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Impact of permeability evolution in igneous sills on hydrothermal flow and hydrocarbon transport in volcanic sedimentary basins
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 intrus...
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Published in: | Solid earth (Göttingen) 2023-06, Vol.14 (6), p.625-646 |
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Main Authors: | , , , , , |
Format: | Article |
Language: | English |
Subjects: | |
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Online Access: | Get full text |
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Summary: | 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. |
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ISSN: | 1869-9529 1869-9510 1869-9529 |
DOI: | 10.5194/se-14-625-2023 |