Simultaneous AVO inversion in PL460 in the Norwegian Northe Sea

Abstract

This thesis treats data conditioning of pre-stack seismic data and simultaneous AVO inversion. The demands of pre-stack reservoir characterization are increasing, and the prediction of lithology and fluid from seismic are becoming important steps in hydrocarbon prospect evaluations. Pre-stack seismic inversion gives access to elastic parameters of the sub-surface, which, with favorable formation properties, can discriminate between lithology and fluids with good fidelity.

The study area is located partly in block 24/3, 25/1 and 25/2 in the Norwegian North Sea. Uplift to the west in connection with the opening of the Atlantic Ocean led to a significant influx of coarse clastic sediments into the North Sea Basin during the Eocene. The general pelagic shale deposition in the area was interrupted by several episodes of sub-marine fan deposition in response to relative changes in sea level and uplift of the hinterland. These sub-marine fans have a good potential for accumulation of hydrocarbons, and large gas fields such as the Frigg and Odin fields are located in the study area.

The seismic inversion process is implemented by the use of a simultaneous AVO inversion algorithm. The convolution step in the algorithm assumes plane wave propagation across the boundaries of horizontally homogeneous layers and does not take into account geometrical divergence, non-elastic absorption, wavelet dispersion, residual moveout and multiple reflections. These issues are addressed in a data pre-conditioning step. It is evident that the data conditioning play a major role in estimating subsurface parameters with good confidence.

The inverted data are quality tested in several different ways, both at the wells and against measured seismic, to insure high inversion confidence. Quantitative analyses are then carried out to test the ability to discriminate between oilsand and brinesand in an Eocene submarine fan. The analyses suggest that the inverted data can be employed in lithology and fluid prediction of Eocene targets in future exploration in block 24/4, 25/1 and 25/2.