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.

Reservoir quality in Lower Permian Rotliegendes Group sandstones in the Jupiter Fields varies considerably between fault blocks due to differing degrees of diagenetic alteration. Average porosities in individual fault blocks range from 12 to 21%, and geometric mean permeabilities vary between 2 and 200 md. Rotliegendes stratigraphy and sedimentology varies little across the area, and therefore does not strongly influence reservoir quality.The two most important diagenetic processes were the formation of authigenic illite and burial compaction. Illite growth mainly occurred during the Middle Jurassic to Early Cretaceous, coincident with maximum burial of the Rotliegendes. Fault blocks with good reservoir quality generally contain (1) relatively low illite abundances, (2) low-potassium illite, (3) increasing illite abundances with depth, (4) illite ages that decrease with depth and (5) comparatively low amounts of burial compaction. Fault blocks with poor reservoir quality all exhibit (1) high illite abundances, (2) high-potassium illite, (3) no consistent trend in illite abundances or age with depth and (4) high amounts of burial compaction. These observations are believed to indicate that reservoir quality was preferentially preserved in fault blocks that were structurally high during the main phase of illite growth. Factors contributing to the preservation of reservoir quality in structurally high fault blocks included differences in pore fluid chemistry, less burial compaction, and, in places, early hydrocarbon charging. Reservoir quality in undrilled fault blocks can be predicted by reconstructing the structural geometries present during the Mid-Jurassic to Early Cretaceous.