Time-Preserving Tomography in Structure Mapping: A Case Study of a Deep Gas Reservoir, Offshore Abu Dhabi

Abstract A deep gas reservoir in an offshore field, Abu Dhabi presents a unique challenge in structure mapping due to significant misties encountered when utilizing both Pre-Stack Time Migration and Pre-Stack Depth Migration seismic data. Rapid variation of velocity in horizontal and vertical directions makes the mistie issue very challenging especially for the wells not included during the velocity model building for PSDM. To address the challenges, improve the structural interpretation and velocity model of the deep gas reservoir, a novel workflow of time-preserving tomography (TPT) has been applied. TPT is a technique that preserves the travel times of seismic events while updating the depth domain horizons and interval velocities by ray tracing. It uses a modified version of the Dix formula that accounts for the vertical and lateral variations of the velocity field. The process may need to be iterated several times until the velocity model and horizons that meet your expectations are obtained. TPT was applied to a deep gas reservoir using seismic data from a 3D survey and initial horizons and velocity models derived from well logs and conventional tomography. The updated horizons and velocity model by TPT showed better consistency with the well data and seismic reflectors than the initial models. The difference between the initial and updated velocity model reveals a polygon-shaped area with significant velocity anomalies, which corresponds to the fault system or a carbonate buildup in the reservoir. The structural mapping and velocity analysis by TPT has enhanced the understanding of the reservoir geology and heterogeneity. This paper presents the first application of TPT to a deep gas reservoir, within the offshore Abu Dhabi oilfields, demonstrating its advantages over conventional structure mapping in updating velocity models and improving the depth domain interpretation. TPT is a robust and efficient workflow that can be applied to other complex reservoirs with similar challenges. The case study highlights the method's ability to reconcile discrepancies and provide a more reliable geological framework for exploration and development strategies.