(2) The formation, migration, and trapping of oil and gas. Explain with the use
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Question
(2) The formation, migration, and trapping of oil and gas. Explain with the use of a neat, labeled, informative sketch the oil/gas accumulation process from start to finish, i.e., from formation of kerogen, oil, & gas in the source rock, to trapping and sealing. You must include: the material from which oil/gas form, the role of temperature (oil window), the density controls and rock property controls (porosity & permeability) on the movement of oil/gas, source rock, reservoir rock, and cap (seal) rock, and oil traps and seals. This may be done with one or two sketches.
Explanation / Answer
Step 1: Diagenesis forms Kerogen
Diagenesis is a process of compaction under mild conditions of temperature and pressure. When organic aquatic sediments (proteins, lipids, carbohydrates) are deposited, they are very saturated with water and rich in minerals. Through chemical reaction, compaction, and microbial action during burial, water is forced out and proteins and carbohydrates break down to form new structures that comprise a waxy material known as “kerogen” and a black tar like substance called “bitumen”. All of this occurs within the first several hundred meters of burial.
The bitumen comprises the heaviest components of petroleum, but the kerogen will undergo further change to make hydrocarbons and, yes, more bitumen…
Step 2: Catagenesis (or “cracking”) turns kerogen into petroleum and natural gas
As temperatures and pressures increase (deeper burial) the process of catagenesis begins, which is the thermal degradation of kerogen to form hydrocarbon chains. Importantly, the process of catagenesis is catalyzed by the minerals that are deposited and persist through marine diagenesis. The conditions of catagenesis determine the product, such that higher temperature and pressure lead to more complete “cracking” of the kerogen and progressively lighter and smaller hydrocarbons. Petroleum formation, then, requires a specific window of conditions; too hot and the product will favor natural gas (small hydrocarbons), but too cold and the plankton will remain trapped as kerogen.
Because the earth is filled entirely by layers of solid (or at significant depths) molten rock, the petroleum it contains cannot exist within a self-contained “lake”, but must decide to live within the small fraction of space (or pores) that exist in these rocks. Like the sponge in your kitchen sink (albeit, less spongy and a bit heavier) certain kinds of rock (mainly sandstone and limestone) contain pores large enough and with enough connections to serve as storage and migration sites for oil or water or any other fluid wishing to call them home. Because most hydrocarbons are lighter than water and rock, those that exist within the earth will tend to migrate upwards until they reach the surface, or are trapped by an impermeable layer of rock
There is a particular window of temperature that the zooplankton must find to form oil. If it is too cold, the oil will remain trapped in the form of kerogen, but too hot and the oil will be changed (through “thermal cracking”) into natural gas. Therefore, the formation of an oil reservoir requires the unlikely gathering of three particular conditions: first, a source rock rich in organic material (formed during diagenesis) must be buried to the appropriate depth to find a desirable window; second, a porous and permeable (connected pores) reservoir rock is required for it to accumulate in; and last a cap rock (seal) or other mechanism must be present to prevent it from escaping to the surface. The geologic history of some places on earth makes them much more likely to contain the necessary combination of conditions
Step 1: Diagenesis forms Kerogen
Diagenesis is a process of compaction under mild conditions of temperature and pressure. When organic aquatic sediments (proteins, lipids, carbohydrates) are deposited, they are very saturated with water and rich in minerals. Through chemical reaction, compaction, and microbial action during burial, water is forced out and proteins and carbohydrates break down to form new structures that comprise a waxy material known as “kerogen” and a black tar like substance called “bitumen”. All of this occurs within the first several hundred meters of burial.
The bitumen comprises the heaviest components of petroleum, but the kerogen will undergo further change to make hydrocarbons and, yes, more bitumen…
Step 2: Catagenesis (or “cracking”) turns kerogen into petroleum and natural gas
As temperatures and pressures increase (deeper burial) the process of catagenesis begins, which is the thermal degradation of kerogen to form hydrocarbon chains. Importantly, the process of catagenesis is catalyzed by the minerals that are deposited and persist through marine diagenesis. The conditions of catagenesis determine the product, such that higher temperature and pressure lead to more complete “cracking” of the kerogen and progressively lighter and smaller hydrocarbons. Petroleum formation, then, requires a specific window of conditions; too hot and the product will favor natural gas (small hydrocarbons), but too cold and the plankton will remain trapped as kerogen.
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