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 petroleum. Show all posts
Showing posts with label petroleum. Show all posts

Oil drilling Rig

What is Rig?

Owais Khattak at
KCA deutag 72 Location Makori East 6 MOL well
A drilling rig is a machine intend to drill hole in the Earth crust. Drilling rigs are massive structures which are used to drill hole for water, oil or natural gas. For water the rigs can be small, moved easily by one person which are termed as auger. But for oil or natural gas motives it can be very huge structures as you can see in the picture where it seems small but is tall about 46 meters. Drilling rigs can be mounted on trucks usually used for water wells or shallow wells. Small to medium-sized drilling rigs are mobile, such as those used in mineral exploration drilling, blast-hole, water wells and environmental investigations. Larger rigs are capable of drilling through thousands of metres of the Earth's crust, using large "mud pumps" to circulate drilling mud (slurry) through the drill bit and up the casing annulus, for cooling and removing the "cuttings" while a well is drilled. Hoists in the rig can lift hundreds of tons of pipe. Other equipment can force acid or sand into reservoirs to facilitate extraction of the oil or natural gas; and in remote locations there can be permanent living accommodation and catering for crews (which may be more than a hundred). Marine rigs may operate thousands of miles distant from the supply base with infrequent crew rotation or cycle.
As you see the rig in the picture where I stands with it, its a huge structure which is intended to drill deep into the Earth crust. This rig is 2000 horse power and able to drill deep because it can lift huge weight of the drill pipes. 

Rig components

Rig is basically made up of five components without which a rig is incomplete which will be discussed below and the components are
  1. Power
  2. Circulation system
  3. Hoisting system
  4. Rotation 
  5. Blowout preventer (BOP)


A rig is always operated with some energy for the whole of the rig to operate which can be generated through some generators or by placing engines. The rig above was using five generators where three were operational for the rig every time because one cannot stop the rig it costs the operation and two were as backup engines. The power is necessary for a rig or its just a tall standing structure.

Circulation system

Circulation system in terms of rig is mud (slurry) which is made up of mud, water or oil which ever type of mud is required for the subsurface formation, and mixture of chemicals which includes gel, barite for increasing weight, caustic soda, defoamation etc. The mud is pumped from the mud tanks by mud pumps and travels through pipes into drill pipes which goes all the way down into the hole in pipes and gets out through holes in bit and returns to surface via annulus. Annulus is the inner diameter of the hole from which mud comes out to the surface bringing cuttings from the bore hole and creating mud cakes around the hole walls. The mud when comes out of the hole it goes to shakers where mud and well cuttings are separated. The mud also exerts hydrostatic pressure on the formation so that any fluid or gas from formation doesn't enters the bore hole.
Mud cake (1mm) produced in the lab with currently used mud which depicts the inner hole scenario.

Hoisting System

Hoisting system is done by the top drive system (TDS) which is held by strings and pulleys atop. The hoisting system is responsible for lifting the heavy weight of the drill pipes. If you cannot pull out or run in the drill pipes you cannot drill the hole. 


Rotation refers to the rotation of drill pipes which in turn rotates the drill bit deep down the hole and cuts the formation. As the drill bit rotates and cuts the formation, if cuttings are not removed from the deep down hole the drill bit can stuck. If not stuck the drill bit will further be crushing the cutting and not the formation this is where the mud works comes in. It lifts up the cutting so that it can drill further and also cools down the bit as friction heats it up and deep the Earth itself is hot.

Blowout preventer (BOP)

BOP is the equipment installed at the surface where drill strings goes in it. The blowout preventer as the name itself is explanatory is used to stop the blowout. The BOP is 1, 2 or 3 stages preventer which is composed or either annulus ram and shear rams, annulus ram, shear rams or upper pipe ram or annulus ram, shear rams, upper pipe ram and lower pipe ram. The BOP is used when the formation pressure exceeds the hydrostatic pressure or else the fluid from the formation will enters the well which is termed as kick. When the kick reaches the surface it will blowout everything within and the rig itself so in order to stop that BOP is installed so that it can stop the pressure from coming out the hole.

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 

Petroleum system

Petroleum System

In order to understand the petroleum generation and extraction petroleum system should be known
Petroleum system starts with the deposition to storage from where the production is obtained.
Petroleum system journey starts with the deposition of organic matter.

Deposition of organic matter

The deposition of organic matter starts when organism starts to die and deposits deep down the ocean floor and the above deposition of clay (finer grains). The clay particles are about 1/256 mm size and is called shale. Organic matter deposited on the ocean floor cannot be oxidized due to the depth factor so they can produce hydrocarbon. Hydrocarbon generation needs the cooking of organic matter at high temperature and pressure and it is obtained when it goes into overburden of deposition by clay particles and greater depths. 

Source rock

In the petroleum system the source rock are the shale (clay that goes under high pressure and temperature which cooks the organic matter). Sometimes limestone can also be the source rock with 1% of organic matter contains. So theses rocks undergo cooking where the temperature and pressure determines what type of fuel will be generated. Despite of temperature and pressure another factor in producing hydrocarbon is the time span required to generate fuel. The time is a critical factor as if the organic matter is cooked for less time it will not generate hydrocarbon and when it is greater than the oil produce will be converted into gas.

Reservoir rock

Reservoir as indicated by the name reserves of hydrocarbon. the hydrocarbon cannot be obtained from the source rock because of the higher pores but are lesser to none interconnection. For the extraction the pores should be interconnected so that it can travel when are extracted. But if there is no reservoir and obtaining fuel from source rock it must be fractured for permeability generation. Reservoir rock are mostly sandstone which have higher porosity and permeability but in some cases limestone also serves as reservoir rock. Limestone all by self is not a good reservoir due to fine particles present which give less permeability but as limestone is calcium carbonate so it can be dissolved in water which are the Karst topography. Only then can limestone have permeability required for hydrocarbon to be obtained.


Primary migration

Migration itself is cleared so the primary migration occurs when hydrocarbon moves from source rock to the reservoir rocks. Primary migration occurs when the source rock is fractured due to tectonic forces (plate movements) or by the overburden squeezing the source rock. As HC (hydrocarbon) have low density they moves upward. 

Secondary migration

Secondary migration is the HC movement within the reservoir rocks. The HC will moves upward in the reservoir rocks.

Seal rock

Trap or seal rocks are those that are present above reservoir rocks as HC movement will always be upward. Seal rock are those that have low to none permeability so that HC cannot escape but are trapped within the reservoir rocks. Shale can be seal rock also as they have porosity but do not have permeability factor so HC will be trapped. Types of traps include stratigraphic and structural.
Stratigraphic traps example is shale as a seal rock and structural traps are fold or faults. 

Time period

The last thing in the petroleum system is time as have said it above already, time is required for HC generation which is always critical. No more time and no less time while cooking of the organic matter or it will not produce the fuel.

Energy Choices, Energy Problems

Energy Choices, Energy Problems 

The Age of Oil and the Oil Crunch 

World energy use, cost and reserves.
Energy usage in industrialized countries grew with dizzying speed through the mid-20th century, and during this time people came to rely increasingly on oil. Eventually, oil supplies within the borders of industrialized countries could no longer match the demand, and these countries began to import more oil than they produced themselves. Through the 1960s, oil prices remained low (about $1.80 a barrel), so this was not a problem. In 1973, however, a complex tangle of politics and war led the Organization of Petroleum Exporting Countries (OPEC) to limit its oil exports. In the United States, fear of an oil shortage turned to panic, and motorists began lining up at gas stations, in many cases waiting for hours to fill their tanks. The price of oil rose to $18 a barrel, and newspaper headlines proclaimed, “Energy Crisis!” Governments in industrialized countries instituted new rules to encourage oil conservation. During the last two decades of the twentieth century, the oil market stabilized. Since 2004, oil prices rose overall, passing the $147/bbl mark in 2008; but the price collapsed in late 2008 when the Great  Recession hit. More recently, the price has hovered around $100/bbl (figure above a). Will a day come when shortages arise not because of politics or limitations on refining capacity, but because there is no more oil to produce? As highly populous countries such as China and India industrialize, the use of fuels accelerates. To understand the issues involved in predicting the future of energy supplies, we must first classify energy resources. As noted earlier, we call a particular resource renewable if nature can replace it within a short time relative to a human life span (in months or, at most, decades). A resource is non-renewable if nature takes a very long time (hundreds to perhaps millions of years) to replenish it. Oil is a non-renewable resource, in that the rate at which humans consume it far exceeds the rate at which nature replenishes it, so we will inevitably run out of oil. The question is, when?

Oil and Gas

Oil and Gas

What Are Oil and Gas? 

For reasons of economics and convenience, industrialized societies today rely primarily on oil (petroleum) and natural gas for their energy needs. Oil and natural gas, both fossil fuels, consist of hydrocarbons, chain-like or ring-like molecules made of carbon and hydrogen atoms. Chemists consider hydrocarbons to be a type of organic chemical.
Some hydrocarbons are gaseous and invisible, some resemble a watery liquid, some appear syrupy, and some are solid. The viscosity (ability to flow) and the volatility (ability to evaporate) of a hydrocarbon product depend on the size of its molecules. Hydrocarbon products composed of short chains of molecules tend to be less viscous (meaning they can flow more easily) and more volatile (meaning they evaporate more easily) than products composed of long chains, because the long chains tend to tangle up with each other. Thus, short-chain molecules occur in gaseous form (natural gas) at room temperature, moderate-length-chain molecules occur in liquid form (gasoline and oil), and long-chain molecules occur in solid form (tar).

Coal: Energy from the Swamps of the Past


Coal, a black, brittle, sedimentary rock that burns, consists of elemental carbon mixed with minor amounts of organic chemicals, quartz, and clay. Typically, the carbon atoms in coal have bonded to form complicated molecules. Note that coal and oil do not have the same composition or origin. In contrast to oil, coal forms from plant material (wood, stems, leaves) that once grew in coal swamps, regions that resembled the wetlands and rain forests of modern tropical to semitropical coastal areas. Like oil and gas, coal is a fossil fuel because it stores solar energy that reached Earth long ago. Significant coal deposits could not form until vascular land plants appeared in the late Silurian Period, about 420 million years ago. The most extensive deposits of coal in the world occur in Carboniferous-age strata (deposited between 286 and 354 million years ago). During the Carboniferous, the continents were assembled in Pangaea, and large areas straddled warm tropical regions where vegetation flourished. Also during this time, sea level was so high that large areas of continents were flooded by shallow seas along which vast swamps grew; the plant debris of these swamps, once buried and heated, turned into coal. Not all coal reserves, however, are Carboniferous during the Cretaceous (144 to 65 Ma), large areas of freshwater coal swamps developed in Wyoming and adjacent states. Coal commonly occurs in beds, or seams, that may be centimetres to meters thick and may be traceable over very large regions. Broad, continuous coal seams develop when sea level rises slowly relative to the land surface so the coastline, and therefore the coal swamp, migrates slowly inland. During such a transgression, the submerged portion of the coal swamp eventually gets buried by layers of other sediments, such as sand and mud. When lithified, the succession turns into beds of sedimentary rock, with the coal occurring as a sedimentary bed, inter-layered with sandstone and shale.

The Formation of Coal 

The formation of coal. Coal forms when plant debris becomes deeply buried.
How do the remains of plants transform into coal? The vegetation of an ancient swamp must fall and be buried in an oxygen-poor environment, such as stagnant water, so that it can be incorporated in a sedimentary sequence without first decaying by reacting with oxygen and/or by being eaten by microbes or larger organisms. Compaction and partial decay of the vegetation transforms it into peat. (Peat, which contains about 50% carbon, itself serves as a fuel in many parts of the world.) To transform peat into coal, the peat must be buried deeply (4–10 km) by overlying sediment. Such deep burial can happen where the surface of the continent gradually sinks, creating a depression, or sedimentary basin, that can collect sediment. Over time, many kilometers of sediment containing numerous peat layers accumulate. At depth in the pile, the weight of overlying sediment compacts the peat and squeezes out any remaining water. Then, because temperature increases with depth in the Earth, deeply buried peat gradually heats up. Heat accelerates chemical reactions that gradually destroy plant fiber and release elements such as hydrogen, nitrogen, and sulfur in the form of gas. These gases seep out of the reacting peat layer, leaving behind a residue concentrated with carbon. Once the proportion of carbon in the residue exceeds about 50%, the deposit formally becomes coal. With further burial and higher temperatures, chemical reactions allow additional hydrogen, nitrogen, and sulfur to escape, yielding progressively higher concentrations of carbon. 

The Classification of Coal 

Geologists classify coal according to the concentration of carbon. With increasing burial, peat transforms into a soft, dark-brown coal called lignite. At higher temperatures (about 100n–200nC), lignite, in turn, becomes dull, black bituminous coal. At still higher temperatures (about 200n–300nC), bituminous coal transforms into shiny, black anthracite coal (also called hard coal). The progressive transformation of peat to anthracite coal, which occurs as the coal layer is buried more deeply and becomes warmer, reflects the completeness of chemical reactions that remove water, hydrogen, nitrogen, and sulfur from the organic chemicals of the peat and leave behind carbon (Fig. 12.6c). Thus, lignite contains only about 50% carbon, bituminous about 70%, and anthracite about 90%. As the carbon content of coal increases, we say the coal rank increases. Notably, the formation of anthracite coal requires high temperatures that develop only on the borders of mountain belts, where mountain-building processes can push thick sheets of rock up along thrust faults and over the coal-bearing sediment, so the sediment ends up at depths of 8 to 10 km, where temperatures reach 300nC. Hot groundwater flowing through the rock may also provide enough heat to produce anthracite. 

Finding and Mining Coal 

The distribution of coal reserves. Vast quantities lie buried in continental sedimentary basins.
Because the vegetation that eventually becomes coal was initially deposited in a sequence of sediment, coal seams interlayered with other sedimentary rocks. To find coal, geologists search for sequences of strata that were deposited in tropical to semitropical, shallow-marine to terrestrial environments the environments in which a swamp could exist. The sedimentary strata of continents contain huge quantities of discovered coal, or coal reserves. The way in which companies mine coal depends on the depth of the coal seam. If the coal seam lies within about 100 m of the ground surface, strip mining proves to be the most economical method. In strip mines, miners use a giant shovel called a dragline to scrape off soil and layers of sedimentary rock above the coal seam. Draglines are so big that the shovel could swallow a two-car garage without a trace. Once the dragline has exposed the seam, miners use smaller power shovels to dig out the coal and dump it into trucks or onto a conveyor belt. Before modern environmental awareness took hold, strip mining left huge scars on the landscape. Without topsoil, the rubble and exposed rock of the mining operation remained barren of vegetation. In many contemporary mines, however, the dragline operator separates out and preserves soil. Then, when the coal has been scraped out, the operator fills the hole with the rock that had been stripped to expose the coal and covers the rock back up with the saved soil, on which grass or trees may eventually grow. In hilly areas, however, miners may use a practice called mountain top removal, during which they blast off the top of the mountain and dump the debris into adjacent valleys. This practice can disrupt the landscape permanently.

Deep coal can be obtained only by underground mining. To develop an underground mine, miners dig a shaft down to the depth of the coal seam and then create a maze of tunnels, using huge grinding machines that chew their way into the coal. Underground coal mining can be very dangerous, not only because the sedimentary rocks forming the roof of the mine are weak and can collapse but also because methane gas released by chemical reactions in coal can accumulate in the mine, leading to the danger of a small spark triggering a deadly mine explosion. Unless they breathe through filters, underground miners also risk contracting black lung disease from the inhalation of coal dust. 

Gas from Coal 

Coalbed Methane As we’ve noted, the natural process by which coal forms underground yields methane, a type of natural gas. Over time, some of the gas escapes to the atmosphere, but vast amounts remain within the coal in pores or bonded to coal molecules. Such coalbed methane, trapped in strata too deep to be reached by mining, is an energy resource that has become a target for exploration in many regions of the world. Obtaining coalbed methane from deep layers of strata involves drilling, rather than mining. Drillers penetrate a coalbed with a hole and then start pumping out groundwater. As a result of pumping out water, the pressure in the vicinity of the drill hole decreases relative to the surrounding bed. Methane bubbles into the hole and then up to the ground surface, where condensers compress it into tanks for storage. 

Coal Gasification Solid coal can be transformed into various gases, as well as solid by-products, before burning. The process of producing relatively clean-burning gases from solid coal is called coal gasification. Coal gasification involves the following steps. First, pulverized coal is placed in a large container. Then, a mixture of steam and oxygen passes through the coal at high pressure. As a result, the coal heats up to a high temperature but does not ignite; under these conditions, chemical reactions break down and oxidise the molecules in coal to produce flammable gases as well as water and CO2. Solid ash, as well as sulphur and mercury, concentrate at the bottom of the container and can be removed before the gases are burned. 

Underground Coalbed Fires 

Coal will burn not only in furnaces but also in surface and subsurface mines, as long as the fire has access to oxygen. Coal mining of the past two centuries has exposed much more coal to the air and has provided many more opportunities for fires to begin; once started, a coalbed fire that progresses underground (sucking in oxygen from joints in overlying rock) may be very difficult or impossible to extinguish. For the past 50 years, a coalbed fire has progressed under the town of Centralia, Pennsylvania, eventually burning coal seams beneath the town itself. The fire produces toxic fumes that rise through the ground and make the overlying landscape uninhabitable, and it also causes the land surface to collapse and sink. And today, over 100 major coalbed fires are burning in northern China. Recent estimates suggest that 200 million tons of coal burn underground in China every year, an amount equal to approximately 20% of the annual national production of coal in China.
Credits: Stephen Marshak (Essentials of Geology)

Unconventional reserves of Hydrocarbons

Unconventional reserves of Hydrocarbons

Oil that can be extracted from reserves in the porous and permeable reservoir rocks of oil traps. Such reserves have come to be known as conventional reserves, because accessing them uses technology that has been around for years. In the past 10 to 15 years, energy companies have begun to increase their focus on extracting hydrocarbons from unconventional reserves, meaning reserves that had previously been left in the ground or disposed of because they cannot be tapped without using new technologies. Let’s look at a few examples of these reserves. 

Natural Gas 

Natural gas consists of volatile, short-chain hydrocarbon molecules (methane, ethane, propane, and butane). Gas burns more cleanly than oil, in that combustion of gas produces only CO2 and water, while the burning of oil not only produces CO2 and water, but also complex organic pollutants. Thus, natural gas has become the preferred fuel for home cooking and heating, and in some localities, for electricity production. It can also be used to run cars and trucks, if the vehicles have been appropriately modified. Natural gas has not yet been used as widely as other hydrocarbons, because gas transportation, which requires high-pressure pipelines or special ships, is quite expensive. But its use is increasing rapidly. As we have seen, gas often occurs in association with oil. Unfortunately, at many oil wells, it is not economical to capture and transport the gas, so this gas vents from a pipe and is burned in a flare where it enters the air. (In localities where rock has been heated to temperatures higher than the oil window, reservoir rock may contain only gas and there may be enough to be worth pumping and capturing by conventional means.) Recently, the use of directional drilling and hydraulic fracturing has made it possible to extract large quantities of gas directly from source rocks. Large reserves of such shale gas underlie states in the northeastern United States, and are currently being drilled to provide energy for east-coast cities (Box 12.2). Intense exploration for shale gas reserves has begun worldwide.

Tar Sands (Oil Sands) 

So far, we’ve focused our discussion on hydrocarbon reserves that can be pumped from the subsurface in the form of a liquid or gas. But in several locations around the world, most notably Alberta (in western Canada) and Venezuela, vast reserves of very viscous, tar-like “heavy oil” exist. This heavy oil, known also as bitumen, has the consistency of gooey molasses, and thus cannot be pumped directly from the ground. It fills the pore spaces of sand or of poorly cemented sandstone, constituting up to 12% of the sediment or rock volume. Sand or sandstone containing such high concentrations of bitumen is known as tar sand or oil sand. Production of usable oil from tar sand is difficult and expensive, but not impossible. It takes about 2 tons of tar sand to produce one barrel of oil. Oil companies mine near-surface deposits in vast open-pit mines and then heat the tar sand in a furnace to extract the oil. Producers then crack the heavy oil molecules to produce smaller, more usable molecules. Trucks dump the drained sand back into the mine pit. To extract oil from deeper deposits of tar sand, oil companies drill wells and pump steam or solvents down into the sand to liquefy the oil enough so that it can be pumped out. 

Oil Shale 

Vast reserves of organic shale have not been subjected to temperatures of the oil window, or if they were, they did not stay within the oil window long enough to complete the transformation to oil. Such rock still contains a high proportion of kerogen. Shale that contains at least 15% to 30% kerogen is called oil shale. Lumps of oil shale can be burned directly and thus have been used as a fuel since ancient times. In general, however, energy companies produce liquid oil from oil shale. The process involves heating the oil shale to a temperature of 500nC; at this temperature, the shale decomposes and the kerogen transforms into liquid hydrocarbon and gas. As is the case with tar sand, production of oil from oil shale is possible, but very expensive.

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 Well Drilling Process with animation

Is There Oil Beneath My Property?

Is There Oil Beneath My Property? First Check the Geological Structure

Accumulations of crude oil and gas require a source rock, which is an organic-rich sedimentary rock that produces petroleum as the organisms decompose following burial. Petroleum migrates from the source rock until it encounters impermeable rock or the earth's surface, where it is lost in seeps. Porous and permeable reservoir rocks allow petroleum to accumulate below impermeable layers. Geologic structures provide traps to further concentrate the accumulating petroleum. Anticlines are the best traps and the first recognized by geologists searching for oil. These structures can form "oil pools" with stacked levels of water (lowest), oil and natural gas (highest) against the impermeable rock above the reservoir.

Other types of traps have been recognized including faults, unconformities, and facies changes. Modern petroleum exploration is sophisticated, but there is no way short of drilling to determine the presence of oil. "Wildcat" wells only have a 1 in 10 chance of discovery, which explains why future oil supplies will be more difficult and costly to find.........

Oil Trap

A set of conditions that hold petroleum in a reservoir rock and prevents its escape by migration.

Major types of Petroleum Traps.  In all cases, impermeable rock encloses or caps the petroleum.

Accumulation of organic matter

It is well documented that oil accumulations are of organic origin and formed from organic matter in sediments. Methane can be formed inorganically and is found in the atmosphere of several other planets, but inorganic methane from the interior of the earth is likely to be well dispersed and thus not form major gas accumulations in the earth’s crust. The organic matter from which petroleum is derived originated through photosynthesis, i.e. storage of solar energy. Sunlight is continuously transformed into such energy on Earth but only a very small proportion of the solar energy is preserved as organic matter and petroleum. The oil and gas which forms in sedimentary basins each year is thus minute in comparison with the rate of exploitation (production) and consumption. In practice petroleum must therefore be regarded as a non-renewable resource even though some petroleum is being formed all the time. Most of the organic materials which occur in source rocks for petroleum are algae, formed by photosynthesis.The zoo plankton and higher organisms that are also represented grazed the algae and were thus indirectly dependent on photosynthesis too. The energy which we release when burning petroleum is therefore stored solar energy. Since petroleum is derived from organic matter, it is important to understand how and where sediments with a high content of organic matter are deposited. The total production of organic material in the world’s oceans is now 5×1010 tonnes/year. Nutrients for this organic production are supplied by erosion of rocks on land and transported into the ocean. The supply of nutrients is therefore greatest in coastal areas, particularly where sediment-laden rivers discharge into the sea. Plant debris is also supplied directly from the land in coastal areas. Biological production is greatest in the uppermost 20–30 m of the ocean and most of the phytoplankton growth takes place in this zone. In clear water, sunlight penetrates much deeper than in turbid water, but in clear water there is usually little nutrient supply. At about 100–150 m depth, sunlight is too weak for photosynthesis even in very clear water. Phytoplankton provides nutrition for all other marine life in the oceans. Zooplankton feed on phytoplankton and therefore proliferate only where there is vigorous phytoplankton production. Organisms sink after they have died, and may decay so that nutrients are released and recycled at greater depths. Basins with restricted water circulation will preserve more organic matter and produce good source rocks which may mature to generate oil and gas. In polar regions, cold dense water sinks to great depths and flows along the bottom of the deep oceans towards lower latitudes. This is the thermal conveyor belt transporting heat to higher latitudes and it keeps the deep ocean water oxidizing. In areas near the equator where the prevailing winds are from the east the surface water is driven away from the western coast of the continents. This generates a strong up welling of nutrient-rich water from the bottom of the sea which sustains especially high levels of primary organic production. The best examples of this are the coast of Chile and off West Africa. Through photosynthesis, low energy carbon dioxide and water are transformed into high energy carbohydrates (e.g. glucose):

CO2 + H2O → CH2O (organic matter) + O2

The production of organic matter is not limited by carbon dioxide or water, but by nutrient availability. 
Phosphorus (P) and nitrogen (N) are the most important nutrients, though the supply of iron can also be limiting for alga production. It is this process of photosynthesis, which started 4 billion years ago, that has built up an atmosphere rich in oxygen while accumulating reduced carbon in sedimentary rocks as oil, gas and coal. Most of the carbon is nevertheless finely divided within sedimentary rocks, for example shales and limestones, in concentrations too low to generate significant oil and gas. Energy stored by photosynthesis can be used directly by organisms for respiration. This is the opposite process, breaking carbohydrates down into carbon dioxide and water again, so that the organisms gain energy. This occurs in organisms at night when there is no light to drive photosynthesis. Also when we burn hydrocarbons, e.g. while driving a car, energy is obtained by oxidation, again essentially reversing the photosynthesis equation quoted above. Oxidation of 100 g glucose releases 375 kcal of energy. Carbohydrates that are produced but not consumed by respiration can be stored as glucose, cellulose or starch in the cell walls. Photosynthesis is also the biochemical source for the synthesis of lipids and proteins. Proteins are large, complex molecules built up of condensed amino acids (e.g. glycine (H2NCH2−COOH)). 
Dried phytoplankton contains 45–55% carbon, 4.5–9% nitrogen, 0.6–3.3% phosphorus and up to 25% of both silica and carbonate. Planktonic algae are the main contributors to the organic matter which gives rise to petroleum. Among the most important are diatoms, which have amorphous silica (opal A) shells.
Diatoms are most abundant in the higher latitudes and are also found in brackish and fresh water. Blue - green algae (cyanobacteria) which live on the bottom in shallow areas, also contribute to the organic material in sediments. In coastal swamps, and particularly on deltas, we have extensive production of organic matter in the form of plants and trees which may avoid being oxidised by sinking into mud or bog. The residues of these higher land plants may form peat, which with deeper burial may be converted into lignite and bituminous coal. But such deposits are also a potential source of gas and oil. Plant matter, including wood, also floats down rivers and is deposited when it sinks to the bottom, usually in a near shore deltaic environment. When the trees rot they release CO2 and consume as much oxygen as the plant produced during the whole period when it was growing. There is thus no net contribution of oxygen to the atmosphere. This also applies to the bulk of the tropical rainforests. Where trees and plants sink into black mud, preventing them from being oxidised, there is a net contribution of oxygen to the atmosphere and a corresponding reduction of CO2 in the atmosphere. All animal plankton (zooplankton) live on plant plankton, and in turn are eaten by higher organisms in the food chain. At each step in the food chain, which we call a trophic level, the amount of organic matter (the biomass) is reduced to 10%. Ninety percent of the production of organic matter is therefore from algae. This is why algae and to some extent zooplankton account for the bulk of the organic material which can be transformed into oil. Larger animals such as dinosaurs are totally irrelevant as sources of oil. The most important of the zooplankton which provide organic matter for petroleum are:
  1. Radiolaria – silica shells, wide distribution, particularly in tropical waters. 
  2. Foraminifera – shells of calcium carbonate. 
  3. Pteropods – pelagic gastropods (snails) with a foot which has been converted into wing-shaped lobes; carbonate shells.
This is the second lowest level within the marine food chain. These zooplanktonic organisms are eaten by crustaceans which themselves are eaten by fish. The total amount of organic matter that can be produced in the ocean is dependent on the nutrient supply from rivers, but river water does not only carry inorganic nutrients. It also contains significant amounts of organic matter, in particular humic acid compounds, lignin and similar substances formed by the breakdown of plant material which are weakly soluble in cold water. When the river water enters the sea, there is precipitation due to the increased pH and lower surface temperature in the ocean. Other plant materials, like waxes and resins, are more chemically resistant to breakdown and are insoluble in water. Such organic particles tend to attach themselves to mineral grains and accompany sediment out into the ocean. Most of the oil reservoirs which have been formed since the Palaeozoic have been uplifted and eroded, and over time vast quantities of oil have flowed (seeped) out onto the land or into the sea. In this sense, oil pollution is a natural process. Only a small proportion of the petroleum that has been formed in source rocks has actually become trapped in a reservoir. One might expect this seepage to have provided a source of recycled petroleum in younger sediments, but petroleum breaks down extremely rapidly when subjected to weathering, oxidising to CO2, and the nutrients (P, N) that were required to form the organic matter are released and may act like a fertilizer. On land, evaporation will remove the lighter components while bacteria will degrade the heavier components. Fossil asphalt lakes consist of heavy substances which neither evaporate nor can be easily broken down by bacteria. In the ocean, the lighter components will dissolve quite rapidly, while the heavier asphalt fraction will sink to the bottom and be degraded and recycled. In uplifted sedimentary basins like the Ventura Basin and the Los Angeles Basin in Southern California there are abundant natural oils seeps both onshore and offshore. On the beaches from Santa Barbara towards Los Angeles there are many natural oil seeps.

Formation of coal, oil and gas

The branch of geology that has the greatest economic importance worldwide is the study of fossil fuels (coal, oil and natural gas): they form by diagenetic processes that alter material made up of the remains of organisms. The places where the original organic material forms can be understood by studying depositional processes, but the formation of coal from plant material and the migration of volatile hydrocarbons as oil and gas require an understanding of the diagenetic history of the sedimentary rocks where they are found.

Coal-forming environments

Vegetation on the land surface is usually broken down either by grazing animals or by microbial activity. Preservation of the plant material is only likely if the availability of oxygen is restricted, as this will slow down microbial decomposition and allow the formation of peat, which is material produced by the decay of land vegetation. In areas of standing or slowly flowing water conditions can become anaerobic if the oxygen dissolved in the water is used up as part of the decay process. These waterlogged areas of accumulation of organic material are called mires, and are the principal sites for the formation of thick layers of peat. Mires can be divided into two types: areas where most of the input of water is from rainfall are known as ombotrophic mires or bogs; places where there is a through-flow of groundwater are called rheotrophic mires or swamps. In addition there are also rheotrophic mires that have an input of clastic sediment, and these are referred to as marshes, or salt marshes if the water input is saline.

The significance of these different settings for peat formation is that these environmental factors have a strong influence on the quality and economic potential of a coal that might subsequently be formed. Bogs tend to have little clastic input, so the peat (and hence coal) is almost pure plant material: the peat can be many metres thick, but is usually of limited lateral extent. Swamp environments can be more extensive, but the through-flowing water may bring in clay, silt and sand particles that make the coal impure (it will have a high ash content). Also, if the water is saline, it will contain sulphates and these lead to the formation of sulphides (typically iron pyrite) in the coal and give the deposit a high sulphur content: this is not desirable because it results in sulphur dioxide emissions when the coal is burnt. The ash and sulphur content are the two factors that are considered when assessing the coal grade, as the lower they are, the higher the grade. A wet environment is required to form a mire and therefore a peat, so environments of their formation tend to be concentrated in the wetter climatic belts around the Equator and in temperate, higher latitudes. In warmer climates plant productivity is greater, but the microbial activity that breaks down tissue is also more efficient. Both plant growth and microbial breakdown processes are slower in cooler environments, but nevertheless the fastest rates of peat accumulation (over 2mm yr 1) are from tropical environments and are ten times the rate of peat accumulation in cooler climes. Coals that originate as peat deposits are known as humic coals, but not all coals have this origin. Sapropelic coals are deposits of aquatic algae that accumulate in the bottoms of lakes and although they are less common, they are significant because they can be source rocks for oil: humic coals do not yield oil, but can be the origins of natural gas.

Formation of coal from peat

The first stage of peat formation is the aerobic, biochemical breakdown of plant tissue at the surface that produces a brownish mass of material. This initially formed peat is used as a fuel in places, but has a low calorific value. The calorific value is increased as the peat is buried under hundreds of metres of other sediment and subjected to an increase in temperature and pressure. Temperature is in fact the more important factor, and as this increases with depth (the geothermal gradient) the peat goes through a series of changes. Volatile compounds such as carbon dioxide and methane are expelled, and the water content is also reduced as the peat goes through a series of geochemical changes. As oxygen, hydrogen and nitrogen are lost, the proportion of carbon present increases from 60% to over 90%, and hence the calorific value of the coal increases. Differences in the degree to which the original peat has been coalified are described in terms of coal rank. Transitional between peat and true coal is lignite or brown coal, which is exploited as an energy source in places. Going on through the series, low-rank coal is referred to as sub-bituminous coal, middle rank is bituminous and the highest rank coals are known as anthracite. In the process of these reactions, the original layer of peat is reduced considerably in thickness and a bed of bituminous coal may be only a tenth of the thickness of the original layer of peat.

Formation of oil and gas

Naturally occurring oil and gas are principally made up of hydrocarbons, compounds of carbon and hydrogen: petroleum is an alternative collective term for these materials. The hydrocarbon compounds originate from organic matter that has accumulated within sedimentary rocks and are transformed into petroleum by the processes of hydrocarbon maturation. This takes place in a series of stages dependent upon both temperature and time. The first stage is biochemical degradation of proteins and carbohydrates in organic matter by processes such as bacterial oxidation and fermentation. This eogenesis eliminates oxygen from kerogen, the solid part of the organic matter that is insoluble in organic solvents. 
Three main types of kerogen are recognised: 
  • Type I is derived from planktonic algae and amorphous organic material and is the most important in terms of generating oil.
  • Type II consists of mixed marine and continental organic material (algae, spores, cuticles) which forms gas and waxy oils.
  • Type III originates from terrestrial woody matter and is a source of gas only. 
Eogenesis occurs at temperatures of up to 408 C and at up to depths of just over 1000 m. At burial depths of between about 1000 and 4000 m and at temperatures of between 408 C and 1508 C, the phase of diagenesis known as catagenesis further transforms the kerogen. This stage of thermal maturation is also known as the ‘oil window’ because liquid petroleum forms from Type I kerogen under these conditions. With increasing temperature the proportion of gas generated increases. Generation of oil by organic maturation of kerogen is a process that requires millions of years, during which time the strata containing the organic matter must remain within the oil window of depth and temperature. At higher temperatures and burial depths only methane is produced from all kerogen types, a stage known as metagenesis. Formation of oil, which is made up of relatively long-chain hydrocarbons that are liquid at surface temperatures, from sedimentary organic matter requires a particular set of conditions. First, the organic matter must include the remains of planktonic algae that will form Type I kerogen: this material normally accumulates in anaerobic conditions in anoxic marine environments and in lakes. Second, the organic material must be buried in order that catagenesis can generate liquid hydrocarbons within the correct temperature window: if buried too far too quickly only methane gas will be formed. The third factor is time, because the kerogen source rock has to lie within the oil window for millions of years to generate significant quantities of petroleum. Gas consisting of short-chain hydrocarbons, principally methane, is formed from Type III kerogen and at higher maturation temperatures. Burial of coal also generates natural gas (principally methane) and no oil. The methane generated from coal may become stored in fractures in the coal seam as coal bed methane, which is a hazard in underground coal mining, but can also be exploited economically.

Oil and gas reservoirs

The hydrocarbons generated from kerogen are compounds that have a lower density than the formation water present in most sedimentary successions. They are also immiscible with water and droplets of oil or bubbles of gas tend to move upwards through the pile of sedimentary rocks due to their buoyancy. This hydrocarbon migration proceeds through any permeable rock until the oil or gas reaches an impermeable barrier.

Hydrocarbon traps

Oil and gas become trapped in the subsurface where there is a barrier formed by impermeable rocks, such as well-cemented lithologies, mudrock and evaporite beds. These impermeable lithologies are known as cap rocks. The hydrocarbons will find their way around the cap-rock barrier unless there is some form of hydrocarbon trap that prevents further upward migration. Structural traps are formed by folds, such as anticlines, especially if they are domeshaped in three dimensions, and by faults that seal a porous reservoir rock against an impermeable unit. Other traps are stratigraphic traps, formed beneath unconformities and in places where the reservoir rock pinches out laterally: porous rocks such as limestone reefs that pass laterally into finer grained deposits and where sand bodies are laterally limited and enclosed by mudrocks are examples of stratigraphic traps. The size and shape of the trap determines the volume of oil and/or gas that is contained by the structure, and hence is an important factor in assessing the economics of a potential oil field. In the absence of traps and caps the hydrocarbon reaches the surface and leaks to the atmosphere. Partial release of hydrocarbons from the subsurface as oil seeps and gas seeps can be important indicators of the presence of hydrocarbons.

Reservoir rocks

Almost all oil and gas accumulations occur underground within the pore spaces of beds of sedimentary rocks. In a few rare cases there are accumulations of hydrocarbons in subterranean caverns formed by dissolution of limestone, but the vast majority of reserves are known hosted between grains in sandstones or within the structures of limestones. For a sedimentary rock to be a suitable reservoir unit, it must be both porous and permeable. Porosity is presented as a percentage of the rock volume. Permeability is expressed in darcy units, with a value of 1 darcy representing a very good permeability for a hydrocarbon reservoir. Some of the best reservoir facies are beds of wellsorted sands deposited in sandy deserts and shallow seas, because these contain a high primary porosity. For similar reasons oolitic grainstones can be good reservoirs, and boundstones formed in reefs have a lot of void spaces within the original structure. There are examples of hydrocarbon reservoirs in deposits of many other environments, including rivers, deltas and submarine fans. Limestones may also have important secondary porosity due to dissolution and diagenetic changes. The reservoir quality of a rock is reduced by two main factors. First, the presence of mud reduces both porosity and permeability because clay minerals fill the spaces between grains and block the throats between them. Second, cementation reduces porosity and permeability by crystallising
minerals in the pore spaces, sometimes to the extent of reducing the porosity to zero.

Economic oil and gas accumulations

Exploration for economic reserves of hydrocarbons requires knowledge of the depositional history of an area to determine whether suitable source rocks are likely to have formed and if there are any suitable reservoir and cap lithologies in the overlying succession. This analysis of the sedimentology is an essential part of oil and gas exploration. Knowledge of postdepositional events is also important to provide an assessment of the thermal and burial history that controls the generation of hydrocarbons.