To kick off our series on the basics of oil and gas exploration and development, we wanted to answer some fundamental questions on the topic, including:
Where do we find most oil or gas, and why?
We’re looking for oil. So … why do we like holes in our rocks?
Which is objectively better, a jelly donut or tiramisu?
Oil and gas accumulations require five components: source, migration, reservoir, trap and seal. If any of them is lacking, there’s no hope. What are these things, and why are they important?
Figure 1: Elements of a conventional hydrocarbon trap (Source: JOGMEC).
Carbon-rich rock that gets heated and squished until hydrocarbons form.
Sedimentary rocks, typically shale but can also be limestone.
Hydrocarbons moving from the squished source to an area with more room for them.
Faults, fractures and even tiny pores in the rock allow hydrocarbons to move about.
The area with more room for the hydrocarbons.
Usually sandstone or limestone with pores that hold hydrocarbons.
Something that causes the hydrocarbons to get stuck as they try to “float” upward in the subsurface.
An upside-down bowl — or dome — as shown below, or another kind of dead end.
A rock layer that acts as a lid, keeping the hydrocarbons stuck in the trap.
Shale commonly makes a fine seal, but salt, though rarer, is even better.
Hydrocarbons? Just a fancy word commonly used to refer to oil, gas and other fossil fuels. It describes their molecular structure, generally a chain of carbon atoms, each of which is surrounded by hydrogen atoms. Firing up the gas grill tonight? You’ll be burning a bunch of these little fellas, C3H8, aka propane.
Figure 2: Propane molecule (Source: Ben Mills, Wikimedia Commons).
Propane happens to be a gas at atmospheric pressure and temperature, as are other short-chain molecules like methane (the most common constituent of natural gas) and butane (think cigarette lighters). Longer chains form liquids such as the one we commonly call octane, a constituent of gasoline that is known to science nerds as 2,2,4-Trimethylpentane, or (CH3)3CCH2CH(CH3)2. That’s a lot of carbon and hydrogen atoms! Okay, enough chemistry…
As you can see, sedimentary rocks are our friends when it comes to oil and gas. Because of this, we tend to look for hydrocarbons in sedimentary basins — places that are (or were) big holes in the earth that were filled with sediments. The Gulf of Mexico is a good example, as is the North Sea. Both are still actively filling with sediment. On the other hand, Texas’ giant and prolific Permian Basin and central Europe’s Pannonian Basin (mainly in Hungary and Romania) are ancient basins that are no longer filling with sediment but are already home to many sedimentary rocks full of oil and gas. Sometimes, it gets even crazier, and the ancient basins get caught up in plate tectonic movements that result in their hydrocarbon-rich sedimentary rocks being uplifted into hills and mountains. Many of Southern California’s onshore fields and most of the Rocky Mountain fields are good examples of this. As you can see from the map below, active and ancient basins cover much of the earth with their sediment at this point.
Figure 3: Global Sedimentary Basins (Source: Enverus).
How do these basins — these big holes in the earth — form? Two main ways: extension and compression, illustrated in the figures below. The hole that forms is technically referred to as “subsidence.”
In extensional basins, subsidence usually results from the earth’s crust thinning or faulting (breaking) — the sort of thing that happens when two tectonic plates pull apart from one another. These basins are typically formed along coasts.
Figure 4: Diagram of extensional basins (yellow) on both sides of the Atlantic Ocean (Source: National Park Service).
In compressional basins, subsidence usually results from two tectonic plates colliding. These collisions can pile up mountains of rock so heavy that they push down the adjacent continental crust. Hence, these basins often form adjacent and parallel to mountain belts.
Figure 5: A very, very simple diagram of one kind of compressional basin (Source: Raynaldi rji, Wikipedia).
Wait! What about the jelly donuts and tiramisu? For that, dear reader, you will have to wait for the next blog in this series, where we will discuss the two major types of oil and gas plays and whether it is better to be conventional or unconventional …
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