A company has found a potential oil field by analyzing existing data. A geologist has been sent into the field to collect data. It looks promising, so seismic and magnetic surveys are completed. Geologists compile all the data using GIS and 3D geologic modelling software, and now the company wants to know where the oil is.
How can geologists use the information they’ve collected to find oil reservoirs deep underground?
The process requires the combination of geologic knowledge with the data specific to the region of the world under consideration.
As we learned previously, several conditions must be met for a productive conventional reservoir to form:
- Presence of source rock.
- Oil- or gas-compatible temperature history and migration pathway for hydrocarbons.
- Presence of reservoir rock.
- Presence of structural or other oil trap.
You will recall from earlier topics that in unconventional oil and gas targets, the source rock is actually the drilling target (reservoir rock) and thus, no migration of hydrocarbons is required between the source rock and reservoir rock. In our discussions below, we will focus on conventional oil and gas systems.
The goal of the exploration geologist searching for a conventional petroleum system is to find underground localities where all of the conditions listed above have been met. Let’s take a closer look at each one.
Source Rock
A source rock once contained the materials that have been “cooked” to produce oil. Most often, source rocks are shales.
Identification of source rocks is accomplished by examining samples of the rocks recovered from boreholes and surface exposures. Once a source rock is identified, an accurate geologic map allows geologists to trace it throughout the area under consideration.
Oil Compatible Temperature History
Remember, hydrocarbons are only generated when temperatures are within the oil and gas windows. Given knowledge of a region’s geothermal gradient, geologists can theoretically calculate the depths of these windows. For a gradient of 25 C per km, the oil window should correspond to a depth of 800 to 5,000 meters, and the gas window should extend to about 9,000 meters depth.
Unfortunately, it’s a bit more complicated than that.
Tectonic forces or underground intrusions of molten rock can result in additional heating of the rock. Alternatively, a period of erosion followed by deposition may have caused a temperature excursion that brought a source rock temporarily out of the oil window.
Even if the rock is in the oil window now, if it was deeper within the earth at one point, all of the hydrocarbons may have been destroyed. In addition, source rocks that are too shallow to be in the oil window may have passed through the window in the past. In this case, oil produced from the source rock in the past may still be present.
As you can see, a detailed knowledge of the geologic history of a region is needed to predict past temperatures. In order to determine the temperature history of each part of the source rock, geologists use geologic maps and a variety of other tools.
Reservoir Rock
As the oil is formed, it is expelled from the source rock into the surrounding layers. After this expulsion, oil tends to rise through the brine that is usually present at depth. Oil moves along fractures and permeable carrier beds until it is trapped beneath an impermeable layer or it escapes at the surface.
Reservoir rocks are most often sandstones and limestones, though smaller amounts of oil can also be stored in the fracture systems of non-porous rocks.
A geologist interested in finding oil will be looking for locations where thick layers of reservoir rocks are present above or adjacent to source rocks. These layers are in a position to receive the oil that was generated by the source rock.
Like source rocks, reservoir rocks can be identified at the surface and traced to depth using a geologic map or model. The most important characteristics of a reservoir rock are high porosity and high permeability.
Oil Traps
Remember, even if a source rock and reservoir rock are present, and oil window conditions have been met, oil is not guaranteed. Unless there is an impermeable barrier, oil will continue to rise until it reaches the surface of the earth and is washed away by rain and other processes.
As we learned earlier, hydrocarbons will only be trapped if three conditions are met:
- An impermeable layer of rock sits above a reservoir rock.
- The impermeable layer has a geometry that creates a space where oil can be trapped.
- The trap existed at the time of oil migration.
There are numerous types of impermeable rocks. Among these, cap rocks are most often shales, anhydrites, or salts.
Geologists trace impermeable layers using geologic maps and models. Structures such as faults, domes, and anticlines are all capable of trapping hydrocarbons. By accurately delineating these structures, geologists are able to determine where hydrocarbons are likely to be trapped.
A knowledge of the geologic history of a region is critical to finding oil traps. For example, if a geologic event that led to the folding of an anticline did not occur until after the oil migrated out of the source rock, there would be no oil in the anticline, despite its ideal trap geometry.
The ultimate goal of the geologist is to use a combination of geologic mapping, knowledge of geologic history, and study of rock units to find productive oil traps.
Images: “Oil pump jack” by crstcbrt via iStock