Quantitative Compositional Characterization of the Biosiliceous Miocene Lark Formation, Danish North Sea and Norwegian Margin

Abstract

Biogenic silica, composed of the tests 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 large controls on reservoir quality including porosity, permeability, and diagenetic phase. 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 or XRF geochemical analysis, Fourier Transform-Infrared Spectroscopy (FT-IR), 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 45 mudstone core samples from three different wells in the Miocene Lark Formation of the North Sea show that the Lark is remarkably homogenous and has a much lower SiO2:Al2O3 ratio (2.1) than the Monterey Formation (3.5) due to a high percentage of mixed layer clay and illite. Biogenic silica proportions derived from normative calculations using ICP-MS geochemistry are in agreement with Full Pattern Matching via XRD, which is suitable for amorphous materials as it does not rely on peak height or area measurement. Alkaline digestions of biogenic silica were inconclusive due to a weak diagenesis of opal-A diatoms to less soluble opal-A’. FT-IR results are yet to be processed. Information provided by this work tests the robustness and applicability of each quantification technique, and has developed an excess silica equation that can be applied to the Miocene Norwegian Margin and Danish North Sea. Based upon an empirically derived biosilica to detritus ratio, the excess silica equation will be used to estimate how much silica was biogenic in nature, even in highly diagenetically altered rocks.

AAPG Datapages/Search and Discovery Article #90339 ©2019 AAPG Pacific Section Convention 2019, Long Beach, California, April 1-3, 2019