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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.

This paper summarizes some of the seismic imaging challenges associated with a producing field in the Arabian Gulf. Overburden effects and their impact on the seismic data quality are discussed. We describe an approach which partially compensates for these effects in the time domain, thereby increasing the utility of this seismic dataset.

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.

The objective of this paper is to overcome the asphaltene risk evaluation usually conducted snapshot basis. We evaluate the temporal change in the asphaltene risks as gas injection proceeds. In reservoirs under gas injection, in-situ fluid component gradually changes by multiple contacting with the injection gas. Those compositional changes affect asphaltene stability and bring difficulty into the risk predictions by asphaltene models. This study aims to reduce the risk uncertainty depending on operational condition changes. Periodical upgrading of asphaltene model is essential for understanding the time-depending changes of asphaltene risks. In the previous study (Yonebayashi et al. 2011), the asphaltene risk was evaluated for an offshore oil field in the Arabian Gulf in 2008 by use of cubic-plus-association equation of state (CPA-EoS) models on the basis of all available date at the time. After the previous study, additional experimental data was accumulated forthe future gas injection plan. Then, the update study was performed by incorporating those newly collected data. Subsequently, both findings in the past and the present were compared with each other. According to the previous study recommendation, additional asphaltene laboratory studies were conducted on the basis of newly collected samples. All Asphaltene On-set Pressure (AOP) detected from the new samples were higher than those of the previous study. Especially, a large difference was observed from the past/present results of the lower reservoir's AOPs even though samples collected from the identical well. Asphaltene precipitation risk was observed to increase largely because the new AOP was detected at the reservoir temperature while no AOPs detected in the previous study. The difference might be occurred by saturation pressure increase. Then, the numerical asphaltene models were revised, and accordingly, the asphaltene risk estimation were updated higher in the lower reservoir. For the upper reservoir, the past/present AOPs were slightly changed to become higher. The reference sample fluids were collected from two different wells showing minor difference of asphaltene contents. Those variations might be caused by geological heterogeneity that could affect on fluid maturity. Then, the risk rating was updated to be slight higher, too. In this paper, through the comparison between the previous and current studies, it was pointed out the importance of regular monitoring asphaltene risks. This study provides the valuable findings of time-lapse evaluation of asphaltene precipitation risks for reservoir under gas injection. The evaluations currently conducted in the industry are snapshots of instantaneous risks. Through entire field life, the risks have varied depending on operating conditions. This study argued the risk-change in the unique field by the identical workflow but using each representative data collected at different times. Finally, this study demonstrated the importance of time-depending fluid dynamics.