--> Diagenetic Modification of Glaciomarine Reservoirs: Insights From an Analog From the Cenozoic Succession in the Victoria Land Basin, Antarctica

AAPG ACE 2018

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Diagenetic Modification of Glaciomarine Reservoirs: Insights From an Analog From the Cenozoic Succession in the Victoria Land Basin, Antarctica

Abstract

Siliciclastic sediments of polar shelves deposited under minimal glacial influence are generally considered as potential reservoir rocks, as they form laterally extensive sand bodies that are regarded as having undergone limited diagenetic modification in the polar environment. However, recent discovery of dense brine in pore water that caused abundant carbonate cement precipitation in the AND-2A core in the Victoria Land Basin (VLB), Antarctica indicates a more complicated diagenetic history than previously expected for glaciomarine reservoirs. Developing the findings from the AND-2A core, this study focuses on assessing the distribution of brine and other subsurface fluids and evaluating their impact on the reservoir quality of sediments with high initial reservoir potential, during burial diagenesis. Although protected from exploration by the Antarctic Treaty, the Cenozoic succession as sampled by several cores in the VLB serves as a useful analog for glaciogenic reservoirs elsewhere. Sandstones were selected within the well-constrained chronostratigraphic and sedimentologic framework of the cores for systematic documentation of diagenetic phases using standard microscopy, point counting and cathodoluminescence microscopy. Secondary carbonate phases in the sandstones were sampled for stable isotope analyses to fingerprint the origin of diagenetic fluids. Petrographic examination revealed widespread secondary carbonates in all sites examined. The carbonates led to dramatic pore space occlusion and reservoir quality deterioration, particularly in the more deeply buried sandstones that formed under minimal/distant glacial influence. Relationships between the distribution of the carbonates and systems tracts within individual high-frequency depositional sequences become less evident with increasing burial depth. Isotopic values of the carbonates are strongly 18O-depleted, consistent with precipitation from the 18O-depleted brine that is present in the AND-2A core. Results from this study suggest that brine is widespread in the subsurface of the VLB. Where the brine has infiltrated, carbonate precipitation has resulted in heterogeneity in porosity and permeability distributions. Reservoir quality of high-latitude shelfal sediments is prone to diagenetic modification by brine infiltration during ice ages. This should be considered in studies of diagenetic evaluation and reservoir characterization of deposits in glaciogenic settings.