Impact of permeability evolution in igneous sills on hydrothermal flow and hydrocarbon transport in volcanic sedimentary basins

Abstract

Abstract. Sills 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 show that thermogenic methane is channeled below the intrusion towards its tip, where hydrothermal vents nucleate and transport methane to the surface. However, these models typically assume impermeable sills and ignore potential effects of permeability evolution in cooling sills, e.g., due to fracturing. Here, we combine a geological field study of a volcanic basin (Neuquén Basin, Argentina) with a hybrid finite-element–finite-volume method (FEM–FVM) of numerical modeling of hydrothermal flow around a sill, including hydrocarbon generation and transport. Our field observations show widespread veins within sills composed of graphitized bitumen and cooling joints filled with solid bitumen or fluidized shale. Raman spectroscopy indicates graphitization at temperatures between 350 and 500 ∘C, suggesting fluid flow within the intrusions during cooling. This finding motivates our modeling setup, which investigates flow patterns around and through intrusions that become porous and permeable upon solidification. The results show three flow phases affecting the transport of hydrocarbons generated in the contact aureole: (1) contact-parallel flow toward the sill tip prior to solidification, (2) upon complete solidification, sudden vertical “flushing” of overpressured hydrocarbon-rich fluids from the lower contact aureole towards and into the hot sill along its entire length, and (3) stabilization of hydrocarbon distribution and fading hydrothermal flow. In low-permeability host rocks, hydraulic fracturing facilitates flow and hydrocarbon migration toward the sill by temporarily elevating porosity and permeability. Up to 7.5 % of the generated methane is exposed to temperatures >400 ∘C in the simulations and may thus be permanently stored as graphite in or near the sill. Porosity and permeability creation within cooling sills may impact hydrothermal flow, hydrocarbon transport, and venting in volcanic basins, as it considerably alters the fluid pressure configuration, provides vertical flow paths, and helps to dissipate overpressure below the sills.