Exploring in the Slyne Basin: a geophysical challenge

Abstract

AbstractThe Slyne Basin lies c. 60 km offshore west of Ireland, in water depths of 200–500 m. It consists of three asymmetric half-graben that are separated by complex structural transfer zones. Sporadic exploration in the basin over the last 20 years has resulted in the drilling of four exploration wells, which have yielded one gas discovery. Well 18/20-1 (Corrib) successfully tested a faulted anticlinal structure and encountered gas in the Triassic Sherwood Sandstone Formation. Although a number of other potential hydrocarbon traps have been identified in the Slyne Basin, the poor quality of the seismic data, plus the presence of complex transfer zones, has generated considerable uncertainty with respect to the correlation of seismic markers. A primary control on the seismic data quality is the presence of near-sea-bed, high-velocity, Tertiary volcanic and Cretaceous chalk layers. These result in very strong and long multiple trains, energy scattering, mode conversion and attenuation. Studies suggest that improved signal penetration can be achieved when the seismic acquisition is focused on the low-frequency end of the spectrum. However, predictive multiple attenuation has proved ineffective because of the complex nature of the multiple generators. An approach based on detailed velocity analysis and the judicious parameterization of more than one pass of Radon demultiple has yielded good results. This approach, coupled with 3D acquisition and processing with its inherent increase in signal-to-noise ratio, has led to a dramatic improvement in the seismic data quality in the Corrib area.