The reservoir properties and diagenesis of the Brent Group: a regional perspective

Abstract

A regional study of Brent Group diagenesis and reservoir properties has been undertaken in order to determine the main controls on porosity and permeability in the sandstone reservoirs. Data from 44 wells from block 211/7 in the north to block 3/8 in the south and spanning current depths from 6700 to 13 400 ft include 9000 porosity, permeability and grain density determinations, quantitative petrographic information from 850 thin sections, and stable isotopic and K/Ar analyses of authigenic phases. The diagenesis of the sediments is similar across the study area and most of the diagenetic phases occur in all formations. The sequence of precipitates and dissolution events reflects early porewater evolution in shallow burial environments and later reactions which were essentially isochemical and controlled largely by increasing burial depth and therefore temperature. Early diagenetic products include siderite, calcite, chlorite and vermicular kaolinite. Only where the calcite cements form concretions or cemented horizons have they a significant effect on reservoir properties. Local dissolution of feldspars and carbonate cements took place on the crest of some fault blocks inferred to have been emergent during the Jurassic and this has caused local enhancement of porosity in some crestal wells. In general, the porosity of each of the reservoir facies decreases systematically with depth but permeability only starts to decrease significantly at depths greater than 10 200 ft (3109.0 m). The general decrease in porosity can be attributed to compaction, together with burial cementation by quartz and iron-rich carbonates. Secondary porosity resulting from feldspar dissolution is increasingly common at depth but there is no net increase in porosity as much of the dissolution was evidently accompanied by the precipitation of authigenic quartz initially with kaolinite but at greater depths with illite. The systematic changes in porosity and the decrease in permeability are compatible with thermally driven dissolution of feldspar and the local reprecipitation of the authigenic silicates; the decrease in permeability corresponds to the presence of increased quantities of illite at depth.

Depth-Dependent Transformation of Kaolinite to Dickite In Sandstones of the Norwegian Continental Shelf

“…A very similar morphological difference between early vermicular and later blocky "kaolinite" has been described repeatedly in diagenetic studies of North Sea sandstones, but without identification of the kaolin polytypes involved (Hurst & Irwin, 1982;Kantorowicz, 1984;Thomas, 1986;Glasmann et al, 1989;Haszeldine et al, 1992;Giles et al, 1992). Polytype identification in such studies is needed to determine whether these two commonly reported morphologies in fact reflect kaolinite and dickite occurrence, respectively.…”

Ankerite Cementation in Deeply Buried Jurassic Sandstone Reservoirs of the Central North Sea

“…Models of North Sea diagenesis have tended either to invoke an essentially isochemical system (e.g., Bjørlykke et al 1989;Giles et al 1992) or significant mass transfer, either from adjacent mudrocks or from Permian evaporite-carbonate successions along episodically active basin faults (e.g., Burley 1993). Recent work suggests that deep-burial diagenesis is influenced by regional flow systems related to the generation and leakage of overpressure compartments (Haszeldine et al, in press).…”

Clay Diagenesis in the Sandstone Reservoir of the Ellon Field (Alwyn, North Sea)

“…Precipitation of clay minerals in the pore spaces and the evolution of clays through burial and temperature were described by many authors; for instance, in the Brent Group reservoirs of the (Middle Jurassic) North Sea, the evolution of illite and kaolin morphologies with depth and temperature is well documented (Kantorowicz, 1984;Thomas, 1986;Glasmann et al, 1989;Giles et al, 1992;Haszeldine et al, 1992). Ehrenberg et al (1993) and Lanson et al (1996) showed the relationship between kaolin morphology (i.e., vermicular or blocky kaolin) and the nature of the kaolin polytype (i.e., kaolinite or dickite).…”