The capabilities and challenges of the seismic method in chalk exploration

Abstract

The success of seismic methods to detect and predict porosity within the chalk fields of the Norwegian North Sea, coupled with the awareness that chalk can behave as both reservoir and seal, has led to the deliberate search for stratigraphically trapped hydrocarbons within the chalk. This effort has relied on seismic methods, such as post-stack inversion and amplitude versus offset (AVO) techniques, to identify porous reworked chalk zones isolated within low porosity pelagic chalk. The excellent conditions for seismic acquisition and processing, slowly varying regional geology, and high correlation between porosity and seismic velocities and density produce reliable porosity predictions far from well control for all inversion techniques. However, the high exploration failure rate for stratigraphic tests demonstrates that migration and seal risk is higher in chalk stratigraphic prospects, and some method of predicting hydrocarbon presence is beneficial. Analysis of available velocity measurements demonstrates that porosity and fluid variations in chalk can be modelled best with the upper bound of Hashin and Shtrikmann (1963), modified with the critical porosity of Nur (1992). This model leads to the conclusion that chalk velocities are sensitive to pore fluid and that hydrocarbon effects may be observable on seismic data. Unfortunately, hydrocarbon effects overprint the larger porosity effects, making resolution of the hydrocarbons signal extremely difficult in practice. An exploration case study demonstrates that the current inability to distinguish the effect of hydrocarbons from that of porosity, in the presence of noise and other complicating factors, seriously limits the ability of seismic methods to directly detect hydrocarbons under normal exploration conditions.