Volcanic Rock Fragments Research Articles

Origin of Grain-Coating Chlorite and Implications for Reservoir Quality in the Triassic Deep-Buried Volcaniclastic Sandstones, Central Junggar Basin, Northwestern China.

The occurrence and genesis of grain-coating chlorite were investigated in order to evaluate the impact of grain-coating chlorite on preserving porosity in the deep-buried Triassic Karamay volcaniclastic sandstones based on thin sections, scanning electron microscopy, and an electron probe. Grain-coating chlorite was formed during the eogenesis, originating from the precursor of smectite through the solid-state transformation (SST) mechanism. The hydration and dissolution of unstable, intermediately basic volcanic rock fragments provided essential Fe2+ and Mg2+ ions for the formation of grain-coating chlorite. Due to relatively high stability and low susceptibility to dissolution, acidic volcanic rock fragments could not promote chlorite formation but resulted in authigenic quartz and clays as pore-filling cements. This process would destroy reservoir properties. Under high hydraulic conditions, medium- to coarse-grained sandstone experienced saltation transport, creating significant velocity differentials and pressure differentials on grain surfaces. Subsequently, clay grains adhere to the surfaces, forming grain-coating chlorite during diagenesis with good continuity. In contrast, pebbly sandstone undergoes rolling transport, resulting in smaller velocity differentials on grain surfaces. This makes relatively ineffective clay adsorption and leads to discontinuous grain-coating chlorite in subsequent stages. Under weak hydraulic conditions, grains and clay particles in fine-grained sandstone undergo suspended transport, lacking mutual movement and velocity differentials. Clay particles cannot effectively cover particles but instead fill the pores between them. Therefore, continuous grain-coating chlorite is more commonly developed in the medium- to coarse-grained sandstones and is crucial for inhibiting quartz cementation with a coverage rate exceeding 80%. Inadequate coatings fail to inhibit quartz cementation effectively, while excessive coatings may block pore throats. Optimal protection of primary porosity could occur only when grain-coating chlorite is moderately developed.

Evolution of reservoir quality related to clay coating overgrowths in tight sandstone in the fluvial facies of the southeastern Sichuan Basin

As a result of silicate overgrowth, clay coatings influence the evolution of reservoir quality during diagenesis. Most studies indicate coatings can resist compaction and inhibit cementation, thus preserving primary porosity and serving as one of the primary controlling factors for improving reservoir quality. However, some research suggests clay coatings have insufficient hardness to resist compaction and could potentially block pore throats and prevent fluid flow within the sandstone, negatively affects reservoir quality. Therefore, to address whether coatings are positive to reservoir evolution, this study employs polarized light microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and porosity-permeability testing to investigate the characteristics of clay coatings within the fluvial facies sandstone of the Xiashaximiao Formation. The microscopy reveals abundant clay coatings were found, mainly in lithic-arenite with 30%–45% feldspar and rock fragments. The underlying Lianggaoshan Formation locally exhibits interbedded siltstones with volcanic rock fragments, providing material sources for the clay coatings formation. SEM and EDX analyses indicate two predominant forms of clay coatings: single-layer (primarily chlorite) and double-layer (inner smectite/illite, outer ferroan chlorite). Single-layer overgrowths primarily consist of thin layers of chlorite (5–15 μm), whereas double-layer overgrowths consist of a thin inner zone (1–5 μm) covering the grain surface and a thicker outer zone (10–20 μm) covering the inner zone, demonstrating characteristics indicative of multi-stage growth. Porosity-permeability testing results the sandstone samples have <10% porosity, which are typically tight sandstones, but those with clay coatings have higher porosity. Nevertheless, sandstones with double-layer overgrowths exhibit lower permeability relative to those with single-layer overgrowths or without overgrowths. Therefore, based on the integration of morphological and physical properties of silicate overgrowths, it is proposed the efficiency of clay coatings in resisting compaction, inhibiting cementation, and preserving primary porosity increases with the number of growth stages, layers, and thickness, which in turn depends on the abundance of source materials. Single-layer coatings facilitate fluid migration, whereas thicker double-layer overgrowths are prone to occluding narrow pore throats, thereby restricting fluid mobility. Consequently, this study establishes a model for the multi-stage evolution of clay coatings overgrowths, elucidating the varying fluid migration behaviors across different stages and providing a more detailed investigation of the implication of clay coatings on reservoir quality, thus offering new theoretical insights into diagenesis and the evolution of tight sandstone reservoirs.

Diagenetic effect on reservoir quality of siliciclastic and volcaniclastic sandstones from a Paleogene volcanic rifted margin, East Greenland

ABSTRACT Siliciclastic and volcaniclastic sediments in a volcanic rifted-margin succession may experience a complex diagenetic history during burial that can have a large impact on sandstone reservoir properties. To understand such changes, variations in initial sediment composition and succeeding diagenetic changes have been studied for a Paleogene outcrop analogue in the Kangerlussuaq area, East Greenland. The nature of the mafic volcanics-bearing succession, which consists of intra-volcanic sandstones, accommodated over quartz-rich pre-volcanic fluvial sandstones, are comparable to the settings of recently discovered hydrocarbon-producing sandstones in the Faroe–Shetland Basin on the conjugate Atlantic margin. Our petrographic and provenance investigations of the pre- and intra-volcanic sandstones are supported by geochemical and X-ray diffraction analyses. The intra-volcanic sandstones were deposited in shallow marine environments with mixed siliciclastic and volcaniclastic input, the latter rich in felsic to mafic volcanic rock fragments and feldspar grains. Similar zircon age distributions of pre- and intra-volcanic sandstones support a continued supply from the same siliciclastic sediment source after the onset of volcanism. Variations in initial detrital grain and pore-fluid (fresh to marine) compositions resulted in different diagenetic changes in the pre- and intra-volcanic sandstones. However, where siliciclastic sandstones were overlain by volcaniclastic rocks rather than massive lava flows, the diagenetic changes resemble those of the intra-volcanic sandstones. The cementing phases are typically quartz, illite (probably illitized kaolinite), and rare anatase in the pre-volcanic sandstones. Chlorite, calcite, zeolite/feldspar, opal/quartz, and titanite are characteristic authigenic phases in the intra-volcanic sandstones. Precipitation of different minerals in the pre- and intra-volcanic sandstones show that the detrital composition (and to a lesser extent depositional environment) played a major role during early and late diagenesis after deep burial (up to 6–8 km). Inter-eruptive siliciclastic units may prove to form highly valuable reservoirs when they are composed of mixed siliciclastic and volcaniclastic material. In the stratigraphically youngest intra-volcanic sandstones and pre-volcanic sandstones overlain by hyaloclastite or tuff, there is a high potential for preservation of interparticle porosity during burial (&amp;lt; 5 km) due to early chlorite rims and the generation of secondary porosity after the dissolution of early zeolite cement.

Landscape inversion episodes in SE China during the Mesozoic−early Cenozoic: Constrained by trace-element contents, Nd isotope geochemistry, and detrital zircon U-Pb geochronology of sedimentary basins

The tectonics and landscape of SE China experienced significant changes throughout the Mesozoic and early Cenozoic, largely in response to variations in the slab dynamics of the paleo-Pacific plate, which was subducting beneath continental Asia. We investigated the Mesozoic Yong’an basin in western Fujian Province of SE China in comparison to the sedimentary records of coeval basins in the region to document how its clastic sediment types and their provenance varied through time during the Mesozoic and what regional geologic processes may have controlled these variations. The average εNd value of samples from the Middle Jurassic Zhangping Formation is −16.6, and its detrital zircons are dominated by 1800 Ma and 2000 Ma grains, sourced from the northern Wuyishan Mountains. These mountains underwent significant rock and surface uplift by the Middle Jurassic and became the main source of clastic sediments in SE China. The Lower Cretaceous Bantou Formation contains pyroclastic rocks and represents fluvial-lacustrine deposits with εNd values of −14.8 to −12.4 and abundant 160−120 Ma detrital zircons, sourced from Late Jurassic granitoid rocks, which were widely exposed at the surface in SE China by this time. The upper Lower and lower Upper Cretaceous Shaxian Formation contains coarse-grained and poorly sorted sandstones-conglomerates with volcanic and granitic rock fragments, and it rests unconformably on the Bantou Formation. The Shaxian Formation represents fluvial- to alluvial-fan deposits, and its formation marks the timing of a rapid uplift of the paleo−Coastal Mountains. The Upper Cretaceous Chong’an Formation (&amp;gt;2000 m thick) contains abundant volcanic and granitic rock clasts and represents alluvial-fan and fluvial deposits. The average εNd values of the Shaxian and Chong’an Formations range between −9.3 and −7.5, and their most abundant detrital zircon ages are between 120 Ma and 80 Ma. By the end of the Late Cretaceous, the paleo−Coastal Mountains constituted a nearly 4-km-high magmatic belt, with much of SE China situated in its rain shadow at a lower elevation to the north. Eocene−Oligocene sedimentary basin rocks in Taiwan have an average εNd value of −10.9 and abundant Phanerozoic detrital zircons. The sediment source for these rocks was the paleo−Coastal Mountains. The Miocene basinal strata in Taiwan have more negative εNd values (−13.0) and contain Jurassic−Cretaceous as well as abundant Paleoproterozoic and Neoproterozoic zircons, indicating that the Wuyishan Mountains were again the main sediment source later in the Cenozoic. Denudation rates in the SE margin of South China were high (0.12−0.10 km/yr) during the Cretaceous (140−60 Ma), while they were very low in SW China and in the interior of South China during the same period. These differences confirm the existence of high coastal mountains in SE China until the Late Cretaceous. Denudation rates in eastern South China, particularly the coastal areas, were very low (0.06−0.02 km/yr) during the late Cenozoic (30−0 Ma), whereas they were the fastest (0.14−0.16 km/yr) in the northern Nanling belt and the Yangtze block farther inland to the north, indicating the surface elevation became higher in the western part of South China but lower in its eastern part in the late Cenozoic. This dynamic landscape evolution of SE China through multiple and major shifts throughout the Mesozoic and early Cenozoic was driven by the subducting slab dynamics and the tectonics of the Tibetan Plateau.

Carbon and oxygen isotope microanalysis of calcite in the Permian Kupferschiefer system, Saale subbasin, eastern Germany

The Kupferschiefer district in Central Europe contains some of the world's highest-grade sediment-hosted stratiform Cu (SSC) deposits. In the Saale subbasin (eastern Germany), high-grade sulfides formed via replacement of calcite cement in the continental sandstones of the uppermost Rotliegend (S1), the overlying organic-rich marine mudstones of the Kupferschiefer (T1) and the Zechstein Limestone (Ca1) units. The spatial distribution of the calcite cement, therefore, had a fundamental role to play in the Cu mineralizing system. In this study, we investigate the origin of the calcite cement (and crosscutting calcite veins) using detailed petrography (cathodoluminescence, CL; scanning electron microscopy, SEM), major element chemistry (electron probe microanalyzer, EPMA), and secondary ion mass spectrometry (SIMS) microanalyses of δ13C and δ18O values in drill core samples (n = 47) from the Saale subbasin. The calcite cement in the S1, T1 and Ca1 has a similar CL response and major element chemistry, suggestive of a common origin. Overlapping δ13C and δ18O values in calcite cement in samples from the S1 and T1 in the Sangerhausen and Wallendorf drill cores also suggest that the calcite cement was derived from fluids of similar composition. The low δ13C values of calcite cement in samples from the S1 (−13‰ to 4.3‰, VPDB) and T1 (−10‰ to 0.7‰) indicates carbonate alkalinity was sourced mainly from seawater-derived fluids and the oxidation of organic matter. The wide range of δ18O values in the calcite cement in the S1 (∼18‰ to 31‰, VSMOW) and T1 (∼ 22‰ to 31‰) samples suggest they are derived from pore fluids with a chemical composition influenced by early diagenetic alteration of detrital clasts, mainly dissolution of volcanic rock fragments, with minor contributions from the influx of meteoric waters and evaporated seawater. The negative δ13C values (down to −15‰) in calcite veins from the T1 and Ca1 indicate sources of carbonate alkalinity derived from organic matter degradation. Our data demonstrate that no isotopic hydrothermal alteration haloes can be inferred from the δ13C and δ18O values in calcite cement associated with the high-grade sulfide mineralization. The lack of systematic isotopic variability in the calcite cement likely indicates the mineralizing fluid flux or temperature was not sufficient to overprint the background sources of isotopic variability, which may help to explain the modest size of SSC deposits in this part of the Kupferschiefer district.

Early-Middle Acheulean Occupation of the Northern Transcaucasian Highland

In the northern part of the Transcaucasian Highland (Lori Depression, Armenia), three stratifi ed sites dating to the Early-Middle Acheulean—Karakhach, Kurtan I, and Muradovo—have long been subject to archaeological studies. On the basis of absolute dates and paleomagnetic records relating to the fi rst two sites, their age falls in the interval between the mid-Early and initial Middle Pleistocene. All three sites yielded a uniform industry with a peculiar toolset (various choppers, picks including chisel-ended ones, handaxes, large scrapers, macro-chisels, and macro-knives), manufactured mostly on natural tabular fragments of local volcanic rocks. Certain indicators of this industry, such as subrectangular and fan-shaped choppers, slab-like chisels, etc., are described. Information on 28 other localities with Acheulean artifacts, including 11 stratifi ed ones, recently discovered in various parts of the Lori Depression and in adjacent areas of the Shirak Depression and the Debed River valley, is provided. It is demonstrated that the lithics from all these sites belong to the Karakhach tradition. Data are cited suggesting that three sites (Yagdan, Agvi-canyon, and Agvorik) are over 2 mln years old, and two more (Kurtan II and Dzhradzor) are at least 1.5 mln years old. It is concluded that people associated with the Karakhach Acheulean tradition had appeared in the northern Transcaucasian Highland ~2.0 Ma BP, then settled widely in this area, and remained there for several hundred thousand years. In my view, this may be explained by the very favorable environmental conditions of the region during the Early Pleistocene, and by the abundance of large rock fragments suitable for tool manufacture.

Silicic frothy xenoliths (xeno-pumice) in recent volcanics from Gran Canaria, Canary Islands

The Quaternary small-volume alkaline magmatic episode on Gran Canaria erupted dominantly basanite and nephelinite lavas and scoria deposits that contain a range of mantle and crustal xenoliths. These xenoliths comprise peridotite nodules, partially melted plutonic and volcanic rock fragments, and a group of light colored, felsic, and commonly frothy quartz-bearing rock fragments (xeno-pumice) that show evidence for intense interaction with their host magmas. Here we study a selection of these felsic and, in part, glassy and vesicular xenoliths from North and North-East Gran Canaria, with the aim to unravel their ultimate origin and learn more about magma storage and ascent within and below the island. Inspection of textures, mineral assemblages and glass compositions reveal one group of felsic xenoliths with fresh to partly altered igneous phenocryst assemblages and relict magmatic textures in addition to δ18O values of 3.6 to 6.6‰. This group is interpreted to be of igneous origin. A second group of frothy felsic xenoliths displays mineralogy and textural characteristics more similar to sedimentary rocks with frequent occurrence of quartz, a mineral usually not present as phenocrysts in magmatic rocks from the Canary Islands. This second group displays relatively high δ18O values (8.1 to 16.8‰), more typical for sedimentary lithologies, and is thus interpreted to represent material derived from the extensive pre-island sedimentary part of the ocean crust. The investigated xenoliths from North Gran Canaria thus provide a snapshot of pre-island sedimentary geology as well as the island's “magmatic” interior. These new data help constrain the available subsurface compositional variations within and below the Canary Islands and will hence be useful in interpreting magma evolution trends and magma storage levels.

Geochemistry of the Turonian-Coniacian strata: New insight into paleoenvironmental conditions of the Tethys, Eastern Pontides, NE Türkiye

The eastern part of the Sakarya Zone, known as the Eastern Pontides, is represented by a south-facing carbonate platform during the Late Jurassic-Early Cretaceous. The shallow marine carbonate sedimentation is masked by hemipelagic sedimentation during the Turonian to Coniacian. The Turonian-Coniacian strata are widely exposed in the Gümüşhane area. In this study, we present new microfacies and geochemical data that can provide new insights into the palaeo-oceanic conditions during the time of their deposition.These strata consist of yellow to gray, thick-bedded, graded calcarenites, calcilutite, pelagic limestone, and monogenic conglomerates. The dominant components are carbonate fragments, including dolomites and limestone, as well as allochthonous bioclasts. Volcanic rock fragments, quartz, cherts, and glauconites are also present, with their abundance varying along the section. The micritic component and planktonic fauna exhibit an increasing abundance in the upward direction, indicating a gradual deepening of the depositional environment. Hence, the analyzed samples can be interpreted as transgressive series deposited on slopes or the deep shelf basin.Furthermore, these strata exhibit distinct V/(V+Ni) and Ni/Co ratios without a notable negative Ce anomaly, suggesting relatively oxygen-reduced conditions. They also show a slight enrichment in alkali elements (Rb and Cs) and post-transition elements (Ga), and LREE, indicating intense weathering. The Ga/Rb and K/Al values further support warm and humid Cretaceous conditions. Thus, the Turonian-Coniacian strata offer valuable information about ancient environments, climate conditions, and the basin evolution of the Tethys Ocean in the Eastern Black Sea region.

Role of depositional and diagenetic controls on reservoir quality of complex heterogenous tidal sandstone reservoirs: An example from the Lower Goru formation, Middle Indus Basin, Southwest Pakistan

The Lower Goru Formation, one of the key hydrocarbon development targets in Pakistan's Middle Indus Basin, offers a unique opportunity to investigate the role of depositional and diagenetic controls on reservoir quality in complex stratigraphic traps. An integrated approach, including thin-section petrography, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), helium porosity, and permeability measurements, was used to analyze the Lower Goru Formation in three gas-producing fields. Based on petrographic interpretation, the sandstones are quartzarenite, sublitharenite, subarkose, and less commonly, litharenite. High permeability and high porosity reservoirs are associated with well-sorted and medium-grained sandstones. Main diagenetic processes include compaction, carbonate cementation, authigenic chlorite and quartz overgrowth. Authigenic chlorite, and quartz cementation were the principal reservoir quality controlling factors. Well-developed chlorite coatings preserved porosity and inhibited quartz cementation. Good quality reservoirs (average porosity: 17%; average permeability: 96 mD) are restricted to tidal channels and tidal inlet in the prograding shoreface environments. Transgressive and tidally reworked deposits show moderate porosity and very low permeability (average porosity: 9.5%; average permeability: 0.07 mD). Middle shoreface and bioturbated storm sands have low porosity and very low permeability (average porosity: 8.7%; average permeability: 0.9 mD) and are considered tight reservoirs. The quality of the studied reservoirs depends on quartz overgrowth, chlorite coating, carbonate cementation and minor detrital clay. Secondary porosity is associated with partial to complete dissolution of feldspar, volcanic rock fragments (VRF), biotite, chamosite ooids, and bioclast grains. This integrated work would be helpful for exploration and field development plans in the stratigraphic traps of the Middle Indus Basin and would lead to new exploration targets, i.e., tight and low permeability reservoirs. The study provides insight into the complex interplay between depositional facies and diagenesis in controlling reservoir quality in the Lower Goru Formation and similar depositional conditions and trapping mechanisms elsewhere.

Fluvial‐Aeolian Interaction and Compositional Variability in the River‐Fed Yarlung Tsangpo Dune System (Southern Tibet)

AbstractRiver‐fed dune fields characterized by relatively low quartz content are much more common on Earth than generally assumed, and the aeolian dune field system in the Yarlung Tsangpo (Upper Brahmaputra) river catchment is considered a typical example. As one of the highest‐altitude fluvial‐aeolian systems on Earth, the dune fields in different wide tracts of the Yarlung Tsangpo valley, made of sand entirely blown from the adjacent river bed, provides an excellent natural laboratory in which to investigate the diverse factors controlling textural and compositional changes induced by aeolian processes. To this aim, the textural (grain‐size, roundness) and compositional (petrography, heavy minerals) properties of multiple size classes of fluvial and aeolian sands were quantified. The results show that sediment composition is markedly controlled by grain size, with greater intrasample than intersample variability. Grain‐size controlled compositional differences cannot be exclusively ascribed to hydraulic‐sorting effects. Independent of their density or shape, quartz, sedimentary rock fragments, zircon, apatite, and epidote grains are observed to be on average smaller than feldspars, volcanic rock fragments, and pyroxenes. This indicates that the original size of minerals in source rocks also played a significant role, limiting their availability in finer (e.g., garnet) or coarser (e.g., apatite) classes. In the river‐fed dune system, the lower content of phyllosilicates and higher grain roundness could represent useful criteria to distinguish aeolian from fluvial sand. The efficient aeolian abrasion of river‐fed sediments on the Tibetan Plateau may be an effective mechanism for the production of dust deposited as vast loess covers during the Quaternary.

The main factor influencing reservoir quality of K1yc tight sandstones within volcano-sedimentary succession in the Dehui fault-depression of Songliao Basin, NE China

The main factor influencing high-quality reservoirs of the Lower Cretaceous Yingcheng Formation (K1yc) sandstones in the Dehui Fault-Depression of the Songliao Basin, NE China within volcano-sedimentary succession is still obscure. In the study area, K1yc sandstone reservoirs has exhibited stronger heterogeneity and more complicated diagenetic alteration than terrigenous clastic sandstones. In this paper, K1yc sandstone reservoirs have been characterized through the aspects of petrology, physical property, and pore-types and -throat size. Accordingly, methods including core analysis, thin section, X-ray diffraction, scanning electron microscopy have been used in the study. Finally, the main factor controlling high-quality reservoirs within volcano-sedimentary successions has been concluded. The results reveal that the abundance of authigenic illite clays with forms of fibrous pore-bridging or lath-like pore filling primarily and rapidly reducing the reservoir performance. The chlorite coatings of framework grains should not effectively retard cementation process. Volcanic rock fragments are widespread within the volcano-sedimentary successions and tend to be dissolved by pore fluids, as then illitization should be the most important origin of authigenic illites. It has a critical significance for predicting high-quality reservoirs in the K1yc tight sandstones.

Using a non-invasive multiple-method approach to characterize rock glaciers in the periglacial critical zone: An example from the San Juan Mountains, Colorado

Anthropogenic climate change threatens water storage and supply in the periglacial critical zone. Rock glaciers are widely distributed alpine aquifers with slower response to temperature increases, that provide the summer water flow of many alpine streams. Knowing the extent and makeup of rock glaciers is necessary to evaluate their potential for water supply. We used non-invasive methods, integrating geological, geomorphological, meteorological, and geophysical information to characterize the internal structure and hydrology of the Upper Camp Bird rock glacier (UCBRG) located on level 3 of Camp Bird Mine in Ouray, Colorado, and assessed the applicability of two electromagnetic induction systems in this highly heterogeneous landform with a history of anthropogenic activity. The time-domain (G-TEM™) system achieved deep subsurface penetration (~100 m) and realistic modeling of the internal structure of the UCBRG: a shell of volcanic rock fragments (<3 m thick; 1–100 Ohm-m), a meltwater component (102–103 Ohm-m), located between 50 and 100 m near the toe (subpermafrost flow), and 1–30 m in the soundings farthest from the toe (suprapermafrost flow within the active layer), and a frozen component (permafrost 50–80 m thick; 103–106 Ohm-m). The frequency-domain system, however, was highly susceptible to local environmental conditions, including anthropogenic objects (i.e., mine carts, lamp posts, tunnel tracks, etc.) and was unable to resolve UCBRG's internal makeup. The non-invasive methodology and general conceptual framework presented here can be used to characterize other alpine aquifers, contributing to the quantification of global water resources, and highlighting the importance of preserving rock glaciers as storage for critical water supply in the future.

Arsenic and fluorine in groundwater in northern Mexico: spatial distribution and enrichment factors.

North-central Mexico has groundwater contaminated with arsenic (As) and fluoride (F). Based on the dispersion patterns of these solutes, their sources are linked to felsic volcanic rock fragments and secondary minerals (clays, iron oxyhydroxides) within the alluvium fill of the aquifers. However, little is known about the effect of the enrichment factors for F and As in this area. Natural enrichment factors include evaporation, Ca/Na, and competitive adsorption and desorption from solid phases. This study used 1237 groundwater quality data measurements from 305 sampling sites collected between 2012 and 2019 in the state of Durango in north-central Mexico. To determine the contribution of enrichment factors to As and F content, the study area was divided into four sections, two being in the mountainous part of the state and two in the high plateaus. The data were compared among sections and analyzed using Spearman correlation and Piper and Block diagrams. The results indicate that the main solute enrichment mechanisms are evaporation and weathering of silicates and evaporites. Among the four sections, As, pH, and HCO3 seemed not to vary, F varied slightly, and nitrate and total dissolved solids varied the most. The lack of variation in As among sections is associated to its strong adsorption to clay minerals and iron oxyhydroxides, whereas the diminished F content in the eastern sections is likely linked to the adsorption of F to precipitating calcite (since groundwater is saturated with respect to calcite (SIcalcite = 0.43) and undersaturated for fluorite (SIfluorite = - 1.16). These processes shed light on the distribution of F and As in this area, and are likely operating in other states in northern Mexico and in semi-arid areas elsewhere.

Differential precipitation mechanism of cement and its impact on reservoir quality in tight sandstone: A case study from the Jurassic Shaximiao formation in the central Sichuan Basin, SW China

Permeability heterogeneity of tight sandstone reservoir is mainly determined by the types and quantities of different cements and dissolution processes that occur during the burial. The thin section description, scanning electron microscopy (SEM), x-ray diffraction (XRD), cathodoluminescence imaging (CL), computed tomography (CT) scans, mercury intrusion tests, electron probe analysis, fluid inclusions and isotopic analysis were used to determine genetic mechanism and its impact on reservoir quality of differential cement combinations in Shaximiao Formation tight sandstone in the Central Sichuan Basin, China. Diagenetic minerals in the Shaximiao Formation (J2s) sandstone comprise calcite, laumontite, chlorite and dissolution. The pore space of J2s sandstone mainly comprises primary pores and secondary pores formed after mineral dissolution. The reservoir sandstones can be divided into six types according to different combinations of the laumontite, chlorite and calcite. Authigenic chlorite coating of J2s sandstones occurred via chloritization of smectite during eodiagenesis. Laumontite cements derived from transformation of volcanic rock fragments, were precipitated in a strong alkaline environment with high pH value (9.5–12) and abundant Ca2+, Si4+, Al3+ during eodiagenesis. The δ13C and δ18O isotopic values and petrological characteristics indicate that calcite cement was related to feldspar dissolution and decarboxylation of organic matter. The different cement combinations were determined mainly by the sedimentary facies and subsequent diagenetic environment evolution. Laumontite precipitation needs a stronger alkaline diagenetic environment than that for chlorite and calcite. The early chlorite coating provided sufficient remaining pore-space for pore-filling laumontite and calcite precipitation. The presence of pore-filling laumontite decreased space for calcite precipitation. However, laumontite is more readily soluble than calcite. The different cement combinations have distinct residual pore space combinations meaning that some intervals of rock have sufficient permeability to act as viable reservoir rock whilst other combinations do not.

Temporal variation in sandstone composition of Miocene Jatiluhur Formation in the Bogor Trough, West Java, Indonesia

Bogor Trough in the West Java are typified by turbidity deposits with the source are mostly characterized by volcanoclastic materials from the southern area. The Trough actually receipt the sediment from both volcanoclastic materials from the south and continental source from the north. But, the discussions of sediments in term of composition and temporal variation are rare to be reported, especially the sediments from the north. This manuscript intends to discuss the temporal variation in detrital compositional and depositional facies of the Neogene sediments that delivered from the north (i.e., Sundaland) into the Bogor Trough, which is represented by Miocene Jatiluhur Formation.&#x0D; A total of 36 selected samples have been taken for identifying the minerals using a polarization microscope. Modal analysis of the Gazzi-Dickinson method was applied for this provenance study of sandstones samples, which are consisting largely of quartz and feldspar, then sedimentary rock and volcanic rock fragments, glaucony, mud chips and skeletal fragments.&#x0D; Sundaland, a continental block highland area in the north, is interpreted to have been the provenance of sediments of the Jatiluhur Formation, which is also considered to be the source area for the Paleogene sediments. Granitic igneous rocks are interpreted as the source of dominance of monocrystalline quartz grains, or the product of long-distance transport of polycrystalline quartz from metamorphic rocks&#x0D; But, however late Miocene samples (upper part of formation) represent that the size and amount of glauconite grains are increasing, and texturally mud supported. Volcanic rocks materials are also observed. The upper part of Jatiluhur Formation records the starvation of sediment discharge into the basin, which has been also promoted for development of carbonate reef Klapanunggal Formation in the self-margin setting, and suggesting that the basin have directly received or indirectly some contemporaneous volcanic provenances sediment from the southern area.

Petrogenesis of Havre Volcano in the Kermadec Arc: 2012 Eruption of a Chemically Homogeneous Rhyolite

The 2012 Havre submarine eruption produced a 1.5 km3 bulk rock volume or 0.52 km3 dense rock equivalent volume of rhyolite emplaced as minor lava flows, a field of sunken seafloor pumice, and a volumetrically dominant pumice raft. This moderately large volume of medium-K (1.4–1.6 wt% K2O) rhyolite pumice is relatively chemically homogeneous (71.5–73.0 wt% SiO2), and no trace element variation or cryptic zoning has been detected despite the textural diversity of pumice material. Radiogenic isotope ratios (87Sr/86Sr 0.703693–0.703744; 206Pb/204Pb 18.7648–18.7781; 208Pb/204Pb 38.587–38.605; 143Nd/144Nd 0.513001–0.513020) demonstrate the Havre rhyolite is sourced from mantle similar to regional eruptive products of the Kermadec arc volcanic front. Providing some further insight into the Havre magmatic system is an abundance of diverse volcanic rock fragments primarily embedded in the banded raft pumice. Embedded rock fragments represent a variety of fresh to hydrothermally altered lavas ranging in composition from basaltic to rhyolitic (50.6–72.3 wt% SiO2) and are likely sourced from varying depths within the volcanic conduit during explosive fragmentation. The diverse embedded volcanic rock fragments, therefore, represent earlier erupted lavas that constructed Havre volcano and are snapshots of the petrogenetic history of Havre. Magnesian augite in basaltic to basaltic andesite embedded rock fragments has a similar compositional range (En55Fs12Wo33 to En39Fs26Wo35) to the previously documented antecrystic clinopyroxene observed in the 2012 rhyolite pumice raft. Herein, we explain how this large volume of chemically homogeneous crystal-poor rhyolite can be generated in an oceanic arc setting based on major and trace element petrogenetic models. Rhyolite-MELTS crystal fractionation models indicate the antecrystic mineral compositions within the Havre pumice of plagioclase (An55–78), and magnesian augites (En53Fs10Wo37 to En40Fs26Wo34) are the primary phases that would crystallize in basaltic to andesitic melt compositions. Modeling indicates that the forerunner basaltic magma must be a relatively dry (∼1 wt% H20) low-K tholeiitic basalt in composition and would require ∼78% crystallization at different pressures to ultimately generate the Havre 2012 rhyolite.

Distribution, Sedimentology and Origin of Mineralogical Assemblages from a Continental Na-bentonite Deposit in the Cretaceous Neuquén Basin (Argentina)

Collected samples of bentonite and associated facies from the Justina deposit of Cretaceous age (Anacleto Formation) have been studied. Facies analysis, mineralogical, and geochemical studies have been carried out using several techniques, including: XRD, FTIR, DTA-TGA, microscopy (OM, SEM-EDX), and chemical analysis. The deposit occurs in a shallow, saline lacustrine environment developed over a fluvial floodplain, with a thickness between 0.21 and 0.8 m intercalated between fine-grained siliciclastic facies. Three mineral assemblages were found. In assemblage 1, the bentonite has low content of detrital minerals and the smectite is sodic. In assemblage 2, the bentonite shows the occurrence of minor analcime and mica, slightly higher detrital mineral content and the smectite is sodic to sodic-calcic. The associated detrital facies (assemblage 3) is dominated by illite and a mixed layer of illite and calcic smectite (R0), subordinately kaolinite + chlorite, and locally low-ordered smectite. As inherited minerals are found: quartz, potassium feldspar, plagioclase, illite-mica, heavy minerals (monazite, zircon, apatite, titanomagnetite) and volcanic rock fragments (andesite, glass). Authigenic minerals are: sodium smectite, analcime, barite, celestine, gypsum, and hematite. A model for the formation of authigenic minerals is proposed, highlighting the formation of sodic smectite from the alteration of volcanic glass of trachyandesitic composition.

Tectonic settings of formation of volcanic and sedimentary rocks of the Itmurundy zone, central Kazakhstan

The paper presents new petrographic and geochemical data from volcanic and sedimentary rocks and first U-Pb ages of detrital zircons from sandstones of the Itmurundy zone of central Kazakhstan. The volcanic rocks are aphyric and porphyric basalts, andesibasalt and andesite. The major element composition of tuff and sandstone are close to that of andesite. The poorly sorted greenish grey sandstones carry numerous fragments of volcanic and sedimentary rocks suggesting its greywacke nature which is probably due to. The greywacke probably formed by the destruction of undissected arc. The distribution of U-Pb ages of detrital zircons spanning 505 to 432 Ma has unimodal character peaked at 445 Ma suggesting formation of the sandstones by the destruction and subsequent transportation of clastic material from a late Ordovician intra-oceanic arc. In geochemical diagrams, the tuffs and sandstone plot close to the volcanic rocks. All chondrite-normalized REE spectra show enrichment in LREE (LaN=38–367, La/YbN=4.0–16.9, La/SmN=2.1–3.3) and moderate to weakly differentiated HREE (Gd/YbN=1.4–4.0). However, the level of REE concentrations in the volcanic rocks, in particular, in basalts, is significantly higher than that in the sandstone and andesite. The primitive mantle normalized trace-element diagrams show peaks at Nb (Nb/Lapm=0.9–1.6, Nb/Thpm=0.8–1.6) in most basaltoids, but troughs at Nb for andesite, tuff and sandstone (Nb/Lapm=0.25–0.31, Nb/Thpm=0.17). The previous and new geochronological, petrographic and geochemical data show that the volcanic and sedimentary rocks of the Itmurundy zone formed in Ordovician time in an intra-plate oceanic setting and in a supra-subduction setting at a Pacific-type convergent margin.

Occurrence and origin of chlorite and associated impact on tight sandstone reservoir quality: A case study of the Xujiahe Formation (NE Sichuan Basin, China)

Chlorite is beneficial to the preservation of primary pores, and its formation mechanism in marine sandstone has been widely reported. However, few studies have focused on the genetic mechanism of various types of chlorite in lacustrine deltaic sandstone. Therefore, the occurrence, diagenetic time and origin of chlorite in Xujiahe tight sandstone were investigated using microscopic images and mineralogical chemical analysis, and three stages of chlorite were identified, namely, grain-coating (Ch1), pore-lining (Ch2), and pore-filling (Ch3) chlorite, which belong to the Fe chlorite, Al chlorite and transitional types, respectively. The variations in morphology and chemical composition reflect differences in the sources and formation mechanisms. Chlorite sandstones are mainly distributed in the underwater distributary channel, which has strong hydrodynamic conditions. The formation of Ch1 is controlled by the sedimentary environment and flocculation. Salinity differences resulted in a larger amount of flocculation deposits in the estuaries during the syngenetic period. In the shallow burial stage, the clay coating underwent in situ transformation under thermal impacts and formed the Ch1. Xujiahe sandstones experienced complex diagenetic processes, which include early compaction, three stages of chlorite cementation, two stages of quartz precipitation, dissolution and two stages of carbonate cementation. Organic acid dissolution of volcanic rock fragments, phyllite, and biotite likely supplied sufficient Fe 2+ for the growth of Ch2 in a relatively closed diagenetic system. As the pore water became weakly alkaline, diagenetic fluid migration originating from the clay mineral transformation in mudstone mainly provided Fe 2+ and Mg 2+ for Ch3 growth in sandstones. Chlorite is considered to have “composite” impacts on deep tight sandstones, with grain-coating and pore-lining chlorites protecting the primary pores by restricting the silica supply and occupying potential quartz nucleation sites; however, grain-coating chlorite caused throat obstruction or even closure under strong mechanical compaction, which reduced the permeability of the reservoir. • Three different types of chlorite have been found in the Xujiahe tight sandstones. • It has been clear that the formation time of chlorite and the paragenetic sequence of authigenic minerals. • The origin and formation mechanism of authigenic chlorite in three types have been investigated. • We considerd that authigenic chlorite has a “composite” impact on deep tight sandstone: preserve primary porosity but reduce the permeability.

From the southern Gangdese Yeba arc to the Bangong-Nujiang Ocean: Provenance of the Upper Jurassic-Lower Cretaceous Lagongtang Formation (northern Lhasa, Tibet)

The stratigraphic records of sedimentary basins situated on the Lhasa block provide crucial evidence to reconstruct the early history of the Tibetan Plateau associated with the successive closure of oceanic branches and related magmatic activities. The Upper Jurassic to Lower Cretaceous Lagongtang Formation of the Nagqu Basin located in the northern part of the Lhasa block chiefly consists of fine-grained quartzose to litho-quartzose sandstones, siltstones and shales deposited in continental shelf to bathyal environments. Provenance data, including sandstone petrography, U-Pb ages and in situ Hf isotopes of detrital zircon, together with paleocurrent indicators, suggest that the Lagongtang Formation was mainly fed by recycling of quartz-rich siliciclastic strata of the central-southern Lhasa block. Fresh felsitic volcanic rock fragments, plagioclase prevailing among detrital feldspars, and zircon grains yielding the same age as that indicated by biostratigraphic analysis testify to penecontemporaneous magmatic activity. Volcanic detritus may have been derived from the Middle Triassic-Lower Cretaceous Yeba and/or Sangri volcanics exposed in the southern Lhasa block to the south as well as from Middle Triassic-Lower Cretaceous magmatic rocks of the central Lhasa block. Although the Nagqu Basin is adjacent to the Bangong–Nujiang suture, no debris from Bangong–Nujiang suture and Qiangtang was found in the Lagongtang Formation here. Our provenance data rule out the hypothesis that the Lagongtang Formation could have been deposited in a forearc basin associated with the southward subduction of the Bangong-Nujiang Ocean or in a basin related to the supposed Shiquanhe-Namco Ocean.