Oil and Gas: Characteristics and Occurrence
Oil and Gas Reservoirs
Hydrocarbons and their associated impurities occur in rock formations that are usually buried thousands of feet or metres below the surface. Scientists and engineers often call rock formations that hold hydrocarbons "reservoirs." Oil does not flow in underground rivers or pool up in subterranean lakes, contrary to what some people think. And, as you've learned, gasoline and other refined hydrocarbons do not naturally occur in pockets under the ground, just waiting to be drilled for. Instead, crude oil and natural gas occur in buried rocks and, once produced from a well, companies have to refine the crude oil and process the natural gas into useful products. Further, not every rock can hold hydrocarbons. To serve as an oil and gas reservoir, rocks have to meet several criteria.
(fig- 5 i) :: a pore is a small open space ::
Characteristics of Reservoir Rocks
Nothing looks more solid than a rock. Yet, choose the right rock-say, a piece of sandstone or limestone-and look at it under a microscope. You see many tiny openings or voids. Geologists call these tiny openings "pores" (fig- 5 i). A rock with pores is "porous" and a porous rock has "porosity." Reservoir rocks must be porous, because hydrocarbons can occur only in pores.
A reservoir rock is also permeable-that is, its pores are connected (fig- 5 2). If hydrocarbons are in the pores of a rock, they must be able to move out of them. Unless hydrocarbons can move from pore to pore, they remain locked in place, unable to flow into a well. A suitable reservoir rock must therefore be porous, permeable, and contain enough hydrocarbons to make it economically feasible for the operating company to drill for and produce them.
(fig- 5 2) :: connected pores give a rock permeability ::
Origin and Accumulation of Oil and Gas
To understand how hydrocarbons get into buried rocks, visualize an ancient sea teeming with vast numbers of living organisms. Some are fishes and other large swimming beasts; others, however, are so small that you cannot see them without a strong magnifying glass or a microscope. Although they are small, they are very abundant. Millions and millions of them live and die daily. It is these tiny and plentiful organisms that many scientists believe gave rise to oil and gas.
When these tiny organisms died millions of years ago, their remains settled to the bottom. Though very small, as thousands of years went by, enormous quantities of this organic sediment accumulated in thick deposits on the seafloor. The organic material mixed with the mud and sand on the bottom. Ultimately, many layers of sediments built up until they became hundreds or thousands of feet (metres) thick.
The tremendous weight of the overlying sediments created great pressure and heat on the deep layers. The heat and pressure changed the deep layers into rock. At the same time, heat, pressure, and other forces changed the dead organic material in the layers into hydrocarbons: crude oil and natural gas.
Meanwhile, geological action created cracks, or faults, in the earth's crust. Earth movement folded layers of rock upward and downward. Molten rock thrusted upward, altering the shape of the surrounding beds. Disturbances in the earth shoved great blocks of land upward, dropped them downward, and moved them sideways. Wind and water eroded formations, earthquakes buried them, and new sediments fell onto them. Land blocked a bay's access to open water, and the resulting inland sea evaporated. Great rivers carried tons of sediment; then dried up and became buried by other rocks. In short, geological forces slowly but constantly altered the very shape of the earth. These alterations in the layers of rock are important because, under the right circumstances, they can trap and store hydrocarbons.
Even while the earth changed, the weight of overlying rocks continued to push downward, forcing hydrocarbons out of their source rocks. Seeping through subsurface cracks and fissures, oozing through small connections between rock grains, the hydrocarbons moved upward. They moved until a subsurface barrier stopped them or until they reached the earth's surface, as they did at Oil Creek. Most of the hydrocarbons, however, did not reach the surface. Instead, they became trapped and stored in a layer of subsurface rock. Today, the oil industry seeks petroleum that was formed and trapped millions of years ago.
A hydrocarbon reservoir has a distinctive shape, or configuration, that prevents the escape of hydrocarbons that migrate into it. Geologists classify reservoir shapes, or traps, into two types: structural traps and stratigraphic traps.
Structural traps form because of a deformation in the rock layer that contains the hydrocarbons. Two examples of structural traps are fault traps and anticlinal traps (fig. 5 3)
(fig. 5 3) :: fault traps and anticlinal traps
A fault is a break in the layers of rock. A fault trap occurs when the formations on either side of the fault move. The formations then come to rest in such a way that, when petroleum migrates into one of the formations, it becomes trapped there. Often, an impermeable formation on one side of the fault moves opposite a porous and permeable formation on the other side. The petroleum migrates into the porous and permeable formation. Once there, it cannot get out because the impervious layer at the fault line traps it.
Anticlinal Traps An anticline is an upward fold in the layers of rock, much like a domed arch in a building. The oil and gas migrate into the folded porous and permeable layer and rise to the top. They cannot escape because of an overlying bed of impermeable rock.
Stratigraphic traps form when other beds seal a reservoir bed or when the permeability changes within the reservoir bed itself In one stratigraphic trap, a horizontal, impermeable rock layer cuts off, or truncates, an inclined layer of petroleum-bearing rock (fig- 54A). Sometimes a petroleum-bearing formation pinches out-that is, an impervious layer cuts it off (fig- 54B). Other stratigraphic traps are lens-shaped. Impervious layers surround the hydrocarbon-bearing rock (fig. 54C) - Still another occurs when the porosity and permeability change within the reservoir itself The upper reaches of the reservoir are nonporous and impermeable; the lower part is porous and permeable and contains hydrocarbons (fig- 54D).
(fig- 54A) (fig- 54B)
(fig- 54C) (fig- 54D)
Many other traps occur. In a combination trap, for example, more than one kind of trap forms a reservoir. A faulted anticline is an example. Several faults cut across the anticline. In some places, the faults trap oil and gas (fig- 55). Another trap is a piercement dome. In this case, a molten substance-salt is a common one-pierced surrounding rock beds. While molten, the moving salt deformed the horizontal beds. Later, the salt cooled and solidified and some of the deformed beds trapped oil and gas (fig- 56). Spindletop was formed by a piercement dome.
(fig- 55) (fig- 56)
From The Primer of Oilwell Drilling, 6th edition Copyright © 2001 Petroleum Extension Service (PETEX®) of The University of Texas at Austin. All rights reserved