So, now we know where the organic material for petroleum come from, we know how it’s converted from it’s original state to a liquid/gas state, we know how it migrates out of the source rock and through other lithologies, now we need to know where it ends up, and how do we get it out. This section will also go into some of the controversies and massive misconceptions about oil drilling, hydrofracking, and contaminations of groundwater. First, let’s get to the science of the whole thing:
In the previous chapter, we explored the concept of a moving mass of petroleum, whether it be liquid or gaseous in character, through different rocks because of the differences in buoyancy in relation to water. So, we now need to understand how and where this petroleum is trapped. First, lets start with a major misconception that a lot of people seemingly have about oil reservoirs: It is not, again, not a big pool of oil in the ground. The oil and gas are trapped within the matrix porosity, or, to say another way, the spaces in between the grains of the material that makes up the rock that it is trapped in. Let’s make an easy example:
We all remember those ball-pits from when you were a child right (figure 1)? The huge padded pits full of multi-colored balls that are absolutely hilarious until you come across a section of wet ones which subsequently sends you into a fury of panicked activity trying to find the nearest exit and shower.
Imagine if those balls had a piece of glass placed on the top of the whole pit (to make sure none float away) and then start pumping oil into the pit. You might want to take the children out first. Or not, doesn’t really affect the outcome of this experiment. Anyway, you’ll noticed that the oil only occupies the space between the balls. This is exactly what happens in an oil reservoir: imagine the plastic balls as quartz grains in a well-sorted mature sandstone with a porosity of about 30% (it’s a high number I know, but for example sake, we’ll take a Navajo sandstone porosity) and the oil is, well, oil. Now, you might be thinking to yourself, “but Cameron, sand is much smaller than those awesome balls, how can any liquid pass through a rock? It’s impossible!” Well, dear reader, I assure you, that you can have a liquid pass through a rock. You just need geologic time scales.
We can take an even more appropriate example: You’re at the beach drinking a beer (figure 2; because you’re a respectable geologist) and you are watching the waves come in and out. You’ll notice that the wave comes up to a certain point and then retreats back again, but you’ve also noticed that the water isn’t just sliding down the beach back into the ocean like if it were on glass, some of it disappears into the beach itself! Curious, you take a handful of that sand, and you press it with all of your might, and some water dribbles out. Having oil migrating, being stored, and then extracted through a sandstone is no different, it’s just it’s under more pressure and the grains are a bit closer together.
Let’s get into where the petroleum resides. There are many complexities upon these styles, but we will go into just the basics of it for now, and if you want to know more, let me know, and I’ll write a much more in-depth report.
First, there are many different elements to a hydrocarbon trap, namely a reservoir rock, top seal, lateral seals, bottom seals and a sympathetic structure for trapping (figure 3).
In the above schematic from Biddle and Wielchowsky, 1994, we can see all of these elements in play with each other. In figure 3A, this is what is known as a structural trap, and in figure 3B, it is a stratigraphic trap. These are just two distinctions that are made in the petroleum industry to better describe and separate the different types of traps and is very important for describing the geologic setting that they occur in.
The structural trap in A has slightly dipping sandstone reservoirs (remember in the past chapter where I described that migration in a sandstone happens in a slightly dipping bed) and is sealed on one side by a fault, the top by a shale, and the bottom with a shale. The petroleum is now trapped.
What are these seals usually made of? Well, a seal is anything that doesn’t allow the hydrocarbons to pass through it, so it could be tight shales, salt, gypsum, a fault, or an unconformity (a boundary of erosion).
The same exact principal is in effect with stratigraphic traps. The only differences between a stratigraphic and structural trap is that the stratigraphic traps are not directly related to a structure such as a fault or anything fault created (such as anticlines).
Figure 4 depicts examples of structural traps from Biddle and Wielchowsky 1994.
For comparison, here are some variations on stratigraphic traps (figure 5; Biddle and Wielchowsky, 1994)
So, now we have all of the petroleum in one spot. How do we get it out? In part 2, we will talk about the main methods of oil and gas recovery, the good, the bad, the unfounded, and the conspiracies.
Biddle K.T., C. Wielchowsky 1994, Hydrocarbon Traps in Magoon, I,. B, and W.G. Dow, eds., 1994, The petroleum system – from source to trap: AAPG Memoir 60.
Traps and oil/gas production with commentary, Part I