Category Mechanical Energy

THE PETROLEUM SYSTEM AS A WORKING HYPOTHESIS

A petroleum system investigation starts with a working hypothesis for generation, migration, and entrapment of petroleum in a province, based on
available geological and geochemical data, which evolves as more data becomes available (Fig. 2A, B). The investigator starts with an oil and gas field map and related field data for the petroleum province of interest. The geographic location of the accumula­tions is important because accumulations located close together are more likely to have originated from the same pod of active source rock (Fig. 4). Accumulations that occur in the same or nearly the same stratigraphic interval are also likely to be from the same active source rock...

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Events Chart

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)...

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Critical Moment

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...

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Location of Burial History Chart

The burial history chart chosen to show the three hydrocarbon events for a petroleum system should be located in the pod of active source rock where, in the judgment of the investigator, much of the oil and gas originated. Usually this location is down-dip from a major migration path to the largest fields.

Petroleum systems are seldom so simple that only one burial history chart adequately describes the same three hydrocarbon events for every location in the pod of active source rock. The investigator chooses the burial history curve that best suits the purpose. If the investigator is presenting (oral or written) a petroleum system investigation, he or she would use the burial history curve down-dip from a major migration path to the largest fields...

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Burial History Chart

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...

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Petroleum System Name

The name of a petroleum system labels the hydro­carbon-fluid system or distribution network (Fig. 2D) in the same way the name Colorado River designates an aqueous distribution system, the river and its tributaries. The name of the petroleum system includes the geological formation names of the source rock followed by the major reservoir rock (Fig. 2C) and then the symbol expressing the level of certainty. For example, the Deer-Boar(.) is the name of a hydrocarbon fluid system whose source rock, the Deer Shale, most likely generated the petroleum that charged one or more reservoir rocks, which in this case is the Boar Sandstone. It is the major reservoir rock because it contains the highest percentage by volume of hydrocarbons in the petroleum system.

A petroleum system can be identified ...

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Cross Section

The petroleum system cross section (Fig. 2B; Table I), drawn at the critical moment, or time when most of the hydrocarbons were generated, shows the geo­graphic and stratigraphic extent of the petroleum system and how each rock unit functions within the system to distribute the oil and gas. Stratigraphically, the petroleum system includes a petroleum source rock, reservoir rock, seal rock, and overburden rock. This cross section is in contrast to structural or stratigraphic cross sections.

The presence of adequate overburden rock in the correct geometry provides (1) the burial needed to thermally mature the source rock, (2) the up dip vector needed for oil and gas to migrate to shallower depths, (3) the burial depth variations needed to form traps for petroleum accumulations, and (4) the ...

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Location of Burial History Chart

The location of the burial history chart is along the cross section line within the pod of active source rock (Fig. 2B, E). At this location, the source rock must be thermally mature (active or spent) otherwise petro­leum would be absent in the conduits or migration paths. The reconstruction of the burial history provides the basis for the times of the onset (O) of generation-migration-accumulation, the depletion (D) of the source rock, and the critical moment (CM).

3.2 Table of Fields

The table showing all the oil and gas accumulations included in the folio sheet provides important information about the petroleum system (Fig. 2C; Table I). First, the discovery dates and sizes of the fields are useful for field-size distributions and discovery-rate modeling...

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