Abstract: State of the Art: Appraisal of HC Generation and Migration

Jean Burrus, Francoise Behar, Jean-Luc Rudkiewicz, Jean Espitalie, Alain Prinzhofer

This paper illustrates recent improvements in the understanding of petroleum systems, based on a combination of experimental geochemistry, numerical modelling and actual case studies.

It is now well established that the maturity of source rocks can be fairly well represented by first order Arrhenius kinetics. Calibration of kinetic parameters against experimental data poses the problem of the indetermination of pre-exponential factors A and activation energies E_i. Practically, this problem can be solved by putting constraints on A values. Applying the kinetic approach, it is found that basement heal flow frequently does not vary much with time, whereas lateral crustal heat flow variation, due to basement heterogeneities, are far more important.

"Compositional" models are promising tools, developed with the objective to represent not only the mass of HC formed, but also the relative proportions of compounds such as dry gas, wet gas, condensate and oil. Simple models consider a limited number of classes, generally 3 to 5, whereas sophisticated n-components models discriminate more than 10 components, including aromatics, saturates and NSO fractions. Although such models require more information, they predict much more precisely the importance of secondary cracking, and can explain survival of liquid hydrocarbons in HP-HT regimes, a prediction beyond the capability of simpler compositional models. A considerable advantage of n-components HC generation models is that they allow to predict petroleum transport properties (density, viscosity), hence to incorporate this knowledge is expulsion-migration studies. Novel interpretation of C isotopes and composition of HC gases appears as a promising way to cross-check the relative contribution of primary versus secondary cracking predicted by n-component models for a given gas accumulation.

Expulsion is largely viewed as a separate phase transport process, sensitive to pressure gradients in the source rock. It is recognized that rich sources expel better than poor sources, and that there must be a minimum HC content in the source before expulsion can start. These minimum thresholds are controversial. Whether they are controlled by HC retention thresholds on organic matter, or by critical saturations in the source pore space, is unclear. Models based on HC saturation thresholds (either by a step function, or by a relative permeability concept) were successfully used to match observed oil/source correlations, taking into account the basin permeability and pressure environments. Compaction models based on effective stress/porosity relations show that effect of HC generation on overpressures is generally second to that of compaction disequilibrium. Exceptions require extremely rich kerogen content and rapid burial.

2D migration models allow to reconstruct qualitatively the dynamic character (time frame, direction, magnitude) of HC migration. Whereas 2D integrated models allow to test qualitatively the critical components of a petroleum system, the future of basin models is definitively towards 3D fluid flow modelling. These tools will estimate quantitavely (and not just qualitatively) the amount, timing and nature of the HC charge delivered by a given kitchen, and its fate during geological times. More simple 1D and 2D tools will however still be used either for rapid calibration of parameters, and when insufficient amount of data will prevent the used of 3D tools.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France