Abstract

In this paper we present the results from an anisotropic converted-wave (C-wave) prestack time migration (PSTM) of a 2D multicomponent (4-C) line over Lomond Field. This is a gas-condensate field 233 km east of Aberdeen, in the Central North Sea, Block 23/21, close to the border between the UK and the Norwegian sectors, on the east flank of the Central Graben. The geologic setting is excellent for testing C-wave imaging tools; the quite complex structure is dominated by a salt-induced anticline (fractured on top) and a large gas cloud (leaked from the fractured reservoir). In addition, a thick overburden of laminated shales is likely to produce polar anisotropic (VTI) effects. The large gas cloud was the main motivation for C-wave acquisition. In fact, the imaging results obtained using P-waves (Figure 1) include a large zone of amplitude dimming, circled in black, and the push-down effects on the above events. This cloud partially obscures the area of the reservoir, which is composed of Paleocene sandstones of the Forties Formation, 200 m in thickness, and sealed by mudstones from the Rogaland Group. The presence of polar (VTI) anisotropy was confirmed by observations of angular-dependent velocity in deviated sonic logs.

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