A Unified Petroleum System
Now that we’ve studied the elements and processes of a functioning petroleum system in detail, let’s take one last look at the big picture:
A petroleum system is an interrelated series of geologic features and processes that produces hydrocarbons, allows them to migrate into a reservoir rock, and traps them in place as an accumulation.
Each of the following events must occur, in the correct order, for a petroleum system to produce and store oil or gas:
- Deposition and burial of organic materials, likely in an anoxic environment, leading to the formation of a source rock with kerogen.
- Burial beneath enough overburden to heat and cook the kerogen within the oil window, gas window, or both to generate hydrocarbons.
- Presence of a migration pathway at the time of expulsion to connect the source rock with a reservoir rock.
- Migration of hydrocarbons into a reservoir rock.
- Accumulation of hydrocarbons by a caprock or seal, in an effective trap that existed at the time of migration.
- Preservation of the oil and gas reservoir from destruction by leakage, overheating, or biological activity.
This is the sort of system we have been looking for and tapping for the last 150 years – a conventional petroleum system. In these conventional systems, buoyant forces keep hydrocarbons in place beneath a caprock. Oil and gas naturally flow from conventional reservoirs into wellbores.
But now unconventional petroleum systems are becoming viable producers, changing some of the variables that petroleum geologists have historically used when deciding where to drill.
Images: “Horizontal and vertical with rock strata” by markrhiggins via iStock; “Illustrations” by Top Energy Training