10.3 Conventional Petroleum (Oil) and Methane (Natural Gas)

Jeff Simpson

Learning Objectives – By the end of this chapter, you should be able to do the following.

  • Describe how petroleum and methane were formed.
  • Draw the geology of a methane and petroleum reservoir.
  • Describe where petroleum and methane are found in the world.
  • List 5 ways we use methane and petroleum.
  • List 4 environmental impacts of using methane and petroleum.
  • List 4 political and social impacts of using methane and petroleum.
  • Describe the production trends and explain why this trend is occurring.

Conventional Petroleum (Oil)

Organic-rich sediments deposited in shallow marine environments formed petroleum and gas,  think shallow seas close to the continents. As the organic-rich sediments are buried, the sediments become , that is, converted into rock (typically , , or ) while the organic matter transforms into . The kerogen then changes into oil. The shale, mudstone, or limestone in which hydrocarbons form are the source rock. Once created, the oil and gas leak out of the source rock and migrate to a different rock, usually located above the source (Fig 10.5). The rock that hosts the hydrocarbons is called the reservoir rock.

Reservoir rocks are typically relatively allowing fluids to enter and move; this also facilitates recovery of the oil or gas. Sometimes, the oil liquids and gases make it all the way to the surface, where they sip. Ancient people used oil from sips in different ways. The oil and gas are oxidized at the surface and the carbon eventually returns to the atmosphere. In other cases, the fluids and gasses do not make it to the surface, they are trapped underground. An impermeable layer (e.g. mudstone or claystone) can seal the reservoir, impeding flow and storing the hydrocarbons underneath the Earth. The liquids and gases that are trapped within reservoirs become layered based on their density, with gas rising to the top, oil below it, and water underneath. [3]

A cross-section of rock layers.  Source rocks of hydrocarbons are below upper rock units, which host the hydrocarbons that migrated from the source rocks. The host rocks are also known as reservoir rocks. 
Figure 10.3.1 –  Migration of oil and gas from source rocks into traps in reservoir rocks. Anticline, Fault and reef or salt Traps are geological structures that create permanent storage for hydrocarbons. Credits: S. Earle



Video 10.3.1 – Oil 101(2:16)




Video 10.3.3 – Oil Sands Production 101(2:18)



Video 10.3.4 – Oil and Gas Formation (3:05)


A trap is a combination of a subsurface geologic structure and an impervious layer that helps block the movement of oil and gas and concentrates it for later human extraction. The development of a trap could be a result of many different geologic situations. Common examples include: an anticline or dome structure, an impermeable salt dome, or a fault-bounded stratigraphic block (porous rock next to non-porous rock). The different traps have one thing in common: they pool the fluid fossil fuels into a configuration in which extraction is more likely to be profitable. Oil or gas in strata outside of a trap renders extraction is less viable.


Where Petroleum Is Found

Petroleum distribution varies widely around the world. This leads to global conflict. When those reserve are gone, they will not be renewed in our lifetime. Countries with proven oil reserves are shown below.

Map of countries with proven oil reserves - according to U.S. EIA, 1/2017 .
Fig 10.3.2 – Map of countries with proven oil reserves – according to U.S. EIA, 1/2017 .


Conventional Methane (Natural Gas)

Figure 10.3.5 – Natural Gas 101 (3:38)


Figure 10.3.5 – Hydraulic Fracturing 101 (2:08)


Figure 10.3.7 – LNG 101 (2:23)


Where Methane Is Found


A map of 48 shale basins in 38 countries, based on US Energy Information Administration data, 2011.
Figure 10.3.3 – A map of 48 shale basins in 38 countries, based on US Energy Information Administration data, 2011.