Alan D Gibbs¹

(1) Midland Valley Exploration Ltd, Glasgow, Scotland

ABSTRACT: Quantifying Uncertainty in Geological Structures

This paper describes techniques which allow a standardised procedure to be applied to faulted and folded structures enabling statistics to be calculated of balance in terms of gap and overlap along restored faults. This represents a direct measure of uncertainty in characterising the structure both at field scale and on a fault compartmental basis. The uncertainty or "error bar" arises from a combination of seismic resolution in fault zones, picking strategy and geological factors such as missing reservoir volume through mass wastage, erosion or the effect of unresolved fault damage zones and sub-seismic faulting. Fault linkages to depth are also analysed to quantify consistency throughout the mapped reservoir horizons. Knowledge of the uncertainty in fault mapping, position and linkages allows risk to be managed through both further targeted technical work and through commercial risk management techniques.

Even in well understood provinces with modern 3D seismic, locating wells in structurally complex regions to optimise recovery is difficult and may result in expensive side tracking or in some cases re-drilling to achieve the desired objectives. An understanding of the uncertainty or inherent error in imaging structurally complex zones and unified database of similar structures, with outcomes, provides a key tool in reducing risk of technical failure and managing commercial risk. Direct benefits arise from quantifying the effect of continued work in the technical cycle relative to an improved understanding of the impact on commercial risk.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado