Hydrocarbon accumulation, entrapment and preservation

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). 

Various terms that are generally used to describe a trap are given in Figxire 5.1. A trap may contain oil, gas or both. Where separate oil and gas phases occur together in the same trap, the gas overlies the oil because it is less dense. The oil-water or gas-water contact is the level below which all pores are completely filled with water. The hydrocarbon-water contact is an equipotential surface. 

Under hydrostatic conditions, hydrocarbons may accumulate in hydrostatic traps, which include structural traps, stratigraphic traps and combination traps (Section 5.1). 

Hydrodynamic conditions influence the system of hydrocarbon migration in a basin, i.e. the volumes of hydrocarbons available for entrapment in a certain part of the basin, and the trapping energy conditions in the basin, i.e. the location of potential trapping positions and the sealing capacity of rocks and faults (Sections 5.2 and 5.3). 

The preservation of hydrocarbons accumulated in a trap that is no longer replenished, depends on the type of trap, the type of accumulated hydrocarbon and the subsequent geological and associated hydrodynamic evolution of the sedimentary basin (Section 5.4). 

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