Some Keys to Hydrocarbon Mass Balance at Basin Scale

J.L. Rudkiewicz, B. Carpentier, and M. Thibaut
Institut Français du Pétrole, 92852 Rueil-Malmaison Cedex France

Hydrocarbon migration modelling aims at reconstructing and whenever possible predicting the trapping of hydrocarbons. Therefore basin modelling can also be used for original oil or gas in place estimation at basin scale. This presentation will focus on this topic and will present its principles and illustrate those on real examples.

This global estimation of hydrocarbon in place goes through several steps, of course in 3D and through geological time. The steps will be illustrated in the paper:
1    definition and estimation of the porosity of the main reservoir units;
2    computation of the surface and pore volume of traps;
3    estimation of the mass of generated hydrocarbons in the various source rocks;
4    estimation of the nature of the expelled hydrocarbons, either oil, condensate or gas;
5    computation of accumulated HC volumes in the previously recognised traps;
6    computation of hydrocarbons lost through capillary leakage;
7    estimation of volumes of hydrocarbons in surface conditions.

Traps are either structural or stratigraphic closures. Faults can be taken into account as being either laterally permeable or impermeable. Moreover, lateral fault transmissivity can vary with geological time or with fault azimuth. Lateral facies changes from reservoir facies to non reservoir facies can also be used to simulate stratigraphic traps. The pore volume of structural or stratigraphic traps changes through geological times, as their size, their spill point and their porosity may change with basin tilting or subsidence.

Simultaneously, the quantity of generated and expelled hydrocarbons from the source rocks of the basin can be computed, based upon compositional kinetics and volume and mass balances within the pore space of the source rock.

Then, the hydrocarbon charge to traps is computed through time. When charge volume exceeds the pore volume, secondary migration can occur through fill and spill along leakage networks. Volume changes can also be due to decreasing pressure and temperature and phase split. This phase split is computed with thermodynamically based equation of states.

At the end of computations, a complete mass balance between generation, expulsion, trapping and leaking allows to discuss the efficiency of migration mechanisms at basin scale.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands