No abstract

No abstract

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.

Advances in seismic acquisition, processing, computing hardware and theory continue to enhance seismic-image quality. However, an investment decision on seismic projects should be based not only technical criteria but a quantifiable expected value above all currently available field data including well information. This presentation will include a case history of a major carbonate oil field demonstrating how this value was estimated before a major reprocessing project and how this value is being achieved. This field contains over 1000 wellbore penetrations. A 3D seismic survey was acquired over the field during 2001–2002, but the reservoir development team believed that these data to date had added limited value. The motivation for evaluating the potential for further investment in seismic data was a multi-billion-dollar field-redevelopment plan. The Value of Information (VOI) exercise to justify a seismic project began with an evaluation of technical issues that limited the use of existing seismic data. Through a targeted fast track reprocessing effort it was determined that the existing survey had been designed and acquired adequately, and that the deficiencies in the dataset at the reservoir level are primarily caused by near-surface and overburden effects. The first-order impact is that mapped seismic surfaces exhibit a "roughness" primarily from the overlying "non-geologic" noise. There was concern that many subtle faults interpreted at the reservoir level could be "non-geologic" artifacts which resulted in reluctance to incorporate these into the reservoir model. Amplitude balancing issues in the original data precluded quantitative assessments such as porosity prediction. The targeted reprocessing also verified that existing algorithms and traditional workflows alone were insufficient to resolve the technical issues. Working with the reservoir development team the key business drivers for reprocessing were identified as follows: Increase individual well productivity and recoveryImage and define new opportunities in current poor-data areasSave on well cost by preventing re-drillsImprove overall field development plan Specific expected value metrics and risks were assigned to the above objectives and a VOI assessment was completed. It was estimated that successfully achieving the above business objectives would result in a potential value at least 15 times the cost of the reprocessing. This resulted in management approval of the full field reprocessing. Following completion of the seismic reprocessing, the project team objectively assessed whether the technical criteria had been achieved and if the business criteria will be achieved. In both cases the team determined that value metrics will be met. The reprocessing has impacted drill-wells as well as field development planning. In addition, the reprocessed seismic data will produce additional potential value as a result of opportunities not recognized at the start of the project.

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.