Seeing Underground: Mapping Below the Surface, Experiment Part 1

Our Experiment: Part 1

First you need to learn how to read a well log. Our sample well log, from a borehole called Drill-Hole 1, shows what a geologist would see in a log from an oil-bearing rock. The top of the diagram is at the level of the land surface, and the downward arrow shows the depth of the borehole. This log contains measurement data for porosity and water saturation. You can see how the measurements change with depth.

Scenic glacier

Porosity, shown in the far-left track on the well log, is a measure of the percentage of open space in the rock; the higher the percentage, the larger the spaces within the rock. (See our porosity activity for more information and details.) Pore space can be an indicator of the rock type. The second track, rock type, is derived from the porosity measurements and illustrates this principle. Rock type 1 has a high porosity (15–30%), while rock type 3 has a low porosity (0–15%). In this example, rock type 1 might be a sandstone and rock type 3 might be a kind of shale.

The amount of water filling the pore space in the rock, called water saturation, is also described as a percentage. This measurement is shown in the third track. The rock may be saturated with water, or oil, or a combination of the two. For example, at 50% water saturation, there may be 50% oil, shown in black. Likewise, if we see 10% water saturation, we can assume we have 90% of the pores filled with oil. Based on the water saturation, we can construct the fourth track determining if there is oil or water in the rocks.

Now it is your turn to interpret well logs and incorporate them with seismic data to create a 3-D model of the underground.

Tools and materials

Well logs (print out sheets)
Markers, crayons, or colored pencils
Pencil
Scissors

What to do

Print out the two well log sheets showing logs from three different boreholes. Holes 1A, InCross, and 3A are located fairly close to each other.

The porosity track in each well log indicates the rock type. Color the porosity tracks for the drill holes on both pages using the key below.
- Purple for porosity less than 15%
- Orange for porosity greater than 15%

You can now see where the rock types change in each borehole.

On both pages, use your ruler to draw lines that connect the rock type changes between the drill holes. This gives an approximate idea of how the rock layers change between the holes.

The water saturation track indicates the presence of water or oil in the rock. Color the water saturation track on both pages for all drill holes using the key below.
- Blue for water saturation greater than 75% (This indicates that most of the liquid in the rock is water.)
- Black for water saturation less than 75% (This indicates that a significant amount of oil may be present in the rock.)

One of the places the water saturation values change is at the same level as a change in porosity (and rock type). You already have a line connecting these changes. Use your ruler to draw an additional line on each page between each of the boreholes, connecting the next markers where the water saturation value changes. Now you have a general idea of how the water saturation values change between the holes. Make sure your pages look like our results and then put them aside for now.

For the next part of this activity, you will work with seismic readings.

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