Rotliegendes gas reservoirs in the Jupiter Fields are compartmentalized by sealing faults. Significant variations in free water levels (i.e. gas-water contacts corrected for capillary pressure effects), hydrocarbon composition and pressure exist between sealed fault blocks. Displacements on many of the sealing faults are low, resulting in ‘clean’, porous and permeable Rotliegendes sandstones being juxtaposed across the faults. Characteristics that are important in determining the sealing potential of intra-Rotliegendes faults include (i) fault-rock cement types and volumes, (ii) intensity of cataclasis, (iii) three-dimensional continuity of the fault and associated damage zone, (iv) authigenic clay content at the time of faulting, and (v) relative orientation, timing and magnitude of later deformational events. Detailed analysis of fault rocks from core indicates cementation is the most effective fault sealing mechanism in the Jupiter Fields. Volumetrically significant cements include salt, anhydrite and quartz. Cataclasis and deformation induced mixing of authigenic clay with fragments of framework grains also contribute to the sealing potential of some faults. Permeabilities in cemented and cataclastic fault rocks are reduced by two or more orders of magnitude compared to undeformed sandstones. 3D seismic mapping indicates NNE-SSW-trending fault zones have the highest along-strike continuity, followed by NW-SE- and E-W-trending fault zones. The NNE-SSW-trending faults also appear to have had the simplest deformational history, with NW-SE- and E-W-trending faults exhibiting evidence of movement during multiple deformational events. Static reservoir data indicate that some NNE-SSW- and NW-SE-trending faults seal over geological time, and it is expected other faults will act as seals or baffles during reservoir depletion.

Azimuthal accelerations of cylindrical surfaces co-axial with the rotation axis have been inferred to exist in Earth's fluid core on the basis of magnetic field observations and changes in the length-of-day. These accelerations have a typical timescale of decades. However, the physical mechanism causing the accelerations is not well understood. Scaling arguments suggest that the leading order torque averaged over cylindrical surfaces should arise from the Lorentz force. Decadal fluctuations in the magnetic field inside the core, driven by convective flows, could then force decadal changes in the Lorentz torque and generate zonal accelerations. We test this hypothesis by constructing a quasi-geostrophic model of magnetoconvection, with thermally-driven flows perturbing a steady, imposed background magnetic field. We show that when the Alfvén number in our model is similar to that in Earth's fluid core, temporal fluctuations in the torque balance are dominated by the Lorentz torque, with the latter generating mean zonal accelerations. Our model reproduces both fast, free Alfvén waves and slow, forced accelerations, with ratios of relative strength and relative timescale similar to those inferred for the Earth's core. The temporal changes in the magnetic field which drive the timevarying Lorentz torque are produced by the underlying convective flows, shearing and advecting the magnetic field on a timescale associated with convective eddies. Our results support the hypothesis that temporal changes in the magnetic field deep inside Earth's fluid core drive the observed decadal zonal accelerations of cylindrical surfaces through the Lorentz torque.

For the past few centuries, the temporal variation in the Earth's magnetic field in the Pacific region has been anomalously low. The reason for this is tied to large scale flows in the liquid outer core near the core-mantle boundary, which are weaker under the Pacific and feature a planetary scale gyre that is eccentric and broadly avoids this region. However, what regulates this type of flow morphology is unknown. Here, we present results from a numerical model of the dynamics in Earth's core that includes electromagnetic coupling with a non-uniform conducting layer at the base of the mantle. We show that when the conductance of this layer is higher under the Pacific than elsewhere, the larger electromagnetic drag force weakens the local core flows and deflects the flow of the planetary gyre away from the Pacific. The nature of the lowermost mantle conductance remains unclear, but stratified core fluid trapped within topographic undulations of the core-mantle boundary is a possible explanation. The Earth's magnetic field is generated by electrical currents flowing within its conducting iron core. These currents, in turn, are driven and maintained against decay by motions in the fluid outer core, likely convective in nature. 1 Core flows produce changes in the magnetic field, including those observed at the Earth's surface, a temporal fluctuation which is referred to as secular variation (SV). 2 Past observations can be used to reconstruct how the magnetic field and its SV have changed with time. 2-4 Fig. 1a shows the mean intensity of the radial component of the SV at the core-mantle boundary (CMB) over the time period 1590-1990 from the model in ref.(4). The SV is not uniformly distributed and is distinctly weaker under a broad region of the 1

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