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Date: 2015-10-07; view: 584.

Unit 1. A Brief Outline of the Petroleum Chain

Chapter 1.

LEARNING OBJECTIVES:

Having worked through this chapter the student will be able to:

· describe the petroleum chain

· describe chemical and physical properties of petroleum

· distinguish between main rock types: reservoir rocks, source rocks andcarrier beds

· describe the petroleum formation process including primary and secondary migration

· distinguish between two main types of oil traps: stratigraphic and structural traps

· describe main characteristic of the cap rock

· list and describe the properties of different types of sedimentary rocks

Petroleum isa complex mixture of hydrocarbons that occurs in the Earth in liquid, gaseous, or solid forms; the term is usually restricted to the liquid form, commonly called crude oil. Both liquid and gaseous phases of petroleum occur naturally underground and, taken to­gether, constitute the most important of the primary fossil fuels.

Petroleum was known to many ancient peo­ples through surface seepages. Excavations in Iran, Iraq, and elsewhere show that bitumen, or asphalt, a form of petroleum, was used to caulk ships, to build roads, and for other purposes. Europeans of the age of exploration found similar seepages of the black liquid in the Americas and in Indonesia.

The first important modern use of petroleum was as an illuminating fuel to replace whale oil in lamps. The first well drilled specifically for oil was that of E.L. Drake at Titusville, Pa., in 1859; within a few decades oil drilling was widespread in the United States, Europe, the Middle East, and East Asia. The development of the automobile gave petroleum a new and swiftly expanding role as the primary source of gasoline. Petroleum also is the source of kerosene, gas oil, lubricating oil, and resid­ual fuel oil. It is refined and processed into numerous other products, such as solvents, paints, asphalt, plastics, synthetic rubber, fi­bres, soaps and cleansing agents, waxes and jellies, medicines, explosives, and fertilizers.

Petroleum is derived from aquatic plants and animals that lived and died hundreds of mil­lions of years ago. Their remains mixed with mud and sand in layered deposits that, over the millennia, were geologically transformed into sedimentary rock. Gradual decomposition of the organic matter contributed to the produc­tion of petroleum in the rock structures. Since the Paleozoic era (from about 570 million to 245 million years ago), the organic mat­ter has been slowly moving from the original compacting clays to more porous and perme­able rocks, such as sandstones and siltstones, where it finally became entrapped. Such en­trapped accumulations of oil are called pools. A series of pools within a common reservoir rock structure or a series of pools in separate but neighbouring reservoir rock formations is commonly referred to as an oil field. A group of oil fields in similar geologic environments is known as a petroleum province.

Crude petroleum, pumped from the Earth through drilled wells, is costly to locate, re­cover, and process, but it is the world's largest energy source, accounting for approximately 39 percent of all energy consumed.

Crude oils vary considerably in their com­positions, but the major components are hy­drocarbons (compounds of hydrogen and car­bon). Despite the variations in the molecular structure of the compounds, most crude oils contain from 82 to 87 percent carbon by weight and from 12 to 15 percent hydrogen. Most crude oils contain a mixture of three se­ries of compounds - paraffin, naphthene, and aromatic. The paraffin series is the broadest, extending from methane, which forms natural gas, through the liquids that are refined into ordinary gasoline, to crystalline waxes. The naphthene series ranges from volatile liquids to tarry asphalts. The chief member of the aro­matic series is benzene. Petroleum also vary in their physical properties, such as volatility, colour, and specific gravity.

Almost all crude oil comes from beneath the Earth's surface, but a few petroleum lakes, or tar pits, are known (a good example is the La Brea Tar Pits in Los Angeles). The extraction of petroleum and natural gas from the Earth involves three basic steps: prospecting, drilling, and recovery. About four-fifths of the world's proved petroleum reserves in land areas have now been developed, and development of offshore deposits has assumed increasing importance since 1960.

In oil exploration, geologic techniques can determine only the existence of rock forma­tions that are favourable for oil deposits, not whether oil is actually there. Drilling is the only sure way to ascertain the presence of oil. With modern rotary equipment, wells can be drilled to depths of more than 7,600 m (25,000 feet). Once oil is found, it may be recovered (brought to the surface) by various methods. Underground pressure created by natural gas or water within the reservoir can force the oil up; if the natural pressure is not great enough, it can be raised by artificially injecting water or steam into the reservoir through a well. Enhanced recovery processes may be utilized to increase production. These processes involve injecting such substances as carbon dioxide, polymers, and solvents into a reservoir to reduce oil viscosity and capillarity. Thermal recovery methods are frequently used to enhance the production of heavy crude oils, whose extraction is impeded by viscous resistance to flow at reservoir temperatures.

Petroleum is transported to refineries by pipelines, which can often carry more than 500,000 barrels per day, or by oceangoing tankers.