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AAPG Bulletin, V. 86, No. 10 (October 2002),Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

The effect of temperature on sealing capacity of faults in sandstone reservoirs: Examples from the Gullfaks and Gullfaks Sor fields, North Sea

Jonny Hesthammer,1 Per Arne Bjorkum,2 Lee Watts3

1University of Bergen, Department of Earth Sciences, Allegaten 41, N-5007 Bergen, Norway; email: [email protected]
2Statoil, N-4035 Forus, Norway; email: [email protected]
3Shell U.K. Exploration and Production, 1 Altens Farm Road, Nigg, Aberdeen, AB12 3FY, United Kingdom; email: [email protected]


Jonny Hesthammer works as a professor in seismic interpretation at the University of Bergen, Norway. Jonny received his M.Sc. degree in structural geology from the University of British Columbia, Canada, and his Ph.D. in structural geology and seismic interpretation from the University of Bergen, Norway.

Per Arne Bjorkum is currently Statoil's chief researcher within exploration. He also holds a position as professor in reservoir geology at Stavanger University College, Norway. Per Arne received his Cand. Real. in petroleum geology from the University of Bergen, Norway, and his Ph.D. in geology from the University of Oslo, Norway.

Lee Watts currently works as a production geologist for Shell UK in the Central Graben area. Lee recently received his Ph.D. in structural geology from the University of Durham, United Kingdom. He carried out work for this study while working for Statoil in Bergen as a summer placement student.


We want to thank Norsk Hydro, Saga Petroleum, and Statoil for permission to publish the results of this research. The article has benefited from stimulating discussions with Asbjorn Thon, Eirik Graue, Tore Odinsen, and colleagues in the Gullfaks Satellites department. R. Knipe, Q. Fisher, B. Trudgill, H. Handschy, and J. Lorenz are thanked for their extensive reviews and comments. Q. J. Fisher, R. J. Knipe, D. Condcliffe, and R. M. Jones provided much needed information on microcharacteristics of shear bands in two Statoil internal reports referred to in this article as Q. J. Fisher et al., 1997, 1998, personal communication.


A comparison of structural core analyses from the Gullfaks and Gullfaks Sor fields, northern North Sea, shows that there are important differences between the two fields that have serious consequences for field development plans. Although the general deformation characteristics are the same, the temperatures within the reservoirs on Gullfaks Sor have exceeded the critical temperatures for accelerated quartz dissolution and precipitation. This has led to reduced porosity and permeability within the reservoir and the many shear bands found within damage zones around the many larger scale faults. This has a larger effect on two-phase flow properties than accounted for in standard reservoir models and has a detrimental effect on the production from compartmentalized high-temperature reservoirs. Prior to the detailed core analyses carried out on Gullfaks Sor, it was assumed that faults in Gullfaks Sor and Gullfaks had similar effects on hydrocarbon flow. However, the important difference became obvious after the drilling of two producers within the Statfjord Formation reservoir in Gullfaks Sor. Due to rapid drop in pressure, the actual production rates are less than 15% of those planned. The drop in pressure likely is caused mainly by quartz cementation of the shear bands, which transforms the bands into low-porosity microporous rock in which the oil capillary entry pressure becomes so high that the oil phase is discontinuous in the field. Hence, during production, oil does not flow easily, resulting in a rapid drop in production.

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