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Activities

Generating, Migrating and Trapping Methane Gas (CH4)

Objectives

  1. To determine the organic theory of hydrocarbon formation.
  2. To illustrate the natural occurence of biogenic gas. Although similar, biogenic decomposition is not the same as catagenesis.
  3. To demonstrate how hydrocarbons migrate and are trapped.

Background Information

Most geologist and geochemists believe that hydrocarbons are formed from the remains of plants and animals. This is called the organic theory of hydrocarbon formation. The remains of plants and animals are covered with layers of sediment in rivers, lakes and seas. Shortly after burial, the organic matter in oxygen poor (stagnant) environments begins to decompose due to bacterial action. A byproduct of this decomposition is methane gas. Since this process often occurs in swamps or marshes the gas is commonly referred to as marsh gas. As the layers of gravel, sand, silt and mud were buried deeper and deeper, they gradually formed into sedimentary rock and some of the organic matter was changed into gaseous and liquid hydrocarbons. These hydrocarbons may migrate through pores in the rock until they become trapped.

  • Review the section in the video on source rocks from approximately 07:55 to 8:55 minutes.
  • Review the principals of the Carbon Cycle (Figure 10).
  • Review the sections, from 8:55 to 10:20 minutes, on Porosity, Permeability, Fluid Migration, Traps and Seals.

Materials

  • Two plastic pails
  • Two 4 litre jugs, preferably 1 glass and 1 plastic
  • A large jar, approximately 4 litres
  • An aquarium heater
  • A thermometer
  • Two rubber stoppers, 1 single hole and 1 two hole
  • A clamp for rubber tubing
  • Some rubber tubing and some glass tubing

Method

Preparation of starter brew
In the jar, collect various types of decomposing organic matter: vegetable compost, and kind of manure, pond bottom muck, etc. Fill the jar to within 8-10 cm of the top with the organic matter and then fill to within 2 cm to the top with water. Cap loosely. Keep in a warm place and shake every day. Anaerobic bacteria are growing inside. The longer the brew sits, the more the bacteria will multiply.

Preparation of methane generator
Cut the bottom off the plastic jug. Make up the tubing as illustrated. If it is not readily available, a piece of glass tubing should be stretched to form a flame jet.

In the whole jug, mix the starter brew, some more fresh manure or decomposing material and water and stir until it is relatively lump-free. If necessary, add water until it is the consistency of thick cream. Place this jug in a pail and add water to the pail to the neck of the jug. Put the aquarium heater into this pail and set the temperature at about 30°C. Keep checking that this temperature is maintained.

Add water to the second pail and insert bottomless jug as shown. Be sure the water comes right up to the next so there is little or no airspace after the stopper is inserted. If necessary, weight it down. Add the stoppers and tubing as shown. Don't forget the tubing clamp.

Process

As gas is generated by the decaying organic matter, it moves through the tubing and collects in the second jug by displacing the water; this is similar to how hydrocarbons generated at depth migrate through the pore spaces in the rock until they are trapped and displace water out of the reservoir. Opening the clamp will allos the gas to flow out of the jug in much the same way that a well may produce gas from a reservoir.

In this generator a lot of gas will be produced right away but, until the oxygen is consumed, most of it will be CO2. If you try to light it the flame probably will be blown out. Keep sampling the gas to prevent an excessive buildup in your reservoir. In about 2-3 weeks (depending on temperature, etc.) biogenic methane gas will start being produced and the jet will light.

This experiment demonstrates a relationship between organic matter and hydrocarbons but it does not represent catagenesis. Anaerobic bacteria would soon be killed by the heat of burial. In a more sophisticated laboratory, actual source rocks can be subjected to high temperatures so that the kerogen is converted to oil. The experiment also provides an analogue to the migration and trapping of hydrocarbons.

Activity Diagram
Click to enlarge

Questions

  1. According to organic theory what are crude oil and natural gas made from?
  2. Why do some sedimentary rocks contain oil and natural gas whereas igneous and metamorphic rocks do not?
  3. Why do most sedimentary rocks not contain any oil or natural gas?
  4. Define organic, hydrocarbon, porosity, permeability, reservoir, trap, seal.
  5. To what part of the hydrocarbon generation system does the starter brew equate? Also the tubing, the second jug and the flame jet?
  6. Is the tubing clamp part of the seal?

Expected Answers

   

    Last Modified: 2004-12-10