by Franklin Foster, Ph.D.
It is commonly assumed that petroleum formation is a function of the increasing burial of the source rock. However, other factors than the burial history; such as the type of originating kerogen, and the geothermal gradient are also important.
|How Crudey sees it
I like it at least 50°C. Where I live under
Lloydminster, it's cooooold - only 21°C.
If I could move closer to the center of the Earth,
it would be warmer and that would help me
grow up faster into a healthy crude oil.
Moving me deeper, makes me warmer, and
speeds up my oil formation. Otherwise,
I'll be down here another 100 million years!
"Kerogen" refers to the organic component of source rock. It began as living organisms, became sediments (comparable to top soil), and then, as they were overlaid, began the change from sediment to sedimentary rock (a process known as "diagenesis"). The original organic material, and the microbial activity it contained, influenced the extent and nature of the hydrocarbons which are eventually produced. The first hydrocarbon generated is methane (sometimes called "biogenic gas").
When the depth of burial increases (this happens quite inconsistently at times, thus we need to know the "burial history". Depth does not automatically increase. There can be periods when the burial depth decreases). When burial depth does increase, the most important thing that happens is that the temperature increases. This is the geothermal gradient. Pioneers in the Lloydminster oil fields used a rule of thumb of 1°F per 100 feet. While the worldwide, average geothermal gradient is 23.5°C per kilometer, this varies from place to place, depending on the thermal conductivity of the substances in the lithology, the amount of ground water, etc.
The basic principle is that an undisturbed formation's temperature increases predictably with depth. The increase in temperature with depth is known as the "geothermal gradient" (G), and is approximately 1.8 degrees Celsius per 100 meters for Canada. The temperature at any given depth can therefore be estimated by the following equation:
Tf = Ts + (D x G)
where: Tf = formation temperature [0Celsius]
Ts = average annual "surface" temperature [approximately 11° Celsius for the Lloydminster region] Note: the term "surface" actually refers to a depth of about 3 meters where the soil temperature remains constant and is not effected by seasonal variations above.
G = geothermal gradient [1.8° Celsius/100 meters]
D = depth [meters]
Field data confirms this with readings of oil temperature of 21°C for a well at 580 m depth.)
Ts + (D * G) = Tf
(11) + (580 * .018) = 21.4
As for pressure, as this article explains, pressure is more a
factor in oil migration than in oil production.
However, reported pressures in the field are approximately 575 psi or 40.4 kg/cm2 at the depth of 580 meters.
Exercise: do the math!
During an appreciable amount of time, thought to be roughly 100 million years, little transformation occurs, as both hydrocarbons and kerogen are metastable under near surface conditions and need sufficient increase in temperature and passage of time for molecular transformations to be initiated. When the temperature has increased to a minimum of 50°C (approximately 2,200 meters depth, locally), the atomic bonds in the kerogen begin to be broken. Oxygen elimination is important during this phase and results in the formation of both CO2 and H2O. The first petroleum products liberated include mostly sulphur, nitrogen and oxygen compounds of high molecular weight, particularly asphaltenes and resins. Gas may also be generated, depending on the original nature of the organic material.
As the temperature increases, more and more bonds are broken, including carbon to carbon bonds. Hydrocarbon molecules, particularly aliphatic chains, form from the kerogen and the previously generated S, N and O compounds. Early hydrocarbons released are C15 to C30 biogenic molecules but later hydrocarbons, which will form the bulk of the oil zone, dilute these with their medium to low molecular weights. This is the principal stage of oil formation.
As burial depth, and more importantly, temperature, increases, the breaking of carbon to carbon bonds occurs more frequently. Light hydrocarbons are produced through this cracking and their proportion increases rapidly in the source rock hydrocarbons and forming petroleum deposits. The overall transformation occurring during this stage (catagenesis) is equivalent to a process of disproportionation. On the one hand, hydrocarbons of increasing hydrogen content are generated. On the other hand, the residual kerogen becomes depleted of hydrogen.
Thus, we have seen that oil and gas are produced from the kerogen in the source rock through a succession of chemical reactions. These reactions are governed by the usual kinetics of chemical reactions. Therefore, the transformation depends primarily on temperature and time. Pressure appears to be only important as an influence on oil migration rather than oil formation, although, as we have seen, burial depth is important to obtain the requisite temperatures.
The respective role of temperature and time has been examined in some laboratory experiments. From these, and from observations in many oilfields throughout the world, it has been concluded that the transformation of organic material is influenced more by temperature than time: i.e. the influence of time is linear, while the influence of temperature is exponential. For these reasons, some very ancient organic sediments have been discovered at depths of less than 200 meters and they have not advanced toward the formation of petroleum.
Because of variations in burial history, the elapsed time from sedimentation until the rock is at sufficient depth to initiate oil formation may vary from a few million years to more than 300 million years. The source rock in the Western Canadian Sedimentary Basin, whether Devonian, or Lower to Middle Cretaceous, produced most of their petroleum in the late Cretaceous and lower Tertiary eras. This means that the principal stage of oil formation began more than 300 million years after the disposition of the Devonian source rocks, but only 40 million years after the disposition of the Cretaceous source rock. One variable which might explain this puzzling phenomenon is depth of burial and the consequent temperature.
In short, the Devonian rocks were too shallow for millions of years; then, when they and the Cretaceous rocks reached sufficient depth, oil formation occurred fairly rapidly. Then, due to the massive erosion for which there is evidence [lower Mississippian to the lower Cretaceous ( a period of almost 200 million years) plus a second section, from the Eocene to the present] and other reasons, the depth of burial decreased.
Further evidence of this derives from field observations where the temperature of oil produced in the Lloydminster region is less than 22°C. This is to be expected given the depth of the oil zones are in the 550 to 650 meter range. The temperature needs to be at least 20°C higher (some fields in other parts of the world produce crudes up to 115°C). To increase the temperature of the Lloydminster reservoirs by the minimum 20°C it would be necessary to increase their depth by another 1,100 meters. Given the current net disposition rate of 1 cm per 100 years, this will take approximately 11 million years. At the minimum temperatures at this level it would take approximately 100 million years for a process of cracking the complex carbon bonds in heavy oil molecules to take place.
In short, Lloydminster's "heavy oil problem" could well be solved in under 100 million years by "letting nature take its course". Just 100 million years from now, Lloydminster could be sitting atop large reservoirs of "light oil". Stay tuned.