A series of plane strain extension experiments measured the close relation on burial depth and stress/strain in synthetic Brent Group sands down to a burial depth of 800m (=8040 kPa), which covers the burial depth for most of the trapforming as found in the North Sea region. We observe that deformation is by grain rolling and grain boundary sliding with little damage to the sand. Peak stress is reached at 1.2% extensions at 10m burial, increasing to 7% at 800m burial by a sub-linearly strain-to-depth trend. The shallow burial has more strain softening and more strain localized to thin shear bands. The softening is mainly an effect of increased porosity during deformation. An initial transient strain hardening is seen in all experiments and seems caused by reorganization of the sand grains into a stress-supporting framework with grain contacts orientated subperpendicular to s1. Velocity hardening in the sand prohibit shear bands to grow at stress maxima: A further porosity increase and more deformation are needed to generate the shear bands.
Strain tests on sand gives unique insight into deformation mechanisms. It is used in scaled experiments to model upper and middle crust behavior during extension and compression [Lohrmann et al. 2003 and ref therein], in triaxial tests to measure rocks strengths and study fault growth [Atkinson & Bransby 1978] and in ring shear experiments to model fault behavior by high to very high strain [Mandl et al. 1977, Torabi et al. 2007]. Deformation of reservoir sand under controlled laboratory conditions gives critical information on reservoir behavior that is hard to get in any other ways. The present paper describes a plain strain test with the purpose to quantify conditions for faulting/shear bands in unconsolidated sand in an extending basin. We investigate how mechanical properties vary with burial depth, strain rate and strain state for sand that is analogue to the Brent Group reservoirs, the main reservoir in North Sea oil fields. Results of the experiment are applied to rotated fault blocks in an oil reservoir to predict the reservoir damage.
2. GEOLOGIC SETTING
Structural traps along the western margin of the giant Oseberg field consist of a number of down faulted terraces several km wide and several tens of km?s long. Reservoir is mid-Jurassic Brent Group, with Statfjord Formation of Triassic age as a secondary target for production wells. The sand of the Statfjord Formation and the Brent Group are separated by sealing shales of the Dunlin Formation. Clearly, there is a depthdependant shift in deformation style from mostly folding or drag in Brent Group to narrow faults in the Triassic.
Fig. 1 Structural traps with domino type faulting in the Triassic and folding in the Jurassic Brent Group (yellow). A drag zone is colored violet. Cretaceous post-rift colored blue.(available in full paper)
The area with drag is wedge shaped, with lower edge approximately at top Statfjord Formation level. The faulting and folding occurred mainly in the late Jurassic time, in which period the overburden was insignificant. Rifting occurred in a sediment starved environment central in a rift basin.