Hydrocarbon accumulation and entrapment under hydrostatic conditions

1 Hydrocarbon accumulation and entrapment under hydrostatic conditions 

The sealing capacity of a rock under hydrostatic conditions is determined by the minimum hydrocarbon-water displacement pressure of the rock, which depends on the radius of the largest connected pore throats in the rock and the oil-water and gas-water interfacial tensions, and in addition on the densities of groundwater and hydrocarbons accumulating in the adjacent reservoir rock. The maximum height of an oil or gas column that can accumulate below a seal is given by Equation 4.17 (Section 4.1.3) 

The sealing capacity of a rock changes with depth. This is because the characteristics of the rock change with depth (e.g. the porosities and permeabilities decrease with depth), the interfacial tensions of oil and gas change (Section 4.3.1) and the densities of groundwater, oil and gas change (Section 4.3.1). At shallower depths ( 2 km) the gas-water interfacial tension of gas is greater than the oil-water interfacial tension, while at depths of more than 2 km the gas-water interfacial tension is similar to the oil-water interfacial tension (Section 4.3.1). 

In a hydrostatic trap, the oil-water and gas-water contacts are generally horizontal, because the equipotential surfaces for oil and gas are horizontal under hydrostatic conditions. Irregular contacts may be observed as a result of 

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