The petroleum-system events chart shows the temporal relationship of the rock units, essential
elements, processes, preservation, and critical moment for each petroleum system in bar graph form (Fig. 2F; Table I). The events chart concept is flexible and is used as a risk chart to evaluate plays and prospects
The events chart shows the following items (Fig. 2F). The rock units include those that are within the stratigraphic extent of the petroleum system. On a certainty scale, the ages of the four essential elements (source, reservoir, seal, and overburden rocks) are usually better established from paleontology or radiometric age dates than those associated with the two processes (trap formation and generation-migration-accumulation). Determining the times over which the traps form and hydrocarbons accumulate (generation-migration-accumulation) is more interpretive because there is less precise temporal information about these processes. Therefore, risk or uncertainty with regard to the times over which the two processes takes place is higher or less certain than for the better established times of development of the four essential elements. This certainty relationship is important if a similar chart is constructed for a complementary play/ prospect or assessment unit.
When an events chart is constructed for a complementary prospect, it becomes a risk chart. The risk chart is derived from the petroleum-system events chart which, in turn, is derived from the summation of the events chart for each oil and gas field in the petroleum system. These oil or gas fields are successful prospects. Unsuccessful prospects are dry holes. For example, if a risk chart for a prospect is similar to the petroleum-system events chart, then it can be concluded that this prospect is more likely to contain petroleum than one that has a dissimilar risk chart. Conversely, if an events chart is constructed for each dry hole within a petroleum system, they should be dissimilar from producing fields. This dissimilarity indicates where the greater uncertainty lies. Used this way, the events chart is a useful analytical tool to deal with uncertainty or risk.
One important issue this simple bar graph addresses is as follows: For an evolving petroleum system to effectively trap migrating hydrocarbon fluids, the trap forming process must occur before or during the generation-migration-accumulation process in order for petroleum to accumulate.
When constructing an events chart, these rules should be followed. First, there is only one pod of active source rock for each petroleum system. Second, every effective reservoir rock needs a seal, no matter how thin. Third, show only reservoir rocks
that contain petroleum accumulations, shows, or seeps. Fourth, show eroded overburden rock with hatcher lines so that it can be incorporated in the modeling exercise. Fifth, the best information for timing of trap formation comes from oil and gas fields. Sixth, the best information for generation – migration-accumulation comes from geological and geochemical information about the source rock that are then incorporated into the burial modeling and kinetics. This information indicates the onset, peak, and end of generation-migration-accumulation or when the active source rock is depleted (spent) or uplifted (inactive source rock). This process takes place over a relatively short period of time. Seventh, preservation time, by definition, starts when generation-migration-accumulation ends and continues to the present. Some petroleum systems have no preservation time. Last, when the critical moment occurs is, as discussed earlier, in the judgment of investigator, but modeling software packages are useful tools as they show the time over which expulsion from the pod of active source rock occurs.
