Summary The Pre-Salt province of the Campos and Santos Basins (SE Brazil) hosts one of the most significant offshore hydrocarbon discoveries of the past 20 years. Several studies have focused on the regional tectonic setting and the depositional and diagenetic characteristics of these Barremian to Aptian lacustrine carbonates. Only recently, a comprehensive tectono-stratigraphic evolution for the deposition of the presalt succession and a model for the time and spatial links between hydrothermal diagenesis and structural evolution was established. Three tectono-diagenetic events were identified through U-Pb geochronology, petrographic and geochemical investigations: Barremian to Aptian, Albo-Cenomanian and Campanian to Maastrichtian. They reveal a diagenetic evolution characterised by an initially strong hydrothermal overprint that progressively attained geothermal equilibrium under deep-burial conditions during the late Cretaceous. These diagenetic episodes are not necessarily coeval with magmatic events but indeed coeval with basin-scale tectonic events linked to changes in the associated tectonic regime, thus characterising events of structural diagenesis. Nevertheless, paragenetic phases related to each of these events are, in most cases, a consequence of the host rock composition.

Abstract Despite all the effort made towards an understanding of the sedimentary, tectonic, and diagenetic evolution of the presalt sequence and the Pre-Salt reservoirs of the North Campos Basin (East Brazil), two knowledge gaps have yet to be filled: 1) a detailed study of diagenesis in the crystalline basement and rift phases, and 2) the timing of diagenetic events that affected the presalt succession. In this study, samples from these geologic units were analysed for mineral composition and paragenetic evolution, fluid temperature and salinity, stable isotope compositions, and LA-ICP-MS derived U-Pb ages of carbonate phases. The U-Pb ages of replacive and vein-filling cements reveal three tectono-diagenetic events, named Barremian-Aptian (BADE, 125-117 Ma), Albo-Cenomanian (ACDE, 103-98 Ma), and Campanian-Maastrichtian (CMDE, 83-70 Ma). Each phase is characterised by distinct minerals, precipitation temperatures, and burial conditions. The hydrothermal qualifier, identified by the temperature contrast between fluid and host rock, was initially high during BADE, then diminished over time (through ACDE) until it achieved equilibrium with the host rocks during CMDE. Diagenetic events are not coeval with magmatism but do coincide with known regional tectonic events described in the literature and are interpreted to be the result of increasing intraplate stresses. Multidisciplinary studies that include diagenetic events constrained by geochronological data will certainly lead to more robust conceptual geologic models, and therefore, to a more reliable management of resources and strategies such as Enhance Oil Recovery (EOR), Carbon Capture, Utilisation and Storage (CCUS), and drinking water. Supplementary material at https://doi.org/10.6084/m9.figshare.c.7103900

Late Ordovician (c. 445 Ma) glacial sandstones form important gas reservoirs in the Illizi Basin, SW Algeria. These reservoirs have a high degree of depositional and diagenetic complexity, such that understanding and predicting reservoir quality (RQ) presents a major challenge to their economic development. Porosity is typically 1-10%, but reaches up to 15% and permeability is typically <10(-15) m(2) (<1 mD), but locally reaches >10(-13) m(2) (>100 mD). The key questions addressed herein concern the development and distribution of this RQ variability, specifically why has good RQ been locally preserved? Primary depositional fabric exerts a strong control on RQ. Muddy sandstones are either highly compacted or pervasively cemented by quartz and microporous illite, and have very poor RQ. Only fine- to medium-grained, moderately well sorted, clean sandstones can contain good RQ, but texturally and mineralogically similar sandstones span a wide range of porosity and permeability. This range is primarily driven by the degree of quartz cementation, with incomplete cementation resulting in the best RQ. Quartz overgrowths in incompletely cemented clean sandstones are patchy and non-luminescent in scanning electron microscopy with cathodoluminescence (SEM-CL), possibly indicating slow growth rates. There is tentative evidence to link incomplete quartz cementation with oil charging of the reservoir. An alternative or additional explanation of RQ preservation may be related to limited silica supply in the centres of the thickest, stacked, clean sandstones, where the better RQ tends to reside. The results of this study imply that sustained high-energy depositional processes, coupled with an early oil charge, are prerequisites for retaining the best RQ. This has important implications for the exploration and development of Late Ordovician glacial sandstones in the Illizi Basin, and potentially similar plays elsewhere.

Journal of Quaternary ScienceVolume 23, Issue 3 p. 307-308 Correspondence Reply: Evidence for episodic dust accretion in SE England Michèle L. Clarke, Michèle L. Clarke School of Geography, University of Nottingham, Nottingham, UKSearch for more papers by this authorAntoni E. Milodowski, Antoni E. Milodowski British Geological Survey, Keyworth, Nottingham, UKSearch for more papers by this authorKevin J. Northmore, Kevin J. Northmore British Geological Survey, Keyworth, Nottingham, UKSearch for more papers by this authorMelanie J. Leng, Melanie J. Leng School of Geography, University of Nottingham, Nottingham, UK NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, UKSearch for more papers by this authorJon E. Bouch, Jon E. Bouch British Geological Survey, Keyworth, Nottingham, UKSearch for more papers by this author Michèle L. Clarke, Michèle L. Clarke School of Geography, University of Nottingham, Nottingham, UKSearch for more papers by this authorAntoni E. Milodowski, Antoni E. Milodowski British Geological Survey, Keyworth, Nottingham, UKSearch for more papers by this authorKevin J. Northmore, Kevin J. Northmore British Geological Survey, Keyworth, Nottingham, UKSearch for more papers by this authorMelanie J. Leng, Melanie J. Leng School of Geography, University of Nottingham, Nottingham, UK NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, UKSearch for more papers by this authorJon E. Bouch, Jon E. Bouch British Geological Survey, Keyworth, Nottingham, UKSearch for more papers by this author First published: 28 February 2008 https://doi.org/10.1002/jqs.1151Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume23, Issue3March 2008Pages 307-308 RelatedInformation

Fluids inclusion studies suggest that the gold mineralization occurring in a silica-rich fault zone in Silurian rocks at Bohaun in Connemara, western Ireland is associated with low temperature, moderate–high salinity fluids more consistent with a basinal brine than an orogenic gold lineage. This contrasts with other gold deposits in western Ireland that are typically orogenic in mineralization style. Remobilisation of pre-existing gold mineralization by low-temperature, highsalinity brines is recognised in a number of gold deposits worldwide. However, at Bohaun there is no evidence for earlier mineralization suggesting that low-temperature fluids can transport gold and potentially form gold deposits independent of other fluids.

The Wildmoor Sandstone Formation, proved in three boreholes drilled at Birmingham University, is dominated by fine- to medium-grained sandstones deposited in a braided river environment, within which channel lag, channel fill and abandoned channel facies are recognized. Minor proportions of aeolian sandsheet are present, as are dolocretes, not previously reported in the formation.The sandstones are feldspathic and lithic arenites, and typically are clay-poor. Early dolomite dominates the diagenetic overprint, and is preferentially developed in channel-lag deposits. Burial diagenetic effects are minor. Late calcite occurs as a pore-filling phase and within fractures.Minor fractures and granulation seams are oriented parallel to the NE-SW Birmingham Fault. 'Conventional' granulation seams, with comminution of detrital material, and more complex seams containing comminuted dolomite cement with a millimetre-wide halo of dolomite cement are present, the latter implying that the sandstone was dolomite-cemented at the time of fracturing.Several scales of heterogeneity will affect groundwater solute transport. The palaeosols and abandoned channel mudstones may act as barriers to vertical flow at the decimetre scale. Dolomite-cemented channel-lag deposits may act similarly at smaller scales. Granulation seams have permeabilities of two-three orders of magnitude lower than their host sandstones, but their limited occurrence may limit their impact on larger scale flow. Matrix permeability is controlled by grain size and dolomite cement.The fines in the fine-grained, ripple cross-laminated sandstones were extensively washed out during coring, and this lithology may be a source of sand yields in some sandstone boreholes. Although no enhancement of particle yields was seen during packer testing, the possibility remains that more comprehensive failure may occur at higher pumping rates.

Dolomitisation and ankeritisation Limestones in the vicinity of the stratabound mineralisation were pervasively dolomitised/ankeritised, and developed vuggy porosity in the presence of a high-salinity brine consistent with fluids derived from adjacent mud and shale-filled basins.

A Microsoft Excel add-in has been developed to aid the relocation of analysis points on petrographical samples between analytical runs on different microscopes and instruments. The general procedure is as follows. Two readily identifiable reference points are defined on the sample. During the first analytical run, the positions of these reference points and of any features of interest are recorded using the co-ordinate system of this instrument. During a subsequent run, which may be on a different instrument with a different co-ordinate system, the positions of the reference points are recorded. The add-in then uses the reference point positions to calculate a set of transformations that maps the co-ordinate system of the first instrument into that of the second. These transformations are then applied to the co-ordinates of the features of interest to calculate their expected positions on the second instrument.

An integrated mineralogical-geochemical study of unconformity-related Au-Pd occurrences within and around the Permo–Triassic basins of southwest England, UK, has confirmed the importance of low temperature (86±13°C), hydrothermal carbonate veins as hosts for the mineralisation. Fluid inclusion data for the carbonate gangue, supported by stable isotope (13C and 18O) and radiogenic (87Sr/86Sr) data, have identified three principal fluids: (1) a reducing calcic brine [>25 wt% salinity, <0.5 NaCl/(NaCl+CaCl2)] originating in the sub-unconformity basement and an expression of advanced mineral–fluid interaction; (2) an oxidising sodic brine [~16 wt% salinity, >0.9 NaCl/(NaCl+CaCl2)] originating in the post-unconformity red beds under evaporitic conditions, and (3) an oxygenated, low salinity groundwater (<3 wt% salinity). The sodic brine is reasoned to be the parent metalliferous fluid and to have acquired its enrichment in Au and Pd by the leaching of immature sediments and intra-rift volcanic rocks within the local Permo–Triassic basins. Metal precipitation is linked to the destabilisation of Au and Pd chloride complexes by either mixing with calcic brines, dilution by groundwaters or interaction with reduced lithologies. This explains the diversity of mineralised settings below and above the unconformity and their affinity with red bed brines. The paucity of sulphide minerals, the development of selenides (as ore minerals and as mineral inclusion in gold grains), the presence of rhodochrosite and manganoan calcites (up to 2.5 wt% Mn in calcite) and the co-precipitation of hematite and manganese oxides are consistent with the overall high oxidation state of the ore fluids. A genetic model is proposed linking Permo–Triassic red beds, the mixing of oxidising and reducing brines, and the development of unconformity-related precious metal mineralisation. Comparison with other European Permo–Triassic basins reveals striking similarities in geological setting, mineralogy and geochemistry with Au, Au-Pd and selenide occurrences in Germany (Tilkerode, Korbach-Goldhausen), Poland (Lubin) and the Czech Republic (Svoboda nad Úpou and Stupná). Though the known Au-Pd occurrences are sub-economic, several predictive criteria are proposed for further exploration.

Abstract Two examples of Dinantian basin-margin carbonates from the United Kingdom provide potential analogues for fracture and porosity distributions in hydrocarbon reservoirs in dolostones. In both examples pore systems have been extensively modified by dolomitization, fracturing and related mineralization. However, the detailed processes and the end results show some significant differences, highlighting the importance of developing an understanding of the specific pore-system modifying processes when characterizing and modelling porosity distributions in these settings. In the first example, predominantly low-porosity and low-permeability limestones in Lower Carboniferous inliers in Leicestershire (Cloud Hill) and south Derbyshire (Ticknall) had their pore systems further degraded by extensive dolomitization. Subsequent fracturing and related mineralization were responsible for significant porosity generation adjacent to fractures. In contrast, in the Sellafield area (west Cumbria), dolomitization was strongly controlled by fractures that were also mineralized by sulphate-rich brines. In the north of the area fractures were filled with an assemblage of barite-fluorite-hematite-calcite that is resistant to corrosion by low-temperature meteoric groundwater. However, in the south of the area the fractures were cemented by anhydrite, which is readily corroded by saline, but sulphatepoor, groundwater formed by percolating meteoric recharge from the east. Progressive dissolution has been ongoing since Tertiary uplift, and has rejuvenated fracture porosity within the dolomitized limestones.

Apatite grains in heavy-mineral separates of samples from the Statfjord Formation (Triassic-Lower Jurassic, North Sea), exhibit variably corroded detrital cores and euhedral diagenetic overgrowths. In the overlying marine deposits of the Nansen Formation, apatite grains show no evidence of either corrosion or overgrowths. Compositions of detrital apatite in both formations are indistinguishable, indicating that the source areas responsible for Statfjord and Nansen deposition included similar apatite-bearing lithologies. Apatite corrosion in the Statfjord Formation is considered to be a response to leaching by acidic groundwaters during sediment storage on the fluvial floodplain. Any variations in the degree of corrosion that may have been generated by differences in floodplain residence time have been homogenized by sediment reworking.Laser-ablation microprobe inductively coupled plasma mass spectrometry and wavelength-dispersive electron microprobe analysis indicate that the authigenic apatite contains high abundances (up to 3.5% total) of the rare earth elements (REE), implying a degree of REE mobilization during diagenesis. REE profiles (NASC normalized) for the authigenic apatites are convex-upwards with enrichment of the MREE. The REE are considered to have been derived from dissolution of other REE-bearing heavy minerals (e.g., garnet) during alluvial storage, with possible contributions from dissolved feldspars (which may account for slight positive Eu anomalies) and REEs adsorbed onto clay surfaces. REE mobilization was probably enhanced by fluoride and/or phosphate complexes, although organic complexing related to pedogenic processes is also a possibility. Through the Statfjord Formation, authigenic apatite displays subtle variations in major-element, minor-element, and trace-element chemistry, which may find application as a tool for diagenetic correlation.

Titanite occurs as a pore filling cement phase in Permian sandstones at the Cock of Arran, Scotland. Wavelength dispersive-electron probe microanalysis and ion probe microanalysis reveals that the titanites contain appreciable concentrations of rare earth and high field strength elements which are partitioned between well developed sector zones. Sector zoning occurs as a result of differential incorporation of impurity elements between different growing titanite crystal faces. In this example the sector zonation is considered to be the result of different crystal faces exposing different lattice site configurations to the growth medium with different propensities for impurity element incorporation. Temporal variations in rare earth element chemistry are also present and suggest that the effective partition coefficients (mineral/fluid) are significantly higher for the HREE than the LREE in this system.

Abstract Diagenetic francolite (carbonate fluor-apatite) occurs as overgrowths on detrital apatite grains in sandstones from the Lower Jurassic Statfjord Formation of the North Sea. The francolite overgrowths are considerably more enriched in the rare-earth elements (up to 1 wt % Ce2O3), Sr and F than the detrital cores. These elements were carried in aqueous solution, probably in the form of complex ligands involving organically sourced carbon and halogens. It is possible that reported aberrant neodymium isotope model ages within the Statfjord Formation are the result of mobilization and relative fractionation of Sm and Nd during diagenesis, rather than a result of changes in provenance.