Seeing Underground: Mapping Below the Surface, Teacher's Notes

Focus question

How do scientists use well logs and seismic information to map the underground and find likely locations of oil reservoirs?

Tools and materials

Well logs (print out sheets)
Markers, crayons, or colored pencils
Ruler
Pencil
Seismic data (print out sheets)
Glue
Scissors
Tape

Main ideas and background information

Without information about the underground layers of the Earth, it is difficult to determine the best places to drill for oil and gas.
There is no single method that is best for gathering information about the underground. Scientists usually use a combination of methods.
Two common methods are well logging and collecting seismic data.
Well logging records information using special instruments and sensors that are lowered into an existing borehole in the ground. With these instruments, it is possible to obtain highly detailed information. However, the information from well logging applies only to the region near the borehole.
Seismic data can be collected by sensors that detect the sound from special explosions or “thumping” vehicles at the surface as it is reflected by the underground layers of the Earth. From these data, scientists can piece together an approximate “picture” of the underground layers over a relatively large area.
When the information from well logs and seismic sources is added together, a more complete representation of the underground can be constructed. With this composite information, especially when it is represented in computerized form, scientists are able to pinpoint likely areas where oil and gas can be found.
For more information about 3-D modeling of underground rock layers, read the CyberGeologist article.

Procedural tips

The well log files are in black-and-white, and the seismic files are in color; however, the activity can be done with only black-and-white printouts. A color printer is not necessary.
The well logs represent three different boreholes that are located fairly close to each other. For example, the distance between Drill Holes 1A and InCross on the printout could represent an actual distance of 3.2 to 4.8 km (2 to 3 mi).
When students draw the layer lines between the drill holes, in step 3 of Part 1, make sure that they understand they are interpolating, or estimating, the general behavior of the underground layers between the holes. At this point, there is no way to know for certain how the layers go up and down between the holes. The only thing we know is that the layer we are tracking at 1A is probably the same layer that shows up near the surface at InCross.
In Part 2, step 1, students are marking the likely lower boundary of a possible oil reservoir.
In Part 2, step 2, the marker bed is a slightly darker color on the printouts. In actuality, this is often a layer of rock with lower porosity or higher density.
Encourage students to use a bright color for the marker bed so it will stand out easily when viewed from a distance.
In Part 2, step 3, we recommend using a glue stick or white glue for backing the pages up against each other.
Students can check the alignment of the features and marks before gluing by holding the two pages up to a window or other source of light. They should not depend on the edges of the paper for the correct alignment.
Starting with Part 2, step 4, make sure students cut carefully along the dashed lines.
For Part 2, step 6, make sure that students cut the slits safely. We recommend that they fold the paper over to start each slit cut, as shown in the activity photograph.
In Part 2, step 7, make sure that the symbol of each tab matches the symbol of each corresponding slit.
In Part 2, step 11, help students get a sense of the three-dimensionality of the seismic cube by imagining what the layers between the pages might look like. In particular, help them sense the shape of the dome marked out by the marker bed.
In Part 2, step 17, help students see the correspondence between the “estimating” lines they drew based on the well logs and the actual underground pattern of the marker bed. Also help them imagine the porous layer of rock beneath the marker bed with its reservoir of oil-saturated rock.
Note that the porous layer of rock beneath the marker bed contains a level saturated by oil (on top, because oil is less dense than water), with a level beneath saturated by water.
Remind students that the process they have just completed of combining the well logs and seismic data into a three-dimensional model is exactly what geologists do with high-powered computers and cutting-edge software.

Discussion Questions

In what ways do the marker beds change from one side of the cube, through the middle, and to the other side of the cube?

From the outside edges of the cube, the marker beds slope upward toward the middle, forming a dome shape.
Imagine that the marker bed was at the surface. How would you describe it? What would it look like?

On the surface, the marker bed would look like a dome-shaped hill.
Do you see any breaks, shifts, or misalignments in the reflection layers? These could be considered faults, or places where the rock has shifted, and the layers are no longer continuous.

Faults can be seen in the interior sections and are common features of the underground.
Imagine yourself going down into the seismic cube. How does the porosity change as you move down through the marker bed? How does the water saturation change?

The porosity is low above the marker bed and high below it. The water saturation is high above the marker bed, low immediately below the marker bed, and then high again.
Which volume of the cube is likely to contain oil? Why?

The volume of high porosity and low water saturation likely contains oil. The porosity allows fluid to accumulate, and the low water saturation suggests that something else, likely oil, has a high saturation
What would the rock beneath the oil reservoir be saturated with? Why?

The rock beneath the oil reservoir would be saturated with water, which is denser than oil. Oil floats on water, so in the rock it would rise above the water.
Where would be the best place for a production oil well? Why?

The best place to drill for oil is where the two cross pieces of seismic data meet, in the middle of the cube. Drilling in this location puts you directly above the portion of the reservoir that is closest to the surface and has the greatest volume of rock with high porosity and low water saturation.

Assessment

Describe, in your own words, how scientists use well logs and seismic information to map the underground and find likely locations of oil reservoirs.

(Students should describe the well log and seismic components of the activity and how the information is used together to create a more comprehensive picture of the underground layers. By looking for regions of high porosity and low water saturation, likely oil reservoir locations can be identified.)

Extensions and further investigations

Have students research current software used by geologists to map the underground layers of the Earth.
Challenge students to design their own seismic cubes, exchange them with others, and find likely locations of oil reservoirs.
Assemble several seismic cubes side by side to create a three-dimensional display of a larger underground region. What patterns emerge?

Correlations with Standards

United States: This activity correlates with portions of the National Science Education Standards (NSES) Content Standard A: Science as Inquiry and Content Standard F: Science in Personal and Social Perspectives, Grades 5–8 and 9–12, and with the following additional standards:

Grades 5-8
Earth and Space Science : D: STRUCTURE OF THE EARTH SYSTEM

Grades 9-12
Earth and Space Science: D: GEOCHEMICAL CYCLES

The activity also correlates with the following portions of the Next Generation Science Standards (NGSS):

Grades 5-8
Performance expectations MS-ESS2-2 (p. 80) and MS-ESS3-1 (p. 83), and disciplinary core ideas ESS2.A (p. 81) and ESS3.A (p. 84).

Grades 9-12
Performance expectation HS-ESS2-3 (p. 122), and disciplinary core ideas ESS2.A (p. 123) and PS4.A (p. 124).

Glossary / vocabulary

anaerobic bacteria: bacteria that can live without oxygen.
fault: a fracture in the rock where the two sides have moved either adjacently up or down relative to one another.
marker bed: a distinctive layer of rock spread over a large area that can be readily identified and correlated.
porosity: a measure of the percentage of pores (open spaces) in a material.
reservoir: a subsurface body of rock with sufficient porosity to store fluids—sedimentary rocks are the most common reservoir rocks.
water saturation: a measure of the percentage of water in a given pore space.

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