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ous, sedimentary, and metamorphic. Igneous rocks have crystallized from a hot, molten liquid. Sedimentary rocks are composed of sediments that were deposited on the surface of the ground or bottom of the ocean or salts that precipitated out of water. Metamorphic rocks have been recrystallized from other rocks under high temperatures and pressures.

Answer the questions:

1.What are types of rocks? 2. How are they formed?

Igneous rocks

Igneous rocks are formed when a molten melt is cooled. Two types of igneous rocks are plutonic and volcanic, depending on where they formed.

Plutonic igneous rocks crystallized and solidified while still below the surface of the earth. Because the rocks that surround the cooling plutonic rocks are good insulators, plutonic rocks often take thousands of years to solidify. When a cooling melt is given a long time to crystallize, large mineral crystals are formed. Plutonic igneous rocks are easy to identify because the mineral crystals are all large enough to be seen by the naked eye. Plutonic rocks formed as hot liquids that were injected into and displaced preexisting rocks in the subsurface. Because of this, plutonic rock bodies are called intrusions.

Volcanic igneous rocks crystallize on the surface of the earth as lava. As the lava flows out of a volcano, it immediately comes in contact with air or water and rapidly solidifies. The rapid crystallization forms very small crystals that are difficult to distinguish with the naked eye.

звать процесс кристалли зации, кристаллизовать

Melt – таять, расплавлять

Surround – окру-

жать; обступать

Identify – опозна-

вать, распознавать;

Displace – вытес-

нять, заменять, замещать

Preexisting – су-

ществующие ранее

Distinguish – на-

ходить отличия; различать, распознавать

particles that were formed by the breakdown of rocks and were transported and deposited as whole particles. Boulders, sand grains, and mud particles are examples. Organic sediments are formed biologically such as seashells. Crystalline sediments are formed by the precipitation of salt out of water. As sediments are buried in the subsurface, they become solid, sedimentary rocks. Sedimentary rocks are the rocks that are drilled to find gas and oil. They are the source and reservoir rocks for gas and oil.

Loose sediments (unconsolidated sediments) become relatively hard sedimentary rocks (consolidated sediments) in the subsurface by the processes of cementation and compaction. No matter how some sediments such as sand grains are packed together, there will be pore spaces between the grains (Fig. 2-2). Once the grains have been buried in the subsurface, the pore spaces are filled with water that can be very salty. Under the higher temperatures and pressures of the subsurface, salts often precipitate out of the subsurface waters to coat the grains. These coatings grow together to bridge the loose grains. This process, called cementation, bonds the loose grains into a solid sedimentary rock. Two common cements are the minerals calcite (СаСОз) and quartz (Si02). Also, as the sediments are buried deeper, the increasing weight of overlying rocks exerts more pressure on the grains. This compacts the sediments that also solidify the rock.

Sedimentary rocks often consist of three parts when examined under a microscope (Fig. 2-3). First, there are sediment grains. These are composed of minerals such as quartz or feldspar or seashells. Second, there are natural cements coating and bonding the grains together.

62

Boulder – осадоч-

ный валун, диаметром не более

256 мм

consolidated sediments – уплотненные осадочные породы

Cementation –

цементирование. скрепление, объединение

Сompaction – уп-

лотнение

Fill – наполнить

Coat – покрывать слоем чего-либо

Bridge – соеди-

нять

Bond – связывать, соединять, скреплять

Exert – оказывать

Examine – иссле-

довать; изучать

Drinking water –

питьевая вода

Seawater – мор-

ская вода

Brackish water –

солоноватая вода

Fresh – свежий

Percolate – проса-

Third, there are pore spaces. The pores are filled with fluids (water, gas, or oil) in the subsurface.

Fig. 2-3 Clastic sedimentary rock under a microscope There is an enormous amount of water below the

surface of the ground, called ground water, in the pores of the sedimentary rocks (Fig. 2-4). Ground water is described by salt content in parts per thousand (ppt). Fresh water contains so little salt (0-1 ppt) that it can be used for drinking water.

Brines are subsurface waters that contain more salt than seawater (35-300 ppt).

Brackish waters are mixtures of fresh waters and brines (1-35 ppt). Below the surface is а boundary called the water table between the dry pores above and pores that are filled with water below. The water table can be on the surface or very deep depending on how much rain falls in that area. Just below the water table, the ground water is usually fresh because of rain water that percolates down from the surface. Deep waters, however, are

63

чиваться, проникать сквозь

Meteoric water –

подземные воды, образовавшиеся при просачивании атмосферных осадков

Cobble –крупная галька (размером

64-256 мм)

Pebble –гравий

Granule – грану-

ла; мелкая частица

(2-4 мм)

Silt – мелкозем, частицы почвы

0,05-0,002 мм в диаметре)

Sorting – сорти-

ровка, классификация

Determine – опре-

делять, устанавливать

Reduce – пони-

жать, сокращать, уменьшать

Hold – сохранять, удерживать

Layering – на-

слаивание

Bedding – напла-

стование; наслоение, слоистость;

usually brines. When a well is drilled, completed, and producing, near-surface fresh waters must be protected. Meteoric water is fresh, subsurface water. Connate water is saline, subsurface water that has been out of contact with the atmosphere for a long time. Connate water is often water that was originally trapped in the sediments when they were deposited.

The sizes of the clastic grains that make up an ancient sedimentary rock are important. The rock is often classified according to the grain size. Sandstones are composed of sand-sized grains whereas shales are composed of finegrained (clay-sized) particles. Also, the size of the grains controls the size of the pore spaces and the quality of the oil or gas reservoir. Larger grains have larger pores between them. It is easier for fluids, such as gas and oil, to flow through larger pores. Clastic grains in sedimentary rocks are classified by their diameters in millimeters (Fig.2-5). They are called boulder, cobble, pebble, granule, sand, silt, and clay-sized particles. The finest grains (i.e., sand, silt, and clay-sized) are the most common.

Sorting is the range of particle sizes in the rock (Fig. 2-6). A well-sorted rock is composed of particles of approximately the same size (Fig. 2-6a). A poorly-sorted, rock is composed of particles with a wide range of sizes (Fig. 2—6b). Sorting is the most important factor in determining the amount of original pore space in a clastic sedimentary rock. Finer-sized particles in a poorly sorted rock occupy the spaces between the larger-sized particles and reduce the volume of the pores. Poorly sorted rocks can hold less fluids and are lower-quality reservoir rocks than well-sorted rocks. Well-sorted sandstones are called clean sands. Because sand grains are light in color, clean sandstones are usually light in color. Poorly sorted sandstones with significant amounts of siltand clay-sized grains are called dirty sands. Because siltand clay-sized particles are usually dark in color, dirty sandstones are dark colored.

Sedimentary rocks are identified by their layering, called stratification or bedding. As the sediments are deposited, there are frequent variations in the amount and composition of sediment supply and the level of the ocean that cause the layering. Sediment layers are origi-

64

залегание

Cause – вызывать

Environment –

окружение

Ripple marks –

рябь

Mud crack – тре-

щина усыхания

Flow mark – знак течения

Metamorphic rocks

Metamorphic rocks are any rocks that have been altered by high heat and pressure. Marble (СаСОз), a metamorphic rock, is metamorphosed limestone (СаСОз), and quartzite (SiO2) is metamorphosed quartz sandstone (SiO2). Since temperatures and pressures become greater with depth, a rock often becomes metamorphosed when buried deep in the earth.

Structure of the Earth’s Crust

The earth is estimated to be about 4.5 billion years old. Even the sedimentary rocks that generated and hold the gas and oil are millions to hundreds of millions of years old. During that vast expanse of geological time, sea level has not been constant. Sea level has been rising and falling. During the rise and fall of sea level, sediments were deposited in layers. Sands were deposited along the ancient beaches. Mud was deposited in the shallow seas offshore. Seashells were deposited in shell beds. These ancient sediments form the sedimentary rocks that are drilled to find gas and oil. The rise and fall of sea level has occurred in numerous cycles (Fig. 2-7). The largest cycles occurred every few hundreds of millions of years. There are shorter cycles on the large cycles and even shorter cycles on them. At least five orders of sea level cycles have occurred, with the shortest occurring every few tens of thousands of years. The shorter cycles are thought to be caused by the freezing and melting of glaciers.

In a typical section of the earth’s crust such as Tulsa, Oklahoma, about 5000 ft (1500 m) of well-layered

66

Alter – изменять, видоизменять

Marble – мрамор

Estimate – оценивать Cycle – цикл; (хронологический) период Order – порядок; последовательность Freeze – замерзать, обледеневать

Glacier – ледник Unproductive – не-

продуктивный

Shield – щит

Ore – руда Iron – железо

Copper – медь

Lead – свинец

Zinc – цинк

Gold – золото Silver – серебро Mine – разрабатывать рудник; добывать Kitchen – зона образования УВ

Interconnected – свя-

занный, взаимосвязанный; соединённый Flank – бок; край, сторона

Barren – пустой, не содержащий полезно-

sedimentary rocks are underlain by very old metamorphic or igneous rocks (Fig. 2-8).

There are about one hundred layers of sedimentary rocks. Sands form the rock sandstone. Mud forms the rock shale. Sea shells form the rock limestone. The unproductive rocks, usually igneous and metamorphic rocks underlying the sedimentary rocks, are called basement rocks. When drilling encounters basement rock, the drilling is usually stopped.

In some areas of the earth, there are no sedimentary rocks, and the basement is on the surface. These areas are called shields, and there is no gas or oil. Every continent of the world has at least one shield area (Fig. 2— 9). A shield, such as the Canadian shield in eastern Canada, tends to be a large, low-lying area. Ore minerals such as iron, copper, lead, zinc, gold, and silver are mined from the basement rock in shield areas. The southwest portion of Saudi Arabia is a shield. All the Saudi Arabian oil fields are located in sedimentary rocks to the northeast of the Arabian Shield.

го ископаемого; безрезультатный (о скважине) ; нефтенепродуктивный