The sediments that accumulate on the sea floor gradually become thicker. It is a very slow phenomenon. From a few meters to a hundred or so meters every million years, the source rock is pushed down gradually under the accumulation of sediments that continue to be deposited. Fortunately, their weight provokes a progressive sinking which leaves room free for further sediments that continue to accumulate. This phenomenon, known as subsidence, is characteristic of sedimentary basins. It is a phenomenon of large amplitude.
With the progressive sinking the sediments can reach several thousand meters in thickness, sometimes more than
8000 meters (8 km!) at the centre of the basin.
The heat increases in the source rock that is gradually pushed down and buried, the temperature of the substratum rising on average by 3 ° C with every hundred meters of extra depth. The organic matter is also crushed more and more by the weight of the sediments, the pressure increasing by 25 bars per 100 meters. Consequently, at a depth of 1 km, the temperature is already 50°C and the pressure 250 bars.
The organic matter is transformed very slowly as the atoms of carbon
and hydrogen are reorganised and brought together. The nitrogen, sulphur and phosphorous, other essential elements for life, are gradually eliminated and the organic matter is transformed into kerogen.
A temperature of around 100°C is necessary for the kerogen to begin to generate liquid hydrocarbons, oil and gas. This corresponds roughly to a burial at between 2200 and 3800 meters. The sinking continues and the production of liquid hydrocarbons attains a maximum, a peak. The liquids produced become lighter and tend more and more towards gas. Between 3800 and 5000 meters, the kerogen begins to produce the lightest of the hydrocarbons, methane gas. Thus, gradually, the source rock produces liquids and finishes by producing gas, before, finally, exhausting its potential. The interval between the depths at which the source rock produces liquids is called the oil window. The interval at which it produces gas is called … the gas window, of course!
The relative proportions of liquid and gas produced depend on the nature of the source rock. For example, if the organic debris of which it is composed is principally of animal origin, it will produce proportionately far more liquids. Conversely, if plant debris dominates, it produces above all gas and not many liquids.
In fact, if we consider oil generated at a depth of 3000 meters, and estimate an average sedimentation of 50 meters every million years, then 60 million years will have been necessary for these dead animals to be transformed into liquid hydrocarbons. It is not surprising that we classify oil as a non-renewable energy, is it? |