Reservoir Capacity of Igneous, Sedimentary, and Metamorphic Rocks

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Updated on May 19, 2015

Determine which category of rock (igneous, sedimentary, metamorphic) has the ability to hold oil and by implication natural gas.

Develop a qualitative data table of the oil holding capacity of various rock samples.Learn how petroleum geologists usereservoir rock formations to figure out where to drill for oil.


  • Sandstone, limestone, marble, and shale rock samples
  • Disposable plastic plates
  • Eyedropper
  • Watch or clock
  • Mineral oil

Research Questions

  • Which rock sample soaked up the oil the fastest?
  • Which rock held the least oil?
  • Which type of rock would probably make the best oil reservoir?
  • Based on the results of this experiment, if you were a petroleum geologist which rock strata would you look for reservoir rock that might contain deposits of oil and natural gas?
  • Why is porosity (pore space) in rock layers important to oil and natural gas accumulation?
  • Why is oil and natural gas called fossil fuels?


  1. Place each rock sample in separate dishes.

  2. Fill the medicine dropper with the mineral oil. Place 5 drops of oil on each rock sample.

  3. Observe and record the time required for the oil to be soaked up by each of the rock samples.

Name of Rock Sample

Time required to Soak up the Oil



  1. Using the data in the table, plot a bar graph of Rock Sample along the x-axis vs. the Time required to soak up the oil along the y-axis

  2. As an extension activity for students in the 7th and 8th grades the actual amount of oil absorbed by the rock samples can be determined by recording the exact amount of oil added to each rock and then carefully measuring, using a measuring cup, how much oil was left after a given amount of time.


Some sedimentary rocks are porous, like a sponge. Tiny particles of sand are held together with rock "cement." Pressure, time and sediments create this natural type of "cement." Oil and natural gas form from decayed plant and animal material. Over time, the many layers of sand and sediments are compacted into sedimentary rock. Tiny spaces, or pores, exist between the particles that enable the rock to hold a liquid.

Petroleum oil is frequently mixed with water underground and, since oil floats on water, the oil tends to migrate upward. Sometimes, though, it comes up against impermeable rock, through which it cannot pass. Then it becomes trapped and slowly accumulates, forming a reservoir between the particles that form rock. Sedimentary (sandstone&limestone) rocks are the most common reservoir rocks because they have more porosity than most igneous and metamorphic rocks.

In summary, a rock with pores is referred to as porous. This means it has tiny holes through which oil and gas may flow. Reservoir rocks must be porous, because oil and natural gas can only become trapped inside the pores.

Mike Calhoun is a consultant for the National Science Teachers Association, a veteran science teacher, and hosts an online science website. Over the years Mike has studied trends in science, education, and finance, conducting research, developing programs, and writing articles on these topics.

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