Physical transportation by diffusion

Diffusion, on the other hand, is a slow and widely-dispersive process. Antonov et al. (1971) measured hydrocarbon diffusion coefficients for a variety of rock types from several hydrocarbon provinces in the former USSR. They discovered that the coefficients of diffusion vary over a wide range (lO -lO cmVs) depending on the particular lithology and geologic conditions. 

The time required for diffusion to occur can sometimes be restrictive. Indeed the time required not only often exceeds the age of the hydrocarbon accumulation but also quite often exceeds the age of the host rock. If this were the dominant process for migration, then the appearance of soil-gas anomalies in the near subsurface would indicate only very shallow accumulations. If a non-steady state exists, where the hydrocarbon signal observed represents only 0.001 times the steady-state signal, then diffusion times could be reduced by a factor of 25 compared to that of the steady-state model. Table 5-V 

Gas concentrations in the near-surfaee rocks before earthquake and (in italics) after earthquake 

Date Strength of shock Distance Well Time of from No. sampling CPI days center to deposit (km) 10- Vol percent (ppm) lO Vol percent (ppm) Vol Percentage percent of hydrocarbon fraction of gas  

Sokolov (1965) calculated diffusion to be sufficient to have resulted in the dissipation of oil fields formed in the Palaeozoic, although to what extent, if any, this has occuned is not known. Furthermore, if any such fields had leakage along faults and fractures or due to erosion of the seal, diffusion might not be able to bring about accumulation before much faster effusive loss caused depletion. Diffusion of benzene into brines adjacent to accumulations has been demonstrated and used as an exploration tool by Zarella et al. (1967). 

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