Text 7 alternative reserves of hydrocarbons

Task1.

Tar Sands (Oil Sands)

So far, we’ve focused our discussion on hydrocarbon reserves that can be pumped from the subsurface in the form of a liquid or a gas. But in several locations around the world, most notably Alberta (in western Canada) and Venezuela, vast reserves of very viscous, tarlike “heavy oil” exist. This heavy oil, known also known as bitumen, has the consistency of heavy molasses, and thus cannot be pumped directly from the ground; it can fill the pore spaces of sand or of poorly cemented sandstone, constituting up to 12% of the sediment or rock volume. Sand or sandstone containing such high concentrations of bitumen is known as tar sand or oil sand.

The hydrocarbon system that leads to the generation of tar sands begins with the production and burial of a source rock in a large sedimentary basin (specifically, a type of basin called a foreland basin, which forms along the continent side of a mountain belt). When subjected to temperatures of the oil window, the source rock yields oil and gas, which migrate into sandstone layers and then up the dip of tilted layers to the edge of the basin, where they become caught in stratigraphic traps. Initially, these hydrocarbons have relatively low viscosity; in the geologic past, they could have been pumped easily. But over time, microbes (bacteria) attacked the oil reserve underground, digested lighter, smaller hydrocarbon molecules, and left behind only the larger molecules, whose presence makes the remaining oil so viscous. Geologists refer to such a transformation process as biodegradation. Generation of tar sand by biodegradation is yet another example of the interaction between physical and biological components of the Earth System.

Production of usable oil from tar sand is difficult and expensive, but not impossible. It takes about 2 tons of tar sand to produce one barrel of oil. Oil companies mine near-surface deposits in open-pit mines, then heat the tar sand in a furnace to extract the oil. Producers then crack the heavy oil molecules to produce smaller, more usable molecules. Trucks dump the drained sand back into the mine pit. To extract oil from deeper deposits of tar sand, oil companies drill wells and pump steam or solvents down into the sand to liquefy the oil enough so that it can be pumped out.

Oil Shale

Vast reserves of organic- rich shale have not been subjected to temperatures of the oil window, or if they were, they did not stay within the oil window long enough to complete the transformation to oil, and thus still contain a high proportion of kerogen. Shale that contains at least 15 to 30% kerogen is called oil shale. Oil shale is not the same as coal, because the organic matter within it exists in the form of hydrocarbon molecules, not as elemental carbon.

Lumps of oil shale can be burned directly, and thus have been used as a fuel since ancient times. In the 1850s, researchers developed techniques to produce liquid oil from oil shale. The process involves heating the oil shale to a temperature of 500°C; at this temperature, the shale decomposes and the kerogen transforms into liquid hydrocarbon and gas. Large supplies of oil shale occur in Estonia, Scotland, China, and Russia, and in the Green River Basin of Wyoming in the United States. As is the case with tar sand, production of oil from oil shale is possible, but very expensive. In addition to the expense of mining and environmental reclamation, producers must pay for the energy needed to heat the shale. It takes about 40% of the energy yielded by a volume of oil shale to produce the oil.

Natural Gas

Natural gas consists of volatile short-chain hydrocarbons, including methane, ethane, propane, and butane. It occurs in the pores of reservoir rock above oil, because it “floats” over the oil. Where temperatures in the subsurface are so high that oil molecules break apart to form gas, gas-only fields develop.

Gas burns more cleanly than oil (burning gas produces primarily carbon dioxide and water, whereas burning oil also produces complex organic pollutants), and thus has become the preferred fuel for home cooking and heating. But gas transportation requires expensive high-pressure pipelines or container ships, so even though gas is much more abundant than oil, the world’s population still consumes more oil than gas. However, in recent years many industries and electrical generation plants have switched to natural gas, so demand for gas has increased.

Gas Hydrate

Until now, we have discussed natural gas as if it occurs only in association with petroleum deposits, and as if it forms only as a result of the breakdown of hydrocarbons at high temperatures in deeply buried sedimentary rocks. However, researchers increasingly recognize that the Earth System holds vast quantities of hydrocarbons in other forms. We will discuss coalbed methane later in this chapter, after we discuss coal. Here, we introduce gas hydrate.

Gas hydrate is a chemical compound consisting of a methane (CH4) molecule surrounded by a cage-like arrangement of water molecules. An accumulation of gas hydrate produces a whitish solid that resembles ordinary water ice. Gas hydrate forms when anaerobic bacteria (bacteria that live in the absence of oxygen) eat organic matter such as dead plankton that have been incorporated into the sea floor. When the bacteria digest organic matter, they produce methane as a byproduct, and the methane bubbles into the cold seawater that fills pore spaces in sediments. Under pressures found at water depths greater than 300 m, the methane dissolves in water and produces gas hydrate molecules; the reaction can occur at shallower depths at colder, polar latitudes. Exploration tests suggest that gas hydrate occurs as layers interbedded with sediment, and/or as a cement holding together the sediment, at depths of between 90 and 900 m beneath the sea floor. Geologists estimate that an immense amount of methane lies trapped in gas hydrate layers. In fact, worldwide there may be more organic carbon stored in gas hydrate than in all other reservoirs combined! So far, however, techniques for safely recovering gas hydrate from the sea floor have not been devised.