Petroleum system logic is the thought process required to develop an integrated interpretation of the processes of petroleum generation, migration, and accumulation. It is illustrated here in frontier area exploration with examples taken from three offshore sales. Details of the application of petroleum system logic vary widely depending on the nature of the exploration problem and the data available. The application of petroleum system logic often allows the explorer to reduce the evaluation problem to the careful assessment of a single factor. The first two examples are of this type. The third example is a comprehensive evaluation illustrating the quantitative treatment of the processes of hydrocarbon generation, migration, and accumulation.
The critical problem in the first example (1986 Offshore Texas) was the prediction of petroleum type in a new growth fault trend. The presence of gas in the new trend was correctly predicted using petroleum system logic to extrapolate information bearing on hydrocarbon type from adjacent previously explored areas. In the second example (1976 Baltimore Canyon), the critical problem was predicting a petroleum charge in a previously unexplored area. Reservoirs, seal, trap, and ease of migration from a thick, mature stratigraphic section were ensured for the Schlee dome. An adequate petroleum charge was predicted to be available because favorable environments for source rock deposition were inferred from a geologic model derived from reflection seismic data. Postsale drilling discovered no petroleum and demonstrated the risk inherent in this mode of prediction.
The third example was taken from the 1983 Norton Sound sale. In part I of this example, the critical problem was determining the likelihood of an oil charge in the area. A reliable answer was anticipated because the determination was based on analyses of samples obtained from favorably located wells that penetrated the whole sedimentary section at a thermally mature location. The most useful evidence was Rock-Eval pyrolysis measurement of the amount of oil generated in the thermally mature section and oil shows in porous rocks in the thermally mature section. These indicated that a negligible volume of oil had migrated out of the mature section. This prediction has been confirmed by drilling. Part II of this example is a comprehensive evaluation of the Stuart subbasin, where the processes of hydrocarbon generation, migration, and accumulation were quantified using rock data from a COST well. The failure of five exploratory wells drilled on four prospects around the Stuart subbasin to find any gas accumulations is explained by this evaluation.
These examples demonstrate that all pertinent data should be considered and that proper interpretation of hydrocarbon shows is often important. When there is a possibility of a limited petroleum charge, quantitative evaluation of the processes of hydrocarbon generation, migration, and accumulation should be considered to aid in prospect or play evaluation.
