An integrated, quantitative approach to assessing fault-seal risk

Abstract

Fault sealing is one of the key factors controlling hydrocarbon accumulations and trap volumetrics and can be a significant influence on reservoir performance during production. Fault seal is, therefore, a major exploration and production uncertainty. We introduce a systematic framework in which the geologic risk of faults trapping hydrocarbons may be assessed. A fault may seal if deformation processes have created a membrane seal, or if it juxtaposes sealing rocks against reservoir rocks, and the fault has not been reactivated subsequent to hydrocarbons charging the trap. It follows from this statement that the integrated probability of fault seal can be expressed as {1 - [(1 - a)(1 - b)]} x (1 - c), where a, b, and c are the probabilities of deformation process sealing, juxtaposition sealing, and of the fault being reactivated subsequent to charge, respectively. This relationship provides an assessment of fault-seal risk that integrates results from the critical parameters of fault-seal analysis that can be incorporated into standard exploration procedures for estimating the probability for geologic success. The integrated probability of fault seal for each prospect can be visualized using the fault-seal risk web, which allows rapid comparison of success and failure cases through construction of prospect risk web profiles. The impact of uncertainty (U) and the value of information (VOI) for each aspect of fault sealing on the overall fault-seal risk may be determined via the construction of data webs and the relation U = [1 - {(Σnw) / n}] x 100, where nw is the value given to each data web parameter and n is the number of data web components. For example, the data web components required to assess fault reactivation risk are the orientation and magnitude of the in-situ principal stresses, pore pressure, fault architecture, and the geomechanical properties of the fault. Risking of the Apollo prospect, Dampier subbasin, North West shelf, Australia is presented as a worked example. Fault-seal risking for the Apollo prospect has been conducted on 10- and 100-ft oil stratigraphic uncertainty to the extent that a drill-or-not-to-drill decision can be made. In the negative case, a decision to divert well costs could be made, thus saving considerable dollar value. Whether to always reduce key uncertainties as identified by the fault-seal risking process should be made in conjunction with VOI studies.