Improved seismic images through full-azimuth depth migration: updating the seismic geological model of an oil field in the pre-neogene base of the Pannonian Basin

Abstract

A seismic survey was conducted in a production oilfield located in Serbia, confined to the Pre-Neogene (Paleozoic) base of the Pannonian basin. It was assumed that additional significant unrecovered residual reserves still exist in this oil field, as well as additional similar undiscovered reservoirs. The further characterization of the existing reservoirs, and the identification and characterization of the new ones, required the implementation of advanced seismic imaging technology. Hence, a new project was designed composed of the following steps: Obtaining the highest possible seismic resolution in the area, and creating an updated, high-definition subsurface model that includes the structural complexities of the geological layers and the azimuthal anisotropic effects (e.g., fracture systems), especially within the target layers. This enables the identification and characterization of the target productive zones, making it possible to accurately design and plan the well placement. A modern, full-azimuth seismic survey was performed with a fairly regular distribution of the source-receiver offsets and azimuths. The dominant fold (number of traces per shot) was about 120. The seismic sources consisted of groups of vibrators with a linear sweep signal, where the frequency range was 6 to 96 Hz and the time duration 15 seconds. Emerson’s EarthStudy 360 full-azimuth angle domain imaging system (Koren and Ravve, 2011) was chosen to facilitate the above-mentioned tasks. This is an advanced subsurface imaging system operating directly in the Local Angle Domain (LAD). The high-resolution images with the unique full-azimuth directional and reflection angle common image gathers obtained by this seismic migration technology make it possible to better define the structural subsurface model and furthermore, to detect fine interlayer fracture systems at the target areas. Both regional faults and low-amplitude sub-seismic faults (fracture indicators at these regions) were mapped. The main imaging characteristics were correlated with existing production wells in the area