The generation, migration, and accumulation of oil and gas in a petroleum system never starts when the source rock is being deposited and seldom extends into the present day. If a source rock is deposited in the Paleozoic, it may be the Mesozoic before it becomes thermally mature and charges adjacent traps, and by the Cenozoic, this source rock is probably depleted. The time over which the process of generation-migration-accumulation takes place could be tens of millions of years. This is a long period of time to chose from if an investigator needs to select the most appropriate moment during this process to make a map and cross section that shows the petroleum system when most (>50%) of the hydrocarbons were migrating and accumulating. To help the investigator with this important exercise, the critical moment was introduced and incorporated into the petroleum system folio sheet.
Geologists use the concept of the critical moment for other exercises. Whenever a map, such as an iso – pach map, is constructed, it is frequently reconstructed to its original thickness at the moment of deposition. The kinematic development of a fold-and-thrust belt occurs over many millions of years, but it is frequently represented by one cross section, or a snapshot in time. A structural cross section of a fold-and-thrust belt reconstructed at the end of the Cretaceous, for example, utilizes the critical moment concept.
The critical moment is the time that best depicts the generation-migration-accumulation of hydrocarbons in a petroleum system. This definition needs an explanation and an example to be better understood. A moment is a brief, indefinite interval of time that is of particular importance. For a camera to take a picture, a moment is less than a second. In geology, the further one goes back in time, the interval becomes thousands or even millions of years. For the petroleum system, moment relates to the shortest measurable time. Critical refers to the moment that best shows, in the judgment of the investigator, the process of generation-migration-accumulation.
Best is a keyword in this definition. Best contains the criteria the investigator should use to select the appropriate moment. The best time needs to fulfill several criteria: (1) it must be within the age of the petroleum system; (2) it must be when most, or more than half, of the hydrocarbons are migrating and accumulating; and (3) it must be shown as an arrow, not an interval, on the burial history and events charts.
The critical moment of a petroleum system can vary depending on its purpose. If the purpose is a petroleum system case study, then the critical moment should be representative of the entire system. However, if the purpose is related to an exploration play or prospect, then the critical moment should be for that part of the pod of active source rock most likely to charge the traps in the play or prospect. Depending on the size, thickness, and variation of the thermal maturity of the pod of active source rock and the objective of the investigator, these could be different best times, none of which are incorrect. In fact, the investigator may need to make numerous burial history charts of a large, thick pod of active source rock that has a wide range of thermal maturity to determine which best moment properly depicts generation-migration-accumulation for the intended audience.
The burial history chart omits important information available in most modeling software packages that explains the critical moment from a different perspective (Fig. 3). This graph shows cumulative volumes of generated or expelled oil and gas.
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Wherever the curves are horizontal, no volume is being added. This graph shows the onset of generation (dashed line) precedes expulsion (solid line) by almost 20 million years. According to this graph, most expulsion occurs more than 15 million years from 195 to 179 Ma. When this cumulative expulsion curve is transformed to a curve showing the distribution of expulsion, it shows peak expulsion at
187.5 Ma (Fig. 3). At this time, at least half of the petroleum is migrating so the critical moment can be selected by the investigator as any time between
187.5 to 179 Ma.
