The effects of anisotropy on AVO behavoir on shallow reflectors from Sleipner
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AVO analysis is an important tool in discriminating between oil, gas and brine saturated rocks in a given reservoir unit. Although, to use AVO analysis can be challenging as there are many pitfalls connected to different effects that may have an influence on the result, e.g anisotropic effects. Often isotropic conditions are assumed, but in general the Earth is anisotropic. By understanding what causes anisotropy and the effects anisotropy has on an AVO analysis, better interpretations can be made in the future and pitfalls avoided. Since 1996, a huge amount of CO2 has been injected into the Utsira sandstone on the Sleipner Øst field in the North Sea and the gas plume has been carefully monitored. Six time-lapse surveys has been conducted from 1994 to 2006, where the 1994 survey is the baseline from before the injection started. The Utsira formation lies within the Nordland Gp. and the thickness may range up to more than 300 meters in certain areas. Within the sandstone, several layers of shale exist and comprises as permeable barriers. The overlying cap rock is what is referred to as the Nordland shales and this is several hundred meters thick. A certain degree of anisotropy within these layers are expected, especially in the area of the injected CO2,as the increased pore pressure and the fluid substitution lead to increased seismic anisotropy. In this Master's thesis the anisotropic parameters δ and ε have been calculated for the three available wells from the Sleipner Øst field. For the primary well 15/9-13 they were calculated to be δ = 0.0066 and ε = 0.0021 for the Utsira sandstone, and δ = 0.0645 and ε = 0.0206 in the overlying shale unit. The uncertainties related to the calculation of these parameters are the P- and S-wave velocities in the Utsira formation and the volumetric shale parameter. Afterwards an AVO analysis were conducted on the synthetic models from the wells for both the isotropic and anisotropic case, and on the two seismic lines provided from the 2006 survey. The AVO analysis was performed by studying the amplitude difference and AVO response for the top Utsira horizon. The seismic analysis was then compared to the synthetic response both for the isotropic and anisotropic case, to see which was the better match.