  According to clues left by the Cassini mission, Titan, one of the two Solar
System bodies with a hydrologic cycle, may harbor liquid hydrocarbon-based
analogs of our terrestrial aquifers, referred to as "alkanofers". On the Earth,
petroleum and natural gas reservoirs show a vertical gradient in chemical
composition, established over geological timescales. In this work, we aim to
investigate the conditions under which Titan's processes could lead to similar
situations. We built numerical models including barodiffusion and
thermodiffusion (Soret's effect) in N_2+CH_4+C_2H_6 liquid mixtures, which are
relevant for Titan's possible alkanofers. Our main assumption is the existence
of reservoirs of liquids trapped in a porous matrix with low permeability. Due
to the small size of the molecule, nitrogen seems to be more sensitive to
gravity than ethane, even if the latter has a slightly larger mass. This
behavior, noticed for an isothermal crust, is reinforced by the presence of a
geothermal gradient. Vertical composition gradients, formed over timescales of
between a fraction of a mega-year to several tens of mega-years, are not
influenced by molecular diffusion coefficients. We find that ethane does not
accumulate at the bottom of the alkanofers under diffusion, leaving the
question of why ethane is not observed on Titan's surface unresolved. If the
alkanofer liquid was in contact with water-ice, we checked that N_2 did not, in
general, impede the clathration of C_2H_6, except in some layers.
Interestingly, we found that noble gases could easily accumulate at the bottom
of an alkanofer.
