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

Abstract

In 1992, a comprehensive 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.

Acquisition, processing and analysis of densely sampled P- and S-wave OBS-data on the mid-Norwegian Margin, NE Atlantic

“…The amount of anisotropy was estimated by Digranes et al (1996) to be 5%. The direction of fast direction corresponds to other measurements of the present-day stress-field in the area, pointing at slight compression from the mid-Atlantic spreading ridge as its main regional cause (Bungum et al, 1990). The crustal structure immediately to the southwest and northeast of the study area has been mapped by Mjelde et al (1997a) and Mjelde et al (1998), respectively.…”

“…Conventional multichannel processing has been successfully performed on the local dataset, which was acquired by use of the high-frequency source and twenty OBSs (Berg et al, 2001; see Section 2). The shot-and receiver spacing were 50 and 200 m, respectively.…”

“…The experiment was carried out in the period August 22 to September 24 1992 by use of R/V Håkon Mosby belonging to the University of Bergen. The experiment was divided in four parts; a regional survey along 7 profiles using 27 OBSs (Mjelde et al, 1997a), a local survey on one structure employing 20 OBSs deployed a sec-ond time (Berg et al, 2001), a circle-shooting around one of the OBSs deployed in the regional survey, and a semi-local experiment where 25 OBSs were employed a third time. This paper focuses on the data acquired during the semi-local survey.…”

“…The location of the OBSs on the seafloor were determined to within 20 m uncertainty by use of acoustic triangulation (Shiobara et al, 1997). The reader is refered to Berg et al (2001) for details concerning the multichannel processing of the local dataset. The quality of the horizontal component stack allowed interpretation of S-wave reflectors on hydrocarbon reservoir scale to be made.…”