Migration of hydrocarbons

Even if all of the elements described so far have been present within a sedimentary basin an accumulation will not necessarily be encountered. One of the crucial questions in prospect evaluation is about the timing of events. The deformation of strata into a suitable trap has to precede the maturation and migration of petroleum. The reservoir seal must have been intact throughout geologic time. If a leak occurred sometime in the past, the exploration well will only encounter small amounts of residual hydrocarbons. Conversely, a seal such as a fault may have developed early on in the field s history and prevented the migration of hydrocarbons into the structure. 

A trap provides an impermeable barrier to the migration of hydrocarbons moving through a porous and permeable sedimentary bed. The trap allows the hydrocarbons to accumulate within the trap. The trap is a prerequisite for the formation of an accumulation of hydrocarbons and, therefore, for a reservoir [26]. 

Fault traps—involve the movement of the reservoir rock formation to a position where the formation across the fault plane provides a seal preventing further migration of hydrocarbons (see Figure 2-48). 

Van Dam, J., 1967, The Migration of Hydrocarbons in a Water Bearing Stratum In Joint Problems of the Oil and Water Industries (edited by P Hepple), Institute of Petroleum, London. 

The primary focus of this chapter is to introduce approaches that limit (or prevent) migration of hydrocarbon and organic contaminants from the vadose zone into underlying water-bearing zones. The more conventional strategies, including a brief discussion of the process, relative cost, practical constraints, and limitations, are presented. Also presented are two typical soil vapor extraction case histories. 

Estimated ultimate recovery is also reported by type of entrapment, of which there are two major types (1) Structural trap—an entrapment in which migration of hydrocarbons in the reservoir rock has terminated... 

Table 4. 1-Tee energy, energy, and entropy of migration of hydrocarbon from water at 25° to nonpolar environment...

Al-Shaieb Z. and Shelton J.W. (1981) Migration of hydrocarbons and secondary porosity in sandstones. AAPG Bull. 65, 2433-2436. 

Littke R., Baker D. R., and Leythaeuser D. (1988) Microscopic and sedimentologic evidence for the generation and migration of hydrocarbons in Toarcian sources rocks of different maturities. Org. Geochem. 13, 549—559. 

Nunn J. A. and Meulbroek P. (2002) Kilometer-scale upward migration of hydrocarbons in geopressured sediments by buoyancy-driven propagation of methane-filled fractures. Am. Assoc. Petrol. Geologists Bull. 86, 907-918. 

Talukdar S., Gallango O., Vallejos C., and Ruggiero A. (1987) Observations on the primary migration of oil in the La Luna source rocks of the Maracaibo Basin, Venezuela. In Migration of Hydrocarbons in Sedimentary Basins (ed. B. Doligez). Editions Technip, Paris, France, pp. 59-78. 

Micelles appear to be larger than the small size of the pore throats in the source rock at a depth of burial corresponding to the main zone of oil and gas generation. According to Tissot and Welte (1984), primary migration of hydrocarbons in micellar solution would definitely be limited down to a depth of about 2000 m. 

After primary migration has taken place, a certain proportion of the generated hydrocarbons remains in the pore system of the source rock (Hunt, 1979). The oil fraction that remains in the source rock will be cracked to gas as the source rock is buried to greater depths and temperatures (Section 3.1.5). The effect of primary migration of hydrocarbons can be indicated by the expulsion efficiency. The petroleum expulsion efficiency is the ratio of the expelled petroleum and the sum of the generated and initial petroleum and can vary from zero (no expulsion) to 1.0 (complete expulsion) (Cooles et al., 1986). The expulsion efficiencies are not uniform in time and space (Leythaeuser et al. 1987b). They depend on the tsrpe of source rock, its richness and thermal maturity and the primary migration mechanism. 

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). 

After having established the present-day hydrocarbon migration characteristics in a basin, evaluation of these characteristics leads to the selection of areas where preferred migration of hydrocarbons takes place, and, consequently, provides insight into the potential basin-wide distribution of hydrocarbons. 

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