Siliceous Cement Research Articles

Very early diagenesis in a calcareous, organic-rich mudrock from Jordan

The diagenesis in the organic-rich Cretaceous to Eocene Al Hisa Phosphorite Formation (AHP), Muwaqqar Chalk Marl Formation (MCM) and Umm Rijam Chert-Limestone Formation (URC) formations of Jordan can be linked directly to the fluctuating sedimentary environment of this shelf depositional system in the Middle to Late Eocene, and its influence on the composition of the deposited sediment and the early burial diagenetic environment. Most cementation was early, mostly within the first 10 m of burial, perhaps entirely within the first 100 m of burial. We propose that the siliceous cements are derived from biogenic silica, probably of diatoms, deposited in a shelf of enhanced productivity. Volumetrically, the most important processes were the redistribution of biogenic opal-A (diatoms) and calcite to form pervasive, layered and nodular cements. The formation of the silica and carbonate cements is closely linked through the effects their dissolution and precipitation have on pore fluid chemistry and pH. The chert beds have a biogenic silica origin, formed through replacement of diatoms and radiolaria by opal-CT, and subsequently by quartz. Calcite cement has carbonate derived from microbial diagenesis of organic matter and calcium derived from seawater. The Mg for early dolomite may have been generated by replacement of opal-CT by quartz, ore dissolution of unstable high Mg calcite bioclasts. The silica and carbonate diagenetic processes are both linked to microbial diagenesis of organic matter, and are intimately linked in both time and space, with pH possibly influencing whether a silica or a carbonate mineral precipitates. The paucity of metal cations capable of precipitating as sulphides is crucial to the creation of acidic pore water favourable to silica precipitation, either as opal-CT, chalcedony or quartz. The lack of clay minerals as a sink for the Mg required for opal-CT polymerisation is the principal factor responsible for the remarkably early silica cementation. All the diagenetic processes, with the probable exception of the opal-CT to quartz transition are early, almost certainly within the first 10 m of burial, possibly much less. A paragenetic sequence is presented here based on these two cores that should be tested against a wider core distribution to see whether this diagenetic history can be generalised throughout the basin. Warm bottom water temperatures probably led to silica diagenesis at much shallower burial depths than occurs in many other sedimentary basins. Silicified layers, in turn, commonly host fractures, suggesting that mechanical properties of the strata began to differentiate at a very early stage in the burial cycle. This has wide implications for processes linking diagenesis to deformation.

Siliceous Cementation of Chlorite-Coated Grains in the Permian Sandstone Gas Reservoirs, Ordos Basin

Siliceous Cementation of Chlorite-Coated Grains in the Permian Sandstone Gas Reservoirs, Ordos Basin

Use of Modern Reservoir Characterization in Mature Fields To Unravel Hidden Reserves

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 177195, “Application of Modern Reservoir Characterization in Mature Fields To Unravel Hidden Reserves,” by M. Rodriguez, F. Abad, L. Rodriguez, and J. Gaibor, Halliburton, and M. Moran and A. Verdezoto, Petroamazonas, prepared for the 2015 SPE Latin American and Caribbean Petroleum Engineering Conference, Quito, Ecuador, 18–20 November. The paper has not been peer reviewed. This paper discusses the successful, fully integrated, 3D Earth modeling (EM) and dynamic reservoir simulation of the Hollin Formation in the Lago Agrio Field within an operator’s first producer field in Ecuador. Early feedback of reservoir heterogeneity helped in understanding key reservoir issues and developing the methods undertaken to solve various problems. The implementation of cutting-edge reservoir-modeling techniques contributed to improving production rates and enhancing recovery. Available Data Lago Agrio has features of a mature field: more than 43 years of production history, 75% of extracted official re-serves P1 and P2 (quantified by the operating company), and a declination from plateau production of 31%, along with a steady decline of the field. New drilling and the workover of existing wells during the reservoir productive life have not been able to increase production. Abundant data exist from a number of wells in the Lago Agrio Field, such as from conventional-log-suite core analysis, pressure/volume/temperature fluid tests, pressure buildup, and a broad production history. Discussion and Results Sedimentological Model. Several authors define three depositional environments for the sediments of the Hollin Formation: fluvial, tidal-dominated, and shallow marine. How-ever, the thickness and lithofacies of these intervals vary from field to field. A sedimentological study was performed with core data, core descriptions, bore-hole well logs, and general knowledge of the Oriente Basin. Petrophysical-Properties Modeling. Diagenetic Analysis. X-ray diffraction and scanning-electron- microscope (SEM) data allow analysis of the mineralogical features in the fluvial, tidal, and marine intervals of the Hollin formation. In the fluvial/tidal Hollin interval, it is observed that quartz is the predominant mineral, followed by clay minerals; calcite and illite minerals were observed toward the top. The most common clay mineral is vermicular kaolinite. Siliceous cement was observed in patches. Toward the top, evidence of calcareous cement is shown. Meanwhile, in the marine Hollin interval, the predominance of glauconite and a significant percentage of ankerite are observed. The existing differential diagenesis between the fluvial/tidal and marine units markedly affects the textural characteristics of the rock and the relationship of flow/storage capacity; therefore, it becomes extremely important to model permeability and rock types (RTs) separately for these units. Porosity Correction. Porosity measurements on core plugs were conducted by various methods and at various pressure confinements. This generated the need to standardize the information before it was used to compare with logs and create models of RTs. In the first in-stance, net confinement pressures were calculated for each reservoir. Then, with the help of the rock- compressibility analysis from a neighboring analog field, the pore volumes were normalized, obtaining porosity changes per each confining-pressure increase. Once core porosity was corrected, the log-core calibration was achieved. The best porosity estimation is given by density neutron because it evidences clay content and calcareous cement correctly.

Influence of Temperature and Added Lime on the Glassy Phase Dissolution in Low-calcium Fly ash Binary Blend

The influence of temperature and added lime on the dissolution of the glassy phase in a binary blend of low-calcium siliceous fly ash and cement. An XRD-based technique is used for quantification of the glassy phase content and the amorphous reaction products in the binary blend. The experimental technique allows for tracking the formation of amorphous reaction products, the availability of lime and the unreacted glassy content. The rate limiting step in the pozzolanic reaction of low calcium fly ash is established to be the dissolution of its glassy phase. Results indicate that increasing the temperature from 25℃ to 40℃ produces a significant increase in the rate of dissolution of the glassy phase of fly ash and an increase in the rate of formation of reaction products. Addition of quicklime is very effective in producing reactive lime in the solution. The rate of dissolution of the glassy phase of fly ash and the rate of pozzolanic reaction are not significantly influenced by the lime content in the system. The proportion of fly ash which remains unreacted which is indicated by the undissolved glassy phase depends on the availability of lime in the system.

Minerals in the Triassic carbonaceous silicites of Sikhote Alin

Over 60 minerals, including native elements, intermetallic compounds, haloids, sulfides, sulfates, arsenides, oxides and hydroxides, silicates, borosilicates, wolframates, phosphates and REE phosphates, were established in Triassic siliceous rocks of Sikhote Alin. Allothigenic and authigenic minerals in the carbonaceous silicites were formed over a long period through several stages. Judging from morphology, chemical composition, and structural position, K-feldspar (K-Fsp), illite, kaolinite, metahalloysite, monazite, xenotime, zircon, rutile, or its polymorphs are the disintegration products of sialic rocks of continental crust. Authigenic sulfides are dominated by diagenetic pyrite (fine-crystalline, microglobular, framboidal, as well as those developed after biogenic siliceous and carbonate fragments), which has been formed prior to precipitation of siliceous cement and lithification of siliceous rocks. Most of other sulfides (sphalerite, galena, chalcopyrite, pyrrhotite, argentite, pentlandite, antimonite, ulmanite, and bravoite), arsenides and sulfoarsenides (arsenopyrite, nickeline, skutterudite, cobaltite, glaucodot, and gersdorffite), wolframates (scheelite and wolframite), intermetallides (Cu2Zn, Cu3Zn2, Cu3Zn, Cu4Zn, CuSn, Cu4Sn, Cu8Sn, Cu4Zn2Ni, Ni2Cu2Zn, Ni4Cd), and native elements (Au, Pd, Ag, Cu, Fe, W, Ni, Se) were crystallized later (during catagenesis after the lithification and brecciation of siliceous beds) from metals involved in the easily mobile fractions of bitumens. Supergene mineral formation was mainly expressed in the sulfide oxidation and replacement of diagenetic pyrite by jarosite and iron hydroxides.

Petrophysical and petrographic evaluation of Sidri Member of Belayim Formation, Badri field, Gulf of Suez, Egypt

Presence of sandstone streaks in Sidri Member within Belayim Formation that lies between two productive zones; Kareem Formation and Hammam Faraun Member, was the main reason to perform this study. It may represent a good hydrocarbon reservoir and will be added to the Egyptian oil production in some wells of Badri field. This Member has high resistivity signature on Electric-logs responses which attracted the attention to investigate its occurrence in the field, to delineate its distribution all-over the area, to evaluate the petrographic and petrophysical characteristics and to evaluate its productivity. Petrographic and petrophysical analyses of these sand zones were undertaken using thin section samples. The electric logs and subsurface geologic data was used to evaluate the main reservoir characteristics of the Sidri sandstone such as lithology, cementation, shale volume, porosity (Φ), effective porosity (Φ eff), estimated permeability (K), fluid saturation, fluid type and Net pay thickness.This study revealed that, Sidri sandstone facies was classified into two mainly sandy facies; blocky sandy facies which located at the northern part of the field and streaky sandy facies at the southern area of the field. These two facies are separated by shaley facies. Some wells were studied to represent the two sandy facies in Sidri Member and these sand intervals have not been tested yet. These sands consist of quartz grains with grey and pink feldspars as accessory minerals, with siliceous and calcareous cementation, with good porosity. Petrophysical evaluation of this sand unit indicated that it is hydrocarbon bearing formation in three wells and water bearing one in other wells. Electrical logs analysis (Resistivity, Density-Neutron, Sonic and Gamma-Ray) revealed that The volume of shale in this sandstone, the effective porosity, the water saturation, the estimated permeability, the hydrocarbon saturation, and the net-pay thickness are varying from 9 to 13%, 19%–22%, 26%–34%, 200 and 600 mD, 66 and 74%, 20–35 feet respectively. The fluid types are oil, gas and water.

Diagenetic differences of the Zhenzhuchong Member of Ziliujing Formation in the Jiulongshan–Jiange area, Sichuan Basin

The rocks of the Zhenzhuchong Member of the Ziliujing Formation, Jiulongshan–Jiange area in the Sichuan Basin, were analyzed by petrography, XRD, and SEM techniques to investigate their diagenetic history and properties, such as authigenic mineral types, evolution of mixed-layer illite–smectite minerals, the clay assembly, and the fraction of mixed-layer clay minerals. The results revealed that the Zhenzhuchong Member has experienced several important episodes of diagenetic alteration since the deposition, including compaction (pressure-solution), cementation, metasomatism, dissolution, fracturing, and infilling of caves and cracks. It was also observed that diagenetic properties of the Jiulongshan area were significantly different from those of the Jiange area. The rock samples from the Jiulongshan area were characterized by the composition of siliceous and calcareous cements, varying amounts of detrital grains, clay matrix and kaolinite replaced by calcites, a certain amount of rarely dissolved early-stage kaolinite, dickite, and infillings by late-stage calcite. On the other hand, for the rock samples from the Jiange area, the dissolution is a common phenomenon with features of abundant aluminosilicates-dissolution pores or components, but the replacement phenomenon has rarely been seen. These rock samples were characterized by the presence of clay mineral cements, quartz, and dolomite infillings. It indicated that there was a great difference of diagenesis between the two areas in the types, phase, and temperature of diagenetic fluids. Revealing the difference would provide theoretical and practical implications for the exploration of high quality oil and gas reservoirs.

Depositional and Diagenetic Controls on Sandstone Reservoirs with Low Porosity and Low Permeability in the Eastern Sulige Gas Field, China

In order to determine the genesis and the factors that control the low-porosity and low-permeability sandstone reservoirs in the eastern Sulige Gas Field in the Ordos Basin, systematic studies on the sedimentary facies and diagenesis were conducted by means of analysis of cores, thin sections, fluid inclusions, X-ray diffraction, cathode luminescence and scanning electron microscope. It was found that the sand bodies of the major gas reservoirs in the Shan1 section (P1S1) and the He8 section (P2H8) were formed during the Permian as sedimentary facies such as braided-channel bars, braided-river channels and point bars of a meandering river. Four types of diagenetic facies developed subsequently: in order from the best to the poorest properties these are type A (weak compaction, early calcite cement-chlorite film facies), type B (moderate compaction, quartz overgrowth-feldspar corrosion-kaolinite filling facies), type C (strong compaction, late calcite cement-quartz corrosion facies) and type D (matrix filling and strong compaction facies). This diagenesis is undoubtedly the main reason for the poor reservoir properties of sandstone reservoirs, but the sedimentary facies are the underlying factors that greatly affect the diagenesis and thus the reservoir performance. Favorable diagenetic facies developed mainly in relatively small lithofacies such as braided-river channels, channel bars and point bars. The vertical distribution of the physical properties and the diagenetic facies of the reservoirs are related to the stratigraphic succession. Most of the sandstones between mudstones and thin beds of sandstone are unfavorable diagenetic facies. Analyses indicate that siliceous cementation can hardly be stopped by hydrocarbon filling. Authigenic chlorite could hardly protect the primary porosity. It not only occupies pore space, but also blocks pathways through sandstone reservoirs, so that it has significant influence on the permeability. Authigenic chlorite cannot be used as a marker for a specific sedimentary facies because it can be formed in different sedimentary facies, but it indicates high hydrodynamic conditions and presence of favorable reservoirs.

Tectonic, sedimentary and metallogenic re-evaluation of basal strata in the Mesoproterozoic Bylot basins, Nunavut, Canada: Are unconformity-type uranium concentrations a realistic expectation?

The Mesoproterozoic Borden, Hunting-Aston, Fury and Hecla, and Thule basins, collectively known as the Bylot basins, are a series of sub-parallel extensional depressions associated with the Mackenzie igneous event at ∼1270Ma. In the Borden basin, basal strata of the Eqalulik Group are dominated by tholeiitic basaltic rocks and siliciclastic sandstones previously interpreted as predominantly terrestrial flows and fluvial deposits respectively. New field data indicate that thick basalt flows in the Nauyat Formation contain cryptic pillow structures and typically have fine-grained, fragmental tops. A subaqueous origin for these flows is supported by the presence of stromatolitic carbonate on the top of several flows, and the presence of peperite and sand-injectite structures at the base of others. Associated quartz arenite units are all of marine origin. Although much of the overlying Adams Sound Formation was previously interpreted as fluvial, there is little evidence for this, except locally at the base of the succession near Paquet Bay. Paleocurrents do not exhibit a strong unimodal pattern to the northwest, as indicated in earlier work, but reflect a less restricted basin, with paleoflow oblique to later graben margins. Most of the Adams Sound Formation was deposited on a tide- and storm-dominated shelf, with local development of large sand-wave complexes. A similar, marine-dominated setting can be demonstrated for basal terrigenous clastic units in both the Hunting-Aston, and Fury and Hecla basins. The latter contains thin sheets of fluvial strata only in the Nyeboe Formation on the south side of the basin, and in the basal 3m of the Whyte Inlet Formation on the northeast margin of the basin. Paleocurrent patterns in all three basins do not reflect confinement between the walls of restricted grabens. This suggests that the early development of the basins may have been related to transtensional extension rather than aulacogen development.Although uranium, present in basement granites north of the Fury and Hecla Strait, may have been scavenged by fluids well after lithification of the Mesoproterozoic basin fill to produce local vein-type uranium deposits, the potential for extensive unconformity-type deposits, analogous those in the Athabasca basin, is exceptionally low. Most of the sandstone units have early siliceous cement, and low initial feldspar content, both of which would have inhibited development of extensive secondary porosity.

Characteristics of the Permian coal-formed gas sandstone reservoirs in Bohai Bay Basin and the adjacent areas, North China

This paper focuses on the physical properties and spatial distributions of the Permian sandstone reservoirs in Bohai Bay Basin and the adjacent areas in the northeast China. The coal-formed gas sandstone reservoirs occur in the Permian Shihezi Formation and the maturity of the sandstone in the lower part is gradually higher than that in the upper part. The high-maturity sandstones are mainly the channel lag deposits and widely distributed in the study area. Due to the low porosity and permeability, however, the sandstones formed only the medium-poor reservoirs. The sandstones contain clay matrix and commonly a small portion of calcareous, siliceous, and authigenic clay cement. On the other hand, severe compaction and cementation caused nearly complete destruction of the primary porosity during diagenesis. Therefore, the main space of the sandstone reservoirs comes from the secondary pores. The distributions of reservoirs are affected by several periods of faulting and denudation. The sandstones of the Lower Shihezi Formation are thicker in the Jizhong and Bozhong depressions, and Luxi areas, and those of the Upper Shihezi Formation are thicker in Jizhong and Jiyang depressions and thinner in Luxi areas.

Correlation between total water absorption and wet compressive strength of compressed stabilised peat bricks

Housing is a great problem in today and future world, the most basic building material for infrastructure construction and houses is the usual bricks or blocks. Although there are approximately 2,500 000 hectares of peat soils in Malaysia, only a small portion of this as being utilised. In this study, peat soil was investigated in tandem with the production of lightweight bricks. Peat soil, sand, cement and hydrated lime mixtures were steam autoclaved under different test conditions to produce brick samples. The purpose for this study was undertaken on the production of compressed peat ‐ siliceous sand, PFA, OPC cement and lime as solid bricks to solving the problems of housing and builds the construction economically and environmentally by utilising local materials. The compressive strength and water absorption of stabilised peat bricks obtained under 6 and 10 Mpa pressure are 5.48 and 7.10 Mpa; 4.75 and 2.6%, respectively. Tests were also conducted to study correlation between wet compressive strength and water absorption; it was found negative values, increase in strength decreased water absorption and hardening of the bricks with time.

Engineering properties of cemented peat bricks

Housing is a great problem in today’s world and the world future. The most basic building material for infrastructure construction and housing is the usual brick or blocks. Conventional bricks, which are burnt clay, create environmental problems. The purpose for this study was undertaken on the production of peat; siliceous sand, composite cement and lime solid bricks to solve the economic problems of housing and construction building by utilizing local materials. The compressive strength, water absorption and density of these bricks were investigated. It was observed that these bricks had sufficient strength for their use in low cost housing development. Tests were also conducted to study the influence of curing type on the increase in strength and hardening of the bricks with time. Key words: Peat, PFA cement, lime, siliceous sand.

The earliest Foraminifera from southern Shaanxi, China

Vase-shaped microfossils (VSMs) described herein mainly occur as isolated individuals in thin bedded siltstone and silty carbonate of the Gaojiashan Member of the upper Ediacaran Dengying Formation (ca. 551–541 Ma). Although these fossils are abundant, chained tests or other types of colonial aggregates have not been observed. Specimens in the siltstones can easily be isolated from the host rocks by ultrasonic vibrators. Compared with the co-occurring fossils Gaojiashania and Conotubus, VSMs are rarely pyritized, yet they are always three-dimensionally persevered with little deformation, suggesting that their tests were sturdy and possibly mineralized. Petrological observation and elemental mapping reveal two types of tests that are respectively calcareous and siliceous in composition. Calcareous tests typically consist of two to three crypto-crystal laminae, somewhat resembling bilamellar walls of foraminifers. Siliceous tests consist of fine-grained particles agglutinated with siliceous cement, similar to agglutinated walls of foraminifers. The Gaojiashan VSMs are broadly similar, at least in gross morphology, to the testate amoebae-like VSMs, but their relative large sizes (600–2400 μm) and possibly mineralized (rather than organic) tests argue against this comparison. They also show some similarities to other protozoans, especially tintinnids. However, tintinnids have robust pesudochitinous loricae consisting of both secreted and agglutinated materials. Moreover, tintinnid loricae differ in shape from the Gaojiashan VSM tests in having a constricted aboral end (sometimes with a caudal appendix) and a flaring oral opening. If the Gaojiashan VSMs are indeed related to foraminifers, they indicate that foraminifers were important players in late Ediacaran communities.

Quantitative research of diagenesis: its effect on pore evolution of the Fuyu oil reservoir in the north Qijia region

From observing cores of 18 wells, identifying 175 ordinary thin sections and 61 thin casting sections, energy spectrum analyses of 37 samples, homogenization temperature measurement of fluid inclusions of 11 samples, we determine the types of diagenesis and pores of the Fuyu oil reservoir in the north Qijia region. We classified the pores and measured their plane porosity using CIA 2000, the software of rock image analysis, calculated the effect of different kinds of diagenesis on porosity, studied the controlling actions of diagenesis to pore evolution quantitatively, combined with burial history, thermal history and their diagenetic environments. Our results show that mechanical compaction and carbonate cementation are the major destructive diagenesed, developed during early diagenesis stages. The reduction in porosity by mechanical compaction and carbonate cementation are about 25% and 8%, while the destructive intensity of siliceous cementation and clay mineral cementation is relatively much smaller, i.e., the reduction of porosity is about 2% and 0.2% Dissolution is constructive diagenesis, the increment of porosity is about 6%. There are four diagenesis evolution stages, during which the porosity reduced from 30%∼38% to 2%∼20%. Mechanical compaction and early cementation are the main diageneses in the early diagenesis stages, when porosity was reduced to 2%∼10%. Dissolution is the main diagenesis of an A I substage of the middle diagenesis stage, when porosity increased 1%∼8%. The dissolution of the A II substage of the middle diagenesis stage affected by late cementation, raised porosity 1%∼5%. The porosity varied slightly during the middle stage B.

Groundwater geochemical evolution in the northern portion of the Guarani Aquifer System (Brazil) and its relationship to diagenetic features

The groundwater flow pattern in the northern portion of GAS (Guarani Aquifer System) is characterized by the existence of four regional recharge areas located in São Paulo, Mato Grosso do Sul and Goiás states. From these areas of recharge the regional flow is radial and directed toward the center of the Paraná Sedimentary Basin. Local discharge occurs in portions of outcrop regions. The groundwater has low mineralization and can be classified as Ca or Ca–Mg–HCO3 type, Na–HCO3 type and Na–HCO3/Cl/SO4 type, this sequence represents the hydrochemical evolution. The mechanisms responsible for this evolution are dissolution of feldspars and removal of the carbonate cement from the sandstone mineral framework, followed by ion exchange, responsible for the increase in the Na concentration and decrease of Ca, and, finally, enrichment in Cl and SO4 derived from underlying aquifer units. The hydrochemical evolution is consistent with diagenetic features that are observed in the sandstones, with the presence of siliceous cement in the outcrop areas, and carbonate cement toward the center of Paraná Basin.

Microstructures in the Cretaceous Bima Sandstone, Upper Benue Trough, N.E. Nigeria: Implication for hydrocarbon migration

Faulting related to movements along major fault zones in the Upper Benue Trough during Albian times, with evidence of deformation in the Cretaceous Bima Sandstone are common especially around the Kaltungo, Gombe, Zambuk and Teli lineaments. Conjugate extensional systems of deformation bands show increased siliceous cementation of the sandstones adjacent to these lineaments. During the Late Cretaceous compressional event, the deformation bands and faults in the Upper Benue Trough were reactivated, resulting into dilational opening of fractures believed to have acted as fluid conduits and/or barriers. These deformation bands which decrease in density away from the major faults are characterized with increasing porosity and permeability in the host sandstone abruptly away from the tectonic barrier. It is proposed here that the master faults of the Benue Trough, linking it with the Anambra Basin and the Niger Delta probably served as conduits for the migration of hydrocarbons into the Cretaceous reservoirs of the Upper Benue Trough and by extension into the Niger Delta.

Análise e Avaliação dos Problemas Existentes na Fachada de Arenito do Museu Júlio de Castilhos, Porto Alegre, RS

The Júlio de Castilhos museum of Porto Alegre, formerly used as residence, was built in 1877 by a military engineer Catão Roxo. This building is one of the few examples of urban architecture from the last part of the nineteenth century. Its façade is covered by arenitic and granitic rocks quarried near the city. The covering shows the effects of weathering processes occurred during many years. Some of these were observed during a façade mapping as loss of rocky materials, discoloration, chemical deposition, desintegration and fracturing. Petrographic analyses of the arenite indicate an arcosean composition, rich in quartz, with plagioclase and alkali-feldspar (microcline) and opaque minerals in lesser amounts. The grains have a fine layer of iron oxide (hematite) clad, with a fringe of siliceous cement. The remained pores are filled by clay minerals, mainly illite. The elevated degree of compactation and diagenesis are responsible for the high coherence of the rock, allowing its use as covering rock. The petrographic features (textures and mineralogical composition) indicate that the arenites used on the façade’s museum, comes from the Botucatu Formation. The recognition of the mineral constituents and the knowledge of regional geology allow the façade’s recuperation through the replacing of unrecoverable pieces by new ones.

Relationships among mechanical, physical and petrographic properties of Siwalik sandstones, Central Nepal Sub-Himalayas

Mechanical and physical characteristics of rocks generally depend on their composition and texture, because they are reflective of their environments during sedimentation, diagenesis and weathering in each location. To understand this, relationships between mechanical and physical indices and composition and texture have been analyzed using several rock samples obtained from Mio-Pleistocene sandstone sequence of the Siwalik Group, widely distributed in foothills of the Nepal Himalayas. Sandstones are quartzo-feldspathic to quartzo-lithic and quartzo-feldspathilithic. Cement, matrix and void vary from 6 to 41%, 0 to 18% and 1 to 12%, respectively. Ferroan calcitic and siliceous cements are the dominant cements, and their ratio to total cement varies from 0.2 to 1.0. Packing density of sandstones ranges from 59 to 95%. Clasts are loosely to closely packed and are weakly to strongly bonded by cement. Results of correlation analyses show that sandstones strongly depend on several indices, that is percent void, strong cement over total cement, strong over weak contacts, concavo–convex contacts and packing density, most of which are measured on thin sections. Contrarily, they depend little on grain size, shape, mineralogical maturity index and the other petrographic indices. Increased interlocking and strong cementing with reduced void tends to possess high densities, low porosity and offer good resistance against deformation. Therefore, proportion of strong cement over total cement, void content, strong over weak contacts, grain-to-void contact, concavo–convex contact and packing density are meaningful indices considering the sandstones of the Siwalik Group. Based on the multiple regression, correlation analyses and then stepwise multiple regression analysis, some regression equations with significant explanatory indices are indicated for estimating mechanical and physical properties. In other words, such relationships may be useful in estimating mechanical and physical properties using the significant petrographic indices.

A Shelduck (Anatidae: Tadorna) from the Pliocene of South Australia

SummaryA complete fossil humerus referable to a shelduck (Anatidae: Tadorna sp.) is reported from an uncertain locality in the ‘Bookmark cliffs’ on the River Murray, a few kilometres upstream from Berri and Lyrup southwest of Renmark, in the Riverland region of South Australia. The fossil was embedded in a piece of pale grey, clean coarse sandstone having a siliceous cement. This is interpreted as being Parilla Sand, of late Pliocene age, which outcrops along the Murray River from Renmark to Loxton.

Another continental pool in the terrestrial silicon cycle

Silicon is the second most abundant element on Earth. It is an important nutrient for phytoplankton and is readily absorbed by terrestrial vegetation; it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates. But the continental cycle of silicon is not well known, and only a few studies have attempted to use silicon stable isotopes (28Si, 29Si and 30Si) to quantify the continental silicon reservoirs. Dissolved silicon in sea and river waters forms a reservoir of mean isotopic value +1.1 per thousand (refs 7, 10). It is enriched in 30Si with respect to the igneous rocks reservoir, which has a mean isotopic value of -0.3 per thousand (refs 4, 9). This enrichment can only be produced by a major fractionation during weathering, and should result in the formation of a continental 30Si-depleted reservoir. Such a reservoir, however, has not been identified to date. Here we analyse silicon isotopes of in situ quartz from a sandstone series in France, using a new-generation secondary ion mass spectrometry apparatus. We show that quartz that precipitates as siliceous cements forms a strongly 30Si-depleted reservoir with isotopic values down to -5.7 per thousand, a more negative value than any previously published for terrestrial samples. Our findings suggest that quartz re-precipitation plays an important role in the biogeochemical cycle of silicon.