Velocity and Reflectivity variations of Overpressured Jurassic Sandstones in the Norwegian Sea

Abstract

The Garn Formation is one of the reservoir sands amongst the Jurassic Sandstones in the Norwegian-Sea. The pore-pressure and its influence on the rock properties within the Haltenbanken province in the Norwegian-Sea has been studied very well over the years. This work uses the Eberhart-Phillips model to study the stress sensitivity and its impact on the velocity and reflectivity variations within brine-saturated Sandstone within the Haltenbanken province in the Norwegian-Sea using Garn Formation as the target Formation. Dataset of six different wells at different in-situ differential stress conditions was selected for this work. The validity of the Eberhart-Phillips model in predicting the velocity variations as a factor of porosity, clay content and the differential (effective) stress was investigated. The results show that the standard Eberhart-Phillips is a good model for brine saturated Garn Formation within the selected Wells and can potentially predict the velocity variations within the brine saturated Garn Formation in this study. AVO modelling of the single interface 1D and the layered 1D model was used to look at the reflectivity variations with angles (offset) of the brine saturated Garn Formation in one of the Wells selected (Well 6505/12-1). The essence was to investigate and differentiate between lithology and overpressure effects on AVO response. The single interface 1D AVO modelling predicts that the effects of lithology was more than the effect of overpressure on reflectivity at the near (0° to 30°) and far (30° to 40°) angles (offsets) while the effects of overpressure on reflectivity could be possibly detected at the ultra-far (above 40°) angles (offsets). In the layered 1D AVO modelling, the lithology effect on the AVO response (reflectivity with offset) was more than the overpressure effect on reflectivity too. Hence, it is difficult to detect the effects of overpressure within the brine-saturated Garn Formation.