Experiment

This activity is based on a project conceived and carried out by SEED volunteers Jeffrey Cook and Ashley Russell.

Background

Geologists searching for oil need to create maps of rock formations to figure out where that oil is. But how can they see what is below the surface of the Earth? And how do they create those maps?

There are a number of ways geologists can acquire the data that tell them what is below the surface. One common method is well logging. Well logging is a process of gathering and recording geological information from deep within the Earth through special instruments lowered into boreholes. Well logs give information about the various layers of rock as the instruments travel down the hole. Geologists then use these logs to make up a layered cross section representing a picture of the Earth around the drill hole. If they have well logs from many drill holes, they can connect the layers and estimate the missing information between them. This then produces a rough three-dimensional (3-D) model of the Earth.

Logging tools

A logging tool about to be lowered into a well

If you are not already familiar with well logging, the SEED activity Build An Electrical Logging Tool can help you understand how it works.

Well logging is useful but it provides information only for the rocks near the borehole. How can geologists fill in the gaps between the boreholes with more accurate information? The answer is by collecting seismic data.

Seismic data is obtained by sensors that record how sound is reflected by the underground layers of the Earth. The sound waves are most commonly created in either of two ways: by large vibrator trucks that “thump” the ground or by planting and detonating specially designed explosives. The sound waves travel through the Earth, and the readings of how fast those sound waves travel give geologists information about the underground layers—the type of rock, where the layers change, and whether they are sitting flat or at an angle. The seismic readings are done in lines. The lines are put together to create cross-sections of the area being studied.

Taken together, the well logs and the seismic data give enough information for geologists to construct a 3-D picture of the subsurface of the Earth using computers and special software programs to place the borehole data and seismic cross sections in the correct order. The resulting 3-D maps of subsurface layers enable them to predict where hydrocarbons might be found.

In this activity, you are the geologist. You have found oil-bearing rocks but are searching for the location of the reservoir, the rocks with most of the oil. You will use well logs to create a cross section of the Earth between boreholes and then build a 3-D model of that area using seismic data. Once those steps are completed you can apply your well log information to the 3-D model to create a map of underground rock layers and identify the location of the oil reservoir.

Let's get started by learning to read a well log.