Successful Application of 3-D Seismic Technology on Middle Cretaceous Carbonate Reservoir—Significant OIP Increasing

Abstract

Abstract 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.