Role of Diffusion in Primary Migration of Hydrocarbons

The following effective diffusion coefficients D(cm2/sec) were determined for the diffusion of light hydrocarbons through the water-saturated pore space of shales: methane (2.12 × 10-6), ethane (1.11 × 10-6), propane (5.55 × 10-7), iso-butane (3.75 × 10-7), n-butane (3.01 × 10-7), n-pentane (1.57 × 10-7), n-hexane (8.20 × 10-8), n-heptane (4.31 × 10-8), and n-decane (6.08 × 10-9). On the basis of these new data, a deterministic, dynamic model was set up to simulate the diffusive transport of light hydrocarbons (C1 to C10) from source rocks. For eight documented source-rock units, representing a wid range of geologic conditions (maturities of 0.40 to 1.35% mean vitrinite reflectance; oil- to gas-prone kerogens), the cumulative amounts of hydrocarbons escaping with time were calculated. Thus, it was shown that diffusion represents an effective process for primary migration of gas but not for oil. The rate of mass transport for gas from source rocks with geologic time can be sufficiently high to account for the origin of commercial-size gas fields. For example, a cumulative amount of 109 kg of methane (1.5 × 109 std m3 or 5.3 × 1010 scf) has escaped by diffusion in 540,000 years from a certain volume (1,000 km2 by 200 m thick) of a high-mature gas-prone Mesozoic source rock in western Canada. The origin of hydrocarbon accumulations with high gas-to-oil ratios in low-mature sediments in geologically young basins ("early gases and condensates") can be explained by an early phase of primary migration predominantly based on diffusion. During the initial stages of the accumulation history of those fields (extending up to millions of years under certain conditions), the reservoir gas changes with geologic time from a methane-rich to a wet-gas composition. At low-maturity levels (below about 0.6% Rm), even oil-prone source rocks yield methane-rich light-hydrocarbon mixtures by migration through diffusion. Compositional trends among reservoir gases of several multiple-pay gas fields in Louisiana represent evidence for diffusive transport of hydrocarbons. The variation in gas composition between the individual pay zones is controlled by increasing distances of diffusive transport of the hydrocarbons from a uniform source rock at depth to their present accumulation sites. In the shallow pay zones, compounds of high diffusivity are enriched. For example, in the Sligo gas field of Louisiana, the methane/ethane and the iso-butane/n-butane ratios increase from 10.2 to 36.0 and from 0.86 to 1.13, respectively, from the deepest to the shallowest of the five productive reservoir sands, which are spread over a a depth interval of 5,500 ft (1,676 m). Diffusion of light hydrocarbons in the subsurface can also have economically adverse effects. For example, existing gas accumulations can be destroyed by dissipation. The rate of this destruction was calculated for the Harlingen gas field, Holland. By diffusive loss through 400 m of shale cap rock, the initial amount of methane in place of 1.93 × 109 std m3 (6.8 × 1010 scf) is reduced by one half over a period of 4.5 million years. This leads us to propose the concept that large gas accumulations can persist through extended periods of geologic time only as dynamic systems reaching some kind of steady-state equilibrium between diffusive loss through the cap rock and continuous replenishment from the source rock.