For a borehole of the Gullfaks field in the northern North Sea 60 days borehole pressure data of two successive registrations (hereafter parts I, II) were analyzed. From correlation with theoretical ocean tides a loading efficiency γ of 0.35-0.40 and of 0.16-0.19 were obtained for parts I and II, respectively. No influence of air pressure or earth tides could be resolved. Assuming an undrained Poisson ratio vuranging between 0.30 and 0.36 and assuming that the vertical loading results in an areal strain εa of the same magnitude as the vertical strain εV a Skempton ratio B of about 0.45 for part I and of about 0.20 for part II were derived. B values are somewhat higher assuming smaller εa. The notably different values for γ and thus, B can be explained by a significant change in reservoir properties, which was induced by several injections in the borehole in the time between the two registrations.

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