Quartz Arenite Research Articles

Craspedia blepharia (Compositae/Asteraceae, Gnaphalieae), a new species from Garibaldi Ridge in Nelson/Tasman, South Island of New Zealand

ABSTRACT Craspedia G.Forst., a genus of tribe Gnaphalieae (Compositae/Asteraceae), is confined to Australia and New Zealand. Twelve species, including Craspedia blepharia Breitw. & Courtney, which we are describing here, are now recognised in New Zealand. Craspedia blepharia is known only from Garibaldi Ridge in Nelson/Tasman, South Island of New Zealand. It is a strong calcicole, confined to exposed, vertical surfaces of limestone cliffs and dolines and quartzose sandstone cliffs which receive calcium-rich seep-water. Its rosettes grow either individually or in dense clumps on cliff faces, thus allowing the plants to receive water from surface water flow. It is distinguished from other New Zealand Craspedia by its rosette leaves clad with a hispid adaxially very dense and abaxially sparse indumentum of very long (2.2–3 mm), usually curved, uniseriate, multiseptate, usually slightly brown or colourless, translucent trichomes that may have a very thin curly, flagellate tip, and leaf margins with a thin white rim formed by the flagellate tips of these long, curved trichomes. We are providing the formal taxonomic treatment together with illustrations and descriptions of its morphology, distribution, habitat, ecology, phenology, and etymology, as well as an assessment of its conservation status.

Depositional and diagenetic controls on the reservoir quality of marginal marine sandstones: An example from the Early Devonian subbat member, jauf formation, northwest Saudi Arabia

The reservoir quality of marginal marine sandstones can vary significantly across different depositional lithofacies. This variation is primarily due to differences in depositional processes and subsequent diagenetic alterations occurring during various stages of burial. To address this issue, this study focuses on the Early Devonian Subbat sandstones of the Jauf Formation, which serve as an important analogue for subsurface gas reservoirs and have potential for CO2 sequestration. An integrated approach was employed, including field observations, petrography, geochemical analyses, and petrophysical analyses, to investigate the impact of depositional and diagenetic processes on reservoir quality. The findings indicate that the sandstones range from poorly to well sorted, very fine to medium-grained arkosic and quartz arenites that were deposited in a range of environments including shoreface-to-offshore transition zones, wave-dominated deltas, fluvial and tidal estuarine channels. The sandstones experienced both shallow and deep burial diagenesis, while key diagenetic events affecting reservoir quality include compaction, carbonate cementation, the formation of authigenic clays (kaolinite, illite, and chlorite), quartz overgrowth, and the dissolution of unstable feldspar and mica grains. Authigenic pore-filling cements and grain-coating clays significantly influence reservoir quality. The effect of quartz cementation was minimal where illite and/or chlorite coatings are well-developed. Mechanical compaction is more pronounced in the shoreface-to-offshore transition compared to the estuarine channels likely due to the higher detrital matrix content (21.7–35%, avg. 26.7%). The well-sorted fluvial to estuarine channel sandstones exhibit the best reservoir quality, with porosity ranging from 15.9% to 25.5% (average 20.4%) and permeability between 433.8 mD and 1261.2 mD (average 720 mD). This high reservoir quality is attributed to the low detrital matrix content (0–1%, average 0.2%) and limited pore-filling cements. In comparison, the moderately sorted, wave-dominated delta sandstones have lower porosity (5.3–11.2%, average 8.2%) and permeability (17.8–80 mD, average 42.8 mD), despite a relatively higher detrital matrix content (0–11.5%, average 2.8%). The poorly sorted shoreface-to-offshore transition sandstones exhibit the lowest reservoir quality, with porosity ranging from 1% to 2% (average 1.5%) and permeability between 0.3 and 1.1 mD (average 0.7 mD). These variations in reservoir quality among different depositional facies are primarily linked to depositional factors (sorting and detrital clay matrix) and shallow to deep burial diagenetic processes (compaction and cementation). A comprehensive understanding of these processes can enhance reservoir quality prediction in the Jauf Formation and similar reservoirs elsewhere.

Fracture length data for geothermal applications

Fracture lengths govern permeability and are unknowns in geothermal assessment. Along their lengths, fracture widths vary due to growth by linkage. Under the influence of diagenesis, narrow widths seal, breaking porosity continuity and reducing open length. The largest range of widths and thus susceptibility to fill occurs where fractures are linked by narrow segments. Outcrops of a geothermal target, Cambrian Potsdam quartz arenite, contain opening-mode fractures having lengths spanning five orders of magnitude from 0.082 mm to 17.9 m. Combined lengths measured at a range of scales can be described by power laws, but at a given image resolution, lengths are best fit by exponential functions. Owing to preferential sealing of small fractures, open fractures follow exponential functions, but values depend on rules for designating fractures as continuous. En échelon segments, offset 10 mm, are connected by narrow fractures or microfractures (hard linked) not evident on outcrop 1-m-elevation LiDAR or 30-m-height drone images. A rule that identifies where narrow but probably connected segments yields lengths that are meaningful for flow simulation. Depending on diagenesis, continuity rules can halve or double average and maximum length values. Length values from outcrop for geothermal applications should be adjusted based on wellsite-specific diagenesis information.

Thermomechanical response and crack evolution of sandstone at elevated temperatures

Understanding the thermomechanical response of rock at high temperatures is crucial for various energy applications such as underground coal gasification and geothermal systems. The study investigated the effects of temperature, mineral composition, and grain size on crack initiation (CI) and crack damage (CD) thresholds in Jodhpur sandstones using uniaxial compressive strength with acoustic emission, Brazilian tensile strength, and thermogravimetric analysis subjected to elevated temperatures. The study revealed distinct patterns in crack initiation stress threshold ratios (CISTR) and crack damage stress threshold ratios (CDSTR) influenced by mineral composition and grain size under temperature treatments. Ferruginous quartz arenite exhibited an inverse relationship between quartz content and crack initiation/damage stress thresholds, while siliceous quartz arenite and subarkose showed a positive correlation. The established variation is attributed to the differing grain boundary strengths among the minerals. Comparative analysis of crack thresholds with the minerals, excluding quartz and feldspar, revealed complex relationships with clay and other minerals. Finer-grained sandstones showed direct proportionality in CI and CISTR with clay content, while coarser sandstones exhibited an inverse relationship. Additionally, the study highlighted differential trends in toughness parameters and CISTR, emphasizing the role of grain size and heat-treatment conditions in governing stress thresholds. Significant chemical changes, including quartz phase shifts and kaolinite/muscovite dehydroxylation, occurred in sandstones at 500–600°C. The presence of kaolinite/hematite in ferruginous quartz arenite caused the increased mass loss in pure O2 due to kaolinite breakdown, while siliceous quartz arenite exhibited a greater mass loss in standard conditions. The findings suggest that quartz content does not consistently enhance rock strength under heat treatment, particularly in the presence of significant clay minerals, leading to an inverse quartz-rock strength relationship in ferruginous quartz arenites. The study provides valuable insights into the thermomechanical behavior of sandstones, which is crucial for assessing rock stability and durability in energy applications.

Geochemical Characterization and Constraints on Provenance, Tectonic setting and Paleoweathering of the Miocene Bhuban Sandstone Exposed in Sakhan Anticline, Tripura Fold Belt, Surma Basin, India

ABSTRACT The petrography of sandstone, major oxides, trace and REE compositions of the Bhuban formation, exposed in the Sakhan Anticline of Tripura Frontal Fold Belt have been studied to determine the source rock, depositional tectonic setup and paleoweathering conditions. The average modal composition of the Bhuban Sandstone (Q89.27F4.40L6.33) classified them as quartz arenite and sub litharenite types. The modal composition data (Q-F-L) indicates a craton interior and recycled source for these sediments and deposited in an active continental basin margin as suggested by the bivariate plot of SiO2 vs. K2O/Na2O. The binary discrimination plot of Hf vs. La/Th suggests acidic and older sedimentary component as source for the studied sandstone. The ternary plot of V-Ni-Th×10 pointed towards felsic igneous provenance. Trace and rare earth elemental ratios, chondrite-normalize REE plot suggests the source of the Bhuban sediments are derived from felsic terrain. The CIA values (66.48-75.41), A-CN-K plot (A=Al2O3, CN=CaO+ Na2O, K=K2O) and distinct depletion of Na2O, CaO, U, Sr and Ba suggests moderate degree of chemical weathering of the source rocks which may indicate humid climatic conditions in the source area.

Geochemistry of Neoproterozoic arenites from the Murwara area, Katni District, Madhya Pradesh: Implications for provenance, weathering and Tectonic Setting

Geochemical analysis in terms of major, trace and rare earth elements of the Neo-Proterozoic arenites of Rewa and Bhander Groups of the Vindhyan Supergroup from Murwara, Katni district, M.P., has been carried out to determine the provenance, weathering and tectonic setting. The arenites are rich in SiO2 content but less in Na2O, K2O and CaO contents, suggesting the dominance of quartz and less amount of feldspars and rock fragments. Geochemically, these rocks are classified as quartz arenites to sub litharenite to sub-arkose. The CIA (75-98) and PIA values for these arenites and A-CN-K diagram indicates a high intensity of weathering in the source area. High LREE/HREE ratios, negative Eu anomaly and La/Sc, Th/Sc, Th/Co and Cr/Th ratio values suggest felsic source of these arenites. The discriminant diagram for these arenites indicates a quartzose sedimentary provenance. Ni vs TiO2 and Th/Sc vs Sc bivariate plots indicate that the felsic rocks have been the source rock for these arenites. The discrimination diagrams based on log (K2O+Na2O) vs SiO2 and Fe2O3+MgO vs TiO2 contents show a passive margin setting. Whereas the results of new discriminant function multi-dimensional diagram for the high silica arenites reveals a change from collision to continental rift setting

Provenance Analysis and Diagenesis Using Integrated SEM-Cathodoluminescence and Optical Microscopy for the Sandstones of the Pirispiki Formation, Kurdistan Region, Northern Iraq

The paragenetic history and provenance of framework grains in marginal marine and deltaic sandstones of the Pirispiki Formation in two sections, (Ora and Kaista), northern Iraq were examined. This was achieved using an integrated approach employing scanning electron microscopy-cathodoluminescence (SEM-CL), and optical microscopy. The Pirispiki Formation is lithologically characterized by alternating layers of thin- and thick-bedded sandstones. These were intercalated with thin laminae of grey-black shale. Petrographic analysis revealed that the dominant detrital grains in the mature quartz arenite sandstone were monocrystalline quartz grains with undulose extinction. The diagenetic analysis of the sandstones documented several processes that occur in three distinct stages. The SEM-CL technique enabled the identification of two distinct quartz overgrowth phases. Quartz cementations are of two types, the earliest phase and the most significant by volume is the Q1 cement characterized by dark non-luminescence which led to the destruction of most of the primary pores and the healing of mechanically induced cracks. Q2 cement is formed after major compaction and is characterized by bright luminescence with sector zoning, revealing an intricate SEM-CL zonation. CL revealed five special textures within detrital quartz grains including healed fractures and zoning. The provenance of the detrital quartz grains indicated the dominance of plutonic and metamorphic sources, with trace amounts originating from volcanic sources. These were mostly derived from the Bitlis Massif Zone in Turkey.

The Folster Lake Formation, Nunavut: remnant of a littoral to fluvio-deltaic source to the Paleoproterozoic-age Penrhyn basin

The Folster Lake Formation consists of Paleoproterozoic-age sedimentary cover units unconformably deposited on deeply weathered Archean basement of the Rae craton on Melville Peninsula, Nunavut. It comprises a lower member, formed of basal conglomerate, hematite-specularite iron-formation, mudstone, siltstone, silty limestone, and cross-bedded arkose, and an upper member dominated by arkose with planar and trough cross-beds, including giant (up to 10 m thick) compound cross-beds, overlain by quartz arenite with heavy mineral laminae. The entire succession has been historically interpreted as a near-shore, tide-dominated succession derived from a source to the southeast. Our field observations, supported by SHRIMP U–Pb analyses on detrital zircon, suggest that the Folster Lake Formation represents the remnants of a 1.91–1.88 Ga sand-dominated delta advancing on a tide-dominated shoreline, sourced by detritus eroded from the Thelon, Queen Maud, and Boothia-Somerset uplift to the west-northwest. This fluvio-deltaic system transported sediments to the Upper Penrhyn Group, which was contemporaneously being deposited in a basin opened to the southeast.

Quartz arenites as signatures of cratonisation: examples from the Archaean Stratigraphic Record, southern Africa

Abstract Holocene quartz-rich sands such as in the Orinoco River of Venezuela and Columbia, develop under a unique set of conditions including tectonic stability, intense weathering and long-term storage of sediment prior to burial. This review paper proposes that a similar set of conditions existed on the Mesoarchaean Earth by 3.0 Ga. Compositionally and texturally mature quartz arenites are present in the Mesoarchaean Hospital Hill Subgroup and correlative lower Mozaan Group, in the cover rocks of the Beitbridge Complex in the Limpopo Belt and at Buhwa in southern Zimbabwe as well as the Neoarchaean Manjeri Formation of Zimbabwe. These quartz arenites are interpreted as first-cycle and developed following the formation of stable cratons under conditions of slow subsidence (<10 cm/1 000 years), intense weathering in a hostile chemical environment related to the ambient carbon dioxide-rich atmosphere, long-distance intrabasinal transport by tides and/or waves and microbial processes that influenced the local chemical environment. Similar examples of Mesoarchaean quartz arenites are documented from the Yilgarn Block in Western Australia, the Slave and Superior provinces in Canada and the Dharwar and Singhbhum cratons in India. On a global scale, cratonisation was diachronous with the Kaapvaal Craton stabilised on a regional scale by 3.0 Ga whereas microcontinents existed over the remainder of Earth.

Diagenetic controls on the quality of shallow marine sandstones: An example from the Cambro-Ordovician Saq Formation, central Saudi Arabia

The Saq Formation is the most important Paleozoic aquifer in central and northern Saudi Arabia. However, owing to the continued depletion of the aquifers and the great depths to which the aquifers are buried in undrilled/underexplored regions, the aquifers porosity and permeability are increasingly becoming an issue of concern. Here, we employed thin-section petrography, petrophysical measurements, X-ray diffraction and scanning electron microscopic analyses, and fluid inclusion microthermometry to investigate the composition and diagenetic evolution of the Saq sandstones. The results of this study indicate that the sandstones are mainly quartz arenite in composition, mostly well sorted, and ranged in grain size from silt to coarse-grained sandstones. We observed three diagenetic stages (early, middle, and late) that impacted the porosity evolution of the Saq sandstones. Mechanical compaction, formation of authigenic kaolinite, and infiltration of smectitic clays occurred during early shallow burial diagenesis. The middle (deep-burial) diagenesis was dominated by mechanical and chemical compaction, formation of authigenic quartz overgrowths, transformation of kaolinite and smectite into illite, and precipitation of late calcite. The late (uplift-related) diagenesis resulted in the formation of authigenic kaolinite, dissolution of the late calcite, and the formation of predominantly pore-filling Fe-oxide (goethite) cement. Fluid inclusion results of this study show that quartz cement mostly formed at temperatures ranging between 151 °C and 175 °C and reached up to 272 °C, with, at least, two phases of overgrowth development. The overall high amounts of rigid grain contents (>65 %), low detrital matrix (<10 %), and coarser grain size have maintained the Saq sandstone quality (average permeability = 2100 mD) despite having been buried to great depths. The findings of this study provide useful insights into the diagenetic evolution of the Saq Formation, which has provided important input data for forward diagenetic modelling of their subsurface equivalent in Saudi Arabia and sandstones of similar depositional settings elsewhere.

Petrography and Geochemistry of the Lower Sylhet Sandstone Member (Therria Sandstone) of Jaintia Group Exposed Along Jowai-Badarpur Road Section, Jaintia Hills District, Meghalaya: Implication for Provenance and Palaeoclimate

ABSTRACT The Paleogene sediments of the Shillong Plateau are exposed in the southern part of Meghalaya and are designated as the Jaintia Group. The Lower Sylhet Sandstone Member (LSSM) (Therria Sandstone) is the lowermost member of the Shella Formation of Jaintia Group. To understand the depositional history of the studied sandstones, petrographic and geochemical approaches have been made. Petrographically, these sandstones are classified as quartz arenite and quartz wacke. They were derived from intermediate to upper rank metamorphic and plutonic source which were deposited in a craton interior and quartzose recycled tectonic setting. The paleoweathering indices of CIA, CIW, PIA and ICV indicate that the weathering at the provenance was intense. The tectonic discrimination diagrams suggest passive margin setting for their sedimentation and climatic condition during the deposition was humid in nature. The studied sediments were deposited in non-marine conditions as evidenced by the geochemical proxies and primary sedimentary structures existing in the LSSM.

Foreland basin development in response to Proto-Tethyan Ocean closure, western North China Block

Closure timing of the northeastern Proto-Tethyan Ocean between the Qilian-Qinling Terranes and the North China Block remains unsolved, with assumptions ranging from end-Ordovician to the Devonian. To address this issue, integrated studies of stratigraphy and geochronology were conducted on Late Ordovician strata in the southwestern Ordos, which recorded the earliest tectonic transition from passive margin to foreland basin in the westernmost North China Block. Stratigraphic and paleontologic syntheses demonstrate that pre-Katian strata are shallow-marine deposits dominated by benthonic faunas. Meanwhile, Katian successions above a paraconformity are characterized by deep-water debrites and turbidites containing abundant planktonic graptolites. Provenance analysis reveals an evolving source from the North China Block basement to the Qilian-Qinling arc terranes at the beginning of Katian (ca. 450 Ma). Detrital zircons from pre-Katian quartz arenites yield ages of ca. 1600–2800 Ma, significantly older than their depositional timing. In contrast, Katian turbidites in the southwestern Ordos and the North Qilian Orogen display similar age patterns dominated by ca. 450–900 Ma ages. These clues imply a basin-filling shift from passive margin to underfilled foreland, separated by forebulge unconformity formation at Sandbian-Katian boundary. Stratigraphic correlation further suggests that sequence of foreland basin-infill transitions was broadly simultaneous along strike more than ~1200 km across the southwestern North China Block, including forebulge flexural uplift, followed by rapid shallow shelf drowning to abyssal plain, until arrival and superimposition of arc-derived turbidites. Along-strike synchronicity of orogenic activities implies that closure of the northeastern Proto-Tethys was nearly synchronous. The L-shaped orogen-parallel foreland encompassing the southwestern North China Block unveils oroclinal bending of the Qilian-Qinling collision belt, which still dominates the geology of present northeastern Tibet. Our new insights provide a stratigraphic constraint for the timing and mode of the initial elimination of the northeastern Proto-Tethys.

Diagenesis, Provenance, Tectonic Setting and Palaeoclimate of the Eocene Kopili Sandstone, Assam, India: A Petrographic Study

Abstract The Eocene Kopili Sandstone outcropping in the North Cachar Hills (NCHs) of Assam, India, was petrographically studied to unravel its diagenetic features, provenance, tectonic setting and palaeoclimate conditions. The modal analysis of the Kopili Sandstone reveals that the detrital mineralogy is dominated by quartz (48.78 to 83.70 %), with variable amounts of rock fragments (0.37 to 7.35 %), feldspar (0.37 to 8.33 %), micas (0.36 to 7.84 %) and subordinate amount of heavy minerals. The matrix and cement content varies from 9.88 to 21.50 % and 1.12 to 19.41 % respectively. The Kopili Sandstone is classified as quartz arenite to sub lithic arenite. Compaction, cementation, dissolution and replacement are the observed diagenetic processes in the studied sandstones. Diagenetic transformations are manifested in the fractured detrital grains, overgrowth of quartz, corrosion of grain boundaries of quartz, modification of grain boundaries, and silica and iron oxide cement precipitation in the interstices of the framework grains. The provenance is of granite-gneissic rock terrain. The detritus in the depositional basin is derived predominantly from craton interior with minor input from recycled orogen. The study discerns humid palaeoclimate conditions for the Kopili Sandstone.

Integrated stratigraphical, hydrogeochemical, and geophysical studies to characterize the shallow groundwater aquifers in Wadi Araba area, western Gulf of Suez, Egypt

The construction of the New Galala City besides great mining (e.g., konoozmisr) and Seawater Reverse Osmosis Desalination Plant projects in the semiarid Gulf of Suez region of Egypt increases the demand for groundwater resources. So, the current research provides integrated stratigraphical, hydrogeochemical, and geophysical studies to investigate the shallow groundwater aquifers in Wadi Araba area on the western side of the Gulf of Suez. The stratigraphic column starts with pre-Cambrian basement rocks which are non-conformably overlain by sedimentary succession of Phanerozoic Erathem. A detailed land magnetic profile has been conducted and processed to estimate the aquifer’s geometrical parameters. This includes the subsurface spatial extension and thickness to delineate the prevailed subsurface structures in the study area. Lithostratigraphically, these aquifers are represented by the Paleozoic- lower Cretaceous siliciclastic-dominated sediments of the Nubia Group and fractured carbonate-dominated facies of the upper Cretaceous Galala Formation. Lithological characters, sedimentary facies (fluvial to shallow and open marine), microfacies types, and diagenesis processes (e.g., compaction, cementation, dissolution, and fracturing) are the main factors that affect the quality of the shallow aquifers in the study area. The main microfacies of the studied aquifers are quartz arenite, subarkose, sublitharenite, mudstone/wackestone, and packstone/grainstone. These facies are characterized by different primary and secondary porosities. According to the water sampling standard methods, nineteen groundwater samples were collected from the available hand-dug wells and springs from the Nubian sandstone aquifer and the Upper Cretaceous aquifer, respectively. Inductively coupled plasma–mass spectrometry (ICP–MS), titration, spectrophotometer and a flame photometer were used to obtain a complete chemical analysis. The results of hydrochemical analyses indicated that total dissolved solids (TDS) and concentrations of major ions as well in the lower Cretaceous aquifer increase toward the Gulf of Suez and in general higher than that of springs of the Upper Cretaceous Aquifer. The predominant factors influencing groundwater chemistry include seawater intrusion, evaporation, mineral alteration, dissolution of fossils, and ion exchange. Groundwater in the study area is unsuitable for drinking except in one sample and moderately suitable for irrigation with caution for higher salinity in samples located near the Gulf of Suez.

Geochemical assessment of mineral sequestration of carbon dioxide in the midcontinent rift

AbstractThis study examines the potential of Midcontinent Rift rocks to facilitate long‐term CO2 sequestration by providing the necessary Ca and Mg for carbonate mineralization. Surface samples were collected from the Oronto and Bayfield‐Jacobsville Groups around Lake Superior and used for petrography and X‐ray diffraction to determine their mineral composition. Also, X‐ray fluorescence was also used to assess their bulk chemical composition. The samples were then exposed to CO2 and deionized water in Teflon‐lined vessels at 90°C, and the resulting leachate fluids were analyzed for the cation released during the testing. SEM microscopy was used to examine the samples for potential mineralization of carbonate minerals. The Oronto Group sediments consist primarily of feldspathic to feldspathic lithic arenites with a chlorite‐dominated matrix, and the primary porosity is blocked by calcite and hematite cement. The Bayfield–Jacobsville sequences are porous quartz arenites to feldspathic quartz arenites that do not contain significant accumulation of Ca‐, Mg‐, and Fe‐bearing minerals. The leachate fluids obtained from Oronto Group samples exhibit a maximum Ca release rate (5.2 × 10−4 mole/cm2.day), indicating rapid calcite cement dissolution and increased porosity and permeability. SEM/EDS microanalysis revealed areas where pore‐filling calcite was preferentially dissolved. Longer‐term rock‐water reactions resulted in induced carbonate mineralization, as evidenced by calcite crystals observed in a sample reacted for 102 days. © 2024 Society of Chemical Industry and John Wiley &amp; Sons, Ltd.

Preliminary assessment of the Cambro-Ordovician reservoir potential from the Shushan basin, north Western Desert, Egypt

This study presents the first-ever preliminary assessment of the potential reservoir intervals from the Cambro-Ordovician Shifah Formation of the Shushan Basin, Western Desert. Seismic data, thin sections, XRD, XRF, and wireline logs were integrated to characterize the potential intervals. The study area is characterized by E-W and ENE-WSW trending steeply dipping normal faults creating a series of horsts, grabens, and half-graben structures. Based on the wireline logs-based petrophysical assessment, three potential reservoir intervals are identified with the Shifah Formation below 15000 ft depth. All the promising intervals show little shale volume (10–20 %) and 70–80 % hydrocarbon saturation, but the porosity is dominantly below 10%. The two potential sandstone intervals are composed of quartz arenites affected by silica cementation and mechanical compaction. These glauconite-bearing sandstones exhibit low-angle cross-lamination and planar lamination, likely deposited in a fluvial to shallow marine depositional environment. Both the tight sandstone intervals exhibit E-W, and NW-SE striking, closed, and partially open fractures which can contribute to the reservoir flow capacity. The third potential reservoir interval is an igneous intrusive body, composed of alkali syenite and exhibits secondary intraparticle porosity due to partial feldspar dissolution and minor fractures. The preliminary assessment presented in this work shed critical insights into the reservoir potential of the Shifah Formation.

Mesoproterozoic age for andalusite in the lower Rocky Cape Group, northwest Tasmania

New U–Th–Pb chemical data from both electron probe microanalysis and U–Pb laser ablation-inductively coupled plasma mass spectrometry on monazite inclusions in giant andalusite porphyroblasts indicate that the lowermost subdivision of the Rocky Cape Group in northwest Tasmania underwent metamorphism at ca 1100 Ma. The Proterozoic Rocky Cape Group is a ∼10 km-thick succession of quartz arenite, siltstone and pelite. These sediments were deposited between ca 1450 and ca 1010 Ma. Metamorphism within the Rocky Cape Group ranges from the prehnite–pumpellyite to upper greenschist facies, locally reaching garnet grade. In the lowermost formation of the Rocky Cape Group, andalusite-bearing mineral assemblages in the Pedder River Siltstone formed prior to or synchronously with monazite development. The andalusite-bearing mineral assemblage indicates relatively low-pressure conditions of <200 MPa, consistent with the stratigraphic thickness of the Rocky Cape Group (∼10 km). However, the presence of the intermediate-temperature biotite–andalusite mineral assemblages (500 °C) in the study area suggests the contribution of additional heat sources. Garnet porphyroblasts were observed ∼2 km away. Similar-age medium-grade metamorphism and magmatism at about 1100 Ma have been observed regionally in the South Tasman Rise and East Tasman Plateau, and in areas that are thought to have been contiguous with northwestern Tasmania such as Dronning Maud Land, central Arizona (Yavapai–Mazatzal) and Belt–Purcell Basin in the southwestern and northwestern Laurentian crustal sequences. The pressure and temperature relationships, mineral associations and regional data suggest additional heat flow was present through the lower part of the Rocky Cape Group. The higher geothermal gradient could be attributed to burial during basin development and deposition of the upper Rocky Cape Group or associated magmatism on the margin of Laurentia during assembly of Rodinia.

Diagenetic imprints in the reservoirs of Agbada Formation, Niger Delta Basin: Implication for reservoir quality and appraisal of calcite cement

The main exploration targets for hydrocarbons in the Niger Delta Basin are within the Agbada sandstone reservoirs. The current study utilizes a multitechnical approach (SEM-MLA, SEM-EDS, XRD and LA-ICP-MS) to evaluate the roles of framework composition and diagenetic processes, particularly calcite cementation on reservoir quality in Miocene intervals of the Agbada Formation sandstone, Niger Delta Basin. Petrographic analysis reveals that the sandstones are feldspatho-quartzose to quartzose sandstones, with a preponderance of quartzose sandstones. The overall diagenetic imprints suggest minimal compaction and lack of pervasive cement, which have contributed significantly to the high porosity. Although the degree of mechanical compaction is generally low, it has comparatively exercised a better control on porosity than the growth of cement, excluding some intervals with considerable cementation (siderite and ferroan calcite) where compactional porosity-loss was minimal. The occurrence of ferroan calcite cement (CaCO3 = 95.9 ± 0.4, MgCO3 = 0.8 ± 0.1, FeCO3 = 2.9 ± 0.2 mol%) is generally small, probably discontinuous and unlikely to represent significant barriers to fluid flow with their localized influence, but understanding their locally significant influence may still contribute to building good reservoir models. Petrographic observations suggest that the precipitation of “poikilitic” calcite was early, but the shale normalized REY patterns do not mimic seawater patterns (Y/Ho ratios; 36.3 ± 1.6) due to possible contributions from clastic or detrital grains and/or incorporation of particulate matter that can preferentially scavenge LREEs from the overlying water column. A possible concern during reservoir production is the occurrence of clay mineral matrix (mostly kaolinitic) anticipated to cause the production of fines and formation damage, but this can be mitigated with proper reservoir management.

Geochemistry of the North Gondwana Paleozoic Araba and Naqus formation siliciclastics, Sinai: implications for provenance, paleoweathering, and tectonic setting

ABSTRACT The Araba and Naqus formations in Ras Milan, Sinai, constitute part of the major continuous Paleozoic clastic belt stretching across northern Gondwana. They are sandstone-dominated successions, resting on Precambrian granitic basement, with lowermost thin basal polymictic conglomerate deposits (1–2 m), marking the unconformity. The Araba Formation section is composed mainly of varicoloured violet to reddish brown, fine to coarse-grained laminated and planner and overturned cross-bedded sandstones, while the overlying Naqus Formation is characterized by chaotically distributed poorly sorted quartz pebbles to cobbles within a moderately-sorted coarse- to medium-grained quartz arenite and by abundance and variety of large-scale tabular and trough cross bedding. Modal QFL classification of Araba and Naqus formation sandstones revealed that they are merely quartzose sandstones (quartz arenites). Geochemistry of the succession shows extremely wide compositional ranges for almost all major elements, but the Araba Formation is relatively enriched in SiO2, TiO2, Fe2O3*, Na2O, P2O5, and depleted in Al2O3, MgO, CaO and K2O relative to Naqus Formation. Only the Araba Formation samples are depleted in the transition elements Ni, Cr and Co and oppositely possess extreme enrichment in the HFSEs (i.e. Zr, Hf, Y, Nb, Ta, Th, U), whereas both formations show variable depletion in the LILE Rb, K and Ba, and excessive REE abundances relative to the PAAS and UCC, and LREE-enriched patterns with marked negative Eu anomalies. The provenance of the Naqus and Araba formations could be an earlier felsic magmatic source, which has undergone extensive sedimentary recycling. They possess high compositional maturity and were subjected to intensive to moderate weathering during humid climatic conditions. Passive margin tectonic setting is implied which denotes the derivation of Araba and Naqus formation siliciclastics from stable continental regions or plate interiors and deposition in intracratonic basins.

Timing of rifting of the Dongkaco microcontinent (Central Tibet) and implications for Neo-Tethyan evolution

Rifting of continental blocks away from northern Gondwana resulted in the development of several Tethyan oceanic branches, the number of which and the timing of tectonic processes are actively debated. Within the Mesozoic Bangong-Nujiang suture, locally exposed Paleozoic strata are potential witnesses of the early development of Tethyan basins, yet their origin and tectonic significance remain poorly understood. In this study, Paleozoic limestone and sandstone units exposed in the Dongkaco area are identified as part of an independent Dongkaco microcontinent, differentiated from the South Qiangtang terrane by the Dongqiao suture on the north and from the Lhasa Block by the Beila suture on the south. The Upper Paleozoic deltaic to shallow-marine feldspatho-quartzose and quartzose sandstones within the microcontinent are interbedded with glaciogenic sequences and exhibit provenance characteristics that align with the Lhasa Block. These stratigraphic and provenance features suggest that during the Carboniferous to Early Permian the Dongkaco microcontinent was attached to the Lhasa Block. The discovery of Middle Triassic (Anisian) radiolarian chert associated with the Beila ophiolite indicates that sea-floor spreading was active within the Beila Ocean at that time. Data presented here show that the detachment of the Dongkaco microcontinent from the Lhasa Block resulted in the Beila Ocean, a branch of Bangong-Nujiang Ocean that expanded through Middle Permian to Early Triassic times. These findings shed new light on the paleogeographic evolution that preceded India-Asia collision and final amalgamation of the Himalayan-Tibetan orogen.