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Limits to the sealing capacity of rock salt: A case study of the infra-Cambrian Ara Salt from the South Oman salt basin

¹ Lehr-und Forschungsgebiet Geologie–Endogene Dynamik, Lochnerstrasse 4-20, Rheinisch-Westfälische Technische Hochschule (RWTH), D-52056 Aachen, Germany; j.schoenherr{at}ged.rwth-aachen.de
² Lehr-und Forschungsgebiet Geologie–Endogene Dynamik, Lochnerstrasse 4-20, D-52056 RWTH Aachen, Germany; j.urai{at}ged.rwth-aachen.de
³ Geologisches Institut, Wüllnerstrasse 2, D-52056 RWTH Aachen, Germany; kukla{at}geol.rwth-aachen.de
⁴ Lehrstuhl für Geologie, Geochemie und Lagerstätten des Erdöls und der Kohle, Lochnerstrasse 4-20, D-52056 RWTH Aachen, Germany; littke{at}lek.rwth-aachen.de
⁵ Midland Valley Exploration Ltd., 144 West George Street, Glasgow G2 2HG, United Kingdom; zsolt{at}mve.com
⁶ Shell EP Int., P.O. Box 11677, Dubai Convention Tower, Dubai, United Arab Emirates; JeanMichel.Larroque{at}shell.com
⁷ Shell Egypt NV, P.O. Box 2681, El Horreya, Heliopolis, Cairo, Egypt; Mark.Newall{at}shell.com
⁸ Petroleum Development Oman LLC, P.O. Box 81, P.C. 113, Muscat, Oman; Nadia.SN.Abry{at}pdo.co.om
⁹ Shell Exploration and Production Company, Pennzoil Place, 700 Milam Street, Houston, Texas 77002; H.Siyabi{at}shell.com
¹⁰ Petroleum Development Oman LLC, P.O. Box 81, P.C. 113, Muscat, Oman; Zuwena.Rawahi{at}pdo.co.om

Johannes Schoenherr received his diploma from the Technical University of Darmstadt, Germany, with main emphasis in structural geology. Johannes is currently a Ph.D. student at Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen University, Germany. His research is focused on the diagenesis and organic geochemistry of intrasalt carbonates and evaporites from the South Oman salt basin. His further interests are in microtectonics involving the geomechanics of rock salt.

Janos L. Urai is currently a professor of structural geology, tectonics, and geomechanics at RWTH Aachen University and program director of the Department of Applied Geoscience, Oman-German University of Technology in Muscat, Oman. He is interested in basic and applied aspects of rock deformation in the presence of fluids at a wide range of scales in hydrocarbon reservoirs.

Peter A. Kukla graduated in geology from Wuerzburg University, Germany, and Witwatersrand University, South Africa (Ph.D.). His professional career included positions at Witwatersrand University (1986–1990), Shell International E&P (1991–2000), and at RWTH Aachen University (since 2000) as full professor of geology and head of the department and director of the Geological Institute, with research focus on applied sedimentology, reservoir geology, and quantitative geodynamics.

Ralf Littke is a professor of geology and geochemistry of petroleum and coal at RWTH Aachen University, Germany. Ralf's current research topics include dynamics of sedimentary basins, with special emphasis on temperature and pressure history; generation of hydrocarbon gases and nonhydrocarbon gases as well as petroleum; transport and accumulation of methane and carbon dioxide; and development of new tools in petroleum system modeling.

Zsolt Schléder received his M.Sc. degree from the Eötvös University, Budapest, Hungary, in 2001 and his Ph.D. from the RWTH Aachen University, Germany, in 2006. He is currently working at Midland Valley Exploration, Ltd., as a structural geologist. His research efforts are focused on deformation and recrystallization mechanisms in rock salt. His current interest is in two- and three-dimensional structural restoration technology.

Jean-Michel Larroque has a Ph.D. in structural geology from Montpellier University (France) and joined the Shell structural geology team in 1988. He had assignments in the United Kingdom, Germany, and Oman as South Oman Exploration team leader. Previously, he was Shell Exploration chief geoscientist for the Middle East and the Caspian. He is now exploration manager for Shell Syria.

Mark J. Newall is a senior exploration geologist in Frontier Exploration in Shell, Egypt. He has a Ph.D. from Liverpool University (1990) and, since joining Shell, has worked as an explorationist in Holland, Malaysia, and Oman, before moving to Cairo in 2005. He is currently exploring for gas in the Nile delta.

Nadia Al-Abry holds a Ph.D. (2002) from the University of Edinburgh. Nadia joined Petroleum Development Oman in October 2002 and since then has been working on the Precambrian intrasalt Ara carbonate stringers first as an exploration team geologist and seismic interpreter and then as a production geologist. Her research interests are in the tectonic evolution of basins and its influence on sedimentation and reservoir architecture.

Hisham A. Al-Siyabi holds an M.S. degree (1994) and a Ph.D. (1998) from the Colorado School of Mines. Hisham joined Petroleum Development Oman in 1999 and, since 2001, has worked as a geologist and seismic interpreter exclusively on the terminal Proterozoic intrasalt Ara stringers. In 2005, Hisham joined Shell Exploration and Production Company in the United States as an exploration geologist.

Zuwena Rawahi is a senior carbonate geologist in Petroleum Development Oman and has been working on the Precambrian stringer play on the South Oman exploration team for the last 3 years. Prior to that, she worked for 7 years on the Shuaiba Formation. Her main interest is related to carbonate sedimentology and diagenesis.

In the South Oman salt basin (SOSB), diapirs of infra-Cambrian Ara Salt enclose isolated, commonly overpressured carbonate reservoirs. Hydrocarbon-impregnated black rock salt shows that it has repeatedly lost and then regained its sealing capacity. The black staining is caused by intragranular microcracks and grain boundaries filled with solid bitumen formed by the alteration of oil. The same samples show evidence for crystal plastic deformation and dynamic recrystallization. Subgrain-size piezometry indicates a maximum differential paleostress of less than 2 MPa. Under such low shear stress, laboratory-calibrated dilatancy criteria indicate that oil can only enter the rock salt at near-zero effective stresses, where fluid pressures are very close to lithostatic. In our model, the oil pressure in the carbonate reservoirs increases until it is equal to the fluid pressure in the low but interconnected porosity of the Ara Salt plus the capillary entry pressure. When this condition is met, oil is expelled into the rock salt, which dilates and increases its permeability by many orders of magnitude. Sealing capacity is lost, and fluid flow will continue until the fluid pressure drops below the minimal principal stress, at which point rock salt will reseal to maintain the fluid pressure at lithostatic values.

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