Feldspar Content Research Articles

Evaluating the influence of meteorite impact events on global potassium feldspar availability to the atmosphere since 600 Ma

Potassium feldspar present in global mineral aerosol (<5%) plays a disproportionate role in modulating the microphysics of mixed-phase cloud. Via exceptional ice nucleation properties, it is capable of changing cloud properties and behaviour. Here we identify times of substantial and abrupt change in the global availability of potassium feldspar since 600 Ma. Normally, weathering and vegetation cover contribute to low availability, with clay dominating mineral aerosol. Periods of maximum availability are reasoned to follow the emplacement and remobilization of ejecta blankets from major meteorite impact events, before returning to background after some hundreds to thousands of years. We review the 44 largest confirmed craters and evaluate the potassium feldspar content of their target rocks, which range from c. 0 to >30%. By combining crater size and tectonic reconstructions, we are able to provide a quantitative and self-consistent assessment of changes to global potassium feldspar availability. Considerable differences in potassium feldspar availability following meteorite impact events are revealed. Different impact events generated dust containing different amounts of potassium feldspar. Differing levels of influence upon climate are hypothesized, and should now be tested by looking at stratigraphic records of these events to reveal the sensitivity of climate to different dust mineralogy. Supplementary material: Figures showing palaeolatitude determinations using various methods and reconstruction models, and estimates of alkali feldspar abundance and distribution across contemporary continental landmasses for meteorite impact events are available at: https://doi.org/10.6084/m9.figshare.c.4253312

A laboratory investigation of the ice nucleation efficiency of three types of mineral and soil dust

Abstract. Surface-collected dust from three different locations around the world was examined with respect to its ice nucleation activity (INA) with the ETH Portable Ice Nucleation Chamber (PINC). Ice nucleation experiments were conducted with particles of 200 and 400 nm in diameter in the temperature range of 233–243 K in both the deposition nucleation and condensation freezing regimes. Several treatments were performed in order to investigate the effect of mineralogical composition, as well as the presence of biological and proteinaceous, organic and soluble compounds on the INA of mineral and soil dust. The INA of untreated dust particles correlated well with the total feldspar and K-feldspar content, corroborating previously published results. The removal of heat-sensitive proteinaceous and organic components from the particle surface with heat decreased the INA of dusts. However, the decrease in the INA was not proportional to the amount of these organic components, indicating that different proteinaceous and organic species have different ice nucleation activities, and the exact speciation is required in order to determine why dusts respond differently to the heating process. The INA of certain dusts increased after the removal of soluble material from the particle surface, demonstrating the low INA of the soluble compounds and/or the exposition of the underlying active sites. Similar to the proteinaceous organic compounds, soluble compounds seem to have different effects on the INA of surface-collected dusts, and a general conclusion about how the presence of soluble material on the particle surface affects its INA is not possible. The investigation of the heated and washed dusts revealed that mineralogy alone is not able to fully explain the observed INA of surface-collected dusts at the examined temperature and relative humidity conditions. The results showed that it is not possible to predict the INA of surface-collected soil dust based on the presence and amount of certain minerals or any particular class of compounds, such as soluble or proteinaceous/organic compounds. Instead, at temperatures of 238–243 K the INA of the untreated, surface-collected soil dust in the condensation freezing mode can be roughly approximated by one of the existing surrogates for atmospheric mineral dust, such as illite NX. Uncertainties associated with mechanical damage and possible changes to the mineralogy during treatments, as well as with the BET surface area and its immediate impact on the number of active sites (ns,BET), are addressed.

Controlling effect of texture on fracability in lacustrine fine-grained sedimentary rocks

Abstract Microtextures in lacustrine fine-grained sedimentary rocks generally include the basic features of quantity, size, shape and arrangement of particles, which are the internal factors that affect the mechanical properties of rocks. In this paper, based on mechanical experiments and a rock slice image analysis technique, fracability is systematically evaluated in terms of textures by using the Mesozoic and Cenozoic lacustrine fine-grained sedimentary rocks in eastern China. Analysis show that the fracability of fine-grained sedimentary rocks is improved with increasing particle quantity and size but is reduced with increasing particle sorting, roundness, vertical distribution uniformity and holistic distribution uniformity. Regarding composition, fracability is reduced with increases in clay minerals and total organic carbon (TOC) but is first reduced and then improved with increases in the content of feldspar or carbonate minerals, i.e., when the mineral content exceeds 30%, fracability is positively correlated with it, but it is negatively or weakly correlated for mineral contents below 30%. Fracability is improved with enhanced the diagenesis strength. Regarding structural aspect, the fine-grained sedimentary rocks with strong laminability and continuity have poor fracability. A comparative analysis also shows that the stronger control factors on fracability are vitrinite reflectance (Ro), particle quantity, size, sorting, roundness and holistic distribution uniformity, while the components (e.g., TOC, clay minerals, and felsic minerals) and laminar development degree (e.g., laminar continuity and laminar quantity) are relatively weak factors. Diagenesis plays an important role in fracability for the fine-grained sedimentary rocks in different target layers, but for the fine-grained sedimentary rocks in a target layer, the texture factors are the intrinsic factors controlling the fracability and should be considered as primary considerations in engineering fracturing operations.

Characteristics, classification and genesis of vertisols under seasonally contrasted climate in the Lake Chad Basin, Central Africa

General properties of Vertisols are known world-wide, but these properties vary in different areas due to specific environmental conditions. The mastery of the specific characteristics of each Vertisols in a given area requires detailed site-scale investigation. This work aims to study the properties of the Lake Chad Basin Vertisols, attempt to highlight their genetic processes and classification. These Vertisols are dark, clayey and massive, very compact, with deep wide-opened cracks and gilgaï micro-relief. They show neutral to slightly alkaline pH, low organic matter content (0.7–2.73%) and high C/N ratio values (7.34–39.64). Ca2+ and Mg2+ are the most represented exchangeable cations (5.92–30.72% and 1.28–5.44%, respectively) on the absorption complex. Smectite is the main clay mineral species, but small contents of kaolinite, illite, quartz and feldspars are also present. The most concentrated elements in the profile are silica (61.07–77.78 wt.% SiO2), aluminium (7.08–15.54 wt.% Al2O3) and iron (1.78–6.92 wt.% Fe2O3). The Si/Al ratio (>2) and chemical index of alteration (CIA) are moderate to high (56.76–88.39) and indicate a moderate to intense weathering. Such properties reflect the pedogenic influence of the environmental factors like contrasting climate, low topographic position and the alluvial parent material. The accumulation of basic cations favours the neosynthesis of smectite minerals typical of a dominant bisiallitisation process. The Vertisols profiles, due to the presence of a perched water table, enable to classify these soils as Epiaquerts and Endoaquerts Vertisols (USDA soil taxonomy) and Gleyics and Fulvics Vertisols (WRB). Their physical characteristics and notably their difficult water management constitute a limitation to agricultural activities.

Fractal analysis of pores and the pore structure of the Lower Cambrian Niutitang shale in northern Guizhou province: Investigations using NMR, SEM and image analyses

Abstract The pore structure and fractal characteristics of the Lower Cambrian Niutitang formation shale in northern Guizhou were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD) observations and nuclear magnetic resonance (NMR). The relationships between mineral compositions, total organic carbon (TOC), porosity, permeability and NMR fractal dimensions of shale samples were discussed, and the properties of organic pores and organic matter were quantitatively analyzed by pore extraction based on SEM images. The shale pore distribution is irregular and complex, with multifractal and anisotropic characteristics. Fractal characteristics of shale samples were studied by NMR and apply to analyze the lithology and physical properties. Db and Dm represent fractal characteristics of bound fluid and movable fluid pore respectively. The quartz content is the negative correlation with Db and positive correlation with Dm. The clay content is the negative correlation with Db and positive correlation with Dm, and the feldspar content is the positive correlation with Db and negative correlation with Dm, whereas TOC content has no correlation with Db but is the negative correlation with Dm. The relationship between feldspar content and fractal dimension depends on the chemical reaction process in the diagenetic stage. Db has no obvious relationship with porosity and permeability, and Dm has negative correlation with porosity and positive correlation with permeability. The fractal dimension Db represents the roughness and complexity of adsorption pores surface. The fractal dimensions Dm represents the roughness and complexity of seepage pores surface. The fractal analysis provides a novel idea for the study of shale reservoir properties and lithology.

Petrography and geochemistry of lunar meteorites Dhofar 1673, 1983, and 1984

AbstractThe Dhofar 1673, Dhofar 1983, and Dhofar 1984 meteorites are three lunar regolith breccias classified based on their petrography, mineralogy, oxygen isotopes, and bulk chemistry. All three meteorites are dominated by feldspathic lithic clasts; however, impact melt rock clasts and spherules are also found in each meteorite. The bulk chemistry of these samples is similar to other feldspathic highland meteorites with the Al2O3 content only slightly lower than average. Within the lithic clasts, the Mg # of mafic phases versus the anorthite content of feldspars is similar to other highland meteorites and is found to plot intermediate of the ferroan‐anorthositic suite and magnesian suite. The samples lack any KREEPy signature and have only minor indications of a mare basalt component, suggesting that the source region of all three meteorites would have been distal from the Procellarum KREEP Terrane and could have possibly been the Feldspathic Highland Terrane. All three meteorites were found within 500 m of each other in the Dhofar region of Oman. This, together with their similar petrography, stable isotope chemistry, and geochemistry indicates the possibility of a pairing.

The early to mid-Holocene marine tephrostratigraphic record in the Nisyros-Yali-Kos volcanic center, SE Aegean Sea

The Kos-Nisyros-Yali volcanic region is one of the centers in the South Aegean Volcanic Arc to exhibit Holocene volcanic activity, with poorly dated thin tephra layers on Yali associated with Neolithic artifacts. This paper helps to resolve the age and character of early to mid - Holocene volcanism in the South Aegean Sea. It presents the lithostratigraphic record of seven newly recovered cores from this region at the SE termination of the South Aegean Volcanic Arc. The cores were scanned for magnetic susceptibility, split, logged and sampled at one centimeter resolution for carbonate content. Selected intervals were analyzed for grain size, organic carbon content, and feldspar content by XRD, in order to identify lithostratigraphic levels with enhanced volcanic minerals. Subsequently, textural and electron microprobe analyses of volcanic glass shards and pumice fragments were carried out on 20 samples. Chronology was inferred from the upper and lower limits of the S1 sapropel and from five AMS 14C dates on foraminifera. In several cores in the S1 interval, mixed volcanic glass types that compositionally correlate with the Nisyros Lower and Upper Pumice, Yali pumices and KPT suggest that the volcanic glass is reworked from previous deposits and transported by turbidity currents. Cores from deep water northeast and northwest of Yali have a single population of tephra at two or more horizons within S1 that geochemically resembles Holocene Yali–4 pumice on land. These beds likely represent air-fall ash that was dispersed at least 10 km from Yali, with ages of 8 and 10 ka. Moreover we report two or more tephra layers within S1 from the Kalymnos Basin that are not reworked and geochemically resemble Santorini tephra. This study thus confirms the occurrence of small early Holocene eruptions at Yali and provides possible evidence for early Holocene eruptions at Santorini.

High‐Frequency Sequence Stratigraphy and Fine‐Scale Reservoir Characterization of the Devonian Sandstone, Donghe Formation, North Uplift of the Tarim Basin

AbstractThe Devonian Donghe Sandstone complex in North Uplift of the Tarim Basin comprises of a series of diachronous sandy intervals deposited from the Late Devonian to Early Mississippian. They are constrained by a Late Devonian to Early Pennsylvanian 2nd‐order supersequence and can be subdivided into five 3rd‐order sequences, namely, S1, S2, S3, S4, and S5, from the oldest to youngest. Cores from four wells, 40 wireline logs, 410 thin sections, and porosity and permeability data from 639 spots from four wells were used to study the sediment provenance, build up the sequence‐stratigraphic model of S5, and characterize the reservoirs at a feet scale. Detrital modes of sandstone from point counting indicate that Donghe Sandstone is directly sourced from recycled orogeny. The low content of feldspar and volcanic rock fragments suggests that Donghe Sandstone is recycled from sediment with a cratonic ultimate source. 1D and 2D chronostratigraphic correlation shows that at least 12 4th‐order high‐frequency sequences (HFSs), from the oldest HFS1 to the youngest HFS12, can be recognized in S5. Each HFS is characterized by a general trend of shallowing‐upward facies assemblage. Sequence boundaries were defined at where regionally correlatable deep‐water facies overlaying shallow‐water facies. There is a general shallowing‐upward trend in the S5 3rd‐order sequence, characterized by a systematically increasing proportion of shallow‐water facies (foreshore and upper shoreface), and a decreasing proportion of deep facies (offshore transition and lower shoreface). The shallowing‐upward trend within both 3rd‐ and 4th‐order sequences is resulted from a combined effect of eustatic sea‐level change, tectonic activity, and sediment supply. The sequence‐stratigraphic model of Donghe Sandstone S5 is similar to the rift‐basin sequence‐stratigraphic model. Sweet spots were defined as porosity >15% and permeability >100md intervals, and their distribution and lateral continuity were investigated. HFS is one of the primary controls on the distribution of sweet spots distribution and can be used to guide hydrocarbon exploration.

Feldspar production from dimension stone tailings for application in the ceramic industry

Brazil has one of the largest reserves of granite in the world and is in part of the top ten producers of dimension stone. The dimension granite extensive mining in this country generates expressive volumes of tailings, among then rock fragments and fine particles, accounting for more than 50% of the material extracted from the quarries. The dimension stone, commercially known as Marrom-Guaíba granite, is an alkali-feldspar quartz syenite rich in potassium and sodium exploited by Serra Geral Mining in the southern part of Brazil. Due to its chemical composition, Marrom-Guaíba tailings have potential application in the ceramic industry. However, the grade of iron bearing minerals (considered a contaminant) is too high. Tailings samples were grinded, milled, chemical analyzed and tested using magnetic separation and froth flotation in order to reduce the iron and quartz content. The results show that the magnetic separation reduced the iron content from 3.20 to 0.48% and increased the feldspar content from 72.20 to 81.23% with 65.31% of mass recovery and 73.48% of metallurgical recovery. Flotation provided small decreased in quartz content, from 4.40 to 3.90%. A suitable feldspar concentrate for ceramic industry was obtained.

Mineralogy and facies variations of Devonian and Carboniferous shales in the Ukrainian Dniepr-Donets Basin

Abstract The mineralogy of Devonian to Carboniferous shales from the Ukrainian Dniepr-Donets Basin (DDB) was investigated during this study. These shales show a high compositional variability in vertical and lateral directions. Furthermore, stratigraphic trends were found to be controlled both by climatic factors as well as by changing detrital input from the hinterland. High kaolinite contents and predominance of kaolinite over illite in the Tournaisian and partly in the lower Visean units are likely a result of intense chemical weathering related to the Hangenberg climatic event at the Devonian/ Tournaisian boundary. In contrast, abnormally high kaolinite contents in upper Visean and Serpukhovian samples at the basin center might be caused by different transport properties of kaolinite and illite, leading to selective concentration of small detrital kaolinite particles, which are often in the sub-micrometer range according to scanning electron microscopy observations. K/Al elemental ratios correlate well with illite/kaolinite ratios for samples in which significant amounts of both clay minerals are present, which enables a pre-evaluation of the relative kaolinite content based on bulk geochemical data. As kaolinite is suggested to decrease the fraccability of shales and to have a great influence on their wetting behaviour, this is useful information for explorational purposes. Higher feldspar contents in Devonian and Tournaisian samples, especially along the NE basin margin and in the shallow NW part of the DDB, are likely related to increased detrital input from magmatic precursors (e.g. in the Voronezh Massif ) during (and shortly after) the active rift stage of the DDB. In general, feldspar contents are higher in proximal positions compared to the basin center, which is likely a result of shorter transport distances of the comparably large feldspar grains. Finally, the presence of expandable clay minerals down to depths of 6 km and the fact that no thermal maturity trend is visible down to these depths, proves, that a low post-depositional heat flow was present in the DDB. This is in good agreement with vitrinite reflectance measurements and thermal modelling results from previous studies, which suggest a low Mesozoic heat flow.

Geochemistry and distribution of sediments in the East Indian shelf, SW Bay of Bengal: implications on weathering, transport and depositional environment

Forty-two substrate sediment samples and three cores procured from the shelf region between Chennai and Cuddalore were analyzed to understand the spatial and vertical sediment distribution. Samples subjected to grain size, bulk and clay mineralogical analyses, REE and trace element geochemistry yielded interesting results about the sediment characteristics with respect to the modern day environment. The present study revealed that the study area is characterized by high energy environment marked by predominant composition of medium sand. Results confirmed the evidence of more illite than kaolinite, smectite, and chlorite in the clay mineral assemblage indicating a terrigenous source. Geochemical data also revealed that the enrichment of light rare earth elements (LREE) in the sediments is due to continental source of Precambrian times. High feldspar content in the sediments which is emphasized by bulk mineralogical data displayed positive Eu anomaly. By using the paleoredox index, the area of interest is considered to have undergone no major changes in its depositional settings.

Petrography of Middle Jurassic to Early Cretaceous sandstones in the Kutch Basin, western India: Implications on provenance and basin evolution

This paper investigates the provenance of Middle Jurassic to Early Cretaceous sediments in the Kutch Basin, western India, on the basis of mineralogical investigations of sandstones composition (Quartz–Feldspar–Lithic (QFL) fragment), Zircon–Tourmaline–Rutile (ZTR) index, and mineral chemistry of heavy detrital minerals of the framework. The study also examines the compositional variation of the sandstone in relation to the evolution of the Kutch Basin, which originated as a rift basin during the Late Triassic and evolved into a passive margin basin by the end Cretaceous. This study analyzes sandstone samples of Jhumara, Jhuran and Bhuj Formations of Middle Jurassic, Upper Jurassic and Lower Cretaceous, respectively, in the Kutch Mainland. Sandstones record a compositional evolution from arkosic to subarkosic as the feldspar content decreases from 68% in the Jhumara Formation to 27% in the Bhuj Formation with intermediate values in the Jhuran Formation. The QFL modal composition indicates basement uplifted and transitional continental settings at source. Heavy mineral content of these sandstones reveals the occurrence of zircon, tourmaline, rutile, garnet, apatite, monazite and opaque minerals. Sub-rounded to well-rounded zircon grains indicate a polycyclic origin. ZTR indices for samples in Jhumara, Jhuran and Bhuj Formations are 25%, 30% and 50% respectively. Chemistry of opaque minerals reveals the occurrence of detrital varieties such as ilmenite, rutile, hematite/magnetite and pyrite, in a decreasing order of abundances. Chemistry of ilmenites in the Jhumara Formation reveals its derivation from dual felsic igneous and metabasic source, while those in Jhuran and Bhuj Formations indicate a metabasic derivation. Chemistry of garnet reveals predominantly Fe-rich (almandine) variety of metabasic origin. X-ray microscopic study provides the percentage of heavy minerals ranging from 3% to 5.26%. QFL detrital modes reflect the evolution of the basin from an active rift to a passive margin basin during the Mesozoic. Integration of results from QFL modal composition of the sandstones, heavy mineral analysis and mineral chemistry, suggests sediment supply from both northern and eastern highlands during the Middle Jurassic. The uplift along the Kutch Mainland Fault in the Early Cretaceous results in curtailment of sediment input from north.

Microscopic Failure Mechanism Analysis of Sandstone Under Triaxial Compression

Mineral composition and microstructure changes under loading greatly influence the macroscopic mechanical strength of rocks. In this study, the chemical and mineral composition of sandstone from a coal mine in China were analyzed with energy dispersive spectrometry and X-ray diffraction. Triaxial compression tests of three sandstone samples with different compositions were carried out by using a servo-controlled rock mechanics testing system to obtain macroscopic failure characteristics and complete stress–strain curves. Fracture morphologies of the sandstone samples were observed by scanning electron microscopy (SEM), and the fracture origins, defects, grain size, pore structure and distribution, microfracture characteristics and grain boundaries were determined. Finally, the fracture mechanism of the rocks was analyzed, and the microstructure and fracture characteristics of three samples were obtained. The results showed that the stress–strain curve of samples under triaxial compression can be divided into four stages: microfractures compressed-closed, elastic deformation, plastic-failure and post-failure. Under triaxial compression, shear failure occurred in the samples, and the slip occurred along the plane of maximum shear stress. The SEM results showed grooves and steps on the fracture surface, where the debris was piled up in angular and granular shapes. Higher quartz and feldspar content is positively correlated with sample strength. There were some microcracks in the fracture surface of the clastic sandstone with higher hydrophilic clay minerals, which indicates the fracture was easily extended during the shear process. Thus, tensile fractures are more likely to be generated along the bedding plane, resulting in the low strength of clastic sandstone.

Identifikasi Mineral Lokal Batu Gayo, Aceh dan pemanfaatannya Sebagai massa raga keramik Stoneware

Indonesia is known to have abundant mineral resources and some of them have not been properly utilized. Frozen rock from Gayo area, Aceh is suspected to have a feldspar content that can be used as a raw material for the manufacture of ceramics. This study is intended to determine the content of Gayo rocks and their compatibility as ceramic materials. For that purpose, has been identified the existing mineral content in these rocks and then made the composition of stoneware ceramic raw materials with triaxial blend  method. different sintering temperature (800, 900, 1000, 1100 and 1250 oC respectively) has been conducted to known the efffect of sintering temperature on ceramic body. From the experiment, it is known that Gayo stone contains 69%of feldspar with specific gravity is 2.6963 + 0.1290 gram / cm3. In the sample test, the results obtained at the sintering temperature of 1250oC achieved 1.09% water absorption, a number which is included in the water impregnation area for stoneware ceramics according to the provisions of ISO 10543-3 / ASTM C373 ie <3%, .The prototype of ceramic objects made with rotary technique showing good quality (not cracked, shapedo not change, solid ceramic body

Identifikasi Mineral Lokal Batu Gayo, Aceh dan pemanfaatannya Sebagai massa raga keramik Stoneware

Indonesia is known to have abundant mineral resources and some of them have not been properly utilized. Frozen rock from Gayo area, Aceh is suspected to have a feldspar content that can be used as a raw material for the manufacture of ceramics. This study is intended to determine the content of Gayo rocks and their compatibility as ceramic materials. For that purpose, has been identified the existing mineral content in these rocks and then made the composition of stoneware ceramic raw materials with triaxial blend method. different sintering temperature (800, 900, 1000, 1100 and 1250 oC respectively) has been conducted to known the efffect of sintering temperature on ceramic body. From the experiment, it is known that Gayo stone contains 69%of feldspar with specific gravity is 2.6963 + 0.1290 gram / cm3. In the sample test, the results obtained at the sintering temperature of 1250oC achieved 1.09% water absorption, a number which is included in the water impregnation area for stoneware ceramics according to the provisions of ISO 10543-3 / ASTM C373 ie <3%, .The prototype of ceramic objects made with rotary technique showing good quality (not cracked, shapedo not change, solid ceramic body

Fluid-rock interaction and dissolution of feldspar in the Upper Triassic Xujiahe tight sandstone, western Sichuan Basin, China

Abstract Secondary porosity in the Upper Triassic Xujiahe tight sandstone of the western Sichuan Basin is mainly the product of feldspar dissolution. In the Xu-4 Member, the upper reservoir of the Xujiahe Formation, feldspars are dissolved to a significant extent and observations indicate that nearly all feldspars have been dissolved completely, with only 1.73% content of feldspar remaining. In the Xu-2 Member, the lower reservoir, feldspars are well preserved; the current content of feldspar is 12.54% on average, and the secondary porosity derived from feldspar dissolution is less than 1%. Kaolinite occurs almost exclusively in the Xu-4, but it is nearly absent in the Xu-2. The K+ content in the Xu-2 is 3.3 times higher than that in Xu-4. The K+/H+ ratio in the Xu-2 is also higher than that in the Xu-4. These differences between the two reservoirs can be attributed to their distinguishing fluid-rock systems. The low K+ content and relatively high δ18O in the Xu-4 formation water are the result of intensive fluid-rock interaction in an open fluid-rock system. The upper Xu-4 is close to the overlying coal-measures of the Xu-5 from which organic acid flowed into the Xu-4. Meanwhile, K+ contained in sandstone migrated out to the mudstones. The resulting low K+/H+ ratio in the formation water of the Xu-4 was responsible for almost all the feldspar dissolution and kaolinite formation. In contrast, due to the relatively closed fluid-rock system in the Xu-2, K+ did not migrate into adjacent rocks and acidic fluids did not invade, which led to K+-rich formation waters maintaining a high K+/H+ ratio. Hence, K-feldspar was well preserved and kaolinite was completely transformed into illite. Therefore, in contrast to the Xu-2 tight sandstone, the Xu-4 sandstone has relatively higher secondary porosity, which favours the formation of better quality reservoirs.

The Upper Oligocene of Montgat (Catalan Coastal Ranges, Spain) : new age constrains to the western Mediterranean Basin opening.

The Oligocene deposits of Montgat are integrated in a small outcrop made up of Cenozoic and Mesozoic rocks located in the Garraf-Montnegre horst, close to the major Barcelona fault. The Oligocene of Montgat consists of detrital sediments of continental origin mainly deposited in alluvial fan environments; these deposits are folded and affected by thrusts and strike-slip faults. They can be divided in two lithostratigraphic units separated by a minor southwest-directed thrust: (i) the Turo de Montgat Unit composed of litharenites and lithorudites with high contents of quartz, feldspar, plutonic and limestone rock fragments; and (ii) the Pla de la Concordia Unit composed of calcilitharenites and calcilithorudites with high contents of dolosparite and dolomicrite rock fragments. The petrological composition of both units indicates that sediments were derived from the erosion of Triassic (Buntsandstein, Muschelkalk and Keuper facies), Jurassic and Lower Cretaceous rocks (Barremian to Aptian in age). Stratigraphic and petrological data suggest that these units correspond to two coalescent alluvial fans with a source area located northwestwards in the adjoining Collserola and Montnegre inner areas. Micromammal fossils (Archaeomys sp.) found in a mudstone layer of the Pla de la Concordia Unit assign a Chattian age (Late Oligocene) to the studied materials. Thus, the Montgat deposits are the youngest dated deposits affected by the contractional deformation that led to the development of the Catalan Intraplate Chain. Taking into account that the oldest syn-rift deposits in the Catalan Coastal Ranges are Aquitanian in age, this allows to precise that the change from a compressive to an extensional regime in this area took place during latest Oligocene-earliest Aquitanian times. This age indicates that the onset of crustal extension related to the opening of the western Mediterranean Basin started in southern France during latest Eocene-early Oligocene and propagated southwestward, affecting the Catalan Coastal Ranges and the northeastern part of the Valencia trough during the latest Chattian-earliest Aquitanian times.

Diagenetic controls on reservoir quality of tight sandstone: A case study of the Upper Triassic Yanchang formation Chang 7 sandstones, Ordos Basin, China

Through a range of petrological techniques, the petrology, diagenesis, pore characteristics, and controlling factors on the regional variations of reservoir quality of the Chang 7 sandstones were studied. These sandstones, mainly arkoses, lithic arkoses, and feldspathic litharenites, were deposited in a delta front and turbidites in semi-deep to deep lacustrine. The detrital constituents were controlled by the provenance and sedimentary condition, which resulted in a spatially variable composition; e.g., high biotite and feldspar contents in the northeast (NE) of the study area, and high contents of rock fragments, especially dolomite, matrix, and quartz in the southwest (SW). Diagenesis includes intense mechanical compaction, cementation, and dissolution of unstable minerals. Diagenetic minerals which were derived internally include quartz, ankerite, ferrous calcite, albite, illite, kaolinite, and chlorite. Thus the original sandstone composition hadfirm control over the development and distribution of cement. Mechanical compaction and late-stage cementations contribute to the porosity loss of sandstones of Chang7 member. The dissolution porosity in major sandstone, slightly higher than primary porosity is principally dependent on the accessibility of acid fluid. The high content of plastic component facilitated the reduction of primary porosity and limited the mineral dissolution. The best reservoir sandstones are found in W, and partly from NE, M districts, with porosity are primary. The relatively high textural maturity of these sandstones reduces the impact of compaction on primary pores, and commonly existed chlorite rims limited the precipitation of pore filling quartz and carbonate cementation in late stage.

The dissolution characteristics of the Chang 8 tight reservoir and its quantitative influence on porosity in the Jiyuan area, Ordos Basin, China

The Chang 8 tight reservoir in the area of Jiyuan holds characteristics such as high feldspar content and common feldspar dissolution phenomenon, which are vital to improve the physical properties of tight reservoirs. This paper simulates the interaction between the Chang 8 tight reservoir rock samples and organic acid, analyzes the types and characteristics of dissolution, explains the dissolution mechanism, and quantitatively calculates the influence of dissolution on the porosity of the reservoir on the basis of identifying the fundamental characteristics of the Chang 8 tight reservoir by utilizing the simulation experiment of fluid–rock interaction with high temperature and high pressure combined with polarized microscope and scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis techniques. With the purpose of making the simulation experiment closer to the actual geological conditions, 0.15 mol/L acetic acid solution with pH = 2.65 is used as the reaction fluid, wherein the simulated temperature range is set to 87–103 °C, and the simulated pressure range is set to 24.70–30.18 MPa. The result of the research shows that intergranular pores and feldspar dissolved pores are the primary pore types. The feldspar dissolved pores are more developed and accounts for 39% of the total porosity. The apparent solution ratio is 37.8%–50.0% with a moderate degree of dissolution. Additionally, carbonate minerals can also be dissolved in an acidic condition. The solution ratio of carbonate minerals generally exceeds that of feldspar. The dissolution degree is most robust at about 95 °C, and the burial depth is about 2370–2710 m by the geothermal gradient calculation, which should be the primary distribution area of the favorable tight reservoir of the Chang 8. Overall, dissolution is the main diagenesis type to improve the porosity that can result in the porosity increasing about 3.57%–3.69% in the tight reservoirs of Chang 8. Therefore, it is also the main controlling factor for the development of the "dessert" in the Chang 8 tight reservoir of western Jiyuan area.

An improved method of characterizing the pore structure in tight oil reservoirs: Integrated NMR and constant-rate-controlled porosimetry data

Tight oil reservoirs have an ultrafine pore structure with a large heterogeneity, which requires the integration of multiple techniques to characterize pore structure characteristics systematically and, therefore, to gain a better understanding of reservoir quality. Tight oil sandstone samples from the Upper Triassic Yanchang Formation in the Ordos basin were measured by nuclear magnetic resonance (NMR), constant-rate-controlled mercury porosimetry (CMP) and field emission scanning electron microscopy (FE-SEM) to investigate the pore structure characteristics and their control on petrophysical properties. Various types of pores from intergranular pores, dissolution pores, and intercrystalline pores to small microfractures occurred in the studied tight sandstones, which indicates that the NMR T2 distributions may be unimodal or bimodal in behaviour with a wide range. The short components of the T2 distributions match the CMP-derived throat size distributions regarding the dimensions, width and shape of the curves. We combined CMP and NMR to uncover the total pore size distribution; the size distributions of the throat and pore body range from 0.01 to 1 μm and from 0.1 μm to tens of microns respectively, which match the FE-SEM results. The pore body-to-throat ratio may be the important factor affecting the free water saturation in the pore body, which, in turn, is probably associated with the mineralogy of tight sandstones, in particular, the quartz, feldspar and clay mineral contents. The pore radius of 0.3–0.5 μm is the percolation threshold. The seepage porosity rather than total porosity overwhelmingly dominates the permeability, which, in turn, relates to the pore types, i.e., mesopores and macropores. An improved permeability prediction model is presented for tight oil reservoirs based on the contribution from different pore size intervals to the permeability. This model has a distinctly better performance than the classic models such as the Timur-Coates and SDR models and may be generalized to other similar reservoirs with ultrafine pore structures and significant volumes of clay minerals.