Abstract

The Donna Terrace West prospect is located offshore of Nordland, Norway. Hydrocarbon-bearing sands are found in the Cretaceous and Jurassic zones between 2 seconds and 5 seconds on time-processed seismic-data. A secondary target lies between 1 second and 2 seconds in the Tertiary. Seismic data in this area show poor reflectivity with a low signal to noise ratio in the lower Tertiary and Cretaceous zones. Timebased processing and imaging are used to determine the structure and fluid content of sands in this area. This strategy is most successful when the lateral velocity-variation in seismic velocity is smooth. At Donna Terrace West, however, the sea floor represents the top of a glacial till layer, and strong velocity-contrasts are present as a result of the topography of this surface.

By placing a component of depth imaging before time processing, wave-equation datuming is used to mitigate the effects of the till layer. First, a model of the layer is constructed, and the recorded wavefields are propagated to a flat datum at the model's base. A single velocity (replacement velocity) is chosen, and the data are propagated back to the recording surface. The resulting reflection events show improved continuity, semblance in velocity analysis is enhanced, and multiples and primaries have better separation in Radon space.

Introduction

The Donna West Terrace prospect is located offshore of Nordland, Norway (Figure 1). Seismic data in this area show poor reflectivity with a low signal to noise ratio in the lower Tertiary and Cretaceous zones. Hydrocarbon-bearing sands are found in the Cretaceous and Jurassic zones between 2s and 5s on time-processed seismic-data. A secondary target lies between 1s and 2s in the Tertiary.

The top of the geologic column is a layer of glacial till (Figure 2). It's surface is highly reflective and causes strong multiple reflections (Figure 3a). Also, this surface is rugose and causes a loss of continuity in underlying reflections (Figure 3b). Velocity in the till layer is roughly constant at 2100m/s (Figure 4a). Reliable processing-sequences for demultiple have been developed by a number of seismic contractors. These are usually based on multiple prediction/subtraction and Radon filters. In this paper, the problem of reflection discontinuity due to the rugose till-layer is addressed.

Time processing and imaging can be used to determine the structure and fluid content of subsurface media and is most successful if lateral velocity-variation is smooth. In areas of strong variation, time processing may be inadequate, and depth processing and imaging must be contemplated. At Donna West Terrace, strong velocitycontrasts are thought to be confined to the till layer, with fairly constant lateral-velocity in the media below. Time processing is preferred to depth processing because of cost, and the deleterious effects of the till layer are accounted for after processing.

By placing a component of depth imaging before time processing (Yilmaz and Lucas, 1986), wave-equation datuming (Berryhill, 1979) can be used to mitigate the effects of strongly variable surface-layers like the till layer in Donna West Terrace.

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