Secondary migration

Figure 3.1 Schematic illustration of primary and secondary migration.

As outlined in Section 4.1, the migration of very finely dispersed oil droplets or gas bubbles with diameters smaller than those of the smallest pore throats of the carrier rock will not be influenced by capillary forces. In addition, according to Tissot and Welte (1984) the very finely dispersed oil droplets will not strictly follow the law of buoyancy. In the initial stages of secondary migration, the hydrocarbons in separate phase may occur as droplets or... 

The secondary hydrocarbon migration system in a basin at a certain moment during its evolution can be characterized by the masses and initial composition of the petroleum hydrocarbons available for secondary migration, the three-dimensional pattern of secondary hydrocarbon migration, i.e. the directions and lengths of the hydrocarbon migration paths, the flux of... 

In addition to the losses of hydrocarbons during secondary migration indicated by Equation 4.25, additional losses of hydrocarbons can be expected to occur by accumulation of small non-economic volumes of hydrocarbons in miniature traps present along the migration path, and by dissolution and diffusion of especially hydrocarbon gases after their expulsion from the source rock (Chapter 3.2 Slvdjk and Nederlof, 1984). 

The loss of separate phase hydrocarbons during secondary migration is indicated by the apparent residual saturation of hydrocarbons, which is on average in the order of 1 - 3% of the total available pore space of the carrier rocks through which hydrocarbons have migrated. 

The process of migration may lead to focussed movement of hydrocarbons into economic accumulations. The secondary migration of hydrocarbons may occur under hydrostatic or hydrodynamic conditions. Under hydrostatic conditions, the hydrocarbons migrate through the water-saturated carrier-reservoir rocks as separate phase hydrocarbons. Under hydrodynamic conditions, the hydrocarbons may be transported in continuous separate phase, in suspension or in aqueous solution. Under both hydrostatic and hydrodynamic conditions, the hydrocarbons ultimately appear as separate phase hydrocarbons before they can accumulate in a trap (Tissot and Welte, 1984). 

On a basin-wide scale, the pattern of secondary migration of separate phase hydrocarbons in a sedimentary basin at a certain time during its evolution is thus determined by... 

By combining the appropriate maps the main characteristics of secondary migration for separate phase hydrocarbons, hydrocarbons in aqueous solution or in very fine suspension, can be identified for each time period. In addition, the influence of periods of increased tectonic activity should be taken into account in the evaluation, because of the direct and indirect effects of tectonic activity on migration, accumulation and entrapment of hydrocarbons (Sections 4.3.4.3 and 5.3.3). 

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