Diagenetic controls on reservoir quality in Permian Rotliegendes sandstones, Jupiter Fields area, southern North Sea

Abstract

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.

Multi-fault analysis scorecard: testing the stochastic approach in fault seal prediction

“…Similar features have been observed throughout the Southern North Sea in both the UK and Netherlands sectors, and have been attributed to oblique shear during Tertiary inversion (Leveille et al 1997a). In places, these features appear to form sealing fault-zone elements that may be attributed to fault-zone cataclasis during shearing (Leveille et al 1997a), or to anhydrite cements that may have been derived from pore fluids originating within the overlying Zechstein evaporites and moved downward into the Rotliegend sands during fault dilation events (Leveille et al 1997b).…”

Conductive faults and sealing fractures in the West Sole gas fields, southern North Sea

“…Thus their observed pre-Rotliegende movement was normal and probably associated with Permian rifting rather than Variscan contraction. Similar control of Rotliegende depositional facies and thickness by what later became Jurassic rift faults has been described by Leveille et al (1997b) from the south-eastern Sole Pit Trough. Top reservoir maps of the West Sole area fields, with platform and well locations.…”

“…Self-cementation typically occurs where deformation took place at temperatures greater than 90 8C and is common in the Southern North Sea Rotliegende (e.g. Leveille et al 1997b). The granulation seams are associated with small-scale faults and invariably have some shear displacement (Fig.…”

Contrasting red bed diagenesis: the southern and northern margin of the Central European Basin

“…Many attempts aimed at understanding porosity and permeability evolution in these sandstone reservoirs, mostly based upon petrographic studies, and important diagenetic processes and their controls have already been identified (e.g. Glennie et al 1978;Drong 1979;Almon 1981;Seemann 1982;Gaupp et al 1993;Lanson et al 1996;Leveille et al 1997). However, reasons for contrasting diagenetic evolution in different basin compartments are still not well understood and prediction of diagenesis thus remains very limited.…”