Brian P. J. Williams

Geochemistry and diagenesis of stratabound calcite cement layers within the Rannoch Formation of the Brent Group, Murchison Field, North Viking Graben (northern North Sea)

Abstract Up to four calcite-cemented horizons (doggers) form impermeable barriers to fluid flow within the Middle Jurassic Rannoch Formation and are correlatable across the Murchison Field. Calcite precipitated during early diagenesis, within high porosity/permeability sandstones at the top of coarsening (shoaling) upward shoreface cycles. Calcite δ13C and δ18O compositions range from -4.1 to -13.4‰ PDB, and -6.6 to -16.7‰ PDB, respectively. Sr concentrations of up to 1334 ppm are consistent with marine carbonate sources (probably shell fragments), but no viable intraformational carbonate source has been identified in the Murchison Field area. Initial 87Sr/86Sr compositions (0.71109–0.71266) are higher than Middle Jurassic seawater (0.7073), and consistent with precipitation from modified porewaters containing significant proportions of continentally derived “meteoric” fluids enriched in 87Sr as a result of basement weathering, or percolation through hinterland soils/unconsolidated detritus. An internal source of 87Sr is not considered viable in view of the high proportion (up to 25‰ clastic constituents) of unaltered detrital alkali feldspar and mica within the Rannoch Formation. Geochemical and isotope data indicate correlations between increasing δ18O composition and increasing iron and magnesium content within calcite. Calcium concentrations decrease with increasing δ18O for calcites. Geochemical data trends can be interpreted differently in terms of either “static” or “evolving” δ18O porewater models. Static δ18O porewater models using Jurassic/Early Cretaceous “meteoric” water ( δ 18 O = −6‰; SMOW ) predict cement precipitation temperatures of 17–77°C. However, δ13C compositions are more depleted than those typical of carbon derived from shell debris, and correlation between decreasing δ18O and decreasing δ13C suggests modification of a meteoric-derived porewater system during burial via contribution of low δ13C carbon derived from a deep basinal source. However, it is difficult to constrain accurately the degree to which porewater δ18O may have evolved during burial. The degree to which porewater δ18O could have evolved from that of a Jurassic/Early Cretaceous meteoric water can be loosely constrained as from δ 18 O ∼ −6‰ SMOW to δ 18 O ∼ −2‰ SMOW if the temperature at which the most 18O-depleted cements precipitated is limited by present-day bottom-hole temperatures (∼110°C) for wells currently at maximum burial depth.

Deformation Structures of Sedimentary Origin in the Lower Limestone Shales (Basal Carboniferous) of South Pembrokeshire, Wales

ABSTRACT Examples of structures ascribed to penecontemporaneous deformation in the Lower Carboniferous of south Pembrokeshire, Wales (United Kingdom), are recorded. The structures occur in units within a neritic calcareous sequence and appear to be developed at a similar stratigraphic level at several localities. Evidence derived from a critical study of the geometry of the deformed bodies and their mutual relationships, in many cases, cannot be completely reconciled with theories of origin involving only foundering or with those which require only sub-horizontal shear deformation and translation. It is suggested that many structures may have originated through a combination of these two modes of deformation. Further support for this hypothesis arises from the strong preferred orientation of the structures observed in many of the deformed units. However, when assessed in the light of regional paleogeography and with the evidence of the directional criteria in associated beds, the orientation of the deformed bodies appears to be more intimately, controlled by local rather than regional factors. /P>

Influence of original fabric on subsequent porosity evolution: an example from the Corallian (Upper Jurassic) reefal limestones, the Weald Basin, southern England

Abstract The Corallian reefs of the Weald Basin, southern England, developed over a prograded oolitic sandbody during a marine transgression. The diagenesis and porosity evolution of these reefs is directly related to their original fabric, a reflection of original environment of formation. A variety of facies-related diagenetic processes and products has been recognized including early marine cementation, skeletal diagenesis and burial cementation. The early marine peloidal cements are abundant in the Corallian subsurface reefs and have played a key role in the overall reservoir quality. These cements, by virtue of occurring in stromatolitic crusts and having abnormally light, stable isotopic values, are interpreted as microbial products. The skeletal diagenesis is mainly controlled by skeletal and matrix permeability with dissolution predominantly occurring in the upper, relatively high-energy part of the reefs and neomorphism commonly developing in the lower part of the reefs which grew in a low-energy setting. Burial cementation has involved both calcite and dolomite precipitation over a wide range of burial depths. Various forms of calcite and dolomite cements have been recognized including relatively early ferroan drusy/poikilotopic calcite and zoned dolomite, and relatively late saddle dolomite and non-ferroan coarsely crystalline calcite. These carbonate cements, by virtue of post-compactional distribution and having depleted δ18O values ( δ 18 O PDB = −6 to −9‰ ) are interpreted as having formed under burial conditions, perhaps in the range of 600 to 2000 m. The porosity evolution is essentially controlled by the combined effects of these diagenetic processes with the best reservoir potential developing in the upper part of the reefs (keep-up highstand systems tract) which underwent extensive skeletal dissolution. Porosity was poorly developed in the middle part of the reefs (catch-up highstand systems tract) due to the extensive early marine cementation. The lower part of the reefs (transgressive systems tract) virtually has no reservoir potential due to the relatively fine-grained sediment texture and intensive compaction.