GEOL 115: The Earth’s Energy Resources

Natural gas comprises largely methane. It can be thermogenic in origin; meaning that it is produced by the same processes that produce oil. Organic matter is exposed to heat and methyl groups break off the larger organic molecules while combining with hydrogen to form methane. Natural gas can also have a biogenic origin. Microbes will reduce combine hydrogen and carbon dioxide to produce methane and water following the reaction

CO₂ + 4H₂ = CH₄ + 2H₂O

In some sedimentary environments microbes can derive energy from this reaction and use it to fuel their existence.

Natural gas has traditionally been less valuable than oil. Recall the table in the PowerPoint file from the lecture that showed that you can get roughly three times the calories per dollar from natural gas that you can from oil (based on prices at the start of the semester). This differential exists for a couple of reasons. 1)We do not use natural gas as a transportation fuel (in the US). 2)Significant amounts of natural gas are produced as a byproduct of petroleum production (associated gas) and, therefore, ample supplies typically exist.

In the absence of a market or close proximity to a pipeline in the United States, Canada, Mexico, South American or Eurasia natural gas has little value. When you look at the pipeline maps you will note that many large natural gas fields (e.g. Urengoy) are in sparsely populated areas, however.

Associated gas (gas produced during the production of oil) will typically be flared if a pipeline is lacking. In recent years it was realized that flaring this stranded gas (gas that cannot find a market) represented a significant CO₂ input to the atmosphere and was a wasting a resource. The amount of gas flared would satisfy 27% of US demand for natural gas. You can go here for a video tour of gas flares around the world (warning: its a big file). The World Bank has made reduction of gas flaring a priority. We will consider a method to increase the value of stranded gas later in the semester.

Although there are seven Liquified Natural Gas (LNG) import terminals and one export terminal operating in the US, there is considerable public resistance to construction of these facilities.

Unconventional gas accumulations are exploited in areas close to markets. These plays include tight gas sands, gas-bearing shales, and coal bed methane. All of these accumulations may occur off structure and may require hydraulic fracturing of the reservoir. These plays have generated tremendous interest in the petroleum industry. The Barnett Shale (TX), is probably the best known gas-bearing shale. Tight gas sands are nearly ubiquitous and this article will give you an idea of their widespread nature. Although a variety of basins produce coal bed methane the activity seems to have been particularly intense in the mountain west (in the USA). These accumulations are possible because of the low permeability of the reservoir. As the gas is generated it either is adsorbed on organic matter in the source rock, migrates into pores in the source rock or migrates into a “reservoir” rock. If the rock is insufficiently permeable to allow much migration of hydrocarbons, the gas will only displace the water in the pore spaces. The pore spaces will fill with gas which can migrate only slowly through the rock. In coal bed methane systems the methane is adsorbed on the organic matter in the coal. In all cases you drill into the rock, fracture it if necessary, establish a pressure gradient (by producing any formation fluid such as water) and produce the gas.