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.

Post depositional modification of sedimentary layers

Sediment is generally deposited as layers that may contain features such as cross-bedding, wave ripples or horizontal lamination formed during deposition: these are referred to as primary sedimentary structures. The original layering and these sedimentary structures may be subject to modification by fluid movement and gravitational effects if the sediment remains soft. Disruption of the sedimentary layers may occur within minutes of deposition or may happen at any time up to the point when the material becomes lithified. Soft-sediment deformation is the general term for changes to the fabric and layering of beds of recently deposited sediment. The deformation structures are mostly formed as a result of sediment instabilities caused by density contrasts and by movement of pore fluids through the sediment. When sediment is deposited in marine environments it is saturated with water, and many continental deposits are also saturated by groundwater. Burial by more sediment usually leads to gradual expulsion of the pore waters, except where the water gets trapped within a layer by an impermeable bed above. This trapped water becomes overpressured, and when a crack in the overlying layer allows fluid to be released it travels at high velocity upwards. Rapidly moving pore water causes fluidisation of the sediment, which is carried upwards with the moving water. Finer sediment can be more easily carried upwards, so the process of elutriation occurs, as fine sand is carried away by the fluid, leaving behind coarser, and more cohesive, material. Liquefaction is a shorter-term process that happens when a mass of saturated sediment is affected by a shock, such as an earthquake, and becomes momentarily liquid, behaving like a viscous fluid. There is usually only very localised movement of sediment and fluid during liquefaction. Soft-sediment deformation takes a variety of forms at various scales and can occur in any sediment deposited subaqueously that retains some water after deposition. They can be loosely grouped into structures due to sediment instabilities, liquefaction, fluidisation and loading, although these are not mutually exclusive categories.


Structures due to sediment instabilities



Slumps and slump scars
Slumps and slump scars form as a result of gravitational instabilities in sediment piles. When a mass of sediment is deposited on a slope it is often unstable even if the slope is only a matter of a degree or so. If subjected to a shock from an earthquake or sudden addition of more sediment failure may occur on surfaces within the sediment body and this leads to slumping of material. Slumped beds are deformed into layers that will typically show a fold structure with the noses of the anticlines oriented in the downslope direction. The surface left as the slumped material is removed is a slump scar, which is preserved when later sedimentation subsequently fills in the scar. Slump scars can be recognised in the stratigraphic record as spoon-shaped surfaces in three dimensions and they range from a few metres to hundreds of metres across. They are common in deltaic sequences but may also occur within any material deposited on a slope.




Growth faults
There is a continuum of process and scale between slump scars and growth faults, which are surfaces within sedimentary succession along which there is relative displacement. Growth faults are considered to be synsedimentary structures, that is, they form during the deposition of a package of strata. They are most commonly found in delta-front successions, where the depositional slope and the superposition of mouth-bar sands on top of delta-front and prodelta muds results in gravitational instabilities within the succession. Failure occurs on weak horizons and propagates upwards to form a spoon-shaped fault (a listric fault) within the sedimentary succession. Movement of the beds above the fault over the curved fault surface results in a characteristic rotation of the beds. Growth faults can be distinguished from post-depositional faulting because a single fault affects only part of the succession, with overlying beds unaffected by that fault.

Structures due to liquefaction



Convolute bedding and convolute lamination
The layering within sediments can be disrupted during or after deposition by localised and small-scale liquefaction of the material. The structures range from slight over-steepening of cross-strata, to the development of highly folded and contorted layers called convolute lamination and convolute bedding. These structures form where the sediment is either deposited on a slight slope or where there is a shear stress on the material due to flow of overlying fluid. The folds in the layering tend to be asymmetric, with the noses of the anticlines pointing downslope or in the direction of the flow. Convolute lamination is particularly common in turbidites, where it can be seen within the laminated and cross-laminated parts of the beds.


Overturned cross-stratification
Sands deposited by avalanching down the lee slope of subaqueous dunes are loosely packed and saturated with water. They are easily liquefied and can be deformed by the shear stress caused by a strong current over a set of cross-beds. Shearing of the upper part of the cross-beds creates a characteristic form called recumbent cross-bedding or overturned cross-stratification.

Structures due to fluidisation



Dish and pillar structures
Soft-sediment deformation structures formed by fluidisation processes are often called dewatering structures as they result from the expulsion of pore water from a bed. Dish structures are concave disruptions to the layering in sediments a few centimetres to tens of centimetres across formed by the upward movement of fluid. They are often picked out by fine clay laminae that are the cause of local barriers to fluid flow within the sediment. In plan view the dish structures form polygonal shapes. Pillar structures, also known as elutriation pipes, are vertical water-escape channels that can be simple tubes or have a vertical sheet-like form. Dish and pillar structures often occur together, although they can form separately.



Clastic dykes
Fluidisation of a large body of sediment in the subsurface can result in elutriation of sediment and the formation of vertical clastic dykes centimetres to tens of centimetres across. These sheet-like vertical bodies are typically made of fine sand and they cross-cut other beds. They form when a fracture occurs above an overpressured bed and the upward rush of pore waters carries sediment with it into the crack. The sand may show some layering parallel to the walls of the dyke but is otherwise structureless. A distinction must be drawn between clastic dykes, which are injected from below, and fissure fills formed by the passive infill from above of fissures and cracks in the underlying layers. Fissure fills form where cracks occur at the surface due to earthquake activity or where solution opens cracks in the process of karstic weathering. They can usually be distinguished from clastic dykes because they taper downwards, can be filled with any size of clast (breccia is common) and can show multiple phases of opening and filling where they are earthquake related. The term 'Neptunian dyke' has been used in the past for these fissure fills.



Sand volcanoes and extruded sheets
Liquified sediment brought to the surface in isolated pipes emerges to form small sand volcanoes a few tens of centimetres to metres across. These eruptions of sand on the surface can be preserved only if lowenergy conditions prevent the sand being reworked by currents. Sand brought to the surface through clastic dykes can also spread out on the surface, usually as an extruded sheet of sandy sediment. These sheets can be difficult to recognise if the connection with an underlying dyke cannot be established. Intrusions forming 'sills' of sand can form, but can also be difficult to identify.

Structures related to loading



Load casts

If a body of material of relatively low density is overlain by a mass of higher density, the result is an unstable situation. If both layers are relatively wet, the lower density mass will be under pressure and will try to move upwards by exploiting weaknesses in the overlying unit, forcing it to deform. Load casts form where the higher density sand has partially sunk into the underlying mud to form downward-facing, bulbous structures: the mud may also become forced up into the overlying sand bed to form a flame structure. As sand is forced downwards and the mud upwards, load balls of sand may become completely isolated within the muddy bed. These load-cast features are sometimes referred to as 'ball-and-pillow structures'. They are common at the bases of sandy turbidite beds and other situations where sand is deposited directly on wet muds.



Diapirism

In cases where the instability due to density differences between layers of unconsolidated sediment results in movements of material on a large scale, the process is known as diapirism. This process can occur in a range of rock and sediment types in a variety of geological settings, but it is most commonly observed where the density contrast is large and the low-density material is relatively mobile. The bulk density of a layer of rock or sediment is determined by two factors: 
  • The density of the minerals and
  • The proportion of the material that is occupied by pore spaces filled with gas or liquid. 
Two types of diapirism are commonly seen in sedimentary successions, salt diapirism and mud diapirism, and they have two important implications for sedimentology and stratigraphy: first, diapiric structures can create local highs on the sea floor that may become the locus for carbonate development and second, diapirism can create subsurface structures that can be traps for hydrocarbons. Halite (NaCl) has a mineral density of 2.17g cm 3, which is considerably lower than most sandstones and limestones, even if they are moderately porous. Halite is solid, but in common with all geological materials it will behave in a plastic manner and deform if put under sufficient heat and/or pressure. The pressure required to cause halite to behave plastically can be generated by only a few hundred metres thickness of overlying strata (overburden) and, due to its lower density, the halite mass will start to move up in areas where the overburden is thinner or weakened by faults. The diapiric movement of salt deforms the overlying strata, a phenomenon that is known as 'salt tectonics'. The effects range from creating swells in the layer of salt, to creating domelike bodies that intrude into the overlying strata, to places where the salt mass breaks through to the surface. In very arid regions the extruded salt may form a mass of halite in the landscape like a very viscous volcanic flow.
The second main form of diapirism occurs where a layer of sediment has a high porosity and its density is reduced due to the presence of a high proportion of water mixed with the sediment. This tends to occur where muddy sediment is deposited rapidly. Mud freshly deposited on the sea floor has about 75% of its mass composed of water. As more sediment is deposited on top, the water is gradually squeezed out, but clay-rich deposits, although they may be porous, have a low permeability because the platelike clay minerals inhibit the passage of fluids through the material. Therefore water tends to become trapped within muddy layers if there is insufficient time for the water to escape. This creates a layer of water-rich, low-density material that may be overlain by denser sediment. This situation most commonly occurs in deltas where fine-grained prodelta facies are overlain by sands of the delta front and delta top as the delta progrades. Mud diapirism (also sometimes called shale diapirism) is therefore a common feature of muddy deltaic successions.

Interpreting past depositional environments

Sediments accumulate in a wide range of settings that can be defined in terms of their geomorphology, such as rivers, lakes, coasts, shallow seas, and so on. The physical, chemical and biological processes that shape and characterise those environments are well known through studies of physical geography and ecology. Those same processes determine the character of the sediment deposited in these settings. A fundamental part of sedimentology is the interpretation of sedimentary rocks in terms of the transport and depositional processes and then determining the environment in which they were deposited. In doing so a sedimentologist attempts to establish the conditions on the surface of the Earth at different times in different places and hence build up a picture of the history of the surface of the planet.



The concept of facies
The term facies is widely used in geology, particularly in the study of sedimentology in which sedimentary facies refers to the sum of the characteristics of a sedimentary unit. These characteristics include the dimensions, sedimentary structures, grain sizes and types, colour and biogenic content of the sedimentary rock. An example would be crossbedded medium sandstone: this would be a rock consisting mainly of sand grains of medium grade, exhibiting cross-bedding as the primary sedimentary structure. Not all aspects of the rock are necessarily indicated in the facies name and in other instances it may be important to emphasise different characteristics. In other situations the facies name for a very similar rock might be red, micaceous sandstone if the colour and grain types were considered to be more important than the grain size and sedimentary structures. The full range of the characteristics of a rock would be given in the facies description that would form part of any study of sedimentary rocks. If the description is confined to the physical and chemical characteristics of a rock this is referred to as the lithofacies. In cases where the observations concentrate on the fauna and flora present, this is termed a biofacies description, and a study that focuses on the trace fossils in the rock would be a description of the ichnofacies. As an example a single rock unit may be described in terms of its lithofacies as a grey bioclastic packstone, as having a biofacies of echinoid and crinoids and with a Cruziana ichnofacies: the sum of these and other characteristics would constitute the sedimentary facies.

Facies analysis
The facies concept is not just a convenient means of describing rocks and grouping sedimentary rocks seen in the field, it also forms the basis for facies analysis, a rigorous, scientific approach to the interpretation of strata. The lithofacies characteristics are determined by the physical and chemical processes of transport and deposition of the sediments and the biofacies and ichnofacies provide information about the palaeoecology during and after deposition. By interpreting the sediment in terms of the physical, chemical and ecological conditions at the time of deposition it becomes possible to reconstruct palaeoenvironments, i.e. environments of the past. The reconstruction of past sedimentary environments through facies analysis can sometimes be a very simple exercise, but on other occasions it may require a complex consideration of many factors before a tentative deduction can be made. It is a straight forward process where the rock has characteristics that are unique to a particular environment. Hermatypic corals have only ever grown in shallow, clear and fairly warm seawater: the presence of these fossil corals in life position in a sedimentary rock may therefore be used to indicate that the sediments were deposited in shallow, clear, warm, seawater. The analysis is more complicated if the sediments are the products of processes that can occur in a range of settings. For example, crossbedded sandstone can form during deposition in deserts, in rivers, deltas, lakes, beaches and shallow seas: a cross-bedded sandstone lithofacies would therefore not provide us with an indicator of a specific environment. Interpretation of facies should be objective and based only on the recognition of the processes that formed the beds. So, from the presence of symmetrical ripple structures in a fine sandstone it can be deduced that the bed was formed under shallow water with wind over the surface of the water creating waves that stirred the sand to form symmetrical wave ripples. The shallow water interpretation is made because wave ripples do not form in deep water but the presence of ripples alone does not indicate whether the water was in a lake, lagoon or shallow-marine shelf environment. The facies should therefore be referred to as symmetrically rippled sandstone or perhaps wave rippled sandstone, but not lacustrine sandstone because further information is required before that interpretation can be made.

Facies associations
The characteristics of an environment are determined by the combination of processes which occur there. A lagoon, for example, is an area of low energy, shallow water with periodic influxes of sand from the sea, and is a specific ecological niche where only certain organisms live due to enhanced or reduced salinity. The facies produced by these processes will be muds deposited from standing water, sands with wave ripples formed by wind over shallow water and a biofacies of restricted fauna. These different facies form a facies association that reflects the depositional environment. When a succession of beds are analysed in this way, it is usually evident that there are patterns in the distribution of facies. For example, beds of the bioturbated mudstone occur more commonly with (above or below) the laminated siltstone or the wave rippled medium sandstone? Which of these three occurs with the coal facies? When attempting to establish associations of facies it is useful to bear in mind the processes of formation of each. Of the four examples of facies just mentioned the bioturbated mudstone and the wave rippled medium sandstone both probably represent deposition in a subaqueous, possibly marine, environment whereas medium sandstone with rootlets and coal would both have formed in a subaerial setting. Two facies associations may therefore be established if, as would be expected, the pair of subaqueously deposited facies tend to occur together, as do the pair of subaerially formed facies. The procedure of facies analysis therefore can be thought of as a two-stage process. First, there is the recognition of facies that can be interpreted in terms of processes. Second, the facies are grouped into facies associations that reflect combinations of processes and therefore environments of deposition. The temporal and spatial relationships between depositional facies as observed in the present day and recorded in sedimentary rocks were recognised by Walther. Walther’s Law can be simply summarised as stating that if one facies is found superimposed on another without a break in a stratigraphic succession those two facies would have been deposited adjacent to each other at any one time. This means that sandstone beds formed in a desert by aeolian dunes might be expected to be found over or under layers of evaporates deposited in an ephemeral desert lake because these deposits may be found adjacent to each other in a desert environment. However, it would be surprising to find sandstones formed in a desert setting overlain by mudstones deposited in deep seas: if such is found, it would indicate that there was a break in the stratigraphic succession, i.e. an unconformity representing a period of time when erosion occurred and/or sea level changed.

Facies sequences/successions
A facies sequence or facies succession is a facies association in which the facies occur in a particular order. They occur when there is a repetition of a series of processes as a response to regular changes in conditions. If, for example, a bioclastic wackestone facies is always overlain by a bioclastic packstone facies, which is in turn always overlain by a bioclastic grainstone, these three facies may be considered to be a facies sequence. Such a pattern may result from repeated shallowing-up due to deposition on shoals of bioclastic sands and muds in a shallow marine environment. Recognition of patterns of facies can be on the basis of visual inspection of graphic sedimentary logs or by using a statistical approach to determining the order in which facies occur in a succession, such as a Markov analysis. This technique requires a transition grid to be set up with all the facies along both the horizontal and vertical axis of a table: each time a transition occurs from one facies to another (e.g. from bioclastic wackestone to bioclastic packstone facies) in a vertical succession this is entered on to the grid. Facies sequences/sucessions show up as higher than average transitions from one facies to another.

Facies names and facies codes
Once facies have been defined then they are given a name. There are no rules for naming facies, but it makessense touse namesthatare more-or-lessdescriptive, such as bioturbated mudstone, trough crossbedded sandstone or foraminiferal wackestone. This is preferable to Facies A, Facies B, Facies C, and so on, because these letters provide no clue as to the nature of the facies. A compromise has to be reached between having a name that adequately describes the facies but which is not too cumbersome. A general rule would be to provide sufficient adjectives to distinguish the facies from each other but no more. For example, mudstone facies is perfectly adequate if only one mudrock facies is recognised in the succession. On the other hand, the distinction between trough crossbedded coarse sandstone facies and planar crossbedded medium sandstone facies may be important in the analysis of successions of shallow marine sandstone. Facies schemes are therefore variable, with definitions and names depending on the circumstances demanded by the rocks being examined. The names for facies should normally be purely descriptive but it is quite acceptable to refer to facies associations in terms of the interpreted environment of deposition. An association of facies such as symmetrically rippled fine sandstone, black laminated mudstone and grey graded siltstone may have been interpreted as having been deposited in a lake on the basis of the facies characteristics, and perhaps some biofacies information indicating that the fauna are freshwater. This association of facies may therefore be referred to as a lacustrine facies association and be distinguished from other continental facies associations deposited in river channels (fluvial channel facies association) and as overbank deposits (floodplain facies association). It can be convenient to have shortened versions of the facies names, for example for annotating sedimentary logs. Miall suggested a scheme of letter codes for fluvial sediments that can be adapted for any type of deposit. In this scheme the first letter indicates the grain size (S for sand, G for gravel, for example), and one or two suffix letters to reflect other features such as sedimentary structures: Sxl is cross laminated sandstone, for example. There are no rules for the code letters used, and there are many variants on this theme (some workers use the letter ‘Z’ for silts, for example) including similar schemes for carbonate rocks based on the Dunham classification. As a general guideline it is best to develop a system that is consistent, with all sandstone facies starting with the letter ‘S’ for example, and which uses abbreviations that can be readily interpreted. There is an additional graphical scheme for displaying facies on sedimentary logs: columns alongside the log are used for each facies to indicate their vertical extent. An advantage of this form of presentation is that if the order of the columns is chosen carefully, for example with more shallow marine to the left and deeper marine on the right for shelf environments, trends through time can be identified on the logs.

Marine and land seismic aquisition


Marine aquisition
Marine seismic acquisition is generally accomplished using large ships with one or multiple air-gun arrays for sources. Air-guns are deployed behind the seismic vessel and generate a seismic signal by forcing highly pressurised air into the water at a given interval. Receivers are towed behind the ship in one or several long streamer(s) that are several kilometres long. Marine receivers are composed of piezoelectric hydrophones, which respond to changes in water pressure. In marine acquisition, seismic vessels sail along predetermined patterns of parallel circuits. The length of straight segments is calculated from fold plots, and must include additional length run in and run out to allow the cable to straighten after each turn. Marine seismic acquisition is faster than conventional land seismic because it does not require jug hustlers to lay and pick up geophones. The advance to 3D seismic acquisition and imaging of the subsurface, introduced in the 1980s, was perhaps the most important step in seismic exploration. The 3D seismic images began to resolve the detailed subsurface structural and stratigraphic conditions that were missing or not discernable from 2D seismic data. With 3D seismic acquisition potential reservoirs are imaged in three dimensions, which allows seismic interpreters to view the data in cross-sections along 360◦ of azimuth, in depth slices parallel to the ground surface, and along planes that cut arbitrarily through the data volume. Information such as faulting and fracturing, bedding plane direction, the presence of pore fluids, complex geological structure, and detailed stratigraphy are now commonly interpreted from 3D seismic data sets. In 2D marine data acquisition a single streamer is deployed, whereas in 3D acquisition multiple streamers are towed behind the boat. 3D acquired data can be processed in a more consistent manner and be further manipulated using modern visualisation tools. In 2D data acquisition the data collection occurs along a line of receivers. The resultant image represents only a section below the line. Unfortunately, this method does not always produce a clear subsurface image. 2D data can often be distorted with diffractions and events produced from offline geologic structures, making accurate interpretation difficult. Because seismic waves travel along expanding wavefronts they have a surface area. A truly representative image of the subsurface is only obtained when the entire wavefield is sampled. A 3D seismic survey where the vessel is towing many streamers (up to 20) at the same time with multiple arrays of airguns is more capable of accurately imaging reflected waves because it utilises multiple points of observations. In the case of 3D survey we have seen that seismic data are sampled from a range of different angles (azimuth) and source-receiver distances (offsets). After seismic processing the data can then be represented as 3D volume images of the subsurface. In 2D processing, traces are collected into CMP gathers, while in 3D, traces are collected into common-cell gathers (binning). To perform 3D binning, a grid is first superimposed on the survey area. This grid consists of cells with dimensions of half the receiver group spacing in the in line direction, equivalent to the CMP spacing in 2D processing, and the line spacing in the cross line direction. In reality, midpoint distributions within a cell are not necessarily uniform since cable shape varies from shot to shot and line to line. Such side drift of the cables is called feathering. Another advanced technique of marine seismic is ocean bottom seismic (OBS) acquisition. It gives the possibility of direct measurement of S-wave data in addition to P-wave data by using ocean bottom cables that have three component geophones (3C) and a hydrophone in addition (thus 4C in total). The 4C cable can be up to 6 km long with 240 stations (i.e. 960 channels since 4C). A typical OBS layout involves 4 or more cables. The optimal choice of acquisition geometry for a 4C survey hinges on both geophysical and financial considerations. Most designs can be classified as either patch or swath. In swath designs, the source lines are parallel to receiver lines, while in patch designs, source lines are perpendicular to receiver lines. Patch design produces seismic data of a relatively wide range of azimuths, whereas the swath design produces data of a limited or narrow range. Moreover, the swath design offers a more uniform sampling of offsets with better near-offset coverage. The use of 4C OBS recording has several advantages over conventional towed streamer technology, which includes:
  1. Dual-sensor summation (3C geophone + hydrophone) for the suppression of receiver-side multiples. 
  2. Utilising P–S wave conversions for enhanced imaging. 
  3. Attenuation of free surface multiples when combined with towed streamer recording.
A comparison of the migrated P–S stack versus the P–P stack. The P–S stack is produced from OBS converted wave data whereas the P–P stack is produced from 3D towed streamer P-wave data. From this comparison it is clear that OBS data can be used to successfully image through a gas chimney.


Land aquisition
A complication in land acquisition is that, unlike marine data, a seismic line is rarely shot in a straight line because of the presence of natural and man made obstructions such as lakes, buildings and roads. The shot points and the receivers may be arranged in many ways. Many groups of geophones are commonly used on a line with shot points at the end or in the middle of the receiver array. The shot points are gradually moved along a line of geophones. The variations in ground elevation in land acquisition causes sound waves to reach the recording geophones with different travel time. The Earth’s nearsurface layer may also vary greatly in composition, from soft alluvial sediments to hard rocks. This means that the velocity of sound waves transmitted through this surface layer may be highly variable. Static corrections a bulk time shift applied to a seismic trace are typically used in seismic processing to compensate for these differences in elevations of sources and receivers and near-surface velocity variations.

Sources and receivers in seismic aquisition

It is essential at this point to understand the principle of seismology. Seismology is based on the transmission of sound waves by the rocks of the crust. Strong earthquakes create pressure waves (natural sources of seismic waves) that are transmitted through the entire Earth and detected by seismographs (receivers) on the other side. Seismic exploration, however, as employed by the petroleum geologist, makes use of artificially generated pulses (seismic sources). The principle is simple; an impulse source sends acoustic energy into the Earth. This energy propagates in many directions and is reflected and refracted when it encounters boundaries between two layers. Sensors (seismic receivers) placed on the surface measure the reflected or refracted acoustic energy. These artificial sources are much weaker than the natural seismic source (earthquake) but are more focused towards areas of specific stratigraphic interest.


Seismic Sources
Different seismic sources are usually used in land and marine acquisitions. In marine environments seismic energy is normally generated using arrays of air-guns, whereas in land seismic one often uses explosives or vibrators. An air-gun is a device that releases highly compressed air (at typically 2,000–5,000 psi) into the water surrounding the gun. A vibrator is an adjustable mechanical source that delivers vibratory seismic energy into the ground. A vibrator source sends a controlled-frequency sweep into the ground. The recorded data are then convolved with the original sweep to produce a usable signal. Dynamite a combination of explosive and detonator, is used as a seismic source. The detonator helps to ignite the explosives. When dynamite ignites, a shock wave propagates with a speed of 3,000–10,000 m/s. It provides an impulsive energy that can be converted into ground motion. It is customary to drill a hole to load dynamite and fill it with heavy mud before shooting. Dynamite can generate usable signal strengths and a bandwidth that covers a wide spectrum of seismic energy. It includes a variety of energy sources based on varying explosive out put parameters to meet geological and climatic conditions.



Seismic Receivers

Hydrophones and geophones serve as receivers for seismic signals. The hydrophone is a device designed for use in detecting seismic energy in the form of pressure changes in water during marine seismic acquisition. It measures pressure variations with the aid of piezoelectric material, which generates a voltage upon deformation. The two piezoelectric elements in one hydrophone are connected and polarised so that voltages due to pressure waves (returning signal) add and voltages due to one-directional acceleration will cancel. In this way the influence of movements due to currents, wave action and so on will be minimised. Hydrophones are combined to form streamers that are towed by seismic vessels or deployed in a borehole. A typical length of a streamer is about 4-6 km where a single receiver section is typically 75 m long and contains 96 hydrophones which are grouped in arrays of a pre-defined length, mostly 12.5 or 25 m. The geophone is a device used in surface seismic acquisition, both onshore and on the seabed offshore, that detects ground velocity produced by seismic waves and transforms the motion into electrical impulses. Geophones, unlike hydrophones, detect motion rather than pressure. Conventional seismic surveys on land use one geophone or a group of geophones per receiver location to detect motion in the vertical direction. The three-component (3C) geophone is used for direct measurements of shear waves at the seafloor. An essential feature of a seafloor seismic acquisition is the four-component (4C) detector unit, which includes a hydrophone and a three-component (3C) geophone. The hydrophone and the vertical geophone measure pressure waves, whereas the two extra horizontal geophones measure particle velocity associated with shear wave energy.


Seismic aquisition
The seismic survey is an essential part of the whole cycle of petroleum exploration and production. Seismic surveys are carried out on land and in transition zone, shallow marine and marine environments in different ways. The basic principle is an impulse source such as dynamite, air-gun or vibrator that sends acoustic energy into the Earth. This energy propagates in many directions. Downward travelling energy reflects and refracts when it encounters boundaries between two layers with different acoustic properties. Sensors or geophones placed on the surface measure the reflected acoustic energy, converting it into an electrical signal that is displayed as a seismic trace. The typical recorded seismic frequencies are in the range of 5–100 Hz. P-waves are the waves generally studied in conventional seismic data. P-waves incident on an interface at other than normal incidence angle can produce reflected and transmitted S-waves. S-waves travel through the Earth at about half the speed of P-waves and respond differently to fluid-filled rocks, and so can provide different additional information about lithology and fluid content of hydrocarbon-bearing reservoirs. The recorded seismic trace is a convolution (∗) of the source signal and the reflectivity sequence of the Earth plus noise. A seismic trace can simply be expressed by the equation below where multiples are not considered. Transmission losses and geometric spreading are not included and the frequency dependent absorptions are also ignored in the equation.

S = W ∗ R + Noises 

where S is the recorded seismic trace, R is the reflectivity and W is the wavelet. A wavelet is a kind of mathematical function used to divide a given function into different frequency components and study each component with a resolution that matches its scale. Accurate wavelet estimation is absolutely critical to the success of any seismic inversion. The inferred shape of the seismic wavelet may strongly influence the seismic inversion results and therefore subsequent assessments of the reservoir quality. Attenuation (amplitude loss) of seismic waves is an important phenomenon and caused by three major factors: 
  • Geometric spreading: progressive diminution of amplitude (proportional to the inverse of propagation distance) caused by the increase in wavefront area. 
  • Intrinsic attenuation: energy losses due to internal friction. 
  • Transmission losses: reduction in wave amplitude due to reflection at interfaces.

Coal sampling techniques for different seams

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

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

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

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


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


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




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

  1. Record card number. 
  2. Map or aerial photograph number on which locality is located. 
  3. Location of sample point, grid reference or reference number. 
  4. Description of the locality, stream section, working face, etc., including dip, strike, coal seam roof and floor contacts. 
  5. Extent of weathering, fracturing, mineralization, etc. 
  6. Lithological description of each ply interval. 
  7. Thickness of each ply interval. 
  8. Designated sample number of each ply interval.
Space canal so be allocated on there cord card for analytical details, that is proximate analysis, to be added later to complete the record. The fresh surface is then sampled as a channel cut from top to bottom, cutting and collecting all material from each ply section in turn. Each ply sample should be sealed in a strong plastic bag immediately after collection to prevent moisture loss and oxidation. All sample bags must be clearly labelled with a designated number, a copy of which should be placed in a small plastic bag inside the sample bag, and another attached to the outside of the sample bag. This number must be recorded on the channel sample record sheet. Because this task is invariably a dirty one, labels get wet, blackened and unreadable very easily, so it is essential therefore that care must be taken to ensure that the sample numbers do not get lost or obliterated during transit to the laboratory, as unidentifiable samples are useless and an expensive waste of time. The advantage of channel ply sampling is that not only can the analysis of the individual plies be obtained, but also by combining a fraction of each ply sample, a whole seam composite analysis can be made. An example of a channel ply sample from a surface exposure is illustrated in Figure 5.3, which shows a channel cut to expose fresh coal, and then a thinner channel (0.25m wide) cut from the fresh coal from which ply samples are collected for analysis.

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

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

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

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

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

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

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

1. Systematic sampling, where increments are spaced evenly in time or in position over the unit. 
2. Random sampling, where increments are spaced at random but a prerequisite number are taken. 
3. Stratified random sampling, where the unit is divided by time or quantity into a number of equal strata and one or more increments are taken at random from each. 

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

Transformaton of Kerogen into Oil with burial and temperature increase


With increasing temperature the chemical bonds in these large molecules (kerogen) are broken and kerogen is transformed into smaller molecules which make up oil and gas. This requires that the temperature must be 80–150◦C over long geological time (typically 1–100 million years). The conversion of kerogen to oil and gas is thus a process which requires both higher temperatures than one finds at the surface of the earth and a long period of geological time. Only when temperatures of about 80–90◦C are reached, i.e. at 2–3 km depth, does the conversion of organic plant and animal matter to hydrocarbons very slowly begin to take place. About 100–150◦C is the ideal temperature range for this conversion of kerogen to oil, which is called maturation. This corresponds to a depth of 3–4 km with a normal geothermal gradient (about 30–40◦C/km). 
In volcanic regions organic matter may mature at much lesser depths due to high geothermal gradients (high heat-flow areas). In large intracratonic sedimentary basins or along passive margins, however, the geothermal gradient may be only 20–25◦C/km and the minimum overburden required to initiate petroleum generation will be correspondingly greater (4–6 km). In general one can say that petroleum can not be generated near the surface except locally through the influence of hydrothermal and igneous activity. Shallow deposits of oil and gas which we find today were actually formed at great depths and either the overburden has been removed by erosion or the hydrocarbons have migrated upwards considerable distances. Large amounts of natural gas, chiefly methane (CH4), may be formed near the surface by biochemical processes. 
Temperature increases with increasing overburden, causing the carbon-carbon bonds of the organic molecules in the kerogen to rupture. This results in smaller hydrocarbon molecules. When kerogen maturation reactions are completed, the kerogen's "organic" components, which may be derived from lipids, fatty acids and proteins, have been converted into hydrocarbons. As the temperature rises, more and more of the bonds are broken, both in the kerogen and in the hydrocarbon molecules which have already been formed. This "cracking" leads to the formation of lighter hydrocarbons from the long hydrocarbon chains and from the kerogen. The removal of gas, mainly CH4, leaves the residual kerogen relatively enriched in carbon. At the outset kerogen (Type I and II) has an H/C ratio of 1.3–1.7. Humic kerogen (TypeIII), which has high initial oxygen contents, gives off mostly CO2 gas and so its oxygen/carbon ratio gradually diminishes. This diagenetic alteration begins at 70–80◦C and as water and CH4 are removed the H/C ratio will fall to about 0.6 and the O/C ratio will become less than 0.1 at about 150–180◦C. In the North Sea basin, most of the oil is generated at temperatures around 130–140◦C, which equates with a depth of about 3.5 km. If temperatures higher than 170–180◦C persist for a few million years, all the longer hydrocarbon chains will already have been broken (cracked), leaving us only with gas mainly methane (dry gas). The kerogen composition will gradually be depleted in hydrogen and move towards pure carbon (graphite) (H/C→0).

What Factors Influence the Maturation of Kerogen?

The term "maturity" refers here to the degree of thermal transformation of kerogen into hydrocarbons and ultimately into gas and graphite. The conversion of kerogen into hydrocarbons is a chemical process which takes place with activation energies of around 50–60 kcal/mol. This energy is required to break chemical bonds in the kerogen which consists of very large molecules (polymers) so that smaller hydrocarbon molecules can be formed. It has been assumed that formation of oil is a first order reaction, the rate of which is an exponential function of time. Understanding the factors which influence the rate of this reaction is of great interest. Four factors are thought to contribute:
  1. Temperature 
  2. Pressure 
  3. Time 
  4. Minerals or other substances which increase the rate of reaction (catalysts) or which inhibit reactions (inhibitors).
Temperature is clearly the most important factor, and hydrocarbons can be produced experimentally from kerogen by heating it (pyrolysis). This reaction is time-dependent and in laboratory experiments, where time is more limited than it is in nature, fairly high temperatures (350–550◦C) have to be used in pyrolysis. Pressure appears to play a minor role but increasing pressure should reduce the rate of the reaction because of the increase in volume involved in the formation of hydrocarbons. There is a relatively small volume increase when kerogen becomes oil, even though oil is lighter than kerogen. This is due to the residual (coke) which remains unaltered. When kerogen is converted directly into gas, or from oil which has been formed first, there is a marked volume increase. This should lead to slower reaction rates under high pressure in a closed system and retard generation of gas. Generation of petroleum, particularly gas, may contribute to the formation of overpressure but in a sedimentary basin the pressure will for the most part be controlled by the flow of water which is the dominant fluid phase. In the source rock however overpressure is likely to develop, causing hydrofracturing which helps to expel the generated petroleum. 
The main cause for overpressuring is, however, not only the increase in fluid volume but the transformation of solid into fluids. When solid kerogen is transformed into fluid oil or gas the ratio between the solid phase and the fluid phase is changed, as expressed by the porosity and the void ratio. Temperature is however the most important factor controlling petroleum generation. It has long been suspected that minerals, particularly clay minerals, might affect the rate of hydrocarbon generation. A number of laboratory experiments have been carried out in which kerogen is mixed with various minerals but the results have not been conclusive. 
The conversion of organic matter begins at 70– 80◦C, given long geological time. Between 60 and 90◦C the transformation of kerogen proceeds very slowly, and it is only in ancient, organic rich sediments that significant amounts are formed. Most of the maturation process occurs between 100 and 150◦C. Here the degree of kerogen transformation is also a function of time. This means that rocks which have been subjected to 100◦C for 50 million years are more mature than rocks which have been exposed to this temperature for 10 million years. As the organic-rich sediment (source rock) is buried in a sedimentary basin, it will normally be subjected to increasing temperature as a function of increasing burial depth. If we know the stratigraphy of the overlying sediment sequence and the geothermal gradient and the subsidence curve, we can calculate the temperature as a function of time. 
At low degrees of maturity we find more of the alkenes (olefins) and cykloalkenes (naphtenes), which have high H/C ratios, while with greater maturity there is an increase in the proportion of aromates and polyaromates (low H/C ratio). Oil thus acquires increasing gas content with increasing maturity. During this transformation of organic matter, water and oxygen-rich compounds are liberated first, then compounds which are rich in hydrogen. This conversion results in enrichment of carbon and the colour of the residual kerogen changes from light yellow to orange, brown and finally black. These gradations can best be registered by measuring light absorption of fossil pollen and spores (palynomorphs). It is also possible to analyse colour changes in other kinds of fossils, for example conodonts. For application in exploration a rapid semi quantitative method has been developed where by these colour changes are estimated from smooth spores examined under transmitted light and compared with a standard colour scale. This parameter is called the Thermal alteration index (TAI) and will give a rough idea of the thermal maturity of the sediments and their temperature history. 
Another way of assessing paleo-temperatures at which alteration took place in sedimentary rocks is to record the degree of carbonisation of other plant remains which are usually present. Vitrinite, which originally was fragments of woody tissue, is a common component of coal but is also found in smaller amounts in marine source rocks. This material is analysed by measuring the amount of light it reflects. It becomes shinier and reflects light better as the degree of carbonisation increases. By measuring the reflectivity of vitrinite particles under a reflected light microscope an exact value is obtained for this maturity parameter, expressed by the reflectivity coefficient R0 (% vitrinite reflectance). If R0 is less than 0.5% in a shale it can not have generated much oil and is classed as immature. Shales with R0 = 0.9–1.0 have been exposed to temperatures corresponding to maximum oil generation. R0 =1.3 represents the upper limit for oil generation, above which the shale will only produce condensate (light oils) or gas. 
For certain source rocks the ratio between extractable alkenes (paraffins) with an even number of carbon atoms per molecule and those with an odd number may also be an expression of maturity. In plant material and in marine algae, one finds a higher abundance of alkenes with an odd number of carbon atoms than in transformed organic matter like waxes and fatty acids. The decrease in this predominance of odd over even in source rocks with increasing maturity is due to the dilution of the original biologically derived n-alkane mixture with a newly generated mixture which has a regular carbon number distribution. This odd/even ratio is normally expressed by means of an index called the Carbon Preference Index (CPI):

CPI = n-alkenes (odd)/ n-alkenes (even) 

It is based on analyses of alkenes with carbon number between 25 and 33 (C25 and C33) in oils. However, organisms also start off with different CPI index values, and land plants have high ratios between odd and even carbon numbers. Bacteria have a predominance of even carbon numbers. The maturation process will cause a shift in the carbon number distribution towards smaller molecules, particularly in the range C13–C18. Oil that comes from carbonate source rocks often has a low CPI index, while oil derived from plants has a high index value. With increasing temperature the CPI index goes towards 1, that is to say, equal even and odd carbon numbers. The isotopic ratio also changes because the bonding between hydrogen and 13C, and between hydrogen and 12C, are not equally stable. Light gases such as methane, are enriched in the light isotope 12C, and the hydrocarbons that remain will therefore have an increasing 13C/12C ratio with increasing temperature. There is isotopic fractionation of carbon, and when kerosene is releasing petroleum, this phase is somewhat enriched in 11C corresponding to the precursor kerosene. Gases, particularly methane, normally have lighter carbon isotopes than kerosene and oil. When methane is formed from larger hydrocarbon molecules by thermal cracking, the 12C–12C bond is less stable than the 13C–12C bond and the product becomes enriched in 12C (low δ 13C).