Caprocks as Real Seals, or Selective Sieves for Hydrocarbons?

A. Prinzhofer, F. Lorant, E. Pernaton, F. Schneider, A. Y. Huc, A. Barre, and C. J. Allegre

There is still no direct way to constrain the age of the hydrocarbons trapped today in a reservoir. Several geochemical evidences support the hypothesis that for the majority of reservoirs, the trapped fluids represent only the last charge of the hydrocarbons generated in the source-rocks. Basin modeling indicates that the ratio between reserves and calculated generated hydrocarbons is generally below 1%. Indeed, the chemistry and isotope values of natural gases in the majority of reservoirs cannot correspond to the sum of the generated products: in the gas from Kansas, the general convergence of the Prinzhofer ^dgr¹³C of C1-C3 with increasing TR, the buffered ^dgr¹³C values of C2 and C3 for dry gases, are not in agreement (experimentally and theoret cally) with bulk isotopic values of gases generated in closed systems. However, a new model of chemical and isotopic fractionation fits the natural case if one assumes that the gas in the reservoir corresponds to an instantaneous product. Another study with the new chronometer Re/Os has been performed on the asphaltenes and oils of the Paris Basin. The calculated ages of the oils are negligible within the uncertainties of the method (0±1 M.y.).

These results would give a new hint for basin evaluation: more than the quality of the source rock and its position in the oil or gas window, the variation of thermal maturation versus time at present becomes a major indicator: for instance, recent erosion in the Paris Basin would explain the small potential for petroleum reserves, the maturation being virtually chilled. Special areas as the Urengoy field (West Siberia) would give much bigger gas fields, because of the real sealing effect of a permafrost. The unusual isotopic signatures found in this area would be correlated with the unusual "cul de sac" for the gas migration path.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California