Geostatistical assessment of time picking uncertainties and their impact on GRV

Nicolas Jeannée (1), Peter Shiner (2), Jacques Deraisme (1)

(1) Geovariances, 49bis av. Franklin Roosevelt, BP91, 77212 Avon Cedex, France
(2) Shell Italia E&P. S.p.A, 66 Via dei Due Macelli, 00187 Roma, Italy.

Abstract

Assessment of gross rock volume (GRV) uncertainty is a key issue in hydrocarbon resource evaluations across the exploration-appraisal-development cycle. Geostatistics provide a variety of techniques for deriving velocity models by combining well information with, for instance, more densely sampled stacking velocity data. Stochastic simulations extend these techniques and, by providing a range of iso-probable velocity models, allow assessment of velocity uncertainty and its impact on GRV distributions in a relatively straightforward manner.

In contrast assessing the impact of TWT interpretation uncertainty is problematic, with difficulty in quantifying this uncertainty meaning that TWT interpretations are commonly treated as having either no uncertainty or having nominally-assigned uncertainties.

The paper presents an innovative approach to quantify TWT uncertainty successfully applied to a complex hydrocarbon field. Moderate-poor seismic data quality make possible a wide range of top reservoir interpretations over many areas of the field. Optimistic / pessimistic interpretations describing this range of uncertainty are used as the basis for a geostatistical analysis of TWT uncertainty, the interpreter’s subjective assessment of the described probability range (10%-50%-90% for instance) providing an empirical confidence interval for the time picking. TWT interpretations are simulated by adding to the best technical case simulations of the TWT uncertainty with a variogram based on the difference between optimistic / pessimistic cases. QCed resulting TWT realizations are then combined with velocity simulations to provide depth simulations and subsequent GRV distributions. The impact of TWT interpretation uncertainty on the latter is quantified and discussed.

AAPG European Region Conference Prague 2004

Improved gross rock volume calculation with better time interpretation - World Oil - June 2005