Uncertainty in Gross Rock Volume Analysis - A Stochastic or Deterministic Approach?

Abstract

Quantifying the uncertainty in volumetrics is of paramount importance especially in mature basins such as the Southern North Sea. One of the key uncertainties is often the size of the gross rock volume (GRV), which is mostly dependent upon the seismic mapping of the reservoir and the time-depth conversion. This paper compares two methods to calculate the GRV uncertainty as a function of TWT uncertainty and velocity uncertainty. A stochastic time-depth conversion is used to estimate the depth uncertainty. Thereafter, the first method uses a stochastic approach creating multiple reservoir realisations using Sequential Gaussian Simulation. From this a GRV expectation curve is made, which gives a high case (P10) and low case (P90) volume. The outcome of the stochastic approach is compared to the outcome of the second method which is introduced in this paper; the plus-minus method. This method uses a simplified deterministic approach. A low case and high case depth map, corresponding to respectively a P90 and P10 GRV, are created by directly adding or subtracting a scaled depth uncertainty map from the base case depth map. Advantages and disadvantages of both methods are discussed. In addition recommendations are given on which method to use under what circumstances.