Prediction of Subsurface Conditions and Hydrocarbon Product Composition Expelled during Primary Migration from Gas-Prone Source Rocks

POELCHAU, HARALD S., WOLF DIETER KUHNE, and DETLEV LEYTHAEUSER, Institute of Petroleum and Organic Geochemistry, Research Centre Julich (KFA), Germany

Expulsion of hydrocarbons from source rocks containing hydrogen-lean (type III) kerogen is most likely to take place in solution within the gas phase. This mode of migration overcomes problems of low permeability for liquids in the source shales. During this process of expulsion oil will become strongly fractionated and skewed toward lower molecular weight hydrocarbons.

Product composition depends on solubility of the hydrocarbon molecules (or oil fractions) in the carrier gas. Solubility in turn is a function of temperature, pressure, and molecular weight. Composition can be predicted either with an empirical equation derived from dissolution experiments or by applying an equation of state for phase behavior of petroleum.

Conversely, where the composition of expelled hydrocarbons is known, the subsurface pressure and temperature conditions at the time of primary migration can be estimated. All simulations done to date indicate pressures in excess of hydrostatic pressure in the source rock. Estimation of composition and subsurface conditions in this type of expulsion system is illustrated with data from a well-constrained case history.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)