Kinetic Model Describing the Composition of Hydrocarbons Generated by Three Major Types of Organic Matter

B. Doligez, P. Ungerer, J. Espitalie, C. Ducreux, O. R. Heum, S. Eggen

The prediction of hydrocarbon composition as a function of thermal history and type of source rock is of great economic importance, but few models have considered this problem. We propose a kinetic model which couples the primary cracking (oil and gas generation from the sedimentary organic matter) with the secondary cracking (degradation of oil into gas and carbonaceous residue). Used for primary as well as secondary cracking, the model involves four hydrocarbon fractions: methane, wet gas, light oil, and heavy oil. The primary cracking is treated by a set of parallel reactions, and the secondary cracking of each hydrocarbon fraction is described by a balanced reaction (except methane, which is considered stable). Progressive expulsion of hydrocarbons without fractionati n is also accounted for.

The kinetic parameters of the model are calibrated with results of pyrolysis experiments. This has been achieved for the three classical types of organic matter. The model reproduces fairly well the pyrolysis experiments for a large range of temperatures (300° to 550°C) and heating times (1 to 48 hr). Model simulations have been performed with typical geologic rates of burial and temperature increase. They show that the model accounts well for the successive stages of heavy oil, light oil, gas condensate, and dry gas formation at depths consistent with well-known trends. In addition to the classical influence of thermal history and organic type on the composition of hydrocarbons, expulsion appears to have a considerable influence in this respect: early expulsion promotes the formation of oil; late expulsion favors gas generation.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.