therein).

The Nabitah lineaments include the N-S trending Neo-Proterozoic deformation associated with the assembly of Gondwana. In the Arabian Plate, Pan African structures form a complex array of shear zones, which general young to the East.

These are mainly reverse structures, which deformed during accretion of older micro-continents.

The WNW-ESE to NW-SE faults are interpreted to represent a strike-slip system associated with the major Gondwana-scale continental shearing event know as the "Najd event". The Najd shear zones are vertically dipping and transect the entire crust. The prevailing steep dips of these structures are an important aspect in the subsequent reactivation during basin formation. Step-overs between Najd shear zones formed pull-aparts and pop-ups at many scales, mainly relaying over the NW-SE Mesopotamian trends.