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By Jeff Danner Jeff has worked in both the chemical and biotech industries and is the veteran of thousands of science debates at cocktail parties and holiday dinners across the nation. In his Common Science blog, Jeff aims to make technological and scientific concepts accessible to all.

Petroleum: 300 Million Years of Sunlight

By Jeff Danner Posted June 1, 2011 at 1:59 am

This is part IV of my series on Petroleum. To start at the beginning read, Nothing Matters More Than Oil, Nothing, Photosynthesis Part I: Oxygen Gets All the Press, and Photosynthesis Part II: Glucose Needs a New Agent. If you want to understand all of the issues, economic, political, and scientific about oil then you have to follow the long journey of a carbon atom, starting as part of a carbon dioxide molecule in the atmosphere, bumping into a chlorophyll-containing organism, being converted to glucose, being further converted to a more complex hydrocarbon, being buried under ground for 300 million years, being extracted from the ground, being refined into gasoline, powering your lawn mower, and finally returning to the atmosphere as a carbon dioxide molecule.

 The impact of petroleum on the world and our daily lives is so pervasive that sitting down to start a blog entry about it is rather daunting. Most people associate petroleum with gasoline and diesel fuel which is correct but is only part of the story. What seems to be farther from everyone’s consciousness is that petroleum is the raw material, and the only feasible raw material, for most of the products which we use in our daily lives here in Chapelboro. Here is a short list of products which come from petroleum: plastics, paint, many pharmaceuticals, shampoo, shoe soles, soaps and cleaning products, fertilizers and pesticides essential to our current model of food production, modern medical devices like stents, heart valves and artificial joints, etc. Imagine trying to live without those items. I make this point because most public discussions of a world in which oil becomes scarce (which will happen, the only debate is when) tend to focus primarily on the transportation fuel issues with only a small afterthought as to the importance of these other vital products. 

The origin of petroleum is a long and complex story. So here is the short version. Starting about 3 billion years ago (See Photosynthesis Part I: Oxygen Gets All the Press) a microorganism called cyanobacteria evolved from its ancestors and was the first organism to perform photosynthesis. Cyanobaceteria and its descendants, including all green plants and algae, generated the oxygen which we breathe. The bacteria and algae from ancient lakes and oceans also used the energy from the sun to capture carbon dioxide from the air and convert into the hydrocarbon molecules. (The energy of the sunlight used in the process is now stored in the chemical bonds of the hydrocarbons storing it for later use). As the algae in the ancient oceans died it fell to the bottom and mixed with mud. As millions upon millions of years passed this algae-mud mixture was covered by additional sediment and sometimes was buried even more deeply through the movement of continental plates. 
 
As you go deeper into the earth the temperature increases and there is no free oxygen. (The lack of oxygen is important. If you heat a hydrocarbon near free oxygen, the hydrocarbon burns and creates smaller, less useful  molecules.)  As the hydrocarbons from the dead algae were heated in the oxygen-free ground they started to decompose and form new complex and useful chemical compounds. At shallower depths and lower temperatures they formed a waxy, non-useful substance called kerogen. At depths of about 1.5 to 2.5 miles, called the “Oil Window”, where temperatures range from 60-120 oC petroleum was formed through a process called pyrolysis. Below the Oil Window where the temperatures are even higher around 120-200 oC (the “Gas Window”) natural gas (methane, CH4) was formed.

Since the oil and gas were smaller and lighter than the original molecules from the dead algae they started to migrate upward towards the earth’s surface. A pocket of gas and/or oil forms when after rising up through something permeable like sand stone, the oil encounters something impermeable like shale and stays there. These pockets of trapped oil and gas are what the oil companies around the world drill for. If the oil does not reach something impermeable it will migrate all the way to surface. People have been using this surface oil for things like asphalt and water proofing since Old Testament times.

I’ll devote the next blog entry in this series to Peak Oil theory, but here is a preview. For several hundred million years oil was slowly forming under the ground. Starting in the late 1800’s we began to extract the oil. As oil drilling technology improved and we learned the geology of the Oil Window, discovery and extraction (I dislike the use of the word “production” when describing the output of an oil well) of petroleum increased. Over time the easy-to-find oil had been extracted making it increasingly more difficult and expensive to locate a new gusher. Consequently the peak year for new oil field discovery was 1966 (the year I was born) and the rate of discovery of new oil fields has been steadily declining ever since. Given that the Oil Window is finite and we have been exploring it and drilling into it ceaselessly for over a century, talk of new finding new “elephant” wells becomes increasingly unrealistic. The best estimate is that approximately half of the world’s oil endowment has already been extracted, the easy half.   Step back and consider this for a moment. It took around 300 million years for the earth to produce petroleum and in nl about 100 years we have extracted and used half of it.)  Extracting the remaining half of the oil will become increasingly less efficient and eventually is will require more energy to remove it from the ground than the energy which can be produced from it. This will bring the end of the age of petroleum.

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