Velocity-model building with a good understanding of anisotropy is one of the important elements in a prestack depth migration (PSDM) project. During a recently conducted PSDM project, we observed that (1) the application of negative delta/epsilon is necessary to achieve reasonable depthing and gather flatness, and (2) azimuthal variation in gather flatness exists in the interval where negative delta/epsilon are necessary. A geologic explanation of these interesting observations was necessary to justify the velocity-model-building result. Investigation of stress magnitude/orientation and theoretical anisotropy of unconsolidated sandstone under nonhydrostatic stress revealed that these observations can be explained by stress-induced anisotropy. This concept may be applicable to other fields, and the existence of azimuthal anisotropy and value of anisotropy parameters can be roughly estimated by the method described.

Faults and fractures were interpreted using attributes that were extracted from a 3-D seismic data set recorded over a Lower Cretaceous Thamama oil field in offshore Abu Dhabi, United Arab Emirates. The Thamama reservoir has good matrix porosity (frequently exceeding 20%), but poor permeability (averaging 15 mD). Because of the low permeability, faults and fractures play an important role in fluid movement in the reservoir. The combination of the similarity and dip attributes gave clear images of small-displacement fault geometry, and the orientation of subseismic faults and fractures. The study better defined faults and fractures and improved geomechanical interpretations, thus reducing the uncertainty in the preferred fluid-flow direction. Two fault systems were recognized: (1) the main NW-trending fault system with mapped fault-length often exceeding 5 km; and (2) a secondary NNE-trending system with shorter faults. The secondary system is parallel to the long axis of the elliptical domal structure of the field. Some of the main faults appear to be composed of en-echelon segments with displacement transfer between the overlapping normal faults (relay faults with relay ramps). The fault systems recognized from the seismic attributes were correlated with well data and core observations. About 13 percent of the fractures seen in cores are non-mineralized. The development of the fault systems was studied by means of clay modeling, computer simulation, and a regional tectonics review. The existing fluid-flow characteristics of individual faults and fractures in the field can be modeled using the present-day stress regime, with the maximum horizontal stress oriented north-northeast. Slip-tendency and dilation-tendency analyses simulating present-day regional stress conditions are indicators of fault and fracture transmissibility. The NNE-striking secondary fault system is parallel to the present-day maximum horizontal stress and could act as a flow conduit in the reservoir.

Current 3-D seismic data with state of the art processing and interpretation technology has revealed a clear picture of the reservoir boundaries, extension and characteristics. Consequently significant Original Oil In Place increment has been recognized in a field offshore Abu Dhabi, U.A.E., which is unusual case in this region. Several approaches were applied to achieve accurate reservoir images. Seismic sequence stratigraphy, AI inversion, waveform classification etc., were utilized to define the reservoir limit. Vertical stretch and map migration methods were applied to investigate the sensitivity to the velocity hence achieve more reliable depth structure map. Co-blending of several 3-D seismic attributes was applied to identify subtle faults and fracture swarms. New structures were identified with a great degree of accuracy through statistical analysis and sensitivity test of depth conversion, with narrow uncertainties ranges to optimize well location and field development plan. Introduction Recently, a 3-D OBC seismic survey was conducted offshore Abu Dhabi, U.A.E. in an attempt to contribute to the optimization of future drilling program and full field development plan. Large geological uncertainties have been recognized on the structure undulation and reservoir lateral extention in a field to be solved utilizing 3-D seismic data. The target reservoir is a Middle Cretaceous reefal limestone of Mishrif Formation with abrupt lithologically changes laterally and vertically, especially stratigraphically changes into a dense, non-reservoir limestone toward the west of the field. The diminishing characteristics are also associated with reduction of thickness from about 850 feet seen at the crest to 100 feet in the western flank wells. Existence of this reefal sequence is limited to this field proven by the non-existence of such reservoir distributed in an adjacent western field, therefore defining the edge of this reservoir existence is one of the major issues to be resolved in relation to Original Oil In Place(OOIP) calculation and optimum full field development. Another technical challenging is a complexity of structural configuration with small undulation since it is implied to be constructed by reef build up with structural deformation by differential compaction. Furthermore reservoir top surface is composed of Karst topography with erosional surface, which could not be captured by previous sparse 2-D seismic data and well information. Therefore, it is obvious that previous OOIP was underestimated conservatively due to the uncertainties of the structure, reservoir characteristics and structural extension. In order to solve aforementioned issues, dedicated efforts were made to interpret 3-D seismic data and obtain precise structural configuration and reservoir characteristics as well as OOIP.

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This reference is for an abstract only. A full paper was not submitted for this conference. Abstract ADMA-OPCO and ZADCO are performing an intensive 3D OBC (Ocean Bottom Cable)seismic survey offshore Abu Dhabi with the twin primary objectives of defining the Thamama reservoir in an undeveloped structure and defining the overlapping Mishrif reservoir in a currently producing field. Survey value is increased by the reservoir characterization information that can be extracted from the seismic data. The structural Thamama target has different seismic acquisition geometry requirements to those of the stratigraphic Mishrif reservoir. OBC acquisition has a huge range of potential geometries and value and costs. This paper describes the processes that were used to reconcile the technical and economic conflicts. Experience from a previous 3D OBC survey highlighted the generic problems in the area:Source-generated noiseStrong multiple interferenceAcquisition footprint New survey design had to overcome these plus deliver high-resolution data(80 Hz at target level) to attain the stratigraphic and reservoir characterization objectives within a finite budget. Team identification of key objectives was critical for the implementation of the successful survey design. Analysis and review of various modeling processes(acquisition geometry, seismic processing responses, acquisition economics)guided the team to an affordable solution that meets the geophysical objectives of a high-quality, high-resolution survey. The process has provided a methodology that will be used as a basis for evaluating of cost versus quality in future surveys.