Drillers discovered that they could increase the permeability of the shale by pumping water down the well under pressure that was high enough to fracture the shale.
These fractures liberated some of the gas from the pore spaces and allowed that gas to flow to the well. This technique is known as " hydraulic fracturing " or "hydrofracing. Drillers also learned how to drill down to the level of the shale and turn the well 90 degrees to drill horizontally through the shale rock unit. This produced a well with a very long "pay zone" through the reservoir rock see illustration. This method is known as " horizontal drilling.
Horizontal drilling and hydraulic fracturing revolutionized drilling technology and paved the way for developing several giant natural gas fields. These enormous shale reservoirs hold enough natural gas to serve all of the United States' needs for twenty years or more. Shale in brick and tile: Shale is used as a raw material for making many types of brick, tile, pipe, pottery, and other manufactured products.
Brick and tile are some of the most extensively used and highly desired materials for building homes, walls, streets, and commercial structures. Everyone has contact with products made from shale. If you live in a brick house, drive on a brick road, live in a house with a tile roof, or keep plants in "terra cotta" pots, you have daily contact with items that were probably made from shale.
Many years ago these same items were made from natural clay. However, heavy use depleted most of the small clay deposits. Needing a new source of raw materials, manufacturers soon discovered that mixing finely ground shale with water would produce a clay that often had similar or superior properties. Today, most items that were once produced from natural clay have been replaced by almost identical items made from clay manufactured by mixing finely ground shale with water.
The best way to learn about rocks is to have specimens available for testing and examination. Cement is another common material that is often made with shale. To make cement, crushed limestone and shale are heated to a temperature that is high enough to evaporate off all water and break down the limestone into calcium oxide and carbon dioxide.
The carbon dioxide is lost as an emission, but the calcium oxide combined with the heated shale makes a powder that will harden if mixed with water and allowed to dry.
Cement is used to make concrete and many other products for the construction industry. Oil shale: A rock that contains a significant amount of organic material in the form of solid kerogen. This specimen is approximately four inches ten centimeters across. Oil shale is a rock that contains significant amounts of organic material in the form of kerogen. This is usually much less efficient than drilling rocks that will yield oil or gas directly into a well.
Extracting the hydrocarbons from oil shale produces emissions and waste products that cause significant environmental concerns. This is one reason why the world's extensive oil shale deposits have not been aggressively utilized. Shale core samples: When shale is drilled for oil, natural gas, or mineral resource evaluation, a core is often recovered from the well. The rock in the core can then be tested to learn about its potential and how the resource might be best developed.
Shale is a rock composed mainly of clay-size mineral grains. These tiny grains are usually clay minerals such as illite, kaolinite, and smectite. Shale usually contains other clay-size mineral particles such as quartz , chert , and feldspar. Other constituents might include organic particles, carbonate minerals, iron oxide minerals, sulfide minerals, and heavy mineral grains. These "other constituents" in the rock are often determined by the shale's environment of deposition, and they often determine the color of the rock.
Black shale: Organic-rich black shale. Natural gas and oil are sometimes trapped in the tiny pore spaces of this type of shale. Like most rocks, the color of shale is often determined by the presence of specific materials in minor amounts.
Sandstone is defined as a rock which is composed of sand-sized grains of various minerals mostly of uniform size and often are smooth and rounded. Shale is a fine-grained sedimentary rock which is formed by the compaction of silt and clay-size mineral particles. These rocks are composed of many distinct minerals. Although often mistaken for a shale, siltstone lacks the laminations and fissility along horizontal lines which are typical of shale.
Siltstones may contain concretions. Unless the siltstone is fairly shaly, stratification is likely to be obscure and it tends to weather at oblique angles unrelated to bedding. Laboratory studies show that simple compression and relaxation of rock samples under realistic conditions does not cause fracturing. Exfoliation joints are most commonly found in regions of surface-parallel compressive stress, whereas this theory calls for them to occur in zones of extension.
Oxidation is the reaction of rock minerals with oxygen, thus changing the mineral composition of the rock. When minerals in rock oxidize, they become less resistant to weathering. Iron, a commonly known mineral, becomes red or rust colored when oxidized.
Exfoliating granite is a granite undergoing exfoliation, or onion skin weathering desquamation. In terms of producing sediment, erosion begins the transportation process by moving the weathered products from their original location. This can take place by gravity massmovement events like landslides or rock falls , by running water. Erosion overlaps with transportation.
Transportation - Sediment can be transported by sliding down slopes, being picked up by the wind, or by being carried by running water in streams, rivers, or ocean currents. The distance the sediment is transported and the energy of the transporting medium all leave clues in the final sediment that tell us something about the mode of transportation.
Deposition - Sediment is deposited when the energy of the transporting medium becomes too low to continue the transport process. In other words, if the velocity of the transporting medium becomes too low to transport sediment, the sediment will fall out and become deposited. The final sediment thus reflects the energy of the transporting medium. Lithification Diagenesis - Lithification is the process that turns sediment into rock. The first stage of the process is compaction. Compaction occurs as the weight of the overlying material increases.
Compaction forces the grains closer together, reducing pore space and eliminating some of the contained water. Some of this water may carry mineral components in solution, and these constituents may later precipitate as new minerals in the pore spaces.
This causes cementation, which will then start to bind the individual particles together. Conglomerate or Breccia depends on rounding. Textures of Clastic Sedimentary Rocks When sediment is transported and deposited, it leaves clues to the mode of transport and deposition.
Examples Beach deposits and wind blown deposits generally show good sorting because the energy of the transporting medium is usually constant. Stream deposits are usually poorly sorted because the energy velocity in a stream varies with position in the stream and time. Sediment Maturity Sediment Maturity refers to the length of time that the sediment has been in the sedimentary cycle.
Types of Clastic Sedimentary Rocks We next look at various clastic sedimentary rocks that result from lithification of sediment. Conglomerates and Breccias Conglomerate and Breccia are rocks that contain an abundance of coarse grained clasts pebbles, cobbles, or boulders. Sandstones A Sandstone is made of sand-sized particles and forms in many different depositional settings.
Mudrocks Mudrocks are made of fine grained clasts silt and clay sized. Biochemical and Organic Sediments and Sedimentary Rocks Biochemical and Organic sediments and sedimentary rocks are those derived from living organisms. Among the types of rock produced by this process are: Biochemical Limestone - calcite CaCO 3 is precipitated by organisms usually to form a shell or other skeletal structure.
Chemical Sediments and Sedimentary Rocks Dissolved ions released into water by the weathering process are carried in streams or groundwater. Among these are: Evaporites - formed by evaporation of sea water or lake water. There are many varsities of such chert that are given different names depending on their attributes, For example: Flint — Black or gray from organic matter.
Sedimentary Structures As mentioned previously, all stages of the sedimentary cycle leave clues to processes that were operating in the past. Stratification and Bedding Because sediment is deposited in low lying areas that often extend over wide areas, successive depositional events produce layers called bedding or stratification that is usually the most evident feature of sedimentary rocks. Rhythmic Layering - Alternating parallel layers having different properties.
Sometimes caused by seasonal changes in deposition Varves. Cross Bedding - Sets of beds that are inclined relative to one another. The beds are inclined in the direction that the wind or water was moving at the time of deposition. Boundaries between sets of cross beds usually represent an erosional surface.
Very common in beach deposits, sand dunes, and river deposited sediment. Graded Bedding - As current velocity decreases, first the larger or more dense particles are deposited followed by smaller particles. This results in bedding showing a decrease in grain size from the bottom of the bed to the top of the bed. Sediment added as a pulse of turbid water. As pulse wanes, water loses velocity and sediments settle. Coarsest material settles first, medium next, then fine.
Multiple graded-bed sequences called turbidites see figure 7. Non-sorted Sediment - Sediment showing a mixture of grain sizes results from such things as rockfalls, debris flows, mudflows, and deposition from melting ice.
Ripple Marks - Water flowing over loose sediment creates bedforms by moving sediment with the flow. Mudcracks - result from the drying out of wet sediment at the surface of the Earth. The cracks form due to shrinkage of the sediment as it dries.
When present in rock, they indicate that the surface was exposed at the earth's surface and then rapidly buried. Sole Marks - Flutes are troughs eroded in soft sediment that can become filled with mud.
Both the flutes and the resulting casts called flute casts can be preserved in rock. Raindrop Marks - pits or tiny craters created by falling rain. If present, this suggests that the sediment was exposed to the surface of the Earth just prior to burial.
Sedimentary rocks are rocks made of lithified sediment. Sediments are grains of rocks, minerals, or mineraloids deposited on the surface of the earth. Reflect on the rock cycle for an indication of the relationships between the rocks that erode to become sediments and sedimentary rocks.
For sediment to become sedimentary rock, it usually undergoes burial, compaction, and cementation. Clastic sedimentary rocks are the result of weathering and erosion of source rocks, which turns them into pieces—clasts—of rocks and minerals.
Once they become pieces, these clasts are free to move away from their source rock and they usually do. They are most often transported by water and deposited as layers of sediment. The burial stage of lithification involves the deposition of more sediment layers top of those that had been deposited earlier.
In a sedimentary basin where sediment is being deposited, it is common for subsidence lowering of the basin to be taking place, either because the crust and lithosphere beneath it are subsiding into the mantle to some extent, or because the surrounding uplands are undergoing uplift relative to the basin, or both. This allows thousands of feet of burial, in some cases tens of thousands of feet of burial, to occur.
As sediments are buried, the weight of overlying material exerts pressure, causing compaction of the sediments. Lithostatic pressure packs the sediment grains closer together and reduces the porosity — space between the sediment grains.
Examples include rock salt and other evaporite deposits. These sediments of salt crystals and other minerals form sedimentary rock without having to undergo burial and compaction. During burial and compaction, sediments will undergo some amount of cementation. Cementation refers to the growth of new minerals between the sediment grains. These new minerals bind the sediment grains together. One form of cementation is growth of quartz rims on the surfaces of pre-existing quartz grains in the sediment.
This new mineral growth is a result of water in the pore spaces that dissolves and precipiates quartz. A second common cementing mineral is hematite, a red or rust-colored iron oxide mineral, which precipitates onto the sediment grains from a combination of dissolved iron and oxygen from water in the pore spaces. A third common cementing mineral is calcite, which also precipitates from ions dissolved in the water in the pore spaces during lithification.
Although there are other cementing minerals, quartz, hematite, and calcite are common cementing minerals that grow between or on the surfaces of the original sedimentary grains. Grains of clastic sediment, which are called clasts are winnowed and modified during the weathering-to-deposition process.
Weathering of minerals will gradually eliminate the physically weaker and chemically more reactive minerals, increasing the relative abundance of more resistant minerals. Quartz tends to become increasingly abundant during the process, due to its common occurrence in the source rocks combined with its hardness and lack of cleavage, which makes it resistant to breaking down physically. Quartz is not easily dissolved or chemically altered, so it is resistant to breakdown by chemical reactions as well.
That is why beach sand is often more rich in quartz than any other mineral. Although feldspar is a fairly hard mineral, it does cleave split apart and is chemically reactive, especially in the presence of water. The most abundant product of chemical reaction of feldspar and water is clay minerals. During the erosion-to-deposition process, clastic sediments lose feldspar and gain a larger proportion of clay.
Other minerals such as amphiboles, micas, and carbonates are relatively soft and chemically reactive and tend to be scarce or absent as sediment grains in mature clastic sediments, although calcite may be present in clastic sedimentary rocks as a secondary, cementing mineral that grew during lithification.
Minerals in chemical sedimentary rocks precipitate from water and usually remain in place or are not transported far before lithification.
Such mineral sediments are subjected to little, if any, erosion and transportation. Therefore, the minerals in chemical sedimentary rocks are not winnowed during the weathering-to-deposition process as are the minerals in clastic sedimentary rocks.
In some cases, during the formation of chemical sediments, the minerals may change as a result of chemical reactions.
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