Feldspar Dissolution Research Articles

Genesis of the Yarikçi Hydrothermal Clay Deposit Within the Mesozoic Metamorphic Units, Mihaliççik, Eskişehir, Turkey

AbstractHydrothermal alteration led to development of the Yarıkçı clay deposit within the Mesozoic chlorite-, muscovite-, chlorite-muscovite-schist, and garnet-graphite phyllite units along NW–SE- and N–S-trending faults in Mihalıççık in western central Anatolia. The geological, mineralogical, and geochemical characteristics and genesis of this economically important clay deposit have not been examined in detail previously. The present study has attempted to fill this gap. Green smectitic and cream kaolinitic claystones are abundant with smaller amounts of gray illite, dark brown Fe oxides, and silica phases occurring as stockwork/fracture infill and stain/coating. These units are covered by a dark, hard, sharp-edged, and thick silica cap. Metamorphic units exhibit cataclastic texture due to tectonic activities. Muscovite is mostly degraded to kaolinite, and feldspars show sericitization and argillization. Kaolinite typically has a platy form with irregular margins and locally sub-rounded, book-like texture suggesting hydrolysis during the hydrothermal injections. The association of Fe oxides, cristobalite/tridymite/quartz, gypsum/anhydrite, and jarosite are indicative of intense hydrothermal activities and development of kaolinite under acidic geochemical conditions. The local enrichment of SiO2, Fe2O3, S, Cu, and Au also supports this suggestion. The leaching of Sr, Rb, Ba, and Zr, and the slight increase in LREE/MREE+HREE ratios together with the negative Eu and Ce anomalies suggest the selective dissolution of muscovite, garnet, feldspar, and pyroxene by the hydrothermal fluids. Thus, abundant claystones of smectite and kaolinite were formed via the increase in Al+Fe+Mg/Si and Al±Fe/Si ratios in the alkaline and acidic environment, respectively, under the tectonic control of hydrothermal activity as seen in the alteration of chlorite, muscovite, and feldspar in metamorphic units.

Element enrichment/depletion during faulting in shale-rich surface exposures of the 2008 Wenchuan earthquake (Mw 7.9) and implications for coseismic temperature

The coseismic frictional temperature is increasingly recognised as an important factor for the transformation mechanism in shallow crustal faulting. We report changes in the mineralogical and geochemical compositions across the fault zone in the Shaba (SB) exposure of the 2008 Wenchuan earthquake (Mw 7.9), which contained the maximum value of vertical displacement and cut through shale-rich strata by the principal slip surface. Assuming Yb to be an immobile trace element, with respect to those in the adjacent damaged zone and wall rocks, Li, Rb, Cs and most light rare earth elements (LREE) exhibited marked enrichments of 143%, 247%, 475% and >60%, respectively; and a 40% depletion of Sr and <5% depletion of most heavy rare earth elements (HREE) was found, responding to mineralogical transformation such as alteration and dissolution of feldspar and formation of abundant clay minerals. In addition, heterogeneous adsorption of clay minerals could result in fractionation between and within LREE and HREE as well as negative Eu anomalies and positive Ce anomalies. The element enrichment/depletion model of the northern segment of the Yingxiu–Beichuan (YX–BC) fault zone was quite different from the middle/southern segments characterising faults in carbonate and granitic rocks. Through model calculation, the calculated trace element compositions (Li, Rb, Sr, Cs, Ba, La, Sm and Pb) of the fault gouge were obtained at 250 °C, 300 °C and 350 °C, and were produced by frictional heating. By comparing with the observed values, we suggest that high temperatures derived from frictional heating or due to buffering of the slip zone temperature by evaporation of pore water during the earthquake within the principal slip surface in the northern segment of the YX–BC fault zone may not have existed. Mechanical crushing and chemical alteration (not under high-temperature conditions) within the fault zone during the coseismic stage may result in mass removal and element enrichment/depletion. KEY POINTS Mineralogical and geochemical compositions were reported in the Shaba exposure of the 2008 Wenchuan earthquake. Li, Rb, Cs and most LREE exhibited marked enrichments in the fault gouge. Fractionation between and within LREE and HREE, negative Eu anomalies and positive Ce anomalies occur in the fault gouge. High temperatures may not have existed within the principal slip surface of the northern segment of the YX–BC fault zone.

Diagenesis and Origination of Carbonate Cements in Deeply Buried Sandstones of the Eocene Es3 Member, Raoyang Sag, Bohai Bay Basin, China

Diagenesis is one of the most predominant factors controlling reservoir quality in the deeply buried siliciclastic sandstones of the third member in the Eocene Shahejie Formation (Es3), in the Raoyang Sag, the Bohai Bay Basin. In this study, thin section, cathodoluminescence (CL), scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectrum, carbon and oxygen isotopes, and fluid inclusion analyses are used to restructure paragenetic sequences and detect origins of carbonate cements recorded in this deeply buried member. Based on petrographic analyses, the Es3 sandstones are identified as lithic arkoses and feldspathic litharenites at present, but derived from original arkoses and lithic arkoses, respectively. Geohistorically, the Es3 sandstones have undergone two diagenetic episodes of eogenesis and mesogenesis. Events observed during eogenesis include chemical compaction, leaching of feldspar, development of chlorite coating and kaolinite, precipitation of the first generation of quartz overgrowth (QogI), dissolution of feldspar, and precipitation of calcite and nonferroan dolomite cement. Mesogenetic alterations include chemical compaction, precipitation of kaolinite aggregate and the second generation of quartz overgrowth (QogII), precipitation of ankerite, development of I/S and illite, and formation of pyrite. Carbon and oxygen isotopic data show that calcite cements are characterized by 13C ( δ 13 C PDB ranging from -0.7‰ to 1.0‰ with an average of 0.1‰) and 18O ( δ 18 O SMOW varying from 12.3‰ to 19.0‰ with an average of 16.2‰); these stable isotopic data combined with Z value (from 114.69 to 122.18) indicate skeletal debris ( δ 13 C PDB ranging from -1.2‰ to -1.1‰ with an average of -1.15‰; δ 18 O SMOW varying from 23.0‰ to 23.2‰ with an average of 23.1‰) and ooids in adjacent carbonate beds involved in meteoric water and seawater from outside jointly served as the carbon sources. For nonferroan dolomite, the δ 13 C PDB value of -4.1‰ is a little bit negative than the calcite, and the δ 18 O SMOW of 14.3‰ is coincident with the calcite, which suggest the nonferroan dolomites come from the diagenetic fluids with a similar oxygen isotopic composition to that of the calcite but modified by the external acidic δ 13C-depleted water. However, the ankerites are actually rich in 12C ( δ 13 C PDB ranging from -10.0‰ to -1.2‰, mean = − 4.3 ‰ ) and 16O ( δ 18 O SMOW varying from 10.1‰ to 19.4‰, mean = 14.9 ‰ ), when combined with the distribution of cutting down along the direction pointing to sand-body center from the margin and microthermometric temperature (Th’s) data mainly varying between 115.2°C and 135.5°C with an average of 96.0°C, indicating the main origination from the Es3 source rocks with effective feldspar buffer action for the acidic fluids in the margins of the Es3 sandstones. In addition, the necessary elements for ankerite such as Fe2+, Ca2+, and Mg2+ ions also come from organic matter and clay minerals during thermal maturation of the Es3 source rocks. The study provides insights into diagenetic processes and origination of carbonate cements in the Es3 sandstones; it will facilitate the cognition of predictive models of deeply buried sandstone reservoirs to some extent, which can reduce the risks involved in oil and gas exploration and development.

Fertilizer derived from alkaline hydrothermal alteration of K-feldspar: A micrometer to nanometer-scale investigation of K in secondary reaction products and the feldspar interface

Global food security concerns have spurred increasing demand for locally sourced and produced K-fertilizers. Various processes have been explored for more than a century; one promising solution is based on the alkaline aqueous alteration of feldspar-rich rocks at elevated temperatures. However, knowledge of the overall physico-chemical reactions comprising dissolution of feldspar and precipitation of secondary phases is still rudimentary, in particular how the feldspar structure evolves at the nm-scale during hydrolysis at alkaline conditions. Here we report on the results of a study aimed at converting potassium feldspars to K-rich fertilizer based on the alteration of sanidine and microcline samples at 190 °C in pH 12 Ca(OH)2 solutions for 24 h. Based on X-ray diffraction and Rietveld refinement, the secondary authigenic minerals that precipitated are primarily composed of Ca-carbonate (calcite, vaterite), and Ca-(Al)-silicates, such as tobermorite and hydrogrossular. Short-term bench top leaching experiments in water prove that the hydrothermal product releases up to two orders of magnitude more K than the unaltered K-feldspar starting material, pointing to its application as a ready-to-use fertilizer for K-deficient soils. Detailed chemical mapping and energy dispersive X-ray spectroscopy (FESEM- and TEM-EDXS) analyses of the precipitates at the μm to nm-scale show that the distribution of K associated with the secondary phases is very heterogeneous, both spatially and in terms of concentrations. Using various analytical transmission electron microscopy (TEM) techniques, e.g., HRTEM, TEM-EDXS, EFTEM, to investigate the structure and chemistry of the feldspar interface, we find no evidence for a change in chemistry or structure at the nm-scale, even though dissolution continuously decreases the volume of each grain. Our observations also show the existence of an amorphous surface altered layer (SAL) of variable thickness (10–~100 nm) forming at the feldspar interface. Nanometer-scale chemical measurements show that this amorphous SAL is rich in K, and therefore may also be an important reservoir of easily leachable K. We hypothesize that it forms continuously and in situ at the expense of the feldspar by a coupled interfacial dissolution-reprecipitation process (CIDR).

Diagenesis of the Vaca Muerta Formation, Neuquén Basin: Evidence from petrography, microthermometry and geochemistry

The Tithonian Vaca Muerta Formation (500 m-thick marine carbonates) in the Pampa de Tril area is a potential unconventional hydrocarbon reservoir but earlier studies rarely focused on its diagenetic history. It constitutes the upper part of the lower Mendoza Group in the Neuquén Basin, western Argentina. Petrographic examinations reveal four generations of calcite (C1, C2, Cb and C3). The microbial lime mudstone (C1, <4 μm), which exhibits dull cathodoluminescence (CL), is the most abundant with mean δ18O value of −7.2 ± 1.3‰ (VPDB) and δ13C of −1 ± 3.3‰ (VPDB) and deemed to have deposited in tropical warm shallow-water settings. The equant calcite cement (C2, 25 μm–140 μm) exhibits red CL and is associated with recrystallization of bioclasts and micritic matrix. The fibrous calcite (Cb, 35 μm–125 μm) fills parallel-to-bedding antitaxial veins (beefs) and shows bright orange CL. Its δ18O and δ13C values are more depleted (−10.7 ± 2.2‰ VPDB and −4.8 ± 3.9‰ VPDB, respectively) relative to those of C1, which is consistent with the occurrence of hydrocarbon fluid inclusions and may imply precipitation during a mid-to late stage of diagenesis within the oil window. The fracture-filling C3 consists of coarse subhedral crystals (40 μm–5 mm) showing bright orange CL, which has been interpreted as precipitated during deep burial conditions at temperatures around 118 ± 26.8 °C based on fluid-inclusion microthermometry. The relative depletion of Fe and Mn from C1 to C3 and the low concentration of U, may imply a relatively closed system of low water/rock interaction and restricted meteoric alteration. The shale normalized (REESN) patterns of C1, Cb and C3 generally exhibit parallel patterns with a distinct positive Eu anomaly and progressive enrichment towards the MREE and HREE. The positive Eu (Eu/Eu*)SN anomalies reflect the impact of dissolution of plagioclase feldspars by circulation of the diagenetic fluids through the surrounding siliciclastics in the basin.The Ce (Ce/Ce*)SN and La (La/La*)SN anomalies of Cb and C3 are consistent with precipitation under reducing conditions of deep burial settings. The Vaca Muerta carbonates are enriched in organic matter (up to 12%) that fills the pores but permeability is insignificant.

Genetic mechanisms of Permian Upper Shihezi sandstone reservoirs with multi-stage subsidence and uplift in the Huanghua Depression, Bohai Bay Basin, East China

Braided river sandstones in the Permian Upper Shihezi (US) Formation in the Huanghua Depression, Bohai Bay Basin, East China, are reservoirs with large accumulations of hydrocarbons. The Upper Paleozoic formations, including the Carboniferous and the Permian strata experienced multi-stage of subsidence and uplift in the past 320 Ma, leading to occurrence of subaerial exposure and deep burial of the Permian US sandstones. Due to complex tectonic evolution, the Upper Paleozoic formations in different tectonic zones of the Huanghua Depression experienced various burial-thermal histories, resulting in the development of long-term open (LTO), early-open to late-closed (EOLC), and long-term closed (LTC) geochemical systems in different Permian US sandstones, as well as different evolutionary patterns of diagenesis and reservoir quality. After extensive diagenetic alteration, the present sandstones are primarily quartzose, subfeldsarenite, and sublitharenite sandstones. Pores in shaly sandstones and fine-grained sandstones were destroyed extensively, only pebbly to medium-grained sandstones can be potential effective reservoirs. The Permian US sandstones with the LTO systems, which are currently overlain by the Paleogene strata and buried less than 2500 m, experienced two stages of meteoric freshwater leaching and are characterized by extensive feldspar dissolution and weak precipitation of authigenic kaolin and quartz. Intergranular and intragranular pores dominate the 10–20% porosity, and permeability ranges from 1mD to 800 mD. The Permian US sandstones with the EOLC systems, which are currently overlain by thick Jurassic and Cretaceous strata on a regional scale and buried deeper than 3000 m, experienced both meteoric freshwater leaching and deep burial leaching and are currently characterized by extensive feldspar dissolution, extensive kaolin precipitation, and weak-moderate quartz cementation. Intragranular secondary pores and kaolin intercrystal pores dominate the 5–15% porosity, and permeability ranges from 0.1 mD to 10 mD. The Permian US sandstones with the LTC systems, which are currently overlain by a thick Permian Shiqianfeng Formation on a regional scale and buried deeper than 3000 m, experienced no meteoric freshwater leaching but only burial dissolution; they are currently characterized by extensive feldspar dissolution and extensive precipitation of clays and quartz cements. Intercrystal pores in the authigenic clay and intragranular secondary pores dominate the 5–10% porosity, and permeability ranges from 0.01mD to 1mD. Since the unconformities, overlying strata, and tectonic evolution relevant to the Permian US sandstone unit can be analyzed using seismic data, the proposed genetic models can support the predrill evaluation of reservoir quality of the Permian sandstones.

Micron to nanometre‐scale pore characterisation of the early oil window Vaca Muerta Formation, Neuquén Basin, Argentina

AbstractThis paper characterises the pore types and their distribution in the Vaca Muerta Formation, Neuquén Basin, Argentina. This formation is now being studied and targeted as reservoir rock, and it is therefore considered an unconventional resource play. There is significant need for a porosity description as part of the unconventional reservoir characterisation. Mineral compositions, grain assemblages, morphology, organic content and distribution, and micron to nanometre‐scale pore types are studied through scanning electron microscopy. Petrographic observations depict the complex response of pores to chemical and mechanical processes, where both destruction of primary porosity and generation of secondary pores are common. Compaction, cementation and infill of migrated organic matter destroy primary porosity whereas dissolution and secondary organic matter generated through maturation create secondary pores. Lamina sets with major detrital components can help identify intervals with higher interparticle and intraparticle porosity. Interparticle and intraparticle porosity are ubiquitous throughout the samples. Interparticle pores are common around rigid detrital grains, like quartz and feldspar, and within detrital clay platelets. Porosity within authigenic clay cements is included, as framework porosity and samples may contain up to a maximum of 30 wt% clay. Partial grain dissolution of coccoliths, feldspars and calcite grains resulted in available intraparticle pore space. Organopores are the least abundant pore types at this location. Organopores were described within migrated organic matter. Although the kerogen type (Type II) and the vitrinite reflectance are optimum for hydrocarbon generation (ca 0.80% Ro) in the Vaca Muerta Formation at the study area, development of organopores is not common. The Vaca Muerta Formation has experienced insufficient thermal maturation (early oil window) and has not yet reached peak oil.

Experimental Study on the Self-Healing Behavior of Fractured Rocks Induced by Water-CO2-Rock Interactions in the Shendong Coalfield

This study experimentally investigated the self-healing behavior, referring to the naturally occurring water permeability decrease, of fractured rocks exposed to water-CO2-rock interaction (WCRI). The experiment was conducted on prefractured specimens of three rock types typical of the Shendong coalfield: coarse-grained sandrock, fine-grained sandrock, and sandy mudrock. During the experiment, which ran for nearly 15 months, all three specimens exhibited decreasing permeabilities. The coarse- and fine-grained sandrock specimens exhibited smooth decreases in permeability, with approximately parallel permeability time curves, whereas that of the sandy mudrock specimen decreased rapidly during the initial stage and slowly during later stages. The sandrock specimens were rich in feldspars, which were dissolved and/or corroded and involved in ionic exchange reactions with CO2 and groundwater, thereby generating secondary minerals (such as kaolinite, quartz, and sericite) or CaSO4 sediments. These derivative matters adhered to the fracture surface, thereby gradually repairing fractures and decreasing the water permeability of the fractured rocks. In comparison, the sandy mudrock had a high content of clay minerals, and the water-rock interaction caused rapid expansions of illite, mixed illite-smectite, and other clay minerals, thereby narrowing the fractures and causing the rapid permeability decrease during the initial stage. In later stages, the derivative matters generated by the dissolution and/or corrosion of feldspars and other aluminum silicate minerals in the mudrock filled and sealed the fractures, causing the slow permeability decreases during the later stages, as in the sandrock specimens. Neutral and basic groundwater conditions facilitated better self-healing of fractured mudrocks rich in clay minerals, whereas acidic groundwater conditions and the presence of CO2 facilitated better self-healing of fractured sandrocks. Thus, this study’s results are of significant value to aquifer restoration efforts in the Shendong coalfield and other ecologically vulnerable mining areas.

Sedimentological analysis and reservoir quality of the early Cenomanian transgressive sandstones, northern Western desert, Egypt

Depicting depositional models of transgressive sandstone reservoirs is challenging due to their internal complexity and limited case studies. This study presents an example for transgressive sandstone reservoirs from the Bahariya Formation at Hayat-Yasser oil field, Shushan Basin (northern Western Desert, Egypt). The upper part of the Bahariya Formation accumulated in an estuarine setting as a consequence of a successive sea level rise. Detailed reservoir characterization, employing sequence stratigraphic and sedimentological approaches, revealed that the studied reservoir consists of a mixed siliciclastic-carbonate transgressive estuarine-shelf succession. Four conventional core samples of this succession at Khalda ridge (Shushan Basin), permit the analysis of their sedimentology and internal architecture (mesoscopic heterogeneities) along correlation lines parallel or slightly oblique to depositional dip. The succession consists of six backstepping parasequence sets that record the overall shift from tidal-dominated to wave-dominated estuaries/bays. The transgressive component of the parasequences sets includes bay mudstones, flood-tidal deltas, and shoreface/barrier deposits; while the regressive component includes estuarine point bars, bay head deltas, and tidal deltas. The reservoir properties show significant variation according to the facies assemblages. The tidally-dominated estuarine channels show low net-to-gross, as it is characterized by lenticular sandstone bodies with several lateral and vertical baffles and barriers. On the other hand, the shoreface/barrier and tidal delta sandstones show high net-to-gross and consist of amalgamated sandstone bodies with little mud barriers or baffles. Petrographically, the upper Bahariya reservoir includes intergranular and secondary porosities. The microscopic heterogeneities are mainly due to the fine grain nature, poor sorting, presence of matrix, and continuous mud barriers or baffles. Destruction of porosity, affecting the reservoir quality, occurred by silica overgrowths, ferroan calcite, grain coating chlorite, and kaolinite cementation, as well as compaction. Feldspar dissolution is the main diagenetic process responsible for the formation of secondary porosity, enhancing the reservoir quality.

Role of cementation and compaction in controlling the reservoir quality of the Middle to Late Jurassic Sandstones, Jara dome, Kachchh Basin, western India

Thin‐section petrography, scanning electron microscopy (SEM) and X‐ray diffraction (XRD) analyses are applied to investigate the various diagenetic controls on the reservoir quality of the Ridge and Athleta Sandstone members (Chari Formation) of the Jara dome, Kachchh Basin, western India. The results show that the sandstones of Jara dome are quartz‐arenite to sub‐arkose, medium‐ to coarse‐grained, sub‐angular to sub‐rounded, poorly to moderately well‐sorted and having low‐ to medium‐sphericity. Diagenetic features such as mechanical compaction, pressure solution, cementation, and dissolution of unstable framework grains are analysed to synthesize the reservoir characteristics. The principal cementing materials are carbonates, iron oxides, silica overgrowth, and authigenic clays, mainly chlorite and kaolinite. Compaction and cementation are the controlling factors for the reduction in porosity and permeability of the sandstones. Dissolution of detrital feldspars is the main cause of porosity enhancement in these sandstones. The relationship between the intergranular volume (IGV) and cement volume indicates that cementation had played a more important role in the destruction of primary porosity than that of compaction. On the one hand, the ubiquitous development of carbonate cement advocates the loss in primary porosity at the early stage of diagenesis, while on the other hand it also prevented the further compaction of sandstone, thus preserving unfilled voids as primary porosity. Precipitation of carbonate cement is followed by development of pore‐lining and pore‐filling clayey cements. The pore‐filling kaolinite resulted in a considerable loss of porosity while the pore‐lining chlorite has helped in retaining the porosity by preventing syntaxial silica overgrowths. The reservoir quality of the studied sandstone is reduced by the authigenic clay cements, early carbonate precipitation and silica overgrowths, nonetheless, it is improved by the dissolution and alteration of unstable grains, in addition to partial dissolution of carbonate cements. The potential of the studied sandstone to act as a reservoir is strongly related to sandstone diagenesis.

Pore structure evaluation of low permeability clastic reservoirs based on sedimentation diagenesis: A case study of the Chang 8 reservoirs in the Zhenbei region, Ordos Basin

Sedimentary diagenesis is one of the most significant factors in the development of low permeability sandstone reservoirs due to its important influence on the formation of pore structure. This study takes Group 8 of the Yanchang Formation in the Zhenbei region of the Ordos Basin as the research object. Methods including casting thin-sections, mercury injection, and scanning electron microscopy were used to analyse core and casting thin-sections to examine the sedimentary diagenetic processes, authigenic mineral composition, pore structure, and physical properties of the reservoirs in the study area. The influence of sedimentary diagenesis on the physical properties of the reservoirs was analysed, and the pore structures in the study area were divided into four types. By determining the development mechanism and distribution characteristics of each type of pore structure, the favourable reservoir areas were also predicted. The results showed that type I (large mesopores with medium throats) and type II (mesopores with fine throats) were the favourable pore structures in the research area. These were mainly formed by the rim cementation of chlorite, the dissolution of feldspar and other soluble components, and a weak to mid compaction. Areas with type I and type II pore structures are considered to have a high reservoir quality, with a porosity of between 10% and 25% and a permeability of between 1 mD and 50 mD. These areas are suggested as regions for optimised exploration and development.

Petrography and authigenic chlorite in the Siegenian reservoir rocks, Berkine Basin, eastern Algerian Sahara

In Berkine Basin, the Siegenian stage is constituted by clayey sandstones and it contains several reservoir levels especially in the perimeter of Zemlet El Arbi, where the sandstone levels can reach 30 m of thickness. The Siegenian reservoir shows a tidal deposit environment, it is located at depths greater than 3200 m, its average thickness is 200 m in the south and it decreases to 30 m because it is strongly affected by Hercynian orogeny mainly in the northwest of the studied area. Petrographic and X-ray diffraction studies indicate the presence of authigenic chlorite, which is the most important cement in Siegenian sandstones. Chlorite appears in different forms: grain coating, pore lining, pore filling and rarely in the form of oolite. It often has a great impact on petrophysical reservoir qualities by inhibiting authigenic quartz overgrowth in porous space and preserving porosity. Other diagenetic phenomena observed are mechanical compaction, alteration and dissolution of feldspars and biotite, precipitation of clay minerals (smectite, berthierine, chlorite, kaolinite and illite) and precipitation of secondary silica, carbonates, pyrite and haematite.

Formation of abnormally high porosity/permeability in deltaic sandstones (Oligocene), Bozhong depression, offshore Bohai Bay Basin, China

The porosity-enhancing capability of secondary porosity in a closed sandstone geochemical system has been suggested to be small in past studies. However, the mechanisms responsible for the formation of abnormally high porosity/permeability remain unclear. The Oligocene Dongying Formation in the Bozhong Depression, offshore Bohai Bay Basin, preserves abnormally high porosity, i.e., 36.4% (average) or 42.1% (median) higher than background values. This work selects samples with abnormally high porosity (AHP), samples with normal porosity (NP) and samples with abnormally low porosity (ALP) and compares their differences in physical properties (porosities, permeabilities and thin section porosities), diagenetic minerals, pore structures, compaction and cementation, detrital compositions and grain texture in an attempt to find the real factors controlling this heterogeneity. An effective export of dissolution by-products (Al3+ and Si4+) from AHP to NP and ALP leads to 2.01% (average) or 2.11% (median) newly created porosity in AHP, which is still significantly lower than the porosity difference between AHP and NP. In comparison, compaction reduction and mechanisms beneficial for compaction reduction should be the major factors controlling AHP formation. The content and distribution pattern of early carbonate cements, compared to the sedimentary factors (grain size and rigid grains), have significant influences on compaction reduction. Only samples with moderate contents (0.5–3.0%) of uniformly point-distributed early carbonate cements and with effective export of dissolution by-products can form AHP. Three formation models for ALP and two formation models for NP are also proposed. In summary, the evolution of porosity/permeability is controlled by early carbonate cements and induced compaction reduction (major causes) and by feldspar dissolution and redistribution of by-products (minor causes). This work provides new constraints on the formation of AHP and heterogeneity and is therefore important for sandstone reservoir quality prediction.

Experimental Investigation of the Impacts of Fracturing Fluid on the Evolution of Fluid Composition and Shale Characteristics: A Case Study of the Niutitang Shale in Hunan Province, South China

Hydraulic fracturing is a widely used technique for oil and gas extraction from ultra-low porosity and permeability shale reservoirs. During the hydraulic fracturing process, large amounts of water along with specific chemical additives are injected into the shale reservoirs, causing a series of reactions the influence the fluid composition and shale characteristics. This paper is focused on the investigation of the geochemical reactions between shale and fracturing fluid by conducting comparative experiments on different samples at different time scales. By tracking the temporal changes of fluid composition and shale characteristics, we identify the key geochemical reactions during the experiments. The preliminary results show that the dissolution of the relatively unstable minerals in shale, including feldspar, pyrite and carbonate minerals, occurred quickly. During the process of mineral dissolution, a large number of metal elements, such as U, Pb, Ba, Sr, etc., are released, which makes the fluid highly polluted. The fluid–rock reactions also generate many pores, which are mainly caused by dissolution of feldspar and calcite, and potentially can enhance the extraction of shale gas. However, precipitation of secondary minerals like Fe-(oxy) hydroxides and CaSO4 were also observed in our experiments, which on the one hand can restrict the migration of metal elements by adsorption or co-precipitation and on the other hand can occlude the pores, therefore influencing the recovery of hydrocarbon. The different results between the experiments of different samples revealed that mineralogical texture and composition strongly affect the fluid-rock reactions. Therefore, the identification of the shale mineralogical characteristics is essential to formulate fracturing fluid with the lowest chemical reactivity to avoid the contamination released by flowback waters.

Experimental alteration of granitic rocks: Implications for the evolution of geothermal brines in the Upper Rhine Graben, Germany

Geothermal energy exploitation in the Upper Rhine Graben (URG) chiefly has targeted faults and fractures within or connected with the crystalline basement, where hot fluids of c. 200 °C circulate at depths of c. 5 km. Formation fluids of the crystalline basement are highly saline, NaCl-dominated brines, whereas shallow crystalline basement water (< 500 m) is only weakly mineralized. In order to contribute to a better understanding of redistribution of elements in fluids and alteration phases during fluid-rock interaction processes we conducted leaching and alteration experiments with granites and monzonites from the Schwarzwald at 70 °C (rock powder, pure water) and 200 °C (texturally intact rock cylinders, 2m-NaCl), respectively, inspired by the P-T-x conditions reported for the URG.The trace element concentrations of the leachates are hereby related to the composition and stability of minerals in the rocks and can be directly linked to the proposed and observed dissolution processes at the different temperatures. In experiments with pure water at 70 °C, representing the recharge or infiltration conditions, water-rock interaction mobilized K, Cl, Fe, Mn, Mg, Cu, Zn, Cr, Sc, V, and Li (dissolution of biotite/chlorite) and Si, Al, K, Ca, Na, Ba, Sc, Pb, Sr, Rb, and Eu (dissolution of plagioclase, K-feldspar, and quartz) into the fluid. According to calculated activity diagrams, alteration at these temperatures may result in the formation of illite, kaolinite, Ca-beidellite, and stilbite. In experiments at higher temperatures of 200 °C, according to reservoir conditions, the net output of most elements was increased, but especially of those contained in feldspar, i.e. K, Ca, Al, Si, Ba, Rb, Cs, and Pb, suggesting that fluid-rock interaction is dominated by feldspar dissolution. This alteration is followed by albitization of feldspar surfaces, chloritization of biotite, and precipitation of illite and hematite. In Si-poor samples, analcime occurs in and around porous reaction zones, in agreement with calculated activity diagrams.The amount of dissolution mainly depends on the temperature and the mineralogy of the rock types, in particular on the anorthite-component of plagioclase. Certain elements (e.g., Na, Fe, Mg, Al) were subsequently incorporated into the solids precipitated during the experiments and thus cannot be considered as conservative.

Towards a global quantification of volcanogenic aluminosilicate alteration rates through the mass balance of strontium in marine sediments

Despite the important role that volcanogenic aluminosilicate (VA) alteration has on elemental cycles in marine sediments, there is no mechanism to arrive at a global assessment of this process. To quantify the VA alteration rates from Japan, New Zealand (NZ), and Costa Rica, we develop a mass balance approach that is constrained by the strontium concentrations and 87Sr/86Sr ratios in pore fluids, authigenic carbonates, and VAs. We derive VA alteration rates ranging from 101 to 103 nmole Sr/m3 bulk sediment/yr with the highest rate obtained for Tuaheni, NZ (Site GeoB20802), which has the youngest sediment. We show that 87Sr/86Sr ratios of VA derived from this mass balance approach are significantly higher than the reported ratios from volcanic glass samples, indicating a concomitant role of feldspar dissolution and/or authigenic clay formation. Most of the strontium released during VA alteration is precipitated as authigenic carbonates, with important implications for carbon inventories. The VA alteration rates derived from this approach can also be used to quantify the release of other critical elements, such as iron that can stimulate formation of Fe‑carbonates and/or fuel microbial activity at depth.

Linking the influence of diagenetic properties and clay texture on reservoir quality in sandstones from NW Borneo

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

Diagenesis and burial history modeling of heterogeneous marginal marine to shoreface Paleocene glauconitic sandstones, Taranaki Basin, New Zealand

ABSTRACTPaleocene marginal marine to shoreface glauconitic sandstones (F-Sands) of the Farewell Formation from the Maui Field in Taranaki Basin, New Zealand, demonstrate a diagenetic evolution driven by major shifts in acidic pore-water composition, rate of burial, and clay-mineral authigenesis. Mechanical compaction is the principal porosity-reducing mechanism during the first 2500 m of burial of the F-Sands. Continued mechanical compaction with long-grain contacts, concavo-convex contacts, and deformed liable grains are common throughout the F-Sands. Late-stage flow of dissolved CO2 in the pore fluids of the Farewell Formation is thought to have been generated from thermal decarboxylation of coaly source rocks. The circulation of these CO2-rich fluids will have dissolved into undersaturated pore fluids and partially catalyzed dissolution of feldspar and quartz, producing ions for the precipitation of kaolinite and chlorite. Timing of the diagenetic reactions, as determined using paragenetic observations, fluid-inclusion analysis, and burial history modeling, suggests that the quartz cements formed at a late stage (&amp;gt; 100°C, corresponding to 0–7 Ma) and is consistent with the migration of hydrocarbons, and associated CO2, into the F-Sand reservoir. Significant secondary porosity is generated through the dissolution of feldspar, which is preserved due to late-stage of occurrence at close to present-day maximum burial. Dissolved solutes in the F-Sands sandstones are being preferentially precipitated in interbedded and surrounding fine-grained heterolithic siltstone to very fine-grained sandstone beds, leading to enhanced heterogeneity and preservation of secondary porosity. This study provides an improved understanding for diagenetic reconstruction of marginal marine to shoreface facies.

Controls on abnormally high porosity in a deep burial reservoir of the Lingshui Formation in the Qiongdongnan Basin, South China Sea

Based on information including porosity and permeability, petrography, the stable isotopic composition of carbonate cements, and homogenization temperatures of aqueous fluid inclusions, we have studied the main factors for the development of abnormally high porosity in the Lingshui Formation reservoir of the Yacheng area. We found the sandstones were mainly subarkose, arkose, and lithic arkose and were texturally and compositionally immature. The research suggested that the sandstones existing close beneath the regional unconformity were formed during the Late Oligocene. Early diagenetic calcite cements leached to form intergranular secondary pores without the precipitation of late-diagenetic calcite cements in most sandstones. The isotopic composition of carbonate cements suggested a significant incursion of meteoric freshwater in the sandstones. Early diagenetic meteoric freshwater leaching reactions provided favorable conduits for the penetration of organic acids during the later period. Thermal fluid activities allowed source rocks to mature rapidly; therefore, the organic acid generation period was extended and feldspars were corroded to form abundant intragranular secondary pores. The abundant corroded minerals and the small amounts of associated authigenic minerals suggested that the dissolution of minerals most likely occurred in an open geochemical system. The dissolution of feldspars and calcite minerals generated an enhanced secondary porosity of approximately 9%–13% in thin sections of these sandstones.

Petroleum emplacement inhibits quartz cementation and feldspar dissolution in a deeply buried sandstone

Whether the emplacement of petroleum in sandstone reservoirs can preserve porosity during burial remains controversial. In the Kessog Field, UK Central North Sea, average porosities of the crestal sections of the fluvial-deltaic Pentland Formation reservoir can exceed 25% despite burial to 4 km or more. The predicted porosity for the reservoir at this depth is only around 14% based on regional data. Oil saturation data, thin-section point counts, grain-size and sorting measurements, reservoir pressure, and SEM images were combined to analyze the cause of the high reservoir porosity. Petroleum emplacement preventing cementation is the most likely mechanism for porosity preservation. Facies variation is not responsible, as the high-porosity sandstones from the crestal well are, in terms of average grain-size (fine-grained) and sorting coefficient (moderately well-sorted), nearly the same as the lower porosity sandstones from the flanks of the field (average porosity 13–15%). Other potential porosity-preservation mechanisms, such as overpressure, grain-coats and feldspar dissolution can be discounted. The sandstones with high oil saturations are characterized by: 1) most porosity being primary as opposed to secondary; 2) there being 2–5% less quartz cement than in the water-saturated sandstones; 3) there being 2–3% more K-feldspar and 2–6% less kaolin than the water-saturated counterparts. This study demonstrates that petroleum emplacement can effectively inhibit quartz cementation and K-feldspar transformation to kaolin in sandstone reservoirs.