Algae, Bacillariophyta Taken by a scanning electron microscope 1 C H A P T E R O N E : The Origins of Oil and Gas www.shell.com/us/alaska
Sandstone Close-up of textured pattern on a sandstone Organic Origins The story of oil and gas begins hundreds of millions of years ago when the Earth was covered in swamplands filled with huge trees and the seas were teaming with microscopic plants and animals. The oil and gas deposits started forming about 350 to 290 million years ago during the Carboniferous Period, which gets its name from the basic element in oil and gas: carbon. A popular belief is that oil comes from dead dinosaurs. It doesn t. The giant reptiles lived mostly from 250 to 65 million years ago, and most scientists believe oil actually comes from the tiniest plants and animals that preceded them. Source and Reservoir Rocks As they died and sank to the ocean floor, the decomposing organisms, along with mud and silt, created hundreds of feet of sediment. Sand, clay and minerals settled over this organic-rich mud and solidified into rocks. The weight of the rocks above pressed the mud into a fraction of its original thickness. Heat from within the Earth cooked the mud s organic remains into a soup of hydrocarbons, the main element of petroleum and natural gas. Those liquids and vapors were emitted from these source rocks, moved upward through the sediment pores, and accumulated between the grains of the sediment, or reservoir rock. The reservoir rocks often contained water, which then pushed the lighter oil and gas upward until they hit an impermeable rock layer, such as mudstone or salt rock, which becomes a seal or cap rock. The oil and gas are thus trapped in reservoir rocks, usually sandstone or limestone. So as you can see, petroleum* reservoirs aren t underground pools as is commonly believed. They are actually rocks soaked in oil and gas, just as water is held in a sponge. (The word petroleum comes from the Latin for rock oil. ) And they re not alone in that sponge-like home. Other substances such as water, salt, carbon dioxide and hydrogen sulfide can get trapped in the rocks too. Oil and gas, however, contain mostly two elements: hydrogen and carbon. How those elements are arranged determines the form of the hydrocarbon. For example, natural gas contains the simplest hydrocarbon, methane, while crude oils can be made up of more complex liquid and solid hydrocarbons. Oil and Gas Varieties The fluids and gases caught in these traps separate into three layers: water at the bottom, oil in the middle and natural gas on top. The oil, sometimes referred to as crude, is usually black or dark brown, but it can also be yellowish or even greenish. The different * The terms petroleum, hydrocarbons and oil and gas are often used interchangeably throughout this text. For definitions and distinctions, refer to the glossary at the end of this chapter. varieties of crude include sweet, (lower sulfur) and sour (higher sulfur). The terms light or heavy refer to the density of the crude and how easily it flows. The heavier the crude, the harder and more expensive it is to refine for use in products. Oil, rather than natural gas, is what produces the gasoline used in cars. Diesel, jet fuels, heating oil, lubricants and asphalt also come from oil. Natural gas is a vapor, but it can be pressurized and cooled to become a liquid (LNG stands for liquid natural gas) for easier transportation. The vapor is colorless and odorless, but gas companies add a chemical to make it smell bad so leaks can be more easily detected. Natural gas is a fuel used to generate electricity; it is also used in plastics, fertilizers, fabrics and other products. The Migration of Hydrocarbons As the tectonic plates of the Earth s crust shifted over millions of years, some of them would rise and others sink, collisions between plates were common as the land became sea and sea became land. Tremendous pressure from thousands of feet beneath the Earth s surface continued to push the oil and gas into areas of less resistance. Sometimes the caps are not perfect seals and petroleum escapes to the Earth s surface as natural seepage, which can be spotted by oily residue on the surface soil and rocks. Underwater seeps can bubble up to the surface and leave an oily sheen. Early consumers such as Native Americans used blankets to skim oil off the surface of streams and lakes to use in medicines and to water-proof canoes. While seeps can signal that there is oil below ground, they also tell us that much of the oil may have already escaped and there may not be much left to find. 2 THE ORIGINS OF OIL AND GAS THE ORIGINS OF OIL AND GAS 3
Geological History of Alaska s Oil Beneath the Earth s Surface Most oil and gas fields are buried anywhere from one to four miles under the Earth s surface, with the record well at more than seven miles. Reservoirs under the ocean can be covered by as much as two miles of water on top of that. Oil and gas are generally trapped together, but sometimes they separate and form fields containing predominately one or the other. If a reservoir lies deeper than three miles, the liquid petroleum can become overcooked, leaving only the natural gas. Types of Traps The Earth s tremendous forces can change the original form of the reservoir rocks, moving them up, down or sideways, and folding the layers into arches or troughs. These shifts can determine the type of trap in which the petroleum resides. A fold trap is formed when rock layers are pushed upward into an arch or anticline. The hydrocarbons move into the uppermost porous layer of the arch until they hit the impervious rock seal. In a fault trap, the reservoir rock is sealed off along a fault or fracture when earth movements shift the impermeable rock layer over the reservoir rock. Another kind of trap occurs when salt or some other non-porous material is pushed up by pressure from within the Earth to create a dome. The salt dome breaks through the layers and when it meets the reservoir rock it blocks the path of the hydrocarbons, which then accumulate around the pillar. Traps are often a combination of these varieties because the forces creating them can deform the rocks in so many ways. 4 THE ORIGINS OF OIL AND GAS THE ORIGINS OF OIL AND GAS 5
Reserves Around the World Glossary While most of the known oil and gas reserves produced in Russia, Canada, China, Brazil, are held in the Middle East, they can be Norway, Mexico, Venezuela, Great Britain, Anticline an arch of stratified rock in which the layers bend downward Reserves the unproduced but economically recoverable oil or gas in a found in many places around the world, such as Australia, Italy, Malaysia and New Zealand. The leading petroleum producers include Saudi Arabia, Iran, Iraq, Kuwait and the United Arab Emirates. Oil is also Nigeria and the United States chiefly Texas, California, Louisiana, Oklahoma, Kansas and Alaska. Offshore reservoirs have been discovered in the North Sea, Africa, South America and the Gulf of Mexico. in opposite directions from the crest. Anticlines form many excellent hydrocarbon traps. Cap a layer of impermeable (non-porous) rock that seals hydrocarbons that have migrated up into reservoirs. Fault trap a trap for oil or gas in which the closure forming the trap results from the presence of one or more faults. formation that has been proved by production. Reservoir (Rock) a subsurface body of rock having sufficient porosity and permeability to store and transmit fluids. Seal a layer of impermeable rock that forms a barrier to hydrocarbons that have migrated upward into a reservoir rock. Seep the leakage of hydrocarbons that have not been sealed into Hydrocarbons organic chemical compounds of hydrogen and carbon reservoirs by impermeable cap rocks. atoms forming the basis of all petroleum products. They may exist as gases, liquids or solids. An example of each is methane, hexane and asphalt. For this document the terms hydrocarbons, petroleum and oil and gas are Source (Rock) a rock rich in organic matter that if heated sufficiently will generate oil or gas. interchangeable. Tectonic pertaining to the structure of the Earth s crust. Methane a colorless, odorless gas that is stable under widely varying pressure and temperature conditions in the absence of other compounds. It is the lightest and most abundant hydrocarbon gas and the principal component of natural gas. Trap a rock formation that contains petroleum that has migrated upward from a source rock and became trapped by an impermeable layer above and sealed into a reservoir. Petroleum a substance occurring naturally in the earth in solid, liquid, or gaseous state and composed mainly of mixtures of chemical compounds of carbon and hydrogen, with or without other nonmetallic elements such as sulfur, oxygen and nitrogen. In some cases, especially in the measurement of oil and gas, petroleum refers only to oil a liquid hydrocarbon and does not include natural gas or gas liquids such as propane and butane. For this document the terms hydrocarbons, petroleum and oil and gas are interchangeable. Sources American Petroleum Institute Schlumberger Oilfield Services Encyclopedia Britannica Society of Petroleum Engineers Oil and Gas UK The Learning Space (http://openlearn.open.ac.uk) Natural Gas Supply Association U.S. Minerals Management Service, Department of the Interior New World Encyclopedia Photo credits for Geological History of Alaska s Oil and Gas timeline: Space, courtesy of NASA; Rock formation, courtesy of seakayakphoto.com; Zebra mussels, by Amy Benson/ U.S. Geological Survey; Sea Lamprey, courtesy of the Environmental Protection Agency; Stoneflies, courtesy of the Environmental Protection Agency; Spider, by James Solomon, USDA Forest Service; Japanese honeysuckle, by John J. Mosesso, National Biological Information Infrastructure (NBII); Amphibian, by John J. Mosesso, (NBII); Crocodile, by Thomas A. Hermann, (NBII); Lizard, by Randolph Femmer, (NBII); Forest, by Randolph Femmer, (NBII); Bird, by John J. Mosesso, (NBII); Flock of birds, by John J. Mosesso, (NBII); Fox, by John J. Mosesso, (NBII); Flowers, by John J. Mosesso, (NBII); Fossilized tracks, by Thomas A. Hermann, (NBII); Monkey, by John J. Mosesso, (NBII); Lion, by Thomas A. Hermann, (NBII); Glacier, by Russels Gletscher, Free Software Foundation, Inc.; Pangeo map, by Kieff, Free Software Foundation, Inc.; Globes, by Antonio Snider-Pellegrini; Earth, courtesy of U.S. Department of Defense; Dead tree, by Magnus Manske; Allosaurus, by Grzegorz Wysocki; Skate, courtesy of U.S. National Oceanic and Atmospheric Administration. 8 THE ORIGINS OF OIL AND GAS THE ORIGINS OF OIL AND GAS 9
Shell in Alaska To find and develop commercial hydrocarbon resources in the Chukchi and Beaufort Outer Continental Shelf. To support communities where we operate in benefiting from any potential offshore activities economically and socially. To respect the way of life of the residents of Alaska.