Uneven transformation of sedimentary silica in Western Siberia (a case study of the Santonian gaizes of the Hayakha formation)

The Upper Cretaceous, potentially gas-bearing gaizes of the Hayakha formation (Santon) of the West Siberian Plate, which covers an area of approximately 2.5 million km², is characterized by a highly irregular degree of crystallization of silica aggregate (opal-cristobalite-tridimite or OCT-phase). This is an important parameter for assessing filtration-capacity properties. The gaize is present in 20-30 m thick strata and is distributed across an area of approximately 2.5 million km². The crystallization of the OCT-phase (ageing) in natural systems can be attributed to a number of factors. The study of the transformation features of the Upper Cretaceous gaize rocks currently necessitates a unified and systematic approach. A series of laboratory techniques, including petrographic description of transparent slides, X-ray diffraction analysis of the total (XRD) and fine fraction (XRD_clay), and scanning electron microscope, were employed to evaluate the extent of opaque aging in cores from over 30 wells. In most of the West Siberian Plate within the Hayakha formation gaizes the quartz content, according to XRD data, ranges from 5-10 to 30-45 %. Additionally, the proportion of OCT-phase is observed to be 30-50 %. However, in the western and south-western regions of the West Siberian plate, quartz constitutes up to 80 % or more, and OCT-phase is undetected by the XRD method. The morphological differences of silica isolations are established on scanning electron microscope images at a magnification of 5,000-6,000 times. In the examined stratigraphic sequences, the areas of rocks exhibiting an elevated degree of OCT-phase ageing do not correlate with the areas displaying indications of transformation of swelling clay minerals (montmorillonite) into illite. On the basis of the accumulated new data, as well as taking into account the mid-Maastrichtian incised relief forms (channels) detected on seismic sections in the western part of the West Siberian Plate, the following conclusion is made: the absence of OCT-phase and anomalously high quartz fraction in gaizes of the Hayakha formation in the western region of the West Siberian Plate are caused by complete crystallization of OCT-phase in the conditions of subaerial exposure (exogenous conditions) of Upper Cretaceous sediments in the Middle Maastrichtian.

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