Geometry of Porosity Break Plane Associated with Bitumen Zone and Timing of its Generation in an Upper Jurassic Reservoir, Offshore Abu Dhabi, UAE

Abstract

Abstract An Upper Jurassic reservoir in "Field A" is characterized by existence of bitumen (tar mat) zone intersecting sublayers in the reservoir and porosity deterioration is observed in the bitumen zone and below bitumen zone. It is obvious that porosity deteriorates significantly at top of the bitumen (Porosity Break Plane: PBP) on core materials and open hole log interpretation, in addition, no flow contribution below PBP was confirmed by production logging data due to its tight nature. Therefore, it is very important to define PBP precisely in order to estimate hydrocarbon reserves and optimize future infill well locations. The thickness of porous interval between top of reservoir and PBP indicates thinning toward the flank area and finally pinching-out in the flank area. The PBP and bitumen have been considered that they were possibly formed near paleo-OWC and the present curved shape of PBP can be attributed structural growth of the field after formation of them by deeper salt swelling. PBP is confirmed at crest and flank wells in different stratigraphic position depending on their structural locations. Based on the detailed structural interpretation of the top reservoir using recently acquired 3D seismic data, the relationship between the top reservoir depth structure and PBP identified in the wells became clear. It is helpful for the PBP mapping and understanding of the bitumen distribution. 3D geological model incorporating PBP was constructed based on the above observation and new structure map derived from 3D seismic interpretation. This workflow helps to improve predictive capability on PBP mapping in non-well control. In addition, based on the structural interpretation of overburden horizons on the 3D seismic data and newly interpreted PBP, the history of structural development and timing of PBP generation were estimated. Based on the back stripping of overburden formations, PBP seems to be flatten in the Shilaif age. This paper discusses the reasonability of this timing.