Geologically Guided Prestack Depth Imaging - a Case History From Greater Burgan, Kuwait

Abstract

Abstract Prestack depth migration (PSDM) can provide superior seismic volumes even in areas of low relief structures. A pilot PSDM study over the Greater Burgan oilfield in Kuwait demonstrated not only the uplift versus prestack time migration (PSTM), but also pointed to areas of further improvement in the velocity model building workflow. A full field (FF) study was subsequently performed, and incorporated the lessons learned from the pilot. The main area for enhancement was the integration of geological information into the velocity model. This included near-surface velocity characterization and incorporation of existing 3D log-based velocity models into the initial PSDM model, plus implicit geological constraints during the model updating workflow. The results were a more geologically plausible velocity model with enhanced imaging versus both the legacy PSTM and pilot PSDM volumes, establishing the FF PSDM volume as the main seismic volume for interpretation, well planning and reservoir characterization for the Greater Burgan field. Introduction The Greater Burgan oilfield is located in southeast Kuwait and is the main production field for Kuwait Oil Company (KOC), covering an area of approximately 850 km2. The geology is well documented (Carman, 1996), and is expressed as a broad, low relief anticlinal dome draped over a basement horst structure. The primary producing reservoirs are the Cretaceous-era Wara and Burgan sands (Figure 1), which are observed as a seismically quiet section with low acoustic impedance on the well logs. Deeper targets also exist in the Jurassic limestone reservoirs (e.g., Marrat). A 3D seismic survey was acquired in 1997–98 covering an area of 1372 km2 using parameters commonly used at that time. This covered the complete Greater Burgan oilfield. Offset and azimuth distribution was mostly restricted to the inline direction (low aspect ratio survey geometry), which, in turn, imposes limitations on the utilization of full-azimuth information in the data processing and imaging sequence. In 2010–11, a small test area was selected for a 3D PSDM pilot study. The aim of the pilot was to provide improved imaging compared to recent PSTM and a better input to ongoing horizontal and multilateral well programs. Particular attention was placed on enhancing the section between the primary reservoirs, the intra-reservoir clastic sequences, and imaging the minor faults that control compartmentalization. Key challenges included the near-surface complexity, characterized by rapid vertical and lateral velocity heterogeneity down to the Rus horizon (the shallowest mapped horizon in the field, between 200 and 500 m in depth, and consisting of high-velocity anhydrites), noise and multiple contamination of the legacy seismic data set, and reliable depth control of low relief structures across the pilot area.