Comparing AVA character between well derived 3D synthetic models and real seismic together with extended elastic impedance studies of the Agat Formation in the Northern Viking Graben

Abstract

This thesis provides the first detailed approach to understanding and comparing amplitude variation with angle (AVA) character between well derived 3D synthetic models and real seismic of the Agat sands in wells 35/3-1, 35/3-2, 35/3-4 and 35/3-7 S in the Northern North Sea. It also investigates the sensitivity in AVA behaviour to varying physical parameters such as: porosity, P-wave velocity, mineral bulk modulus and thickness. These were plotted against hydrocarbon saturations where key relationships were observed. Altogether a clear Class IV AVA Top Agat response prevailed when comparing well derived 3D synthetic models against real seismic data, which remained consistent for all fluid substituted brine, oil and gas scenarios. Furthermore, brine saturated sands characteristically showed little to no variation in AVA effect when physical parameters were varied, whereas gas saturated sands were significantly softer with a greater sensitivity to AVA changes. Minor reductions in the P-wave velocity of the overlying shale by as little as 7.5% revealed polarity reversals whereas as a similar increase changed the AVA class from IV to III. Moreover, enhanced porosities created a more prominent AVA class IV response with the same also being true for a reduced mineral bulk modulus scenario or in other words a shaley sand scenario. Extended Elastic Impedance (EEI) volumes were generated and proved to be a predictive tool for extrapolating lithology and fluid effects away from well data. Although sand bodies were identified as relatively isolated, upside potential was identified to the southwest of well 35/3-7 S where undrilled fluid and lithology anomalies prevailed.