Abstract: Determining Ultimate Petroleum Potential of Sedimentary Basins by Hydrous Pyrolysis

Michael D. Lewan

Over the last decade, major efforts have been made in developing computer programs to model petroleum generation in sedimentary basins. Most of these programs have emphasized the timing of petroleum generation using conventional pyrolysis (e.g., Rock-Eval) data and the thermal histories of basins based on heat-flow calculations or vitrinite reflectance trends. While the inclusion of mathematical expressions and statistical operations in these modeling programs portray a quantitative approach, the intuitive assumptions and numerous degrees of freedom of these programs makes the validity of their results questionable. This concern becomes most apparent in attempts to determine the ultimate petroleum potential of a sedimentary basin. Determining the ultimate petroleum potential is not on y important in assessing and ranking frontier basins, but it is also important in assessing the magnitude of undiscovered petroleum accumulations and in determining shifts in drilling strategy from exploration to exploitation in established petroleum basins. Although the occurrence of petroleum accumulations in a sedimentary basin is controlled by secondary-migration pathways and development of reservoir traps, the amount of petroleum expelled from subsiding source rocks within a basin is a critical consideration upon which the efficiencies of the other considerations are gauged. This ultimate petroleum charge in a sedimentary basin is sometimes considered unimportant because of over estimates based on unrealistic yields from conventional pyrolysis methods (e.g., Rock-Eval). The high tem eratures (250-600°C), unnatural conditions (no water with inert carrier gas), and high heating rates (>25°C/min) used in these pyrolysis methods are the cause of the excessively high yields, which also allow unnaturally high yields from large volumes of organic-lean rocks (<2.0 wt. % organic carbon) to be included in the ultimate petroleum charge of a basin. Research on the mechanisms and processes controlling petroleum expulsion from source rocks indicates that realistic ultimate petroleum charges may be obtained by heating potential source rocks in the presence of liquid water between 250 and 360°C for 72 hr. This laboratory method is referred to as hydrous pyrolysis. Expelled oils generated by this method also provide information on the quality of natural crude oi s and on thermal maturity parameters indicative of expulsion. Data from hydrous pyrolysis experiments are currently being used to assess the ultimate petroleum potential of the Illinois and Uinta basins.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994