Biogenic silica, composed of the frustules of diatoms and other siliceous algae, is a major constituent of marginal and equatorial oceanic sediments. Highly siliceous mudstones, like the Monterey Formation in California, are important source and reservoir rocks for oil and gas exploration. The proportion of biogenic silica to detritus has a large influence on reservoir quality, including porosity, permeability, and the rate of diagenetic change. Quantification of biogenic silica is therefore integral to understanding biosiliceous rocks and their geomechanical properties. However, quantification of biosilica is challenging as two of the diagenetic phases, opal-A and opal-CT, are crystallographically amorphous or poorly ordered. Consequently, traditional methods of quantification of minerals, like x-ray diffraction (XRD) via peak height, must be calibrated and tested against other techniques, such as ICP-MS/OES or

XRF geochemical analysis, Fourier Transform-Infrared Spectroscopy (FTIR), and wet-alkaline digestions of the biosilica.

This study advances biosilica quantification methods by evaluating and comparing the effectiveness of these four techniques. Analysis of 47 mudstone core samples from three wells in the Oligo-Miocene Lark Formation of the North Sea show that the formation is remarkably homogenous and has a lower SiO₂/Al₂O₃ ratio (2.66) than the Monterey Formation (3.5) due to a high percentage of mixed-layer clay and illite. The biogenic silica proportions derived from normative calculations using ICP-MS/OES geochemistry, FTIR, and Full Pattern Matching via XRD are in agreement, and indicate that the Lark Formation samples range from 3–38 % biogenic silica. The normative geochemical calculation, termed the ‘Excess Silica Equation’, is based upon an empirically derived biosilica-to-detritus ratio. The excess silica can be used to estimate how much silica was biogenic in nature, even in highly altered rocks. Quantitative XRD via full Pattern Matching (FullPat) is suitable for amorphous material, as it does not rely on peak height or area measurement. Alkaline digestions of biogenic silica were inconclusive due to changes in solubility by weak diagenesis of opal-A diatoms to less soluble polymorph, opal-A’.

Information provided by this work tests the robustness and applicability of each quantification technique, and furthers the compositional understanding of the Lark Formation. Despite the similarity in biogenic silica estimations between the three techniques, the mineral percentages inferred by FTIR are not in agreement with FullPat, nor the geochemical data. FTIR estimated mineral composition overestimates calcite, pyrite, and quartz percentages, and underestimates mixed-layer clay (montmorillonite) and illite, most likely because of the similarity in the chemical bonds in phyllosilicates in the region between 880 cm⁻¹ and 1270 cm⁻¹. FullPat and normative geochemical calculations of ‘excess’ silica are ranked as the optimal biogenic silica quantification techniques. FTIR is considered to be more statistically robust; yet discrepancies in mineral phase quantification decrease confidence in

the method.