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Geological modelling of clay diagenesis in sandstones

  • Andrew Hurst (a1) and Hilary Irwin (a2)

Abstract

Porewater composition is the main control on diagenetic reactions in sandstones. Porewater has two possible contrasting primary sources: (i) fresh meteoric water, which is dilute and acidic, (ii) sea-water, which is alkaline and more concentrated than meteoric water. During burial, unstable minerals equilibrate with these porewaters, thus increasing the concentrations of dissolved species. A simple manometer model is used to describe the diagenesis of interconnected (fluvial or deltaic) sandstones. This model illustrates the following geological relationships: (a) a hydraulic head causes meteoric waters to penetrate deep into sedimentary basins, typically generating authigenic kaolinite; (b) decrease of the hydraulic head (by lowering the land level or by raising sea level) causes concentrated brines to rise within the basin, typically forming illitic cements; (c) enclosed sandstones (marine facies) are isolated from meteoric water flux and only receive fluxes when fault-induced or when uplifted. Kaolinite morphology and distribution are identified as being flux- or diffusion-controlled.

Resume

La composition de l'eau des pores exerce le contrôle majeur sur les réactions diagénétiques des grès. L'origine de cette eau est double (i) de l'eau fraîche météorique, diluée et acide (ii) de l'eau de mer, alcaline et plus concentrée que l'eau méteorique. Pendant l'enfouissement, les minéraux instables s'équilibrent avec ces eaux des pores augmentant ainsi les concentrations des espèces en solution. Un modèle à base d'un simple manomètre est utilisé pour représenter la diagénèse des grès (fluviaux ou maritimes) interconnectés. Ce modèle met en evidence les relations géologiques suivantes: (a) un niveau hydraulique entraîne la pénétration des eaux météoriques loin dans les bassins sédimentaires générant des kaolinites authigènes, (b) la diminution du niveau hydraulique (par diminution du plan terrestre ou augmentation du plan marin) provoque l'élévation des saumures dans le bassin formant des ciments à base d'illites, (c) des grès enfermés (facies marin) sont isolés du flux d'eau météorique et reçoivent uniquement ces flux s'ils présentent des défauts ou s'ils sone surélevés. On démontre que la morphologie et la distribution des kaolinites sont contrôlées par le flux ou la diffusion.

Kurzreferat

Eine Schlüsselfunktion über diagenetische Reaktionsabläufe in Sandsteinin stellt die Zusammansetzung von Porenwasser dar. Dieses hat zwei mögliche, gegensätzliche Primärquellen: (i) Oberflächensüßwasser, welches elektrolytarm und sauer ist (ii) Meerwasser, welches alkalisch und elektrolytreicher als Oberflächenwasser ist. Während der Absenkung streben instabile Minerale mit den Porenwässern ein Gleichgewicht an, sodaß die Konzentration gelöster Arten ansteigt. Für den modellhaften Diageneseablauf miteinander verbundener Sandsteinne (fluviatiles- oder Deltasediment), wird ein einfaches Manometermodell verwendet. Dieses veranschaulicht die folgenden geologischen Zusammenhänge (a) ein Wasserüberdruck veranlasst das Oberflächenwasser tief in die Sedimentbacken vorzudringen, wobei kennzeichnenderweise authigene Kaolinite entstehen (b) eine Verringerung des Wasserüberdrucks (durch Absinken der Landoberfläche oder Anstieg des Meeresspiegels) veranlasst konzentrierte Salzlösungen innerhalb des Beckens aufzusteigen und typische illitische Verkittungen auszubilden (c) eingeschlossene Sandsteine (marine Facies) sind von der Strömung des Oberflächenwassers isoliert und werden von dieser nur dann erreicht, wenn Verwerfungen entstanden sind oder Hebungen auftreten. Die Morphologie und Verteilung von Kaoliniten wird als strömungs- oder diffusionskontrollierter Vorgang bestimmt.

Resumen

La composición de las soluciones intersticiales es el control principal de las reacciones diagenéticas en areniscas. Las soluciones intersticiales tienen dos posibles origenes: (1) aguas meteóricas diluidas y ácidas (2) agua del mar, alcalina y mas concentrada que la meteórica. Durante el enterramiento los minerales inestables se equilibran con las soluciones intersticiales incrementandose la concentracion de especies disueltas. Un modelo sencillo manométrico se ha usado para representar la diagénesis de areniscas (fluviales o deltaicas) interconectadas. Este modelo ilustra las siguientes relaciones geológicas: (a) una presión hidraulica fuerza a las aguas meteóricas a penetrar profundamente en las cuencas sedimentarias, formando caolinita autigénica (b) el descenso de la presión hidrostatica (por disminucion del nivel terrestre o por elevación del nivel del mar) provoca un ascenso de salmueras concentradas en la cuenca, formando un cemento rico en ilita (c) las areniscas confinadas (facies marina) son aisladas del flujo de aguas meteóricas y solo reciben soluciones cuando se producen fallas o emersiones. La morfología y distribución de la caolinita estan controladas por los procesos de flujo o difusión.

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References

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