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Petroleum 2: How do they know where to drill?

© Geoff Habiger

Last week I covered how the oil and gas that we use is first created and how that petroleum is trapped within reservoirs in the rock. Knowing how the petroleum is formed and eventually trapped is an important first step for the petroleum geologist. It gives the geologist a firm foundation for knowing where to begin looking for petroleum. But how does a geologist know that the petroleum is where they think it is? And once we find a deposit, how do oil companies get the petroleum out of the ground?

First, I want to look briefly at how geologists search for oil. Edwin Drake(1819 - 1880) of the Pennsylvania Rock Oil Company at Titusville Pennsylvania drilled the first producing oil well in the United States in 1859. That first well was dug because other companies, mining for salt, found oil-bearing rocks as they mined the salt. People knew the oil was there, but Edwin Drake was the first person to get the oil out by drilling for it. Similarly, many of the earliest wells were dug in areas where we already knew oil existed, by drilling into rocks that could be observed to seep oil at certain localities. But this method was a hit-or-miss endeavor. Sometimes, wells would be dug that didn’t produce any oil. This becomes a very costly method for drilling for oil. Geologists needed to come up with better methods for exploring and locating petroleum deposits.

To find the petroleum deposits, the geologist uses several tools to study the geology to help locate a petroleum deposit. Studying the structure of the rocks is an important step. Geologists will do this by looking at the structure visible above the ground. They will also use techniques to look at the structures under the ground. The best way to study the underground structures is to send seismic waves into the ground and use devices to record these seismic waves as they bounce off the rocks. This method is similar to how geologists know the structure of the earth’s interior by looking at how earthquake waves bounce off the layers of the earth, but on a much smaller scale. Large trucks (like thos at the left) are used that shake the ground, like a man-made earthquake. This shaking sends seismic waves through the ground that bounce off the different rock layers. These seismic waves are picked up by geophones that record the seismic waves on a log. The seismic log shows the different rock layers and any faults, fractures or other structures.

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