Search

A quest for life on Mars (LoMars) started in the early 1960s when the most prestigious scientific journals published several research articles. According to Elsevier’s Scopus database, the rise in annual literature production started in the late 1990s, most likely associated with the launch of the National Aeronautics and Space Administration’s (NASA) first rover, Sojourner, in 1996. The number of articles on Mars will likely continue to rise sharply, given that the launch and landing of the Mars 2020 Perseverance Rover are critical to discovering and understanding the present or past life on the planet. Thus far, the USA has dominated in the number of citations and collaborations related to Mars research, given its facilities equipped with relevant infrastructure and researchers’ capacity to explore the Solar System beyond Earth. Most of these frequently cited papers report observational and theoretical research results. However, a highly cited article is based on analytical studies of a unique Martian meteorite (i.e., Allan Hills 84001) found in Antarctica. It is expected that the future sample return mission associated with the Perseverance Rover caching system could increase the LoMars research exponentially in the coming decades if Martian samples are successfully brought to Earth. Based on the total number of publications on LoMars, the most influential institute, the author, and the journal are Caltech, C.P. McKay, and Icarus, respectively. Both the institute and the author are directly affiliated with NASA, indicating the leadership offered by the organization in LoMars research.

Recognizing ancient deltas' depositional architecture is challenging due to the complex interplay of the fluvial, waves, and tidal processes. The integration of field-based sedimentological and trace fossils studies of the Late Cambrian-Early Ordovician Barik Formation in Central Oman allowed better identification of different lithofacies architectures and depositional process signatures. The Barik Formation is a major tight-gas reservoir in the Oman Sedimentary Basins, where it is over 800-m thick. The only outcrop analogue is in the Qarn Mahatta Humaid area, where ca. 50-m thickness of the formation is exposed and extends laterally over 5 km. This study revealed the presence of four lithofacies associations, including (a) mouth bar/shoreface, (b) tidal flat, (c) tidal channel, and (d) delta distributary channels using 12 logged sections. The depositional features identified in the field include tidal, wave/storm, and fluvial. The tidal signatures are represented by mud-drapes, large-scale reactivation surfaces, sigmoidal cross-bedding, and climbing and flaser bedding, predominantly in the mixed mud-sand tidal flats of the lower delta plain and delta distributary channels of the upper to lower delta plains. The wave/storm signatures are represented by wave ripples and small-scale hummocky cross-stratification in the mouth bar/shoreface of the delta front platform. The fluvial and subaerial signatures are represented by the occurrence of the scour surfaces with lag deposits of channel-fill and the development of desiccation mud-cracks. The Barik Formation delta sandstones form amalgamated sheet-like bodies prograding to the north-northeast and deposited along a ramp-type low relief basin. This prograding delta represents a highstand system tract. This study presents an example of the ancient fine-grained hybrid-energy prograding delta, where fluvial processes operated beside the tidal and wave/storm processes over a broad and low relief shelf in a vegetation-free system. This work is supported by His Majesty's Trust Fund Grant # SR/SCI/ETHS/15/01 to Sultan Qaboos University (SQU)

A quest for life on Mars (LoMars) started in the early 1960s when several research articles were published by the most prestigious scientific journals. The rise in annual literature production started in the late 1990s, most likely associated with the launch of the National Aeronautics and Space Administration’s (NASA) first Sojourner Rover in 1996. This trend predicts that the articles on Mars would further increase in the future given the fact that the recent launch and landing of the Mars 2020 Perseverance Rover is of utmost importance to find and understand the present or past life on the planet. So far, USA dominated the citations and collaborations with the rest of the world on Mars research given its facilities equipped with relevant infrastructure and researchers’ capacity to explore the Solar System beyond Earth. Majority of these frequently cited papers report results of observational and theoretical research; however, the highly cited paper (MCKAY D. S. et al. 1996) is based on analytical studies of a unique Martian meteorite (i.e., Allan Hills 84001) found in the Antarctica. It is expected that the sample return mission associated with the Perseverance Rover could possibly increase the LoMars research exponentially in the coming decades if Martian samples are successfully brought back to Earth. Based on the total number of publications on LoMars, highly influential institute, author, and journal represent Caltech, McKay C. P., and Icarus, respectively. Both the institute and the author are directly affiliated with the NASA, suggestive of the leadership offered by the organization in LoMars research.

The Messinian lagoonal carbonate–evaporite sequence of Cyrenaica, north‐east Libya, hosts: (i) opaline nodules in gypsiferous microbial‐rich mudstone; (ii) nodules replacing poikilotopic gypsum cementing bioclastic carbonates; and (iii) bedded porcelanite with large lenticular gypsum pseudomorphs intercalated with recrystallized microbial mats. Optical microscopy, scanning electron microscopy, backscattered electron images – secondary electron images, X‐ray fluorescence, X‐ray diffraction and cathodoluminescence techniques were employed to elucidate on the early stages of chert diagenesis and demonstrate the role of moganite in the formation of varieties of chalcedony during silicification. The opaline nodules composed of opal‐A, exhibit shrinkage cracks lined by isopachous fibrous quartzine underlain by a thin birefringent zone. This suggests that quartzine may develop by crystallization and conversion of opal‐A through a transitional moganite. The quartzine is followed by amorphous silica gel that converts into length‐fast chalcedony. By contrast, lutecite evolved by silica replacement of gypsum through a transitional moganite phase, which forms an amorphous zone in apparent optical continuity with the fibrous lutecite; as moganite advances by pseudo‐crystalline terminations controlled by the gypsum cleavage. In gypsified oyster shells, intercrystalline organic matter enveloping pseudomorphed shell microstructures enabled moganite nucleation that merges syntaxially around the gypsum crystal rim. The associated lutecite occurs as radial‐fibrous beekite aggregates that are consistent with early shell silicification. The bedded porcelanite consists of opal‐CT groundmass undergoing recrystallization and enclosing micro‐cavities lined by quartzine. The associated pseudomorphs display phases of gypsum replacement by moganite and lutecite at the outer margin, followed by dissolution and cavity filling by quartz varieties in response to fluid flushing. In all cases, organic matter and microbes played a vital role in the silicification process.

Understanding the origin of grain-coating clays and its effects on the inhibition of cementation by quartz overgrowths plays a crucial role in porosity preservation versus destruction in deeply buried sandstones. This study aims to decipher the origin of grain-coating clays and their effect on the quartz overgrowth cementation of the mouth-bar/shoreface and tidal channel sandstones of the lower Paleozoic Barik Formation in the Huqf region of Central Oman. The study is based on petrographical and mineralogical investigations using the conventional optical microscope, scanning electron microscope (SEM) equipped with a dispersed energy spectrometer (EDS), and X-ray diffraction (XRD). The results reveal that the grain-coating clays are predominantly mesogenetic-transformed illite and, to some extent, chlorite. The illitic clays occur as 2–5-μm-thick rims covering the entire detrital grain surface, as well as at grain-to-grain contacts. The illitic clays display honeycomb and cornflake textures with filamentous termination. The chloritic clays occur as 1–3-μm-thick rims, apparently covering the entire detrital grains and at grain-to-grain contact; they display honeycomb-like and platelet morphologies. Such textural morphologies demonstrate the development of illite and chlorite through a transformation process from smectitic clay precursors. The smectitic clay precursors are typically near-surface clay fractions that infiltrated mechanically into the mouth-bar/shoreface and tidal channel facies association sandstones through the tidal hydrodynamic pumps. The predominant occurrence of illite over chlorite is attributed to the availability of potassium ions that are sourced through the extensive dissolution of detrital potassium feldspar grains. The illitic and chloritic clay rims covering the entire detrital grains effectively inhibited the growth of quartz cement by limiting the nucleation sites on the detrital grains. This study may also serve as an analogue model for a similar depositional environment to better understand grain-coating clays’ origin and its role in controlling cementation by quartz overgrowths in deeply buried sandstone reservoirs globally.

Sedimentological parameterization of rock heterogeneity and prediction of shallow-marine siliciclastic reservoir quality remains a challenge globally to date, especially in regions with complex depositional histories, in that, variations in petrographic and petrophysical properties can occur within the same sedimentary sequence of similar age and rock type. Because exploratory drilling strategies and reservoir models can significantly be augmented through direct observation of outcrops that are representative of equivalent reservoir stratigraphic intervals in the offshore zone, outcrop sedimentological characterization can help improve the understanding of subsurface reservoirs of similar strata in surrounding petroliferous basins. To this end, we analyzed well-exposed successions of the mid Mio–Pliocene shallow-marine sandstone deposits of the Sandakan Formation, Borneo, through conventional field investigation, petrographic and petrophysical studies of different sandstone facies types to predict reservoir quality and heterogeneity within different depositional settings. On the basis of these evaluations, the studied sandstone was grouped into three qualitative reservoir rock classes: Class I (O = 18.10–43.60%; K = 1265.20–5986.25 mD), Class II (O = 17.60–37%; K = 21.36–568 mD) and Class III (O = 3.4–15.7%; K = 3.21–201.30 mD). The petrographic and petrophysical results reveals a consistent rise in both gamma ray and permeability values due to a change in depositional environment from proximal lower shoreface to upper shoreface of some successions within the sequence. The change in depositional setting for each sandstone type establishes certain variation in classes within the same facies type. For instance, the permeability may indicate low values due to the presence of bioturbation and increased mud content owing to the burrows with bimodal grain classification, regardless of change in gamma ray readings. This phenomenon signifies that there exists no strict correlation between gamma ray and permeability profile despite containing good quality reservoir rocks and appropriate classification. Our study is of paramount significance to the development of initial exploration strategies in regions where similar siliciclastic strata can be potential reservoirs because it serves as analogs to offshore hydrocarbon bearing packages.

Controls on the diagenesis of ancient glaciogenic sandstone reservoirs are still poorly-constrained. This study examines and documents the types, the spatial and the temporal distribution of diagenetic alterations and their subsequent impact on reservoir quality of sandstones from the outcropped Late Carboniferous, glacio-lacturine Al Khlata Formation in Wadi Daiqa, Northern Oman. An integration of textural and petrographic with geochemical analysis revealed that sandstones have undergone various diagenetic histories during eo- and mesogenetic stages. Eodiagenetic alterations are dominated by mechanical compaction and rarely by kaolinization of chemically unstable framework grains such as feldspars, micas and calcite cements. In contrast, mesodiagenesis is extensively dominated by the occurrence of the chemical compaction, quartz overgrowths, illitization of grains coating clay and micas and rarely calcite cements. Paragenetic sequence revealed that compaction has played significant role than cementation in destroying porosity and thus reservoir quality. Such an extensive compaction (mechanical and chemical) and scarcity of early cementation are attributed to the high rate of sedimentation due to glacier retreat and melting. The availability of meteoric waters as a result of glacier melting was responsible for silicate framework grains dissolution and kaolinite formation.

Reservoir studies in the Sandakan sub-basin NW Borneo are challenging and inexplicable due to complex diagenetic effects such as compaction, cementation, dissolution and depositional means including grain size, sorting and packing with reference to clay texture. These diagenetic effects together impart significant amounts of uncertainty on reservoir quality assessment. In this regard, we documented the mineralogical, diagenetic and reservoir properties of sandstones from the Sandakan Formation, NW Borneo through thin sections, SEM, XRD, XRF, He-porosimeter and poroperm. In order to analyze the influential parameters, we used matrix, grain rearrangement and clay coating around detrital grains for the reservoir quality. Our results show that the sub-litharenites to quartz arenite that dominate in the sandstones are drawn from recycled orogenic sand. Furthermore, reservoir properties of the sandstones were ascertained to be controlled by diagenesis, micro quartz with clay coatings and pressure dissolution of feldspar with the alteration of lithic fragments. The result shows that the physical compaction, rearrangements and ductility of grains were the reasons that reduced porosity and permeability. The main reason for porosity reduction in the present study is pressure dissolution that resulted in the form of chemical compaction induced by micas with grain contacts. Nonetheless, through SEM and petrography, clay coatings around the grains and dissolution of feldspar were found to optimally contribute to the preservation of reservoir properties. Dissolution of feldspar and kaolinization resulted in the formation of secondary porosity that allows meteoric water to percolate through the open and detachment spaces of quartz or other grains. This percolation is also responsible in enhancing the pore throat radii in the reservoir media of the analyzed sandstones. These traits are obliging to diagenetic effects on porosity and permeability, which are linked with the physical compaction and clays coating to reduce and preserve the reservoir properties respectively of same type of reservoirs globally.

Listwaenites are highly altered ultramafic rocks that are potentially associated with economic mineralization and research on these is extremely important worldwide. In the present study, the classification of mineralized listwaenites developed along the serpentinite–amphibolite interface of the Semail Ophiolite, its associated lithology and the zones of alteration and mineralization in the Fanjah Saddle of the Central Oman Mountains region of the Sultanate of Oman are carried out, using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite data. The developed band ratioing, Principal Component Analysis (PCA) and the Spectral Angle Mapper (SAM) supervised classification and image processing techniques applied on the ASTER data set have proved their capability for better interpretation and identification of hydrothermally altered rocks and associated mineralization. The hyperspectral tools (Minimum Noise Fraction (MNF), Pixel Purity Index (PPI) and nD-visualizer) extracted end member spectra and SAM classification clearly show the occurrence of minerals and their spatial distributions. The promising results are verified and confirmed in the field by identification of alteration and mineralization such as listwaenites, silicification, serpentinization and talc alteration and are validated further through laboratory analysis. The confirmation of the occurrence of base metal mineralization along the serpentinite–amphibolite interface in listwaenites suggests that detailed investigation in this and other arid regions which have similar geological conditions may locate mineral deposits. The hyperspectral tools applied on ASTER satellite data show that these can be used as a powerful tool to explore the listwaenites and the potential associated mineralization in other arid geographical regions worldwide.

This study aims to unravel the spatial and temporal distribution of diagenetic alterations of the Mheiherrat Member, the Rudeis Formation (lower Miocene) of the Gulf of Suez rift, Egypt within depositional facies and sequence stratigraphy. The Mheiherrat member is represented by shoreface calcarenites and hybrid arenites (transgressive and highstand systems tracts; TST and HST; respectively) and deltaic rudites and coarse-grained calcarenites and hybrid arenites (lowstand systems tracts; LST). Petrographic, stable O- and C-isotopes, mineral chemical and geochemical analyses have revealed that the arenites are pervasively cemented by eogenetic carbonates and, to small extent, by zeolite and pyrite as well as by telogenetic palygorskite. The shoreface TST and HST calcarenites and hybrid arenites were dominantly cemented by microcrystalline grain-rimming and inter- and intragranular pore-filling calcite (δ 18 O V-PDB = −3.6‰ to −0.3‰ and δ 13 C V-PDB = −2.3‰ to −0.7‰) and rhombic dolomite (δ 18 O V-PDB = −3.9‰ to +0.9‰ and δ 13 C V-PDB = −2.5‰ to −0.7‰). These cements are interpreted to be formed by marine to brackish waters with δ 18 O V-SMOW −1.2‰ to +3.2‰ at temperature of 20–55 °C. The deltaic LST coarse-grained calcarenites and hybrid arenites were dominantly cemented by coarse-crystalline, inter- and intragranular pore-filling calcite (δ 18 O V-PDB −4.4‰ to −2.3% and δ 13 C V-PDB = −2.8‰ to −1.3‰), which are interpreted to have precipitated from pore waters with δ 18 O V-SMOW +3.5‰ to +5.5‰ at temperatures of greater than 55 °C. Such pervasive cementation by carbonates is attributed to the occurrence of abundant intrabasinal carbonate grains. The carbonate cement texture is suggested to be controlled by changes in pore-water chemistry owing to transgression and regression events. This case study revealed that better understanding of factors controlling the extent and textural habits of carbonate cements can be achieved when it is linked with depositional facies and sequence stratigraphy.

The outcrop of shallow marine sandstone, Belait formation at Meragang Beach, Brunei Darussalam have long been regarded as tide-, wave- and storm-dominated environment that reveals the reservoir heterogeneities and variation in sand body geometry. Intensive outcrop study methods were available from previously conducted work on Belait Formation. But for this study the priority was to acquire gamma-ray profile at 5-10 cm scale by using Digital Total GR Count Detector and re-logged of Sedimentological log (in cm) of 311.8 m thick outcrop to describe the depositional environment and sedimentary facies architecture. The sediments deposited in a tide-dominated environment mostly interpreted in all cycles which were characterized by sub-depositional environment which includes; tidal channel, muddy tidal flat and mixed tidal flat sub-environment with sedimentary facies of planner cross-bedding, trough cross-bedding, herringbone cross-stratification, wave ripples and thin parallel lamination with coal fragmented bed and carbonaceous material. The gamma-ray profile for tide dominated environment shows different shapes; block left-boxcar, bell shape, funnel shape and irregular trend. In wave- and storm-dominated are wave ripples, thin parallel lamination with mud drapes, large planner cross-bedding, trough cross-bedding, highly bioturbated (skalithos) sandstone and hummocky cross-stratification. Whereas, the gamma-ray profile shows different shapes; left-boxcar, right-boxcar, funnel shape and irregular trend and in storm-dominated it show only left-boxcar with no sharp edges on top and bottom. This approach combines excellent data quality to interpretate different depositional environment and its impact on sandstone quality which implies that the outcrop gamma-ray log can use to characterize the rock deposited in different depositional environment in different facies. It also reveals that the changes in gamma-ray profile with in the sandstone depend upon the environment in which the sandbody deposited that reveal in reservoir heterogeneities in subsurface well correlation.

Parameters controlling the diagenetic evolution of passive margin, marine turbidites, which are important targets of hydrocarbon exploration, are poorly constrained in the literature. This study aims to unravel the conditions of diagenesis and its impact on the reservoir quality evolution from late Cretaceous canyon-filling turbiditic sandstones of the onshore portion of Espirito Santo Basin, eastern Brazil. Kaolinization (δ 18 O = +13.3‰ to +15.2‰; δ D = −96.6‰ to −79.6‰) and dissolution of framework silicate grains is attributed to meteoric water incursion during eodiagenesis in response to a considerable fall in relative sea-level. Eogenetic alterations also include cementation by siderite (average δ 18 O = −7.2‰; δ 13 C = +9.3‰) and pyrite. Progressive sediment burial (present depths = 1530–2027 m) resulted in the formation of poikilotopic calcite, ferroan dolomite-ankerite (average δ 18 O = −7.9‰; δ 13 C = +2.9‰), minor amounts of quartz overgrowths and in partial dickitization of kaolinite. Isotopic values of calcite and dolomite-ankerite follow two trends of co-variance of δ 13 C with decreasing δ 18 O and increasing temperature. From a composition closer to marine (≈0‰), one trend goes towards positive δ 13 C values (up to +22.4‰ for calcite; +18.6‰ for dolomite-ankerite), indicating increasing input of carbonate from methanogenic fermentation. The other trend develops towards negative δ 13 C values (down to −17.2‰ for calcite; 15‰ for dolomite–ankerite), suggesting increasing contribution from thermal decarboxylation with increasing temperature and depth. Despite the presence of various cement types, mechanical compaction was more important than cementation in reducing depositional porosity in the onshore Urucutuca sandstones.

The present study demonstrates the capability of multi-spectral data acquired from advanced spaceborne thermal emission and reflection radiometer (ASTER) satellite to explore the areas of massive carbonate deposits and associated rock formations for geological application. The extent of interdependence among VNIR, SWIR and TIR bands of ASTER spectral regions has been studied for discrimination of rock formations and identification of minerals of eastern mountain region (Saih Hatat window) of Sultanate of Oman and processed through digital image analysis and classification. Visual interpretation techniques have been employed to discriminate major quartz-rich silicates, carbonates and mafic ophiolite rock formations on the satellite image by carrying out subsequent image enhancement technique and principal component analysis (PCA). Color composite using nine VNIR and SWIR ASTER spectral bands by exposing the results of band ratios of (band 7 + band 9)/band 8 for limestone (CaCO3); (band 6 + band 8)/band 7 for dolomite (CaMgCO3); and band 2/band 1 for mafic-rich (Fe3+) rock formations differentiated the carbonates and ophiolite formations of the study region. The band ratios of 6/8 developed for quartz-rich silicates (shale, schist, sandstone, graywackes) of autochthonous Unit ‘A’ of Late Proterozoic to Early Ordovician and Tertiary age, 9/7 for the carbonates (limestone and dolomite) of Autochthonous rock Unit ‘B’ of Late Permian to Triassic age and 1/2 for mafic ophiolites (harzburgite, harzburgite with dunite) of Samail Nappe discriminated the different rock formations and increased the visual interpretations. It has well delineated the gray limestone and yellow dolomite of Autochthonous Unit ‘A’. The subsequent PCA realized on the 6 SWIR spectral bands enables very good validation and discrimination of quartz-rich silicates, carbonates and mafic ophiolite rock formations defined on previous image rationing techniques and existing geological map, and provides information comparable to surficial formations previously not well recognized. It is capable of distinguishing the ancient and recent alluvial fans consisting of clay, silt, sand and conglomerate formations of Tertiary age from the Autochthonous Unit ‘A’. Furthermore, the ASTER TIR spectral indices have been applied for assessing the effectiveness of TIR spectral bands on identification of quartz-rich silicates, carbonates and mafic-rich minerals and to evaluate the discriminated rock formations. The results agree well with existing geological maps and other published data. The study results show that the combination of visual interpretation, previous field knowledge and digital image processing techniques applied on the ASTER spectral regions have proved beneficial in studying carbonates and associated rock formations of eastern mountain region of Sultanate of Oman and can thus be used as a powerful tool to explore massive carbonate deposits or for geological mapping of other geographical regions where similar geological questions need to be resolved.

Hydrothermal alteration of Proterozoic granitic rocks in the Äspö underground laboratory, southestern Sweden, resulted in the formation of chlorite with large variations in textural and chemical characteristics, which reflect differences in formation temperatures, fluid composition, and reaction mechanisms. The mineral assemblage associated with chlorite, including Ca-Al silicates (prehnite, pumpellyite, epidote, and titanite), Fe-oxides, calcite, albite and K-feldspar, suggests that chloritization occurred at temperatures of between 200–350°C during various hydrothermal events primarily linked to magmatism and rock deformation. Petrographic and electron microprobe analyses revealed that chlorite replaced biotite, amphibole and magnetite, and hydrothermal chlorite phases filled fractures and vugs in the granitic rocks. While fracture-filling chlorite reduces fracture permeability, chloritization reactions in the host granite resulted in the formation of new localized microporosity that should thus be taken into consideration when evaluating the safety of the granitic basement rocks as a repository for nuclear waste. It is also important to take into account that similar alteration reactions may occur at the site of stored nuclear waste where temperatures in excess of 100°C might be encountered.

Petrographic examinations and electron microprobe analyses of Proterozoic granitic rocks, SE Sweden aimed to characterize and unravel the mechanisms and conditions of plagioclase alterations. The ...

Sequence stratigraphy of fluvial deposits is a controversial topic because changes in relative sea level will eventually have indirect impact on the spatial and temporal distribution of depositiona ...

Magmatic titanite in Proterozoic granitic rocks from southeastern Sweden has been subjected to hydrothermal dissolution and replacement by finely crystalline Fe-dominant chlorite and TiO 2 (± quartz ± calcite). These alteration patterns require the redistribution of the highly immobile Ti and Al on the thin-section scale. The chloritization of titanite involved the formation of an intermediate Al–Fe-rich phase probably comprising intimately intergrown, submicroscopic chlorite and titanite. The creation of microporosity by the hydrothermal alteration of titanite may enhance the diffusive flux of matter, and hence the rate of reactions in granitic rocks. The alteration of titanite is most pronounced in red-stained granitic rock zones enriched in partly dissolved biotite and magnetite, which could have acted as local source of Al, Fe and Mg needed for chlorite formation.

The distribution of diagenetic alterations in Triassic fluvio-deltaic, quartzarenitic to sublitharenitic, lowstand systems tract (LST) sandstones of the Gres a Voltzia Formation, anastomosing fluvial, quartzarenitic transgressive systems tract (TST) sandstones of the Gres a Roseaux Formation, and shallow marine, quartzarenitic to sublitharenitic, TST sandstones of the Gres Coquiller Formation, the Paris Basin (France), can be linked to transgression and regression events, and thus to the sequence stratigraphic context. Near-surface eogenetic alterations, which display a fairly systematic link to the depositional facies and sequence stratigraphic framework, include: (i) cementation by meteoric water calcite ( δ 18 O=−8.9‰ and δ 13 C=−9.1‰) in the fluvio-deltaic, LST sandstones, (ii) cementation by mixed marine–meteoric calcite ( δ 18 O=−5.3‰ to −2.6‰ and δ 13 C=−3.9‰ to −1.3‰) and dolomite ( δ 18 O=−4.6‰ to −2.6‰ and δ 13 C=−2.9‰ to −2.3‰) in the foreshore, TST sandstones and below parasequence boundaries (PB), and transgressive surface (TS), and in the shoreface, TST sandstones below maximum flooding surfaces (MFS), being facilitated by the presence of carbonate bioclasts, (iii) dissolution of detrital silicates and precipitation of K-feldspar overgrowths and kaolinite, particularly in the fluvio-deltaic, LST sandstones owing to effective meteoric water circulation, and (vi) formation of autochthonous glauconite, which is increases in abundance towards the top of the fluvio-deltaic, LST sandstones, and along TS, and in the shoreface, TST sandstones, by alteration of micas owing to the flux of seawaters into the sandstones during transgression, whereas parautochthonous glauconite is restricted to the TS sandstones owing to marine reworking. Mesogenetic alterations, which include cementation by quartz overgrowths and illite, display fairly systematic link to fluvio-deltaic, LST sandstones. This study has revealed that linking of diagenesis to transgression and regression events enables a better understanding of the parameters that control the spatial and temporal distribution of diagenetic alterations in sandstones and of their impact on reservoir quality evolution.

Elucidation of diagenetic alterations in the Petrohan Terrigenous Group (fluvial; highstand systems tract HST) sandstones and Svidol Formation (tide-dominated deltaic and tidal flat, transgressive systems tract TST and highstand systems tract HST, respectively) sandstones and calcarenite, Lower Triassic, NW Bulgaria was constrained within a sequence stratigraphic framework. Eogenetic alterations in the fluvial HST sandstones include (i) formation of grain-coating infiltrated clays as a result of percolation of mud-rich surface waters into underlying coarse-grained and permeable channel-fills and crevasse splay sandstones; (ii) formation of pseudomatrix by mechanical compaction of mud intraclasts that were incorporated into the coarse-grained channel sandstones during their lateral avulsion; and (iii) cementation by calcite (δ 18 O VPDB = − 6.5‰ to − 3‰; δ 13 C VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ 18 O VPDB = − 6.1‰ to − 0.3‰; δ 13 C VPDB = − 7.2‰ to − 5.8‰) in the crevasse splay and floodplain sediments. Mesogenetic alterations that are encountered in the fluvial HST sandstones include (i) illitization of grain-coating clays, mud intraclasts, and mica, possibly because of simultaneous albitization of feldspars; (ii) cementation by calcite (δ 18 O VPDB = − 14.5‰ to − 8.4‰; δ 13 C VPDB = − 7.7‰ to + 0.6‰) and dolomite (δ 18 O VPDB = − 15.8‰ to − 5‰; δ 13 C VPDB = − 7.9‰ to + 1.5‰); and (iii) limited amounts of quartz overgrowths in the channel sandstones owing to occurrence of thick grain-coating clays. Conversely, the tide-dominated deltaic TST sandstones and the tidal flat HST calcarenite were pervasively cemented by calcite (δ 18 O VPDB = − 6.6‰ to − 3.1‰; δ 13 C VPDB = − 5.1‰ to + 0.6‰) and siderite (δ 18 O VPDB = − 7.2‰ to − 5.7‰; δ 13 C VPDB = + 0.3‰ to + 0.9‰) particularly below marine and maximum flooding surfaces, due to the presence of abundant bioclasts and prolonged residence time of the sediments under certain geochemical conditions along these surfaces. The remaining open pores were cemented during mesodiagenesis by calcite (δ 18 O VPDB = − 6.6‰ to − 3.1‰ and δ 13 C VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ 18 O VPDB = − 6.6‰ to − 3.1‰ and δ 13 C VPDB = − 5.1‰ to + 0.6‰). This study shows that constructing a conceptual model for the distribution of diagenetic alterations is possible by integration of diagenesis with sequence stratigraphy. The model shows that tide-dominated deltaic TST sandstones and tidal flat HST calcarenite were pervasively cemented by carbonates during near-surface eodiagenesis, owing to the presence of abundant bioclasts. Conversely, fluvial LST sandstones remained poorly cemented during near-surface eodiagenesis due to the lack of bioclasts, but were cemented by mesogenetic calcite, dolomite and quartz overgrowths instead.

The spatial and temporal distribution of diagenetic alterations has been constrained in relationship to depositional facies and sequence stratigraphy of the Upper Ordovician glaciogenic quartzarenite sandstones in the Murzuq Basin, SW Libya, which were deposited during the Haritanian glaciation when the basin was laying along the continental margin of Gondwana. Eogenetic alterations encountered include: (i) replacement of detrital silicates, mud matrix and pseudomatrix by kaolinite in paraglacial, tide-dominated deltaic, in foreshore to shoreface (highstand systems tract; HST) and in post-glacial, Gilbert-type deltaic (lowstand systems tract; LST) sandstones, particularly below the sequence boundaries (SB). Kaolinite formation is attributed to the influx of meteoric water during relative sea level fall and basinward shift of the shoreline. (ii) Cementation by calcite ( δ 18 O VPDB = − 3.1‰ to + 1.1‰ and δ 13 C VPDB = + 1.7‰ to + 3.5‰) and Mg-rich siderite in the paraglacial, tide-dominated deltaic and foreshore to shoreface HST sandstones, in the glacial, tide-dominated estuarine (transgressive systems tract; TST) sandstones and in the post-glacial, shoreface TST sandstones is interpreted to have occurred from marine pore-waters. (iii) Cementation by Mg-poor siderite, which occurs in the post-glacial, Gilbert-type deltaic LST sandstones and in the paraglacial, tide-dominated deltaic and foreshore to shoreface HST sandstones, is interpreted to have occurred from meteoric waters during relative sea level fall and basinward shift of the shoreline. (iv) Pervasive cementation by iron oxides has occurred in the glacial, shoreface–offshore TST sandstones and post-glacial, shoreface TST sandstones immediately below the maximum flooding surfaces (MFS), which was presumably enhanced by prolonged residence time of the sediments under oxic diagenetic conditions at the seafloor. (v) Formation of grain-coating infiltrated clays mainly in the glacial, fluvial incised-valley LST sandstones and in the post-glacial, Gilbert-type deltaic LST sandstones as well as, less commonly, in the paraglacial, foreshore to shoreface HST sandstones and in the tide-dominated deltaic HST sandstones below the SBs. Mesogenetic alterations include mainly the formation of abundant quartz overgrowths in the glacial, fluvial incised-valley LST sandstones, post-glacial, Gilbert-type deltaic LST sandstones and glacial, shoreface TST sandstones, in which early carbonate cements are lacking. Illite, chlorite and albitized feldspars, which occur in small amounts, are most common in the glacial, tide-dominated estuarine TST sandstones and paraglacial, shoreface HST sandstones. This study demonstrates that the spatial and temporal distribution of diagenetic alterations and their impact on reservoir-quality evolution in glacial, paraglacial and post-glacial sandstones can be better elucidated when linked to the depositional facies and sequence stratigraphic framework.

The origin and timing of siderite cementation have been constrained in relation to depositional facies and sequence stratigraphy of Upper Ordovician glaciogenic sandstones from the Murzuq basin, SW Libya. Optical microscope, backscattered electron imagery, and carbon and oxygen stable isotope analysis have revealed that siderite is of eo- and mesogenetic origin. Eogenetic siderite is Mg-poor with a mean composition of (Fe91.7Mg1.5Ca0.3Mn6.5)CO3, and occurs in paraglacial, tide-dominated deltaic highstand systems tract (HST) sandstones, in paraglacial, foreshore to shoreface HST sandstones and in postglacial, Gilbert-type deltaic lowstand systems tract (LST) sandstones. This siderite is typically of meteoric water origin that influxed into the LST and HST sandstones during relative sea level fall and basinward shift of the strandline. Mesogenetic siderite, which engulfs and thus postdates quartz overgrowths and illite, is Mg-rich with a mean composition of (Fe72.2Mg21.7Ca0.8Mn5.3)CO3 and occurs in the paraglacial, tide-dominated deltaic HST sandstones, in paraglacial foreshore to shoreface HST sandstones, in glacial, tide-dominated estuarine transgressive systems tract (TST) sandstones, in postglacial, Gilbert-type deltaic LST sandstones, and in postglacial, shoreface TST sandstones. δ18OV-PDB values of this siderite, which range between −22.6 and −13.8‰, suggest that precipitation has occurred from evolved formation waters with δ18O values between −14.0 and +1.0‰ and was either meteoric, mixed marine–meteoric and/or marine in origin by assuming postdating quartz overgrowths and illite temperature between 80 and 130 °C.

Development of a predictive model for the distribution of diagenetic alterations and related evolution of reservoir quality of sandstones was achieved by integrating the knowledge of diagenesis to sequence stratigraphy. This approach allows a better elucidation of the distribution of eogenetic alterations within sequence stratigraphy, because changes in the relative sea level induce changes to: (i) pore water chemistry, (ii) residence time of sediments under certain near-surface geochemical conditions, (iii) variations in the detrital composition, and (iv) amounts and type of organic matter.This thesis revealed that eogenetic alterations, which are linked to sequence stratigraphy and have an impact on reservoir quality evolution, include formation of: (i) pseudomatrix and mechanically infiltrated clays in fluvial sandstones of the lowstand and highstand systems tracts (LST and HST, respectively), (ii) kaolinite in tide-dominated deltaic and foreshore-shoreface sandstones of HST, Gilbert-type deltaic sandstones of LST and fluvial deltaic sandstones of LST, (iii) kaolinite and mechanically infiltrated clays in sandstones lying below sequence boundary, (iv) K-feldspar overgrowths in fluvial deltaic LST, (v) glaucony towards the top of fluvial deltaic LST immediately below and at transgressive surface (TS) and in foreshore and shoreface transgressive systems tracts (TST) below parasequence boundaries (PB) and maximum flooding surface (MFS), (vi) framboidal pyrite and extensive cementation by calcite and dolomite in foreshore and shoreface and tide-dominated deltaic TST, and shoreface and tidal flat HST bioclastic-rich arenites particularly in the vicinity of PB, TS and MFS, (vii) pervasive cementation by iron oxide in shoreface-offshore and shoreface sandstones of TST immediately below the MFS, (viii) zeolites and palygroskite in shoreface sandstones of TST and HST, particularly above PB, and (ix) cementation by siderite in Gilbert-type deltaic sandstones of LST, tide-dominated deltaic and foreshore-shoreface sandstones of HST and in tide-dominated deltaic sandstones of TST, particularly at MFS. Moreover, this thesis revealed that the distribution of eogenetic alterations strongly control, and thus provide information for constraining the distribution patterns of mesogenetic alterations, such as illitization of mechanically infiltrated clays and dickitization of kaolinite, and hence of related reservoir quality evolution of sandstones during progressive burial.

Meteoric-water flux and formation of kaolinite owing to the dissolution of detrital silicates are common features of continental and paralic sandstones. In deep-water marine sandstones, meteoric-water flux is commonly considered unlikely to occur. However, the study of deep-water, marine sandstones of the Shetland–Faroes Basin on the British continental shelf revealed widespread and extensive dissolution and kaolinitization of mica and feldspar grains, which are attributed to meteoric-water flux during a sea-level lowstand. We suggest that this apparently enigmatic meteoric-water flux mechanism is likely to have occurred by hyperpycnal flow. Hyperpycnal flow occurs when river effluent directly transfers into sediment gravity flow, and enters seawater as a mixture of sediment and fresh water. The likelihood for hyperpycnal flows increases at times when rivers and distributary channels reach the shelf edge, and their flows are delivered directly onto the deepwater slope.

The application of sequence stratigraphy methods to glaciogenic deposits is more problematic than in normal paralic deposits because changes in the relative sea level are strongly influenced by an interplay between glacial advance and retreat, and concomitant isostatic loading and rebound of the continental shelf. The study of outcrop and subsurface sections of the Upper Ordovician glaciogenic Melaz Shuqran and the Mamuniyat formations, Murzuq Basin SW Libya, yielded important information on understanding changes in the relative sea level related to glacier movements. Three depositional sequences were recognized. Depositional sequence one (DS-1) corresponds to the entire Melaz Shuqran Formation that was bounded below by subglacial erosion surface (i.e. sequence boundary, SB) formed during glacier advance into shallow water areas. This surface correlates with a transgressive surface (TS) in deep water areas and records an initial rise in the relative sea level owing to glacier advance and loading of the continental shelf. Transgression occurred due to the slower rate of eustatic sea-level fall than the isostatic loading. The transgressive systems tract (TST) comprises shoreface sandstones and offshore mudstones/diamictites with ice-rafted debris. Further rise in relative sea level was associated with glacial retreat, which resulted in sediment starvation in deep water areas and the formation of a condensed section. These deposits represent glacial depositional systems. The highstand systems tract (HST) comprises prograding deltaic deposits, which was formed when the rate of sediment supply exceeds the rate of relative sea level rise. The HST deposits represent paraglacial depositional systems. Depositional sequence two (DS-2) corresponds to the lower and middle part of the Mamuniyat Formation, and is bounded below by deep erosional surface (SB) that cuts into DS-1 as an incised valley, which was formed during glacial advance across the continental shelf. Incision occurred due to the faster rate of eustatic sea-level fall than the rate of isostatic loading. Glacier retreat and the concomitant rise in relative sea level resulted in the deposition of braided fluvial sediments in a lowstand systems tract (LST) and tide-dominated estuarine sediments in a TST. These deposits represent glacial depositional systems. The HST sediments represent prograding foreshore to shoreface deposits, which were formed when the rate of sediment supply exceeded the rate of relative sea-level rise. These deposits represent paraglacial depositional systems. Depositional sequence three (DS-3) corresponds to the upper part of the Mamuniyat Formation and is bounded below by an erosional surface (SB) that was formed as a result of isostatic rebound, associated with relative sea-level fall. DS-3 LST comprises Gilbert-type, deltaic conglomeratic sandstones grading upwards into sandstones. Subsequent relative sea-level rise during the late Ordovician resulted in deposition of shoreface TST sandstone. Further rise in the relative sea level resulted in sediment starvation in deep water areas and the formation of thin hardground layer (i.e. condensed section) that marks the upper boundary of late Ordovician deposits. This study provides predictive model for the spatial and temporal distribution of ancient glaciogenic depositional facies that may have important implications in hydrocarbon explorations.

A study of the distribution of diagenetic alterations and their impact on reservoir-quality evolution in four deep-water turbidite successions (Cretaceous to Eocene) from basins in active (foreland) and passive margins revealed the impact of tectonic setting, depositional facies, and changes in the relative sea level. Diagenetic modifications encountered in the turbiditic sandstones from the passive margin basins include dissolution and kaolinitization (kaolin has δ18OV-SMOW = +13.3‰ to +15.2‰; δDV-SMOW = -96.6‰ to -79.6‰) of framework silicates, formation of grain coating chloritic and illitic clays, cementation by carbonates and quartz, as well as the mechanical and chemical compaction of detrital quartz. Kaolinitization, which is most extensive in the lowstand systems tracts, is attributed to meteoric-water flux during major fall in the relative sea level. Preservation of porosity and permeability in sandstones from the passive margin basins (up to 30% and 1 Darcy, respectively) is attributed to the presence of abundant rigid quartz and feldspar grains and to dissolution of carbonate cement as well as mica and feldspars. Diagenetic modifications in turbidites from the foreland basins include carbonate cementation and mechanical compaction of the abundant ductile rock fragments, which were derived from fold-thrust belts. These diagenetic alterations resulted in nearly total elimination of depositional porosity and permeability. The wide range of δ13CV−PDB values of these cements (about -18‰ to +22‰) in passive margin basins is attributed to input of dissolved carbon from various processes of organic matter alterations, including microbial methanogenesis and thermal decarboxylation of kerogen. The narrower range of δ13CV−PDB values of these cements (about -2‰ to +7‰) in the foreland basins suggests the importance of carbon derivation from the dissolution of carbonate grains. The generally wide range of δ18O values (about -17‰ to -1‰) of the carbonate cements reflect the impact of oxygen isotopic composition of the various fluid involved (including marine depositional waters, fluxed meteoric waters, evolved formation waters) and the wide ranges of precipitation temperatures. Results of this study are anticipated to have important implication for hydrocarbon exploration in deep-water turbidites from passive and active margin basins and for pre-drilling assessment of the spatial and temporal distribution of reservoir quality in such deposits.

International audience; This study aims to unravel the impact of diagenetic alterations on porosity loss of foreland-basin turbiditic hybrid arenites and associated siliciclastic sandstones of the Eocene Hecho Group (south-central Pyrenees, Spain). In this succession, hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement (delta18O VPDB = –10.0 per thousand to –5.8per thousand ; Th, mode = 80° C; salinity mode = 18.8 wt% eq. NaCl), Fe-dolomite (delta18O VPDB = –8.5 per thousand to –6.3 per thousand ) and trace amounts of siderite. The extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains, which provided nuclei and sources for the precipitation and growth of carbonate cements. Other diagenetic alterations, such as pyrite and albitization, had no impact on reservoir quality. Scarce early diagenetic cements, coupled with abundant ductile carbonate and siliciclastic framework grains, have led to rapid porosity loss owing to compaction. Conversely, abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction. Reservoir quality was affected by mesogenetic cementation by quartz overgrowths, calcite and dolomite intergranular pressure dissolution of quartz grains, and formation of fracture-filling calcite cement (delta 18O V-PDB values from –10.4 per thousand to –7.8 per thousand ; Th temperatures of circa 150° C), which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism. The results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition, particularly the proportion and types of extrabasinal noncarbonates, extrabasinal carbonates, and intrabasinal carbonate grains. These insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones. Degradation of reservoir quality makes these hybrid arenites, calclithites, and sandstones suitable as tight gas reservoirs, but only if fracture porosity and permeability develop during tectonic deformation.

The Palaeocene, deep-water marine sandstones recovered from six wells in the Shetland-Faroes Basin represent lowstand, transgressive and highstand systems tract turbiditic sediments. Mineralogic, petrographic, and geochemical analyses of these siliciclastics are used to decipher and discuss the diagenetic alterations and subsequent reservoir quality evolution. The Middle-Upper Palaeocene sandstones (subarkoses to arkoses) from the Shetland-Faroes Basin, British continental shelf are submarine turbiditic deposits that are cemented predominantly by carbonates, quartz and clay minerals. Carbonate cements (intergranular and grain replacive calcite, siderite, ferroan dolomite and ankerite) are of eogenetic and mesogenetic origins. The eogenetic alterations have been mediated by marine, meteoric and mixed marine/meteoric porewaters and resulted mainly in the precipitation of calcite (δ18OV−PDB=−10.9‰ and −3.8‰), trace amounts of non-ferroan dolomite, siderite (δ18OV−PDB=−14.4‰ to −0.6‰), as well as smectite and kaolinite in the lowstand systems tract (LST) and highstand systems tract (HST) turbiditic sandstone below the sequence boundary. Minor eogenetic siderite has precipitated between expanded and kaolinitized micas, primarily biotite. The mesogenetic alterations are interpreted to have been mediated by evolved marine porewaters and resulted in the precipitation of calcite (δ18OV−PDB=−12.9‰ to −7.8‰) and Fe-dolomite/ankerite (δ18OV−PDB=−12.1‰ to −6.3‰) at temperatures of 50–140 and 60–140 °C, respectively. Quartz overgrowths and outgrowth, which post- and pre-date the mesogenetic carbonate cements is more common in the LST and TST of distal turbiditic sandstone. Discrete quartz cement, which is closely associated with illite and chlorite, is the final diagenetic phase. The clay minerals include intergranular and grain replacive eogenetic kaolinite, smectite and mesogenetic illite and chlorite. Kaolinite has been subjected to mesogenetic replacement by dickite. The K-feldspar and plagioclase grains have been albitized. Dissolution of calcite cement and of framework grain (feldspar, volcanic fragments and mud intraclasts) has resulted in a considerable enhancement of reservoir quality.