Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Sill intrusions emplaced in organic-rich sedimentary rocks trigger the generation and migration of hydrocarbons in volcanic sedimentary basins. Based on seismic and geological observations, numerical modeling studies of hydrothermal flow around sills have shown that thermogenic methane is channeled towards the intrusion tip, where it rises to the surface in a hydrothermal vent. However, these models typically assume impermeable sills and ignore potential effects of permeability evolution in cooling sills, e.g., due to fracturing. To address this issue, we combine a geological field study of a volcanic basin (Neuquén Basin, Argentina) with hybrid FEM/FVM numerical modeling of hydrothermal flow around a sill, including hydrocarbon generation and transport. Our field observations show that graphitized bitumen veins and cooling joints filled with solid bitumen or fluidized shale are common within the studied sills. Raman spectroscopy indicates graphitization at temperatures between 350–500 °C, evidencing fluid flow within the intrusions shortly after solidification. This finding motivates our modeling study, which investigates flow patterns around intrusions that become porous and permeable upon solidification. The results show three distinct flow phases affecting the transport of hydrocarbons generated in the contact aureole: (1) Contact-parallel flow toward the sill tip before solidification, (2) upon complete solidification, sudden vertical “flushing” of overpressured hydrocarbon-rich fluids from the lower contact aureole through the hot sill, and (3) slow rise of hydrocarbon-rich fluids above the sill center, and backward-downward flow near the sill tip. We conclude that permeability creation within cooling sills may be an important factor for hydrothermal flow and hydrocarbon transport in volcanic basins, as it considerably alters the fluid pressure configuration and flow patterns by dissipating overpressure below the sills. This could, for instance, lead to a reduced potential for hydrothermal venting.
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