Task 3 -Formation of Oil and Gas deposits
Oil and gas are almost always formed in marine environments (Skinner et al., 1999), although can also be formed in lakes and river systems (AAD, 2009). Typical environments that form oil and gas deposits have low levels of oxygen, which prevents the decay of organic material (Dickey, 1996). When organisms such as phytoplankton and bacteria die, their remains sink to the bottom, and are mixed in with the underlying sediment (Skinner et al., 1999). Over a long period of time, these organisms form a thick layer of organic rich sludge. With continual sedimentation, the older deposits are buried and are then subjected to increased temperatures (AAD, 2009). When these deposits get to a depth of approximately 2 kilometres, they are exposed to temperatures of 40°C to 60°C (Skinner et al., 1999). Gentle heating at temperatures between 40°C and 100°C is conducive to oil formation, with temperatures between approximately 100°C and 200°C needed to produce gas (Skinner et al., 1999). Heating above 200°C, or burial greater than 7 kilometres results in the destruction of natural gas, as methane reacts with steam to produce carbon dioxide (AAD, 2009). Figure 1 below graphically displays the ideal depth and temperatures needed to produce oil and gas.
Figure 1. Ideal depth and temperatures needed to produce oil and gas. [Image taken from Skinner et al., 1999).
Although oil develops in clay-rich sediments, hydrocarbons are usually found in sandstones (Skinner et al., 1999). This is due to the migration process of oil and gas. As oil and gas have lower densities, they become buoyant and move upwards (Nichols, 2009). As deposits such as sandstones have a high porosity, hydrocarbons are able to migrate up through the large pores between the grains. Although sandstones are the most common rock that contains oil and gas deposits, limestones and dolostones may also provide the opportunity for oil and gas to migrate upwards (Skinner et al., 1999). These permeable deposits that allow for the migration of oil and gas are known as reservoir rocks (Hyne, 2001).
When oil and gas continues to rise, it may encounter an impermeable layer known as a cap rock (Nichols, 2009). If there is a suitable trap surrounding the cap rock, oil and gas are able to accumulate in underground pools. Many oil and gas pools are found beneath structural traps such as anticlines, which result due to folds or faults prior to the accumulation of hydrocarbons (Skinner et al., 1999). Stratigraphic traps also allow oil and gas to accumulate, and may exist due to an unconformity sealing the trap or the pinching out of the reservoir rock (Nichols, 2009). Figure 2 shows examples of both structural and stratigraphic traps.
As gas is has a lower density than oil, a layer of gas is often observed directly underneath the cap rock, with oil underlying the gas (Nichols, 2009). This can also be observed in figure 2. Although a large amount of oil has been recovered from these pools, the majority is in fact are carried away in groundwater to the surface or into the ocean (Skinner et al., 1999).
Figure 2. Examples of oil and gas traps. ‘A’ represents a structural trap produced from folding, and ‘B’ is an example of a structural trap resulting from fracturing. ‘C’ and ‘D’ are both stratigraphic traps formed due to and unconformity sealing the trap and the pinching out of the reservoir rock respectively. [Image taken from Skinner et al., 1999).
Australian Antarctic Division, 2009. Mining. Available at: http://www.aad.gov.au/default.asp?casid=6561
. Accessed 11th May 2010.
Dickey, J.S. 1996. On the rocks: Earth science for everyone. John Wiley & Sons, USA.
Hyne, N.J. 2001. Nontechnical guide to petroleum geology, exploration, drilling, and production. 2nd Edition. PennWell Corporation, USA.
Nichols, G. 2009. Sedimentology and stratigraphy. 2nd Edition. Blackwell Science, Malden, Mass.
Skinner, B.J., Porter, S.C. and Botkin, D.B. 1999. The Blue Planet. 2nd Edition. John Wiley & Sons Inc, USA.