To investigate the relationship between oil, gas, and water in a hydrocarbon reservoir and to make predictions about where oil and gas would be located in an oil and gas field.

For porous and permeable rocks to become an effective reservoir for oil and gas (collectively known as hydrocarbons), the overlying seal must have a convex upwards geometry. One way to imagine this is to think of an open umbrella filling with rain that is falling upwards and filling it from below. In nature, the oil rises because of its relatively lower density than the surrounding rocks within the reservoir. The volume of petroleum in a reservoir depends on the amount of petroleum in the source and the horizontal area beneath the seal and the vertical height of the closed area.

Students explore the spatial relationships between water, oil, and gas using a model of a hydrocarbon reservoir. Students explore maps and cross sections for an oil and gas field.

500 mL clear plastic soda bottle, vegetable oil, water, protractor, graph paper.

- In this model, a clear soda bottle will be used to represent an oil and gas reservoir. Ask students to fill the 500 mL bottle to approximately half full. Have them add vegetable oil until the bottle is three-fourths full. Finally ask them to screw on the cap. The water, vegetable oil, and air in this model represent water, crude oil, and natural gas.
- Ask students to slowly turn the bottle upside down. They must ensure that bubbles do not form in the oil.
- Ask them to sketch the model and explain the appearance of the contents of the reservoir.
- Have students complete the following:
- If a well is drilled into an oil and gas reservoir, in what sequence would materials be encountered?

*Students should observe that water is on the bottom followed by oil and then gas (air).* - Explore the relationship between the surface area of the oil and the angle of the oil reservoir. You should tilt the bottle at different angles and make and record observations.
- How does the angle at which the reservoir is inclined affect the thickness of the reservoir?

*The greater the angle of tilt, the larger the surface area covered by the oil.*

- If a well is drilled into an oil and gas reservoir, in what sequence would materials be encountered?

- Have students examine the data in the following table. Point out that it shows the results of drilling of four wells. The elevation of the top of each well was 5000 ft. above sea level.

Feature Well A Well B Well C Well D Elevation of top of VanSant Sandstone 3650 ft 3850 ft 3950 ft 3900 ft Elevation of base of VanSant Sandstone 3450 ft 3700 ft 3825 ft 3800 ft Result Water (dry Hole) Oil at 3850 ft; water at 3800 ft Gas at 3950 ft; oil at 3875 ft Gas at 3900 ft; oil at 3875 ft - Provide students the following map, which shows the locations of the wells. It also shows the top of the VanSant Sandstone layer. The results of drilling are shown by various symbols on the map.

- Have students draw a cross section through the oil field along the east-west line labeled A-A'. Tell them to plot both the top and the bottom of the VanSant Sandstone layer. They should do this by placing the edge of a sheet of paper along the line of the cross section. The positions of the geologic structure in the map can then be marked along this line. The depth of the top and bottom of the VanSant sandstone can be found by subtracting the respective depths from the 5000 ft elevation of the surface. From these measurements the subsurface geology, the water zone, the oil zone, and the gas zone can be determined.
- Ask students to use their results to show the elevation of the gas, oil, and water within the cross section.
- Have students show the extent of the oil and gas by coloring them green and red on the map. You may want to show them the following example from another oil and gas field.

Source: EarthComm - Tell students that a field geologist has recommended drilling at two more sites, E and F on the contour map. Ask students to use their maps and cross sections to determine whether or not these are good places for drilling.

*Drilling at E is a good recommendation, but drilling at F will only produce a dry well.*