Oil and Gas Traps

In a typical trap, gas accumulates on top of the reservoir as a gas cap over the oil leg which in turn overlies the water-saturated zone in the reservoir. This occurs because natural gas is lighter than oil which is lighter than water. However, all three fluids are often intermingled in parts of the reservoir. Porosity is the ability of a rock to hold oil and gas like water in a sponge. Permeability indicates how easily fluids can flow through the rock.

A trap requires three elements:

• a porous reservoir rock to accumulate the oil and gas—typically sandstones, limestones and dolomites

• an overlying impermeable rock to prevent the oil and gas from escaping

• a source for the oil and gas, typically black waxy shales.

Petroleum Communication Foundation. Our Petroleum Challenge: Exploring Canada's Oil and Gas Industry, Sixth Edition. Calgary: Petroleum Communication Foundation, 1999. With permission from the Centre for Energy.