TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractRecent mapping of fault patterns from picked surface attribute analysis ('difference' maps, spectral decomposition, isochron maps) and time slices over a giant offshore field in Abu Dhabi has recognized a complex multi-set pattern of faults at reservoir level. As well as providing additional seismic scale faults, linear features (proposed subseismic faults and fracture systems) have been identified. These patterns are best described in terms of strike-slip geometries; however, many display components of both dip-slip and strike-slip over their movement history. In different parts of the field dominant fault directions displaying dextral transtension, dextral transpression, trapdoor hinge faulting and oblique-slip keystone graben generation are observed. Fault throw statistics and segment growth history in the most important mapped zones typify components of strike-slip along with local shear and segment termination at cross-fault zones. The prevalent fault trends are roughly orthogonal NE-SW and NW-SE systems that dissect the field. Superimposed on this geometry is an array of distinctly en-echelon WNW-ESE and WSW-ENE structures that link to steep zones at deeper levels. Within sub-regions of the field other trends are also present, including NNE-NNW to N-S, NNW-SSE and E-W. These sets do not form a radial pattern; they are distributed spatially within field domains and involve complex systems and layerrelated rheologically controlled deformation.

From structural analysis of 3D seismic data over a giant field, offshore Abu Dhabi, a complex pattern of intersecting, broadly conjugate, strike-slip dominated en-echelon fault systems are observed from ‘basement’ levels up to latest Cretaceous horizons. Over geologic time these fault systems exerted fundamental controls on depositional patterns within the study area. Use of new attribute techniques, isochron mapping, flattening on key intra-Cretaceous horizons, cinematic time-slice ‘movies’ and a review of the available vertical seismic sections has indicated that many apparent velocity anomaly zones are controlled by deep-seated fault systems developed as strike-slip dominated ‘flower structures’. Spatial continuity of these fault zones, the most prominent of which trend ~NE-SW, exhibit a linear nature on a scale of several kilometers to tens of kilometers; have flower-zone geometry in cross-section but with increasingly en-echelon nature at Upper Cretaceous levels; low apparent normal offsets on long fault segments; anastomosing, convergent and divergent, patterns at different stratigraphic levels; wrench offset with respect to other deep fault systems; throw terminations with rapidly decreasing displacement gradients along en-echelon zones into the cross-over zones with other fault systems. There are four prominent fault systems that exert control on the depositional systems:NE to NNE trending ‘Fiqa’ direction, also delineating local depositional highs/lows and Mishrif ‘reef’ margin;NW-SE zones defining depositional areas of step-wise thickening/thinning;Conjugate WNW-ESE and WSW-ENE en-echelon strike-slip zones, andNNW-SSE fault zones displaying local control on thinning or thickening. Temporal fault activity on these zones is linked to sedimentary thickening and thinning patterns, though deeper level faults provided only ‘soft’ links to the sedimentary sequences of early Cretaceous age. By late Cretaceous times the transtensional / transpressional fault systems provided ‘hard’ links to the depositional systems (e.g. Fiqa channel system). Introduction Until recently most large fields in the offshore UAE region were not covered by seismic data and as a consequence details of the distribution of sedimentary sequences was limited to 1d well data with extrapolation into the inter-well areas. Investigation of the recently acquired 3D seismic volume over an offshore field has allowed previously unattainable details regarding the distribution and thickness of Mesozoic sequences and facies, that were impossible to quantify from well data alone, to be made. In this paper the description of some of the larger-scale effects and the broader features of such facies belts is provided, along with discussion of the structural controls on their distribution. Consequently, the distribution of present day trends in reservoir quality, sequence distribution and coincidence of mapped fault zones with facies belts can be more confidently evaluated. Following a review of salient background geology, stratigraphy, methodology and the general faulting pattern in the study area this paper aims to cover the following topics:Structural controls on deposition in the Thamama, Mishrif, Halul and Fiqa sequences;Review the areal coincidence of facies belts at different stratigraphic levels;Role of reactivation in temporal control of depositional trendsDiscussion of velocity effects of Fiqa channels, regional correlation with plate tectonic development and links with deep basement trends of the Arabian Shield and surrounding areas.

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 ∼100–130o 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 sequencesSoft 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 ofNE-wards tilting,amplified doming of the crest of the structure, andactivity on fault-controlled panels adjacent to the major lineaments under the field.