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Abstract: Imaging of the Valhall Field, Norway, Using 4-C Seismic Data

Rodriguez-Suarez, Carlos; Robert R. Stewart; Xinxiang Li and John C. Bancroft - The University of Calgary

A 2-D seismic line using 4-C receivers (a 3-C geophone and a hydrophone) laid on the sea bottom was acquired over the Valhall Field, offshore Norway¹. The field, operated by Amoco Norway Oil Co. and its partners, is located in the southernmost part of the Norwegian North Sea. Water depth in the area is about 70 m. The main reservoir is a redeposited chalk of Maastrichtian age with porosity in excess of 40%, permeability between 2 to 15 mD, and abrupt thickness variation (O to 80 m). The trap is structural /stratigraphic and the reservoir depth around 2,400 m².

The quality of conventional seismic data is poor because the overburden layers (Tertiary marine shales) are highly gas charged, causing scattering and attenuation of P-waves. Converted (P-S) waves were then used to obtain a better image. As S-waves do not propagate in fluids, a cable with eight receiver units was fixed on the sea floor, and a source vessel traversed directly overhead and parallel to the cable. Then the cable was moved to a new position, and a new line was acquired¹.

The vertical component of the geophone and the hydrophone gave both a chaotic or reflection-free zone at the middle of the section and an uninterpretable image for the target (Fig. 1). Observations in published data from Valhall¹,² permit us to conclude that the strong reflection at 2.7 s is related to the top of the chalk. A pull-down effect is observed in the central part of the section, around 1.5 s.

Four processing flows were applied to the radial receiver component: 1) conventional CDP processing (for possible P-S-S waves from P-S conversion at the sea bottom), 2) common conversion point (CCP) asymptotic approximation binning algorithm based on a Vp/Vs ratio of 2.0, 3) converted wave (P-S) Previous HitDMONext Hit, and 4) equivalent offset Previous HitmigrationNext Hit (EOM). The last method, developed by CREWES, creates an intermediate pre-stack data volume that is sorted into common scatter point (CSP) gathers, which provide Previous HitmigrationTop velocity information.

The final result for the radial component processed for P-S events was quite good: a continuous and interpretable image of the reservoir target was obtained (Fig. 2). This section was obtained using the EOM technique mentioned above, with a Vp/Vs ratio of 2.5. A largely continuous reflection, with relatively high frequency, can be followed between 5.0 and 5.5 s most of the line. It probably relates to the top of the chalk¹,² . Some normal faulting seems to be present, which is consistent with the believed tectonic evolution of this area². The quality of the line is good, even if its central part has somewhat less continuity than on the structure?s flanks. This may be due to the P-wave downgoing path inside the gas rich region and/or some fault-imaging problems.

Amplitude spectra show the presence of very strong notches in all components and at the hydrophone. These notches, which are more harmful for the radial component data (as it has a lower frequency content), are likely caused by energy reverberation in the water layer. An algorithm is being developed to attenuate the reverberation using pressure (hydrophone) and velocity (geophone) measurements.

We are grateful to Amoco Norway and their partners in the Valhall license for the permission to present this data.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil