Non traditional petroleum resources
Traditionally, petroleum geologist has been concerned with the exploration of natural gas and crude oil as the sole source of energy. However, other non traditional petroleum resources hold large amounts of energy and include coal-bed methane, shale gas, oil shales, gas hydrates and tar sands. Their extraction has been difficult mainly since the cost involved in production petroleum from the traditional sources is low therefore inhabiting technological research on their extraction. With the increase in the production cost of the traditional petroleum resources, focus has shifted to development of production methods for the non traditional methods.
Solid and plastic hydrocarbons
These are mostly found as in sedimentary rocks of varied ages. They are mostly viscous but their viscosity would decrease as temperature increases. The hydrocarbons come as pools or lakes on the surface of the earth and distributed in pores and veins on the subsurface. They may occur as secondary deposits or inspissated deposits. Inspissated deposits refer to heavy hydrocarbons which are devoid of the light fraction. This happens where a combination of erosion, uplift and migration brings liquid oil accumulation to the earth’s surface. Secondary deposits may arise when oil flows extensively from oil leak. Secondary deposits are a combination of heavy degraded oil and sediment.
Solid hydrocarbon comes about with dissipation of crude oil at the earth’s surface where its composition progressively changes with the loss of lighter components. There are two types of solid hydrocarbons including waxy solid hydrocarbons and asphaltic solid hydrocarbons. Waxy solid hydrocarbons result from paraffinic crude oil insipissation. Generally, they are plastic substances with their colors ranging from dark brown to waxy yellow. Asphaltic solid hydrocarbons result from naphthenic crude oil insipissation. Both of these are composed of carbon, hydrogen, sulfur, oxygen, and nitrogen as well as inorganic compounds. Their only difference is in the molecular composition.
Tar sands
These are heavy, gluey oil deposits that occur near or at the surface of the earth and have API ranging from 5-15o. They generally occur in highly permeable sands. They undergo a gradational change from the typical crude oil, through heavy oil and to tar becoming heavier and incorporating higher amounts of inorganic impurities. They are more aromatic and sulfurous compared to the crude oils. They have less saturated hydrocarbons but more asphaltenes and resins.
In most cases, tar sands occur where the major basin cross over the Precambrian shields. They either occur in traps that are erosionally breached or migrate to surface seepages from the deep traps. Major deposits of tar sand may be found in Malagasy, Trinidad and Alberta. In extracting oil from the tar sands, subsurface extraction or surface mining approach may be used. After quarrying the tar sands, the constituent oil is extracted using steam or hot water to disaggregate the sand and separate the oil. In case the traditional open cast extraction is not possible due to a thick overburden, in situ extraction must be done either by dissolving oil through solvent injection or heating to reduce the viscosity of the oil.
Oil shales
These are small-grained sedimentary rocks that when heated yield oil. Unlike tar sands, their oil is contained in complex kerogen structures from where it is distilled. Oil shales contain organic components such as kerogens and bitumen as well as inorganic components like quartz, mica, feldspar, carbonates, accessory minerals, pyrite and clays. Oil shales come in two types with well known biological origin. These are Torbanite available from Scotland, and Tasmanite available from Tasmania in Australia as well as in North Alaska.
Oil is extracted from the shales through in-situ extraction method when underground or surface quarrying. Crushed shale is heated in a retort at 425 to 475o centigrade. Kerogen pyrolysis drives of the gas and oil while spent shale would collect the retort’s bottom.
Coal-bed methane and shale gas
Rarely would you have well-defined fields having gas in shales rather it comes in crooked fracture systems and siltstone bands. This gas has over 10% ethane, is commonly wet and has high calorific value. Coal-bed methane differs from shale gas in that it has lower calorific value thanks to the production of dry gas only by the coals. In most cases, coal beds are more permeable than the shales due to easier artificial or natural fracturing.
