Tuning Effect on Near and Far Offset Stacks: Mapping the Thickness Beneath Seismic Resolution of a Shallow Thin Layer in Block 2/4.

Abstract

Analysis of high-amplitude anomalies (bright spots) are the most used basic method of identifying hydrocarbon accumulations in seismic signals. When this method is applied to thin-bedded strata, the result can be misleading. The interference between the closely spaced reflectors can cause destructive or constructive changes on the initial amplitude, commonly referred to as the tuning effect. The main problem in such settings is to distinguish if the amplitudes are affected by changes in lithology or saturation, or if the changes can be related to tuning effects. In this thesis, the interpretation of a shallow thin bed in Block 2/4 in the Central Graben, North Sea, shows high-amplitude anomalies in association with two crossing ice scours. The anomalies are related to trapped gas against ice scours. With this assumption, it is believed that the gas forms a thinning wedge. It is also considered that the thickness of the layer is beneath the seismic resolution, with signals from the top and bottom of the gas saturated layer interfering in a constructive and destructive manner.The aim of this thesis is to measure the thickness of the layer, as well as investigating to what extent tuning influences the seismic signals. RMS-amplitude maps have been extracted from both near and far offset stacks. AVO analysis based on near and far offset stacked data have been performed. Due to the assumption about thinning, it was important to investigate if the response was a gas or a tuning effect. Tuning curves for different saturations and offset have been made and the relationships from the curves are used for analyzing the amplitude change across one high amplitude triangular pocket.The shape of the ice scour is not determined properly, thus three models are discussed. The AVO-analysis shows the same response in the entire layer. The interpretation of the anti tuning effect gave unreliable results. The transition between the amplitudes on the left and right of the tuning thickness should be smooth in order to be considerable. It is not likely that the thickness jumps from 6 to 14 meters, without having signs of ice scours nearby. Therefore, the maximum thickness of the gas saturated layer is suggested to be 9.5 m. It is assumed that the constructive interference on the edges of the pocked dims the initial high-amplitudes caused by gas.