Energy & Resource Development of Continental Margins (eBook)

Don Walsh, Teh Fu Yen (Herausgeber)

eBook Download: EPUB

2013 | 1. Auflage
249 Seiten
Elsevier Science (Verlag)
978-1-4831-8943-7 (ISBN)

Infos

Energy & Resource Development of Continental Margins is a collection of papers that addresses issues in the development of energy technology and exploration of energy sources. The title first covers concerns in geological and geochemical explorations. Next, the selection deals with the issues in utilizing novel methods for energy resource recovery. The text also tackles environmental constraints and pollutant analysis. The last section talks about the social impact and safety of onshore and offshore activities. The book will be of great use to scientists, engineers, and technicians involved in the research, development, and implementation of energy technology.

Textauszug

1

EXPLORING FOR OIL AND GAS ON THE CONTINENTAL SHELF

Crandall D. Jones,     Exxon Company, U.S.A.

Publisher Summary

The practice by the cartographers of vertical exaggeration has fostered the mistaken belief that the continent is surrounded by submarine cliffs. In actuality, there is a total of 853 thousand square miles of continental shelf surrounding the United States that slopes gently into the abysal deep. The vast majority of it presents no obstacles in drilling for oil and gas. Except by drilling, there is no way to confidently measure the reserves of an area. Offshore drilling normally requires a number of successful wells in an area before the endeavor can be considered economically feasible. But, because of the lengthy time lag between discovery and production, the exploration of the remaining frontier areas should be rapidly pursed. The continental shelf is usually defined as that portion of the continent that extends from the shoreline seaward to a depth of approximately 200 meters (600 feet). The continental slope is the portion of the continent lying between ocean depths of 600 and 6,000 feet.

Introduction

The practice by cartographers of vertical exaggeration has fostered the mistaken belief that the continent is surrounded by submarine cliffs. In actuality, there is a total of 853 thousand square miles of continental shelf surrounding the United States which slopes gently into the abysal deep. The vast majority of it presents no obstacles in drilling for oil and gas. Except by drilling, there is no way to confidently measure the reserves of an area. Offshore drilling normally requires a number of successful wells in an area before the endeavor can be considered economically feasible. But, because of the lengthy time lag between discovery and production, exploration of our remaining frontier areas should be rapidly pursed.

The Continental Shelf

The continental shelf is usually defined as that portion of the continent which extends from the shoreline seaward to a depth of approximately 200 meters (600 feet). The continental slope is the portion of the continent lying between ocean depths of 600 and 6,000 feet. That portion of the North American continental shelf, bordered by the United States, comprises some 853,300 square miles, with the continental slope providing an additional 450,000 square miles. The continental shelf varies in width from area to area. For example, in the Gulf of Mexico and in the Bering Sea, there are broad continental shelves which extend more than a hundred miles from shore before the water depth exceeds 600 feet. In other areas, notably the West Coast, the continental shelf is much narrower. However, of the combined total of 1,300,000 square miles which constitute the area of the continental shelf and slope adjacent to the borders of the United States, only 3% has been leased for exploration for gas and oil to date.

Because maps attempting to depict the earth’s surface in three dimensions must resort to considerable vertical exaggeration, many people have the mistaken idea that the continent is surrounded by precipitous submarine cliffs. Cartographers depart from the use of true scale in many maps, for instance, some of the maps published by the National Geographic Society. Comparison of these maps and true scale cross-sections often reveal that the vertical exaggeration of the former can be misleading.

There are three areas in the U.S. in which the outer-continental shelves have been explored fruitfully. They are the Gulf of Mexico (the most productive discovery to date), Southern California, and the Upper Cook Inlet of Alaska. Additionally, some exploration has taken place along the coasts of Northern California, Oregon and Washington, and in the Gulf of Alaska. Exploration of areas off the Atlantic Coast and off much of Alaska has only just commenced.

Formation of Reservoirs

Oil and gas are complex hydrocarbon compounds formed by the alteration of organic matter which has accumulated and been buried in finegrained sediments deposited on the sea floor. This sediment-organic matter “sandwich” is subjected to pressure from the continued deposition of sediments in the basin, and to heat from the earth over a period of millions of years. Heat and pressure, along with bacterial action, transform the organic matter into oil and gas. Gradually, over a long period of time, the gaseous or liquid hydrocarbons migrate from the source beds into more porous and permeable sedimentary rocks which act as reservoirs. If favorable trapping mechanisms exist, further migration through the reservoir rock is prevented, causing the oil and gas to form a captive pool. These so-called “pools” are not caverns filled with oil, nor are they underground lakes or streams. The oil or gas is found between the grains of sand or limestone that make up the reservoir rock much as water is held by a kitchen sponge.

Figure 1 is a two-dimensional cross-section through a series of arched alternating beds of sand and shale. Geologists call this an anticline. The beds of sand are porous and permeable, while the beds of shale are impermeable. The gas, oil, and water have separated naturally, according to their relative densities. Gas, being the lightest, goes to the top, followed by oil, then water. In the lower sandstone bed, there is no oil, only gas. Sometimes oil occurs without the presence of free natural gas. All too frequently, the trap is filled only with water.

Fig. 1

There are a number of different types of traps (Fig. 2). Sometimes the beds of alternating sand and shale have been displaced along a fault, which forms an impermeable barrier and, thus, traps the oil. Salt domes which flow up from an underlying thick bed of salt sometimes cause numerous reservoir traps to form arround the periphery of the dome. Then, there is the typical Stratigraphie trap, formed by the tilting and erosion of a series of layers, and the subsequent deposition of further layers. When the lowermost of the overlying layers is an impermeable shale, a trap is formed.

Fig. 2

The four examples discussed are merely a few of many types of traps which exist.

Location of Reservoirs

In petroleum exploration, offshore geology is frequently inferred from a projection of onshore conditions and occurrence. The presence or absence of reservoir beds, as well as the presence or absence of hydrocarbon source beds can sometimes be successfully projected, but it is difficult to project the presence of traps. Magnetic surveys are sometimes carried out, using an airborne magnetometer. The purpose of this type of survey is to estimate the amount of sedimentary rocks which may be present, and to determine their depth. Oil and gas are usually generated and trapped in sedimentary rocks which are the sandstones, shales, and limestone deposited in the ocean. A gravity meter is ocassionally used, as well. It measures the pull of the earth’s gravity, which varies from place to place, depending on the density of the rocks. Thus, the resulting gravity map, which gives an indication of the structural grain of the ocean floor, can be useful in detecting the presence of salt domes, since the salt is less dense than the beds of rock surrounding it. In some areas, scuba divers and small, manned submarines have examined the sea floor to determine the attitude of dip of the beds which outcrop and to obtain samples of these beds for age-determination.

Sometimes, core and stratigraphic test holes are punched or drilled into the ocean floor to determine age and rock type. Recovered samples are frequently analyzed for the presence of hydrocarbons to ascertain whether or not beds are present which could be sources of oil and gas. Deep holes are sometimes drilled in areas where traps are not believed to exist. These so-called “strat tests” are undertaken by groups of companies to acquire information which can be used by geochemists to predict the absence or presence of hydrocarbons in the sedimentary basin. If a basin becomes productive and is trapped by many wells, the subsurface information obtained can be used to project the geologies of nearby undrilled areas, lending greater confidence in the prediction of traps.

By far the most often used device for location of traps is the reflection seismograph. Sea-going vessels are commonly equipped with this equipment which emit sharp bursts of energy which travel through the water to penetrate various layers of the Earth until they are reflected and return to be picked up by a series of sensitive receivers affixed to cable which is towed by the vessel. Knowing the precise time the sound sources are emitted, the velocity of the sound waves through water and various layers of sedimentary rock, as well as the precise time the reflected signals are received by the geophone, it is possible to calculate the depth to the reflecting horizon. In the early days of marine seismic exploration, dynamite was used as a sound source. Today, the sound source is provided by the sudden release of compressed air or by the explosion of a mixture of propane and air within a rubber sleeve. The explosion causes the sleeve to expand and contract quickly, providing the proper sound source without danger to ocean life.

It costs about $400 per mile to make a seismic survey offshore, and a similar amount to process the...

Produkt-Details

• Erscheint lt. Verlag: 22.10.2013
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Themenwelt: Recht / Steuern ► Wirtschaftsrecht
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• Themenwelt: Wirtschaft ► Betriebswirtschaft / Management ► Rechnungswesen / Bilanzen
• Themenwelt: Betriebswirtschaft / Management ► Spezielle Betriebswirtschaftslehre ► Immobilienwirtschaft
• ISBN-10: 1-4831-8943-0 / 1483189430
• ISBN-13: 978-1-4831-8943-7 / 9781483189437