Geologic settings of resources
Geologic settings of resources refer to the natural environments and processes that lead to the formation and accumulation of valuable materials within the Earth's crust. This includes hydrocarbons, such as petroleum and natural gas, which originate from the decomposition of organic matter, requiring specific conditions for their formation, such as burial under sediment and pressurization over time. These hydrocarbons often collect in porous rock formations or are trapped by impermeable layers, creating reservoirs that can be mined for energy.
Additionally, minerals and metallic ores, crucial for various industrial applications, are also extracted from geological formations. These resources can be found in both surface and subsurface layers, with mining methods tailored to access different depths and types of deposits. For instance, the mining of metals like copper and iron typically involves extracting materials from ancient marine ecosystems, where sediments rich in these minerals have accumulated over millions of years.
Exploration techniques, including seismic surveys, play a pivotal role in locating these resources by analyzing the physical properties of geological layers. Given the finite nature of traditional hydrocarbon reserves, there is increasing interest in unconventional sources, such as oil sands and oil shale, which present their own extraction challenges. As global demand for these resources continues, understanding the complex interplay of geological processes and resource formation becomes ever more critical.
Geologic settings of resources
Buried organic matter accumulates in porous and fractured sediment, forming reservoirs of hydrocarbons that can be harvested for commercial use. Petroleum and natural gas are hydrocarbon substances that form from the accumulation, decomposition, pressurization, and heating of organic matter. Minerals and metallic ores are other resources that are harvested from the earth’s crust by mining. Deep-sea hydrothermal environments also include metallic ores and hydrocarbon reserves that are later available at the continental surface because of the shifting of ocean systems.
Hydrocarbon Reservoirs and Traps
Petroleum and natural gas are hydrocarbon-rich fuel sources harvested from geological sediment. Hydrocarbon fuels form as organic matter decays and sinks through the upper layers of the earth’s crust, infiltrating the rocks beneath.
Hydrocarbon fuels form only under certain specific conditions. First, organic matter must be buried during decomposition, preferably by soft sedimentary material such as sand and mud. After burial, the carbon residue of decomposing organisms is subjected to increasing pressure, while gases and heat emanating from decomposition raise the temperature within these pockets of buried material. As this occurs, the resulting petroleum and gas begin rising to the surface because they are less dense than the overlying medium.
Petroleum that cannot return to the surface because it is sealed by the overlying rock forms a petroleum trap. Traps are the source of most of the world’s petroleum and result from tectonic movement of the earth’s crust, which causes physical deformations in the rock surrounding a developing petroleum reservoir. The most common petroleum trap is formed when tectonic movement causes the sediment to buckle and fold. The underlying petroleum moves toward the surface because it is more buoyant than the underlying rock but cannot reach the surface because the overlying sediment is impermeable to the oil.
Hydrocarbon Exploration
Hydrocarbon exploration involves a variety of methods to locate petroleum and other hydrocarbon fuel reservoirs. In some cases, petroleum or natural gas may break through to the surface of the crust under natural conditions, creating a seep, which is a rupture in the earth’s surface that slowly releases oil or natural gas, or both. When a seep is located, engineers can utilize equipment to contain and further develop the seep, extracting hydrocarbon fuel from the connected reservoir.
Engineers also locate reservoirs by using seismic surveys. Seismic technology involves using machines that can propagate seismic waves, which are waves of acoustic energy that can travel through solid rock. Seismic waves are usually propagated on a low frequency, as these types of force waves are more suitable to traveling through solid materials and will travel significant distances before diminishing. As seismic waves move through the sediment, they are also reflected back to the source of the wave as they collide with materials under the surface. By measuring and analyzing the reflection of seismic waves, engineers can determine the location of pockets within the crust that might contain certain types of materials. This method is therefore used to find hydrocarbon traps that can then be explored and harvested by mining.
Geologists can increase their chances of finding new reservoirs by analyzing existing harvesting operations. For instance, a number of productive reservoirs are contained within dolomitic limestone deposits, a sedimentary rock that forms from ancient marine environments. The shallow marine environments containing dolomitic limestone also support the formation of petroleum reservoirs because environmental conditions favor the burial and compression of large quantities of organic matter. In addition, dolomitic limestone is a porous rock that lends itself to fracturing, thereby reducing density and providing space for the development of a hydrocarbon trap. Given this, engineers and geologists can seek rocks and minerals indicative of dolomitic limestone deposits or other types of rock that develop in similar environments; engineers can then concentrate exploration efforts in these areas, increasing the chance of finding productive reservoirs.
Unconventional Petroleum Resources
Conventional oil traps and reserves compose a finite resource that will be exhausted given current trends in human hydrocarbon consumption. Engineers have begun looking into alternative methods for finding and harvesting additional petroleum and natural gas. Unconventional petroleum exploration is too cost prohibitive and technically complex to constitute a significant alternative to conventional sources, but research is ongoing to find new ways of developing alternative systems to supplement existing petroleum sources.
Oil sands are deposits of petroleum blended with find sedimentary material. Harvesting petroleum from oil sands requires a complex and expensive process that separates and removes the hydrocarbons contained within the surrounding sediment. The process is vastly inferior to removing petroleum from conventional traps because it requires a greater initial investment of energy to power the purification process and uses a large amount of water, which becomes polluted during the process.
Oil shale is another potential source of hydrocarbons. Oil shale comprises collected and extracted petroleum contained within porous shale rock, a type of sedimentary rock that contains hydrocarbons tightly bound in the structure of the rocks. Through combustion, the petroleum within shale can be extracted, though combustion is far less efficient and produces greater pollution than standard methods of petroleum harvesting.
Mineral and Ore Resources
Minerals used in industrial and chemical processes are harvested from a variety of geological settings. Among the most common types of mineral resources are metals, including copper, manganese, iron, magnesium, platinum, and gold.
These minerals are harvested through mining and by chemically processing various rocks to remove and purify metallic deposits. The precious metals, including gold, platinum, and silver, are mined and refined for use in the construction of cosmetic items and jewelry; they also have other applications. Nonprecious metals, such as iron, lead, and nickel, are commonly used in industrial processes and in the production of consumer items.
Most minerals and ores are harvested by mining, which is the process of removing sediment, rock, and other materials from the earth’s crust. Surface mining is used to harvest sediment from the upper layers of the crust and surface, while subsurface mining utilizes tunnels extending into the lower layers of the crust to harvest materials that are found at greater depths. Various metallic ores and other minerals are found at both the surface and lower depths, so both types of mining are important for retrieving minerals and ore.
Many metallic ore deposits, like petroleum deposits, result from ancient marine ecosystems where metallic compounds gathered at the bottom of a sea or ocean and were subsequently concentrated as the water receded. Iron, for instance, is harvested largely from banded iron deposits, which are rocks that formed in prehistoric seas, such as the Western Interior Seaway, which once covered large portions of North America.
Banded iron rocks consist of layers of iron-rich minerals, like magnetite, alternating with layers of other, nonmetallic sedimentary rock. Most banded iron deposits are found in sediment that was once part of prehistoric oceans. Geologists believe that similar iron-rich rock deposits are forming on the ocean floor in modern seas. Much of the iron used in construction and other industrial applications is refined and processed from banded iron ore deposits.
As mentioned, dolomite and dolomitic limestone are often found associated with petroleum reserves. These carbonate rocks are also a source of other minerals used in industrial applications. Dolomite, for instance, contains high levels of magnesium bound into the mineral structure of the rock. Chemical processing can separate this magnesium, which can then be processed to yield magnesium metal for a variety of applications.
Many dolomite deposits also contain a layer of bauxite, an aluminum-rich sedimentary rock that is used to provide aluminum for industrial and manufacturing processes. Bauxite provides the majority of the world’s aluminum and is generally harvested by strip mining, a type of surface mining that strips the top layer of sediment from the crust. Bauxite forms in sedimentary environments, where aluminum becomes blended with various silicon-rich rocks, like granite and shale. To remove and process the aluminum within bauxite, engineers subject samples of bauxite to a heated, acidic environment, thereby melting and separating the aluminum in the rock.
Copper and lead, two of the most useful metals for industrial applications, are generally harvested by strip mining areas with high concentrations of metallic ore. Lead is most commonly derived from galena, a naturally occurring form of lead sulfide, whereas copper is extracted from chalcocite, an ore that forms from copper sulfide. In both cases, rocks are harvested through strip mining operations and then subjected to intense heat and chemical environments that melt and separate the metals in the ore.
Resources and
Many metallic ores derive from ongoing marine geologic processes that lead to the buildup of metallic ore through millions of years. Hydrothermal vents, which are pockets of trapped water that develop under the surface of ocean sediment, are thought to be important in the formation of many metallic rocks.
The water and gas trapped within the vents are heated by volcanic material and become enriched with compounds dissolved from surrounding rock. Eventually, this heated water erupts to the surface, producing a jet of superheated solution. Water leaking into the hydrothermal chamber from elsewhere fuels the system, leading to jets that may erupt multiple times each day for thousands of years.
As water erupts from the vent’s funnel, it meets the cold water of the surrounding oceans, facilitating chemical reactions that cause minerals and metals to precipitate and fall to the surface surrounding the vent. In millions of years, waves of deposited minerals and metals develop into thick deposits that may contain many kilograms of metallic ore. Seafloor spreading then distributes these mineral deposits around the ocean. As the oceans shift and shrink in response to tectonic movements, portions of former oceanic sediment are revealed at the surface. Beneath the top layer of sediment, the earth’s crust often contains sediment that was once part of marine environments, which may include sediment derived from hydrothermal vent systems.
Deep ocean environments also are rich in mineral resources but are difficult to explore because of the pressure and other environmental factors that make it difficult for divers or diving technology to operate at these depths. One example can be found in the manganese nodules that cover much of the ocean floor around hydrothermal vent environments.
Manganese nodules develop over thousands of years from the accumulation of manganese-rich sediment released from the vents. The chemical nature of this metallic substance and the activities of organisms maintain these nodules at the surface of the ocean, despite the deposition of further sediment. Though there is no system for efficiently harvesting manganese from deep oceanic environments, the deposits represent a large, untapped reserve of manganese that may be harvested in the future.
Principal Terms
carbonate: material that contains carbon as a key structural or chemical ingredient
evaporite: type of mineral or rock that results from the accumulation of saline sediments in environments where an aqueous solution is evaporating from the surface
hydrocarbon: organic compounds consisting of hydrogen and carbon molecules
hydrothermal vent: chamber or fissure in the oceanic crust that releases water into the ocean that has been heated by geochemical processes arising from magmatic substances beneath Earth’s surface
metallic ore: sedimentary rock containing minerals rich in metals and often used in the harvest and derivation of metal for industrial processes
petroleum: liquid or semiliquid hydrocarbon substance that results from the decay of organic material within the earth’s surface as it is heated and pressurized by geochemical forces
sediment: material consisting of fine portions of rocks and other minerals dissolved from larger rocks and carried within a gaseous or liquid fluid
seep: an area where water, gas, petroleum, or another fluid emerges from subterranean chambers onto the surface of the earth
seismic waves: low-frequency acoustic energy that travels through solid rock and other structures
tectonic movement: gradual movement of plates, which are portions of the earth’s crust and lithosphere connected at deep sedimentary cores and that move as units
Bibliography
Evans, Anthony M. An Introduction to Economic Geology and Its Environmental Impact. Malden, Mass.: Blackwell Science, 1997. Introductory geological textbook discussing the economic uses of minerals and geologic resources and the environmental impact of harvesting geologic materials worldwide. Contains detailed discussions of mining in the petroleum industry and the harvest of coal and metallic minerals and rocks.
Hyne, Norman J. Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production. 2d ed. Tulsa, Okla.: Pennwell, 2001. General introduction to petroleum mining and processing. Contains references to numerous locations utilized in the production of petroleum and their geological history.
Monroe, James S., Reed Wicander, and Richard Hazlett. Physical Geology. 6th ed. Belmont Calif.: Thompson Higher Education, 2007. General text in geologic sciences written for the introductory student of Earth science. Contains references to geologic settings commonly used to harvest petroleum, natural gas, coal, and other mineral resources.
Plummer, Charles C., Diane H. Carlson, and David McGeary. Physical Geology. 13th ed. Columbus, Ohio: McGraw-Hill Higher Education, 2009. Introductory text covers the basic theories, research practices, and principles of geology. Contains information on petroleum mining and processing and references to numerous other types of mineral resources and their industrial uses.
Pohl, Walter L. Economic Geology: Principles and Practice. Hoboken, N.J.: Wiley-Blackwell, 2011. Advanced textbook outlining the economic use and development of geologic research. Contains discussions of coal, petroleum, and natural gas harvesting and processing and detailed discussions of the harvesting and processing of metallic minerals.
Thomas, Larry. Coal Geology. Hoboken, N.J.: John Wiley & Sons, 2002. Detailed technical textbook covering all major aspects of coal formation, geologic structure, geologic setting, and its mining and usage in the production of energy. Chapter 7 contains a detailed discussion of the global setting of coal resources and the processes used to harvest coal in various environments.
Wenk, Hans-Rudolf, and Andrei Bulakh. Minerals: Their Constitution and Origin. New York: Cambridge University Press, 2004. Intermediate-level textbook covering the basic structure, formation, and mining of minerals. Includes a discussion of economically important minerals and their functions in industrial applications.