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AAPG Bulletin


AAPG Bulletin, V. 87, No. 3 (March 2003), P. 507-524.

Copyright 2003. The American Association of Petroleum Geologists. All rights reserved.

An integrated, quantitative approach to assessing fault-seal risk

Richard M. Jones,1 Richard R. Hillis2

1Woodside Energy, 1 Adelaide Terrace, Perth, Western Australia, 6000, Australia; email: richard.joneswoodside.com.au
2National Centre for Petroleum Geology and Geophysics, Australian Petroleum Cooperative Research Centre, University of Adelaide, South Australia 5005, Australia; email: rhillisncpgg.adelaide.edu.au


Richard joined Woodside in September 2000 and is currently working in the Trap and Pressure Team, New Ventures. He graduated with a joint B.Sc. degree (with honors) in geology and economics from Keele University (1992) and a Ph.D. from Keele University (1996). He has worked extensively in the area of fault and top seal evaluation and has been involved with seals research programs in Europe, the United States, and Australia. Current interests include structural modeling, seal evaluation, wellbore stability, and Liverpool FC. Richard is a member of AAPG, PESA, and PESGB.

Richard holds the State of South Australia Chair in Petroleum Reservoir Properties/Petrophysics at the National Centre for Petroleum Geology and Geophysics (NCPGG), Adelaide University. He graduated with a B.Sc. degree (with honors) from Imperial College (London, 1985), and a Ph.D. from the University of Edinburgh (1989). After seven years at Adelaide University's Department of Geology and Geophysics, Richard joined the NCPGG in 1999. His main research interests are in petroleum geomechanics and sedimentary basin tectonics. He has published approximately 50 papers and has consulted to many Australian and international oil companies in these topics. Richard is a member of AAPG, American Geophysical Union, Australian Society of Exploration Geophysicists, European Association of Geoscientists and Engineers, Geological Society of America, Geological Society (London), Petroleum Exploration Society of Australia, Society of Exploration Geophysicists, and Society of Petroleum Engineers.


The authors are extremely grateful to colleagues at the National Centre for Petroleum Geology and Geophysics, Adelaide, Woodside Energy, and Shell for manuscript discussions. Russell Davies, Rob Knipe, Gavin Lewis, Frank Krieger, and James MacKay are thanked for providing constructive and focused reviews that improved earlier versions of the manuscript. The fault-seal risk web as presented herein has evolved from a risk-web concept presented by former colleagues of the first author at Rock Deformation Research, University of Leeds. The integrated fault-seal risking procedure detailed in this paper forms part of the propriety APCRC Seals Consortium. The consortium members Woodside Energy, BHP Billiton, JNOC, Origin Energy, ChevronTexaco, Exxon-Mobil, Globex Energy, Santos, Anadarko Petroleum Corporation, and OMV are thanked for their permission to publish.


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)]} (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 − {(nw) / n}] 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 columns to allow integration with volumetric probabilistic statements. The critical parameter for fault-seal risking at the Apollo prospect is the ability of disaggregation zone faults (low shale gouge ratio fault gouge) to support increasingly large hydrocarbon columns. Evaluation of the individual components for Apollo fault sealing indicates a = 0.3 (10-ft column) and a = 0.1 (100-ft column), b = 0.2, and c = 0.1. The overall probability of the Apollo trap-bounding fault sealing a 10-ft oil column is 0.4 or 40% (seal condition moderately unlikely). The likelihood that the fault seals oil columns greater than 100 ft is 0.3 (seal condition unlikely). Data web error margins for the Apollo prospect are 20% (juxtaposition uncertainty), 26% (fault-rock process uncertainty), and 27% (fault reactivation uncertainty). Recalculating each parameter by its uncertainty, for a 10-ft oil column, the upper value of integrated fault-seal risk is 0.5 (seal condition intermediate), and the lower value is 0.3 (seal condition unlikely). The upper value of integrated fault-seal risk for a 100-ft oil column is 0.3 (seal condition unlikely), and the lower value is 0.2 (seal condition very unlikely). The variation in the Apollo final risk calculation reflects the lack of prospect-specific data. The greatest VOI benefit for Apollo fault-seal prospectivity is sedimentary architecture data.

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