Precipitation Of Calcite Cement Research Articles

Diagenesis of carbonate density-flow deposits controlled by differential uplift of platform segments: examples from the Cretaceous of the Gargano Promontory, Italy

ABSTRACT Thin-section investigation (polarized-light, cathodoluminescence, and ultraviolet microscopy) combined with isotopic (δ18O, δ13C,87Sr/86Sr) analyses of bulk carbonate samples form the basis for evaluating the diagenetic alteration of Albian–Cenomanian and Maastrichtian density-flow deposits off two segments of the Apulia Carbonate Platform in the Gargano Promontory, Italy. We propose that differential platform uplift south and north of the Mattinata Fault controlled the diagenesis of density-flow deposits during Albian–Cenomanian times. In both cases, the: i) abundant blocky cement and vuggy pores in clasts, and ii) remnant blocky cement on allochems in the corresponding matrix samples with interparticle pores, indicate disintegration of at least partially cemented deposits before failure and reworking into density flows. The abundant rudist fragments suggest that they were sourced from the margin and upper slope. However, the δ13C compositions of the density-flow deposits south and north of the Mattinata Fault are different, and geochemical modeling based on presumed marine and terrestrial δ13C compositions indicates: A) The marine δ13C values of deposits south of the Mattinata Fault suggest that the margin- and upper-slope deposits were subjected to predominantly marine-burial diagenesis before failure. Albian–Cenomanian Sr-isotope ages support the marine-burial diagenetic scenario where strontium was redistributed locally during calcitization of aragonitic allochems and during precipitation of calcite cements. However, post-uplift precipitation of vadose cement in pores formed during marine-burial diagenesis has lowered the δ13C and increased the 87Sr/86Sr ratio in many of the samples. B) Twenty-five km north of the Mattinata Fault, the negative δ13C values suggest that oxidation of terrestrial plants supplied 12C-enriched CO2 to the pore-water carbon pool during subaerial exposures, thus lowering the δ13C compositions of the margin- and upper-slope deposits from values obtained during marine-burial diagenesis. This diagenetic model requires that residual aragonitic and high-Mg calcitic allochems were available in the deposits during penetration of meteoric water. However, the Albian–Cenomanian Sr-isotope ages and the geochemical modeling support a predominantly marine-burial scenario, with intraformational redistribution of strontium during meteoric diagenesis. A similar diagenetic model is envisaged for the Maastrichtian density-flow deposits south of the Mattinata Fault, but the less negative δ13C and geochemical modeling suggest less influence of meteoric diagenesis before reworking. The majority of separate- and touching-vug pores likely also formed during marine burial before failure and reworking of the margin- and upper-slope deposits. The large range in porosity (4–31%) of density-flow deposits with predominantly interparticle pores are inferred to reflect varying degrees of compaction caused by variable overburden thickness as well as sedimentary processes. The results of the present study indicate that the geochemical imprint of meteoric diagenesis can be expected to vary in density-flows deposited along coeval segments of carbonate platforms in tectonically active regions.

Deciphering the diagenetic evolution of the lower Carnian succession in the Gorno and Pian Resinelli sulphide mining districts (Southalpine Domain, N Italy)

The stratabound Mississippi Valley-type (MVT) sulphide deposits of the Gorno and Piani Resinelli mining districts are hosted in the lower Carnian carbonate succession, mainly peritidal limestones, of the Lombardian Basin (Southalpine Domain, Northern Italy). In addition to the formation of the well-known sulphide deposits, these carbonate sediments experienced a complex diagenetic evolution, including different stages of dolomitization, silicification, dissolution, brecciation, calcite cement precipitation. Stratigraphic and petrographic features suggest that, apart from an early calcite cementation and a first penecontemporaneous dolomitization affecting the supratidal beds, diagenetic processes and sulphide precipitation, are intimately and genetically associated. They document the occurrence of a Carnian hydrothermal system developed shortly after deposition at shallow burial depth, confirmed by fluid inclusion microthermometry and U-Pb radiometric datings.

Diagenetic characterization and the influence on the porosity properties of a mixed bioclastic-siliciclastic rift lacustrine deposit of the Sergipe-Alagoas Basin, northeastern Brazil

Bioclastic and siliciclastic deposits of the Morro do Chaves Formation, Early Cretaceous of the Sergipe-Alagoas Basin, northeastern Brazil, show depositional and diagenetic analogies with the Brazilian Pre-salt rift reservoirs. A detailed petrographic study carried out on 221 thin sections from four well-cores were used to define the stratigraphic and sedimentological controls of the diagenetic events on coquinas supported by cathodoluminescence and EDS-MEV analysis. The study of cement distribution allowed identification in high- and low-frequency cycles of the subaerial exposure zones and intervals with porosity and permeability enhancement. The eodiagenetic processes and products show a good correlation with the maximum regressive events marked in the lower carbonate-dominated sequence, by the occurrence of nodules, micritization, pseudomicrokarst, mechanical compaction, three types of calcite fringes, coarse calcite cement, and three dissolution events. This sequence shows less than 25% of terrigenous constituents, thus favoring a pervasive early calcite cement precipitation and consequently an occlusion of the porosity. The upper sequence displays a predominance of poikilotopic and blocky calcite cement related to the mesogenetic stage. This sequence shows a dominance of mixed carbonate and siliciclastic constituents, which reflects directly the reduced early cementation, and the higher porosity and permeability relatively to the lower sequence. This study on cement occurrence and distribution on mixed deposits and their relationships with sedimentological and stratigraphic controls adds new and useful data for helping in the understanding of the complex coquina successions of the analogous lacustrine bioclastic reservoirs of the Pre-salt interval. The diagenetic model can support further studies regarding to predictive models of porosity and permeability distribution in other bioclastic lacustrine reservoirs.

LA‐ICP‐MS U–Pb dating of calcite cement in Upper Triassic tight‐gas sandstone reservoirs, western Sichuan Basin, SW China

AbstractThe absolute timing of carbonate cement formations can provide insights into porosity evolution and hydrocarbon accumulation in reservoirs. In this study, scanning electron microscopy‐based cathodoluminescence (SEM‐CL), quantitative mineralogical analysis and laser ablation inductively coupled plasma‐mass spectrometry U–Pb dating were performed on calcite cements from sandstones in the fourth member of the Upper Triassic Xujiahe Formation in the western Sichuan Basin. Two samples yielded U–Pb ages of 149.5 ± 31.1 Ma and 190.0 ± 180.5 Ma (2σ), suggesting late diagenetic precipitation of calcite cements. The backscattered electron and SEM‐CL images indicated that the cements precipitated after feldspar dissolution. Combined with palaeo‐temperature evolution, these results suggest that the timing of calcite cement precipitation occurred between the Late Jurassic to Early Cretaceous. Rapid accumulation of sediment in the western basin in response to the Yanshanian Orogeny may have caused stratum overpressure, resulting in geofluid activity and calcite precipitation in the Xu4 sandstone.

Shallow‐marine carbonate cementation in Holocene segments of the calcifying green alga Halimeda

AbstractEarly‐diagenetic cementation of tropical carbonates results from the combination of numerous physico‐chemical and biological processes. In the marine phreatic environment it represents an essential mechanism for the development and stabilization of carbonate platforms. However, diagenetic cements that developed early in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. When lithified sediment samples are studied, this complicates the recognition of processes involved in early cementation, and their geological implications. In this contribution, a petrographic microfacies analysis of Holocene Halimeda segments collected on a coral island in the Spermonde Archipelago, Indonesia, is presented. Through electron microscopical analyses of polished samples, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant High‐Mg calcite (3.9 to 7.2 Mol% Mg), bladed Low‐Mg calcite (0.4 to 1.0 Mol% Mg) and mini‐micritic Low‐Mg calcite (3.2 to 3.3 Mol% Mg). The co‐existence and consecutive development of fibrous aragonite and equant High‐Mg calcite results initially from the flow of oversaturated seawater along the aragonite template of the Halimeda skeleton, followed by an adjustment of cement mineralogy towards High‐Mg calcite as a result of reduced permeability and fluid flow rates in the pores. Growth of bladed Low‐Mg calcite cements on top of etched substrates of equant High‐Mg calcite is explained by shifts in pore water pH and alkalinity through microbial sulphate reduction. Microbial activity appears to be the main trigger for the precipitation of mini‐micritic Low‐Mg calcite as well, based on the presumable detection of an extracellular polymeric matrix during an early stage of mini‐micrite Low‐Mg calcite cement precipitation. Radiocarbon analyses of five Halimeda segments furthermore indicate that virtually complete intragranular cementation in the marine phreatic environment with thermodynamically/kinetically controlled aragonite and High‐Mg calcite takes place in about 100 years. Collectively, this study shows that early‐diagenetic cements are highly diverse and provides new quantitative constraints on the rate of diagenetic cementation in tropical carbonate factories.

Rift and salt-related multi-phase dolomitization: example from the northwestern Pyrenees

The Meillon (Callovo-Oxfordian) and Mano (Tithonian) Formations are dolomitized carbonate reservoirs that actively produce oil and gas (Aquitaine Basin, France). In this study, the dolomitization conditions of their counterparts exhumed in the northwestern Pyrenees are detailed using a combination of field observations, petrography, fluid inclusion microthermometry, elemental and isotopic geochemistry, and carbonate U–Pb geochronology. Dolomitization occurred in several stages spanning from the Neocomian (pre-rift) to the Albian (syn-rift, associated with mantle exhumation and active salt tectonics). Both formations were first massively dolomitized in near-surface to shallow burial conditions during the Berriasian-Valanginian, likely triggered by the influx of marine-derived waters. Between the Barremian and the Albian, the Early Cretaceous rifting caused the upward influx of hot fluids associated with the partial to complete recrystallization of the initial dolomites. During the Albian, subsequent dolomites precipitated in both formations as high-temperature (T > 160 °C) vein- and pore-filling cement. Distinct fluid inclusion chlorinities and rare earth element patterns between the Meillon and Mano Formations point to fluid compartmentalization during this stage. Whereas dolomite cements indicate the involvement of evaporite-derived brines in the Meillon Formation, precipitation was likely related to clay-derived water in the Mano Formation. Lastly, a final episode of dolomite cementation occurred only in the vicinity of faults and volcanic intrusions during the Albian when the highest temperatures were recorded in both formations (T > 250 °C). These saddle dolomites precipitated from hydrothermal water with a mixture of mantle-, crustal-, and evaporite-derived waters channeled by faults and active diapirs. Subsequent quartz and calcite cement precipitation reveals a temperature decrease in a post-rift to inversion setting (post-Cenomanian) and indicates fluid compartmentalization between both formations. This study highlights the major control exerted by rifting, combined with the presence of diapiric salt, on dolomitization, making carbonate platforms of modern salt-rich passive margins potential targets for exploration.

Influence of faulting on porosity and permeability features of carbonate rocks, Kardeh fault, NE Iran

In the southern part of the Kopeh Dagh basin, NE Iran, outcrop of the Kardeh fault damage zone in Mozduran carbonate formation provides an opportunity for investigation of the impact of faulting on permeability and porosity. For this purpose, three methods were focused on: 1) gas injection in oriented cores prepared parallel and perpendicular to fault strike and parallel to fault dip, 2) microscopic analysis and 3) determining variation of the degree of fractures connectivity in the fault damage zone. Results of tests show air-porosity and air-permeability increase from fault core toward host rock. Using microscopic studies, we attribute these variations to bioclasts destruction and filling, precipitation of calcite cement in fractures and pores, development of compaction structures such as stylolites and particle abrasion. Analyzing variation of the fractures connectivity in the fault damage zone in macro-scale shows degree of connectivity increases from host rock toward fault core (Y and X nodes). Clear dependence between this parameter and fluid flow is reflected by connected linear springs developed in the damage zone in close proximity to the fault core. Results of this study show that permeability in a fault zone is a scale dependent feature.

Sedimentology, diagenetic evolution, and sweet spot prediction of tight sandstone reservoirs: A case study of the third member of the Upper Paleogene Shahejie Formation, Nanpu Sag, Bohai Bay Basin, China

Substantial progress has been made in the exploration of the tight petroleum resources in Bohai Bay Basin, as a series of tight oil reservoirs were discovered. Among these reservoirs, a new breakthrough was achieved in the Nanpu Sag, proving a previously unconfirmed prospect of tight oil. Mineralogical, petrographic and fluid inclusions analyses were performed to investigate physical properties (composition, pore system, porosity, and permeability), controlling factors, and features of tight sandstones of the Upper Paleogene Es3 (the third member of the Shahejie Formation) in the Gaoshangpu Structural Belt of the Nanpu Sag. Our study indicates that the Es3 tight sandstones are mainly feldspathic litharenites and had low values for porosity (average value 10.1%) and permeability (average value 0.8 mD). The pore system is dominated by secondary dissolution pores (average thin porosity 3.1%). The deterioration of porosity is greater due to compaction (average compaction rate 68.8%) rather than cementation (average cementation rate 16.3%). Two peaks in the homogenization temperature were observed; the first ranges from 116 °C to 136 °C and exhibits yellow fluorescence, while the other peak varies from 156 °C to 165 °C, showing blue fluorescence, which indicates two stages of hydrocarbon charging at 26 Ma and 3 Ma, respectively. Tight sandstone is identified during the Mesodiagenesis phase. Eodiagenesis included compaction, precipitation of calcite cements and quartz overgrowth. Mesodiagenesis mainly exhibited compaction and grain dissolution (mainly feldspar). The main controlling factors can be summarized as follows, with the second factor being the most important: (a) a high energy deposition environment (distributary channels); (b) intense dissolution; and (c) early hydrocarbon charging and overpressure. This study provides a new prospect of the tight sandstone petroleum system in northeastern China.

Application of Non-Extensive Statistical Physics on the particle size distribution in natural carbonate fault rocks

This contribution focuses on the results of combined generalized statistical mechanics and microstructural analyses of carbonate fault rock samples, which derive from active extensional fault zones cropping out in peninsular Italy. Specifically, this work is aimed at deciphering the role played by both pre-existing structural elements and structural diagenesis on the dimension and distribution of the largest survivor grains. The studied extensional fault zones were exhumed from shallow crustal depths during the Plio-Quaternary downfaulting of the Apennines. They crosscut Mesozoic platform limestones and dolostones. Despite the different host rocks, all carbonate fault rocks exhibit very similar cataclastic textures but dissimilar pore characteristics. In this work, we apply for the first time the principles of the Non-Extensive Statistical Physics to the study of fault rocks. By considering the Tsallis entropy, which is expressed by the Q-value representing the hierarchical organization of the system, we consider the crossover point value, V0, computed for both 1cm2 and 9cm2 representative fault rock images at the transition from Q-exponential to exponential behaviors. As a result, we document that the limestone grain-supported rocks, which mainly localize in the outer fault cores, are always characterized by smaller V0 values with respect to the dolostone ones. These data are interpreted as due to both burial and thrusting-related pressure solution seams, which pervasively formed in the limestone host rocks. Conversely, the amount of V0 variations computed from outer to inner fault cores are associated to the effects of widespread calcite cement precipitation, which mainly occurred in the limestone matrix-supported fault rocks. By considering the weighted, average areas of survivor grains with dimension larger than V0 a as a proxy for sorting, we document an increase from outer to inner fault cores in all the study fault zones. At a close view, all dolomite-rich fault rock samples show very similar median values, whereas the matrix-supported fault rock samples of the inner limestone fault cores are characterized by smaller median values relative to the grain-supported of the limestone outer fault cores. This discrepancy is interpreted as also due to widespread cementation processes, which formed aggregate grains in the fine-grained fault rocks encompassing the main slip surfaces of the limestone fault cores.

Degradation of fracture porosity in sandstone by carbonate cement, Piceance Basin, Colorado, USA

Cretaceous Mesaverde Group sandstones contain opening-mode fractures lined or filled by quartz and, locally, calcite cement. Fracture occlusion by quartz is controlled primarily by fracture size, age and thermal history. Fracture occlusion by calcite is highly heterogeneous, with open and calcite-sealed fractures found at adjacent depths. In the Piceance and in other basins, processes that control the distribution of these calcite cements have been uncertain. Using pore and fracture cement petrography, fluid inclusions, and isotopic and elemental analysis, we show that host-rock calcite distribution and remobilization govern porosity degradation and occlusion of fractures >1 mm wide by calcite. Fluid-inclusion analyses indicate calcite cement precipitation at 135–165°C. 87 Sr/ 86 Sr ratios of calcite and the presence of porous albite suggest that detrital feldspar albitization released Ca 2+ , driving carbonate cement precipitation. In host rock, both albite and calcite content decreases with depth along with greater fracture porosity preservation. Although the cement sequence Fe-dolomite → ankerite → calcite is widespread, Fe-dolomite and ankerite occur as host-rock cements only, with detrital dolomite as preferred precipitation substrate. We find that the rock-mass calcite cement content correlates with fracture degradation and occlusion, and can be used to accurately predict where wide fractures are sealed or open. Thematic collection: This article is part of the Naturally Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/naturally-fractured-reservoirs

Petrological and geochemical constraints on fluid types and formation mechanisms of the Ordovician carbonate reservoirs in Tahe Oilfield, Tarim Basin, NW China

As one of the major hydrocarbon source rocks, deeply buried (4500–7000 m) Ordovician marine carbonates are important reservoir rocks in Tahe Oilfield, Tarim Basin, NW China. Most of the observed diagenetic features are genetically related to the meteoric water due to the multistage structural uplifts and subsequent paleo-exposure, weathering and erosion experienced by the carbonate strata. There is very limited study on the petrological and geochemical constraints on fluids types and formation mechanisms of the Ordovician carbonate reservoirs in this region. In order to investigate to the mechanism and characteristics of karstification, this paper integrated petrographic studies, elements and C, O, Sr isotopes geochemistry and fluid inclusions, and revealed the origin and types of diagenetic fluids, with which the dissolution and development of secondary porosity enhances reservoir properties of the Ordovician carbonates. Through this study, vugs and pores generated from epigenetic meteoric dissolution were observed to partially destroyed due to subsequent compaction, filling and cementation. Meteoric water, hydrothermal fluids and formation water are the three main types of diagenetic fluids in the Ordovician carbonate rocks of Tahe Oilfield. This paper investigated the values of key parameters related to perception, and figured out the precipitation types of calcites by methods of comparisons. Comparing to carbonate matrix, some calcite cements exhibit the features of relatively negative values of δ18O, a relatively wide range of δ13C, higher ratios of 87Sr/86Sr, and low to medium homogenization temperatures. These may indicate the precipitation of calcite cement from meteoric water. Respectively high ΣREE and obvious positive Eu anomaly, lower δ18O, higher δ13C value, higher 87Sr/86Sr ratios, combined with abnormal high homogenization temperatures of some calcites could confirm their precipitations from hydrothermal fluids. Respectively higher δ18O values and lower 87Sr/86Sr ratios of some calcite cements suggest their precipitations from formation water. The dissolution of meteoric water is the most important karstification and controlled by carbonic acid solution recharge conditions. Hydrothermal activity can promote the thermochemical sulfate reduction (TSR) and have certain dissolution ability. The paper established the models of meteoric water and hydrothermal fluids karstification. In practice, the results would work as a useful reference for the exploration of deeply burial carbonate reservoirs and hydrocarbon in carbonate strata of basins around the world.

Application of Stable Strontium Isotope Geochemistry and Fluid Inclusion Microthermometry to Studies of Dolomitization of the Deeply Buried Cambrian Carbonate Successions in West-Central Tarim Basin, NW China

Detailed petrographic, geochemical (O-C-Sr isotopes) and fluid inclusion studies of the deeply buried Cambrian carbonates in the West-central Tarim Basin revealed three types of crystalline dolomites (fine-crystalline, nonplanar-a(s), dolomite (RD1), fine- to medium-crystalline, planar-e(s) dolomite (RD2), and medium- to coarse-crystalline, nonplanar-a dolomite (RD3)), medium- to coarse-crystalline, nonplanar-a saddle dolomite cement (CD) and early and later-stage calcite cement. The occurrence of RD1 along low-amplitude stylolites points to link with pressure dissolution by which minor Mg ions were likely released for replacive dolomitization during early- to intermediate-burial seawater dolomitization. The increasing crystal sizes of RD2 and RD3 with irregular overgrowth rims suggests intense recrystallization and replacement upon the RD1 or remaining precursor limestones by dolomitizing fluids during late intermediate burial dolomitization. The overlap of δ18O, δ13C and 87Sr/86Sr values of RD1-RD3 and CD dolomite with coeval seawater values, suggests that the principal dolomitizing fluids that precipitated these dolomites was connate (Cambrian) seawater preserved in the host limestones/dolomites. Their high 87Sr/86Sr ratios suggest influx of radiogenic strontium into the Cambrian seawater. Two regimes of fluid flow are recognized in the study area: firstly, influx of magnesium-rich higher-temperature basinal brines along deep-seated faults/fractures, resulting in cementation by CD dolomite. Secondly, the incursion of meteoric waters, mixing with ascending higher-temperature basinal brines, and an increase in Ca2+/Mg2+ ratio in the fluids probably results in the precipitation of calcite cement in vugs and fractures.

Petrographic and geochemical characterization of the early formative stages of Northern Adriatic shelf rocky buildups

A peculiar type of marine rocky outcrops is generated under temperate conditions through the cementation of carbonate and siliciclastic sediments on the northwestern Adriatic shelf. The lithification process results into the genesis of punctiform outcrops (tegnùe) that serve as substrate to substantial coralligenous epibenthic growth. These buildups have been often described as beachrock. More recently, an alternative genetic mechanism, implying precipitation of carbonate cements in the marine phreatic zone, has been hypothesized for the rocky outcrops off the coast of Chioggia (south of Venice). These rocky outcrops are settled along the levee of meandering channels inherited from Pleistocene fluvial systems.Petrographic analyses of the Chioggia rocky outcrops allowed distinguishing three (3) main lithofacies: 1) biofouled sandstone blocks; 2) cross-stratified slabs and 3) bioclastic carbonates. These lithofacies have a common substrate mainly made by allochemic sandstone and sandy allochem limestone consisting in rounded grains of limestones and dolostones. The host sediments are cemented by a thin micrite coating followed by isopachous rims of scalenohedral and bladed calcite. Geochemical analyses revealed a sensible decrease in Mg contents between the early and late generations of isopachous cements. Vadose cements are missing suggesting that the precipitation of calcite cements occurred in marine phreatic zone without supra-tidal exposure. The lack of aragonite cements suggests that seawater was mixing with meteoric groundwater. The mixing of groundwater induced a partial dissolution of high-Mg calcite and precipitation of a later stage low-Mg calcite.The lithification of the morphologies inherited from Pleistocene fluvial systems provided the nucleus for the development of the coralligenous buildups observed in the North Adriatic Sea shelf. It is possible that diagenetic processes, other than those normally responsible for the formation of beachrocks worldwide, have been active in the northern Adriatic Sea starting from the post-Last Glacial Maximum transgression. It is therefore likely that these processes are more relevant for the coralligenous epibenthic growth in temperate siliciclastic shelves than previously thought.

Growth‐faults from delta collapse – structural and sedimentological investigation of the Last Chance delta, Ferron Sandstone, Utah

AbstractInvestigations of syn‐sedimentary growth faults in the Last Chance delta (Ferron Sandstone, Utah, USA) show that fault‐bounded half‐grabens arrested high amounts of sand in the mouth bar and/or distributary channel areas. Fault‐controlled morphology causes changes in routing of the delta top to delta front drainage towards the long axis of half‐grabens. Faulting was spatially and temporally non‐systematic, and polyphase, with 3D cusp/listric fault geometries instigated by linkage of variously oriented segments. Hanging wall rollover folds consisting of wedge‐shaped syn‐kinematic sand attest to rapid <1‐m slip increments on faults followed by mild erosion along crests of fault blocks and sedimentary infill of adjacent accommodation. Triangle‐zones in prodelta to delta front muds are located underneath steeper faults and interconnected rotated fault‐flats. Their geometry is that of antiformal stack duplexes, in an arrangement of low‐angle‐to‐bedding normal faults at the base, replaced by folded thrusts upwards. These faults show a brittle, frictional flow deformation mechanism ascribed to early compaction of mud. For syn‐kinematic sand, there is a change from general granular/hydroplastic flow in shear zones to later brittle failure and cataclasis, a transition instigated by precipitation of calcite cement. Extensional faulting in the Last Chance delta was likely controlled by gravity driven collapse towards the delta slope and prodelta, as is commonly observed in collapsing deltas. The trigger and driving mechanism is envisioned as localized loads from sand deposited within distributary channels/mouth bars and fault‐controlled basins along the delta top. A regional tilt and especially displacement of compacted mud below sand bodies towards less compacted muds also contributed to the faulting.

Quantifying the impact of early calcite cementation on the reservoir quality of carbonate rocks: A 3D process-based model

The reservoir properties of carbonate rocks are controlled by both deposition and diagenesis. The latter includes the early precipitation of calcite cements, which can exert a strong control on the evolution of subsequent diagenetic pathways. We quantify the impact of early marine cement growth in grainstones on evolving pore space by examining trends in the relationship between cementation and permeability using a 3D process-based model (Calcite3D). The model assumes varying proportions of polycrystalline and monocrystalline grain types, upon which we grow isopachous and syntaxial calcite cement types, respectively. We model two syntaxial cement shapes, compact and elongated, that approximate the geometries of typical rhombohedral calcite forms.Results demonstrate the effect of cement competition: an increasing proportion of monocrystalline grains creates stronger competition and a reduction in the impact of individual grains on final calcite cement volume and porosity. Isopachous cement is effective in closing pore throats and limiting permeability. We also show that the impact of syntaxial cement on porosity occlusion and therefore flow is highly dependent on monocrystalline grain location and the orientation of crystal axes. This demonstrates the importance of diagenetic overprint in controlling the evolution of rock properties, but also that this process can be essentially random. We also show that diagenesis alone can create notable heterogeneity in the permeability of carbonates.While Calcite3D is successful in modelling realistic changes in cement volumes and pore space morphology, modelled permeabilities (0.01−30D) are above the range reported in reservoir grainstones due to the very high permeability of the initial synthetic sediment deposit (58.9D). Poroperm data generated by Calcite3D, however, exhibits a linear relationship between the logarithms of porosity and permeability with a high coefficient of determination, as observed in natural media.

Life beneath ammonite shells – A unique Late Cretaceous habitat for the trace maker of Chondrites and its impact on taphonomy of the shells

In a unique middle Campanian horizon at Rzeżuśnia, southern Poland, the trace fossil Chondrites occurs densely and preferentially on the lower side of horizontally oriented ammonite moulds but it is absent in the surrounding sediment and on the upper side of the moulds. The ammonite shells created an ideal microenvironment for the Chondrites trace maker, with a higher organic matter content and shallower redox boundary than in the sediment outside the shells. Detailed microscopic (microfacies, SEM, CL and XRD) analyses revealed different taphonomic histories of shell for the upper and lower side of the ammonite specimens. Probably, the chemosymbiotic worm-like trace maker uses the burrow tunnels for pumping the hydrogen sulfide-rich water. This changes the pH of pore water only below the horizontally lying ammonite shells and enabled the aragonite dissolution. Slower rate of sedimentation in combination with downward migration of geochemical zones cause iron sulfide oxidation, calcite cement precipitation and glauconitization.

Origin and evolution of microporosity in packstones and grainstones in a Lower Cretaceous carbonate reservoir, United Arab Emirates

AbstractMicroporosity in carbonate reservoirs is generated by the complex interplay between depositional and diagenetic processes. This petrographical, SEM, fluid-inclusion and isotopic study of a Lower Cretaceous carbonate reservoir, Abu Dhabi, UAE, revealed that: (1) micritization of ooids and skeletal fragments, which resulted in spheroidal (rounded) micrite, accounts for most microporosity in peloidal packstones and grainstones; and (2) transformation of spheroidal micrite into subhedral/euhedral micrite and microspar, known as aggrading neomorphism, could happen via precipitation of syntaxial calcite overgrowths around micrite (micro-overgrowths) and not only, as suggested previously in the literature, by recrystallization involving the dissolution (of micrite) and reprecipitation (of microspar). Precipitation of calcite cement around micrite (i.e. destruction of microporosity) is more extensive in the water zone than in the oil zone, which is possibly contributing to the lower porosity and permeability of the carbonate reservoir in the water zone. Similarity in bulk oxygen isotopic values of micritized packstones and grainstones in the water and oil zones (average δ18OV-PDB= −7.2‰ and −7.8‰, respectively) is attributed to: (1) a small difference in temperatures between the crest (oil zone) and the flanks (water zone); and (2) calcite precipitation around micrite occurred prior and subsequent to oil emplacement. Bulk carbon and strontium isotopic compositions of micritized packstones and grainstones in the water and oil zones (average δ13CV-PDB= +3.7‰ and average87Sr/86Sr ratios = 0.707469) indicate that calcite cement was derived from marine porewaters and/or dissolution of the host limestones. The minimum formation temperatures of bulk micrite/microspar, which are inferred based on paragenetic relationships, fluid-inclusion microthermometry and oxygen isotope data, are around 58–78°C.

Diagenetic evolution of Tortonian temperate carbonates close to evaporites in the Granada Basin (SE Spain)

The Granada Basin (SE Spain) is a small basin located in the central part of the Betic Cordillera, structured as such in the late Tortonian and initially connected to the Atlantic Ocean and to the Mediterranean Sea. During the late Tortonian, normal marine conditions prevailed, leading to the deposition of skeletal carbonate sediments on platforms around structural highs. The marine connections were later interrupted, first to the Atlantic Ocean and then to the Mediterranean Sea, and a thick evaporite sequence, marking the transition from marine to continental conditions, was deposited during the latest Tortonian. In this work, the diagenetic evolution of the Tortonian temperate carbonates (TTC), underlying and close to the evaporite bodies, is revealed and discussed. The diagenetic study includes petrographic analyses (conventional petrography, cathodoluminescence, and fluorescence), geochemical analyses (major, minor and trace elements, and δ13C and δ18O stable isotopes), and microthermometry of fluid inclusions. In the TTC, marine diagenetic processes such as micritization and fibrous calcite-cement precipitation and mechanical compaction took place during or just after deposition (Eogenesis). An initial burial event (Mesogenesis 1) is characterized by: 1) stabilization of the temperate-water carbonates by freshwater, and 2) porosity occlusion via precipitation of low-Mg bladed and syntaxial/mosaic calcite cements. The TTC were then subaerially exposed (or got close to the surface) during evaporite deposition and underwent pedogenesis, Mg-smectite infiltration, and pyrite formation (Telogenesis 1). Subsequent brine-related diagenetic alterations, such as dolomitization and silica, halite, and sylvite replacements of carbonate grains occurred during a second burial episode (Mesogenesis 2) concomitant with the Messinian lacustrine deposition, this being followed by chemical compaction (stylolite formation). Finally, the area was uplifted and the TTC exhumed. Microstalactitic (dripstone) and fibre/whisker calcite cement precipitation and extensive dissolution relate to this Pliocene–Quaternary late event (Telogenesis 2). In the study case diagenetic history is closely linked to basin evolution, as diagenetic pathways of carbonate rocks were related to major geodynamic events, including basin restriction leading to evaporite deposition, and several episodes of subsidence and uplift. Up to now, only very few diagenetic studies have attempted to demonstrate this correlation between diagenetic history and basin evolution.

Fault-controlled and stratabound dolostones in the Late Aptian–earliest Albian Benassal Formation (Maestrat Basin, E Spain): Petrology and geochemistry constrains

Fault-controlled hydrothermal dolomitization of the Late Aptian to earliest Albian Benassal Fm shallow water carbonates resulted in the seismic-scale stratabound dolostone geobodies that characterize the Benicàssim case study (Maestrat Basin, E Spain). Petrological and geochemical data indicate that dolomite cement (DC1) filling intergranular porosity in grain-dominated facies constituted the initial stage of dolomitization. The bulk of the dolostone is formed by a replacive nonplanar-a to planar-s dolomite (RD1) crystal mosaic with very low porosity and characteristic retentive fabric. Neomorphic recrystallization of RD1 to form replacive dolomite RD2 occurred by successive dolomitizing fluid flow. The replacement sequence DC1-RD1-RD2 is characterized by a depletion in the oxygen isotopic composition (mean δ18O(V-PDB) values from −6.92, to −8.55, to −9.86‰), which is interpreted to result from progressively higher temperature fluids. Clear dolomite overgrowths (overdolomitization) precipitated during the last stage of replacement. Strontium isotopic composition suggests that the most likely origin of magnesium was Cretaceous seawater-derived brines that were heated and enriched in radiogenic strontium and iron while circulating through the Paleozoic basement and/or Permo-Triassic red beds. Burial curves and analytical data indicate that the replacement took place at burial depths between 500 and 750 m, and by hydrothermal fluids exceeding temperatures of 80 °C. Following the partial dolomitization of the host rock, porosity considerably increased in dolostones by burial corrosion related to the circulation of acidic fluids derived from the emplacement of the Mississippi Valley-Type deposits. Overpressured acidic fluids circulated along faults, fractures and open stylolites. Saddle dolomite and ore-stage calcite cement filled most of the newly created vuggy porosity. Subsequent to MVT mineralization, precipitation of calcite cements resulted from the migration of meteoric-derived fluids during uplift and subaerial exposure. This late calcite cement destroyed most of the dolostone porosity and constitutes the main cause for its present day poor reservoir quality.

Multiphase partial and selective dolomitization of Carnian reef limestone (Transdanubian Range, Hungary)

AbstractPartially dolomitized carbonate successions provide a good opportunity to understand the commonly multistage process of dolomitization. Petrographic methods, fluid inclusion microthermometry and stable isotope measurements were applied to reconstruct the diagenetic evolution and dolomitization of a partially dolomitized Carnian reef limestone from the Transdanubian Range, Hungary. The diagenetic history began with reef diagenesis and formation of dolomite micro‐aggregates in microbial fabric elements; this was followed by the development of euhedral porphyrotopic dolomite crystals through overgrowths around the previously formed dolomite micro‐aggregates during the earliest burial stage. Increasing burial resulted in the extension of the dolomite patches via formation of finely crystalline replacement dolomite. From the Late Norian, when the Carnian reef carbonates reached the depth of 1·0 to 1·8 km, the diagenetic evolution continued in an intermediate to deep‐burial setting. Contemporaneously, an extensional regime was established, leading to fracturing. The progressive burial resulted in the recrystallization of the pre‐existing dolomite with increasing temperature, while saddle dolomite cement was precipitated in fractures. In connection with the Alpine Orogeny, intense denudation took place during the Late Cretaceous, accompanied by fracturing. Similar tectonically controlled denudation and fracturing occurred in several stages during the Cenozoic. As a result of these processes, the studied Carnian carbonates were raised to a near‐surface position or became subaerially exposed, leading to dedolomitization of the last dolomite phase and precipitation of calcite cement in cavities and fractures. This study revealed that by investigating partially and selectively dolomitized rock types, it is possible to document and understand those stages of the multiple dolomitization process which can barely be detected in the completely dolomitized rock bodies. Recognition of the dolomitization phases could provide the basis for the analysis of their relations with the depositional, diagenetic and tectonic processes, and stages of basin evolution.