Tectonic History and Basement Fault Control on Structural Development of an Offshore Field, Abu Dhabi
Authors: Edwards Ewart , Al-Rougha Hamad Bu , Sultan Akmal , Khouri Naeema 
Published: December 2007

Structural analysis of 3D seismic data over an offshore field in Abu Dhabi, involving a detailed study of the early Cretaceous reservoir sequence, the shallow cover and the deeper stratigraphy of the area, has shown that spatial fault patterns change subtly through the different stratigraphic packages present. This relates to the rheological properties, but consistency is shown in both the fault locations and in the resolved tectono-kinematic signature of the fault systems. The main fault system present comprises a rhombohedral array of NW-SE to WNW-ESE zones intersected by a broadly NE-SW system, at ∼100-130° to each other. Elements of these faults controlled depositional facies in the early to late Cretaceous (e.g. Thamama depocentres, Mishrif edge, Salabikh basin) though often only structurally soft-linked at the time of deposition. Hard linkages characterize the later Oman orogenic event as flower zones generating the Fiqa channels. Fewer of the faults continue up through the Tertiary sequence, where most of the breaks reflect fold-related arching of the cover sequences, but many coincide with locations of linear features and intersections at deeper levels.

Within the reservoir sequence the dense-reservoir alternations are fractured in a manner that responded to the mechanical layering. On a seismic sequence scale the faults display a similar effect whereby their geometry, density, connectivity and style within the damage zones display a brittle-ductile contrast, and where fault branches, tip zones or nucleation sites indicate structural linkage and deformation of a multi-layer rheology. A distinct effect is illustrated by changes in fault orientation, style and linkage across the fine-grained Nahr Umr seal sequence.

From the seismic structural analysis a basement-influenced faulting model, whereby localized readjustment of elements within a chequer-board of steep, deep faults occurred during Mesozoic-Cenozoic tectonic events, linked to distal mountain building episodes (Oman Mountains, Zagros event), is proposed. Repeated use of the antecedent framework is reconciled with a highly linked, low-offset, strike-slip dominated system that propagated into cover sequences as variably hard to soft-linked architectures:

Hard linkages exist across stronger, stiffer carbonate sequences Soft linkages prevail through more ductile shale-rich sequences

Within the offshore area many of the structures are salt-cored, particularly the smaller, tight dome-shaped fields and piercements. Large fields define broader, very open folds, each envisaged to be underlain by a deep ‘salt pillow’, postulated to be governed by the intersection of basement lineaments: they do not show distinct alignment/asymmetry typical of onshore fields. Initial growth of such traps during the Oman obduction event was largely by upward-propagating foreland-reactivated faulting, however, the later growth of these structures, when the trap amplification occurred, is viewed as a response to mild salt pillowing at depth above fault intersections triggered by Zagros-related events. Flattening of the seismic surfaces in the Tertiary sequence illustrate the timing of this fold modification coupled with the tilting of the fields to the northeast, with an effect to relocate the trap crests. This can be reconciled with the distribution of bitumen zones at reservoir levels that exhibit components of (i) NE-wards tilting, (ii) amplified doming of the crest of the structure, and (iii) activity on fault-controlled panels adjacent to the major lineaments under the field.