Distribution and Origin of a Tar mat in the S. Field (Abu Dhabi, U.A.E.)

Abstract

Abstract This study has been conducted to investigate the distribution and origin of bitumen (tar mat) occurring in the Thamarna Zone B reservoir (Upper Kharaib Fm., Lower Cretaceous) of the S. Field in Abu Dhabi. The main objective was to assess the continuity of the bitumen rich interval and its possible role as an intra-reservoir permeability barrier potentially responsible for the pressure anomalies recognized at the field scale during oil production. Detection of bitumen was performed on 68 wells using petrophysical data, Rock-Eval and electrical-log derived methods. The result provides a realistic 3D image of the tar mat distribution at the field scale. Biomarker analysis suggests that there is no difference in origin nor maturity between the tar mat and the overlying oil. Comparison between bitumen location, sedimentological facies, petrophysical properties, field structural history, present-day and paleo-OWCs were used to decipher the process and timing of the tar mat deposition. Results, Observations and Conclusions A tar mat (4 to 23 ft thick) is present in the crestal part of the field and is located at the base of Thamarna Zone B, increasing the role of vertical permeability barrier formed by the underlying "Dense Limestone". The bitumen is clearly associated with the low porosity zone located at the base of the Thamarna B and seems to be never present in the water leg nor related to the present day or paleo-OWCs. The formation of the tar mat is probably post-Dammam (Eocene) in age and is likely to be due to flowing down of a gravity segregated heavy fluid which is eventually trapped in the low porosity base of the reservoir. Applications The results demonstrate that the bitumen rich interval is not responsible for the pressure drop during production and will not play the role of permeability barrier between the central and northern part of the field where gas injection is planned. The provided 3 D distribution of the bitumen volume allows a better estimate of the OIP (oil in place). Technical Contributions