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Abstract: Clastic Diagenesis and Porosity Prediction in Sedimentary Basins

BJØRLYKKE, KNUT
Department of Geology, University of Oslo, Oslo, Norway

The properties of sedimentary rocks change continuously during burial in response to diagenetic processes. Compaction of sandstones and shales is driven mainly by mechanical stress from the overburden down to 2-3 km depth (>70-100°C). At greater depth compaction is mainly chemical and reactions, and the rates are controlled by thermodynamics and kinetics. The distribution of porosity, permeability, thermal conductivity, and rock mechanical properties are critical input parameters for basin modeling. Prediction of these parameters requires a clear understanding of diagenetic processes. Data from Jurassic reservoirs in the North Sea Basin and the Mid-Norwegian Shelf show that porosity and permeability vary greatly at similar burial depths and fluid pressures due to the effect of primary textural and mineralogical composition. Biogenic carbonate and silica accumulating on the sea floor can be important precursors for cement.

Dissolution of feldspar and mica and precipitation of kaolinite are most common in shallow marine and fluvial sandstones that have been subjected to high fluxes of meteoric water at shallow depth. This is less common in distal turbiditic sandstones and sandstones from desert environments in the North Sea Basin.

The amount of diagenetic illite in Jurassic reservoir sandstones from the North Sea and Haltenbanken, offshore Norway, shows a strong increase at depths >4 km (130-140°C), and is accompanied by a decrease in the precursor minerals (K-feldspar and kaolinite). Sandstones containing little or no K-feldspar contain unaltered kaolinite, suggesting that a local source of potassium is required for the illitization.

The rate of quartz cementation can be modeled as an exponential function of temperature, but is also strongly influenced by grain coatings such as chlorite or bitumen affecting the surface area available for quartz cementation. Mass transport of silica is mainly driven by local dissolution and diffusion over shorter distances. Calculations show that compaction-driven fluid fluxes are several orders of magnitude too low to supply significant silica for quartz cementation. The rate-limiting process is in most cases that of quartz nucleation and precipitation, which also control the rate of dissolution along stylolites and at grain contacts. This implies that the magnitude of effective stress at grain contacts and the degree of overpressure play a minor role. The initial mineralogical and textural parameters required for diagenetic modeling are the most difficult to predict. They can only be obtained from provenance, facies, and early diagenetic reactions, including meteoric water diagenesis and early marine diagenesis. 

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