The petrophysical characteristics of the Brent sandstones

Minor amounts of authigenic pyrite are common throughout the Middle Jurassic Brent Group, often being well developed within the delta‐plain fades of the Ness Formation. Petrographic analyses of pyrite samples from the Lyell and Murchison fields reveal two stages of pyrite authigenesis, both of which occurred during an early stage of burial. The first phase of pyrite authigenesis is represented by the development of nodules and finely‐disseminated cement within shale, mudstone, and siltstone fades, and can include the development of bladed marcasite nodules. These cements are interpreted as having formed within organic‐rich sediment soon after deposition, and pre‐date all other authigenic precipitates (typically calcite, quartz and kaolinite). “Early” pyrite cements display a wide range of δ34S(‐14.9 to +42.5% CDT), and some nodules analysed display isotopically‐heavy cores and considerably lighter margins. These isotopically‐zoned nodules are problematical. They could be interpreted as forming via progressive “closed system” bacteriogenic reduction and isotopic fractionation of sea‐water sulphate, with incorporation of heavy H2S into the earliest‐formed pyrite occurring as a result of its upward diffusion through the sediment column, following isotopic fractionation at depth. However, there are a number of problems with this type of interpretation. These include the fact that diffusion will favour the 32S isotope, so that the first‐formed cements should still be isotopically “light” not “heavy”; also, there is a mass‐balance problem, in that no low δ34S sulphides were detected. Coarsely‐crystalline pyrite euhedraform nodular cements mainly within sandstones (δ34S ‐2.6 to +12.8%CDT), and are interpreted as a “later” diagenetic phase. This second phase of sulphide cementation also pre‐dates the main phases of quartz and kaolinite authigenesis within sandstones, but post‐dates an early phase of kaolinite, and may have been partly coeval with some early calcite authigenesis. High organic contents and the early establishment of reducing conditions led to early formation of pyrite within fine‐grained brackish and marine sediments. The lowest measured end‐member δ34S of sandstone‐hosted pyrite cements is less isotopically depleted than that within fine‐grained argillaceous fades, perhaps indicating that sandstone‐hosted pyrite cements began to form subsequent to the onset of burial and bacteriogenic reduction within mudstones. Compaction of marine/brackish‐water shales and mudstones (or sulphate diffusion from these fades) may have supplied an already isotopically‐fractionated source of sulphate to porous sandstone lithologies, where “later” pyrite cements precipitated.

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“…Wide-ranging 634S (-27%0 to +7 1.7%0 CDT) have also been recorded for Brent Group pyrites by Boyce et aZ., (1993), andMcConville et ul. (1993).…”

Section: Reductionsupporting

confidence: 52%

THE OCCURRENCE AND δ³⁴S OF AUTHIGENIC PYRITE IN MIDDLE JURASSIC BRENT GROUP SEDIMENTS

Prosser

Daws

Fallick

et al. 1994

Journal of Petroleum Geology

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Open and restricted hydrologies in Brent Group diagenesis: North Sea

Haszeldine

Brint

Fallick

et al. 1992

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Diagenetic cements in the mid Jurassic Brent Group sandstones of the North Sea have been studied in the Thistle, Murchison, Dunlin and Alwyn South (3/13a-1) oilfields. Volumetrically important cements start with early siderite, kaolinite and calcite, and continue to later kaolinite, ankerite, quartz and illite. The mineralogies of early siderites and calcites are homogeneous in formations with different depositional porewaters. Carbonate concretions nucleated on biotite grains, and their carbon supply was very local and facies controlled. Strontium was derived from dissolution of silicates and shell debris and 87 Sr/ 86 Sr rapidly became homogeneous in different formations. Oxygen was derived from meteoric water and was homogeneous throughout, even in marine deposits. These data suggest an open-flow system with uniform meteorically-derived pore fluids and local ion supply throughout the sandstones. Diagenetically later ankerite formed below 1.1 km and shows mineralogies which are less homogeneous, with 87 Sr/ 86 Sr variable. Cement ions could all have been very locally supplied from within the formation. Late kaolinite could also have been formed locally from feldspars within the formation. The exception to this was quartz cement, its large volumes indicating import across formations or from outside the Brent. Fluid inclusions in quartz show abnormally hot temperatures equating to 80–100°C at 2.2 km. These can be interpreted in two ways. First to indicate a large circulation of hot evolved meteoric water within the basin. This would have been induced by tensional fracturing of the crust related to the final phase of North Atlantic rupture in the Palaeocene and Eocene. However, fluid inclusion temperatures within quartz overgrowths show a systematic increase of temperature with present-day burial depth, suggesting that these temperature records could have been reset, and are now too hot. If these temperature data are rejected, a second interpretation is possible. The δ 18 O isotopic signatures show that late diagenetic silicates and carbonates could have grown in meteoric water, unmodified in its δ 18 O value, in a normal geothermal gradient. Illite grew in a slowly flowing fluid, which rapidly evolved to todays δ 18 O values, as individual groups of pores attempted to equilibrate with the rock around them Oil accumulation halted diagenesis and was permitted by buoyant trapping as this porewater system became stagnant.

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Permeability heterogeneity within the Jerudong Formation: an outcrop analogue for subsurface Miocene reservoirs in Brunei

Prosser¹,

Carter²

1997

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The Jerudong Formation (early Tortonian) cropping out at Punyit Beach (Negara Brunei Darussalam) comprises a heterolithic succession of deltaic strata. Probepermeameter data have been used to evaluate the spatial nature of permeability heterogeneity within shoreface, lagoon and distributary channel environments. Strata at outcrop comprise the same lithologies as those at depth in the abandoned Jerudong Field, and provide an analogue for parts of the Jerudong Formation and equivalents in other Bruneian oilfields.Identified permeability heterogeneities are associated with contrasts of up to three orders of magnitude, occurring at facies, genetic facies unit, bed-set, bed and laminar scales. Distributary-channel facies display the best outcrop permeability (>7000 mD). They include trough cross-stratified fills of relatively straight (often tidal) distributaries, and laterally accreted point bar sandstones deposited within meandering channels. These facies display upward decreasing permeability trends. Shoreface facies may also display high (>6000 mD) permeability, but are characterized by upward increasing permeability and are significantly more heterogeneous than channel facies. Lagoon fill successions are highly heterolithic, comprising sandstones, siltstones and mudstones with a wide range of permeabilities. 5 cm spaced outcrop permeability data are more variable, and display 'average' permeabilities several orders of magnitude larger than occur within similar facies encountered in the subsurface.The spatial scale, frequency and magnitude of permeability variation within the Jerudong Formation indicate that facies packages observed at outcrop are of sufficient scale (4-10 m thickness) to provide potentially useful scaled elements for flow simulation.A model using 0.25 m scale grid blocks has allowed assessment of intra-facies permeability variation (associated with bedding/cross-bed sets) upon recovery within different channel fills. The model predicts both trough cross-stratified and laterally accreted channel fills are heterogeneous with respect to waterflood, and preferential imbibition into their lower parts. Trough cross-stratified channels flood more rapidly than laterally accreted channel fills, where more fingering of the flood front occurs. Lagoon fill deposits display variable waterflood characteristics dependent upon sand content. Within both simulations, heterolithic lagoon fill facies appear to effectively compartmentalise modelled reservoir successions with respect to vertical cross flow.

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The petrophysical characteristics of the Brent sandstones

Cited by 5 publications

References 22 publications

THE OCCURRENCE AND δ³⁴S OF AUTHIGENIC PYRITE IN MIDDLE JURASSIC BRENT GROUP SEDIMENTS

THE OCCURRENCE AND δ³⁴S OF AUTHIGENIC PYRITE IN MIDDLE JURASSIC BRENT GROUP SEDIMENTS

Open and restricted hydrologies in Brent Group diagenesis: North Sea

Permeability heterogeneity within the Jerudong Formation: an outcrop analogue for subsurface Miocene reservoirs in Brunei

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The petrophysical characteristics of the Brent sandstones

Cited by 5 publications

References 22 publications

THE OCCURRENCE AND δ34S OF AUTHIGENIC PYRITE IN MIDDLE JURASSIC BRENT GROUP SEDIMENTS

THE OCCURRENCE AND δ34S OF AUTHIGENIC PYRITE IN MIDDLE JURASSIC BRENT GROUP SEDIMENTS

Open and restricted hydrologies in Brent Group diagenesis: North Sea

Permeability heterogeneity within the Jerudong Formation: an outcrop analogue for subsurface Miocene reservoirs in Brunei

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THE OCCURRENCE AND δ³⁴S OF AUTHIGENIC PYRITE IN MIDDLE JURASSIC BRENT GROUP SEDIMENTS

THE OCCURRENCE AND δ³⁴S OF AUTHIGENIC PYRITE IN MIDDLE JURASSIC BRENT GROUP SEDIMENTS