Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Showing posts with label reservoir. Show all posts
Showing posts with label reservoir. Show all posts

Download Geoscience Books

Geoscience Books:

We are grateful to Qazi Sohail Imran for providing Geoscience books to our community. Qazi is from Islamabad Pakistan and is a Former Research Geophysicist at King Fahd University of Petroleum and Minerals. He is contributing to his oil and gas community with the believe that "Knowledge is power and knowledge shared is power multiplied".Follow the link above the images to download the books. However if the link is not working or you have any other query, just mention it in comment or email us here , we will fix it for you.

1. Name: Sedimentology and Stratigraphy by Gary Nichols 
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2. Name: Physical Geology- Earth Revealed. 9th Edition by C.C. Plummer   
     Download Here

3. Name: An Introduction to Structural Geology and Tectonics by Stephen Marshak
    Download Here

4. Name: 3-D Structural Geology by R.H.Groshong
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5. Name: Structural Geology of Rocks & Regions 3rd Edition - Davis Reynolds
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6. Name: Geological Field Techniques Edited By Angela L. Coe
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7. Name: Seismic Stratigraphy, Basin Analysis and Reservoir Characterisation_Vol37       by Paul C.H. Veeken
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8. Name: General Dictionary of Geology by Alva Kurniawan, John Mc. Kenzie,                  Jasmine Anita Putri
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9. Name: The Handy Geology Answer Book
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10.   Name: Sedimentary Basin Formation-Presentation
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11. Name: Facies Models Response to Sea Level Change by Walker and James
       Dowload Here

12. Name: AAPG Memoir 33 - Carbonate Depositional Environments
      Download here

13. Name: Petroleum Formation and Occurrence by Tissot, B.P. and Welte, D. H        Download Here

14. Name: Basin Analysis-Principles and Applications by Allen     
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15. Name: Sedimentary Rocks in the Field by Tucker
         Download Here

16. Name: Exploration Stratigraphy 2nd Edition - Visher
      Download Here

17. Name: Petroleum Geology of Pakistan by Iqbal B. Kadri
      Download Here

18. Name: The Geological Interpretation Of Well Logs by Rider
      Download Here

19. Name: Digital Signal Processing Handbook by Vijay K. Madisetti
      Download Here

20. Name: AAPG Memoir 88 - Giant Hydrocarbon Reservoirs of the World                           Download Here

21. Name: Deep-Water Processes and Facies Models-Implications for Sandstone               Reservoirs by  Dr. Shanmugam
      Download Here


Different shale distributions in low resistivity log response.

                   First, we will start with a small introduction about the resistivity logs                                           

Resistivity log

Technique : produce a current in the adjacent formation and measure the response of the formation to that current.

Resistivity logs are used to:

• determine hydrocarbon-bearing versus water bearing zones

• indicate permeable zones

• determine porosity

By far the most important use of resistivity logs is the determination of hydrocarbon-bearing versus water-bearing zones. Because the rock’s matrix or grains are non-conductive and any hydrocarbons in the pores are also non-conductive, the ability of the rock to transmit a current is almost entirely a function of water in the pores. As the hydrocarbon saturation of the pores increases (as the water saturation decreases), the formation’s resistivity increases. As the salinity of the water in the pores decreases , the rock’s resistivity also increases.

Resistivity tools principle : there are two types of resistivity tool , The dual lateral log ( DLL ) and the induction log ( DIL ) both types measures the resistivity in three zones simultaneously.

LLD looks deep into reservoir

LLS Looks shallow into the reservoir

MSFL reads the resistivity close to the wellbore.

Low Resistivity response :

High deep resistivity means : HCs or Tight streak  { low porosity }

Low deep resistivity means : Shale or wet sand.

Shale: Shale is defined as a fine-grained, indurated detrital sedimentary rock formed by the consolidation (by compression or cementation) of clay, silt, or mud.

It is characterized by a finely stratified structure of laminae ranging from 0.1 to 0.4 mm thick. Shale contains an appreciable content of clay minerals or derivatives from clay minerals, with a high content of detrital quartz; containing at least 50% silt, with 35% clay or mica fraction, and 15% chemical or authigenic materials

In petrophysical analysis, shale volume is one of the key answers used later to correct porosity and water saturation for the effects of clay bound water, (CBW).

Shale distribution in shaly sand :


Shale can be distributed in several different ways, as shown below.

Laminated shale is a special case in petrophysical analysis. Standard models for porosity and saturation do not work.

Dispersed shale is usually composed of from clay minerals that form in place after deposition due to chemical reactions between the rock minerals and the chemicals in the formation water.

Structural shale is usually deposited as particles, grains, or clasts during the initial depositional phase. For example, the flooding of a river valley can carry mud or shale from surrounding areas.

Different shale distributions have different effect on the sand reservoir.

In a sand reservoir contain structure shale : it will affect the reservoir porosity

In a sand reservoir contain laminae shale : it will affect only the net pay of the reservoir

In a sand reservoir contain : it will affect the porosity and permeability of the reservoir and also it will lead to a shortcut in the resistivity log response , which may result in a miss lead in the interpretation of the reservoir porosity and saturation  , it could be interpreted as sand bearing water instead of a sand contain dispersed shale.

So, the question here is how to differentiate between them and to avoid this wrong interpretation ?!

Let’s assume that you have a 100% clean sand reservoir. So the total porosity of this reservoir is 30% and the sand grains will represent 70% of the volume of the reservoir

Hint : Porosity of sandstone is 30 % and porosity of shale is 10%

Case 1 :

In the case of the presence of structure shale ,

So in this case shale grains will replace sand grains ( volume of 70% ) , the shale will bring its 10% porosity with it.

In other words , The porosity will be enhanced by 10% in the volume of 70% of the sand

So , the porosity will increase by 70/10 and the total porosity will be = 37 %

Case 2 :

In the case of the presence of laminae shale , in this case shale will replace the whole reservoir ( 100 & ) and also will bring its own 10% porosity.

In other words , the porosity will be reduced from 30% to 10%
Case 3 :

In the case of the presence of dispersed shale , in this case shale we will replace the porosity  volume it self ( 30 % ) and as usual it will bring its own porosity.

In other words , the porosity will be reduced into 3% ( 30 / 10 )Summarized figure for the different shale distributions in shaly sand reservoir and it’s effect on the reservoir porosity.
Shale distribution model proposed by Thomas and Stieber (Tyagi et al. 2009). Here Vshale is the volume of shale, φtotal is the total porosity, φmax is the maximum porosity, and φsh is the porosity in shale

Conclusion :


So, we can differentiate between the three different types of shale distribution and according to the type we can make the right interpretation for the porosity and the saturation of the sand reservoir , also we will avoid the miss leading interpretation in the shortcut in the resistivity log.

Photo Credits: Ahmed Adel
Originally blog is written by Ahmed Adel 

Oil Exploration and Production

Oil Exploration and Production

Birth of the Oil Industry 

In the United States, during the first half of the 19th century, people collected “rock oil” (later called petroleum, from the Latin words petra, meaning rock, and oleum, meaning oil) at seeps and used it to grease wagon axles and to make patent medicines. But such oil was rare and expensive. In 1854, George Bissel, a New York lawyer, came to the realization that oil might have broader uses, particularly as fuel for lamps, to replace increasingly scarce whale oil. Bissel and a group of investors contracted Edwin Drake, a colourful character who had drifted among many professions, to find a way to drill for oil in rocks beneath a hill near Titusville, Pennsylvania, where oily films floated on the water of springs. Using the phony title “Colonel” to add respectability, Drake hired drillers and obtained a steam-powered drill. Work was slow and the investors became discouraged, but the very day that a letter arrived ordering Drake to stop drilling, his drillers found that the hole, which had reached a depth of 21.2 m, had filled with oil. They set up a pump, and on August 27, 1859, for the first time in history, pumped oil out of the ground. No one had given much thought to the question of how to store the oil, so workers dumped it into empty whisky barrels. This first oil well yielded 10 to 35 barrels a day, which sold for about $20 a barrel (1 barrel equals 42 gallons). Within a few years, thousands of oil wells had been drilled in many states, and by the turn of the 20th century, civilization had begun its addiction to oil. Initially, most oil went into the production of kerosene for lamps. Later, when electricity took over from kerosene as the primary source for illumination, gasoline derived from oil became the fuel of choice for the newly invented automobile. Oil was also used to fuel electric power plants. In its early years, the oil industry was in perpetual chaos. When “wildcatters” discovered a new oil field, there would be a short-lived boom during which the price of oil could drop to pennies a barrel. In the midst of this chaos, John D. Rockefeller established the Standard Oil Company, which monopolized the production, transport, and marketing of oil. In 1911, the Supreme Court broke down Standard Oil into several companies including Exxon (Esso), Chevron, Mobil, Sohio, Amoco, Arco, Conoco, and Marathon some of which have recombined in recent decades. Oil became a global industry governed by the complex interplay of politics, profits, supply, and demand. 

The Modern Search for Oil 

Wildcatters discovered the earliest oil fields either by blind luck or by searching for surface seeps. But in the 20th century, when most known seeps had been drilled and blind luck became too risky, oil companies realized that finding new oil fields would require systematic exploration. The modern-day search for oil is a complex, sometimes dangerous, and often exciting procedure with many steps. Source rocks are always sedimentary, as are most reservoir and seal rocks, so geologists begin their exploration by looking for a region containing appropriate sedimentary rocks. Then they compile a geologic map of the area, showing the distribution of rock units. From this information, it may be possible to construct a preliminary cross section depicting the geometry of the sedimentary layers underground as they would appear on an imaginary vertical slice through the Earth.

To add detail to the cross section, an exploration company makes a seismic-reflection profile of the region. To obtain a seismic profile, a special vibrating truck or a dynamite explosion sends seismic waves (shock waves that move through the Earth) into the ground. The seismic waves reflect off contacts between rock layers, just as sonar waves sent out by a submarine reflect off the bottom of the sea. Reflected seismic waves then return to the ground surface, where sensitive instruments (geophones) record their arrival. A computer measures the time between the generation of a seismic wave and its return, and from this information defines the depth to the contacts at which the wave reflected. With such information, the computer constructs an image of the configuration of underground rock layers and, in some cases, can “see” reserves of oil or gas. 

Drilling and Refining 

If geological studies identify a trap, and if the geologic history of the region indicates the presence of good source rocks and reservoir rocks, geologists make a recommendation to drill. (They do not make such recommendations lightly, as drilling a deep well may cost over $50 million.) Once the decision has been made, drillers go to work. These days, drillers use rotary drills to grind a hole down through rock. A rotary drill consists of a pipe tipped by a rotating bit, which is a bulb of metal studded with hard metal prongs. As the bit rotates, it scratches and gouges the rock, turning it into powder and chips. Drillers pump “drilling mud,” a slurry of water mixed with clay and other materials, down the center of the pipe. The mud flows down, past a propeller that rotates the drill bit, and then squirts out of holes at the end of the bit. The extruded mud cools the bit head, which otherwise would heat up due to friction as it grinds against rock, then flows up the hole on the outside of the drill pipe. As it rises, the mud carries “rock cuttings” (fragments of rock that had been broken up by the drill bit) up and out of the hole. Mud also serves another very important purpose its weight counters the pressure of the oil and gas in underground reservoir rocks. By doing so, it prevents hydrocarbons from entering the hole until drilling has been completed, the hole has been “finished” (by removing the drill pipe and sealing the walls of the drill hole with concrete), and the hole has been capped. Were it not for the mud, the natural pressure in the reservoir rock would drive oil and/or gas into the hole. And if the pressure were great enough, the hydrocarbons would rush up the hole and spurt out of the ground as a gusher or blowout. Gushers and blowouts can be disastrous, because they spill oil onto the land and, in some cases, ignite into an inferno. Early drilling methods could produce only vertical drill holes. But as technology advanced, drillers developed methods to control the path of the drill bit so the hole can curve and become diagonal or even horizontal. Such directional drilling has become so precise that a driller, using a joystick to steer the bit, and sensors that specify the exact location of the bit in 3-D space, can hit an underground target that is only 15 cm wide from a distance of a few kilometres. Drillers use derricks (towers) to hoist the heavy drill pipe. To drill in an offshore hydrocarbon reserve, one that occurs in strata beneath the continental shelf, the derrick must be constructed on an offshore-drilling facility. These may be built on huge towers rising from the sea floor, or on giant submerged pontoons. Using directional drilling, it’s possible to reach multiple targets from the same platform. On completion of a hole, workers remove the drilling rig and set up a pump. Some pumps resemble a bird pecking for grain; their heads move up and down to pull up oil that has seeped out of pores in the reservoir rock into the drill hole. You may be surprised to learn that simple pumping gets only about 30% of the oil in a reservoir rock out of the ground. Thus oil companies may use secondary recovery techniques to coax out more oil (as much as 20% more). For example, a company may drive oil toward a drill hole by forcing steam into the ground nearby. The steam heats the oil in the ground, making it less viscous, and pushes it along. In some cases, drillers create artificial fractures in rock around the hole by pumping a high-pressure mixture of water, various chemicals, and sand into a portion of the hole. This process, called hydrofracturing (or “fracking”) creates new fractures and opens up pre-existing ones. The sand left by the fracturing fluid keeps the cracks from closing tightly, so they remain permeable. The fractures provide easy routes for the oil to follow from the rock to the well. Once extracted directly from the ground, “crude oil” flows first into storage tanks and then into a pipeline or tanker, which transports it to a refinery. At a refinery, workers distil crude oil into several separate components by heating it gently in a vertical pipe called a distillation column. Lighter molecules rise to the top of the column, while heavier molecules stay at the bottom. The heat may also “crack” larger molecules to make smaller ones. Chemical factories buy the largest molecules left at the bottom and transform them into plastics.

Where Does Oil Occur? 

Reserves are not randomly distributed around the Earth. Currently, countries bordering the Persian Gulf contain the world’s largest reserves in 25 supergiant fields. In fact, this region has almost 60% of the world’s reserves. Reserves are specified in barrels (bbl); 1 bbl 42 gallons 159 liters. Why is there so much oil in the Middle East? Much of the region that is now the Middle East was situated in tropical areas between latitude 20n south and 20n north between the Jurassic (135 Ma) and the Late Cretaceous (65 Ma). Biological productivity was very high in these tropical regions, so the muds that accumulated there were very organic rich and lithified to become excellent source rocks. Thick layers of sand buried the source rocks and eventually became porous sandstones that make excellent reservoir rocks. Later, mountain-building processes folded the layers into large anticlines, which are excellent traps. The Middle East is not the only source of oil. Reserves also occur in sedimentary basins formed along passive continental margins, such as the Gulf Coast of the United States and the Atlantic Coasts of Africa and Brazil, as well as in intracratonic and foreland basins within continents.

Oil inclusions in quartz cement, sandstone reservoir

Quartz with oil inclusions from Zhob, Balochistan, Pakistan
Inclusions of small drops of oil may be trapped in quartz cement and show up well in fluorescent light. Fluid inclusion data from quartz helps to constrain the temperature for quartz cementation and in sandstone reservoirs it has been demonstrated quite clearly that quartz cementation continues after oil emplacements in a reservoir. The lowest fluid inclusion temperatures in quartz cement indicate an onset of quartz cementation close to 70–80◦C. In the Ula Field (North Sea basin), fluid inclusion temperatures in quartz cement range from 86◦C to 126◦C, the highest temperature being close to the present day reservoir temperature which is also the maximum burial depth and temperature. Fluid inclusions from 24 samples from 11 different reservoir units from the North Sea and Haltenbanken also show temperatures from about 80◦C to values close to the present reservoir temperatures. This shows that quartz cementation occurs as a continuous process at a rate controlled by the temperature. There is no evidence that quartz cementation is episodic, controlled by the supply of silica, or that quartz cementation stops after the sandstones have become oil-saturated. In a water-wet reservoir precipitation can still continue in the remaining water around the grains. At high oil saturation, the transport of silica by advection as well as by diffusion becomes much less efficient. The continued growth of quartz cements after oil emplacement results from the closed system nature of quartz cementation in sandstones. In an oil-wet system, however, quartz can not precipitate on the grain surfaces and oil or bitumen may become very effective coatings. Asphaltic oil and bitumen formed by bio-degradation or other processes may preserve good reservoir quality, particularly if the heavy oil only occurs as a grain coating. In summary, quartz cementation is controlled by the slow kinetics (high activation energy) for quartz cementation and normally a minimum temperature of 70–80◦C is required. This is however also dependent on the pH. In sedimentary basins marine porewater starts out with a pH close to 7 but quickly becomes more acid due to the build of CO2 and other reactions with the minerals present. At 3-4 km depth the pH may typically be 4.5–5 but at 120◦C the pH is close to neutral. The rate of quartz cementation is then lowered by the pH but increased by higher temperatures. At very high pH quartz cementation may occur at the surface and silcrete is fine-grained quartz formed in soils due to concentration of porewater by evaporation, and quartz is also forming in some alkaline African lakes. Authigenic illite consists of thin hair- or plate-like minerals and it is fairly obvious that they would have a detrimental effect on reservoir quality by reducing the permeability. SEM images are routinely taken from dried-out cores where the illite appearance is no longer representative of its morphology in the reservoir. When cores are dried without destroying the delicate illite morphology the pore space often looks like it has been filled with rockwool. Illite can often be seen to grow at the expense of kaolinite and may also form by alteration of smectite. Although authigenic illite may also be observed on fractures and other places where there are no obvious precursor minerals, it is most commonly found as a replacement of an earlier Al-rich mineral phase. Because of the low Al-solubility in porewater, illite will in most cases precipitate where the source of Al is available locally from a dissolving mineral. Calculations suggest that the solubility of aluminium is only about 1 ppm at 150◦C and that organic acids have little effect in terms of increasing its solubility. The formation of illite from smectite via mixed layered minerals is well known and occurs in sandstones in the temperature range of 70–100◦C. Sandstones with abundant smectite are poor reservoir rocks at the outset, and illitisation of such rocks may itself slightly improve reservoir quality as illite has a lower specific surface area than smectite. In better sorted and potentially good reservoir rocks kaolin minerals (kaolinite or dickite) are the most important precursors for illite. However, the formation of illite requires potassium, and K-feldspar is usually the only significant source present in the sediment. 

KAlSi3O8  + Al2Si205(OH)4 = KAl3Si3O10(OH)2+ 2SiO2 +H2O 
K-Feldspar + Kaolinite            = Illite                        + Quartz

The reaction between K-feldspar and kaolinite occurs at about 130◦C and above this temperature these two minerals are no longer thermodynamically stable together. In the North Sea basin and at Haltenbanken this corresponds to a burial depth of about 3.7–4 km. A sharp increase in the illite content in sandstone reservoirs is observed. If the matrix is well-cemented the rate of diffusion is reduced, and the minerals are then able to co-exist at higher temperatures if they do not occur close together. In sandstones with little or no K-feldspar, however, kaolin remains stable at greater depth as it is not dissolved and replaced by illite. The formation of illite can therefore be predicted from the sandstone provenance with respect to K-feldspar supply, and from the early diagenesis and freshwater flushing with respect to the distribution of kaolinite. If a sandstone is derived from an albite-rich gneiss the K-feldspar content is likely to be too low and much of the kaolinite would then not be illitised. Similarly, not much illite will be formed in sandstones with little kaolinite or smectite. Both in Haltenbanken and the North Sea there are Jurassic reservoirs where plagioclase is the dominant feldspar and where the low K-feldspar content is unable to supply the necessary potassium for illitisation of kaolinite. The low illite content in such reservoirs preserves better permeability. This is a direct function of the provenance and could be due to erosion of albite gneisses rather than granitic gneisses. The distribution of authigenic illite in sedimentary basins like the North Sea and Haltenbanken shows that illite formation is strongly controlled by the present day burial depth and temperature. The increase in illite content at about 3.7–4.0 km is usually very sharp, indicating a temperature-controlled reaction rather than a high kinetic reaction rate when the association of kaolinite and K-feldspar becomes thermodynamically unstable. Basin loading from thick Pleistocene sequences in these areas suggests that the illite formed recently. K-Ar dating of illite gives variable ages for the formation of illite. This is probably because even very small amounts of detrital (older) mica or feldspar will produce too-old ages.

Prediction of Reservoir Quality

Reservoir Quality

The properties of all reservoir rocks are continuously changing, from the time the sediments are deposited through to their burial at great depth and during any subsequent uplift. This is a combined function of mechanical compaction and of chemical processes involving dissolution and precipitation of minerals. At any given burial depth the properties depend on the composition of the sandstones when at shallow depth, and on their temperature and stress history during burial. Practical prediction of the porosity and permeability during exploration and production is only possible if the processes that change these parameters are understood. It should be realised that the starting point for the diagenetic processes is the initial sandstone composition. This is a function of the rocks eroded (provenance), transport, and depositional environments. Diagenetic models must therefore be linked to weathering and climate, sediment transport, facies models and sequence stratigraphy, and should be integrated in an interdisciplinary basin analysis. Diagenesis is often considered a rather specialised field of sedimentology and petroleum geology, but it embraces all the processes that change the composition of sediments after deposition and prior to metamorphism. The most important factor in predicting reservoir quality at depth is the primary clastic composition and the depositional environment. The diagenetic changes also determine the physical properties of sandstones, such as seismic velocities (Vp and Vs) and the compressibility (bulk modulus). This is also critical when predicting physical rock changes during production. 
The main diagenetic processes are:
  1. Near-surface diagenesis. Reactions with fresh groundwater (subsurface weathering). In dry environments, with saline water concentrated by evaporation. Sand may also be cemented with carbonate cement near the sea floor. 
  2. Mechanical compaction, which reduces the porosity by packing the grains closer together and by grain deformation and fracturing, increasing their mechanical stability. Mechanical compaction is a response to increased effective stresses during burial and follows the laws of soil mechanics. 
  3. Chemical diagenesis (compaction), which includes dissolution of minerals or amorphous material and precipitation of mineral cement. The clastic minerals in the primary mineral assemblage are not in equilibrium, and there is always a drive towards thermodynamically more stable mineral assemblages. Kinetics determine the reaction rates, which for silicate reactions are extremely slow so temperature plays an important role. 
  4. Precipitation of cement (quartz cement) will increase the strength of the grain framework and prevent further mechanical compaction. The sandstone is then over consolidated not due to previously higher stress, but due to cementation. Further compaction will then mostly be controlled by the rate of dissolution and precipitation.

Early Diagenesis

As soon as sediments are deposited, early diagenetic reactions start to modify the primary sediment composition. At very shallow burial depth (<1–10 m), sediments have the maximum potential to react with the atmosphere or water, both by fluid flow and diffusion. Transport of dissolved solids by diffusion and fluid flow (advection) is most efficient near the surface; in the case of diffusion within about 1 m of the seabed. The potential for sediments to change their bulk composition after deposition is therefore much higher at shallow depth than at greater burial. Near the surface on land, and also in the uppermost few centimetres of the seabed, the conditions may be oxidising, while at greater depth in the basin they are always reducing. Precipitation of minerals due to pore water concentration by evaporation can only occur on land or at shallow depth within enclosed basins. On land, sediments are exposed to air and fresh (meteoric) water. Weathering is partly due to reactions with oxygen in the atmosphere and partly due to dissolution of minerals in freshwater, which is initially under saturated with respect to all the minerals present. These are soil-forming processes which can be considered to be examples of early diagenesis. In desert environments groundwater and occasional rainwater may become concentrated through evaporation, causing precipitation of carbonates and also silicates. Coatings of red or yellow iron oxides and clays frequently form on desert sand and this may subsequently retard or prevent quartz cementation at greater depth. In the sea, the water above the seabed is normally oxidising. Only where there is poor water circulation (poor ventilation) is the lower part of the water column likely to be reducing, though the phenomenom is more widespread in lakes and inland seas like the Black Sea. However, even below well oxygenated water, oxidising conditions extend in most cases for only a few centimetres into the sediments, since oxygen is quickly consumed by the oxidation (decay) of organic matter in the sediment. This is for the most part facilitated biologically by bacteria. Accumulating sediments normally contain sufficient organic matter to serve as reducing agents in the pore water. This organic matter is comprised of both the remains of bottom fauna and of pelagic organisms, including algae, accumulating on the sea floor, and also often includes terrestrial plant debris transported into the basin.

Hydraulic fracturing or fracking

Hydraulic fracturing

Hydraulic fracturing also referred as hydrofracturing, hydrofracking, fracking or fraccing is a well stimulation technique in which rock is fractured by a hydraulically pressurized liquid made of water, sand and chemicals. Hydraulic fracturing is just like a dyke or sill formed in a rock where the hot magma intrudes and fractures the host rock. In hydraulic fracturing pressurized fluid is injected into the well bore hole to create cracks in the deep rock formations. The cracks or fractures in the formation of interest where oil and gas exists will more freely flow through the cracks. As the pressure is then removed from the well to extract the oil and gas, some of the hydraulic fracturing proppants is still in the fractures which don't allow the formation to close down and fractures remain open. 

Risks in hydraulic fracturing

As hydraulic fracturing is the technique which allows large number of oil and gas reserves extraction which are not accessible because of the low permeability of the formation bearing oil and gas. There are a lot of arguments regarding hydraulic fracturing. The environmental impacts of hydraulic fracturing includes the contamination of ground water, depleting fresh water, degrading air quality, potentially triggered earthquakes, noise pollution, surface pollution and the consequential hazard to public health and environment. Increase in seismic activity following hydraulic fracturing along dormant or previously unknown faults are sometimes caused by the deep injection disposal of hydraulic fracturing flow back and produced formation brine (a byproduct of both fractured and nonfractured oil and gas wells).

Advantages and disadvantages of fracking

Fracking has an advantage that it fractures the formation so that more and more oil and gas recovery can be done in a less time consuming allowing more permeability in the formation.
The disadvantage is also of the very much concerns as much damage can be to the planet. Air to water freshness lost and also triggering earth quakes.

Oklahoma earthquakes from the fracking

Oklahoma state had an average rate of earthquakes in the back year at 2008, the average rate of earthquakes of magnitude 3.0 was 2 per year. As hydraulic fracturing technique was in use for hydrocarbon recovery, increased to count of 109 in 2013. Prior to that last year it was recorded about 585 earthquakes.