Derisking Shallow Gas as Exploration Target by Seismic Characterisation

Abstract

The Netherlands is the first country in the North Sea area in which shallow gas is developed and has proven to be a valuable resource. Amongst 3 successfully producing fields and 5 more proven accumulations, EBN has identified more than 150 additional leads from shallow seismic anomalies (bright spots). The play consists of relatively clean reservoir sands deposited in a Cenozoic fluvio-deltaic system. Seals are formed by shales. Whereas most of the gas samples reveal a biogenic origin, gas chimneys and associated faults might indicate that a thermogenic origin is well possible, as is the scenario of secondary biogenic gas. While flat spots, velocity pull-down effects, attenuation, gas chimneys and pockmarks at the seabed can be recognized in relation to the amplitude anomalies, gas saturation remains the largest subsurface uncertainty due to the non-linear behaviour of seismic P-wave velocity with saturation. EBN has developed a system in which the individual leads are ranked based on their geometrical and seismic characteristics. This paper presents the results of the modelling work that has been done aiming to further explore the relation of the bright spots to gas saturation, including the analysis of seismic data in the pre-stack domain (AVO).