Three-component OBS-data processing for lithology and fluid prediction in the mid-Norway margin, NE Atlantic

Abstract

In 1992, acomprehensive three-component ocean bottom seismic survey was performed in the central and northern area of the Vøring Basin, offshore mid-Norway, NE Atlantic. An important part of the data acquisition program consisted of a local survey with 20 Ocean Bottom Seismographs (OBS) dropped at approximately 200 m interval in 1300 m water depth. The main purpose of the local survey was to acquire densely sampled P- and S-wave reflection data above a seismic flatspot anomaly observed earlier, in order to more accurately predict if hydrocarbons could be related to it. The conventional reflection data processing methods applied to the vertical components included predictive deconvolution in order to attenuate low frequency ringing, near offset mute and a series of constant velocity stacks in order to obtain the optimal velocity function. The final result is a “trouser” shaped, high resolution V_Z stacked section with minor influence of water multiples. The inline (V_x ) component contains no strong multiples, and extensive near trace muting was hence not necessary to apply for this component. Velocity analysis together with ray-tracing modelling indicate that P-S-converted shear waves (reflections) represent the dominant mode. The results of the interpretation and modelling indicated a V_p/V_s-ratio of approximately 2.6 in the overburden, which suggests domination of partly unconsolidated shale, while the V_p/V_s-ratio in the assumed reservoir was approximately 1.8, which indicates a more sand dominated facies. Outside the flatspot area a higher V_p/V_s-ratio ratio (approximately 2.0) was estimated, indicating that hydrocarbons could be present in the assumed reservoir.