The majority of geoscientists believe that most oil has an organic origin, derived ultimately from the remains of organisms (mainly microscopic marine phytoplankton) that were buried with ancient sediments deposited on the sea floor. Although there are trace amounts of this buried organic matter in almost all sedimentary rocks, unique geological and geochemical conditions are required to generate, expel and trap significant amounts of oil. The typical sequence of events leading to the formation of oil from sedimentary rocks, and the subsequent accumulation of oil in a reservoir, is outlined below. However it should be kept in mind that there are numerous other geological and geochemical scenarios that may also result in economic accumulations of oil.
Requirement #1 Formation of an organic-rich, oil-prone source rock for oil
FIGURE 1
Under normal marine conditions (Figure 1), the water column in the ocean is well-aerated, and the phytoplankton which live in the upper levels of the water column are efficiently scavenged by other animals living in the water, and on or in the sediment on the seafloor. Very little organic matter escapes the scavengers under these conditions, and as a result, only trace amounts of organic matter are buried with the sediments on the seafloor. These sediments are organic-poor, and will not become a source for oil.
FIGURE 2
There are two main causes or conditions which can result in the enhanced burial and preservation of organic matter in marine sedimentary rocks (Figure 2). These include (1) periods of high biological productivity in the surface waters of the oceans (e.g. algal bloom events), and/or (2) development of anoxic conditions in the bottom water layers close to the seafloor. Most organisms are unable to tolerate anoxic conditions, and hence relatively more organic matter becomes buried with the sediments on the seafloor due to the absence of scavengers. These sediments are organic-rich, and have the potential to become a source for petroleum.
Requirement #2 The formation of kerogen
FIGURE 3
The next stage in the formation of oil requires burial of the organic-rich sediment under more layers of sediment (Figure 3). During the first kilometer or so of burial, the organic matter undergoes a series of chemical and physical changes that results in the rearrangement of the organic matter into a solid substance called kerogen. The kerogen forms a more-or-less continuous, three-dimensional organic network within the rock, and consists predominantly of the elements carbon and hydrogen, with lesser amounts of oxygen, sulphur and nitrogen. These elements are bonded together in a wide variety of molecules, many of which will eventually be broken away from the kerogen during the process of oil generation.
Requirement #3 Burial to depths where temperatures are high enough to "cook" the kerogen, and produce oil
As the organic-rich source rock is buried progressively deeper beneath more layers of sediment, the temperature of the rock increases. This temperature increase is due to the natural increase of temperature with depth below the surface of the earth (the geothermal gradient). As the rock is buried to depths of about 3-4 km, it usually reaches temperatures of about 100-150° C. At this point, the chemical bonds between the various molecules in the kerogen start to break, resulting in the release of several types of molecules, but most are carbon- and hydrogen-bearing molecules (referred to as hydrocarbons). This process is sometimes described as "cooking" of the source rock, and a source rock at this point in its journey from shallow to deep burial is said to be thermally mature and in the main zone of oil generation (often referred to as the "oil kitchen" or "oil window"). As the temperature increases, more bonds are broken and more oil is generated.
Requirement #4 Expulsion of oil from the source rock into an adjacent more porous and permeable layer
FIGURE 4
Once the amount of oil in the source rock builds up to some threshold volume, it will be expelled from the source rock into adjacent porous and permeable layers (Figure 4). This movement of oil from the source rock into an adjacent "carrier" bed is referred to as primary migration. Primary migration is the least understood step in the overall process of oil generation, migration and entrapment in a reservoir.
Requirement #5 Migration of the oil into a sealed trap
Once the oil has made its way from the source rock into a porous and permeable unit (usually sandstone or crystalline carbonate rocks), it will migrate upwards through the water-filled pore spaces in the rock due the effect of buoyancy (oil has a lower density than water, and thus will try to float on top of the water). If the oil encounters an impermeable layer of rock that prevents further upward migration, it will be trapped in the porous reservoir unit (Figure 5). Oil will continue to enter the carrier bed and migrate to the trap until the kerogen in the source rock is no longer able to produce more liquid hydrocarbons. After this point in the thermal evolution of the source rock, the kerogen is only capable of producing the small hydrocarbon molecules of natural gas. The source rock is now overmature with respect to oil generation.
FIGURE 5
Requirement #6 Timing is everything
Oil generation and expulsion from the source rock must occur after the formation of a suitable trap. If no trap is encountered by the migrating oil, then the oil will eventually reach the surface or near-surface and be destroyed by weathering and/or bacterial degradation.