Fluid-Rock Interactions in a Paleo-Geothermal Reservoir (Noble Hills Granite, California, USA). Part 1: Granite Pervasive Alteration Processes away from Fracture Zones

Only few data from geothermal exploited reservoirs are available due to the restricted accessibility by drilling, which limits the understanding of the entire reservoir. Thus, analogue investigations are needed and were performed in the framework of the H2020 MEET project. The Noble Hills range, located along the southern branch of the Death Valley pull-apart (CA, USA), has been selected as a possible granitic paleo-reservoir. The aim is to characterize the pervasive alteration processes affecting this granite, away from the influence of the faults, in terms of mineralogical, petrophysical and chemical changes. Various methods were used as petrographic, geochemical and petrophysical analyses. Mineral changes, clay mineralogy, bulk rock chemical composition, calcite content and porosity were determined on different granite samples, collected in the Noble Hills granite, far from the faults and in the Owlshead Mountains, north of the Noble Hills, considered as its protolith. In order to complete the granite characterization, the metamorphic grade has been studied through the Noble Hills granite body. This complete characterization has allowed distinguishing the occurrence of three stages of alteration: (1) a pervasive propylitic alteration characterized by calcite-corrensite-epidote-K-white mica assemblage, (2) a more local one, only present in the Noble Hills granite, producing illite, kaolinite, illite/smectite, calcite and oxides, characteristic of the argillic alteration, which overprints the propylitic alteration and (3) weathering evidenced by the presence of montmorillonite in the Owlshead Mountains, which is considered as negligible in both granites. Alteration was also outlined by the correlation of the loss on ignition, representing the hydration rate, to porosity, calcite content and chemical composition. Moreover, the Kübler Index calculated from illite crystals allowed to identify a NW-SE temperature gradient in the Noble Hills.

Quantifying the diagenetic impact in the late Ediacaran and Early Palaeozoic of the Avalon Peninsula using illite “crystallinity”

The Avalon Peninsula, Newfoundland, Canada, defined as the type zone of Avalonia is believed to have been impacted by several orogenetic and deformation events since the Neoproterozoic. Previous studies determined the lowest degree of metamorphism reached in the successions was of the prehnite-pumpellyite or greenschist facies. We sampled and measured thirteen clastic sedimentary sections ranging from the late Ediacaran to the Early Ordovician and analysed the illite “crystallinity” of 331 samples using the Kübler index. Our results show diagenetic zones occur related to lithology, age and burial depth, respectively, and regional setting. Samples adjacent to the fault zones bounding the Holyrood Horst experienced among the highest degree of metamorphism (anchizone) in the study area. The lowest degree of thermal alteration occurs in the high stratigraphic sections at the centre of the horst structure where shallow diagenetic conditions are preserved. Fault zones, most probably active during the Acadian Orogeny, may have served as potential paths for hot fluids in bounding areas of the horst, whereas the centre of the horst remained almost unaffected by any metamorphic overprint. The thermal impact decreases from the Bonavista Peninsula to the study area from greenschist facies to anchizonal and diagenetic. The study area experienced lower metamorphic conditions than major regions of Avalonia south of the study area on the mainland of New Brunswick and Maine and eastwards in Europe. The thermal impact is in part consistent with a few other areas of Avalonia, such as the Mira terrane and the Antigonish Highlands in Nova Scotia.

Reaction progress of clay minerals and carbonaceous matter in a contact metamorphic aureole (Torres del Paine intrusion, Chile)

This study reports on reaction processes in a transition zone from contact to regional metamorphism by using Raman spectroscopy on carbonaceous matter (RSCM), illite “crystallinity” (Kübler index, KI), chlorite geothermometry, and thermal modeling. The thermal effect due to the emplacement of the Torres del Paine intrusion (TPI, assembly time of ca. 150 kyr) had different consequences for inorganic and organic compounds of the host rock. The thermal alteration of the pre-intrusive regional metamorphosed host rock is documented by elevated RSCM temperatures, high-temperature chlorite generations, and the appearance of epidote and retrograde Fe-rich chlorite. Microprobe analysis on chlorite indicates incomplete re-equilibration as evidenced by various chlorite populations of individual contact metamorphic samples. This study indicates that the maturity of organic matter is the most reliable and unequivocal indicator on timescales of several thousand years to determine the lateral extension of the TPI contact aureole. Raman geothermometry reveals that the lateral extension of the contact-influenced zone expands up to a distance of 1.5 km and, thus, expands to ca. 1.1 km further out than the macroscopically mappable hornfels contact aureole. The best match between measured (Raman geothermometry) and calculated (thermal modeling) ΔTmax values (ΔT=54 ∘C) is achieved with a total intrusion assembly time of 150 kyr, a magmatic temperature of 800 ∘C, a two-batch model (batch repose time of 10 kyr) with five pulses per batch, short heating durations (3 kyr), and long pulse repose times (15 kyr).

Diagenetic Clay Minerals and Their Controls on Reservoir Properties of the Shahbazpur Gas Field (Bengal Basin, Bangladesh)

Clay mineralogy and diagenesis affect the reservoir quality of the Neogene Surma Group in the Hatiya trough of Bengal Basin, Bangladesh. X-ray diffraction and scanning electron microscopic analyses of diagenetic clay minerals from Shahbazpur#2 well reveal that on average illite is the dominant clay mineral (50%), followed by chlorite (24%), kaolinite (23%) and smectite (2.50%). The absence of smectite at Core-2 (3259.80 m to 3269 m) results from the total transformation of smectite to illite owing to burial depth and high K–feldspar. The diagenetic changes are a result of chemical processes such as cementation, chlorite authigenesis, dissolution, alteration and replacement that have significantly affected the reservoir properties. Cementation plays an important role in reducing reservoir properties with pore and fracture filling cement. The relative percentage of illite and smectite minerals (>90% illite in I/S mixed layer) and Kübler index value (0.34° to 0.76° Δ2θ) indicate a diagenetic zone with subsurface temperatures of 120–180 °C in the studied samples. The temperature range determined using clay percentages and the Kübler index as a geothermometer is supported by observed diagenetic features such as quartz overgrowths, smectite to illite transformations and chlorite coatings. The diagenetic features cause variable reservoir porosity and permeability that are critical in planning exploration and development programs of this field or analog fields across the Bengal Basin.

A new collection of clay mineral ‘Crystallinity’ Index Standards and revised guidelines for the calibration of Kübler and Árkai indices

ABSTRACTA new set of clay mineral ‘Crystallinity’ Index Standards (CIS) is available for improved calibration of the half-peak-width values of the X-ray diffraction 001 illite reflection (the Kübler index) and the 002 chlorite reflection (the Árkai index), two widely used indices for determining the state of prograde diagenesis and low-temperature metamorphism. Calibration using mudrock standards removes the numerical differences between laboratories caused by variations in sample preparation, machine settings and measurement methods, thus avoiding erroneous grade determinations. The number of standards available has been increased to nine. These can be used to obtain Kübler index values for each CIS sample and Árkai index values for upper anchizonal and epizonal samples. The diagenetic and lower anchizonal mudrocks are not suitable for Árkai index measurements due to the absence of chlorite or overlap by the 001 kaolinite reflection. Applying the new Kübler-equivalent upper and lower boundary limits of the anchizone placed at 0.32°2θ and 0.52°2θ, respectively (Warr & Ferreiro Mählmann, 2015), the nine standards from the Palaeozoic mudrocks of southwest England now comprise two diagenetic, two lower anchizonal, three upper anchizonal and two epizonal grade samples. These range from weakly cleaved mudstones to strongly foliated slates.

The thrust contact between the Canastra and Vazante groups in the Southern Brasília Belt: structural evolution, white mica crystallinity and implications for the Brasiliano orogeny

ABSTRACT: Two regional thrust-sheets of Neoproterozoic metasedimentary rocks occur in the Southern Brasília Belt, northwest Minas Gerais. The lower one comprises the Vazante Group, that is formed in the studied area, from base to top, by the Serra do Garrote (metapelites interlayered with carbonaceous phyllite), Serra do Poço Verde (beige to pink stromatolitic metadolomite with interlayered greenish slates), Morro do Calcário (gray stromatolitic metadolomite interlayered with gray slates) and Serra da Lapa (phyllite with dolarenitic lenses interlayered with slates) formations. The upper thrust sheet consists of the Canastra Group (Paracatu formation): laminated sericite phyllites and carbonaceous phyllites interlayered with quartzite. The Braziliano orogeny resulted in four phases of contractional deformation, associated with low-grade metamorphism. The first two (D1 and D2) are ductile, while the third and fourth ones (D3 and D4) are brittle-ductile. D1 developed a slaty S1 cleavage subparallel to the primary layering, with shallow to steep dips to NW. D2 developed a crenulation cleavage (S2) that dips moderately to NW and is associated with tight to isoclinal folds. D3 and D4 phases developed crenulations and open folds and kink bands. S3 dips steeply to NW, while S4 has moderate to steep dips to NE and SW. White mica crystallinity (Kübler index) measurements in metapelites indicate that both the Canastra and Vazante groups reached anchizone/epizone conditions, and metamorphic discontinuities along thrusts indicate that the peak of metamorphism is pre or syn-thrusting.

ÍNDICE DE CRISTALINIDADE DA ILITA E CARACTERIZAÇÃO DOS ARGILOMINERAIS DA FORMAÇÃO PIMENTA BUENO – BACIA DOS PARECIS

ResumoA Bacia dos Parecis é uma das maiores bacias intracratônicas brasileiras e poucos são os estudos relacionados à sua gênese. A Formação Pimenta Bueno, objeto desse estudo, é constituída, da base para o topo, de calcários, folhelhos, siltitos e arenitos. O presente estudo tem como principal objetivo estimar o provável estágio diagenético/metamórfico em que essas rochas foram submetidas, por meio do estudo de difratogramas de raios-X da fração argila e dos parâmetros do Índice de Kübler (Índice de Cristalinidade da Ilita). Os resultados indicam que essas rochas encontram-se na Epizona e, a média das medidas da largura a meia altura (FWHM) é de 0,16°. A análise de difratometria de raios – X indicou a presença de clorita, ilita, pirofilita, caulinita e esmectita (montmorilonita). Além das espécies bem definidas, identificou-se, também, argilominerais interestratificados de ilita/esmectita e clorita/esmectita.Palavras Chave: Cristalinidade da ilita; Bacia dos Parecis; Formação Pimenta Bueno.AbstractINDEX CRYSTALLINITY OF ILLITE AND CHARACTERIZATION CLAY MINERALS OF PIMENTA BUENO FORMATION – PARECIS BASIN. The Parecis basin is one of the largest Brazilian intracratonic basins and few are the studies related to its genesis. Pimenta Bueno Formation, object of this study, is represented, from bottom to top, by limestones, shales, siltstones and sandstones. To achieve the objectives of this study, knowing the diagenetic stage and / or metamórphic degree to which were subjected the Parecis Basin rocks, were developed diffraction studies of X-ray of the clay fraction, followed by the calculation of the Kübler Index parameters (crystallinity of iIlita). The results indicated that these rocks are at the Epizone and the average width measured at half height (FWHM) is 0.16 °. The X-ray diffraction indicated the presence of the chlorite, illite, pyrophyllite, kaolinite and smectite (montmorillonite). Likewise were identified interstratified phases of Illite/ smectite and chlorite / smectite.Keywords: Crystallinity of Ilita; Parecis Basin; Pimenta Bueno Formation.

Tectonothermal evolution in the core of an arcuate fold and thrust belt: the south-eastern sector of the Cantabrian Zone (Variscan belt, north-western Spain)

The tectonothermal evolution of an area located in the core of the Ibero-Armorican Arc (Variscan belt) has been determined by using the conodont colour alteration index (CAI), Kübler index of illite (KI), the Árkai index of chlorite (AI) and the analysis of clay minerals and rock cleavage. The area is part of the Cantabrian Zone (CZ), which represents the foreland fold and thrust belt of the orogen. It has been thrust by several large units of the CZ, what resulted in the generation of a large number of synorogenic Carboniferous sediments. CAI, KI and AI values show an irregular distribution of metamorphic grade, independent of stratigraphic position. Two tectonothermal events have been distinguished in the area. The first one, poorly defined, is mainly located in the northern part. It gave rise to very-low-grade metamorphism in some areas and it was associated with a deformation event that resulted in the emplacement of the last large thrust unit and development of upright folds and associated cleavage (S1). The second tectonothermal event gave rise to low-grade metamorphism and cleavage (S2) crosscutting earlier upright folds in the central, western and southern parts of the study area. The event continued with the intrusion of small igneous rock bodies, which gave rise to contact metamorphism and hydrothermal alteration. This event was linked to an extensional episode due to a gravitational instability at the end of the Variscan deformation. This tectonothermal evolution occurred during the Gzhelian–Sakmarian. Subsequently, several hydrothermal episodes took place and local crenulation cleavage developed during the Alpine deformation.