Introduction to Petroleum Systems

The first stage of oil and gas development is known as exploration. During this phase, geologists are trying to figure out where to drill for new reserves of oil and gas, or how to expand an existing oil or gas field.

What petroleum geologists are really looking for are complete petroleum systems.

A petroleum system is a self-contained collection of geologic formations that has produced oil, allowed it to migrate, and trapped it in place until the present. Since each of these steps requires a very specific set of geologic conditions, complete petroleum systems are relatively uncommon.

Here’s a brief overview of the geologic elements and processes that make up a petroleum system:

Essential elements of a petroleum system:

  1. Source: A rock rich in organic matter from which hydrocarbons have been generated or are capable of being generated. Typically fine grained, relatively impermeable materials.
  2. Overburden: Rock in the subsurface that overlies an area of interest.
  3. Migration pathway: The path of movement of hydrocarbons into reservoir rock or other area of accumulation. Commonly porous formations as well as leaky faults.
  4. Reservoir: A subsurface rock with adequate porosity and permeability to store and transmit fluids. Typically sedimentary rocks.
  5. Seal: A rock forming a barrier next to a reservoir rock such that fluids cannot migrate out of the reservoir. Commonly impermeable rocks such as shale or salt.
  6. Trap: Arrangement of rocks that allows for the accumulation of hydrocarbons beneath a seal. Often defined as structural traps or stratigraphic traps or a combination of both.

Essential processes for a petroleum system:

  1. Generation: Organic materials are deposited in a sedimentary basin. Under a specific set of conditions, these materials are transformed into a solid substance called kerogen. Temperatures within a set of specific ranges, called the oil and gas windows, convert kerogen into oil and gas within a source rock.
  2. Migration: Pressurized oil and gas are expelled from the source rock and flow along migration pathways to a reservoir rock.
  3. Accumulation: The upward movement of the oil through the reservoir rock is stopped by an impermeable layer, the caprock or seal rock. These layers may be in their original depositional orientation, or they may have been deformed by tectonic forces, forming a structural trap.
  4. Preservation: Once the hydrocarbons are generated, migrate, and accumulate, they must be preserved – sometimes for millions of year – until the time that a company drills into the reservoir to produce the resource.

Don’t worry if you didn’t catch all the details – we’ll cover each of these processes in detail.

Why It Matters

Petroleum geology allows us to begin to identify how and why drillers make decisions on where to drill. Understanding the key development-process differences between conventional and unconventional resources is critical for citizens so they can separate rumor from reality.

Learning Objectives

  • Describe how petroleum is generated and under what conditions.
  • Describe how petroleum moves underground after it has been created.
  • Describe the conditions that led to the creation of unconventional sources.
  • Name unconventional hydrocarbon sources based on their geologic type.

Images: “Petroleum Systems Illustration” by Top Energy Training