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"Petroleum Geology, Drilling, and Production" Both crude oil and natural gas are mixtures of molecules formed by carbon and hydrogen atoms. There are many different types of crude oils and natural gases, some more valuable than others. Heavy crude oils are very thick and viscous and difficult or impossible to produce, whereas light crude oils are fluid and relatively easy to produce. Less valuable are sour crude oils that contain sulfur and sour natural gases that contain hydrogen sulfide. Some natural gases burn with more heat than others, contain natural gas liquids and gasoline, and are more valuable. In order to have a commercial deposit of gas or oil, three geological conditions must be met. First, there must be a source rock in the subsurface of that area that generated the gas or oil at some time in the geological past. Second, there must be a separate, subsurface reservoir rock to hold the gas or oil. Third, there must be a trap on the reservoir rock to concentrate the gas or oil into commercial quantities. The uppermost crust of the earth in oil- and gas-producing areas is composed of sedimentary rock layers. Sedimentary rocks are the source and reservoir rocks for gas and oil. These rocks are called sedimentary because they are composed of sediments. Sediments are particles such as sand grains that were formed by the breakdown of pre-existing rocks and transported, seashells, or salt that precipitated from water. The sedimentary rocks that make up the earth’s crust are millions and sometimes billions of years old. During the vast expanse of geological time, sea level has not been constant. Many times in the past, the seas have risen to cover the land and then fallen to expose the land. During these times, sediments were deposited. These sediments are relatively simple materials such as sands deposited along beaches, mud on the sea bottom, and beds of seashells. These ancient sediments, piled layer upon layer, form the sedimentary rocks that are drilled to find and produce oil and gas. The source of gas and oil is organic matter buried and preserved in ancient sedimentary rocks. These rocks contain not only inorganic particles such as sand grains and mud but also dead plant and animal material. The most common organic-rich sedimentary rock (the source rock for most of the gas and oil) is black shale. It was deposited as organic-rich mud on an ancient ocean bottom. In the subsurface, temperature is the most important factor in turning organic matter into oil. As the source rock is covered with more sediments and buried deeper in the earth, it becomes hotter and hotter. The minimum temperature for the formation of oil, about 150°F (65°C), occurs at a depth of about 7000 ft (2130 m) below the surface (Fig. 1-1). Oil is generated from there and down to about 300°F (150°C) at about 18,000 ft (5500 m). The reactions that change organic matter into oil are complex and take a long time. If the source rock is buried deeper where temperatures are above 300°F (150°C), the remaining organic matter will generate natural gas. Gas and oil are relatively light in density compared to water that also occurs in subsurface sedimentary rocks. After oil and gas have been generated, they rise due to buoyancy through fractures in the subsurface rocks. The rising gas and oil can intersect a layer of reservoir rock. A reservoir rock is a sedimentary rock containing billions of tiny spaces called pores. A common sedimentary rock is sandstone composed of sand grains similar to those on a beach or in a river channel. Sand grains are like spheres, and there is no way the grains will fit together perfectly. There are pore spaces between sand grains on a beach and in a sandstone rock. Limestone, another common sedimentary rock, is deposited as shell beds or reefs, and there are pores between shells and corals. The gas and oil flow into the pores of the reservoir rock layer. Fig. 1-1 Generation and migration of gas and oil Any fluid (water, gas, or oil), either on the surface or in the subsurface, will always flow along the path of least resistance, the easiest route. In the subsurface, the path of least resistance is along a reservoir rock layer. This is because most of the pore spaces interconnect, and the fluid can flow from pore to pore up the angle of the rock layer toward the surface. The ease in which the fluid can flow through the rock is called permeability, and the movement of gas and oil up the angle of the reservoir rock toward the surface is called migration. Because of migration, gas and oil can end up a considerable distance, both vertically and horizontally, from where it was originally formed (Fig. 1-1). As the gas and oil migrate up along the reservoir rock, they can encounter a trap. A trap is a high point in the reservoir rock where gas or oil is stopped and concentrated. Because the pores in the reservoir rock are filled with water, gas and oil will flow to the highest part of the reservoir rock. One type of trap is a natural arch in the reservoir rock (Fig. 1-2) called a dome or anticline. In the trap, fluids separate according to their density. The gas is the lightest and goes to the top of the trap to form the free gas cap. 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