Articles de revues : « Cementation (Petrology) »

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Olsen, Casper, Thanong Hongdul et Ida Lykke Fabricius. « Prediction of Archie’s cementation factor from porosity and permeability through specific surface ». GEOPHYSICS 73, n^o 2 (mars 2008) : E81—E87. http://dx.doi.org/10.1190/1.2837303.

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Based on Archie’s cementation factor as measured on North Sea chalk and on published data, we explore how the cementation factor depends on other physical properties of the chalk. A relationship between cementation factor and specific surface with respect to bulk volume is obtained for chalk. This leads to how Archie’s cementation factor may be predicted from porosity and permeability for chalk as well as for sandstone. A common relationship between cementation factor and specific surface with respect to bulk volume calculated from porosity and permeability is obtained. As an alternative, the [Formula: see text]-factor in Archie’s equation is related to specific surface with respect to bulk volume, but the relationship is less clear than the simple relationship between cementation factor and specific surface.

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Han, Tongcheng, Zhoutuo Wei et Li-Yun Fu. « Cementation exponent as a geometric factor for the elastic properties of granular rocks ». GEOPHYSICS 85, n^o 6 (1 novembre 2020) : MR341—MR349. http://dx.doi.org/10.1190/geo2020-0250.1.

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A geometric factor properly describing the microstructure of a rock is compulsory for effective medium models to accurately predict the elastic and electrical rock properties, which, in turn, are of great importance for interpreting data acquired by seismic and electromagnetic surveys, two of the most important geophysical methods for understanding the earth. Despite the applications of cementation exponent for the successful modeling of electrical rock properties, however, there has been no demonstration of cementation exponent as the geometric factor for the elastic rock properties. We have developed a workflow to model the elastic properties of clean and normal granular rocks through the combination of effective medium modeling approaches using cementation exponent as the geometric factor. Based on the dedicated modeling approaches, we find that cementation exponent can be adequately used as a geometric factor for the elastic properties of granular rocks. Further results highlight the effects of cementation exponent on the elastic and joint elastic-electrical properties of granular rocks. The results illustrate the promise of cementation exponent as a geometric link for the joint elastic-electrical modeling to better characterize the earth through integrated seismic and electromagnetic surveys.

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Zhu, Haihua, Guangchen Liu, Dakang Zhong, Tingshan Zhang, Jun Lang et Jingli Yao. « Diagenetic controls on reservoir quality of tight sandstone : A case study of the Upper Triassic Yanchang formation Chang 7 sandstones, Ordos Basin, China ». Earth Sciences Research Journal 22, n^o 2 (1 avril 2018) : 129–38. http://dx.doi.org/10.15446/esrj.v22n2.72251.

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

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Al-Tahini, Ashraf M., Carl H. Sondergeld et Chandra S. Rai. « Effect of cementation on ultrasonic velocities in sandstones ». GEOPHYSICS 72, n^o 2 (mars 2007) : E53—E58. http://dx.doi.org/10.1190/1.2431327.

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We determine the acoustic velocities for samples cored from the Jauf and Unayzah sandstone formations of Saudi Arabia. We use microstructural analysis including thin sections and point counting to quantify cementation. Velocities in these formations are strongly controlled by the combination of porosity and cementation. Consequently, rocks of similar porosity but with different cementation materials display different velocities. The objective of this study is to understand the effect of cementation on the acoustic velocity. Cements in these two formations have variable compositions and properties. Pure quartz overgrowth plays a major role in increasing velocities while clay and clay coatings play a minor role. We found that clay coatings inhibit the quartz overgrowth cement leading to a decrease in velocities. Understanding the influence of various cementation types on velocity, and thus elastic properties in sandstone cores, enables an understanding of the variation of sonic velocities and moduli across these formations. The uniqueness of this study is that we emphasize the quantification of the role of cement and not just mineral volume.

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Iden, K. « Fracture Cementation in the North Sea ». Mineralogical Magazine 62A, n^o 2 (1998) : 675–76. http://dx.doi.org/10.1180/minmag.1998.62a.2.22.

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El-Sayed, Mahmoud Kh. « Beachrock cementation in Alexandria, Egypt ». Marine Geology 80, n^o 1-2 (avril 1988) : 29–35. http://dx.doi.org/10.1016/0025-3227(88)90070-9.

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Grude, Sissel, Jack Dvorkin et Martin Landrø. « Permeability variation with porosity, pore space geometry, and cement type : A case history from the Snøhvit field, the Barents Sea ». GEOPHYSICS 80, n^o 1 (1 janvier 2015) : D43—D49. http://dx.doi.org/10.1190/geo2014-0064.1.

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Laboratory permeability data from the brine-filled Tubåen Formation in the Snøhvit field show an order of magnitude permeability variation for approximately the same porosity. This variation in permeability is explained by a modified Kozeny-Carman equation that exploits the relationships among permeability, porosity, cementation, and pore geometry. The expression correlates the slope in a logarithmic plot of porosity versus permeability with the amount of contact cement and sorting, and the intercept with the grain size. Additional information about sorting and/or cementation can be used to better constrain the slope of the plot. Based on this equation, we found that the grain size and the amount of contact cement increased with depth in the lowermost Tubåen 1–3 sandstone units, this led to an increasing permeability with depth, in the same porosity range. The permeability variation in the shallowest Tubåen 4 sandstone unit was affected by sorting to a larger degree than the remaining Tubåen intervals, which influenced the cementation factor, porosity, and permeability simultaneously. These findings were supported by the depositional environment of the formation, a petrology study of grain size and sorting and a rock-physics study. The rock-physics study indicated that the samples with higher permeability had higher elastic moduli compared with the samples with lower permeability. This correlation between permeability and elastic moduli can be explained by the increasing amount of contact cement for the stiffer, high-permeability samples.

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Barker, D. S., et K. L. Milliken. « CEMENTATION OF THE FOOTPRINT TUFF, LAETOLI, TANZANIA ». Canadian Mineralogist 46, n^o 4 (1 août 2008) : 831–41. http://dx.doi.org/10.3749/canmin.46.4.831.

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Pedersen, T. « Fracture Cementation Mechanisms in the North Sea ». Mineralogical Magazine 62A, n^o 2 (1998) : 1149–50. http://dx.doi.org/10.1180/minmag.1998.62a.2.267.

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Riches, P., I. Traub-Sobott, W. Zimmerle et U. Zinkernagel. « Diagenetic peculiarities of potential Lower Jurassic reservoir sandstones, Troms 1 area, Off Northern Norway, and their tectonic significance ». Clay Minerals 21, n^o 4 (octobre 1986) : 565–84. http://dx.doi.org/10.1180/claymin.1986.021.4.09.

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AbstractIn the Troms 1 area, sediments of Early to Middle Jurassic age, ranging from alluvial plain deposits at the base, passing through coastal plain/tidal flat sediments up into high-energy nearshore shallow-marine sands, mark a transgression. The sandstones, classified as mineralogically and texturally mature quartz-arenites, are potential reservoir rocks in the eastern part of the area. The apparent supermaturity, however, is of secondary origin because unstable detrital components were dissolved during diagenesis. The succession of complex diagenetic processes was: (i) mechanical compaction and simultaneous pressure solution, (ii) partial dissolution with corrosion of detrital quartz and dissolution of unstable fragments, (iii) silica cementation, (iv) calcite cementation, (v) partial carbonate dissolution, (vi) kaolinite/Fe-carbonate cementation in the remaining pore space. Porosity and permeability of the sandstones are controlled by the degree of silicification and by dissolution processes. Two dissolution stages led to partial ‘skeletonization’ of the detrital framework and to elimination of unstable detrital grains. The first stage was a basic process leading to corrosion of detrital quartz and creating transitory secondary porosity; the second stage was acidic leading to the present preserved secondary porosity. Diagenetic dissolution channels formed. The degree of diagenetic alteration was much higher than normally observed in sandstones of such burial depth. Hydrothermal solutions rising from deep-seated faults may have led to this unusual alteration and triggered a rift-related type of complex diagenesis.

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Thornber, M. R., E. Bettenay et W. G. R. Russell. « A mechanism of aluminosilicate cementation to form a hardpan ». Geochimica et Cosmochimica Acta 51, n^o 9 (septembre 1987) : 2303–10. http://dx.doi.org/10.1016/0016-7037(87)90283-3.

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Platt, J. D. « Controls on Clay Mineral Distribution and Chemistry in the Early Permian Rotliegend of Germany ». Clay Minerals 28, n^o 3 (septembre 1993) : 393–416. http://dx.doi.org/10.1180/claymin.1993.028.3.05.

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AbstractAuthigenic clay minerals in the German Rotliegend formed mainly at burial depths >1.5 km. Illite is the dominant cement, although kaolinite, dickite and early radial chlorite are abundant locally. Illites contain more AI and late chlorites more AI and Fe in sequences showing extensive grain dissolution. This relationship between clay chemistry and grain dissolution suggests that clay cementation is linked to grain dissolution. Sequences at relatively shallow burial depths (<3-5 km) contain less clay cement. In the more deeply buried sections, increased illite and kandite cementation, together with extensive grain dissolution, is evident where the Rotliegend is juxtaposed against Carboniferous Coal Measures. Faults also acted as important conduits for acidic fluids. Illite and kandite growth occurred at similar depths and from waters of similar isotopic composition (618O = 1-6‰ SMOW) throughout most of the Southern Permian Basin. However, the timing of illite growth varied between areas and corresponded to periods to tectonic activity.

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Zha, Xiaojun, Fuqiang Lai, Xuanbo Gao, Yang Gao, Nan Jiang, Long Luo, Yingyan Li et al. « Characteristics and Genetic Mechanism of Pore Throat Structure of Shale Oil Reservoir in Saline Lake—A Case Study of Shale Oil of the Lucaogou Formation in Jimsar Sag, Junggar Basin ». Energies 14, n^o 24 (14 décembre 2021) : 8450. http://dx.doi.org/10.3390/en14248450.

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The shale oil reservoir of the Lucaogou Formation in the Jimsar Sag has undergone tectonic movement, regional deposition and complex diagenesis processes. Therefore, various reservoir space types and complex combination patterns of pores have developed, resulting in an intricate pore throat structure. The complex pore throat structure brings great challenges to the classification and evaluation of reservoirs and the efficient development of shale oil. The methods of scanning electron microscopy, high-pressure mercury injection, low-temperature adsorption experiments and thin-slice analysis were used in this study. Mineral, petrology, pore throat structure and evolution process characteristics of the shale oil reservoir were analyzed and discussed qualitatively and quantitatively. Based on these studies, the evolution characteristics and formation mechanisms of different pore throat structures were revealed, and four progressions were made. The reservoir space of the Lucaogou Formation is mainly composed of residual intergranular pores, dissolved pores, intercrystalline pores and fractures. Four types of pore throat structures in the shale oil reservoir of the Lucaogou Formation were quantitatively characterized. Furthermore, the primary pore throat structure was controlled by a sedimentary environment. The pores and throats were reduced and blocked by compaction and cementation, which deteriorates the physical properties of the reservoirs. However, the dissolution of early carbonate, feldspar and tuffaceous minerals and a small amount of carbonate cements by organic acids are the key factors to improve the pore throat structure of the reservoirs. The genetic evolution model of pore throat structures in the shale oil reservoir of the Lucaogou Formation are divided into two types. The large-pore medium-fine throat and medium-pore medium-throat reservoirs are mainly located in the delta front-shallow lake facies and are characterized by the diagenetic assemblage types of weak compaction–weak carbonate cementation–strong dissolution, early medium compaction–medium calcite and dolomite cementation–weak dissolution. The medium-pore fine throats and fine-pore fine throats are mainly developed in shallow lakes and semi-deep lakes. They are characterized by the diagenetic assemblage type of strong compaction–strong calcite cementation–weak dissolution diagenesis. This study provides a comprehensive understanding of the pore throat structure and the genetic mechanism of a complex shale oil reservoir and benefits the exploration and development of shale oil.

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Barnes, Heather, Johnny R. Hinojosa, Glenn A. Spinelli, Peter S. Mozley, Daniel Koning, Tyler G. Sproule et John L. Wilson. « Detecting fault zone characteristics and paleovalley incision using electrical resistivity : Loma Blanca Fault, New Mexico ». GEOPHYSICS 86, n^o 3 (27 avril 2021) : B209—B221. http://dx.doi.org/10.1190/geo2020-0375.1.

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We have combined electrical resistivity tomography (ERT), geologic information from boreholes and outcrops, and hydrogeologic data to investigate field-scale fault-zone cementation of the Loma Blanca Fault in the Rio Grande Rift. We have collected electrical resistivity data from 16 transects and geologic samples from 29 boreholes (completed as groundwater wells to 30 m depth) across and around the fault. The 2D ERT profiles, whose interpretations are constrained by geologic data, indicate (1) a high resistivity zone in cemented portions of the fault below the water table and (2) in the unsaturated zone, a low-resistivity feature along the cemented portions of the fault. The high-resistivity zone below the water table is consistent with a 10% reduction in porosity due to the fault zone cementation. With the same porosity in the unsaturated zone, the low-resistivity feature in the cemented fault zone is consistent with saturation >0.7, in contrast to saturation 0.2–0.7 for sediment outside of the cemented fault zone. In addition, subsurface samples and ERT profiles delineate a buttress unconformity (i.e., steeply dipping erosional contact) corresponding to a paleovalley margin. This unconformity truncates the cemented fault zone and separates Pliocene axial-fluvial sand (deposited by an ancestral Rio Grande) from late Quaternary sand and gravel (deposited by the Rio Salado, a Rio Grande tributary). The cemented fault zone in the southern portion of the study area is a hydrogeologic barrier; north of the buttress unconformity, where the cemented fault zone has been removed by erosion, the fault is not a hydrogeologic barrier. The integration of geologic, geophysical, and hydrogeologic observations is key to developing our understanding of this complex system, and it allows us to demonstrate the utility of ERT in detecting subsurface fault-zone cementation.

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Salomon, Eric, Atle Rotevatn, Thomas Berg Kristensen, Sten-Andreas Grundvåg, Gijs Allard Henstra, Anna Nele Meckler, Richard Albert et Axel Gerdes. « Fault-controlled fluid circulation and diagenesis along basin-bounding fault systems in rifts – insights from the East Greenland rift system ». Solid Earth 11, n^o 6 (5 novembre 2020) : 1987–2013. http://dx.doi.org/10.5194/se-11-1987-2020.

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Abstract. In marine rift basins, deep-water clastics (>200 m) in the hanging wall of rift- or basin-bounding fault systems are commonly juxtaposed against crystalline “basement” rocks in the footwall. A distinct feature of such fault systems is therefore the juxtaposition of relatively highly permeable, unconsolidated sediments against relatively low-permeable basement rocks. Due to limited surface exposure of such fault zones, studies elucidating their structure and evolution are rare. Consequently, their impact on fluid circulation and diagenesis within and proximal to the fault zone as well as into the hanging wall strata are also poorly understood. Motivated by this, we here investigate a well-exposed strand of a major basin-bounding fault system in the East Greenland rift system, namely the Dombjerg Fault which bounds the Wollaston Forland Basin, northeast (NE) Greenland. Here, syn-rift deep-water clastics of Late Jurassic to Early Cretaceous age are juxtaposed against Caledonian metamorphic basement. Previously, a ∼ 1 km wide zone of pervasive pore-filling calcite cementation of the hanging wall sediments along the Dombjerg Fault core was identified (Kristensen et al., 2016). In this study, based on U–Pb calcite dating, we show that cementation and formation of this cementation zone started during the rift climax in Berrisian–Valanginian times. Using clumped isotope analysis, we determined cement formation temperatures of ∼ 30–70 ∘C. The spread in the formation temperatures at similar formation age indicates variable heat flow of upward fluid circulation along the fault in the hanging wall sediments, which may root in permeability variations in the sediments. Calcite vein formation, postdating and affecting the cementation zone, clusters between ∼ 125 and 100 Ma in the post-rift stage, indicating that fracturing in the hanging wall is not directly related to the main phase of activity of the adjacent Dombjerg Fault. Vein formation temperatures at ∼ 30–80 ∘C are in a similar range as cement formation temperatures. Further, similar minor element concentrations of veins and adjacent cements indicate diffusional mass transfer into fractures, which in turn infers a subdued fluid circulation and low permeability of the fracture network. These results imply that the cementation zone formed a near-impermeable barrier soon after sediment deposition, and that low effective permeabilities were maintained in the cementation zone even after fracture formation, due to poor fracture connectivity. We argue that the existence of such a cementation zone should be considered in any assessments that target basin-bounding fault systems for, e.g., hydrocarbon, groundwater, geothermal energy, and carbon storage exploration. Our study highlights that the understanding of fluid flow properties as well as fault-controlled diagenesis affecting the fault itself and/or adjacent basinal clastics is of great fundamental and economic importance.

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Saul, Matthew, et David Lumley. « The combined effects of pressure and cementation on 4D seismic data ». GEOPHYSICS 80, n^o 2 (1 mars 2015) : WA135—WA148. http://dx.doi.org/10.1190/geo2014-0226.1.

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Time-lapse seismology has proven to be a useful method for monitoring reservoir fluid flow, identifying unproduced hydrocarbons and injected fluids, and improving overall reservoir management decisions. The large magnitudes of observed time-lapse seismic anomalies associated with strong pore pressure increases are sometimes not explainable by velocity-pressure relationships determined by fitting elastic theory to core data. This can lead to difficulties in interpreting time-lapse seismic data in terms of physically realizable changes in reservoir properties during injection. It is commonly assumed that certain geologic properties remain constant during fluid production/injection, including rock porosity and grain cementation. We have developed a new nonelastic method based on rock physics diagnostics to describe the pressure sensitivity of rock properties that includes changes in the grain contact cement, and we applied the method to a 4D seismic data example from offshore Australia. We found that water injection at high pore pressure may mechanically weaken the poorly consolidated reservoir sands in a nonelastic manner, allowing us to explain observed 4D seismic signals that are larger than can be predicted by elastic theory fits to the core data. A comparison of our new model with the observed 4D seismic response around a large water injector suggested a significant mechanical weakening of the reservoir rock, consistent with a decrease in the effective grain contact cement from 2.5% at the time/pressure of the preinjection baseline survey, to 0.75% at the time/pressure of the monitor survey. This approach may enable more accurate interpretations and future predictions of the 4D signal for subsequent monitor surveys and improve 4D feasibility and interpretation studies in other reservoirs with geomechanically similar rocks.

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Evans, J. « Quartz dissolution during shale diagenesis implications for quartz cementation in sandstones ». Chemical Geology 84, n^o 1-4 (juillet 1990) : 239–40. http://dx.doi.org/10.1016/0009-2541(90)90224-u.

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Egeberg, Per Kristian, et Girish C. Saigal. « North Sea chalk diagenesis : cementation of chalks and healing of fractures ». Chemical Geology 92, n^o 4 (octobre 1991) : 339–54. http://dx.doi.org/10.1016/0009-2541(91)90078-6.

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Aubert, Irène, Philippe Léonide, Juliette Lamarche et Roland Salardon. « Diagenetic evolution of fault zones in Urgonian microporous carbonates, impact on reservoir properties (Provence – southeast France) ». Solid Earth 11, n^o 4 (5 juillet 2020) : 1163–86. http://dx.doi.org/10.5194/se-11-1163-2020.

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Abstract. Microporous carbonate rocks form important reservoirs with permeability variability depending on sedimentary, structural, and diagenetic factors. Carbonates are very sensitive to fluid–rock interactions that lead to secondary diagenetic processes like cementation and dissolution capable of modifying the reservoir properties. Focusing on fault-related diagenesis, the aim of this study is to identify the impact of the fault zone on reservoir quality. This contribution focuses on two fault zones east of La Fare anticline (SE France) crosscutting Urgonian microporous carbonates. Overall, 122 collected samples along four transects orthogonal to fault strike were analyzed. Porosity values have been measured on 92 dry plugs. Diagenetic elements were determined through the observation of 92 thin sections using polarized light microscopy, cathodoluminescence, carbonate staining, SEM, and stable isotopic measurements (δ13C and δ18O). Eight different calcite cementation stages and two micrite micro-fabrics were identified. As a main result, this study highlights that the two fault zones acted as drains canalizing low-temperature fluids at their onset and induced calcite cementation, which strongly altered and modified the local reservoir properties.

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Hughes, Alexandra T., Brian D. Smerdon et Daniel S. Alessi. « Hydraulic properties of the Paskapoo Formation in west-central Alberta ». Canadian Journal of Earth Sciences 54, n^o 8 (août 2017) : 883–92. http://dx.doi.org/10.1139/cjes-2016-0164.

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In an effort to better understand the hydraulic properties of the Paskapoo Formation, hydraulic conductivity and porosity were evaluated for a region in west-central Alberta. Whereas previous studies have focused mainly on sandstone units in the lower portion of the Paskapoo Formation, in southern and central parts of the province, this study focuses on the middle to upper portions. Hydraulic conductivity values were determined by air permeametry for seven drill cores from the area between Hinton and Fox Creek, Alberta. Thin-section petrology and porosity analyses using photomicrographs were also conducted for three of the seven drill cores. Results confirm previous findings that the Paskapoo Formation has heterogeneous hydraulic properties, with horizontal hydraulic conductivity values ranging from 10−10 to 10−5 m/s (determined by air permeametry) and porosity values ranging from 0.02% to 15.3%. The first measurements for the upper sandstone units are provided (1.1 × 10−9 – 2.6 × 10−5 m/s and 0.08%–15.3%) and numerous measurements of the middle siltstone–mudstone unit (1.1 × 10−10 – 4.9 × 10−8 m/s and 0.02%–1.8%) for the northwestern portion of the Paskapoo Formation. Qualitative petrologic analysis suggests that the degree of cementation, rather than grain size, is the dominant control on the hydraulic properties of this portion of the formation. This study determined primary hydraulic properties for both the highly conductive units often considered as aquifers and the low-conductivity units considered as aquitards or confining layers. When combined with previous findings, this study helps expand the understanding of the Paskapoo Formation and provides critical data for assessing groundwater resources.

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Holail, H. M., M. N. Shaaban et A. S. Mansour. « Cementation of holocene beachrock in the Aqaba and the Arabian Gulfs : Comparative study ». Carbonates and Evaporites 19, n^o 2 (septembre 2004) : 142–50. http://dx.doi.org/10.1007/bf03178477.

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PLECAS, I., S. DIMOVIC et I. SMICIKLAS. « Influence of bentonite and zeolite in cementation of dry radioactive evaporator concentrates ». Applied Clay Science 43, n^o 1 (janvier 2009) : 9–12. http://dx.doi.org/10.1016/j.clay.2008.07.003.

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Rimstad, Kjartan, et Henning Omre. « Impact of rock-physics depth trends and Markov random fields on hierarchical Bayesian lithology/fluid prediction ». GEOPHYSICS 75, n^o 4 (juillet 2010) : R93—R108. http://dx.doi.org/10.1190/1.3463475.

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Early assessments of petroleum reservoirs are usually based on seismic data and observations in a small number of wells. Decision-making concerning the reservoir will be improved if these data can be integrated and converted into a lithology/fluid map of the reservoir. We analyze lithology/fluid prediction in a Bayesian setting, based on prestack seismic data and well observations. The likelihood model contains a convolved linearized Zoeppritz relation and rock-physics models with depth trends caused by compaction and cementation. Well observations are assumed to be exact. The likelihood model contains several global parameters such as depth trend, wavelets, and error parameters; the inference of these is an integral part of the study. The prior model is based on a profile Markov random field parameterized to capture different continuity directions for lithologies and fluids. The posterior model captures prediction and model-parameter uncertainty and is assessed by Markov-chain Monte Carlo simulation-based inference. The inversion model is evaluated on a synthetic and a real data case. It is concluded that geologically plausible lithology/fluid predictions can be made. Rock physics depth trends have influence when cementation is present and/or predictions at depth outside the well range are made. Inclusion of model-parameter uncertainty makes the prediction uncertainties more realistic.

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Cooper, Mark R., et Robert H. Hunter. « Precision serial lapping, imaging and three-dimensional reconstruction of minus-cement and post-cementation intergranular pore-systems in the Penrith Sandstone of north-western England ». Mineralogical Magazine 59, n^o 395 (juin 1995) : 213–20. http://dx.doi.org/10.1180/minmag.1995.059.395.06.

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AbstractThe application of serial lapping, imaging and image processing, and three-dimensional reconstruction techniques are discussed using the quartz-cemented Penrith Sandstone as an example. The study involves the characterization of the three-dimensional pore system microgeometry using combined back-scattered scanning electron and cathodoluminescence microscopy and focuses on the definition of post-compaction and post-cementation intergranular porosity networks. Sample preparation, section spacing and re-orientation and data presentation are described. Aspects of the application of serial section datasets and their limitations are discussed in relation to permeability studies.

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Han, Tongcheng, Shengbiao Liu, Li-Yun Fu et Han Yan. « Understanding how overpressure affects the physical properties of sandstones ». GEOPHYSICS 86, n^o 4 (15 juin 2021) : MR203—MR210. http://dx.doi.org/10.1190/geo2020-0776.1.

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Overpressure is one of the major difficulties that we have to face when exploring the deep earth. Geophysical remote sensing methods, especially seismic and electrical methods, are widely used to predict overpressure for derisking the drilling of deep boreholes in the earth’s crust. However, there are major uncertainties in relating the measured geophysical parameters to the physical properties of crustal rocks, needed for reliable overpressure estimation. We address this knowledge gap through dedicated laboratory measurements of elastic velocity, electrical conductivity, and porosity with increasing pore pressure and the analyses and interpretation of the experimental results. We find a linear increase in the porosity and an exponential increase and decrease in the electrical conductivity and compressional wave velocity, respectively, with the increasing pore pressure. We also find an exponential reduction in the cementation exponent and effective pore aspect ratio (PAR) inverted from the pore pressure induced electrical conductivity and compressional wave velocity, respectively. We further determine that electrical and elastic rock properties are affected more significantly by the changing cementation exponent and effective PAR by increasing the pore pressure rather than by dilating porosity. The results not only strengthen our understanding of the influencing mechanism of overpressure on the physical properties of crustal rocks, but also provide new insights for the more reliable detection of overpressure zones from geophysical remote sensing.

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Mahon, Keith I., T. Mark Harrison et Marty Grove. « The thermal and cementation histories of a sandstone petroleum reservoir, Elk Hills, California ». Chemical Geology 152, n^o 3-4 (novembre 1998) : 227–56. http://dx.doi.org/10.1016/s0009-2541(98)00115-6.

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Mahon, Keith I., T. Mark Harrison et Kevin D. McKeegan. « The thermal and cementation histories of a sandstone petroleum reservoir, Elk Hills, California ». Chemical Geology 152, n^o 3-4 (novembre 1998) : 257–71. http://dx.doi.org/10.1016/s0009-2541(98)00116-8.

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Ma, Baoquan, Shaochun Yang, Hong Zhang, Qingdong Kong, Chunting Song, Ya Wang, Qinglin Bai et Xidong Wang. « Diagenetic facies quantitative evaluation of low-permeability sandstone : A case study on Chang 82 reservoirs in the Zhenbei area, Ordos basin ». Energy Exploration & ; Exploitation 36, n^o 3 (7 novembre 2017) : 414–32. http://dx.doi.org/10.1177/0144598717738813.

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Quantitative characterization of diagenetic facies has great significance for reservoir evaluation and prediction. In order to find out the method to evaluate diagenetic facies, the author took Chang 82 reservoir low-permeability sandstone in Zhenbei area of Ordos basin as research object and divided the reservoir into six types of diagenetic facies by analysis of casting thin section, scanning electron microscope, cathode luminescence, and physical property. According to 14 quantitative evaluating parameters which were related with petrology characteristic, diagenesis strength, pore structure, etc. quantitative evaluation of diagenetic facies of low-permeability sandstone reservoir was done by data envelopment analysis. The result showed that in the Chang 82 reservoir low-permeability sandstone in Zhenbei area of Ordos basin, quantitative representative indexes of diagenetic facies ranged from 0 to 1.00. Various diagenetic facies and their indexes had interval corresponding relation. The diagenetic facies of weak corrosion with chlorite mat and the diagenetic facies of corrosion of unstable components had the best reservoir quality. Their diagenetic facies indexes ranged from 0.66 to 1.00. The reservoir quality of the diagenetic facies filled with kaolinite was not as good as the former. The indexes ranged from 0.50 to 0.66. The diagenetic facies of quartz secondary enlargement and the diagenetic facies of clay mineral cementation replacement had poor reservoir quality. Their diagenetic facies indexes ranged from 0.30 to 0.40. The diagenetic facies of carbonate cementation had the poorest reservoir quality. It hardly possessed fluid storage capability. After comparing diagenetic facies indexes, absorption strength and remaining oil saturability, the perfect corresponding relation between quantitative evaluation results and reservoir quality could be verified.

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Gal, Doron, Jack Dvorkin et Amos Nur. « A physical model for porosity reduction in sandstones ». GEOPHYSICS 63, n^o 2 (mars 1998) : 454–59. http://dx.doi.org/10.1190/1.1444346.

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The experimental elastic moduli‐porosity trends for clean sandstones can be described by the modified upper Hashin‐Shtrikman (MUHS) bound. One geometrical (but not necessarily geological) realization is: as porosity decreases, the number of the pores stays the same and each pore shrinks while maintaining its shape. This concept of uniform porosity reduction implies that permeability is proportional to the effective porosity squared, and that formation factor is proportional to the inverse of the effective porosity. The effective porosity here refers to the part of the pore‐space that dominates fluid flow. The proposed relations for permeability and formation factor agree well with the experimentally observed values. These laws are different from the often used forms of the Kozeny‐Carman equation and Archie’s law, where permeability is proportional to the total porosity cubed and formation factor is proportional to the inverse of the total porosity squared, respectively. We suggest that the uniform porosity reduction concept be used in consolidated rocks with porosities below 0.3. The transition from high‐porosity unconsolidated sands to consolidated sandstones can be described by the cementation theory: the MUHS moduli‐porosity curves connect with those predicted by the cementation theory at the porosity of about 0.3. This scheme is not appropriate for modeling other porosity reduction mechanisms such as glass bead sintering because, during sintering, the pores do not maintain their shapes, rather they gradually evolve to rounder, stiffer pores.

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Singh, J., P. A. Cilli, A. Hosa et I. G. Main. « Digital rock physics in four dimensions : simulating cementation and its effect on seismic velocity ». Geophysical Journal International 222, n^o 3 (3 juin 2020) : 1606–19. http://dx.doi.org/10.1093/gji/ggaa271.

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SUMMARY Porosity exerts a strong control on the mechanical and hydraulic properties of rocks, but can often only be imaged indirectly from the surface using geophysical measurements, such as seismic velocity. Understanding and quantifying the relationship between seismic velocity and porosity is therefore a fundamental goal of many rock physics models. Simulating the geological processes that control porosity to generate digital rocks, and numerically modelling wave propagation to estimate their elastic properties, allows for flexible and rapid calibration of velocity–porosity trends. Here, the initial deposition of two digital carbonate sediments are simulated: grainstone (near spherical grains) and coquina (anisotropic shell fragments). The gradual precipitation of cement is then simulated, resulting in a suite of 3-D volumes of varying porosity with otherwise constant and known mineral and grain phases. These models are then used as input to a 3-D acoustic staggered-grid finite difference simulation of wavefield propagation, from which we estimate bulk seismic velocity and calculate the estimated bulk modulus. The resulting bulk modulus varies systematically with respect to porosity within the physical limits imposed by the Hashin–Shtrikman bounds. The samples exhibit anisotropy in the measured velocity consistent with structural anisotropy due to the settling of elongate grains under gravity. We use the resulting bulk velocity–porosity trends to test competing rock physics models, including one that accounts for varying effective pore-aspect ratio with porosity. The results validate the hypothesis that there is a power-law relationship between effective pore aspect ratio and porosity. This relationship is consistent with similar results obtained from a suite of natural carbonate grainstones examined in the laboratory. The results show the optimal rock physics model to be relatively insensitive to the degree of anisotropy in the fabric of the starting material, and may now be used with more confidence to link observed changes in effective pore aspect ratio to changes in porosity due to a range of geological processes, for example fracturing, dissolution and compaction, where other process-based models are available.

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Fabián, M., Jana Ficeriová, J. Briančin et J. Harvanová. « Alternative methods of obtaining silver from waste solutions using cementation ». Mineral Processing and Extractive Metallurgy 122, n^o 3 (septembre 2013) : 133–36. http://dx.doi.org/10.1179/1743285513y.0000000035.

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Du, V. V., M. L. Polyakova, V. I. Grebenev, V. P. Zhludov, V. A. Groshev et S. M. Terent'ev. « Soldering and cementation of hard-alloy metal cutting tool ». Chemical and Petroleum Engineering 22, n^o 9 (septembre 1986) : 421–23. http://dx.doi.org/10.1007/bf01799302.

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Wang, Baobao, Xiucheng Tan, Wenjie Su, Wei Yan, Di Xiao, Mushi Guo, Shoukang Zhong, Yuqing Xu et Mengying Yang. « Genesis and pore evolution of dolomite reservoir in the Majiagou Formation, Ordos Basin, China ». Energy Exploration & ; Exploitation 40, n^o 1 (20 octobre 2021) : 155–73. http://dx.doi.org/10.1177/01445987211049302.

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In gypsum–carbonate rock assemblages, multistage and complex fluids control the formation of dolomite reservoirs that are a focus of hydrocarbon exploration. It is difficult to determine the types of dolomite reservoirs and their formation mechanisms due to the diverse rock assemblages and multiple stages of diagenesis. In this study, we investigated the petrology, reservoir physical properties, and geochemistry of the 6th sub-member of member five of the Majiagou Formation (i.e. Ma56) in the Ordos Basin, China. These data were used to determine the nature and types of gypsum–carbonate rocks, and constrain their reservoir characteristics and diagenetic history, and fluid-related mechanisms that led to dolomite reservoir development and preservation. The Ma56 was deposited on a restricted evaporatic platform in the North China Craton, and contains three main types of dolomite reservoirs with variable types of reservoir space. Dolomite reservoir formation was closely related to penecontemporaneous dolomitization, karstification, and differential cementation. Early large-scale dolomitization produced dolomitized carbonate sediments that were resistant to compaction and dissolution, which was conducive to the preservation of primary and secondary pores. The intermittent exposure and dissolution of mound–shoal facies sediments, due to high-frequency sea-level fluctuations, was the dominant mechanism for formation of secondary dissolved pores and high-quality reservoirs. During burial, differential cementation occurred due to interaction between fluids and pore size, which determined the extent of reservoir preservation. In general, the studied dolomite reservoirs have undergone multistage diagenesis and alteration, which led to complex and multistage development of the reservoir porosity. However, the reservoir lithology and pore space developed mostly in the depositional to penecontemporaneous stages. Our results provide new insights into the origins of deeply buried dolomite reservoirs in carbonate–evaporite successions.

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Demirkıran, Nizamettin, Ahmet Ekmekyapar, Asım Künkül et Ahmet Baysar. « A kinetic study of copper cementation with zinc in aqueous solutions ». International Journal of Mineral Processing 82, n^o 2 (mars 2007) : 80–85. http://dx.doi.org/10.1016/j.minpro.2006.10.005.

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Swarbrick, R. E. « Reservoir diagenesis and hydrocarbon migration under hydrostatic palaeopressure conditions ». Clay Minerals 29, n^o 4 (octobre 1994) : 463–73. http://dx.doi.org/10.1180/claymin.1994.029.4.06.

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AbstractFluid inclusion studies from two contrasting fields in the Northern North Sea reveal hydrostatic pressure conditions during the formation of secondary quartz overgrowths which trapped both brine and petroleum inclusions. In these fields, dating of authigenic illite, part of the diagenetic sequence and broadly coeval with quartz cementation, permits comparison with modelled pressure conditions. Pressure modelling indicates the likelihood of several periods of overpressure in the past, interspersed with normal pressure. The timing of diagenesis coincides with one of these periods of hydrostatic pressure. Furthermore, as both fields are now highly overpressured, the cause and timing of overpressure generation post-date the diagenesis. A model is proposed to link the pressure history and diagenesis.

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Xu, Huiyuan, Dujie Hou, Stefan C. Löhr, Quanyou Liu et Simon C. George. « Early diagenetic pyrite cementation influences molecular composition of sedimentary organic matter in the Dongying Depression, China ». Organic Geochemistry 144 (juin 2020) : 104019. http://dx.doi.org/10.1016/j.orggeochem.2020.104019.

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Flinn, Derek, et Allan Pentecost. « Travertine-cemented screes on the serpentinite seacliffs of Unst and Fetlar, Shetland ». Mineralogical Magazine 59, n^o 395 (juin 1995) : 259–65. http://dx.doi.org/10.1180/minmag.1995.059.395.10.

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AbstractScrees on high serpentinite sea cliffs in Shetland have been internally cemented by deposits of brucite, hydromagnesite and aragonite since the last glaciation. These minerals have been deposited from solutions originating as rain (often contaminated with salt spray), which fell on the serpentinite screes and dissolved Mg and minor amounts of Ca from the more finely divided and powdery scree components. As the solutions filtered down through the screes they deposited cements in the form of spherulitic layers of brucite and hydromagnesite with minor amounts of aragonite. At sea level the Mg-rich solutions mingled with sea water impregnating the scree and there cementation within the scree took place by the crystallization of plush-type layers of aragonite on the serpentinite fragments.

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de Kuijper, A., R. K. J. Sandor, J. P. Hofman et J. A. de Waal. « Conductivity of two‐component systems ». GEOPHYSICS 61, n^o 1 (janvier 1996) : 162–68. http://dx.doi.org/10.1190/1.1443936.

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We present measurements and computer simulation results on the electrical conductivity of nonconducting grains embedded in a conductive brine host. The shapes of the grains ranged from prolate‐ellipsoidal (with an axis ratio of 5:1) through spherical to oblate‐ellipsoidal (with an axis ratio of 1:5). The conductivity was studied as a function of porosity and packing, and Archie’s cementation exponent was found to depend on porosity. We used spatially regular and random configurations with aligned and nonaligned packings. The experimental results agree well with the computer simulation data. This data set will enable extensive tests of models for calculating the anisotropic conductivity of two‐component systems.

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Glover, Paul W. J. « Archie's law – a reappraisal ». Solid Earth 7, n^o 4 (29 juillet 2016) : 1157–69. http://dx.doi.org/10.5194/se-7-1157-2016.

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Abstract. When scientists apply Archie's first law they often include an extra parameter a, which was introduced about 10 years after the equation's first publication by Winsauer et al. (1952), and which is sometimes called the “tortuosity” or “lithology” parameter. This parameter is not, however, theoretically justified. Paradoxically, the Winsauer et al. (1952) form of Archie's law often performs better than the original, more theoretically correct version. The difference in the cementation exponent calculated from these two forms of Archie's law is important, and can lead to a misestimation of reserves by at least 20 % for typical reservoir parameter values. We have examined the apparent paradox, and conclude that while the theoretical form of the law is correct, the data that we have been analysing with Archie's law have been in error. There are at least three types of systematic error that are present in most measurements: (i) a porosity error, (ii) a pore fluid salinity error, and (iii) a temperature error. Each of these systematic errors is sufficient to ensure that a non-unity value of the parameter a is required in order to fit the electrical data well. Fortunately, the inclusion of this parameter in the fit has compensated for the presence of the systematic errors in the electrical and porosity data, leading to a value of cementation exponent that is correct. The exceptions are those cementation exponents that have been calculated for individual core plugs. We make a number of recommendations for reducing the systematic errors that contribute to the problem and suggest that the value of the parameter a may now be used as an indication of data quality.

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Salazar, Jesús M., Gong Li Wang, Carlos Torres-Verdín et Hee Jae Lee. « Combined simulation and inversion of SP and resistivity logs for the estimation of connate-water resistivity and Archie’s cementation exponent ». GEOPHYSICS 73, n^o 3 (mai 2008) : E107—E114. http://dx.doi.org/10.1190/1.2890408.

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Knowledge of initial water saturation is necessary to estimate original hydrocarbon in place. A reliable assessment of this petrophysical property is possible when rock-core measurements of Archie’s parameters, such as saturation exponent [Formula: see text] and cementation exponent [Formula: see text], are available. In addition, chemical analysis of formation water is necessary to measure connate-water resistivity [Formula: see text]. Such measurements are seldom available in most applications; if they are available, their reliability may be questionable. We describe a new inversion method to estimate [Formula: see text] and Archie’s cementation exponent from the combined use of borehole spontaneous-potential (SP) and raw array-induction resistivity measurements acquired in water-bearing depth intervals. Combined inversion of resistivity and SP measurements is performed assuming a piston-like invasion profile. In so doing, the reservoiris divided into petrophysical layers to account for vertical heterogeneities. Inversion products are values of invaded and virgin formation resistivity, radius of invasion, and static spontaneous potential (SSP). Connate-water resistivity is calculated by assuming membrane and diffusion potentials as the main contributors to the SSP. Archie’s or dual-water equations enable the estimation of [Formula: see text]. The new combined estimation method has been successfully applied to a data set acquired in a clastic formation. Data were acquired in a high permeability and moderately high-salt-concentration reservoir. Values of [Formula: see text] and [Formula: see text] yielded by the inversion are consistent with those obtained with a traditional interpretation method, thereby confirming the reliability of the estimation. The method is an efficient, rigorous alternative to conventional interpretation techniques for performing petrophysical analysis of exploratory and appraisal wells wherein rock-core measurements may not be available.

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Klein, B., et N. E. Altun. « Gold losses by cementation and thermal reduction in the gold recovery circuits ». International Journal of Mineral Processing 98, n^o 1-2 (janvier 2011) : 24–29. http://dx.doi.org/10.1016/j.minpro.2010.09.001.

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Caja, Miguel Ángel, Ihsan S. Al-Aasm, Rafaela Marfil, Meaza Tsige, Tomás Martín-Crespo et Ramón Salas. « Multiphase carbonate cementation related tofractures in the Upper Jurassic limestones, Maestrat Basin (Iberian Range, Spain) ». Journal of Geochemical Exploration 78-79 (mai 2003) : 33–38. http://dx.doi.org/10.1016/s0375-6742(03)00045-1.

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Liu, Xiao-Dong, Li-Guang Sun, Zhong-Qi Cheng, San-Ping Zhao, Ke-Xin Liu, Xiao-Hong Wu, Zhou-Qing Xie et al. « Paleoenvironmental implications of the guano phosphatic cementation on Dongdao Island in the South China Sea ». Marine Geology 247, n^o 1-2 (janvier 2008) : 1–16. http://dx.doi.org/10.1016/j.margeo.2007.03.014.

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Jørgensen, Niels Oluf. « Methane-derived carbonate cementation of marine sediments from the Kattegat, Denmark : Geochemical and geological evidence ». Marine Geology 103, n^o 1-3 (janvier 1992) : 1–13. http://dx.doi.org/10.1016/0025-3227(92)90006-4.

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Yang, Zhi, Sheng He, Furong Wang, Zhiliang He, Hengzhi Wu et Xianlong Meng. « Carbonate cementation-dissolution in deep-seated sandstones near the overpressure top in central Junggar Basin, Xinjiang, NW China ». Chinese Journal of Geochemistry 28, n^o 1 (11 janvier 2009) : 86–96. http://dx.doi.org/10.1007/s11631-009-0086-x.

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Muñoz-López, Daniel, David Cruset, Irene Cantarero, Antonio Benedicto, Cédric M. John et Anna Travé. « Fluid Dynamics in a Thrust Fault Inferred from Petrology and Geochemistry of Calcite Veins : An Example from the Southern Pyrenees ». Geofluids 2020 (25 septembre 2020) : 1–25. http://dx.doi.org/10.1155/2020/8815729.

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Petrographic and geochemical analyses (δ18O, δ13C, 87Sr/86Sr, clumped isotopes, and elemental composition) coupled with field structural data of synkinematic calcite veins, fault rocks, and host rocks are used to reconstruct the episodic evolution of an outstanding exposed thrust zone in the Southern Pyrenees and to evaluate the fault behavior as a conduit or barrier to fluid migration. The selected thrust displaces the steeply dipping southern limb of the Sant Corneli-Bóixols anticline, juxtaposing a Cenomanian-Turonian carbonate unit against a Coniacian carbonate sequence. Successive deformation events are recorded by distinct fracture systems and related calcite veins, highlighting (i) an episodic evolution of the thrust zone, resulting from an upward migration of the fault tip (process zone development) before growth of the fault (thrust slip plane propagation), and (ii) compartmentalization of the thrust fault zone, leading to different structural and fluid flow histories in the footwall and hanging wall. Fractures within the footwall comprise three systematically oriented fracture sets (F1, F2, and F3), each sealed by a separate generation calcite cement, and a randomly oriented fracture system (mosaic to chaotic breccia), cemented by the same cements as fracture sets F1 and F2. The formation of fractures F1 and F2 and the mosaic to chaotic breccia is consistent with dilatant fracturing within the process zone (around the fault tip) during initial fault growth, whereas the formation of the latest fracture system points to hybrid shear-dilational failure during propagation of the fault. The continuous formation of different fracture systems and related calcite cementation phases evidences that the structural permeability in the footwall was transient and that the fluid pathways and regime evolved due to successive events of fracture opening and calcite cementation. Clumped isotopes evidence a progressive increase in precipitation temperatures from around 50°C to 117°C approximately, interpreted as burial increase linked to thrust sheet emplacement. During this period, the source of fluid changed from meteoric fluids to evolved meteoric fluids due to the water-rock interaction at increasing depths and temperatures. Contrary to the footwall, within the hanging wall, only randomly oriented fractures are recognized and the resulting crackle proto-breccia is sealed by a later and different calcite cement, which is also observed in the main fault plane and in the fault core. This cement precipitated from formation fluids, at around 95°C, that circulated along the fault core and in the hanging wall block, again supporting the interpretation of compartmentalization of the thrust structure. The integration of these data reveals that the studied thrust fault acted as a transverse barrier, dividing the thrust zone into two separate fluid compartments, and a longitudinal drain for migration of fluids. This study also highlights the similarity in deformation processes and mechanisms linked to the evolution of fault zones in compressional and extensional regimes involving carbonate rocks.

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de Lima, Olivar A. L., Michael Ben Clennell, Geraldo Girão Nery et Sri Niwas. « A volumetric approach for the resistivity response of freshwater shaly sandstones ». GEOPHYSICS 70, n^o 1 (janvier 2005) : F1—F10. http://dx.doi.org/10.1190/1.1852771.

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The dc electrical response of freshwater-saturated shaly sandstone is analyzed on the basis of effective volume conductivity concepts for a concentrated mixture of “solid grains” in a continuous electrolyte. The bulk conductivity of this model is physically controlled by (1) the effective porosity and the average electrical tortuosity of its free-pore electrolyte, and (2) the amount and concentration of bound water that coats charged solid surfaces (mainly of clays), and a corresponding averaged electrical surface tortuosity. The latter is combined in an equivalent volume conductivity that is mainly due to the electrical double layers of charges developed at the clay-electrolyte interfaces. Analytical expressions, based on effective medium and general mixture theories, are developed to describe both the whole rock conductivity and the specific conductivities of its constituent elements. The derived equations for the bulk conductivity of the system describe, with sufficient precision, experimental core data over a large range of water conductivity. The equations are also written in a modified Archie-Winsauer form, wherein their coefficients are shown to be strongly dependent on the electrolyte and matrix conductivity, the effective medium porosity, and the cementation exponent. The scheme is shown to describe satisfactorily laboratory data for clay gels, shales, and shaly sandstones saturated with saline to fresh water. The scheme can also be used for interpreting resistivity logs of wells in freshwater aquifers. To accomplish this, we use the simultaneous measurements of formation resistivity at two different values of salinity: the freshwater virgin zone obtained from a deep induction log or laterolog and the mud filtrate–invaded zone inferred from a short normal or microfocused log. It must be assumed that either the effective porosity or the cementation exponent, and the native water and mud filtrate resistivities are known from other sources. The new method is being applied to the interpretation of logging data from wells drilled in the Recôncavo-Tucano Basin, Bahia, northeast Brazil.

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Vernik, L. « Predicting lithology and transport properties from acoustic velocities based on petrophysical classification of siliciclastics ». GEOPHYSICS 59, n^o 3 (mars 1994) : 420–27. http://dx.doi.org/10.1190/1.1443604.

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Based on the recently developed petrophysical classification of siliciclastics, which takes into account the amount of the volumetric clay content C and textural position of clay, it is shown that acoustic velocities can be fairly accurate tools in predicting lithology, porosity, and ultimately, transport properties of these rocks. Four major petrophysical groups of carbonate‐ and organic‐poor siliciclastics are distinguished: (1) clean arenites (C < 2 percent), (2) arenites and arkoses (C = 2–15 percent), (3) wackes (C = 15–35 percent), and (4) shales (C > 35 percent). The compressional velocity versus porosity relation for consolidated rocks in each of these groups is found to be linear with very high correlation coefficients. This allows for remarkably accurate porosity estimates or lithology prediction in consolidated siliciclastics from acoustic velocities compared to the widely used time average (Wyllie) equation or its improved modification (Raymer equations), both of which neglect textural factors, or recently proposed relations based on the critical porosity concept. The transforms proposed display fundamental trends subject to only a second‐order regional effects, such as details of mineralogy, grain size distribution, and authigenic clay development. These trends primarily reflect the processes of chemical diagenesis, including pressure solution, cementation, and mineral phase transformation. The processes of lithification of unconsolidated sediments by physical compaction and initial cementation are characterized by a steeper slope of the velocity‐porosity transform because of a more pronounced velocity increase compared to the porosity reduction at this stage. The use of the [Formula: see text] ratio versus velocity relation for lithology prediction is limited compared to the [Formula: see text] versus porosity plots; however, if both porosity and lithology are unknown, the velocity ratio can still be used for discriminating between predominantly grain‐supported reservoir rocks (clean arenite, arenite and arkose) and clay matrix‐supported (wacke, shale) rocks. Finally, a strong correlation between porosity and permeability of clean arenites is weakened somewhat in arenites. Nonetheless, even in the latter case, an order of magnitude accuracy in permeability assessment based on porosity can be achieved.

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Hossain, Zakir, Tapan Mukerji, Jack Dvorkin et Ida L. Fabricius. « Rock physics model of glauconitic greensand from the North Sea ». GEOPHYSICS 76, n^o 6 (novembre 2011) : E199—E209. http://dx.doi.org/10.1190/geo2010-0366.1.

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The objective of this study was to establish a rock physics model of North Sea Paleogene greensand. The Hertz-Mindlin contact model is widely used to calculate elastic velocities of sandstone as well as to calculate the initial sand-pack modulus of the soft-sand, stiff-sand, and intermediate-stiff-sand models. When mixed minerals in rock are quite different, e.g., mixtures of quartz and glauconite in greensand, the Hertz-Mindlin contact model of single type of grain may not be enough to predict elastic velocity. Our approach is first to develop a Hertz-Mindlin contact model for a mixture of quartz and glauconite. Next, we use this Hertz-Mindlin contact model of two types of grains as the initial modulus for a soft-sand model and a stiff-sand model. By using these rock physics models, we examine the relationship between elastic modulus and porosity in laboratory and logging data and link rock-physics properties to greensand diagenesis. Calculated velocity for mixtures of quartz and glauconite from the Hertz-Mindlin contact model for two types of grains are higher than velocity calculated from the Hertz-Mindlin single mineral model using the effective mineral moduli predicted from the Hill’s average. Results of rock-physics modeling and thin-section observations indicate that variations in the elastic properties of greensand can be explained by two main diagenetic phases: silica cementation and berthierine cementation. These diagenetic phases dominate the elastic properties of greensand reservoir. Initially, greensand is a mixture of mainly quartz and glauconite; when weakly cemented, it has relatively low elastic modulus and can be modeled by a Hertz-Mindlin contact model of two types of grains. Silica-cemented greensand has a relatively high elastic modulus and can be modeled by an intermediate-stiff-sand or a stiff-sand model. Berthierine cement has different growth patterns in different parts of the greensand, resulting in a soft-sand model and an intermediate-stiff-sand model.

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Gkortsas, Vasileios-Marios, Lalitha Venkataramanan, Kamilla Fellah, David Ramsdell, Chang-Yu Hou et Nikita Seleznev. « Comparison of different dielectric models to calculate water saturation and estimate textural parameters in partially saturated cores ». GEOPHYSICS 83, n^o 5 (1 septembre 2018) : E303—E318. http://dx.doi.org/10.1190/geo2018-0100.1.

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Accurate estimation of water saturation is central in predicting capillary pressure and relative permeability, under special core analysis in the laboratory. We have explored the use of dielectric measurements at different frequencies to estimate water saturation. In addition to water saturation, dielectric measurements are sensitive to the distribution of water and oil in a porous system, reflected by the apparent cementation factor [Formula: see text], which describes the water phase tortuosity. We have performed an experimental study to benchmark water saturation from dielectric measurements on eight carbonate cores and estimated their cementation exponent [Formula: see text] and saturation exponent [Formula: see text] in Archie’s equation from dielectric data. All cores went through a series of drainage/imbibition steps, creating varying saturations of brine/fluorocarbon. Fluorocarbon was chosen because it is invisible to proton nuclear magnetic resonance (NMR). Therefore,NMR porosity represents only the water-filled porosity and can be used to benchmark dielectric water-filled porosity. Three dielectric models were used for the comparison of the dielectric water-filled porosity with the one from NMR, i.e., the complex refractive index model (CRIM), bimodal model, and Stroud-Milton-De (SMD) model, and very good agreement of 1.5 porosity units on average is found. Despite its simplicity, CRIM predicted well the water-filled porosity in this experiment. However, it cannot provide information about the texture, which is captured by bimodal and SMD models. We also estimated [Formula: see text] and [Formula: see text] based on [Formula: see text] found from bimodal and SMD models, and good agreement with [Formula: see text] from resistivity data was shown. This is the first time to our knowledge that such a rich set of dielectric and NMR measurements was acquired at different saturation stages in a surface laboratory. This study is useful in benchmarking the water saturation from dielectrics, comparing different dielectric models, and demonstrating feasibility of estimating textural parameters.

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