--> Evaluation of Reservoir Potential of Glaciomarine Sandstones in the Victoria Land Basin, Antarctica: Insights from Diagenesis in Cape Roberts Project Drillcores

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Evaluation of Reservoir Potential of Glaciomarine Sandstones in the Victoria Land Basin, Antarctica: Insights from Diagenesis in Cape Roberts Project Drillcores

Abstract

Glaciogenic reservoirs are effective and producible, but not yet as well understood as clastic reservoirs from other settings. This work investigates the diagenesis of glaciomarine sandstones from the Cape Roberts Project (CRP) 1, 2A, and 3 cores in a temporal context of regional depositional processes, paleoclimate, and basin evolution to assess controls on reservoir quality. CRP-1, -2A, and -3, drilled in the Victoria Land Basin, Antarctica in 1997, 1998, and 1999, recovered a nearly complete stratigraphic transect through c. 1500 m of dipping strata extending from the uppermost Eocene through Lower Miocene. Systematic petrographic documentation of diagenetic phases in 110 thin sections was aided by standard petrography, point counting microscopy methods, and cathodoluminescence microscopy. A complex paragenesis is revealed in the sandstones, including syndepositional to burial cementation. High porosities (>40%) are observed as a result of modest compaction, and dissolution of labile constituents, such as dolomite cements, feldspars, volcanic rock fragments, orthopyroxene and other ferromagnesian minerals. However, secondary carbonate, occurring as replacement and cement phases in microcrystalline, fibrous, blocky, and poikilotopic forms, occludes primary pore space, leading to very low porosities (approaching 0%) in some intervals. Patterns in sandstone texture and composition become more complex with the transition from ice-free to iceberg-influenced marine shelfal environments at the Eocene-Oligocene boundary (Oi-glaciation at c. 34 Ma), indicating the initiation of Antarctic icehouse. High porosity sandstones are dominant in highstand systems tracts in temperate periods, whereas low porosity sandstones dominate more frigid, lowstand systems tract intervals, showing increasing matrix and cements, and limited physical and chemical weathering. The patterns are consistent with previous studies showing that glaciation had waxed and waned during the Oligocene cooling trend, and that the permanent Antarctic cryosphere was established during cooling associated with Mi-1 glaciation at c. 24 Ma. Diagenetic patterns in CRP cores demonstrate high reservoir potential in glaciomarine sandstones. However, predicting reservoir quality in glaciogenic clastic reservoirs is complex, with basin evolution, paleoclimate, glacial dynamics, and subsurface fluid history all exerting fundamental controls.