Tar Origin and Prediction in an Offshore Oil-Field Abu Dhabi

Abstract

This study shows the successful integration of analyses performed in an oil field offshore Abu Dhabi to explain the origin and distribution of a Tar zone. These analyses were performed at different scales and led us to propose a hypothesis that helps to optimize well placement and reservoir development. The study was executed in five overlapping phases starting with a geochemical characterization (Tegelaar et al., 2021) to describe the nature of the Tar found in the field using rock and fluid samples. The second phase was a structural analysis (Needham., 2021) focused on understanding the chronology and role of the faults and structural deformation on the migration process and Tar deposition. In parallel, detailed petrographic analyses (Rossi., 2021) were performed to establish the Diagenetic and oil charge history; fluid inclusion data were of key importance to the final conclusions. Petroleum systems modelling (Grimmer et al., 2022) was performed using input from the preceding phases to validate concepts. Finally, an integration phase was performed to bring together ideas from each component and conclude with a unique description for the tar distribution and corresponding uncertainty. There are two types of Tar found in an Upper Jurassic Formation of this offshore Abu Dhabi field. Tar Type 1 occurs as massive or patchy and mainly in the Mid-Lower reservoir, commonly in coarser grained lithology where it occludes porosity. Tar Type 2 (stylobitumen) occurs in non-reservoir facies (mainly micrite and stylolite) in the Lower Reservoir. The study highlighted that the field was exposed to a paleo-temperature peak higher than the current reservoir temperature that transformed both types of tar into pyrobitumen, a solid carbon form with no potential for exploitation. Predicting the distribution of Tar type 1 is important to the field development as it occurs within the producing zone. The proposed concept of flocculation indicates that it was created mainly during Eocene times. Two conceptual models where presented, both leading to similar predicted tar distributions that allow the optimization of future well locations. The study proposed a new description of the tar found in this field offshore Abu Dhabi and a novel concept that has already proven valuable in the well planning process of the wells drilled since the study was completed.