Components of a Petroleum System
There are five critical components needed in order to maintain a petroleum system where hydrocarbons can be found. These five features include a mature source rock, a migration pathway, a reservoir rock, a trap, and a seal. In order to drill for and recover oil and gas, a complete petroleum system is necessary.
What is a Seal?
A seal is a relatively impermeable, a seal is a layer of rock that forms a barrier or cap above and around a reservoir rock. Commonly composed of shale, chalks, clays, anhydrite or salt, a seal helps prevent fluids from migrating beyond the reservoir. It is sometimes also referred to as a cap rock.
Where are Seals Located?
Seals are closely associated with the traps they surround. Some common traps associated with petroleum systems include structural traps such as folds, anticlines, faults, and diapirs (Figures 1, 2, & 3). Stratigraphic pinchout traps are also common in petroleum systems as well (Figure 1(c)). Traps can have top, lateral and bottom seals. In figure 1 (a), the hydrocarbons are trapped in an anticline and sealed with a top, lateral and bottom seal. Figure 1 (b) shows hydrocarbons trapped by a fault, surrounded by top, lateral, and bottom seals again. Figure 1 (c) shows hydrocarbons trapped in a stratigraphic pinch-out, completely surrounded by the seal. Figure 1 (d) depicts hydrocarbons trapped by a tight unconformity, incased by top and lateral seals. Figure 2 represents a salt diaper trap, in which hydrocarbons are trapped against the salt dome and beneath the overlying top seal. Similarly, Figure 3 represents a shale diaper trap in which hydrocarbons within a heavily fractured shale dome migrate up and are trapped by the overlying top and lateral seals.
Figure 1 (A) - In an anticline trap, hydrocarbons (black) have migrated to the top of the reservoir rock (brown), and are trapped by the surrounding gray seal. (B)- In a fault trap, the rapid movement of the fault has severed and displaced the reservoir rock. Gravity has allowed the hydrocarbons to migrate upwards within the reservoir layer, where they are trapped by the fault and surrounding seal rock. (C)- In a stratigraphic pinchout trap, gravity has allowed the hydrocarbons to migrate to the tip of the reservoir rock. Surrounding seal rock that cuts off the pinchout prevents the hydrocarbons from further migration. (D)- In an unconformity trap, hydrocarbons have migrated to the top of the reservoir rock and are trapped laterally and above by the surrounding seal rock involved in the unconformity.
Figure 2- Salt diapir hydrocarbon trap. A salt dome is created by the movement of salt due to increased pressure and temperature from the deposition of rock layers above it. Oil migrated from the lower green layer into the light yellow reservoir rock layer. Oil is then trapped between the salt dome and the overlying seal, creating a salt diapir trap.
Figure 3- Hydrocarbons trapped within a heavily fractured shale diapir have migrated through the fractures to the top of the shale dome where they are trapped by an overlying seal rock.
How Thick Can Seals Be?
Rock seals can vary from layers of a few feet thick, to several hundreds of feet thick. Although thicker seals are usually more effective for hydrocarbon accumulation, if a seal has low permeability, is ductile, laterally continuous, and has high capillary entry pressures it will still be very effective regardless of its thickness.
Effective Seals for Hydrocarbon Accumulation and Exploration
Capillary Entry Pressures
Typically, effective seals have capillary entry pressures that range from 14-20,000psi.
1) Shaly siltstone and sandstone, anhydrite-filled dolostone, and cemented sandstone. Seals of these lithologies are typically most ductile and therefore very effective.
2) Sandy shales
3) Clay-mineral-rich shale, silty shales, and dense mudstones.
4) Evaporites and kerogen-rich shale are most effective seals for containing oil.
Clay Smears and Shale Gouge
When dealing specifically with fault traps, there are a few other aspects to consider when estimating the quality of the seal. When a fault displaces layers of shale that are rich with clay content, clay smears and shale gouge can appear. Fault traps themselves are not always reliable when it comes to the petroleum system and hydrocarbon accumulation. In order for geophysicists and geologists to trust a fault trap prospect, they need to have a good understanding of the seals involved.
Clay Smear Factor
A clay smear is formed when the fault causes a ductile and continuous smear of a high clay content shale. To calculate the clay smear factor, geophysicists use the equation: CSF = (T) throw/ (H) thickness CSF = clay smear factor (T) throw = the vertical component of separation caused by the fault (H) thickness = the thickness of the shale bed The user defines the critical clay smear factor to use. A clay smear factor of three is commonly used as a standard critical clay smear factor because in general, it tends to define a continuous smear. Therefore, as long as the throw is less than or equal to three times the thickness of the shale bed, the smear is going to be continuous (Figure 4 (b)). If the throw is greater than three times the shale bed thickness, it will become discontinuous (Figure 4 (a)). Choosing higher clay smear factors will result in a longer, continuous smear calculation along the fault. It’s important to narrow the clay smear factor down a few times in order to accurately define the clay content available (Faerseth, 2006). This calculation will help estimate how low the permeability of the fault rock is. If the fault rock ends up being low in permeability, its effectiveness as a seal increases.
Shale Gouge Ratio
The shale gouge ratio is the most commonly used algorithm for estimating clay content involved in fault trap seals. The shale gouge ratio is used by geophysicists when probable clay-rich shale mixing has occurred along the fault.
To calculate the shale gouge ratio, geophysicists use the equation:
SGR= ((Σ Vcl ∆z))/(T)throw x 100
SGR= shale gouge ratio Σ Vcl ∆z = volume fraction of clay in an interval of thickness z T= throw across the fault
Essentially this equation helps average the clay content of the section that has been displaced past a certain point. Similar to the clay smear factor, the shale gouge ratio involves the throw interval across the fault. Within this interval, the average clay content is calculated for a single point using the equation listed. This calculated clay content is then used by geoscientists to contour the clay interval across the fault. If the contour reveals a thick clay interval, it is a more reliable seal.
Case studies of Petroleum Systems and their seals
In this section I will briefly discuss three different case studies and will focus on the seals involved in each. I will provide 3 external links to journals that go more in depth about each case study.
Downey, MW. (1984). “Evaluating Seals for Hydrocarbon Accumulation”. https://pubs.geoscienceworld.org › article-pdf › aapg_1984_0068_0011_1752 Faerseth, Roald B..(2006). “Shale smear along large faults: continuity of smear and the fault seal capacity”. Journal of the Geological Society. 163, 741-751. https://doi.org/10.1144/0016-76492005-162 Freeman, B et al. (2016). “Fault seal prediction: the gouge ratio method”. Structural Geology in Reservoir Characterization.127, 19-25. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1020.6598&rep=rep1&type=pdf
- Downey, MW. (1984). “Evaluating Seals for Hydrocarbon Accumulation”. https://pubs.geoscienceworld.org › article-pdf › aapg_1984_0068_0011_1752