Detrital Quartz Research Articles

Experimentally determined Si isotope fractionation between zircon and quartz

Abstract The silicon isotope composition of detrital quartz and zircon have the potential to inform us about secular changes to the silica cycle and weathering reactions on Earth. However, inferring source melt Si isotope composition from out-of-context minerals is hampered by the fact that, to-date, there is limited Si isotope equilibrium fractionation data for minerals. Here, we report experimental data to constrain Si isotope equilibrium fractionation between zircon and quartz, using two fundamentally different strategies, but with the same experimental design. First, zircon and quartz were hydrothermally synthesized from Zr(OH)4 and SiO2 at 1.5 GPa and temperatures of 725, 800, and 900 °C. The second experimental strategy utilized the three-isotope method; the starting materials consisted of natural zircon and isotopically-labelled SiO2. Three sets of hydrothermal time-series experiments were conducted at the same pressure and temperatures as the direct synthesis experiments. For all experiments, quartz and zircon were separated and 30Si/28Si and 29Si/28Si ratios were measured by solution multi-collector inductively coupled plasma mass spectrometry. The three-isotope method, which provides the best indicator of equilibrium fractionations, yields the following relationship: Δ 30 S i ( q t z - z r c ) = ( 0.53 ± 0.14 ) × 10 6 / T 2 where Δ30Si(qtz-zrc) is the relative difference in 30Si/28Si between quartz and zircon in permil, T is temperature in K, and the error is 2 s.e. This relationship can be used to calculate the fractionation between zircon and other phases, and to estimate the Si isotope composition of the melt from which a zircon crystallized. The results may be used to assess equilibrium-disequilibrium isotope fractionations between quartz and zircon and co-existing phases in igneous rocks. These data can also be applied to out-of-context zircon (and quartz) to estimate the isotope composition of the host rock. Zircons crystallizing from a melt derived from purely igneous sources – i.e., without the involvement of “weathered” material – are expected to display a δ30SiNBS-28 (permil deviation of the 30Si/28Si from the NBS-28 standard) range from −0.7 to −0.35‰. Deviations from this range indicate assimilation of non-igneous (i.e., sedimentary) material in the melt source.

Diagenesis and reservoir quality of sandstones in the Eocene Shahejie Formation, Raoyang Sag (Jizhong Depression), Bohai Bay Basin, China

Deeply buried (3.5–4.5 km) fan deltaic and braided deltaic sandstones of the Eocene Shahejie Formation are important hydrocarbon reservoirs in the Raoyang Sag of the Bohai Bay Basin (North China). An integrated approach incorporating petrophysics, thin‐section petrography, scanning electron microscope (SEM), cathodoluminescence (CL), and well log analysis was applied to investigate diagenesis, diagenetic minerals, and their impacts on reservoir quality. The sandstones of the Sha‐3 (the third Member of the Shahejie Formation) are dominated by lithic arkoses and arkoses with a low‐moderate compositional maturity and a moderate textural maturity. The pore systems are dominated by secondary pores resulting from dissolution of framework grains, micropores among authigenic clays, and minor amounts of primary intergranular pores. Chemically unstable framework grains such as feldspar and lithic fragments experienced alteration and resulted in formation of authigenic kaolinite. Dissolution and alteration of feldspars and rock fragments and pressure solution of detrital quartz grains were the main sources for quartz cements. Mechanical compaction and cementation by calcite, dolomite, quartz, kaolinite, and mixed‐layer illite/smectite destroyed primary pores, whereas dissolution of framework grains generated secondary pores. Five types of diagenetic facies are identified based on framework mineralogy, texture, diagenetic minerals, and pore systems. These comprise the following: (a) quartz‐cemented sandstone, (b) tightly compacted sandstone, (c) carbonate‐cemented sandstone, (d) clay‐mineral‐cemented sandstone, and (e) clean dissolved sandstone. Diagenetic processes and evolution sequences for the five diagenetic facies were reconstructed based on textural relationships from thin section, SEM, and CL studies. The reservoir quality evolution of various diagenetic facies is predicted by considering variations in grain size, sorting, shape, and matrix content of sandstones. This work, which investigates the diagenetic sequence of sandstones, will provide insights into reservoir quality prediction for low‐permeability sandstones with similar tectono‐depositional settings.

OH in detrital quartz grains as tool for provenance analysis: Case studies on various settings from Cambrian to Recent

Detrital quartz grains from Paleozoic and Mesozoic sandstones from North Africa and central Europe, respectively, and from recent siliciclastic sediments of the Elbe River from Germany were analysed by IR spectroscopy with respect to their OH defect content. Sample sets were carefully chosen to cover different stratigraphic units from different localities and according to previous findings that indicate a significant change in the source region in the respective sedimentary system. The validity of the new method is compared to heavy mineral and zircon age spectra analysis from previous studies. Results reveal that the OH defect inventory in quartz shows in all investigated sedimentary successions significant internal variations from sample to sample and thus may be used as a tool to identify changes in the source region. The degree of changes observed with the new method does not necessarily reflect the magnitude in differences observed by other methods (such as heavy minerals and/or zircon age spectra), underlining the potential as complementary tool for provenance analysis. The new tool is also tested to estimate mixing proportions between the Variscan and the Scandinavian signal in the Elbe River, resulting in a surprisingly high contribution of the Nordic source.

Quartz and zircon decoupling in sandstone: Petrography and quartz cathodoluminescence of the Early Triassic continental Buntsandstein Group in Germany

AbstractThis study illustrates how decoupling of quartz and zircon can be used advantageously in provenance research. Thirty‐eight fine‐grained to coarse‐grained arkose samples of the Early Triassic intracontinental Buntsandstein Group from the Central European Basin in Germany were analysed for their petrography and 1200 grains in 23 of these for their detrital quartz cathodoluminescence characteristics. The samples represent the Hessian and Thuringian sub‐basins and the Eichsfeld–Altmark Swell separating them. The Hessian Sub‐basin includes more metamorphic lithoclasts with a larger content of plutonic grains than are found further east in the Thuringian Sub‐basin. More than 90% of the detrital quartz from the eastern Thuringian Sub‐basin produce medium to bright blue cathodoluminescence colours and corresponding spectra that are typical for igneous or high‐temperature metamorphic origin. Differently, the quartz from the Hessian Sub‐basin mostly luminesces brown and dark to medium blue, typical for a low‐temperature metamorphic origin. Quartz from the Eichsfeld–Altmark Swell and the western Thuringian Sub‐basin is a mixture between these origins. The quartz indicates different catchments for the sub‐basins, possibly the Bohemian Massif and the Massif Central, with converging transport routes on and close to the eastern fringe of the swell. Taking published zircon data from the same samples into account, light mineral‐zircon grain‐size shifts are up to 2 Φ units. That can be explained by mineral decoupling due to different transport modes for quartz and zircon and different zircon‐size availability in the source areas, exaggerated by combined aqueous–aeolian transport, as well as sample preparation‐induced sorting. This study concludes that submerged highs significantly can influence continental sediment transport. Hence, vast, flat continental areas with submerged morphological highs and a seemingly straightforward transportation pattern may be more complex than expected. The results also illustrate that analysis of detritus that has been affected by different dominating transport modes, and further sorting during sampling and preparation can reveal additional source information.

OH defects in quartz as a provenance tool: Application to fluvial and deep marine sediments from SW Japan

Hydrogen-related point defects (OH defects) in the crystal lattice of detrital, monocrystalline quartz have been proposed as an indicator to distinguish different sediment sources, making it a potential tool for provenance analysis. This study tests this novel technique on samples from SW Japan by comparing the results of OH defect analyses with those of “classical” provenance studies. OH defects of 188 detrital, monocrystalline quartz grains from five rivers (Fuji, Tenryu, Nagara, Yoshino, and Shimanto) and four offshore wells drilled in the Nankai Trough area (ODP site 1177; IODP sites C0002, C0007, C0012) are characterized using Fourier-transform infrared (FTIR) spectroscopy. Considerable differences in OH defect concentrations are identified between individual river samples. A general increase of values from east to west is observed. Possible links to the respective source geology are established. Very low OH defect contents (<1 wt. ppm water) of sediment from the Fuji and Tenryu rivers may be a result of high deformation and exhumation in the Izu-Honshu collision zone. Moderate to high OH defect contents of Nagara and Yoshino river samples (up to 50 wt. ppm water) are similar to the values of the global average in continental crust, possibly due to a mixing of various source lithologies. High and very diverse OH defect contents of Shimanto River sediments (up to 211 wt. ppm water) are likely an effect of extensive sediment reworking and mixing in the Nankai accretionary wedge. The OH defect signal is shown to be traceable through the offshore sedimentary record. In the western Shikoku Basin (ODP site 1177), turbidite deposits at 15 Ma and 7 Ma show a very similar signal as recent Shimanto river sediment, whereas eastern Shikoku Basin sediments (IODP Site C0012), at 15 Ma and 6 Ma strongly resemble present day Nagara and Yoshino river sediments. Sediments deposited in the Nankai trench wedge at 1 Ma (IODP Site C0007) indicate a shift towards lower OH defect concentrations. This may be due to increased, trench parallel sediment flux from the low-OH defect Fuji and Tenryu river sources, as proposed by previous studies. This study presents the first OH defect dataset in detrital quartz from rivers and offshore drill sites in a subduction zone environment, introducing OH defects as a new useful tool for sediment provenance analysis in active convergent margin settings.

Quartz types and origins in the paleozoic Wufeng-Longmaxi Formations, Eastern Sichuan Basin, China: Implications for porosity preservation in shale reservoirs

Abstract Quartz in shales has been demonstrated to exert a significant effect on shale gas production by influencing rock mechanical behavior and reservoir porosity; however, quartz types and origins in shales are complicated and challenging to study due to their fine-grained nature. In this study, we propose combining thin section images, scanning electron microscopy (SEM) images, energy dispersive spectroscopy (EDS) maps, cathodoluminescence (CL) images and geochemical data to investigate quartz types and origins in shale formations. Shale samples from the Ordovician Wufeng-Silurian Longmaxi Formations, Eastern Sichuan Basin, China were used to study quartz types, origins and the effect of quartz cementation on porosity development. Samples from the Wufeng-Longmaxi Formations have TOC contents in the range of 0.5–4.7 wt%, averaging 2.7 wt%, and are currently in the thermally overmature stage. Five types of quartz, including silt-size detrital quartz, siliceous skeletal fragments, quartz overgrowth, clay matrix-dispersed microquartz, and aggregates of euhedral quartz, were identified. The ternary diagram of Fe–Al–Mn, the cross-plot of SiO2 versus Zr, and the cross-plot of Si versus Al suggest that the dissolution and reprecipitation of siliceous skeletons may provide the major source of silica for quartz cementation in the lower part of the Wufeng-Longmaxi Formations, and released silica as a result of smectite to illite transformation is also important for authigenic quartz formation in the upper part of the Wufeng-Longmaxi Formations. Our SEM images suggest that the rigid framework formed by quartz cementation not only prevents primary interparticle pores from compaction but also favors the generation of organic matter-associated pores.

Sedimentary and diagenetic controls on reservoir quality of low-porosity and low-permeability sandstone reservoirs in Chang101, upper triassic Yanchang Formation in the Shanbei area, Ordos Basin, China

There are many reservoirs with low porosity and low permeability in the world, and many large oilfields have been found in these reservoirs. Unlike conventional reservoirs, the genesis of low-porosity and low-permeability reservoirs and the controls of relatively high-porosity and high-permeability zones are still unclear. This study takes the Chang101 reservoir sandstones of the Shanbei area in the Ordos Basin, China, a typical low-porosity and low-permeability, as an example. Chang101 sandstones are fine-grained to medium-grained, moderately to well sorted arkose and lithic arkose deposited in a delta plain environment. It is a typical low porosity and low permeability reservoir with an average porosity of 10.21% and an average permeability of 2.88 mD. Pore types mainly include primary intergranular pores and secondary dissolved pores. The main controls on the reservoirs are the sedimentary environment and diagenesis. The sedimentary environment determines the initial properties of the reservoir sandstones, including the material composition and texture that determine the primary porosity and permeability. Diagenesis further transforms the reservoir quality to develop heterogeneity in the porosity and permeability. The reservoir sandstones have undergone complex diagenetic processes and are currently in stage A of mesodiagenesis. Compaction accounts for the major reduction of the primary porosity of the sandstones. Different types of cementation have both positive and negative influences on reservoir quality. Carbonate cementation, quartz overgrowths and some authigenic clay cementation(i.e., I/S and illite) are major pore-filling components, that reduce the reservoir quality. An appropriate amount of chlorite resists compaction and prevents quartz overgrowth, but excessive chlorite coatings fill the pores and block the pore throats. The laumontite cement is not only resistant to compaction but also easily dissolved to form a large number of effective secondary pores. The dissolution of feldspar and rock fragments plays a role in contributing to connecting the isolated intergranular pores and increasing the secondary pores. Relatively high porosity and high permeability zones are deposited in high-energy environment, mainly delta plain distributary channel sandstones with more detrital quartz, fewer rock fragments and coarser grains than other sandstones, were deposited in high-energy environment. There is weaker compaction and less or no carbonate cement, with the preservation of the primary pores by chlorite coatings and an increase in the pores by the dissolution of laumontite cement.

The Geochemistry and Bioturbation of Clay Sediments Associated with Amalgamated Crusts at the Gagua Ridge

Based on the analysis of geochemical and mineralogical compositions, deep sea clay sediment characteristics and their material sources were examined in the eastern flank of the Gagua Ridge. The mineralogy mainly consists of detrital clay minerals, quartz, and authigenic phillipsite. There is scarce biogenic debris (siliceous or calcareous). The consolidated sediments are more enriched in Si, Al, K, Na, Li, Sc, Cr, Rb, and Cs than the associated crusts and nodules. The unmixed sediment samples were mainlycontributed by Asian eolian dust. The onset of the outer Fe-Mn crust growth nearly coincides with the Central Asia aridification event at ~3.5 Ma, which resulted in an abrupt increase in eolian flux of Asian dust. Intensified surface primary productivity is assumed to bring more metals to deep waters, and eventually facilitate the outer Fe-Mn crust formation. Authigenic phillipsite may come from the alteration of local basic volcanic glasses and cause excess Al, high Al/Ti, and low Si/Al ratios. However, phillipsites hardly affect the abundance of rare earth elements (REEs) and their patterns. In addition, the investigation of two kinds of burrows inside the consolidated sediments reveals that the inner nodules of the amalgamated crusts may remain on the oxic sediment surface, due to frequent benthic activities.

Sedimentology of Marl and Marly Limestone Sequence of Upper Cretaceous Kawagarh Formation from Northern Kalachitta Range, Attock Hazara Fold and Thrust Belt, Pakistan

Upper Cretaceous Kawagarh Formation is well exposed in the Attock Hazara Fold and Thrust Belt (AHFTB) and shows significant lateral and vertical variations in lithology. The present work deals with the sedimentological studies of marl and marly limestone sequence of Kawagarh Formation exposed at the Bagh Neelab, Ghariala north and Sojhanda villages in Northern Kalachitta Range. Detailed petrographic studies of marly limestone and hard marl substrate show that planktons and oysters are the main skeletal constituents of studied samples and clay and detrital quartz mainly composed the non skeletal fraction. X-Ray diffraction analyses of selected marl samples confirm the petrographic data. On the basis of skeletal and non skeletal content, two microfacies—marl microfacies and Planktonic microfacies are constructed. The faunal content, their paleoecology and detrital content of microfacies suggest that marl and marly limestone sequence of Kawagarh Formation was deposited over the mid and outer ramp settings.

Mineralogy and organic geochemistry of phyllite from the Dewon–Pokrzywna deposit, the Opava Mountains (SW Poland)

Phyllites from the Dewon–Pokrzywna deposit in the Opava Mts., SW Poland, were investigated by XRD (Rietveld method), XRF, EPMA, SEM, and ATR-FTIR from the perspective of their potential usage as a buffer and/or backfill material in a geological repository of radioactive waste. Organic matter dispersed in the phyllite matrix was analysed by GC-MS. Fine-grained Mg-Fe-muscovite (13 to 29 wt.%), Fe-ripidolite (10 to 25 wt.%), detrital quartz (20 to 46 wt.%), and albite (7 to 28 wt.%) ± microcline, illite or illite/smectite, and kaolinite are major minerals in phyllite samples. The chlorite/muscovite ratio ranges from 0.65 to 1.1. Mg-annite inherited from the precursor rock is a minor constituent. Detrital ilmenite is a dominant accessory mineral. Ancylite-(Ce) occurs in quartz-calcite-ripidolite veins. Two types of phyllite have been distinguished based on the proportion of phyllosilicates to silt fraction: argillaceous (47 to 55 wt.% phyllosilicates) and silt-rich (28 wt.% phyllosilicates). Argillaceous phyllite shows elevated content of alumina and moderate concentration of silica. It is highly enriched in Fe compared to phyllites from other localities worldwide. The BET specific surface area of argillaceous phyllite ranges from 1.73 to 3.64 m 2 /g. Whole-rock chemical composition, mineral assemblages, chlorite geothermometry, and the occurrence of aliphatic hydrocarbons suggest that argillaceous phyllite originated from a pelagic pelite protolith under low-temperature (260–370°C) greenschist to subgreenschist facies conditions. Persistent biomarkers are indicative of bacterial degradation of planktonic organic matter suspended in a high water column. Enrichment in Fe-rich chlorite and Mg,Fe-muscovite, low volume of interconnected pores with dominant mesopores suggest that argillaceous phyllite from the Dewon–Pokrzywna deposit is a potential candidate for a buffer and/or backfill material.

Mineralogical and chemical composition of Ediacaran-Cambrian pelitic rocks of The Tamengo and Guaicurus formations, (Corumbá Group - MS, Brazil): Stratigraphic positioning and paleoenvironmental interpretations

The Tamengo and Guaicurus formations, upper units of the Corumbá Group, are placed at Ediacaran-Cambrian transition due to biostratigraphy and chemostratigraphy data. The Tamengo Formation is composed mainly of limestones with interbedded mudrocks (siliciclastic, carbonate and mixed siliciclastic-carbonate mudrocks) and record occurrences of macro-fossils Cloudina and Corumbella. The Guaicurus Formation is characterized as an extensive and homogeneous siliciclastic siltstone package. This paper focus in sedimentary, petrographic, mineral (XRD) and chemical (XRF) characterizations of mudrocks and siltstones in order to define stratigraphic positioning of the Tamengo and Guaicurus formations, and indicate depositional environment. To this purpose, three areas were studied in the vicinity of Corumbá (MS-Brazil): Laginha and Corcal quarries and an outcrop along MS-243 road. The rocks of the Tamengo Formation, under microscope, present fluid diffusion features as well as irregular laminations, calcite laminas as evaporate pseudomorphes and biogenic mats. In contrast, siltstones of the Guaicurus Formation are persistently laminated, well sorted and homogenous. Both mudrocks interbedded in carbonate layers and siltstones are composed of detrital quartz, mica and diagenetic clay minerals. Furthermore, the major oxides compositions are broadly similar, except for Na2O, higher in the Guaicurus Formation. The presence of carbonate rocks and mudrocks within the Tamengo Formation imply environment favorable to carbonate formation with episodically immature siliciclastic supply, from very fine-grained rocks of a nearby area, interrupting carbonate depositions. The Guaicurus Formation, distinctively, is a result of an uninterrupted immature siliciclastic supply from a nearby source area, broadly similar to the Tamengo Formation, and was deposited under a low energy setting in a high water level, below fair-weather wave. Petrographic features and diagenetic clay mineral paragenesis – illite + chlorite + smectite – within the Tamengo Formation indicate fluid circulation event. This event resulted in a complex diagenetic history with a possible retrograde diagenesis recorded in the mudrocks of the Tamengo Formation. The siltstones of the Guaicurus Formation present neither petrographic evidences of fluid diffusion nor presence of smectite along the paragenesis illite + chlorite, constraining the event in the Tamengo Formation. After these characterizations, the research revealed that the superior portion of Corcal quarry (L7) exposes rocks from the Tamengo Formation.

Rare earth elements in Fe-Mn nodules from southern Baltic Sea – a preliminary study

Between 1976–1990, the Polish Geological Institute performed geological works in the Polish Maritime Areas. During these works, 260 occurrences of concretions were recorded from 7,500 sampled sites. In 1980, the threshold that separates the Bornholm Basin from the Słupsk Furrow was mapped. Numerous Fe-Mn nodules on the seabed were found in that area. The results of detailed analyses of nodule samples collected from four sites are presented in this paper. Analyzed nodules represent discoidal D, irregular I, and transitional D-I types. The nodules are characterized by varied chemical composition of main oxides (Fe, Mn). The maximum Fe2O3 content is 26.63% and MnO 23.18%. Total average amount of REE + Y in the samples is approximately 165.11 ppm, ΣLREE 145.72 ppm and ΣHREE 19.39 ppm. The LREE content is enriched in comparison to HREE. The majority of nodules consist of Fe-Mn oxy-hydroxide minerals with very low crystallinity (practically amorphous phases). The main confirmed Mn-phases are birnessite and todorokite. Other main components of the nodules are: detrital quartz, albite, microcline, glauconite and muscovite, clinochlore, and clay minerals: illite and chlorite. The rate of growth of Fe-Mn nodules has been estimated using a cobalt chronometer. The nodule growth rate ranges from 0.006 to 0.134 mm/yr –1. Based on the Fe, Mn and (Cu + Co + Ni) contents, the origin of studied nodules is determined as hydrogenetic, while using REE (Cesn/Cesn · vs. Nd) – as diagenetic.

Maastrichtian to early Paleocene sea level changes and climatic evolution on the southeastern Tethys margin

Abstract An integrative multi proxy approach based on lithology, microfacies, bulk rock composition, total organic carbon (TOC), and δ13Ccarb was used to estimate southeastern Tethys sea level fluctuations during the deposition of the Maastrichtian-Danian oil shales in Jordan. Clay mineral assemblages were used as proxies to determine climatic variation induced sediment supply during the Cretaceous/Paleogene boundary (K/Pg) interval. The results from two cores (OS-02, Jafr Basin, southeast Jordan; OS-28 Azraq-Hamza Basin, central-east Jordan) reveal four major intervals of distinctive bulk rock composition reflecting sea level changes. Interval I (earliest Maastrichtian) with a low sea level and the formation of dolomites and a high detrital quartz input predates the oil shales. Intervals II and III in core OS-02 and interval II in core OS-28 represent the highest relative sea level following the Early Maastrichtian sea level rise on the Arabian Platform. Intervals III (core OS-02) and II (core OS-28) coincide with the highest organic matter deposition. Interval IV (core OS-02) and intervals III and IV (core OS-28) relate to the sea level regression-transgression cycle in the latest Maastrichtian through Danian and are marked by a large drop in the TOC content. Increased terrestrial influx during sea level lows is evident from two peaks/intervals of major negative δ13Ccarb values in core OS-28. Based on clay mineralogy it can be differentiated between warm/humid, warm/cool seasonally alternating, and cool/arid climates. When bulk rock composition is used as a sea level proxy, clay mineral assemblages from both cores reveal the dominance of warm and humid climate or, alternatively, warm/cool seasonal alternations during high sea levels (II and III core OS-02; II and IV core OS-28), while periods of arid and cool climate correspond to lower sea levels. The sea level changes and climatic variations deduced from the Jordanian oil shale successions are in agreement with the long term variations from the Egyptian Western Desert sections (southeastern Tethys), and the Tunisian El-Kef section (southweastern Tethys). Moreover, the sea level changes identified relate to the regional changes on the Arabian platform during the Maastrichtian-Danian.

Pore characterization and the controls of organic matter and quartz on pore structure: Case study of the Niutitang Formation of northern Guizhou Province, South China

Quartz and organic matter (OM) are two important components in marine shales that have significant effects on shale gas reservoir quality, particularly on pore structure. However, a limited number of in-depth studies exist on characterizing the OM and quartz. In this study, types of quartz and OM in the organic-rich marine Niutitang shales and their impacts on pore structure were investigated using X-ray diffraction analysis, scanning electron microscopy, nitrogen adsorption, and geochemical analyses. Three OM types (scattered OM, stripped OM, and interstitial OM), four types of OM pores (primary OM pores, convoluted OM pores, shrinkage OM pores, and thermogenic OM pores) and two quartz types (detrital quartz and authigenic quartz) were identified. The Lower Member (LM) and Upper Member (UM) of the Niutitang Formation have different types of OM and quartz, resulting in the differences of pore structure characteristics. Scattered OM and convoluted OM pores and extrabasinal detrital quartz are widespread in the UM, whereas interstitial OM and thermogenic OM pores and authigenic quartz are common in the LM. Quartz in the LM shales positively correlated with TOC, as well as TOC positively correlating with excess-Si, which may indicate that quartz may be mostly biogenic in origin. The biogenic authigenic quartz can act as a rigid framework, which can resist compaction and preserve the internal pore structure and provide enough space to be in-filled by OM. Abundant OM filled inter-particle pores formed by authigenic quartz is the principal matrix for OM pore development. There are positive correlations among TOC and quartz with pore structure parameters and shale with higher contents of TOC and quartz lead to preservation of some primary pores and development of OM pores, indicating that the pore structure in the Niutitang shale may be mainly controlled by OM and quartz. This study suggests that the shale in the LM would more likely contain better pore systems and provide favorable reservoir spaces for shale gas due to its high content and favorable types of OM and quartz.

Petrogenesis, provenance, and rare earth element geochemistry, southeast desert phosphorite, Jordan

Representative samples from the Al-Hisa Phosphorite Formation (AHP) in the SE desert of Jordan, are selected from three sections: Batn El-Ghoul, Nagb Etayyeg, and Zgaimat Al-Hasah. The samples are investigated geochemically and petrographically. Geochemistry is discussed through the analysis of the major, trace and rare earth elements (REEs), loss on ignition (LOI), sulphur, and organic matter. The studied sections differ from the AHP in central Jordan by having highly reduced thicknesses, omission of underlying formations, abundant detrital quartz sand and age. The phosphate particles are of authigenic and biogenic origin. Early reworking from phosphate mud is evident from the shape and sorting of the phosphate particles. Major and trace elements are distributed into five factors: upwelling, provenance, redox potential, sea level changes, and resistates. The studied sediments were formed under oxic marine conditions because their REEs patterns preserve the REEs seawater linear pattern: depletion of LREEs, a negative Ce anomaly, and an enrichment of HREEs. This signature seems to have persisted through reworking and late diagenetic cementation, dolomitization and silicification. The SE desert phosphorite-siliciclastic sequence is better correlated with the Paleocene Jalamid Formation of northwestern Saudi Arabia because of similarities in age and lithology due to being both deposited on the western flanks of the Sirhan Paleohigh and the Rutba Paleohigh, respectively. The siliciclastics were delivered from a mixed mafic-felsic-older cycle siliciclastic provenance, and subjected to intensive chemical weathering under tectonic quiescence giving rise to mature quartzarenites.

Murihiku rocks as potential petroleum reservoirs in Zealandia

ABSTRACTDetailed petrographic analysis has been undertaken on Jurassic to earliest Cretaceous Murihiku rocks from Southland and South Auckland locations. They comprise lithic (primarily volcanic) and feldspathic (plagioclase-rich) sandstones that generally contain little detrital quartz, and classify as feldsarenites, lithic-feldsarenites, feldspathic litharenites and litharenites. The oldest (Early Jurassic) sandstones are the most quartz-rich. Sandstones become more lithic up-section, with very high volcanic contents occurring within the younger samples (Late Jurassic to earliest Cretaceous). The higher quartz content in the oldest strata is likely to reflect contributions from plutonic or metamorphosed rocks into the forearc during the Early Jurassic, a contribution that decreased with time. However, some quartz enrichment in Early Jurassic sandstones may also have occurred by preferential sorting of quartz grains during transport into the depositional environment. Generally, Murihiku sandstones have poor reservoir properties based on the petrographic and petrophysical results but they may constitute a viable fractured reservoir target.

Amorphous silica and its effects on shale reservoir: A case study about Yanchang formation lacustrine shale, Ordos Basin

ABSTRACTThe content of amorphous silica in Yanchang formation shale ranges from 1.48% to 19.89%, with an average of 11.77%. The Chang 7–3 submember (the submember 3 of the seventh member of Upper Triassic Yanchang formation) shale contains much larger content of amorphous silica. Amorphous silica appears as opal rim closely adjacent to detrital quartz, cement among clay minerals, and cement aside detrital minerals. Silica originates from clay mineral transformation and pressure solution of detrital quartz. Mineral composition, growth space, and overpressure affect the content of amorphous silica. Amorphous silica reduces the brittleness of shale reservoir, decreases the density of Chang 7–3 submember shale, and causes an increase in AC values for Chang 7–3 submember shale.

Diagenetic evolution of the upper Devonian phosphorites, Alborz Mountain Range, northern Iran

The upper Devonian, mixed siliciclastic‑carbonate stratigraphic succession of the Geirud Formation in the Alborz Mountain Range, north of Iran, hosts numerous phosphorites and phosphatic rocks. These phosphorites, which represent one of the most globally important deposits in the Devonian, have gained little attention regarding their genesis and diagenetic evolution. This study focuses mainly on the post-depositional changes that occurred during shallow to deep burial diagenetic evolution and subaerial weathering of the Geirud phosphorites. The phosphorites consist of phosphatic massive mudclasts, detritus-rich lithoclasts, bioclasts and bones formed by several stages of phosphogenesis and erosion, reworking, winnowing and condensation associated with Famennian sea level changes. The phosphatic clasts consist of early diagenetic, poorly crystalline, carbonate fluorapatite (CFA) and pyrite cementing detrital quartz, illite, muscovite, rutile and zircon. Phosphatic clasts are intensely compacted with sutured grain contacts, along which dissolution and reprecipitation of chemically-refined, well crystalline CFA with a bright blue cathodoluminescence and illite occurred during deep burial diagenesis. Similarly, quartz shows deformation planar features; overgrowth cement was developed as a result of pressure dissolution in phosphatic sandstones, followed by precipitation of ferroan dolomite, ankerite and chlorite in secondary porosity. Subaerial weathering led to oxidation of pyrite and generation of acidic pH condition, under which Ca and P from phosphates, Fe and S from pyrite, Fe, Ca, Mg and Mn from ferroan carbonates and Al and Si from aluminosilicates were released to form goethite, psilomelane, calcite, gypsum and crandallite.The PAAS-normalized rare earth elements (REEs) show bell-shaped patterns with middle (REEs) enrichment, positive Eu anomaly and a weak negative Ce anomaly. This implies recrystallization of CFA in phosphorites and phosphatic rocks during deep burial diagenesis, deviating from the modern seawater-like REE patterns. In this regard, deep burial diagenesis affecting the upper Devonian Geirud phosphorites is possibly a common process in the Proterozoic-Paleozoic phosphorites in northern Iran and elsewhere in the world.

Quartz types in shale and their effect on geomechanical properties: An example from the lower Cambrian Niutitang Formation in the Cen'gong block, South China

Abstract We identified the sources of quartz in shales of the Niutitang Formation in the Cen'gong block, northern Guizhou Province, China, based on thin-section studies using a scanning electron microscope (SEM), rock cores and field observations. By analyzing the petrology, total organic carbon (TOC), major elements, and illite “crystallinity”, we investigated the formation of silica in the Niutitang shales. Our results show that the quartz in the Niutitang shales consists mainly of detrital quartz, biogenic quartz, and quartz that transformed from clay minerals. The amount of excess SiO2 in the Niutitang shales was approximately 5–35%, and the Al/(Fe + Al + Mn) and Si/(Si + Al + Fe) ratios ranged from 0.45 to 0.85 and from 0.7 to 0.91, respectively, which indicates that the silica was not solely biogenic. The values of excess SiO2, Al/(Fe + Al + Mn), Si/(Si + Al + Fe), and TOC increased with increasing quartz content, which implies that increasing quartz concentrations were associated with gradual increases in biogenic quartz and decreases in detrital quartz. Quartz from different sources can result in varying rock geomechanical properties and fracture abundance. Compared with pure biogenic quartz or detrital quartz, an appropriate proportion of mixed biogenic quartz and detrital quartz can make rocks more brittle and therefore lead to more fractures.

The effect of thermal maturity on geomechanical properties in shale reservoirs: An example from the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin

Abstract Shale reservoirs are characterized by low matrix permeability and therefore require effective models of geomechanical properties to optimize drilling and hydraulic fracturing strategies. Both initial rock composition and thermal maturity are potentially critical controls on geomechanical properties. We investigate the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin, a major shale gas target that spans a wide range in rock compositions and thermal maturity to identify relationships between these parameters and geomechanical properties. Core hardness measurements and dipole sonic and density log data were used to characterize the geomechanical properties. Major element chemical analysis, X-ray diffraction analysis and LECO combustion were used to determine mineralogy, bulk rock chemistry and total organic carbon (TOC) content and to distinguish biogenic from detrital silica. Scanning electron microscopy (SEM) images with complementary energy-dispersive spectroscopy (EDS) maps were obtained for representative samples to document the rock fabric and distribution of organic matter and minerals. Hardness and Al2O3 concentrations are strongly negatively correlated in all cores, regardless of thermal maturity, suggesting that clay minerals are the most significant factor controlling geomechanical properties. Biogenic silica is positively correlated to hardness. Detrital silica is negatively correlated to hardness, an artifact of the positive correlation between detrital clay minerals and detrital quartz. The positive correlations between CaO content and hardness in all cores suggest the brittle behavior of carbonate minerals. Increased thermal maturity from immature to oil window results in greater hardness for rocks of similar geochemical compositions. We propose that this results from: (1) enhanced mechanical compaction; (2) carbonate cementation; (3) increased stiffness of kerogen; and (4) partial conversion of kerogen into expelled hydrocarbons that reduces the load-bearing function of the organic matter. At maturities greater than oil window, thermal maturity does not exert a major control on the rock strength; thus, there are no significant changes in hardness between samples from oil window and dry gas window. Geomechanical properties can be related to mineralogy, which exerts a major impact on the geomechanical properties. Quartz cements sourced from biogenic silica rather than smectite-to-illite transition are the primary contributor to the rock strength in the Duvernay Formation. However, it is noteworthy that thermal maturity should also be considered as an important factor when predicting geomechanical properties from mineralogy.