4.5 What are Oil Sands vs. Oil Shales? Petroleum Geology

Oil sands, also called tar sands, are usually mixtures of clay and sand combined with water and varying amounts of a black, highly viscous tar known as bitumen. The term sand can be misleading, because not all deposits are associated with sands and sandstones. Some occur in other materials, including shale and limestone. The oil in these deposits is very similar to heavy crude oil pumped from wells. The major difference between conventional oil reservoirs and oil sand deposits is in the viscosity, or resistance to flow, of the oil they contain. In oil sands, the oil is much more viscous and cannot simply be pumped out.

Substantial oil sand deposits occur in several locations around the world. The two largest are the Athabasca oil sands in the Canadian province of Alberta, and the Orinoco River deposit in Venezuela.

Currently, oil sands are mined at the surface in a manner similar to the strip mining of coal. The excavated material is then heated with pressurized steam until the bitumen softens and rises. Once collected, the oily material is treated to remove impurities and then hydrogen is added. This last step upgrades the material to a synthetic crude, which can then be refined. Extracting and refining oil sand requires a great deal of energy, nearly half as much as the end product yields. Nevertheless, oil sands from Alberta’s vast deposits are the source of about one-third of Canada’s oil production.

Obtaining oil from oil sand has significant environmental drawbacks. Substantial land disturbance is associated with mining huge quantities of rock and sediment. Moreover, large quantities of water are required for processing, and when processing is completed, contaminated water and sediment accumulate in toxic disposal ponds.

About 80% of the oil sands in Alberta are buried too deeply for surface mining. Oil from these deep deposits must be recovered by in situ technologies. Using drilling technology, steam is injected into the deposit to heat the oil sand, reducing the viscosity of the bitumen. The hot, mobile bitumen migrates toward producing wells, while the sand is left in place. In Situ technology is expensive and requires certain conditions, such as a nearby water source. Production using In situ techniques already rivals open pit mining and in the future may well replace mining as the main source of bitumen production from the oil sands. Challenges facing in situ processes include increasing the efficiency of oil recovery, management of water used to make steam, and reducing the costs of energy required for the process.

What is oil shale and how is it different from oil sands? Oil shale contains enormous amounts of untapped oil. Worldwide, the US Geological Survey estimates that there are more than 3,000 billion barrels of oil contained in shales that would yield more than 10 gallons of oil per ton of shale. But this figure is misleading because only a small percentage of that is recoverable with present technology. Still, estimated U.S. resources are about 14 times as great as those of conventionally recoverable oil, and estimates will probably increase as more geological information is gathered.

Roughly half of the world’s supply is in the Green River formation in Colorado, Utah, and Wyoming. Within this region the oil shales are part of sedimentary layers that accumulated at the bottoms of two vast, shallow lakes during the Eocene epoch, between 57 million to 36 million years ago.

Oil shale has been suggested as a partial solution to dwindling fuel supplies. However the heat energy in oil shale is about 1/8 that in crude oil, owing to the large proportion of mineral matter in the shales.

This mineral matter adds cost to mining, processing, and waste disposal. Producing oil from oil shale has the same problems as producing oil from oil sands. Surface mining causes widespread land disturbance and presents significant waste disposal problems. Moreover, processing requires large amounts of water, something that is in short supply in the semi-arid region occupied by the Green River formation.

.
.
.