ProbGRV: a workflow for geostatistical depth conversion and probabilistic assessment of gross rock volume

Abstract

Understanding and estimating the probability of geological success (Pg) and original hydrocarbon-in-place (OHIP) are an integral part of the geoscientist’s role from exploration though to development. This paper presents a workflow developed at Santos for the systematic assessment of two key elements in this process: the probability of structural closure (Pcl); and, the estimation of the uncertainty of gross rock volume (GRV). There are two distinct parts to the workflow. Firstly, Kriging and sequential Gaussian simulation (SGS) are used in map-based, seismic time-to-depth conversion to generate multiple equi-probable depth maps (in the order of 1,000+ realisations). Numerous depth conversion methods are considered where appropriate. Secondly, every depth map realisation is validated for structural closure around a target location (e.g. a proposed or existing well). Primary outputs from the workflow include estimates of Pcl, and probability distributions of GRV and pool area for valid maps, which may then be used directly in estimating geological risk and OHIP. Secondary outputs from the workflow include basic statistics associated with depth conversion uncertainty, such as target depth prognosis and the distribution of the depth to lowest closing contour. The ultimate objective is to provide the interpreter with a greater appreciation of the range of possible outcomes given the uncertainty in structural geometry. This workflow, named probGRV, has been designed and developed within Santos using commercially available geophysical software and in-house proprietary software. During the past few years, the workflow has been implemented on key assets at various stages of the prospect/field life cycle and in both pre- and post-drilling campaigns for audit purposes. Case studies from the Cook Field in the onshore Eromanga Basin and the Finucane South Field in the offshore Carnarvon Basin are shown to illustrate the methods and results.