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Faults slip in response to changes in stress or fluid pressure, and these changes can be natural or anthropogenic. Estimating the likelihood of fault slip for a given change in loading is critical for safe geological storage and energy extraction in faulted rocks, as well as effective communication of risks to policy makers and the public. The energy transition and decarbonization are urgent and essential tasks: we will only be successful if we manage to balance public perceptions of risk with the technical challenges inherent to the exploitation of faulted rock. To accomplish both, we need to do a better job of quantifying the uncertainties in our mechanical and geometrical data. Measures of fault stability include slip (Ts) and dilation (Td) tendency, and fracture susceptibility (Sf, the change in fluid pressure to push effective stress to failure). The input values for any of these measures are always uncertain, and they are uncertain to varying degrees. For example, while the vertical stress can be well constrained from wireline density log data, the maximum horizontal stress is generally much harder to quantify from any source.Probabilistic models of fault stability for the Mississippian (Lower Carboniferous) Limestone underlying much of northern England are presented. This is an undrilled target for geothermal energy. Fault maps are derived from published geological maps and recently reprocessed seismic reflection data. Stress and pressure constraints are derived from legacy onshore hydrocarbon wells and wireline logs. Coefficients of friction and cohesive strength remain poorly constrained, not only in terms of their magnitude, but critically in the shapes of their statistical distributions. In addition, the applicability of simplified indices of fault stability (Ts, Td, Sf) to complex natural fault zones is questionable, and our predictions could be improved through weighting by information derived from long-term seismological records. This contribution raises two key points: 1) Laboratory data rarely include repeat measurements from quasi-identical samples of the same rock, and therefore the statistical distributions of friction or cohesion are poorly known. This is important because skewed distributions – and the direction of that skewness (high or low) – can significantly affect predictions of fault stability; 2) What is the best way to weight the calculated probability of slip in complex natural fault zones to account for geometrical weakening? Specifically, is total fault length more or less important than fault smoothness (roughness)?

Determination of mineral texture and diagenetic features in mudstones is crucial to reveal the history of their pore systems and provides key information to predict their future sealing ability, reactivity and storage capacity for sequestered CO 2, hydrogen storage or nuclear waste disposal. To understand the spatial transport and storage of fluids, it is necessary to map the distribution of minerals and fractures in three dimensions (3D). This study proposes a novel, multi-scale three-dimensional (3D) imaging method, i.e., a combination of synchrotron- sourced micro- x-ray tomography and lab- sourced nano-tomography, to investigate the sedimentology and diagenetic features of the Bowland Shale, one of the most volumetrically important mudstone-dominated systems in the UK. Diagenetic minerals have been identified and characterised, including pyrite, calcite, kaolinite, illite, chlorite, dolomite, ankerite and authigenic quartz (micro-sized quartz and quartz overgrowths). Multi-scale 3D images provide detailed information about dolomite-ankerite zonation and carbonate dissolution pores. These features cannot be observed or quantified by conventional 2D methods, and they have not been reported in this subject area before. Using these results, potential reactions during carbon storage and other subsurface storage applications are predicted.

In this study, we propose a multiple hypotheses approach to improve interpretations of limited remotely sensed datasets, such as sparsely exposed outcrops, subsurface datasets, or planetary objects using semi-quantitative scoring and ranking of observable features. This method is demonstrated using an outcrop example from the Broken Beds of the Upper Jurassic–Lower Cretaceous Purbeck Limestone Group exposed along Britain's Jurassic Coast. Four published hypotheses regarding their origin are refined, represented in matrix form, scored and ranked based on carefully selected outcrop features. Semi-quantitative scoring utilises knowledge of likely processes governing the occurrence of a range of features, some of which might be ignored or down-played to favour a single hypothesis. Furthermore, by integrating expertise from different sub-disciplines (e.g. basin analysis, sedimentology, diagenesis), we also consider the combined evidence of multiple features. This new method results in an interpretation that favours a multi-process origin for the Broken Beds due to evaporite dissolution, overpressure release and tectonic folding, with identified uncertainty, all useful to guide further data collection.

The calculation of a reliable temperature dependent dolomite solubility product constant (Ksp°−dol) has been the subject of much research over the last 70 years. This study evaluates log10(aCa2+/aMg2+) using PHREEQC (Pitzer approach) for a screened subset (n=11,480) of formation waters in the U.S. Geological Survey National Produced Waters Geochemical Database V2 (PWGD), an extensive inventory of 165,960 formational waters from a range of sedimentary lithologies in North America up to 6.6 km depth (Blondes and others, 2016). Through extensive ground-truthing against datasets sourced from Texas Gulf Coast basin and the Mississippi Salt Dome basin we establish that both the geochemical data from the PWGD and a new geothermal model of the US that is used to determine temperatures at-formation-depth to be reliable data sources. The vast majority (at least 90%) of PWGD samples have log10(aCa2+/aMg2+)-temperature values that are interpreted to be indicative of calcite-dolomite equilibrium and controlled by bulk mineral equilibria rather than Mg-calcite surface phases. Using statistical models with different parameterizations (different Maier-Kelly formulas, mixed-effects models with various random effects and linear models) log10(aCa2+/aMg2+) values (outcome variable) are regressed against temperature (fixed effect) calculating Ksp°−dol between 0-200°C using the well constrained calcite solubility product (Ksp°−cal). Local effects that modify log10(aCa2+/aMg2+) values are evaluated through the addition of random effects to the mixed model which improves the statistical reliability of the Ksp°−dol estimate and enables the determination of Ksp°−dol for local dolomite phases. The nature of these local effects is open to interpretation, but we suggest the primary influence on log10(aCa2+/aMg2+) values is the stoichiometry of the equilibrium dolomite phase within individual fields that systematically modifies log10(aCa2+/aMg2+) values. We discount the influence on log10(aCa2+/aMg2+) values from the ionic strength of the solution, the equilibration with anhydrite and chlorite group minerals, the illitization of smectite and albitization of feldspar. For the dolomite solubility equation; CaMg(CO3)2 (s) ↔ Ca2+(aq)+ Mg2+(aq) + 2CO3(2-)(aq) (1)the mixed-effects model chosen as most representative yields a pKsp°−dol (log10Ksp°−dol); pK(sp-dol)= 1.47545×10^1 - 6.24959×10^-2∙T(K) - 3.99350×10^3∙1/T(K) (2)At 25°C pKsp°−dol = -17.27±0.35, which is close to prior estimates including the most recent experimental value reported by B´en´ezeth and others, 2018 (pKsp°−dol = -17.19±0.3). This validates this study’s approach and enables conclusions to be drawn via a meta-analysis of a contaminated, though expansive dataset.

Understanding and predicting architecture and facies distribution of syn-rift carbonates is challenging owing to complex control by climatic, tectonic, biological and sedimentological factors. CarboCAT is a three-dimensional stratigraphic forward model of carbonate and mixed carbonate–siliciclastic systems that has recently been developed to include processes controlling carbonate platform development in extensional settings. CarboCAT has been used here to perform numerical experiment investigations of the various processes and factors hypothesized to control syn-rift carbonates sedimentation. Models representing three tectonic scenarios have been calculated and investigated, to characterize facies distribution and architecture of carbonate platforms developed on half-grabens, horsts and transfer zones. For each forward stratigraphic model, forward seismic models have also been calculated, so that modelled stratal geometries presented as synthetic seismic images can be directly compared with seismic images of subsurface carbonate strata. The CarboCAT models and synthetic seismic images corroborate many elements of the existing syn-rift and early-post-rift conceptual model, but also expand these models by describing how platform architecture and spatial facies distributions vary along-strike between hanging-wall, footwall and transfer zone settings. Synthetic seismic images show how platform margins may appear in seismic data, showing significant differences in overall seismic character between prograding and backstepping stacking patterns.

The dominant paradigm for petrogenesis of high-temperature fault-controlled dolomite, widely known as “hydrothermal dolomite” (HTD), invokes upwelling of hot fluid along faulted and fractured conduits from a deep over-pressured aquifer. However, this model has several inherent ambiguities with respect to fluid sources and their dolomitisation potential, as well as mechanisms for delivering enough of these reactive fluids to form substantial volumes of dolomite. Here, we use generic 2D and 3D reactive transport simulations of a single transmissive fault system to evaluate an alternative conceptual model whereby dolomitisation is driven by seawater being drawn down into the subsurface and heated. We examine the evolution of fluid chemistry and the distribution of diagenetic alteration, including predictions of the rate, distribution, and temperature of HTD formation, and consider the possible contribution of this process to the Mg budget of the world's oceans. The simulations suggest that it is possible for convection of seawater along the fault damage zone to form massive dolomite bodies that extend hundreds of metres vertically and along the fault within a timescale of a few tens of thousands of years, with no significant alteration of the country rock. Dolomitisation occurs as a gradient reaction by replacement of host limestones and minor dolomite cementation, and it results in the discharge of Mg2+-poor, Ca2+-rich fluids to the sea floor. Fluids sourced from the basement contribute to the transport of heat that is key for overcoming kinetic limitations to dolomitisation, but the entrained seawater provides the Mg2+ to drive the reaction. Dolomite fronts are sharper on the “up-flow” margin where Mg2+-rich fluids first reach the threshold temperature for dolomitisation, and the “down-flow” dolomite front tends to be broader as the fluid is depleted in Mg2+ by prior dolomitisation. The model demonstrates spatial contrasts in the temperature of dolomitisation and the relative contribution of seawater and basement-derived fluids which are also commonly observed in natural fault-controlled dolomites. In the past, such variations have been interpreted in terms of major shifts in the system driving dolomitisation. Our simulations demonstrate that such changes may also be a product of emergent behaviour within a relatively stable system, with areas that are dolomitised more slowly recording the effect of changes in fluid flow, heat, and solute transport that occur in response to diagenetic permeability modification. Overall, our models robustly demonstrate that high-temperature fault-controlled dolomite bodies can form from mixed convection and act as a sink for Mg in the circulating seawaters. In addition, comparison of our 3D simulations with simplifications to 2D indicate that 2D models misrepresent critical aspects of the system. This has important implications for modelling of systems ranging from geothermal resources and mineralisation to carbonate diagenesis, including hydrothermal karstification and ore genesis as well as dolomitisation.

[Variability of dolomitizing fluid sources across the Western Canadian Sedimentary Basin, showing that basinal brines interacted with the Precambrian basement and immature basal clastic aquifers in the West Shale Basin and Peace River Arch area, and with basal carbonate aquifers in the East Shale Basin. , Abstract Dolomitization in the Western Canadian Sedimentary Basin has been extensively researched, producing vast geochemical datasets. This provides a unique opportunity to assess the regional sources and flux of dolomitizing fluids on a larger scale than previous studies. A meta‐analysis was conducted on stable isotope, strontium isotope (87Sr/86Sr), fluid inclusion and lithium‐rich formation water data published over 30 years, with new petrographic, X‐ray diffraction, stable isotope and rare‐earth element (REE+Y) data. The Middle to Upper Devonian Swan Hills Formation, Leduc Formation and Wabamun Group contain replacement dolomite (RD) cross‐cut by stylolites, suggesting replacement dolomitization occurred during shallow burial. Stable isotope, REE+Y and 87Sr/86Sr data indicate RD formed from Devonian seawater, then recrystallized during burial. Apart from the Wabamun Group of the Peace River Arch (PRA), saddle dolomite cement (SDC) is more δ18O(PDB) depleted than RD, and cross‐cuts stylolites, suggesting precipitation during deep burial. SDC 87Sr/86Sr data indicate contributions from 87Sr‐rich basinal brines in the West Shale Basin (WSB) and PRA, and authigenic quartz/albite suggests basinal brines interacted with underlying clastic aquifers before ascending faults into carbonate strata. The absence of quartz/albite within dolomites of the East Shale Basin (ESB) suggests dolomitizing fluids only interacted with carbonate strata. We conclude that replacement dolomitization resulted from connate Devonian seawater circulating through aquifers and faults during shallow burial. SDC precipitated during deep burial from basinal brines sourced from basal carbonates (ESB) and clastic aquifers (WSB, PRA). Lithium‐rich formation waters suggest basinal brines originated as residual evapo‐concentrated Middle Devonian seawater that interacted with basal aquifers and ascended faults during the Antler and Laramide Orogenies. These results corroborate those of previous studies but are verified by new integrated analysis of multiple datasets. New insights emphasize the importance of basal aquifers and residual evapo‐concentrated seawater in dolomitization, which is potentially applicable to other regionally dolomitized basins.]

During the Cretaceous, high global sea‐level and low latitudinal temperature variations led to the growth of epeiric carbonate platforms. Platform‐scale dolomitization of these platforms is not common, reflecting the low Mg/Ca ratio of seawater and a humid climate. This study describes the processes governing pervasive dolomitization of a land‐attached carbonate platform within the Iberian Basin. Dolomite is planar to sub‐planar with a geochemical signature consistent with dolomitization from penesaline seawater. Dolomitization was most pervasive during a 1 Myr period in the middle Cenomanian, by repeated reflux of seawater from brine pools formed on the top of a southward‐prograding carbonate platform. Tilting and structural reorganization in the Upper Cenomanian led to a reversal in polarity of the platform, and dolomitization was restarted by the northward reflux of seawater. Rising relative sea‐level and oceanic acidification led to back‐stepping of the platform such that the supply of dolomitizing fluids was cut off. In the Lower Turonian, pervasively dolomitized rudist rudstone facies in the south of the study area indicate that dolomitization restarted, either penecontemporaneously or later, from highly evaporated Campanian–Maastrichtian seawater. A systematic increase in dolomite crystal size up‐section ties broadly, but not entirely, to stratigraphy. It is possible that these textural differences reflect changes in fluid chemistry, limestone permeability or precursor rock texture. However, the lack of stratigraphic conformance, and the preservation of the earliest‐formed dolomite only in the oldest sediments, could indicate a progressive recrystallization of early‐formed dolomite through repeated reflux of brines. As such, the succession appears to preserve a fossilized record of dolomite recrystallization through time during the Cenomanian–Turonian. The results of this study therefore provide a record of the progressive dolomitization of a carbonate platform and demonstrate the important interplay of climate and basin‐scale tectonics on dolomite distribution and crystallinity.

Coral carpets are biostromes that form laterally continuous but low-relief (typically less than 1-metre-high) coral communities with a lack of clear internal zonation. There are few locations that allow analysis of such systems on a regional (tens of kilometre) scale in the rock record. This study describes extensive, well-exposed, coral-rich bodies (coral carpets), deposited on the Moroccan northwest Atlantic shallow-marine margin during the Cretaceous (early Hauterivian). The coral-rich sequence reaches 50 m in thickness, with individual carpets between 10 and 100 cm thick, extending over an area of more than 150 km 2. Logging and regional correlation, integrated with thin-section analysis, has allowed the evaluation of facies distribution, geometries and temporal evolution. The results provide information on allogenic and/or autogenic driving processes for coral development, and allows the generation of conceptual models for their growth. The Tamanar Fm. corals reflects a shallowing up sequence and the recovery of carbonate productivity following stressed environmental conditions in the late Valanginian. Temporally, coral morphological response relates to high-frequency relative sea level changes that affected water energy and turbidity; thick-branching and massive forms grew in shallow high energy environments whilst platy/flat forms grew in poorly lit and turbid environments. However, local-scale (over hundreds of metres) lateral changes in morphology and facies can be explained by autogenic factors independent of sea level change; varying sedimentation, hydrodynamism and turbidity across the extensive open marine platform. The termination of coral deposition is marked by a hardground surface, indicating slow rates of sedimentation and a transgression, which is followed by an influx of clastic sediments onto the platform, resulting in the deterioration of carbonate platform health, as conditions became intolerable for coral growth. These coral-rich outcrops contribute to the relatively sparse documentation of Hauterivian corals globally and suggest niche palaeoecological and palaeoclimatic relationships that allowed corals to develop despite stressed platform conditions. The low diversity, low-relief coral carpets may provide a proxy for the recovery and demise of corals in response to environmental stress on modern carbonate platforms and future climate change projections.

Porosity formed after compaction has been recognised for many years in carbonate rocks, but there is little consensus as to the origin of the fluids responsible for dissolution, and the processes by which pores are generated. In particular, it is often hard to identify the source of fluids that are below calcite saturation, and in sufficient volumes, to explain the apparent volume of porosity. This study directly addresses this challenge through analysis of core data from a number of oil fields across north Oman, which produce from the lower Cretaceous (Aptian) Shuaiba Formation. The Shuaiba Formation in all these fields exhibits high total porosity, comprising pores that range in size from less than a micron to centimetres in diameter. There is an increase in the effective porosity upwards within the succession; permeability is highest in the upper 15 m of the formation, beneath the top-Shuaiba unconformity. Since this unconformity represents a hiatus of over 5 million years (My), caused by platform emergence, previous studies often interpreted porosity modification to be due to dissolution by groundwater during penecontemporaneous exposure of the platform. Nevertheless, paragenetic relationships indicate that most of the secondary porosity that was formed early in the burial history was either occluded by drusy calcite cements, or destroyed by compaction. Furthermore, many of the drusy calcite cements are etched, indicating that dissolution took place after cementation. Consistent with this, many of the pores formed on the margins of tectonic fractures and stylolites, indicating that some pore-formation post-dates compaction. The top of the Shuaiba Formation in all fields is between 600 and 1600 m, following uplift in the late Cretaceous and early Oligocene by inversion along deep-seated, strike-slip faults. Possible causes of dissolution are cooling of upward-rising brines fluxed along faults and corrosion by organic acids prior to hydrocarbon emplacement. It is unclear, however, as to the extent to which these fluids could dissolve carbonate before reaching equilibrium. A third possible fluid is groundwater. Sulphur isotope data from paragenetically late pyrite, is consistent with bacterial sulphate reduction, indicative of relatively cool temperatures (

Emergent surfaces in the Mississippian (Asbian to Brigantian) carbonate platform succession of North Wales record periods of plant colonisation and peat formation that led ultimately to the local development of coals. Examination of bituminous coals on three emergent surfaces within Cefn Mawr Quarry reveals information on palaeoclimate that is not available from study of the limestones alone. Three coal seams in the Asbian Loggerheads Limestone Formation were identified and the lowest one studied in detail. Vitrinite reflectance data from alternating bands of vitrite and duroclarite microlithotypes, the distribution of pyrite within them, and the sharp contacts between them, suggest that there were abrupt changes in marine influence during the development of the peats that formed the coals. It is inferred that local palaeoclimate alternated between periods of high and low rainfall, the amount of rainfall influencing the extent to which seawater encroached into the peats, with higher rainfall suppressing the ingress of saline waters into groundwater. On the basis of modern peat growth rates, the timescale of the alternation indicated by each duroclarite-vitrite couplet is suggestive of an annual cycle, such as would arise in a monsoonal climate. The low proportion of ash in the three coals, the preservation of internal lamination, the low diversity of spore species in the lowest coal compared with the over- and underlying mudrock, and the presence of rhizoconcretions in palaeokarstic limestone beneath the lowest and highest coals, demonstrate that the peat swamps were isolated from the hinterland and autochthonous. This study demonstrates that a wider application of palynology and coal petrology is an important contribution to the study of marine carbonate successions of any age where terrestrial organic matter, formed during emergence, has been preserved.

The Mississippian Derbyshire and North Wales carbonate platforms were formed in similar tectonic settings within the Pennine and East Irish Sea Basin, respectively. The Derbyshire Platform was surrounded by sub-basins to the north, west, and south whilst the North Wales Platform, 130 km west, had a simpler land-attached geometry. Comparison of these age-equivalent platforms allows the controls on sedimentation, at an important juncture in Earth history, to be evaluated. Both platforms are dominated by moderate-to-high-energy, laterally discontinuous facies, with weak evidence for facies cyclicity, suggesting multiple controls on deposition. Influx of siliciclastic mud on the North Wales Platform led to perturbations in carbonate accumulation; along with abundant palaeosols and coal beds this implies a more humid climate, or shallower water depths compared to the Derbyshire Platform. On both platforms, exposure surfaces can rarely be correlated over >500 metres except for a regionally correlative palaeokarstic surface at the Asbian-Brigantian boundary. This exposure event appears to coincide with a significant regional facies change. Given the lack of evidence for ordering and cyclicity within the strata, the Asbian-Brigantian boundary may mark a significant event that could reflect onset of a transitional climate, prior to the second glaciation event in the Late Palaeozoic Ice Age.

Fault-controlled dolomitization has been documented in Lower Carboniferous (Viséan) platform carbonates at various localities in the Pennine Basin and North Wales. The largest of these dolomite bodies (approx. 60 km2) occurs on the Derbyshire Platform, on the southern margin of the Pennine Basin. This study tests the hypothesis that dolomitization occurred at this locality during deposition, platform drowning, and the earliest stages of burial, coincident with the transition from a late syn-rift to post-rift regime. It also assesses the importance of syn-rift volcanism on dolomitization. Planar, fabric-retentive dolomite with single-phase (i.e., low temperature) fluid inclusions occurs along NW–SE and E–W oriented faults, and in platform margin facies and in proximity to the Masson Hill Volcanic Complex. Oxygen isotope data are consistent with dolomitization from seawater, but slightly depleted δ13C values reflect mixing with magmatic fluids. Volcanic activity is likely to have produced a thermal drive for fluid circulation on the platform margin, and post-depositional alteration of basalts by CO2-rich fluids could have led to alteration of olivine and release of magnesium to convecting seawater. Consequently, the large volume of dolostone on the southern margin of the Derbyshire Platform is attributed to the increased geothermal gradient and a localized increase in the Mg/Ca ratio of dolomitizing fluids at this locality, compared to elsewhere in the Pennine Basin. The results suggest that syn-rift carbonate platforms in volcanically active areas of rift basins have a greater potential for dolomitization from seawater than non-volcanic platforms in the same basin.

Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurring where limestone (CaCO3) is replaced by dolomite (CaMg (CO3)2) under a wide range of crystallization temperatures and fluids. The processes governing its formation have been well studied, but the controls on the position of dolomitization fronts in ancient natural settings, particularly in a fault-controlled hydrothermal system (HTD), have received remarkably little attention. Hence, the origin and evolution of HTD dolomitization fronts in the stratigraphic record remain enigmatic. Here, a new set of mineralogical and geochemical data collected from different transects in a partially dolomitized Cambrian carbonate platform in western Canada are presented to address this issue. Systematic patterns of sudden decrease in the magnesium content (mol% MgCO3) and increase in porosity were observed towards the margin of the body. Furthermore, fluid temperatures are cooler and δ 18 Owater values are less positive at the dolomitization front than within the core of the body. These changes coincide with a change from poorly ordered, planar-e dolomite with multiple crystal zonations at the margin, to an unzoned, well-ordered, interlocking mosaic of planar-s to nonplanar dolomite in the core of the body. These phenomena are hypothesized to reflect dynamic, self-limiting processes in the formation and evolution of HTD dolomitization fronts through (i) plummet of dolomitization potential at the head of dolomitizing fluids due to progressive consumption of magnesium and fluid cooling; and (ii) retreat of dolomitization fronts towards the fluid source during subsequent recrystallization of the dolomite body, inboard of the termination, once overdolomitization took place. This new insight illustrates how dolomitization fronts can record the oldest phase of dolomitization, instead of the youngest as is often assumed. Formation of porosity is interpreted to occur as the result of acidification-induced grain leaching during the development of dolomitization fronts. This mechanism, coupled with retrogradation of dolomitization fronts, may help to explain the apparent enhancement of porosity in proximity to dolomitization fronts.

Fault‐controlled hydrothermal dolomitisation in tectonically complex basins can occur at any depth and from different fluid compositions, including ‘deep‐seated’, ‘crustal’ or ‘basinal’ brines. Nevertheless, many studies have failed to identify the actual source of these fluids, resulting in a gap in our knowledge on the likely source of magnesium of hydrothermal dolomitisation. With development of new concepts in hydrothermal dolomitisation, the study aims in particular to test the hypothesis that dolomitising fluids were sourced from either seawater, ultramafic carbonation or a mixture between the two by utilising the Cambrian Mount Whyte Formation as an example. Here, the large‐scale dolostone bodies are fabric‐destructive with a range of crystal fabrics, including euhedral replacement (RD1) and anhedral replacement (RD2). Since dolomite is cross‐cut by low amplitude stylolites, dolomitisation is interpreted to have occurred shortly after deposition, at a very shallow depth (

The Asmari reservoir properties in various hydrocarbon fields across the Dezful Embayment (SW Iran) are modified by dolomitization. Petrographic and geochemical analyses of the dolomitized Asmari Formation units highlight their origin in the Bibi Hakimeh and Gachsaran oilfields. The petrographic studies revealed that dolomite in the Asmari Formation can be classified texturally (based on crystal sizes) into three main groups: (i) very fine crystalline (D1, 150 μm). Petrography, fluid inclusions, stable isotopes (δ13C, δ18O), radiogenic isotopes (86Sr/88Sr) and major and trace elemental analyses were used to interpret the origin and paragenetic sequence of each dolomite group. Pervasive dolomitization prevails in the upper part of the Asmari Formation where very fine and medium crystalline dolomites with partially and fully fabric-retentive and fabric destructive textures, replaced micrite and allochems of the platform-top facies, respectively. The parent fluids of the very fine dolomites originated from contemporaneous Oligo-Miocene seawaters produced in a sabkha environment. The fine to medium dolomites (D2) most likely shared the same seawater origin and relate to refluxing, sabkha-derived fluids. The medium to coarse crystalline dolomite has more depleted oxygen isotopes than the former, very fine to fine crystalline, dolomites suggesting a burial dolomitization origin. The estimated high salinities (≈19 to 20 wt% NaCl) together with compatibility of oxygen and strontium isotope data with evaporative seawater values, suggest Oligo-Miocene seawater was the parent fluid for precipitation of coarse and cement phase of the medium crystalline dolomites. This study reveals that reactivated basement faults with associated fractures have probably played an important role in convecting and heating evaporative seawater along their planes. This heated seawater provided the required fluid for recrystallization of coarser matrix and cement phases of medium (D2) and coarse (D3) dolomites at depth. Therefore, the interaction of climate, sea-level changes, and tectonics played a critical role in formation and development of pervasive dolomitization in the Asmari Formation. Additionally, dissolution (partly as a consequence of dolomitization) was one of the key processes during early diagenesis in the formation of different types of porosity such as biomolds, vugs and intraparticles porosity. This study shows that the reservoir quality improves from finely crystalline dolomite towards samples dominated by coarsely crystalline dolomites.

An appreciation of the diagenetic processes impacting basinal mudstones is important for understanding the properties of mudstones and the nature of fluids which might have been expelled onto adjacent carbonate platforms. This study utilizes petrographic and mineralogical data from mudstones from the Serpukhovian Morridge Formation in the Widmerpool Gulf, UK to investigate the paragenetic succession and associated mineral precipitation during diagenesis of mudstones. Authigenic minerals in the studied samples contain kaolinite, calcite, non-ferroan and ferroan dolomite, pyrite, ankerite, and quartz. The Si/Al ratios in most samples are above 2.5, indicating that most of the studied samples contain excess silica. The widespread presence of authigenic kaolinite and quartz provides important information on the sources for dissolved silica in pore water. The presence of pyritized radiolarians indicates that the dissolution of siliceous radiolarian skeletons is the most likely principle silica source within the system. The illitization of kaolinite also provided a possible silica source for quartz overgrowth during burial diagenesis. It has been long-proposed that the supply of base metals (i.e. Ba, Zn and Mg) for mineralization on the adjacent Derbyshire Platform came from juxtaposed basins, but evidence for this has been limited. The results of this study show that the diagenetic evolution of mudstones in the late Carboniferous links closely to the mineralization observed on the Derbyshire Platform. In the Widmerpool Gulf, the source of magnesium incorporated into dolomite is likely from basinal brine that also provided a source of barium for the barite mineralization. Based on the burial history model, the studied samples were buried to 1.3 km with a temperature of 82 °C in the late Carboniferous. Hydrocarbon was generated during maturation of organic matter and migrated with basinal brines through factures into surrounding formations during basin inversion. This mixed fluid likely contributed to the mineralization on the Derbyshire Platform.

In structurally-controlled dolomitization systems, there is a general consensus that the formation of dolostone-limestone transition, termed here as “dolomitization fronts”, is governed by either the presence of an ultra-low permeability zone (fluid barrier) or changes in dolomitization potential and kinetics. However, the actual processes controlling the abrupt termination of dolostone bodies and their corresponding morphology and dimension are still relatively poorly understood. To address these challenges, we aim to (i) review the different origin and styles of structurally-controlled dolomitization fronts in the stratigraphic record and (ii) provide a standardized framework and quantitative insight to describe and interpret dolomitization fronts.To achieve this, field observations across geologic timescales and geodynamic settings are complimented with published data to document different styles of structurally-controlled dolomitization fronts. The results should that the following morphologies are associated with both tabular and columnar dolostone bodies: (i) lateral contact/bed-perpendicular fronts; (ii) vertical contact/bed-parallel fronts; and (iii) complex-shaped fronts at the distal part of dolostone bodies. This morphological information, when coupled with detailed petrography, mineralogical and geochemical data could help to accurately reveal the governing processes behind the termination of dolostone bodies and their corresponding reaction fronts geometries. Our review shows that the first front type is primarily controlled by the interplay between intrinsic properties of the host rocks, dolomitizing fluids, and self-organization process. In contrast, the second front type is governed by the presence of laterally continuous depositional, diagenetic, or structural fluid barriers, creating a significant permeability contrast across beds. The formation of complex-shaped fronts is interpreted to be controlled by a combination of original lithological composition and kinetics.This overview provides the first multi-study categorisation of ancient dolomitization fronts and the controls on their formation at a range of scales. This improves our understanding of low temperature metasomatic processes, and their termination, in sedimentary systems. Furthermore, it highlights how accurate interpretation of the origin and styles of dolomitization fronts can improve our understanding of dolomitization processes, paleofluid flow, and distribution of economic resources in dolomitized carbonate platforms, which can be challenging to determine from the dolostone bodies themselves, where they have undergone multiple phases of recrystallization and diagenetic overprinting.

The Upper Jurassic Arab Formation, onshore southern UAE, comprises an upward-shallowing succession of slope to basinal wackestones (lower Arab D Member) oolitic grainstone (upper Arab D Member) and peritidal - subtidal carbonate-evaporites (Arab A-C Members). This study focuses on the upper Arab D Member, the volumetrically most important reservoir unit in the field, which is dominated by oolitic and skeletal oolitic grainstones with high porosity and moderate to low permeability. Through much of the upper Arab D Member, porosity has been inverted so that ooids are porous and interparticle macropores cemented by pre-compactional non-ferroan calcite. Cementation is most pervasive within cross-bedded skeletal oolitic grainstones, occluding primary interparticle macropores and biomoulds and penetrating the margins of altered ooids. In contrast, bioturbated pack-grainstones are often only partially cemented, and retain a well-connected primary interparticle macropore network. Ooid alteration and porosity inversion in oolitic grainstones is often explained by open-system recrystallization by meteoric fluids during platform emergence. In this study, however, deposition took place in an arid basin that was eventually desiccated and hence the supply of groundwater would have been limited. The high concentrations of Mg and Sr within both the bulk rock limestone and cements suggest recrystallization could have occurred by marine pore fluids whilst, petrographical and geochemical homogeneity, including a narrow range of δ 18 O calcite (−5%0 ± 1%0), can be explained by closed-system stabilization of aragonite. Subsequent calcite cemented fractures and drusy, sparry pore filling calcite contain petroleum and aqueous inclusions with homogenisation temperatures up to 150 °C The preservation of the highest macropore volumes on the palaeocrest of the field, and an off-flank reduction in porosity and permeability, suggests that oil emplacement terminated calcite cementation. The results of this study demonstrate that a major reorganization of porosity can occur during the stablisation of metastable grains without invoking open-system circulation of meteoric water. Overall, the data provides an insight into possible controls on so-called porosity inversion, a poorly understood phenomenon in many hydrocarbon-bearing oolitic grainstone reservoirs.

This research forms the basis for the applicability of the Dunham (1962) classification of carbonated rock lithofacies to the analysis of the giant Ballık travertine architecture, while reconstructing lateral and vertical environmental changes. This study provides an analogue for spring-related deposits encountered offshore Brazil and Angola by linking macroscopically travertine lithofacies distribution to depositional environments. The analysis is based on rock-building constituents such as gastropods, charophytes, intraclasts, phytoclasts, coated grains, dendrites etc., forming micro-sedimentary fabrics with different structures such as packstone, grainstone, wackestone and boundstone, these latter closely associated with crust of dendrites and phytoherm of reeds and bryophytes. Our findings indicate that the Ballık travertine area consists of a “Lower” and an “Upper Domain” reflecting different depositional environments. More specifically, the “Lower Domain” consists from west to east of a laterally complex amalgamation of extended pool, marsh pool and flood plain environments that formed from a mixture of spring and ground waters. The extended pond environment characterised by a boundstone facies of stromatolites in the west evolves eastward into a marsh pool and flood plain. This is because CO2 degassing and water temperature decreased as the water depth of the Lower Domain reduced towards the east. The marsh pool environment includes packstone to grainstone lithofacies and abundant wackestone lithofacies made of phytoclasts, whose crusts exhibit pustular fabrics. Moreover, the flood plains along with the marsh pool consist dominantly of packstone to grainstone lithofacies with many gastropods and intraclasts, interfingered with wackestone lithofacies made of phytoclasts. Irregular clotted fabrics, along with coated grains with radial fibres, high lime mud content with bioturbation are also present. The “Upper Domain” displays a laterally less heterogeneous palaeoenvironmental distribution with flooded slope and flood plain deposits. The eastern part of the “Upper Domain” is characterized by a systematic alternation of these environments, with intercalations of wackstone lithofacies made of phytoclasts, packstone to grainstone lithofacies made of intraclasts and lime muds as well as coated grains. The flood plain has coated grains having peloidal nuclei and coatings of sparry laminations and clotted fabric of peloids representing intraclasts, whereas, the flooded slope possesses coated grains with coatings of dendrites and nuclei of peloids, boundstone of stromatolites which have flat-laminated and columnar-laminated fabrics indicating a laminar discharge away from the spring(s). Alluvial fan and palustrine deposits with abundant bryophytes and reeds frequently interfinger with marsh pool environment in the “Lower Domain”, and with the flood plain and flooded slope environments in the “Upper Domain”. The results illustrate well how environmental changes identified in the two different domains have induced heterogeneity in reservoir-based depositional architecture. © 2020 Association Francaise pour l'Etude du Quaternaire. All rights reserved.

Trace and rare earth elements (REEs) are considered to be reliable indicators of chemical processes for the evolution of carbonate systems. One of the best examples of ancient carbonate successions (Berdiga Formation) is widely exposed in NE Turkey. The Lower Cretaceous limestone succession of Berdiga Formation may provide a case study that reveals the effect of ocean paleoredox conditions on diagenetic alteration. Measurement of major, trace and REEs was carried out on the Lower Cretaceous limestones of the Berdiga Formation, to reveal proxies for paleoredox conditions and early diagenetic controls on their geochemistry. Studied micritic limestone microfacies (MF-1 to MF-3) indicate deposition in the inner platform to a deep shelf or continental slope paleoenvironment during the Hauterivian-Albian. The studied limestone samples mainly exhibit low Mg-calcite characteristics with the general chemical formula of Ca98.35–99.34Mg0.66–1.65(CO3). They are mostly represented by a diagnostic REE seawater signature including (1) slight LREE depletion relative to the HREEs (ave. 0.72 of Nd/YbN and ave. 0.73 of Pr/YbN), (2) negative Ce anomalies (Ce/Ce* = 0.38–0.81, ave. 0.57), (3) positive La anomaly (La/La* = 0.21–3.02, ave. 1.75) and (4) superchondritic Y/Ho (ave. 46.26). Studied micritic limestones have predominantly low Hf (bdl to 0.5 µg/g), Sc (bdl to 2 µg/g) Th (bdl to 0.9 µg/g) contents suggesting negligible to minor shale contamination. These findings imply that micritic limestones faithfully record chemical signals of their parental and diagenetic fluids. The succession also exhibits high ratios of Eu/Eu* (1.01–1.65, ave. 1.29 corresponding to the positive Eu anomalies), Sm/Yb (1.26–2.74, ave. 1.68) and La/Yb ratios (0.68–1.35, ave. 0.9) compared to modern seawater and wide range of Y/Ho ratios (29.33–70.00, ave. 46.26) which are between seawater and hydrogenetic Fe-Mn crusts. Several lines of geochemical evidence suggest water-rock interaction between parental seawater and basaltic rocks at elevated temperatures triggered by hydrothermal activity associated with Early Cretaceous basaltic magma generation. The range of Ce/Ce* values is suggestive of mostly oxic to dysoxic paleoceanographic conditions, with a sudden change to dysoxic conditions (Ce/Ce* = 0.71–0.81), in the uppermost part of the MF-1. This is followed by an abrupt deepening paleoenvironment with a relative increase in the oxic state of the seawater and deposition of deeper water sediments (MF-2 and MF-3) above a sharp transition. The differences in microfacies characteristics and foraminifera assemblage between MF-1 and overlying facies (MF-2 and MF-3) may also confirm the change in paleoceanographic conditions. Therefore, REEs data obtained from studied limestones have the potential to contribute important information as to regional paleoceanographic conditions of Tethys during an important period in Earth history.

Despite the increasing interest in geologic co-sequestration of CO2 and H2S, the long-term consequences of the chemical interactions involved in this process remain largely unknown on a reservoir scale. A Mississippian aged CO2-H2S reservoir in LaBarge Field, Wyoming, USA is an ideal study site to investigate mineral and fluid reactions related to gaseous H2S and CO2. We conducted two reaction path models based on mineralogical, fluid, gas, and stable isotope compositional data to discern the role of CO2 influx upon the generation of H2S through thermochemical sulphate reduction (TSR). We discriminate between two models-one in which TSR is triggered by temperature at a given burial depth and one where TSR is triggered by ingress of CO2. The reaction path model based upon burial-controlled TSR and later CO2 influx is consistent with mineralogical observations and stable isotope measurements from drill cores. The models show that CO2 influx leads to calcite precipitation which is only limited by the calcium concentration in the fluid. This modelling approach is useful in constraining the timing of fluid flux in the reservoir and gives further insight into the mineralogical consequences of the gas, water, and rock interactions occurring in the reservoir. In terms of geologic co-sequestration this implies that the addition of CO2 into a reducing carbonate system can result in calcite precipitation, instead of anhydrite as previously thought. Furthermore, it is only limited by the availability of Ca2+ and will therefore not diminish the amount of H2S in the system.

Constructing geocellular models of carbonate rocks using standard software is challenging since most of modelling packages are designed, first and foremost, to represent siliciclastic depositional systems, where rock properties are strongly facies-controlled. The distribution and components of carbonate depositional facies vary drastically across the geological timescale as a result of paleoclimate and its effects on carbonate-producing biota. Furthermore, reservoir architecture is less strongly controlled by depositional environment than in clastic settings, and rock physical properties, including fracture networks, are controlled by both primary components and their subsequent diagenetic alteration. This means that rock property distribution is less predictable than in siliciclastic systems, and less well represented by geocellular models that are designed to represent sedimentary architecture. In other words, in carbonate systems, the depositional and diagenetic history needs to be reconstructed in order to successfully model reservoir properties. In this study a geocellular model was created by using a well-characterised outcrop analogue obtained from the Hammam Faraun Fault (HFF) Block, located on the eastern coast of the Gulf of Suez in Sinai, Egypt. This model integrates sedimentological, petrophysical, diagenetic, and structural information into a single database. The workflow utilizes the regional tectonic history, upscaled lithological logs, and two-stage facies modelling (reflecting in and ex situ depositional facies) and resulted in the creation a realistic model of remobilized carbonates that were deposited on the slope of a carbonate platform during a period of tectonic instability. Diagenetic overprinting was achieved using probability functions to reflect the history of burial, rifting, and the spatial relationship of stratabound and non-stratabound dolostone bodies. The study demonstrates a workflow for modelling mass-transport carbonate facies and multistage fault-related diagenesis so that flow controlling facies and diagenetically altered poroperm and fracture networks are accurately represented using commercially available modelling software, and in particular demonstrates how diagenetically controlled geobodies can be captured using simple algorithms.

There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of which host economic mineral deposits or hydrocarbons, but there remains a lack of consensus as to how they form. In particular, multiple phases of diagenetic overprinting can obscure geochemical fingerprints. Study of a Cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds. The Hammam Faraun fault (HFF) block within the Suez Rift, Egypt, hosts both massive and stratabound dolostone bodies. Non-fabric-selective massive dolostone is limited to the damage zone of the fault, while fabric-selective stratabound dolostone bodies penetrate nearly 2 km into the footwall. Oligo-Miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the HFF. Escape of fluids from the incipient HFF fault into the lower Thebes Formation led to differential, stratabound dolomitization. Once the HFF breached the surface, fluid circulation focused along the fault plane to form younger, massive dolostone bodies. This study provides a snapshot of dolomitization during the earliest phases of extension, unobscured by subsequent recrystallization and geochemical modification. Contrary to many models, stratabound dolomitization preceded non-stratabound dolomitization. Fluids were hydrothermal, but with little evidence for rapid cooling and brecciation common to many HTD bodies. These results suggest that many of the features used to interpret and predict the geometry of HTD in the subsurface form during later phases of structural deformation, perhaps overprinting less structurally complex dolomite bodies

Understanding the processes that drive fluid flow in sedimentary basins has important implications for models of metallic ore and petroleum migration and accumulation in the subsurface. Potential drive mechanisms include gravity-driven flow, compaction-driven flow, and density-driven convection. In this study, we demonstrate that the brine hosted in Ordovician sandstone in the Illizi Basin in Algeria is genetically linked to Triassic-Liassic evaporites deposited >400 km to the north in the Berkine Basin. This observation confirms that long distance, lateral, brine migration has occurred within the basin in the past. We assess the hydrogeologic record preserved in aqueous fluid inclusions within the Ordovician sandstone, document a marked increase in formation water salinity during cooling and exhumation, and evaluate the drive mechanisms for late-stage remobilization of deep brines within the basin. It is hypothesized that the release of overpressure during exhumation of the Illizi Basin may have been a critical contributor to updip fluid flux. This model could be applicable to other exhumed basins worldwide.

Late Visean platform interior carbonate strata of northern England and Wales have traditionally been interpreted to comprise shallowing-up cyclothems, the upper boundaries of which are marked by unconformities that record subaerial exposure events. These strata were deposited during what is understood to be a greenhouse-to-icehouse transition, and the cyclothems are assumed to reflect glacioeustatic sea-level oscillations associated with this climatic change. Importantly, these interpretations reflect a qualitative model which assumes that facies respond in a simple way to water depth variations and so migrate and stack in a predictable pattern as eustatic sea-level fluctuates. Exposure surfaces and most facies are, however, laterally discontinuous across the platform tops. Quantitative statistical analysis and simple 1D stratigraphic forward modelling suggest that the observed vertical facies stacking patterns on the Derbyshire and North Wales Platforms are not cyclical. The most ordered arrangement of facies that could be interpreted as “ideal cycles” in each section are variable within and across the platforms. This observational and analytical evidence suggests that instead of a simple, dominant external glacioeustatic forcing on sedimentation, a model commonly proposed for the Late Visean carbonate strata of northern England and Wales, strata were deposited as a facies mosaic, under variable hydrodynamic conditions influenced by autogenic processes as well as local environmental fluctuations in salinity and temperature, and perhaps in conjunction with glacioeustasy and syn-rift tectonic processes which occurred from metre-scale to kilometre-scale, as suggested by the ideal cycles.

The Mauddud Formation, a late Albian-aged carbonate oil reservoir, is one of the main oil producers in Kuwait, however, it exhibits unreliable oil production from the Bahrah Field. The Mauddud Formation in the Bahrah field is divided into two sub-structures by a series of E-W faults and shows a slight degradation in oil quality (~5API) within the up-dip structure. Intersections of the E-W faults with NW-SE faults are associated with deep-seated seismic pipes, which culminate in pull-up features, creating the overlying sag-like features. Additionally, the Mauddud Formation shows evidence of baroque (saddle) dolomite in fracture cements and replacing clasts. Fluid inclusions within the baroque dolomite have abnormally high salinity (>20 wt% NaCl eq), high homogenisation temperatures (up to ~100°C) and 3He/4He ratios ~0.3 times higher than atmospheric values. Taken together, this seismic and petrographic evidence suggests that the Mauddud Formation experienced an ingress of radiogenic, crustal fluids, potentially sourced from the basement. Seismic pipes, therefore, are interpreted to be the product of the upward movement of hydrothermal fluids that breach seal rocks, causing oil to escape and subsequent calcite cementation. An integrated map of a basement magnetic anomaly and current exhalative features on the present-day surface shows evidence of a relationship with fault locations on the level of the Mauddud Formation that were rejuvenated and associated with the remigration of oil. This study provides an exceptional example of a seal breach in one of the world’s most prolific hydrocarbon provinces. It also indicates the importance of a multidisciplinary, multi-scale approach extending from the basement to the surface in order to understand seal breach in carbonate petroleum systems. The overall data is used to assess the main factors that effected the integrity of the seal when this latter is breached by hydrothermal fluids.

Reservoirs that host CO2-H2S bearing gases provide a key insight into crustal redox reactions such as thermochemical sulphate reduction (TSR). Despite this, there remains a poor understanding of the extent, duration and the factors limiting this process on a reservoir scale. Here we show how a combination of petrography, fluid inclusion, rare earth element (REE) and carbon (δ13C), oxygen (δ18O) and sulphur (δ3424 S) stable isotope data can disentangle the fluid history of the world's largest CO2 accumulation, the LaBarge Field in Wyoming, USA. The carbonate hosted LaBarge Field was charged with oil around 80 Ma ago, which together with nodular anhydrite, represent the reactants for TSR. The nodules exhibit two distinct trends of evolution in δ13C with both δ34S and δ1827 O that may be coupled to two different processes. The first trend, was interpreted to reflect the coupled dissolution of anhydrite and reduction to elemental sulphur and the oxidation of organic compounds and associated precipitation of calcite during TSR. In contrast, the second trend was interpreted to be the result of the hydrothermal CO2 influx after the cessation of TSR. In addition, mass balance calculations were performed to estimate an approximate TSR reaction duration of 80 ka and to identify the availability of organic compounds as the limiting factor of the TSR process. Such an approach provides a tool for the prediction of TSR occurrence elsewhere and advancing our understanding of crustal fluid interactions.

Dolomitization is commonly associated with crustal-scale faults, but tectonic rejuvenation, diagenetic overprinting and a fluid and Mg mass-imbalance often makes it difficult to determine the dolomitization mechanism. This study considers differential dolomitization of the Eocene Thebes Formation on the Hammam Faraun Fault block, Gulf of Suez, which has undergone a simple history of burial and exhumation as a result of rifting. Stratabound dolostone bodies occur selectively within remobilized sediments (debrites and turbidites) in the lower Thebes Formation and extend into the footwall of, and for up to 2 km away from, the Hammam Faraun Fault. They are offset by the north–south trending Gebel fault, which was active during the earliest phases of rifting, suggesting dolomitization occurred between rift initiation (26 Ma) and rift climax (15 Ma). Geochemical data suggest that dolomitization occurred from evaporated (ca 1.43 concentration) seawater at less than ca 80°C. Geothermal convection is interpreted to have occurred as seawater was drawn down surface-breaching faults into the Nubian sandstone aquifer, convected and discharged into the lower Thebes Formation via the Hammam Faraun Fault. Assuming an approximate 10 Myr window for dolomitization, a horizontal velocity of ca 0.7 m/yr into the Thebes Formation is calculated, with fluid flux and reactivity likely to have been facilitated by fracturing. Although fluids were at least marginally hydrothermal, stratabound dolostone bodies do not contain saddle dolomite and there is no evidence of hydrobrecciation. This highlights how misleading dolostone textures can be as a proxy for the genesis and spatial distribution of such bodies in the subsurface. Overall, this study provides an excellent example of how fluid flux may occur during the earliest phases of rifting, and the importance of crustal-scale faults on fluid flow from the onset of their growth. Furthermore, this article presents a mechanism for dolomitization from seawater that has none of the inherent mass balance problems of classical, conceptual models of hydrothermal dolomitization.

Fault-controlled dolostone bodies have been described as potential hydrocarbon-bearing reservoirs. Numerous case studies have described the shape and size of these often non fabric selective dolostone bodies within the vicinity of crustal-scale lineaments, usually from Palaeozoic or Mesozoic carbonate platforms, which have undergone one or more phases of burial and exhumation. There has been little attention paid, however, to fault-strike variability in dolostone distribution or the preferential localization of these bodies on particular faults. This study focuses on dolostone bodies adjacent to the Hammam Faraun Fault (HFF), Gulf of Suez. This crustal-scale normal fault was activated in the Late Oligocene, coincident with the onset of extension within the Suez Rift. Dolomitization in the prerift Eocene Thebes Formation occurred in the immediate footwall of the HFF forming two massive, non facies selective dolostone bodies, ca. 500 m wide. Facies-controlled tongues of dolostone on the margins of the massive dolostone bodies extend for up to 100 m. The geochemical signature of the dolostone bodies is consistent with replacement by Miocene seawater, contemporaneous with the rift climax and localization of strain along the HFF. A conceptual model of dolomitization from seawater that circulated within the HFF during the rift climax is presented. Seawater was either directly drawn down the HFF or circulated from the hanging wall basin via a permeable aquifer towards the HFF. The lateral extent of the massive dolostone bodies was controlled by pre-existing HFF-parallel fracture corridors on the outer margins of the damage zone of the fault. The behaviour of these fracture corridors alternated between acting as barriers to fluid flow before rupture and acting as flow conduits during or after rupture. Multiple phases of dolomitization and recrystallization during the ca. 10 Ma period in which dolomitization occurred led to mottled petrographical textures and wide-ranging isotopic signatures. The localization of dolomitization on the HFF is interpreted to reflect its proximity to a rift accommodation zone which facilitated vertical fluid flow due to perturbed and enhanced stresses during fault interaction. It is possible that the presence of jogs along the strike of the fault further focused fluid flux. As such, it is suggested that the massive dolostones described in this study provide a window into the earliest stages of formation of fault-controlled hydrothermal dolostone bodies, which could have occurred in other areas and subsequently been overprinted by more complex diagenetic and structural fabrics.

This work evaluates a suite of carbonate rocks from Albian age in the Campos Basin — Brazil, complemented by data from the literature, totaling 472 samples with detailed description of diagenetic features, quantitative mineralogy analyses, and P- and S-wave velocities (Vp and Vs) measured at three ranges of effective pressure loading: low (5–7.5 MPa), moderate (20 MPa) and high (40–50 MPa) values. Digital image analysis (DIA) was applied on microtomography (μCT) images to quantitatively describe the macro-mesopore system of the Albian carbonates, and was extended to characterize different textures from literature data to estimate reference values for carbonates. The methodology utilized to predict the aspect ratio of microporosity assumes three pore-space scales in two representative scenarios: 1) measured macro-mesopore aspect ratio from DIA, and 2) predicted microporosity aspect ratio, using Vp measurement as the main input parameter. The differential effective medium model (DEMM) is combined with analytical theories of data analysis to characterize microporosity. Shear modulus and microporosity aspect ratio calibrated by this methodology were used to predict Vs, which was compared to experimental data, resulting in a good match for all samples. Polynomial curves are fitted with a variety of carbonate textures by velocities at effective pressure and bulk porosity crossplots, establishing important relationships for velocity prediction. The effects of effective pressure on the pore system within dry plugs of Albian samples were evaluated by combining triaxial measurements at 0–10 MPa, relative pore volume reduction (RPVR) and microporosity aspect ratio prediction. According to the results, micropores that exhibit low aspect ratio tend to close with stress and cause an increase on Vp and Vs. A wide textural heterogeneity of data base and different digital image analysis and resolutions were employed successfully, combining rock physics methodologies and concepts to characterize carbonate pore system as microporosity and pressure effects.

Determination of the distribution and mechanism for carbonate-dominated mass transport sediments is often compromised by the scale and access to exposures. Consequently, many studies lack the resolution to capture the heterogeneity and dimensions of mass transport deposits. This study documents the size, shape, and stratal assemblage of remobilized carbonates in the Eocene Thebes Formation in the Hammam Faraun Fault Block (HFFB) of western Sinai, revealing the complexities of carbonate mass transport deposits at sub-seismic scale. Present day pseudo three-dimensional exposure of the Thebes Formation in a large fault block, formed during the opening of the Gulf of Suez, allowed for lateral and down-dip measurement of slope and basinal facies in the field and from photos. Remobilized facies were digitized in the photos and evaluated using image analysis software, a technique with a wide range of applications to outcrop studies of sedimentary architecture. Debris flow deposits in the lower section of the Thebes Formation comprises clasts with differing fossil assemblages. A relative sea level rise at the start of upper Thebes Formation deposition resulted in basinal sediments comprising periodic incursions of high-density turbidite grainstones encased within a background of planktonic foraminiferal wackestones. Foraminiferal assemblages of remobilized facies imply multiple sources on the carbonate platform, demonstrating the effect of short-lived tectonism on slope instability and deposition of mass transport deposits. The results of the study confirm that tectonism associated with the Syrian Arc Fold Belt, which altered the style of basin sedimentation between Egypt and Syria, persisted into the Eocene at least as far south as Wadi Araba in the western desert and Hammam Faraun in Sinai. In addition, the shape, size, and extent of the two dominant remobilized facies, debris flows and grainstone turbidites are influenced by their mud-rich versus grainy compositions.

This paper summarises the results of an intense reservoir appraisal programme that was conducted within a sour gas reservoir, onshore UAE, in order to determine the controls on porosity presence, distribution and effectiveness. This multiwell study was conducted over less than 5 years in order to define and predict porosity sweet-spots, and prepare for first gas, which was achieved in 2015. Focus is on the Arab D Member of the Arab Formation, the thickest and volumetrically most significant reservoir interval. It comprises an upward-cleaning succession of lime mudstone, peloidal and skeletal packstone and oolitic grainstone, culminating in a clay-rich, brecciated emergent surface. The entire succession shallows upwards from very low energy, basinal mudstones into cross-bedded oolitic grainstones that were deposited in a shoal complex that protected a gently agitated lagoon. Low skeletal diversity and the abundance of ooids are indicative of slightly elevated salinities; the abundance of anhydrite within the overalying Arab A/B/C and Hith Formation is indicative of basin dessication.The lowermost Arab D Member is dominated by lime mudstones and is highly microporous. A very low matrix permeability is locally enhanced by short, partially cemented, vertical fractures formed perpendicular to stylolites, usually cutting thin skeletal floatstone beds with abundant biomoulds. The main reservoir interval is the overlying upper Arab D Member. Porosity occurs principally as intraparticle micorporosity in ooids but in the uppermost upper Arab D Member, interparticle cement volumes are lower and macropores are preserved. The best reservoir properties occur within this bioturbated oolitic grainstone facies, and in cross-bedded oolitic grainstone near the palaeo-crest of the field, probably because early oil emplacement inhibited cementation. There is good evidence that the base of the upper Arab D Member and the lower Arab D member are in communication. Although there is evidence of thermochemical sulphate reduction in the Arab D Member, it is volumetrically minor and unlikely to have generated the high volumes of H2S that are known to occur in the field.

During the Cretaceous, a humid global climate, calcitic seas, high relative sea-level and low amplitude changes in relative sea-level largely prevented large-scale dolomitization in many carbonate successions. However, the well-exposed shallow-water carbonate sediments of the Upper Albian–Lower Turonian Zebbag Formation on the Jeffara Escarpment, southern Tunisia, are pervasively dolomitized. This study considers why dolomitization was so widespread in this region during a period of Earth history when platform-scale dolomitization is rare. Marine conditions were established in the Upper Albian, evidenced by stacked upward-shallowing packages of shallow subtidal to peritidal carbonate sediments in the basal Rhadouane Member. A gradual increase in the volume of subtidal sediments in the Cenomanian Kerker Member, culminated in deposition of laterally extensive marls, during maximum flooding of the platform in the Lower Turonian. The overlying Gattar Member was then deposited in shallower water as relative sea-level fell. The entire Zebbag Formation is pervasively replaced by stratabound, fabric-retentive, dolomite, except within the marl at the top of the Kerker Member, which is only partially dolomitized. Petrographic textures indicate dolomitization largely post-dated marine cementation and platform emergence but pre-dated chemical compaction. Slightly more positive oxygen isotope signatures, slightly elevated concentrations of Sr and a near-absence of evaporites are consistent with dolomitization by reflux of mesohaline sea water. An upward-decrease in major element concentrations and higher 87Sr/86Sr compared to Upper Cretaceous sea water suggest that basal, Albian siliciclastic beds acted as aquifers facilitating dolomitization by fluxing fluids offshore. Dolomitization is interpreted to have resulted from multiple fluxes of sea water over periods of 0·5 to 2·5 Ma. The unusually high volume of dolostone for a platform of this age most probably reflects deposition within an arid climate belt, where an efficient reflux system was facilitated by basal, permeable siliciclastic strata.

The Precambrian–Cambrian Athel Silicilyte is a 400 m thick, salt-encased siliceous succession in the South Oman Salt Basin. It is a self-sourcing hydrocarbon reservoir and comprises up to 95% microcrystalline quartz and exhibits wavy discontinuous lamination, comprising thin, alternating organic-rich and silica-rich layers. Textures and geochemical fingerprinting indicate that it is a primary precipitate formed by microbially mediated precipitation of silica from sea water, within the water column at the sulphidic/oxic interface. The unique occurrence of the Athel Silicilyte in the terminal Proterozoic implies that optimal conditions for this style of silica precipitation occurred only briefly. Basin anoxia, coupled with the growth of microbial mats, low pH and high silica pore water saturations, created optimal chemical conditions for silica precipitation. Volumes of microcrystalline quartz are highest within the transgressive and early highstand systems tract and towards the centre of the Athel Basin. At the basin margins, and within the late highstand systems tract, volumes of microcrystalline quartz decreased as the volume of detrital sediment increased. Mass-balance calculations indicate that silica-enriched sea water would have been supplied to the basin by infrequent marine incursions that replenished ambient sea water in the upper part of the water column. In conclusion, precipitation of the Athel Silicilyte was driven by the coincidence of basin restriction, limited clastic input, availability of organic matter and water column anoxia. The observation that there are few documented examples of chert deposits younger than ca 700 Ma, prior to the Cambrian explosion, suggests that although silica budgets within marine basins probably remained high prior to the evolution of silica-secreting organisms, direct precipitation from sea water was restricted. This is tentatively related to the gradual increase in alkalinity of sea water through the Palaeo-Proterozoic and Meso-Proterozoic, such that silica precipitation could only occur through the coincidence of basin anoxia and low siliciclastic input.

This paper reports the discovery of extensive middle Eocene bryozoan reef complexes along the paleoshelf edge of the Great Australian Bight (GAB). The complexes form the earliest carbonate deposit in the GAB, which is the largest Cenozoic cool-water carbonate province on Earth. The bryozoan reef mounds, previously misidentifi ed as volcanic bodies, were deposited parallel to the shelf margin for more than 500 km along strike. Individual reef mound complexes are 60?150 km long, as wide as 15 km, and as thick as 200 m, and dwarf all previously described examples. Superimposed on the distal margin of an underlying Paleocene to mid-Eocene siliciclastic delta complex, the reef mounds provide a critical insight into changing paleoenvironments of the Australo-Antarctic Gulf ca. 43 Ma, coinciding with global and continent-wide climatic and tectonic events. The rapid growth and demise of reef mound?building bryozoans raises new questions regarding the interplay of Southern Ocean opening, ocean currents, and biosphere interactions.

Network models of porous media are beneficial for predicting the evolution of macroscopic mass transport properties. This work proposes a novel bi-regular network model based on truncated octahedral support. With the model, pore systems with different pore coordination spectra for a given average coordination number can be constructed to match experimental data. This feature and the maximum allowed pore coordination of 14 make the proposed model more realistic and flexible than the existing models based on cubic supports. Limestone is used as a study material to illustrate the model performance. Experimental pore space data for this material obtained by X-ray tomography (CT) are used for constructing microstructure-informed model realisations. It is demonstrated that with these realisations the experimentally measured permeability of the material can be predicted. The model is further used to assess the effects of pore connectivity and porosity on the permeability. It is shown that the effect of pore connectivity is substantially stronger than the effect of porosity. A strategy for calculating the evolution of permeability with damage-related pore space changes is also described. The results reveal the effects of the mechanical properties of the medium on the permeability evolution as a function of damage evolution. Developments of this strategy are suggested for deriving mechanism-based constitutive laws for engineering applications.

Petroleum exploration in many North African intracratonic basins targets Early Paleozoic sandstones as the primary reservoir objective. These sandstones are often characterized by highly variable reservoir quality (0.0001–1000 mD), and the ability to predict and selectively target areas of enhanced porosity and permeability is crucial to unlock the hydrocarbon potential. The objective of this study is to characterize the primary controls on reservoir quality in an Ordovician field in the Illizi Basin of Algeria through detailed core and petrographic analysis, and establish if variations in thermal history across the field have a material impact on reservoir quality. The best reservoir quality is observed in facies where primary intergranular porosity has been preserved in fine to coarse grained quartzarenites with less than 1% fibrous illite. These lithologies are most commonly found within the high-energy, tidally reworked, post-glacial facies sandstones of the uppermost Ordovician succession. Observed differences in quartz cement volume within compositionally and texturally similar samples from the southern and northern parts of the field are interpreted to reflect variations in thermal exposure due to deeper burial. This interpretation is supported by field-wide numerical modelling of sandstone diagenesis. This study indicates that subtle variations in thermal history can have a material impact on the spatial trends in reservoir permeability. Thermal history, therefore, is an important consideration in reservoir quality studies in exhumed basins where variations in present-day burial depth will be a poor guide for evaluating reservoir quality risk across a basin or play.