The purpose of the burial history chart is to show the essential elements, and three important hydrocarbon events, which are (1) the onset (O) of generation – migration-accumulation, (2) the partially spent or depleted (D) active source rock, and (3) the critical moment (CM) of the petroleum system (Fig. 2E; Table I). The top of the oil and gas windows, and the lithology and name of the rock units involved should also be shown. This chart uses sedimentologic and paleontologic evidence in the overburden rock to reconstruct the burial or thermal history of the source rock. The onset of generation-migration – accumulation usually occurs when the source rock reaches a thermal maturity at a vitrinite reflectance equivalence of 0.6% Ro and ends when the source rock is either uplifted before all the hydrocarbons can be expelled or is depleted as the source rock is more deeply buried. The location of the burial history chart is shown on the petroleum system map and cross section.
In this example, the Deer Shale (rock unit 3) is the source rock, the Boar Sandstone (4) is the reservoir rock, the George Shale (5) is the seal rock, and all the rock units above the Deer Shale (4, 5, 6) comprise the overburden rock. The burial history chart is located where the overburden rock is thickest, and indicates that the onset of generation-migration-accumulation started 260 Ma in Permian time and was at maximum burial depth 255 Ma. Oil generation ended about 240 Ma because the source rock is depleted. The critical moment as judged by the investigator is 250 Ma because modeling indicates most (>50%) of the hydrocarbons have been expelled and are accumulating in their primary traps. However, the investigator would be correct to have chosen anytime between 250 and 240 Ma, but 250 Ma was chosen because the best geologic information is available to reconstruct the map and cross section. The time of generation-migration – accumulation ranges from 260 to 240 Ma and is the age of the petroleum system.
As mentioned in the cross section discussion, knowing the age and thickness for each increment of overburden rock is crucial for any modeling exercise. Each increment needs to be bounded by time lines whose dates are supported by paleontologic dates, isotopic age dates, or other means of dating strata. As the number of increments in the overburden rock increases, the details of the burial history of the source rock will be better understood. For instance, in the previous example, the overburden rock is Permian, undifferentiated. Suppose, however, that paleontologic dates indicate that 95% of the overburden rock is Early Permian and that the rest is Late Permian. This increase in increments in the overburden rocks will change the time when the underlying source rock becomes thermally mature. In this example, the change in time that the source rock becomes mature might be considered insignificant, but in other examples the difference could be significant.