Classes of sedimentary rocks

Classes of sedimentary rocks

kacimo samy   9:45 ص

Classes of sedimentary rocks

Geologists divide sedimentary rocks into four major classes, based on their mode of origin. 

(1) Clastic sedimentary rock consists of cemented-together clasts, solid fragments and grains broken off of preexisting rocks (the word comes from the Greek klastos, meaning broken); (2) biochemical sedimentary rock consists of shells; (3) organic sedimentary rock consists of carbon-rich relicts of plants or other organisms; and (4) chemical sedimentary rock is made up of minerals that precipitated directly from water solutions. Let’s now look at these major classes in more detail.

Clastic Sedimentary Rocks Formation

Nine hundred years ago, a thriving community of Native Americans inhabited the high plateau of Mesa Verde, Colorado. In hollows beneath huge overhanging ledges, they built multistory stone-block buildings that have survived to this day. Clearly, the blocks are solid and durable they are, after all, rock. But if you were to rub your thumb along one, it would feel gritty, and small grains of quartz would break free and roll under your thumb, for the block consists of quartz sand grains cemented together. Geologists call such rock a sandstone. Sandstone is an example of clastic sedimentary rock. It consists of loose clasts, known as detritus, that have been stuck together to form a solid mass. The clasts can consist of individual minerals (such as grains of quartz or flakes of clay) or of fragments of rock (such as pebbles of granite). Formation of sediment and its transformation into clastic sedimentary rock takes place via the following five steps.

• Weathering: Detritus forms by disintegration of bedrock into separate grains due to physical and chemical weathering. 
• Erosion: Erosion refers to the combination of processes that separate rock or regolith (surface debris) from its substrate. Erosion involves abrasion, falling, plucking, scouring, and dissolution, and is caused by moving air, water, or ice. 
• Transportation: Gravity, wind, water, or ice carry sediment. The ability of a medium to carry sediment depends on its viscosity and velocity. Solid ice can transport sediment of any size, regardless of how slowly the ice moves. Very fast-moving, turbulent water can transport coarse fragments (cobbles and boulders), moderately fast-moving water can carry only sand and gravel, and slow-moving water carries only silt and clay. Strong winds can move sand and dust, but gentle breezes carry only dust. 
• Deposition: Deposition is the process by which sediment settles out of the transporting medium. Sediment settles out of wind or moving water when these fluids slow, because as the velocity decreases, the fluid no longer has the ability to carry sediment. Sediment is deposited by ice when the ice melts. 
• Lithification: Geologists refer to the transformation of loose sediment into solid rock as lithification. The lithification of clastic sediment involves two steps. First, once the sediment has been buried, pressure caused by the weight of overlying material squeezes out water and air that had been trapped between clasts, and clasts press together tightly, a process called compaction. Compacted sediment may then be stuck together to make coherent sedimentary rock by the process of cementation. Cement consists of minerals (commonly quartz or calcite) that precipitate from groundwater and fill the spaces between clasts. 

Classifying clastic sedimentary rocks

Say that you pick up a clastic sedimentary rock and want to describe it sufficiently so that, from your words alone, another person can picture the rock. What characteristics should you mention? Geologists find the following characteristics most useful. 

• Clast size. Size refers to the diameter of fragments or grains making up a rock. Names used for clast size, listed in order from coarsest to finest, are: boulder, cobble, pebble, sand, silt, and clay. 
• Clast composition. Composition refers to the makeup of clasts in sedimentary rock. Clasts may be composed of rock fragments or individual mineral grains. 
• Angularity and sphericity. Angularity indicates the degree to which clasts have smooth or angular corners and edges. Sphericity, in contrast, refers to the degree to which the shape of a clast resembles a sphere. 
• Sorting. Sorting of clasts indicates the degree to which the clasts in a rock are all the same size or include a variety of sizes. Well-sorted sediment consists entirely of sediment of the same size, whereas poorly-sorted sediment contains a mixture of more than one clast size. 
• Character of cement. Not all clastic sedimentary rocks have the same kind of cement. In some, the cement consists  predominantly of quartz, whereas in others, it consists predominantly of calcite. 

With these characteristics in mind, we can distinguish among several common types of clastic sedimentary rocks. This table provides common rock names specialists sometimes use other, more precise names based on more complex classification schemes. The size, angularity, sphericity, and sorting of clasts depends on the medium (water, ice, or wind) that transports the clasts and, in the case of water or wind, on both the velocity of the medium and the distance of transport. The composition of the clasts depends on the composition of rock from which the clasts were derived, and on the degree of chemical weathering that the clasts have undergone. Thus, the type of clasts that accumulate in a sedimentary deposit varies with location. To see how, let’s follow the fate of rock fragments as they gradually move from a cliff face in the mountains via a river to the seashore. Different kinds of sediment develop along the route. Each kind, if buried and lithified, would yield a different type of sedimentary rock.

To start, imagine that some large blocks of granite tumble off a cliff and slam into other blocks already at the bottom. The impact shatters the blocks, producing a pile of angular fragments. If these fragments were to be cemented together before being transported very far, the resulting rock would be breccia (a in above figure). Later, a storm causes the fragments (clasts) to be carried away by a turbulent river. In the water, clasts bang into each other and into the riverbed, a process that shatters them into still smaller pieces and breaks off their sharp edges. As the clasts get carried downstream, they gradually become rounded pebbles and cobbles. When the river water slows, these clasts stop moving and form a mound or bar of gravel. Burial and lithification of these rounded clasts produces conglomerate (b in above figure). If the gravel stays put for a long time, it undergoes chemical weathering. As a consequence, cobbles and pebbles break apart into individual mineral grains, eventually producing a mixture of quartz, feldspar, and clay. Clay is so fine that flowing water easily picks it up and carries it downstream, leaving sand containing a mixture of quartz and some feldspar grains this sediment, if buried and lithified, becomes arkose (c in above figure). Over time, feldspar grains in sand continue to weather into clay so that gradually, during successive events that wash the sediment downstream, the sand loses feldspar and ends up being composed almost entirely of durable quartz grains. Some of the sand may make it to the sea, where waves carry it to beaches, and some may end up in desert dunes. This sediment, when buried and lithified, 

becomes quartz sandstone (d figure below). Meanwhile, silt and clay may accumulate in the flat areas bordering streams, regions called floodplains that become inundated only during floods. And some silt and mud settles in a wedge, called a delta, at the mouth of the river, or in lagoons or mudflats along the shore. The silt, when lithified, becomes siltstone, and the mud, when lithified, becomes shale or mudstone (e figure below).

Biochemical Sedimentary

Rocks The Earth System involves many interactions between living organisms and the physical planet. Numerous organisms have evolved the ability to extract dissolved ions from seawater to make solid shells. When the organisms die, the solid material in their shells survives. This material, when lithified, comprises biochemical sedimentary rock. Geologists recognize several different types of biochemical sedimentary rocks, which we now describe.

Limestone (biochemical)

A snorkeler gliding above a reef sees an incredibly diverse community of coral and algae, around which creatures such as clams, oysters, snails (gastropods), and lampshells (brachiopods) live, and above which plankton float (a figure above). Though they look so different from each other, many of these organisms share an important characteristic: they make solid shells of calcium carbonate (CaCO3). The CaCO3 crystallizes either as calcite or aragonite. (These minerals have the same composition, but different crystal structures.) When the organisms die, the shells remain and may accumulate.  

Rocks formed dominantly from this material are the biochemical version of limestone. Since the principal compound making up limestone is CaCO3, geologists refer to limestone as a type of carbonate rock. Limestone comes in a variety of textures, because the material that forms it accumulates in a variety of ways. For example, limestone can originate from reef builders (such as coral) that grew in place, from shell debris that was broken up and transported, from carbonate mud, or from plankton shells that settled like snow out of water. Because of this variety, geologists distinguish among fossiliferous limestone, consisting of visible fossil shells or shell fragments (b figure above); micrite, consisting of very fine carbonate mud; and chalk, consisting of plankton shells. Experts recognize many other types as well. Typically, limestone is a massive light-gray to darkbluish-gray rock that breaks into chunky blocks it doesn't look much like a pile of shell fragments (c figure above). That’s because several processes change the texture of the rock over time. For example, water passing through the rock not only precipitates cement but also dissolves some carbonate grains and causes new ones to grow.

Chert (biochemical).

If you walk beneath the northern end of the Golden Gate Bridge in California, you will find outcrops of reddish, almost porcelain-like rock occurring in 3- to 15-cm-thick layers (a figure above). Hit it with a hammer, and the rock would crack to form smooth, spoon-shaped (conchoidal) fractures. Geologists call this rock biochemical chert; it’s made from cryptocrystalline quartz (crypto is Greek for hidden), meaning quartz grains that are too small to be seen without the extreme magnification of an electron microscope. The chert beneath the Golden Gate Bridge formed from the shells of silica-secreting plankton that accumulated on the sea floor. Gradually, after burial, the shells dissolved, forming a silica-rich gel. Chert then formed when this gel solidified. 

Organic Sedimentary Rocks 

We've seen how the mineral shells of organisms (CaCO3 or SiO2) can accumulate and lithify to become biochemical sedimentary rocks. What happens to the “guts” of the organisms the cellulose, fat, carbohydrate, protein, and other organic compounds that make up living matter? Commonly, this organic debris gets eaten by other organisms or decays at the Earth’s surface. But in some environments, the organic debris settles along with other sediment and eventually gets buried. When lithified, organic-rich sediment becomes organic sedimentary rock. Since the dawn of the industrial revolution in the early 19th century, coal, one type of organic sedimentary rock, has provided the fuel of modern industry and transportation, for the organic chemicals in the rock yield energy when burned. Coal is a black, combustible rock consisting of over 50 to 90% carbon. The remainder consists of oxygen, nitrogen, hydrogen, sulphur, silica, and minor amounts of other elements. Typically, the carbon in coal occurs in large, complex organic molecules made of many rings note that the carbon does not occur in CaCO3. Coal forms when plant remains have been buried deeply enough and long enough for the material to become compacted and to lose significant amounts of volatiles (hydrogen, water, CO2, and ammonia); as the volatiles seep away, a concentration of carbon remains  (b figure above).

Chemical Sedimentary Rocks 

The colourful terraces, or mounds, that grow around the vents of hot-water springs; the immense layers of salt that underlie the floor of the Mediterranean Sea; the smooth, sharp point of an ancient arrowhead these materials all have something in common. They all consist of rock formed primarily by the precipitation of minerals from water solutions. We call such rocks chemical sedimentary rocks. They typically have a crystalline texture, partly formed during their original precipitation and partly when, at a later time, new crystals grow at the expense of old ones through a process called recrystallization. In some examples, crystals are coarse. In others, they are too small to see. Geologists distinguish among many types of chemical sedimentary rocks, primarily on the basis of composition.

Evaporites: the products of salt-water evaporation.  

In 1965, two daredevil drivers in jet-powered cars battled to be the first to set the land speed record of 600 mph. On November 7, Art Arfons, in the Green Monster, peaked at 576.127  mph; but eight days later Craig Breedlove, driving the Spirit of America, reached 600.601 mph. Travelling at such speeds, a driver must maintain an absolutely straight line; any turn will catapult the vehicle out of control. Thus, high-speed trials take place on extremely long and flat racecourses. Not many places can provide such conditions the Bonneville Salt Flats of Utah do. The salt flats formed when an ancient salt lake evaporated. Under the heat of the Sun, the water turned to vapour and drifted up into the atmosphere, but the salt that had been dissolved in the water stayed behind. Salt precipitation occurs where salt-water becomes supersaturated, meaning that it has exceeded its capacity to contain more dissolved ions. In supersaturated salt-water, ions bond to form solid grains that either settle out of the water or grow on the floor of the water body. Supersaturated salt-water develops where evaporation removes water from a water body faster than the rate at which new water enters. This process takes place in desert lakes and along the margins of restricted seas (figure above). For thick deposits of salt to form, large volumes of water must evaporate. Because salt deposits form as a consequence of evaporation, geologists refer to them as evaporites. The specific type of salt minerals comprising an evaporite depends on the amount of evaporation. When 80% of the water evaporates, gypsum forms; and when 90% of the water evaporates, halite precipitates. 

Travertine (chemical limestone).  

Travertine is a rock composed of crystalline calcium carbonate (CaCO3) formed by chemical precipitation from groundwater that has seeped out at the ground surface either in hot- or cold-water springs, or on the walls of caves. What causes this precipitation? It happens, in part, when the groundwater degasses, meaning that some of the carbon dioxide that had been dissolved in the groundwater bubbles out of solution, for removal of carbon dioxide encourages the precipitation of carbonate. Precipitation also occurs when water evaporates, thereby increasing the concentration of carbonate. Various kinds of microbes live in the environments in which travertine accumulates, so biologic activity may also contribute to the precipitation process. Travertine produced at springs forms terraces and mounds that are meters or even hundreds of meters thick, such as those at Mammoth Hot Springs (a in figure above). Travertine also grows on the walls of caves where groundwater seeps out (b in figure above). In cave settings, travertine builds up beautiful and complex growth forms called speleothems.

Dolostone.  

Another carbonate rock, dolostone, differs from limestone in that it contains the mineral dolomite (CaMg[CO3]2), which contains equal amounts of calcium and magnesium. Where does the magnesium come from? Most dolostone forms by a chemical reaction between solid calcite and magnesium-bearing groundwater. Much of the dolostone you may find in an outcrop actually originated as limestone but later changed into dolostone as dolomite crystals replaced calcite. This change may take place beneath lagoons along a shore soon after the limestone formed, or a long time later, after the limestone has been buried deeply.

Chert (replacement).  

A tribe of Native Americans, the Onondaga, once lived off the land in eastern New York State. Here, outcrops of limestone contain layers or nodules (lenses or lumps) of a black chert (a in figure above). Because of the way it breaks, the tribe’s artisans could fashion sharpedged tools (arrowheads and scrapers) from this chert, so the Onondaga collected it for their own toolmaking industry and for use in trade with other people. Unlike the deep sea (biochemical) chert described earlier, the chert collected by the Onondaga formed when cryptocrystalline quartz gradually replaced calcite crystals within a body of limestone long after the limestone was deposited; geologists call such material “replacement chert.” 

Chert comes in many colours (black, white, red, brown, green, gray), depending on the impurities it contains. Petrified wood is chert that forms when silica-rich sediment, such as ash from a volcanic eruption, buries a forest. The silica dissolves in groundwater, and then later precipitates as cryptocrystalline quartz within wood, gradually replacing the wood’s cellulose. The chert deposit retains the shape of the wood and the growth rings within it. Some chert, known as agate, precipitates in concentric rings inside hollows in a rock and ends up with a striped appearance, caused by variations in the content of impurities incorporated in the chert (b in figure above).

Basics of Geology, Rocks and Minerals

Basics of Geology, Rocks and Minerals

kacimo samy   12:24 ص

Sedimentary Rocks

Sedimentary Rocks

kacimo samy   3:28 ص

Sedimentary Rocks:

Sediments are loose rock particles, produced by one of three mechanisms:

1) Weathering of preexisting rocks, followed by transportation and deposition (produce clastic sedimentary rocks such as sandstones, breccias, and conglomerates).

2) Chemical precipitation of minerals from water (produce chemical sedimentary rocks such as limestone and evaporites).

3) Accumulation of biological matter such as shells and plant fragments (such as coal).

Sediments that are buried may harden into sedimentary rock through a process know as lithification (a combination of compaction and cementation)

From Sediment to Sedimentary Rock

Prior to lithification, sediment experiences two major events:

1)  Transportation- The movement of sediment away from its source rock by water, wind, or ice

Rounding of particles occurs due to abrasion during transport.  Rounding increases as transport distance increases.

Size sorting occurs by transport agents, especially running water.  Sediment size decreases as transport distance increases.

2)  Deposition- The environment of deposition is the location where deposition occurred.  Examples of environments of deposition are:

River Channel

Lake Bottom

Delta

Beach

Deep Ocean

Desert Dunes

Classification of Sedimentary Rocks by Origin

1) Clastic (or detrital) sedimentary rocks- Form from the cementation of sediment grains that come from pre-existing rocks.  This is the most common sedimentary rock type.  Clastic sedimentary rocks are classified by grain size, and to a lesser extent by chemical composition.

Boulder - >256 mm

Cobble - 64 to 256 mm

Pebble - 2 to 64 mm

Sand - 1/16 to 2 mm

Silt - 1/256 to 1/16 mm

Clay - <1/256 mm

Breccia- A coarse-grained clastic rock composed of angular rock fragments cemented together (poor rounding).

Breccia. © Marli Miller, University of Oregon.

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Conglomerate- A coarse-grained clastic rock made of rounded gravel cemented together (good rounding).

Conglomerate, © Marli Miller, University of Oregon.

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Sandstone- A medium-grained clastic rock.

Sandstone, Public Domain Image, USGS (Minerals in your World Project).

Sandstones are subclassified based on the chemical composition:

Quartz sandstone - Contains >90% quartz grains.

Arkose - Contains mostly feldspar and quartz grains mixed together.

Graywacke - Contains sand grains surrounded by dark, fine-grained matrix (they are "dirty" sandstones).

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Siltstone- A fine-grained clastic rock.  Siltstones have a gritty feel, and the individual grains are visible with a hand lens.

Siltstone, Public Domain Image, USGS (Minerals in your World Project).

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Shale- A fine-grained clastic rock containing silt and clay-sided grains.  Shale spits into thin layers, a behavior known as fissile.  Shales feel smoother than siltstones.

Shale, Public Domain Image, USGS (Minerals in your World Project).

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Mudstone/Claystone- Mudstone is made of silt and clay-sized grains.  They are blocky (non-fissile).

2) Chemical (and biochemical) sedimentary rocks- Chemical sedimentary rocks form by the precipitation of minerals from water (this process may or may not involve the actions of organisms).  In contrast to the clastic textures of the rocks discussed above, chemical sedimentary rocks have crystalline textures.

Limestone- A chemical sedimentary rock composed mainly of calcite (CaCO₃).

Fossiliferous limestone, Public Domain Image, USGS (Minerals in your World Project).

Most limestones are biochemical, but many are inorganic.

They often contain easily recognizable fossils (fossiliferous).

Chemical alteration of limestone in Mg-rich water can produce dolomite, CaMg(CO₃)₂.

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Chert- A hard, compact, fine-grained chemical sedimentary rock composed almost entirely of silica (SiO₂).

 

Chert, Public Domain Image, USGS (Minerals in your World Project).

Chert can occur as layers or as lumpy nodules within other sedimentary rocks, especially within limestone.

Some cherts are primary precipitates, but others formed by replacement of pre-existing material by silica.

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Evaporates- Evaporites are chemical sedimentary rocks that grow upward from seas and salt-rich lakes due to water evaporation.

Delicate evaporites

© Larry Fellows, Arizona Geological Survey.

Common evaporite deposits are gypsum, CaSO₄•2(H₂O), and halite (NaCl).

Organic sedimentary rocks- Fossil fuels are sedimentary rocks with a biological origin.

Coal- Coal is a sedimentary rock formed from the compaction of partially decayed plant material (requires stagnant water and rapid burial).

Oil and natural gas- “Cooking” below Earth's surface can change organic solids into oil and natural gas.  These fossil fuels rise and accumulate in porous overlying rocks.

Sedimentary Structures

Sedimentary structure- A features within a sedimentary rock that provides clues about the environment of deposition.

1) Bedding: Series of horizontal layers within an outcrop of rock.  The most common sedimentary structure.

Shale with interbedded limestone. © Marli Miller, University of Oregon.

2) Ripple marks: Small ridges formed on the surface of sediment layer by moving wind or water.  Symmetrical ripples represent water wave ripples, whereas assymetrical ripples represent water current or wind current ripples.

Sand dunes of Death Valley, © Marli Miller, University of Oregon.

Ripples on sandy beach in southern Alaska, © Marli Miller, University of Oregon.

Ripple marks on sandstone of the Triassic Chinle Formation, © Marli Miller, University of Oregon.

3) Cross-bedding- Series of thin, inclined layers within a horizontal bed of rock that represent the preservation of migrating dunes.  You are looking at the side of a bed rather than the top of the bed.

Cross-bedding structures in the Navajo Sandstone of Zion National Park, © Marli Miller, University of Oregon.

4) Graded bedding- A type of bedding where grain size gets smaller from bottom to top due to sorting of grains under water.  This type of bedding results from underwater landslides (called turbidity currents) initiated by earthquakes.

Graded bedding in matrix supported conglomerate from the Pliocene Copper Canyon formation, © Marli Miller, University of Oregon.

5) Mud Cracks/Dessication Cracks- Polygonal cracks formed in mud that dries upon exposure to air (common in dry lake beds and tidal flats).

Mud cracks in the bed of the Amargosa River in California's Death Valley National Park, © Michael Collier.

Ancient mudcracks (shrinkage cracks) preserved in red-brown mudstone near the base of the Watahomigi Formation, Public Domain Image, United States Geological Survey.

6) Fossils- Any evidence of past life preserved in rock.  Hard parts (shells, bones) are the most easily preserved parts of organisms.

Fossils can give detailed information about the environment of deposition.

Trilobite. © Oklahoma University

Animal footprints in Coconino Sandstone of Aubrey Cliffs, Arizona- Courtesy United States Geological Survey.

Sediment Supply and the Importance of Big Rivers

Sediment Supply and the Importance of Big Rivers

kacimo samy   10:40 م

Sediment supply is controlled primarily by tectonics and climate. In geologically simple areas, where the basin is fed directly from the adjacent margins and source-area uplift is related to basin subsidence, supply considerations are likely to be directly correlated to basin subsidence and eustasy as the major controls of basin architecture. Such is the case where subsidence is yoked to peripheral upwarps, or in proximal regions of foreland basins adjacent to fold-thrust belts. However, where the basin is supplied by long-distance fluvial transportation, complications are likely to arise. Where the rate of sediment supply is high, it may overwhelm other influences to become a dominant control on sequence architecture. Many sedimentary basins were filled by river systems whose drainage area has been subsequently remodeled by tectonism, and it may take considerable geological investigation to reconstruct their possible past positions. For example, stratigraphic successions may occur that cannot be related to the evolution of adjacent orogens. In North America, dynamic topographic processes have generated regional uplifts and continental tilts that have resulted in deep erosion and large-scale continental fluxes of detrital sediment. For example, much of the detritus derived by uplift and erosion of the Grenville orogen of eastern North America during the late Precambrian may have ended up contributing to the thick Neoproterozoic sedimentary wedges on the western continental margin. Detailed study of detrital zircons from sedimentary rocks of this age in the western Canadian Arctic indicated that 50% of them are of Grenville age. A major west-flowing river system was established during the late Proterozoic which transported this detritus some 3,000 km across the continental interior. Much of the thick accumulations of late Paleozoic and Mesozoic fluvial and eolian strata in the southwestern United States had been derived from Appalachian sources, and this was confirmed by the detrital-zircon. Tertiary river system draining from the continental interior of North America into Hudson Bay, ultimately delivering sediment to the Labrador Shelf. This has been supported by the studies of Cenozoic landforms and sediments. 

Major river systems may cross major tectonic boundaries, feeding sediment of a petrographic type unrelated to the receiving basin, into the basin at a rate unconnected in any way with the subsidence history of the basin itself. The modern Amazon river is a good example. It derives from the Andean Mountains, flows across and between, and is fed from several Precambrian shields, and debouches onto a major extensional continental margin. From the point of view of sequence stratigraphy, the important point is that large sediment supplies delivered to a shoreline may overwhelm the stratigraphic effects of variations in sea level. A region undergoing a relative or eustatic rise in sea level may still experience stratigraphic regression if large delta complexes are being built by major sediment-laden rivers. Effects of upstream controls on the development of fluvial graded profiles, fluvial style and the development of nonmarine sequences downstream. Upstream controls may also be significant in the case of deep-marine deposits. Major episodes of submarine-fan sedimentation in the North Sea and Shetland-Faeroes basins correlate with pulses of Iceland plume activity, which caused magmatic underplating of the continental margin, and uplift, erosion, and enhanced sediment delivery to offshore sedimentary basins. A significant example of this long-distance sedimentary control is the Cenozoic stratigraphic evolution of the Texas-Louisiana coast of the Gulf of Mexico. This continental margin is fed with sediment by rivers that have occupied essentially the same position since the early Tertiary. The rivers feed into the Gulf Coast from huge drainage basins occupying large areas of the North American Interior. Progradation has extended the continental margin of the Gulf by up to 350 km. This has taken place episodically in both time and space, developing a series of major clastic wedges, some hundreds of metres in thickness. The major changes along strike of the thickness of these clastic wedges is also evidence against a control by passive sea-level change. Highly suggestive are the correlations with the tectonic events of the North American Interior; for example, the timing of the Lower and Upper Wilcox Group wedges relative to the timing of the Laramide orogenic pulses along the Cordillera. It seems likely that sediment supply, driven by source-area tectonism, is the major control on the location, timing and thickness of the Gulf Coast clastic wedges. A secondary control is the nature of local tectonism on the continental margin itself, including growth faulting, evaporite diapirism and gravity sliding. Variations in deep-marine sediment dispersal in the Gulf of Mexico show very similar patterns to the coastal and fluvial variations. Large-scale submarine-fan systems are therefore dependent, also, on considerations of long-term sediment supply variation, which may be controlled by plate-margin tectonism, in-plane stress regime and dynamic topography.

In arc-related basins volcanic control of the sediment supply may overprint the effects of sea-level change. Sediment supply and tectonic activity overprinted the eustatic effects and enhanced or lessened them. If large supplies of clastics or uplift overcame the eustatic effects, deep marine sands were also deposited during highstand of sea level, whereas under conditions of low sediment input, thin-bedded turbidites were deposited even during lowstands of sea level.

Other examples of the tectonic control of major sedimentary units are provided by the basins within and adjacent to the Alpine and Himalayan orogens. Sediments shed by the rising mountains drain into foreland basins, remnant ocean basins, strike-slip basins, and other internal basins. But the sediment supply is controlled entirely by uplift and by the tectonic control of dispersal routes. For example, the Oligocene Molasse of the Swiss proforeland basin was deposited by rivers flowing axially along the basin, and that these underwent reversal in transport directions as a result of changes in the configuration of the basin and the collision zone during orogenesis. The shifting of dispersal routes through basins and fault valleys within the Himalayan orogen of central and southeast Asia. Some of the major rivers in the area (Tsangpo, Salween, Mekong) are known to have entirely switched to different basins during the evolution of the orogen. Much work remains to be done to relate the details of the stratigraphy in these various basins to the different controls of tectonic subsidence, tectonic control of sediment supply, and eustatic sea-level changes.

Shallow marine environments of terrigenous clastic deposition

Shallow marine environments of terrigenous clastic deposition

kacimo samy   3:00 ص

The continental shelves and epicontinental seas are important sites of deposition of sand and mud in the world’s oceans and account for over half the volume of ocean sediments. These successions can be very thick, over 10,000 m, because deposition may be very long-lived and can continue uninterrupted for tens of millions of years. They occur as largely undeformed strata around the edges of continents and also in orogenic belts, where the collision of continental plates has forced beds deposited in shallow marine environments high up into mountain ranges. This chapter focuses on the terrigenous clastic deposits found in shallow seas; carbonate sedimentation, which is also important in these environments.

Sediment supply to shallow seas

The supply of sediment to shelves is a fundamental control on shallow marine environments and depositional facies of shelves and epicontinental seas. If the area lies adjacent to an uplifted continental region and there is a drainage pattern of rivers delivering detritus to the coast, the shallow-marine sedimentation will be dominated by terrigenous clastic deposits. The highest concentrations of clastic sediment will be near the mouths of major rivers: adjacent coastal regions will also be supplied with sediment by longshore movement of material by waves, storms and tides. Shallow seas that are not supplied by much terrigenous material may be areas of carbonate sedimentation, especially if they are in lower latitudes where the climate is relatively warm. In cooler climates where carbonate production is slower, shelves and shallow seas with low terrigenous sediment supply are considered to be starved. The rate of sediment accumulation is slow and may be exceeded by the rate of subsidence of the sea floor such that the environment becomes gradually deeper with time.

Characteristics of shallow marine sands

Detritus that reaches a shallow sea is likely to have had a history of transport in rivers, may have passed through a delta or estuary, or could have been temporarily deposited along a coastline before it arrives at the shelf. If there is a long history of transport through these other environments the grain assemblage is likely to be mature. Texturally, the grains of sand will have suffered a degree of abrasion and the processes of turbulent flow during transport will separate the material into different grain sizes. The compositional maturity will probably be greater than the equivalent continental deposits, because the more labile minerals and grains (such as feldspar and lithic fragments) are broken down during transport: shallow marine sands are commonly dominated by quartz grains. In polar areas, the sediment supplied is much less mature because cold weather reduces chemical weathering of the grains and glacial transport does not result in much sorting or rounding of the clasts. The detrital component is often complemented by material that orginates in the shallow marine environment. Shallow seas are rich in marine life, including many organisms that have calcareous shells and skeletons. The remains of these biogenic hard parts are a major component of shelf carbonate deposits, but can also be very abundant in sands and muds deposited in these seas. Whole shells and skeletons may be preserved in mudrocks because they are low-energy deposits. In higher energy parts of the sea, currents move sand around and a lot of biogenic debris is broken up into bioclastic fragments ranging from sand-sized, unidentifiable pieces up to larger pieces of shelly material and bone. Bone is also the origin for phosphates that can form as authigenic deposits in shallow marine settings: these phosphates are relatively rare. However, another authigenic mineral, glauconite, is a common component of sandstones and mudrocks formed on shelves and epicontinental seas and is considered to be a reliable indicator of shallow marine conditions. The characteristic dark green colour of the mineral gives sediments rich in it a distinctly green tinge, although it is iron-rich and weathers to a rusty orange colour. ‘Greensands’ are shallowmarine deposits rich in glauconite that are particularly common in Cretaceous strata in the northern hemisphere. Shallow seas are environments rich in animal life, particularly benthic organisms that can leave traces of their activity in the sediments. Bioturbation may form features that are recognisable of the activities of a particular type of organism, but also results in a general churning of the sediment, homogenising it into apparently structureless masses. Primary sedimentary structures (wave ripples, hummocky cross-stratification, trough crossbedding, and so on) are not always preserved in shelf sediments because of the effects of bioturbation. Bioturbation is most intense in shallower water and is frequently more abundant in sandy sediment than in muddy deposits. This is because the currents that transport and deposit sand may also carry nutrients for benthic organisms living in the sand: many organisms also prefer to live on and within a sandy substrate. The abundance of calcareous shell material in shallow-marine sandstones makes calcium carbonate available within the strata when the beds are buried. Groundwater moving through the sediments dissolves and reprecipitates the carbonate as cement. Shelly fossils within sandstones are therefore sometimes found only as casts of the original form, as the original calcite or aragonite shells have been dissolved away. Sandstone beds deposited in shallow marine settings also typically have a calcite cement.

Shallow marine clastic environments

The patterns and characteristics of deposition on shelves and epicontinental seas with abundant terrigenous clastic supply are controlled by the relative importance of wave, storm and tidal processes. The largest tidal ranges tend to be in epicontinental seas and restricted parts of shelves, although in some situations the tidal ranges in narrow or restricted seaways can be very small. Open shelf areas facing oceans are typically regions with a microtidal to mesotidal regime and are affected by ocean storms. Two main types, storm-dominated shelves and tide-dominated shelves, can be recognised in both modern environments and ancient facies: these are end-members of a continuum and many modern and ancient shelves and epicontinental seas show influence of both major processes. The majority of modern shelves are storm dominated (80%): the remainder are mainly tide dominated (17%), with just a small number (3%) of shelves influenced mainly by ocean currents. These ocean-current-dominated shelves are generally narrow (less than 10 km) and lie adjacent to strong geostrophic currents: sandwaves and sand ribbons form on them, and as such they are similar to tidal shelves, but the driving current is not of tidal origin. The detailed characteristics of sands deposited on modern shelves can be determined directly only by taking shallow cores that provide a limited amount of information: indirect investigation by geophysical techniques, such as shallow seismic profiles, can also yield some information about the internal structures. Not all sandy deposits occurring on modern shelves have been formed by processes occurring in the present day: the sea-level rise in the past 10 k/yr, the Holocene transgression, has drowned former strand plain and barrier island ridges, along with sands deposited in the shoreface, leaving them as inactive relics in deeper water.

Coal sampling techniques for different seams

Coal sampling techniques for different seams

kacimo samy   6:34 ص

The sampling of coal can be a difficult task in that coal is a heterogeneous material. Samples are the representative fractions of a body of material that are acquired for testing and analysis in order to assess the nature and composition of the parent body. They are collected by approved methods and protected from contamination and chemical change. Such samples should be differentiated from those materials collected in ways that may not be truly representative of the coal from which they have been collected. These materials may still be useful but should be regarded as specimens rather than samples. Coal samples may be required as part of a greenfield exploration programme to determine whether the coal is suitable for further investigation, or as part of a mine development programme, or as routine samples in opencast and underground mines to ensure that the quality of the coal to be mined will provide the specified run of mine product. 

In situ coal samples are taken from surface exposures, exposed coal seams in opencast and underground workings, and from drill cores and cuttings. 

Ex situ samples are taken from run of mine coal streams, coal transport containers and coal stockpiles. 

Sampling may have to be undertaken in widely differing conditions, particularly those of climate and topography. It is essential that the sample taken is truly representative as it will provide the basic quality data on which decisions to carry out further investigation, development, or to make changes to the mine output will be made. It is important to avoid weathered coal sections, coals contaminated by extraneous clay or other such materials, coals containing a bias of mineralization, and coals in close contact with major faults and igneous intrusions.

In-situ sampling

Several types of in situ samples can be taken, dependent upon the analysis required

Grab sampling

Generally this is a most unsatisfactory method of obtaining coal for analysis, as there are no controls on whether the coal is representative, and can easily lead to a bias in selection, for example the bright coal sections attract attention. However, grab samples can be used to determine vitrinite reflectance measurements, as an indicator of coal rank.

Channel sample

Channel samples are representative of the coal from which they are taken. If the coal to be sampled is a surface exposure, the outcrop must be cleaned and cut back to expose as fresh a section as possible. Ideally the full seam section should be exposed, but in the case of thick coals (especially in stream sections), it may be possible to see only sections of the roof and coal immediately below, or the floor and coal immediately above. To obtain a full seam section under these circumstances, two or more overlapping channels will need to be cut, and the overlap carefully recorded. The resultant samples will consist of broken coal and will not preserve the lithological sequence. In opencast workings, the complete seam section should be exposed, and is less likely to be weathered than natural surface exposures. In underground workings, the seam will be unweathered, but the whole seam section may not always be seen, due to the workings only exposing the selected mining section of the seam.

To carry out a channel sample, the coal is normally sampled perpendicular to the bedding. A channel of uniform cross-section is cut manually into the coal seam, and all the coal within the cut section is collected on a plastic sheet placed at the base of the channel. Most channels are around 1.0m across and samples should not be less than 15kg per meter of coal thickness. Such channel samples will provide a composite quality analysis for the seam, that is an analysis of all the coal and mineral matter present in the seam as a whole. Although this is suitable for general seam quality assessment, more detailed analysis of the seam from top to bottom may be required. To achieve this, a channel ply sample is taken, which entails a similar procedure as for the whole seam channel sample except that the seam is divided into plies or subsections. Coal seams are rarely homogeneous throughout their thickness, most are divisable into distinct lithological sections. Plies are lithological subdivisions of the seam, each of which has a uniform character. When the lithology changes, such as at a clay parting in the seam, a separate ply is designated. Where the roof and floor of a seam are exposed, ply samples of at least 0.25m of roof material immediately above the seam and 0.25m of floor underlying the seam should be included in the samples. This will allow the effects of dilution on coal quality to be assessed. In general the thickness of coal plies should be a minimum of 0.1m and a maximum of 1.0m. In the case of banded coals containing alternating thin (<0.1m) layers of bright coal/dull coal/clay, the seams may be sampled as a series of composite plies, with the details of the individual layers shown on the record sheet. An interbedded non-coal ply greater than 0.25m in thickness may be regarded as a seam split and recorded as such. Ply samples should be at least 2.0kg where possible, it may be that the sample will be split into two fractions and one stored for later use. Once the outcrop or face is cleaned, a shallow box-cut is made for the total thickness of the exposed coal seam. Once this is completed, the seam is divided into plies, each of which is measured and recorded on are cord sheet. The channel sample record sheet should show the following information.

1. Record card number. 
2. Map or aerial photograph number on which locality is located. 
3. Location of sample point, grid reference or reference number. 
4. Description of the locality, stream section, working face, etc., including dip, strike, coal seam roof and floor contacts. 
5. Extent of weathering, fracturing, mineralization, etc. 
6. Lithological description of each ply interval. 
7. Thickness of each ply interval. 
8. Designated sample number of each ply interval.

Space canal so be allocated on there cord card for analytical details, that is proximate analysis, to be added later to complete the record. The fresh surface is then sampled as a channel cut from top to bottom, cutting and collecting all material from each ply section in turn. Each ply sample should be sealed in a strong plastic bag immediately after collection to prevent moisture loss and oxidation. All sample bags must be clearly labelled with a designated number, a copy of which should be placed in a small plastic bag inside the sample bag, and another attached to the outside of the sample bag. This number must be recorded on the channel sample record sheet. Because this task is invariably a dirty one, labels get wet, blackened and unreadable very easily, so it is essential therefore that care must be taken to ensure that the sample numbers do not get lost or obliterated during transit to the laboratory, as unidentifiable samples are useless and an expensive waste of time. The advantage of channel ply sampling is that not only can the analysis of the individual plies be obtained, but also by combining a fraction of each ply sample, a whole seam composite analysis can be made. An example of a channel ply sample from a surface exposure is illustrated in Figure 5.3, which shows a channel cut to expose fresh coal, and then a thinner channel (0.25m wide) cut from the fresh coal from which ply samples are collected for analysis.

Pillar samples

In underground coal mining, samples of large blocks of undisturbed coal are taken to provide technical information on the strength and quality of the coal. These pillar samples are taken when a specific problem may have arisen or is anticipated. Such samples are taken in much the same way as whole-seam channel samples except that extra care is required not to disturb the cut-out section of coal during removal. Samples are then boxed and taken to the laboratory. Pillar sampling is a long and arduous business and is undertaken only in special circumstances, such as when mining becomes difficult or new roadways or faces are planned.

Core samples

Core sampling is an integral part of coal exploration and mine development. It has the advantage of producing non-weathered coal including the coal seam floor and roof, and unlike channel samples, core samples preserve the lithological sequence within the coal seam. First, the borehole core has to be cleaned if drilling fluids have been used, and then lithologically logged. Following this, the lithological log should be compared with the geophysical log of the borehole to select ply intervals and to check for core losses and any other length discrepancies. Once the core has been reconciled to the geophysical logs and the ply intervals have been selected, sampling can commence. Core ply samples are taken in the same ways for surface channel ply samples, again a ply sample of the coal seam roof and floor (up to 0.25m) is taken to determine dilution effects. Then the individual plies are sampled, making sure no core is discarded. As in the case of surface samples, bright coal tends to fragment and make up the finer particles that may easily be left in the core tray. The samples are bagged and labelled as for surface ply samples, and the sample numbers recorded on the core-logging sheet. Large diameter cores may be split lengthways with a bolster chisel and then one of the halves ply sampled, the other being retained for future analysis.

Cuttings samples

This method of sampling is considerably less accurate than that of core sampling. As with core samples, cuttings are unweathered and are a useful indicator as to the general nature of the seam. Air flush and mud flush noncore drilling is a quicker operation than core drilling and will produce cuttings for each horizon encountered in drilling. In the case of mud flush cuttings they will need to be washed to remove any drilling fluid before sampling. Cuttings are usually produced for every metre drilled, those cuttings returns that are all coal may be collected, bagged and numbered in the same way as channel samples. The depth to the top and bottom of the seam sampled should be determined from the geophysical log. The drawback with using cuttings samples is that only a general analysis of the seam can be made, and even this is unlikely to be truly representative. Contamination from strata above the coal also may be included, and a close study of the geology will determine whether this is so.

Specimen samples

Orientated specimens of coal may be collected so that their precise orientation can be re-created in the laboratory. The dip and strike of the coal is marked on the specimen before removal. This method is commonly used for studies of the optical fabric of the coal, or of the structural features in the coal.

Bulk samples

Bulk samples are taken from outcrops, small pits or minishafts (i.e. 2m diameter shaft excavations). A bulk sample is normally 5–25t and is taken as a whole seam channel sample on a large scale. Such a bulk sample is taken in order to carry out test work on a larger scale, which is designed to indicate the coal's likely performance under actual conditions of usage. Steam coal samples are taken for small combustion tests in a pulverized fuel (pf) rig, to simulate conditions in a PF boiler. Pulverized coal firing is the combustion of powdered coal suspended as a cloud of small particles in the combustion air. Substantially more heat is released per unit volume in PF boilers than in stoker type boilers. Coking coal samples are taken to carry out moving wall oven tests, that is to determine how much the coal swells when it is combusted, thus putting pressure on the oven walls, which are constructed of uncemented brickwork. High-pressure coals are undesirable, and are normally blended with low-pressure coals to reduce the problem. In the United States, low-volatile coking coals (volatile matter (VM)=20–25%,SI=9) are high-pressure coals, whereas in general, high-volatile coals do not have such high pressures. It is significant that Gondwana coking coals are low-pressure coals, an important factor in Australia being able to export coking coals. Bulk samples are collected from a site already channel sampled, loaded into drums, numbered and shipped to the selected test centre.

Sample storage

In the majority of cases, the channel and core samples will be required immediately for laboratory analysis. However, there are circumstances where duplicate coal samples for future reference are taken. Usually the channel plies are divided into two or the cores are split and one half retained. If the duplicate samples are to be put into storage, this presents a problem because the exposure of the coal to air will allow oxidation to take place during storage and this will result in anomalous quality results when analysed at a later date. The usual procedure to prevent oxidation of samples is to store them under nitrogen or in water. To store in nitrogen, place a tube connected to a pressurized cylinder containing nitrogen in a plastic sample bag, then add the coal sample, flush the sample with nitrogen regulating the flow by means of a flow meter. The nitrogen has to fill the spaces between the coal fragments, so flushing with nitrogen is required for several minutes. One difficulty with this method is that nitrogen is lighter than air so inevitably some is lost in the process. Once the bag has been thoroughly flushed, it should be heat-sealed; no other form of sealing is anywhere near as effective. The coal samples can be as received or air dried and can be in the form of lump or crushed coal. It should be noted that for all in situ and ex situ samples, the top size to which any sample is crushed to is important in determining the weight of the sample required. The size of the sample is calculated as: 5.24×mean particle size=xkg (where mean particle size is top size×bottom size). 5.24 is an empirically determined number quoted in BS1017-1 and Australian Standard 4264 (Appendix 1). A cheaper method of storage is by immersing the channel or core sample in the form of lump coal in water. This method has the advantage over storing in nitrogen in that it preserves fluidity of the coal, but it does present handling problems when the sample is required. The sample will have to be air dried before analysis can begin. Samplescanbekeptbythesemethodsfor1–2yrbefore analysis.

Ex-situ sampling

The object of collecting coal samples after mining is to determine the quality of coal actually being produced. This coal may differ significantly from the in situ seam analysis in that not all of the seam may be included in the mining section, or that more than one seam may be worked and fed to the mine mouth and mixed with coal from other seams. In addition there may be dilution from seam roof and/or floor contamination that becomes part of the mined coal product. The mined coal is broken up and therefore contains fragments that vary a great deal in size and shape. Representative samples are collected by taking a definite number of portions, known as increments, distributed throughout the total quantity of coal being sampled. Such increments represent a sample or portion of coal obtained by using a specified sampling procedure, either manually or using some sampling apparatus. The various practices used in collecting ex situ samples and the mathematical analysis of the representativeness of samples, i.e. quality control. Increments are taken using three methods.

1. Systematic sampling, where increments are spaced evenly in time or in position over the unit. 

2. Random sampling, where increments are spaced at random but a prerequisite number are taken. 

3. Stratified random sampling, where the unit is divided by time or quantity into a number of equal strata and one or more increments are taken at random from each. 

It is good practice that whatever the method used, duplicate sampling should be employed to verify that the required precision has been attained. Ex situ coal sampling is carried out on moving streams of coal, from rail wagons, trucks, barges, grabs or conveyors unloading ships, from the holds of ships and from coal stockpiles.

Hand sampling from streams is carried out using ladles or scoops, the width of the sampler should be 2.5 times the size of the largest lump likely to be encountered; however, this type of sampling is not suitable for coal larger than 80mm. For larger samples mechanical sampling equipment is used, where moving streams of coal (conveyors) are sampled by: (a) falling stream samplers, which make either a line a traverse across the coal convey or in a straight line path perpendicular to the direction of flow, or opposite to the direction of flow, or in the same direction of flow, or they make a rotational traverse by moving in an arc such that the entire stream is within the radius of the arc; (b) cross-belt samplers, which move across the belt pushing a section of coal to the side while the belt runs; (c) the stop belt method, whereby the conveyor is stopped and all coal occurring within a selected interval, usually a couple of metres, is collected. The correct increment selection occurs when all the elements of the transversal cross-section are intercepted by the sampling cutter during the same length of time. This should avoid any increase in error. These sampling systems are checked for bias by using a reference sampling method as recommended by BSISO 13909 or ASTM D2234. Wagons and trucks are sampled by taking samples from their tops by means of probes, or by sampling from bottom or side door wagons during discharge,or sampling from the exposed face of coal as the wagons or trucks are tipping into bunkers or ships, or wagons being emptied via tipplers. Ships are sampled either from conveyors loading and unloading coal, at a point where bias can be avoided, or from the hold of the ship. Samples from the hold, are taken every 4m of the depth of the coal within the hold.Itisimportanttoestimatetheproportionoffine and lump coal in the consignment. It should be noted that free moisture, if present, will tend to settle towards the bottom of the hold. This increase of moisture with depth makes it difficult to collect samples for moisture content determination. 4. Sampling from barges is the same as for ships except that if the depth of coal is less than 4m it should be sampled onboard during unloading, once the bottom of the barge is partially uncovered. 5. When the preferred procedure of sampling from a conveyor belt during stocking and unstocking cannot be used, then the stockpile is sampled based on collecting increments spaced as evenly as possible over the surface and layers of the stockpile. Sampling is by means of probes or by digging holes. If the stockpile is known to consist of different coals piled in separate areas of the total pile, a separate gross sample must be taken from each such area. The stockpile should be divided into a number of portions, each 1000t or less from which a separate sample with a specified number of increments is taken. This normally takes along time to accomplish, but can be speeded up if automated auger units are employed. It is important that all levels in the stockpile are sampled.

Marcellus shale

Marcellus shale

kacimo samy   8:36 ص

What is Marcellus shale?

Marcellus shale is a sedimentary rock which has exposure in eastern North America. It is found in the Appalachian basin where black shale of Devonian period called Marcellus shale. The name Marcellus is from the nearby village named Marcellus, New York, United States. This formation is extended into much of the Appalachian basin. These shales were discovered to be the containing most abundant gas reserves which is estimated to be about 1.9 trillion cubic feet of gas. It is alot of gas reserves but is spread at large geographical area.

Stratigraphical position of Marcellus formation

Marcellus formation is the lowest unit of the Devonian age Hamilton group and is divided into several sub units. As it is shale so is dominant by black shales but it also has limestone interbedded and lighter shales which are the result of sea level variations at the time of its deposition.

Production of gas from Marcellus

Marcellus shales has production of gas which is in trillion cubic feet and many wells are drilled in this formation. These shales yields gas production which occurs not for a long time when its production is declined gradually but some wells sustained which produced for decades. 

How gas can be produced more from Marcellus shales?

Marcellus shales as wells declines with time so new technology and techniques can help produce large amount of gas from Marcellus shales. A vertical well drilled in these shales will gradually decline the production so in order to a sustained well, Horizontal drilling and hydraulic fracturing technique should be used which in some new wells produced over a million cubic feet of gas per day.

Gas occurring in the Marcellus shales

Gas in Marcellus shales occurs in three ways

• Within pore spaces of the shales. 
• Within vertical fractures of the shales. 
• Adsorbed on mineral grains.

Most recoverable gas is present in the pore spaces of the shales. Gas has difficulty in escaping the pores because of the tiny spaces and poorly interconnection of the pores. Therefore a vertical well in these shales would be expected to intersect few fractures within the shales therefore a horizontal drilling will intersect maximum number of fractures which in turn will produce maximum amount of gas from these shales.

Hydraulic fracturing

Hydraulic fracturing is another technique that can be used in Marcellus shales to enhance the production. Hydraulic fracturing uses high pressure water or gel which is inserted in the formation and it fractures the formation. To remain the formation open after releasing the pressure of the water or gel, sands or other propant is induced into the formation which allows the fractures to remain open.

Shale

Shale

kacimo samy   6:45 ص

Shale

Shale is a sedimentary rock composed of fine grained sediments mainly silt and clay particles ranges at size of 1/256 millimetre. Shale is in a category of mudstone but is different from other mudstone as the rock comprises laminae (thin beds) and is also fissile (breaks into particles). 

Composition

Shale is composed of clay size particles, it includes fine grains as illite, kaolinite and smectite. Other particles can also be in the shale which are finer grains of quartz, feldspar and chert. Shale also contains organic material which are marine plants and animals deposited at the sea floor. Other constituents can also be carbonate particles, iron oxide minerals, sulphide minerals and heavy minerals. These particles are the indicator of shale depositional environment.

Uses

Shale properties have important uses, as it contain organic material that can produce oil and gas when cooked at proper depth with high pressure and temperature. Shale can also be used as constituent material for producing cement. Limestone and shale are crushed and heated together to a temperature greater enough to eliminate all water content and breaks limetone into calcium oxide and carbon dioxide. With mixture of shale and calcium oxide a powder is formed that harden when water is mixed in it. 

Other than this shale can also be used to produce clay by mixing fine grained shale with water. This clay is then used to produce brick which is the building block of any building.

Colour of shale

Black and grey

The colour black and grey in a sedimentary rock is the indicator, presence of organic material in the rock. The organic material present in the shale is deposited by the animal and plant debris at the ocean floor. This organic material is not decayed because of the oxygen deficient environment or otherwise all the organic matter would have decayed. The oxygen deficient environment also provide proper conditions for production of sulphide minerals as pyrite. This organic material with oxygen deficient environment produces oil and gas. For production of oil and gas proper burial depth and pressure, temperature is important.

Red, brown and yellow

The colour as brown, red and yellow of a shale is the indicator of oxygen rich environment and presence of iron oxide or iron hydroxide such as hematite, goethite or limonite. Less amount of this can give the colour throughout the rock body. Hematite gives red colour and goethite or limonite presence give the colour of brown or yellow.

Green colour

Green colour of the shale are by the mica or other minerals present in the shale consist of greenish colour which turns the shale green.

Oil and gas shales

Oil shale

The term oil shale refers to sedimentary rocks of clay and silt size that contain bituminous material (Kerogens) which releases petroleum like liquid when heated in a process of pyrolysis. Oil shale production started million of years ago as that of the oil generations. Oil shale generated by the deposition of organic debris at bottom of lake or sea but when buried the heat and pressure was not great enough to turn it into oil and gas source rock. These rock catch fire without adding any chemical because of its oil. 

Shale gas

Shale gas is referred to gas that is trapped in the shale and cannot escape shale (source rock) to the reservoir rock. Marcellus shale of the Marcellus formation have a lot of gas which can be recovered by unconventional method.

Conventional oil and gas recovery

Conventional Method 

Organic shales are the source rock for most oil and gas production. So these organic material is turned into petroleum when buried at great depths with high pressure and temperature. The petroleum then moves from source rock into reservoir rock by the squeezing of shales through overburden. These when migrated to reservoir rock moves to height due to its low density till the time when it is trapped at a certain blockage where petroleum cannot moves further. So for extraction of such oil and gas a well is drilled above the trap and oil is extracted.

Unconventional oil and gas

Unconventional or horizontal drilling 

Unconventional oil and gas recovery is the technique used by extracting oil and gas from the tiny pores of shales which are not inter connected so therefore a directional drilling method is used where well is drilled horizontal into the source rock. The source rock is fractured by hydraulic fracturing and the recovery is made for the trapped oil and gas in the shales.

Hydraulic properties

Hydraulic properties are characteristics of a rock such as porosity and permeability which intends on the properties of holding and transmitting liquids through it.

The shales are made up of tiny particles so its pore spaces will also be small. The small pores can trap water or oil and gas but their movement within the rock is difficult. As no movement of liquids can be allowed through shales but due to the small pores present, it can take large amount of liquids. Shales can serve as a seal rock but not as a reservoir.

Expansive properties

Clay minerals in some soils of shales have the tendency to absorb and release large amount of water. These soil swells when it absorbs water and shrinks on release. These derived soils must be check for the construction of building or road to reduce maintenance cost at later stages.

Slope stability

Shales are often associated with landslides as weathering of shales turns them into clay material that when wet have low shear strength. On a hill side and wet shales, will creep or move rapidly along the slope.