Petroleum is a mineral substance basically comprised of hydrocarbons and generated from the natural accumulations of organic matter of a faunal and/or floral provenance. Petroleum is a gaseous, liquid or semi-solid substance, present in the pore space of porous rocks, termed to as reservoir rocks that are mostly of sedimentary origin.
1) Clastic Sedimentary Rocks:
Sedimentary rocks yields from the deposition of sedimentary particles, termed as Clastic material or detritus (from the Latin word 'worn down'), comprising of mineral grains and rock fragments. Sedimentary particles are derived from the weathered and fragmented older rocks, igneous, metamorphic or sedimentary, generally by some chemical changes. Sediment including loose mineral detritus or debris is termed to as Clastic sediment (from the Greek term 'klastos' that means broken). Some Clastic sediment comprises of the accumulations of skeletal parts or shells of dead organisms, generally fragmented, and is termed to as bioclastic rocks. The particles of Clastic sediment might range broadly in size, and the predominant grain-size fraction is the main basis for categorizing Clastic sediments and Clastic sedimentary rocks. As illustrated in the table below, Clastic sediments can be categorized into four main classes: gravel, sand, silt and clay, where mud is the mixture of clay and silt, possibly including as well some very fine sand. The narrower the grain-size range of a given sediment, the better its 'sorting'. Both the grain size and sorting encompass direct implications for the sediment permeability to fluids.
Table: Definition of grain-size and terminology for sediments and sedimentary rocks
2) Non-Clastic Sedimentary Rocks:
A) Chemical Deposits:
Some of the sedimentary rocks have little or no Clastic particles. Such sediment, made by the precipitation of minerals from solution in water, is chemical sediment. It makes by means of either biochemical or purely chemical (that is, inorganic) reactions. The main porosity of such rocks is practically zero, and their possible porosity is completely dependent on the growth of secondary porosity, mainly in the form of micro-fractures.
B) Biogenic Deposits:
Sedimentary rocks generally have fossils, the remains of plants and animals which died and were buried and preserved in the sediment as it accumulated. Sediment comprised mostly or totally of fossil remains is termed as biogenic sediment. If the fossil debris has not been homogenized via chemical processes, the deposit can be regarded as the bioclastic sediment.
The major examples of non-Clastic rocks are limestone, dolomite, salt, gypsum, chert and coal. Chalk is a special kind of biogenic limestone, comprised of the sheletal parts of pleagic coccolithophorid algea, known as coccoliths. The main kinds of sedimentary rocks and their chemical compositions are illustrated in the list below, having main sedimentary rock kinds and their chemical composition of categories.
Sandstone is basically a siliciclastic rock made up of sand, generally quartzose or arhosic, cemented by silica, calcium carbonate, iron oxide or clay.
Shale is a fissile rock, generally having a laminated structure, made up by consolidation of clay or mud (mostly siliciclastic).
c) Argillite (mud rock):
Argillite is basically a compact sedimentary rock comprised mostly of siliciclastic mud.
Dolomite is a carbonate rock, comprising mostly of the mineral dolomite (that is, calcium magnesium carbonate)
Limestone is a carbonate rock comprising completely or mainly of the mineral calcite.
Calcarenite is sandstone comprised of carbonate grains, generally a Clastic variety of limestone.
Marl is a friable rock comprising of calcium carbonate and siliciclastic mud/clay.
h) Salt (rock salt):
It is a chemical rock comprised of the mineral halite.
Gypsum is a chemical and evaporitic rock comprised of the mineral gypsum.
Anhydrite is a chemical and evaporitic rock comprised of the mineral anhydrite.
The Origin and Habitat of Petroleum:
Source Rock and generation of Petroleum:
Local big concentrations of organic matter in sedimentary rocks, in the form of coal, oil or natural gas are termed as the fossil fuels.
A rock rich in the primary organic matter is termed as a source rock, as it is able of discharging huge amounts of hydrocarbons in natural burial conditions. Generally this is a shale or mud rock which itself is a very general rock kind, comprising around 80% of the world's sedimentary rock volume. The organic carbon-rich shale and mud rock are typically black or dark grayish in colour, which points out a non-oxidized primary organic matter.
Most of the hypotheses regarding the origin of petroleum have been advanced over the last years. Presently, the most favored one is that oil and gas are made up from marine phytoplankton (that is, microscopic floating plants) and to a lesser degree from algae and foraminifera. In the ocean, phytoplankton and bacteria is the principal of organic matter buried in the sediment. Most of the organic matter is trapped in clay mud which is slowly transformed into shale under burial. Throughout this conversion, the organic compounds are transformed (mostly via the geothermal heat) into petroleum, stated as gaseous, liquid or semisolid natural substances that comprise mostly of hydrocarbons.
In terrestrial sedimentary basins, it is plants like bushes, trees and grasses which contribute to most of the buried organic matter in mud rocks and shales. Such large plants are rich in waxes, resins and lignins, which tend to remain solid and make coal, instead of petroleum.
Most of the organic carbon-rich marine and lake shales never get to the burial temperature level at which the original organic molecules are transformed into hydrocarbons making oil and natural gas. Rather, the alteration procedure is limited to some wax-like substances having large molecules. This material, that remains solid, is known as Kerogen, and is the organic substance of so-called oil shales. Kerogen can be transformed into oil and gas via further burial through mining the shale and subjecting it to heat it in a retort.
Petroleum is produced whenever the Kerogen is subjected to an adequate high temperature in the procedure of the sediment burial. The modification of Kerogen to petroleum is identical to other thermal-cracking reactions that generally need temperatures more than 60oC. At lower temperatures, throughout the early diagenesis, natural biogenic methane known as marsh gas is produced via the action of microorganisms which live near the ground surface.
A temperature range between regarding 60oC and 175oC is most favorable for the generation of hydrocarbons, and is generally known as the oil window.
At temperatures much above 175oC, the generation of liquid petroleum ceases and the formation of gas becomes dominant. Whenever the formation rock temperature surpasses 225oC, most of the Kerogen will have lost its petroleum-producing capacity.
The long and complex chain of chemical reactions comprised in the conversion of raw organic matter into crude petroleum is known as maturation. Additional chemical changes might take place in the oil and gas even after these have been produced or accumulated. This describes, for illustration, why the petroleum taken from different oil fields has dissimilar properties, in spite of a common source rock. Similarly, primary differences in the source composition might be reflected in the chemistry of the petroleum.
Two kinds of proof support the hypothesis that petroleum is a product of the decomposition of the natural organic matter.
Oil source rocks are mainly marine shales and mud rocks. Sampling of mud on the continental shelves and all along the bases of continental slopes has illustrated that the shallowly buried mud contains up to 8% organic matter. Identical or even higher total organic-carbon (TOC) content characterizes numerous ancient marine shales. Geologists conclude thus that oil is originated mainly from the organic matter deposited in the marine sediments.
The fact is that most of the world's biggest hydrocarbon fields are found in the marine sedimentary rock successions representing ancient continental shelves. Though, some lake sediments might be just as oil-prone as marine source rocks. Most of the oil fields in various parts of the world are in ancient lacustrine deposits (made at the bottom or all along the shore of lakes, as geological strata).
Petroleum Migration and Accumulation:
The accumulation of petroleum takes place in only those areas, where geological conditions have given the unique combination of both the hydrocarbon prone source rocks and hydrocarbon traps.
Hydrocarbons are less dense as compare to water. Once discharged from the source rock, they therefore tend to migrate upwards in the direction of the minimum pressure, till they either escape at the ground surface, or an impervious barrier, termed as atrap.
In a trap, the oil and gas accumulate through displacing pore water from the porous rock. The top might be imperfectly sealed that signifies that gas and possibly as well some oil might 'leak' to yet higher lying traps or up to the ground surface. The part of the trap which includes hydrocarbons is known as a petroleum reservoir.
Water usually underlines the hydrocarbons in a trap. The water bearing part of the trap is known as an aquifer, and is hydraulically linked with the reservoir. This signifies that any pressure change in the aquifer will as well influence the reservoir, and the depletion of the reservoir will make the aquifer expand to this space.
Both oil and gas are generated altogether, in varying proportions, from the source rock that yields in a primary gas cap above the oil in the reservoir. Similarly, a secondary gas cap might develop whenever the reservoir pressure has decreased and the lightest hydrocarbon start to bubble out from the oil. Some 'leaky' or limited-capacity traps might segregate oil and gas which have been produced altogether, in such a way that these accumulate in separate reservoirs.
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