Integrated Study of Tar Mat Formation Through Structural Restoration and Basin Modelling in an Oil Field Offshore Abu Dhabi, UAE and Qatar

Abstract

A Tar Mat Zone (TMZ) is present in the Upper Jurassic reservoir in a studied field; these features are commonly observed in other fields in the Middle East. Since the tar mat is widely distributed in the field and acts as a pressure barrier, understanding of its formation and distribution is important for field development. To investigate the structural development history of the TMZ, tar formation mechanism and its distribution, integrated structural restoration and basin modelling studies were conducted. A total of eleven horizons were used for structural restoration to constrain the timing of the reservoir and TMZ geometry. A total of fifteen 2D lines were utilized to build a pseudo-3D structural restoration and to analyze the restoration results. In addition, a basin modelling study was conducted to confirm that the timing of tar mat formation inferred from the structural restoration study to ensure a consistent interpretation in terms of thermal history and hydrocarbon migration timing. The timing of the onset of folding of the reservoir was interpreted to be between 150 Ma and 120 Ma and for TMZ between 120 Ma and 108 Ma, which is earlier than estimates from simple horizon flattening. In addition, the current TMZ geometry can be reconstructed based on the results of the structural restoration. Then the reconstructed TMZ geometry can be utilized as a trend to estimate the TMZ surface, especially in areas without wells. The basin modelling results indicate that the source rock (Oxfordian to Kimmeridgian) to the southeast of the field became oil mature during the Early Cretaceous and oil expulsion commenced during the Aptian. Importantly, this is synchronous with or later than the trap formation in the field defined by the structural restoration study, enabling hydrocarbons to be trapped in the rapidly growing structure. Slowing of oil expulsion rate from 116 Ma to 92 Ma coincided with the onset temperature of thermochemical sulphate reduction (TSR) of anhydrite with hydrocarbons in the reservoir. It is likely that TMZ was formed by the TSR-related process of hydrocarbon alteration and sour gas production on a paleo-oil water contact during this period. This integrated approach revealed not only the geometry of TMZ distribution from evaluating the structural evolution of the field, but also the timing and mechanism of TMZ formation from the thermal history, which complemented each other to provide a more robust, reliable, and reasonable interpretation.