The Geology of Oil Traps Activity

The following are my notes for the exercise that resulted in the Oil Traps and Deltas in the Sandbox post.

Trapping Oil

Crude oil is extracted from layers of sand that can be deep beneath the land surface, but it was not created there. Oil comes from organic material, dead plants and animals, that sink to the bottom of the ocean or large lakes. Since organic material is not very dense, it only reaches the bottom of ocean in calm places where there are not a lot of currents or waves that can mix it back into the water. In these calm places, other very small particles like clay can also settle down.

Formation-of-oil-traps — Figure 1. Formation of sandstone (reservoir) and shale (source bed).

Over time, millions of years, this material gets buried beneath other sediments, compressing it and heating it up. Together the organic material and the clay form a type of sedimentary rock called shale. As the shale gets buried deeper and deeper and it gets hotter and hotter, and the organic matter gets cooked which causes it to release the chemical we know as natural gas (methane) and the mixture of organic chemicals we call crude oil (see the formation of oil and natural gas).

Formation-of-oil-traps2 — Figure 2. The trapping of oil and natural gas by a fault.

Shale beds tend to be pretty tightly packed, and they slowly release the oil and natural gas into the layers of sediment around them. If these layers are made of sandstone, where there is much more space for fluids to move between the grains of sand, the hydrocarbons will flow along the beds until they are trapped (Figure 2).

In this exercise, we will use the wave tank to simulate the formation of the geologic layers that produce oil.

Materials

Wave tank
Play sand (10x 20kg bags)
Colored sand (2 bags)

Observations

For your observations, you will sketch what happens to the delta in the tank every time something significant changes.

Procedure

Fill the upper half of the tank with sand leaving the lower half empty.
Fill the empty part with water until it starts to overflow at the lower outlet.
Move the hose to the higher end so that it creates a stream and washes sand down to the bottom end — observe the formation of the delta.
Observe how the delta builds out (progrades) into the water.
After about 10 minutes dump the colored sand into the stream and let it be transported onto the delta.
After most of the colored sand has been transported, raise the outlet so that the water level in the tank rises to the higher level. — Note how the delta forms at a new place.
After about 10 more minutes dump another set of colored sand and allow it to be deposited on the delta.
Now lower the outlet to the original, low level and observe what happens.
After about 10 minutes, turn off the hose and drain all of the water from the tank.
When the tank is dry, use the shovel to excavate a trench down the middle of the sand tank to expose the cross-section of the delta.

Analysis

1. How did changing the water level affect the formation of the delta.

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2. When you excavated the trench, did you observe the layers of different colored sand in the delta? Draw a diagram showing what you observed. Describe what you observed here.

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3. Was this a realistic simulation of the way oil reservoirs are formed. Please describe all of the things you think are accurate, and all of the things you think are not realistic?

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The Formation of Oil and Natural Gas

When organic material is buried it is compressed and “cooked” because the deeper you go beneath the surface of the earth the hotter it gets. This causes the breakdown of the organic matter and the production natural gas and oil. The stages of decomposition are:

Diagenesis:

Decomposition of biological material produces methane gas. At slightly higher temperatures and pressures the organic matter is converted to kerogen – an unorganized (amorphous) material of carbon, hydrogen, and oxygen.

Catagenesis:

At higher temperatures and pressures kerogen is altered and the majority of crude oil is formed. During this phase and the next, the larger molecules break down into simpler molecules such as octane and propane (a process called cracking).

Metagenesis:

In the final stage of alteration (at higher temperatures and pressures) of kerogen and crude oil, natural gas (mostly methane) is produced and residual carbon is left in the source rock.

Oil does not come from dinosaurs.

phytoplankton — Phytoplankton (image from NASA).

There’s a nice article in the New York Times on the fact that oil, petroleum, did not come form dead dinosaurs, but rather from the microscopic plankton that died and fell to the ocean floor.

The idea that oil came from the terrible lizards that children love to learn about endured for many decades. The Sinclair Oil Company featured a dinosaur in its logo and in its advertisements, and outfitted its gas stations with giant replicas that bore long necks and tails. The publicity gave the term “fossil fuels” new resonance. – Broad, 2010

It’s easy to forget how pervasive is the idea that oil comes from dinosaurs. Broad’s article is a nice reminder that:

Today, a principal tenet of geology is that a vast majority of the world’s oil arose not from lumbering beasts on land but tiny organisms at sea. It holds that blizzards of microscopic life fell into the sunless depths over the ages, producing thick sediments that the planet’s inner heat eventually cooked into oil. It is estimated that 95 percent or more of global oil traces its genesis to the sea. – Broad, 2010

How do we know?

[I]n the 1930s. Alfred E. Treibs, a German chemist, discovered that oil harbored the fossil remains of chlorophyll, the compound in plants that helps convert sunlight into chemical energy. The source appeared to be the tiny plants of ancient seas. – Broad, 2010

phyto-bloom — Phytoplankton bloom off the Carolina coast. (Image from NASA).

We tend to find a lot of oil in the deltas of the great rivers because the rivers provide nutrients for the microorganisms to survive and layers of sand and clay sediments that trap the oil and natural when they’re produced.

The article also ties the location of oil production to the geography of plate tectonics.

[W]hen Africa and South America slowly pulled apart in the Cretaceous period, forming the narrow beginnings of the South Atlantic. Big rivers poured in nutrients. A biological frenzy on the western shores of the narrow ocean ended up forming the vast oil fields now being discovered and developed off Brazil in deep water. – Broad, 2010

Scientists tracking oil plumes

filter-1140m-watermk1 — Filter after 10L of plume water was passed through it -- visible oil! (from Joye, 2010)

As oil continues to leak from the damaged well in the Gulf of Mexico and the surface slick is affecting more and more of the coastline, scientists now using research vessels to track the underwater plumes spreading at depth throughout the gulf.

station34b_watermark — Dissolved oxygen, CDOM and beam attenuation with depth (from Joye, 2010).

Satellite imagery from NASA only shows what’s at the surface. To find the underwater plumes, researchers on boats lower instruments on cables that measure the chemistry of the water. Certain chemicals, like colored dissolved organic matter (CDOM) are produced when there is a lot of oil in the water.

Dr. Samantha Joye, from the University of Georgia, is the lead scientist on one such vessel. She started the Gulf Oil Blog where she describes her ongoing work in the gulf and answers readers questions. It is an excellent resource. A great demonstration of science in action, working on a practical problem but using techniques and methods developed over time for answering more abstract questions.

blog-5-28-10-wake-of-oil — Oil in the wake of a ship (from Joye, 2010)

Oil slick

gulf_amo_2010115 — Oil slick in the Gulf of Mexico on April 25th, 2010. Image from NASA.

The scale of the disaster caused by the oil leaking into the Gulf of Mexico from the damaged oil rig is increasing day by day. We are preparing to go on the end-of-year adventure trip soon, but I’m wondering if students might be interested in heading down to the Gulf coast to volunteer in the clean-up.

oil-ships — Scale of the slick. Image from NASA (April 25th).

NASA’s Earth Observatory has some amazing imagery on its page on the oil leak. Many of the images also show the mouth of the Mississippi and its delta, which tie directly into our observations in the sandbox. The impact of the oil spill also brings up the topic of density differences in fluids, something we’ve seen in the making bread jars, but applied to a much larger scale.

Oil traps and deltas in the sandbox

sandbox-colored-sand — Red and green sand added for marker beds.

The sandbox was built to be a wave tank so we could look at interference patterns and wave properties. But if you tilt it a little, and put in a few holes on the lower end, you can get sandbox to look at the formation of streams, deltas and the sedimentary layering that traps oil and natural gas.

Using the holes at the bottom end the students started with a low “sea-level”, raised it and lowered it. At the end of the run, they drained all the water and sliced the tank to see the depositional layers in cross-section.

We added red and green sand to try to make marker beds before each change in base level. The marker beds worked reasonably well, but it would have been better to have sand with different densities that could be sorted by the stream flow and depositional environment. It also helps to get the colored sand wet, to make a slurry, otherwise the grains will float on the water.

The shifting lobes of the delta showed up very well (see the animation) and some nice river features showed up as well. What I want to do sometime is to have students build coastlines and have waves erode them away creating typical coastal features.

My students were even able to demonstrate the tank for their presentation, because it really only takes half an hour to get all the features if you know what you’re aiming for.

Sources

The exercise these results are based on is posted as The Geology of Oil Traps Activity.

Methane hydrates for energy

Despite the fact that methane is a powerful greenhouse gas itself and burning it produces carbon dioxide there is currently quite a bit of research on extracting methane hydrates from the sea floor as an alternative to the traditional fossil fuels because there is just so much of it. Discovery Channel has an interesting video on the topic where they burn some methane hydrate ice.

Methane releases from the arctic and sea-floor could also trigger rapid climate change. Recent discoveries suggest that global warming is warming the arctic so much that the permafrost is melting an releasing a lot of methane into the atmosphere. If the arctic atmosphere continues to warm, more methane will be released, causing more warming …. This positive feedback loop would accelerate global warming. Some scientists worry that warmer ocean waters can melt methane hydrates at the sea floor releasing them into the atmosphere in a similar positive feedback loop.