MURRAY, T., and A. D. GIBBS
Midland Valley Exploration Ltd, Glasgow, UK

Abstract: New Techniques for Calculating Reservoir Volumetrics in Complex Thrust Folded Areas

Accurate reservoir volume estimation presents a challenge in thrust-fold areas. Available techniques are based on a gridded map approach, which provides problems in representing overturned sections. Techniques working on projected areas will consistently underestimate volumes. Further problem arises when primary depositional characteristics of the reservoir need to be interpolated across the reservoir volume.

An alternative is to build a 3D volume and to calculate the volume directly using a triangle and tetrahedral element technique. A new approach is presented, which provides improvements to the accuracy of reservoir volumetrics. The workflow comprises the following:

1. The reservoir top and base are built as 3D triangulated surfaces. The use of a 3D triangle mesh allows the structure to be represented as a single continuous surface.
2. The hangingwall fold is then unfolded using volume conservation flexural unfolding techniques: either line length preservation; or surface area preservation.
3. The fault cut-offs are restored to check that there is no missing rock volume.
4. With each of the fault blocks in the reservoir unfolded, it is then possible to interpolate sedimentary attributes across the structure weighting attribute variations relative to the undeformed rather than the deformed state.

The volume can then be sliced at intervals and each slice multiplied by the surface area of the slice to provide a volume for the reservoir. The approach is illustrated by a case history to demonstrate the differences (~20%) in volume estimation using this and traditional techniques.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas