Abstract: Deformation bands significantly alter the local petrophysical properties of sandstone reservoirs, although it is not known how the intrinsically variable characteristics of sandstones (e.g. grain size, sorting and mineralogy) influence the nature and distribution of deformation bands. To address this, cataclastic deformation bands within fine- and coarse-grained Triassic Sherwood Sandstone at Thurstaston, UK were analysed, for the first time, using a suite of petrographical techniques, outcrop studies, helium porosimetry and image analysis. Deformation bands are more abundant in the coarse-grained sandstone than in the underlying fine-grained sandstone. North- and south-dipping conjugate sets of cataclastic bands in the coarse-grained sandstone broadly increase in density (defined by number/m 2 ) when approaching faults. Microstructural analysis revealed that primary grain size controls deformation band density. Deformation bands in both coarse and fine sandstones led to significantly reduced porosity, and so can represent barriers or baffles to lateral fluid flow. Microstructural data show preferential cataclasis of K-feldspar grains within the host rock and deformation band. The study is of direct relevance to the prediction of reservoir quality in several petroleum-bearing Lower Triassic reservoirs in the near offshore, as deformation band development occurred prior to Carboniferous source-rock maturation and petroleum migration.

Abstract: The Vedder (Oligocene) and Kreyenhagen (Eocene) sandstones at the Greeley oil field consist of arkosic to subarkosic arenites and wackes deposited in shallow marine environments. Burial depths of the Vedder sandstones exceed 3150 m and the reservoir temperature is 124°C. The Kreyenhagen sandstones are buried to greater than 3920 m and the reservoir temperature is estimated to be c. 135°C. These sandstones are currently at or very near their deepest burial depths. The textural relationships of the diagenetic minerals suggest syndepositional formation of glauconite, phosphate and pyrite, followed by early precipitation of pore-lining clay coatings and carbonate cements along with framework-grain fracturing and possibly dissolution. With increasing burial, dissolution of the framework grains continued, accompanied by the albitization of feldspars, the formation of K-feldspar and quartz overgrowths, the precipitation of kaolinite and other clays and possibly the precipitation of late carbonate cements. Finally, hydrocarbon migration and the formation of pyrite occurred during late diagenesis. Porosity preservation and reservoir quality are primarily the result of plagioclase dissolution occurring as the strata approached their current burial depths. Mass balance calculations indicate the significant export of aluminium out of the sands. Thus secondary porosity produced by plagioclase dissolution has replaced the primary porosity destroyed by compaction, and now accounts for the majority of the porosity in these rocks.

The Permo-Triassic Sydney basin is located between a craton to the west and a volcanic source to the east. Systematic variations in sandstone composition, both in space and time, are characteristic patterns in the Sydney basin. A sharp change in sandstone composition is observed in the Late Permian from a quartzo-feldspathic subarkose to volcanic arenite. This change is attributed to the syndepositional development of a volcanic source along the eastern margin of the basin. Throughout the latter part of the Late Permian until Early Triassic time, the sediments were primarily derived from the volcanic source with a secondary mode from the craton. During this time, sandstone composition shows a gradual change in maturity from volcanic arenite to quartz arenite. This maturation was mostly related to chemical weathering in a temperate-humid climate in the source area. Petrographic data from the Sydney basin show that even temperate-humid climate can generate quartz arenite from a mixed cratonic-volcanic provenance and obliterate the plate tectonic signatures. In the Middle Triassic, the volcanic source was virtually shut off, and a quartz arenite unit was derived from the deeply weathered craton. Spatial variations in sandstone composition within correlative units and cyclicity within the broad maturity trend are also observed. Sandstones rich in volcanic grains to the east and sandstones rich in cratonic quartzose grains to the west, are related to the proximity of the respective source terrain. The cyclicity in compositional trends is related to a subtle variation in the duration of chemical weathering and fluvial dynamics.