Tar pit

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Tar pit at Tierra La Brea, Trinidad Mother-of-the-Lake.jpg
Tar pit at Tierra La Brea, Trinidad
An anticlinal trap is feeding the tar pit on the surface through the vertical fracture in the strata (indicated by the red arrow). Once the crude oil reaches the surface, evaporation takes place and lighter hydrocarbons are vaporized, leaving behind sticky asphalt. Tar Pit Formation Image.png
An anticlinal trap is feeding the tar pit on the surface through the vertical fracture in the strata (indicated by the red arrow). Once the crude oil reaches the surface, evaporation takes place and lighter hydrocarbons are vaporized, leaving behind sticky asphalt.

Tar pits, sometimes referred to as asphalt pits, are large asphalt deposits. They form in the presence of petroleum, which is created when decayed organic matter is subjected to pressure underground. If this crude oil seeps upward via fractures, conduits, or porous sedimentary rock layers, it may pool up at the surface. [1] The lighter components of the crude oil evaporate into the atmosphere, leaving behind a black, sticky asphalt. [1] Tar pits are often excavated because they contain large fossil collections. [1]

Contents

Tar pits form above oil reserves, and these deposits are often found in anticlinal traps. In fact, about 80 percent of petroleum found on Earth has been found in anticlinal traps. [2] Anticlines are folds in stratigraphic layers in which each half of the fold dips away from the crest. Such structures are usually developed above thrust faults or in tectonic regions where the land is bending and folding. If the structure above the concave-down fold (arch) is a non-porous rock or aquitard, such as shale, it is considered an anticlinal trap. [2] The figure in this section is a cartoon cross-section diagram that shows oil stuck in an anticlinal trap. If there is a fault or fracture in the overlying strata above the oil reserve, the oil may migrate to the surface. This is possible by capillary fringe and because oil is less dense than water. [3]

Chemistry

Tar pits are pools of asphalt. However, at the beginning of their formation, they were not always sticky and dense. The pools were composed of crude oil that originated below Earth's surface. Crude oil is a mixture of heteroatom compounds, hydrocarbons, metals, and inorganic compounds. [4] Heteroatom compounds are organic molecules that contain elements that are not carbon or hydrogen, while hydrocarbons contain only carbon and hydrogen. [4] Crude oil is less viscous than asphalt because it contains a higher percentage of light hydrocarbons. [5] Light hydrocarbons include the following alkanes: methane, ethane, propane, and butane. [6] These molecules have very low molecular weights. Crude oils may also contain some inorganic impurities, such as CO2, H2S, N2, and O2. [6] At the surface, these light molecules may evaporate out of the crude oil, leaving behind the heavier, stickier molecules. Asphalt, or bitumen, usually contains hydrocarbon molecule chains with 50+ carbon atoms. [5] The longer the hydrocarbon chain, the more viscous it becomes, and the boiling point increases. [5]  

Evaporation is an important process in the formation of tar pits. A reservoir of light crude oil on Earth's surface can be reduced by up to 75% of the initial volume just after a few days, forming asphalt as the resulting product. For medium crude oils, the volume may be reduced by 40%. [7] Crude oils will evaporate differently depending on their chemical composition. [4] The average composition of a bitumen sample by weight is 80.2% carbon, 7.5% hydrogen, 7.6% oxygen, 1.7% nitrogen, and 3.0% sulfur. [2]

Notable tar pits

La Brea Tar Pits

The La Brea Tar Pits are located in Los Angeles. The petroleum that is seen on the surface is sourced from the Salt Lake Oil Field reservoir and the oil sands in the Repetto and Pico formations. [8] These oil deposits were formed during the Miocene Epoch when marine plankton organisms accumulated in an ocean basin. [9]  Over time, sediments buried the organisms 300 to 1000 meters below Earth's surface, subjecting them to high pressures. This process turned the organic matter into oil. The 6th Street Fault that cuts through the Salt Lake Oil Field is the conduit that feeds the La Brea Tar Pits. [8] Petroleum migrated to the surface over time, trapping and preserving animals and plants for the past 50,000 years. [9]

Carpinteria Tar Pits

The Carpinteria Tar Pits are located in Tar Pits Park in Carpinteria, California. These tar pits were predicted to have formed during the Pleistocene. During an excavation project, 25 plant species were recovered along with 55 species of birds and 26 species of mammals. [10]  Springs of tar still ooze to the surface through fractures in the underlying stratigraphic layers of marine shale. [10]

Binagadi Asphalt Lake

The Binagadi Asphalt Lake is located in Azerbaijan, or in the Caucasus, a region between the Black Sea and the Caspian Sea. This tar pit is known for preserving the heads and bodies of multiple cave lions, a mammal that flourished in the Pleistocene. [11] A well-preserved horse skull was also found in the Binagadi asphalt lake. It is estimated to be 96-120 thousand years old. It is on display at the Natural-Historical Museum of Azerbaijan in Baku, Azerbaijan. [12]

Pitch Lake

Pitch Lake in Trinidad and Tobago is a large, bowl-like depression filled with bitumen. The lake has a maximum depth of 250 feet with an area of 100 acres, making it the largest deposit of solid bitumen on Earth. [13] The lake is cold and dense near the shores, and it has a top layer that can be walked on. Underneath this skin, the asphalt is continuously churning. The lake gets gradually softer and hotter near the center where the bitumen begins to bubble. The gas being released in the middle of the lake is largely methane and an ample amount of carbon dioxide. [14]

Pitch Lake formed thousands of years ago in the presence of tectonic activity. Movement along a fault created a fracture that tapped into an oil and gas reservoir deep within the crust. The oil and gas seeped upward to Earth's surface through the fracture over time, creating Pitch Lake. Due to this constant replenishment of the oil and gas, the lake has a slight current.  The current is largely unnoticed because the top layer of Pitch Lake is mostly solid. [13]

Fossils

Tar pits are characteristic of their many fossils. This is the case because the thick, sticky asphalt traps animals. [15]   Once animals step into the tar, they become immobilized and begin sinking immediately if the asphalt is warm and sticky enough. Predators that see these helpless animals usually would advance into the tar pits with the hope of catching their next meal. As a result, prey are usually found beneath the predator during excavation projects. [15]

The bones and hard parts of the animals are well preserved because they are buried rapidly after the organism's death. Beneath the surface, the hard parts are engulfed with asphalt, and they are protected from climate variations like rain, wind, or snow that may accelerate weathering processes. Asphalt also lacks oxygen and water, so major decomposing organisms like aerobic fungi and bacteria are absent.[ citation needed ]

In the La Brea Tar Pits, more than one million bones have been recovered since 1906. 231 vertebrate species, 234 invertebrate species, and 159 plant species have been identified. [9] The most frequent large mammal found in the La Brea Tar Pits is the dire wolf, one of the most famous prehistoric carnivores that lived during the Pleistocene. [16] Fossils from saber-toothed cats and coyotes were also abundant. [9] Additional fossils are constantly being discovered through continued excavation projects. [9]

Living organisms

Life was found in a ca. 28,000-year-old sample of natural asphalt in the La Brea Tar Pits. [17] Hundreds of new species of bacteria were discovered that have the ability to thrive in environments with little to no water or air. They contain special enzymes that can break down hydrocarbons and other petroleum products. [17] The origin of the bacteria in these natural asphalt pits is unknown, but it is believed that they evolved from preexisting soil microorganisms that survived an asphalt seepage event thousands of years ago. The soil microorganisms had to adapt and undergo genetic changes to help tolerate the harsh, new environment, which ultimately gave rise to new bacterial species. [17]

In one study, the predominant bacteria found in the La Brea Tar Pits were of the Gammaproteobacteria class in the Chromatiales order, more simply referred to as purple sulfur bacteria. [17] Purple sulfur bacteria do not use water as their reducing agent, so oxygen is not produced during respiration. Instead, they use sulfur in the form of sulfides as their reducing agent. Other bacteria discovered in the tar pits were of the Rubrobacteraceae family. These bacteria are known for being some of the most radiation-resistant organisms on the planet. [17]  

Pitch Lake, another asphalt pit in Trinidad and Tobago, is also a habitat for microbial communities of archaea and bacteria. Bacterial microorganisms from the orders Burkholderiales and Enterobacteriales have been found living in microliter-sized droplets of water recovered from the lake. [18] The biomass in Pitch Lake lake was reported up to 107 cells per gram of asphalt. [19] Many of these microbes survive on sulfur, iron, methane, or other hydrocarbons. [19] Ongoing research is being conducted in Pitch Lake because it mimics the environment found on the surface of Saturn’s largest moon, Titan. The discovery of extremophiles in Pitch Lake provides insight into the possibilities for microbial life in the hydrocarbon lakes that are found on Titan. [19]

Contributions to greenhouse gases

Tar pits are formed by the fractionation of crude oil at the surface. The lighter hydrocarbons of the crude oil, which include methane (CH4), ethane (C2H6), and propane (C3H8), evaporate, leaving behind larger hydrocarbons that make up the chemical composition of asphalt. This is a concern because methane, ethane, and propane are either major greenhouse gases and/or photochemical pollutants. [20] The La Brea Tar Pits emit about 500 kg of methane per day. [20] The emissions are the highest along the 6th Street Fault, which is the conduit that feeds the tar pits with crude oil from the sediments underneath Earth's surface. [20] It was also discovered that methane is evaporating out of the nearby soil, affecting the physiology of the native grasses. The La Brea Tar Pits have the highest natural gas flux measured for any onshore seepage zone in the United States. [20]  On a global scale, geologic CH4 and C2H6 emissions estimates from gas seepage in sedimentary rock are in the order of 50-70 Tg/year and 2-4 Tg/year respectively. These values are roughly half of the global CH4 and C2H6 emissions from anthropogenic fossil fuel combustion, which are approximately 100-150 Tg CH4/year and 6-8 Tg C2H6/year. [20] These hydrocarbon emissions can be contributed to oil biodegradation and methanogenesis within the tar pits. [20]  

The natural geologic sources of methane and other hydrocarbons should be considered when modeling atmospheric greenhouse gases. Not all sources of hydrocarbons in the atmosphere are a result of anthropogenic emissions. [20]

Dangers of tar pits

Hydrocarbon seepage in urban or industrialized areas present a geologic hazard due to the explosive nature of hydrocarbons. On March 24, 1985, a pocket of methane gas passed through a small opening between the floor slab and foundation walls of a Ross clothing department store in Los Angeles, only about a mile north of the La Brea Tar Pits. This methane pocket created an explosion that injured 21 people. [21] This event increased awareness of the potential dangers of methane pockets and hydrocarbon seepage in the area. [21]

Key to paleoplant behavior

Tar pits are excellent preserving agents, and they also have the ability to provide carbon isotope data for trees that have fallen into the asphalt. Looking at carbon isotope data in prehistoric trees can reveal information about plant responses to different amounts of carbon dioxide in the paleoatmosphere. [22] Samples of Juniperus trees from the Last Glacial Period were recovered from the La Brea Tar Pits, and they revealed that the ratio of intercellular and atmospheric CO2 was similar between glacial and modern trees. Since the amount of carbon dioxide during the Last Glacial Period was between 180 and 200 ppm (409.8 ppm today), [23] there was less carbon available for photosynthesis. The Juniperus trees had to enhance CO2 uptake to survive under limiting carbon conditions. It is likely that the trees’ stomatal conductance and chloroplast demand for CO2 was higher during this period to increase their carbon consumption. [22] When moving into the following Interglacial Period, the Juniperus trees’ stomatal conductance and chloroplast demand for CO2 decreased as a result of higher temperatures and higher CO2 concentrations in the atmosphere. This response to fluctuating carbon levels is seen in plants over time. For example, increased stomatal conductance is observed in modern C3 plants grown in low CO2 environments. [22] It is also hypothesized that the wetter climate during the Last Glacial Period may have increased the nitrogen availability to plants, which therefore increased the concentration of nitrogen in leaves. This change may have increased the Juniperus trees’ photosynthetic capacities. [22]

History of tar pits and humans

A woman's body was recovered from the La Brea Tar Pits in 1914.  Only the skull and parts of the skeleton were preserved, and she was determined to have died about 9,000 years ago. [24]   She was between the ages 18–24 at death, and she was 4 feet and 8-10 inches tall. [24] This is the only reported instance of human remains found within tar pits. [25]

For thousands of years, Native Americans used tar from the La Brea Tar Pits as an adhesive and binding agent. [1] They would use it as waterproof caulking to line their boats and baskets. When Europeans arrived at the tar pits, they began mining and extracting the tar for roofing material in nearby towns. [1]

Related Research Articles

<span class="mw-page-title-main">Bitumen</span> Form of petroleum primarily used in road construction

Bitumen is an immensely viscous constituent of petroleum. Depending on its exact composition it can be a sticky, black liquid or an apparently solid mass that behaves as a liquid over very large time scales. In American English, the material is commonly referred to as asphalt. Whether found in natural deposits or refined from petroleum, the substance is classed as a pitch. Prior to the 20th century, the term asphaltum was in general use. The word derives from the Ancient Greek word ἄσφαλτος (ásphaltos), which referred to natural bitumen or pitch. The largest natural deposit of bitumen in the world is the Pitch Lake of southwest Trinidad, which is estimated to contain 10 million tons.

<span class="mw-page-title-main">Hydrocarbon</span> Organic compound consisting entirely of hydrogen and carbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may be similar to that of gasoline or lighter fluid. They occur in a diverse range of molecular structures and phases: they can be gases, liquids, low melting solids or polymers.

<span class="mw-page-title-main">Petroleum</span> Naturally occurring combustible liquid

Petroleum or crude oil, also referred to as simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name petroleum covers both naturally occurring unprocessed crude oil and petroleum products that consist of refined crude oil.

<span class="mw-page-title-main">Ethane</span> Organic compound (H3C–CH3)

Ethane is a naturally occurring organic chemical compound with chemical formula C
2H
6. At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petrochemical by-product of petroleum refining. Its chief use is as feedstock for ethylene production.

<span class="mw-page-title-main">La Brea Tar Pits</span> Paleontological research site in Los Angeles

The La Brea Tar Pits is an active paleontological research site in urban Los Angeles. Hancock Park was formed around a group of tar pits where natural asphalt has seeped up from the ground for tens of thousands of years. Over many centuries, the bones of trapped animals have been preserved. The George C. Page Museum is dedicated to researching the tar pits and displaying specimens from the animals that died there. La Brea Tar Pits is a registered National Natural Landmark.

<span class="mw-page-title-main">Oil sands</span> Type of unconventional oil deposit

Oil sands, tar sands, crude bitumen, or bituminous sands, are a type of unconventional petroleum deposit. Oil sands are either loose sands or partially consolidated sandstone containing a naturally occurring mixture of sand, clay, and water, soaked with bitumen, a dense and extremely viscous form of petroleum.

<span class="mw-page-title-main">Tar</span> Dark viscous organic liquid

Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat.

<span class="mw-page-title-main">Pitch Lake</span> Largest natural deposit of asphalt in the world, located in Trinidad and Tobago

The Pitch Lake is the largest natural deposit of asphalt in the world, estimated to contain 10 million tons. It is located in La Brea in southwest Trinidad, within the Siparia Regional Corporation. The lake covers about 100 acres and is reported to be 250 feet deep.

<span class="mw-page-title-main">Petroleum coke</span> Solid carbon-rich material

Petroleum coke, abbreviated coke, pet coke or petcoke, is a final carbon-rich solid material that derives from oil refining, and is one type of the group of fuels referred to as cokes. Petcoke is the coke that, in particular, derives from a final cracking process—a thermo-based chemical engineering process that splits long chain hydrocarbons of petroleum into shorter chains—that takes place in units termed coker units. Stated succinctly, coke is the "carbonization product of high-boiling hydrocarbon fractions obtained in petroleum processing ". Petcoke is also produced in the production of synthetic crude oil (syncrude) from bitumen extracted from Canada's tar sands and from Venezuela's Orinoco oil sands. In petroleum coker units, residual oils from other distillation processes used in petroleum refining are treated at a high temperature and pressure leaving the petcoke after driving off gases and volatiles, and separating off remaining light and heavy oils. These processes are termed "coking processes", and most typically employ chemical engineering plant operations for the specific process of delayed coking.

Heavy crude oil is highly viscous oil that cannot easily flow from production wells under normal reservoir conditions.

<span class="mw-page-title-main">Petroleum seep</span> Place where natural hydrocarbons escape

A petroleum seep is a place where natural liquid or gaseous hydrocarbons escape to the Earth's atmosphere and surface, normally under low pressure or flow. Seeps generally occur above either terrestrial or offshore petroleum accumulation structures. The hydrocarbons may escape along geological layers, or across them through fractures and fissures in the rock, or directly from an outcrop of oil-bearing rock.

<span class="mw-page-title-main">Asphaltene</span> Heavy organic molecular substances that are found in crude oil

Asphaltenes are molecular substances that are found in crude oil, along with resins, aromatic hydrocarbons, and saturates. The word "asphaltene" was coined by Boussingault in 1837 when he noticed that the distillation residue of some bitumens had asphalt-like properties. Asphaltenes in the form of asphalt or bitumen products from oil refineries are used as paving materials on roads, shingles for roofs, and waterproof coatings on building foundations.

<span class="mw-page-title-main">Methane</span> Hydrocarbon compound (CH₄) in natural gas

Methane is a chemical compound with the chemical formula CH4. It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes it an economically attractive fuel, although capturing and storing it is hard because it is a gas at standard temperature and pressure.

<span class="mw-page-title-main">Coal Oil Point seep field</span> Marine petroleum seep area near Goleta, California

The Coal Oil Point seep field (COP) in the Santa Barbara Channel offshore from Goleta, California, is a marine petroleum seep area of about three square kilometres, within the Offshore South Ellwood Oil Field and stretching from the coastline southward more than three kilometers (1.9 mi). Major seeps are located in water depths from 20 to 80 meters. The seep field is among the largest and best studied areas of active marine seepage in the world. These perennial and continuous oil and gas seeps have been active on the northern edge of the Santa Barbara Channel for at least 500,000 years. The combined seeps in the field release about 40 tons of methane per day and about 19 tons of reactive organic gas ; about twice the hydrocarbon air pollution released by all the cars and trucks in Santa Barbara County in 1990. The liquid petroleum produces a slick that is many kilometres long and when degraded by evaporation and weathering, produces tar balls which wash up on the beaches for miles around.

<span class="mw-page-title-main">Salt Lake Oil Field</span> Oil field under Los Angeles, California, United States

The Salt Lake Oil Field is an oil field underneath the city of Los Angeles, California. Discovered in 1902, and developed quickly in the following years, the Salt Lake field was once the most productive in California; over 50 million barrels of oil have been extracted from it, mostly in the first part of the twentieth century, although modest drilling and extraction from the field using an urban "drilling island" resumed in 1962. As of 2009, the only operator on the field was Plains Exploration & Production (PXP). The field is also notable as being the source, by long-term seepage of crude oil to the ground surface along the 6th Street Fault, of the famous La Brea Tar Pits.

<span class="mw-page-title-main">Asphalt volcano</span> Ocean floor vents that erupt asphalt instead of lava

Asphalt volcanoes are a rare variety of submarine volcano (seamount). They were unknown before 2003. Several examples have been found along the coasts of the United States and Mexico and elsewhere, some still showing activity. Asphalt volcanoes resemble other seamounts however they are made entirely of asphalt. The structures are thought to form above geologic faults through which petroleum seeps from deeper in the Earth's crust.

<span class="mw-page-title-main">McKittrick Tar Pits</span> Series of natural asphalt lakes situated in the western part of Kern County in southern California

The McKittrick Tar Pits are a series of natural asphalt lakes situated in the western part of Kern County in southern California. The pits are the most extensive asphalt lakes in the state.

<span class="mw-page-title-main">Carpinteria Tar Pits</span> Natural asphalt lakes in Carpinteria, California, US

The Carpinteria Tar Pits are a series of natural asphalt lakes situated in the southern part of Santa Barbara County in southern California, US.

<span class="mw-page-title-main">Pyrobitumen</span> Type of solid, amorphous organic matter

Pyrobitumen is a type of solid, amorphous organic matter. Pyrobitumen is mostly insoluble in carbon disulfide and other organic solvents as a result of molecular cross-linking, which renders previously soluble organic matter insoluble. Not all solid bitumens are pyrobitumens, in that some solid bitumens are soluble in common organic solvents, including CS
2, dichloromethane, and benzene-methanol mixtures.

<span class="mw-page-title-main">Gas venting</span> Disposal of unwanted methane gas from fossil fuels

Gas venting, more specifically known as natural-gas venting or methane venting, is the intentional and controlled release of gases containing alkane hydrocarbons - predominately methane - into Earth's atmosphere. It is a widely used method for disposal of unwanted gases which are produced during the extraction of coal and crude oil. Such gases may lack value when they are not recyclable into the production process, have no export route to consumer markets, or are surplus to near-term demand. In cases where the gases have value to the producer, substantial amounts may also be vented from the equipment used for gas collection, transport, and distribution.

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