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Formation of Oil and Gas Fields
With time, the chambers generated by deep plumes at continental the subsequent oceanisation of continental crust and the formation
margins usually migrate towards continents. Autonomous sedimentary of passive continental margins, which retained oil-bearing basins
basins and alkaline intrusions related to this process are also successively and fragments of the island-arc framing of marginal seas.
displaced towards the interior parts of platforms, with a simultaneous
increase in magma alkalinity. Unlike the volcanosedimentary (trap) The scale of pools undoubtedly indicates the influx of hydrocarbons
depressions, which have a relatively rapid development regime, from deep zones, and their geological settings rule out the existence
sedimentary basins are formed slowly during long geological periods. of ‘producing’ sedimentary sequences beneath these pools. In fact,
For example, the sedimentary basin in the southern Caspian Sea has the vertical migration governed the localisation of oil and gas fields
been forming since the Riphean era, and its formation continues today. in sedimentary depressions.
8
In oceanic ridges, abyssal fluids are
This is evident from the conjugation of this basin with earthquakes that recorded as hydrocarbon-rich hydrothermal vents and liquid
correlate with sea-level fluctuations. Oil and gas fields are largely formed hydrocarbon seeps on the seafloor. Hydrocarbons accompany the
in such sedimentary depressions at underwater continental margins. formation of a sedimentary depression along the Juan de Fuca
Most productive among them are depressions related to rapid Ridge. Thus, they model the initial stage of black shale formation.
subsidence, where the shallow-water sediments are replaced upsection Judging from the presence of oil hydrocarbons (alkanes and alkyl
by the deepwater facies. Oceanic depressions host complex saliferous benzols) in hydrothermal vents of the ridge, oil can also be
horizons associated in a certain manner with gas and oil generation. generated within the ridge. However, the oil is likely confined to the
Metasomatic processes include sulphidisation, sulphatisation and native deep sedimentary sequence of the depression, where the fluid
sulphur formation during the intricate evolution of fluids of magma pressure prevents the loss of hydrogen. In this respect, oceanic-
chambers (regime I → regime II) that governs the concentration of margin ridges of the Juan de Fuca type occupy an intermediate
hydrocarbons and ores. The present-day massive sulphide mineralisation position between the classic mid-oceanic ridges, which are not
in oceanic ridges is always accompanied by the development of accompanied by depressions, and the shelf-continental margins of
hydrocarbon-rich hydrothermal microbial systems. oceans and seas, where sedimentary sequences serve as the main
reservoirs for oil pools.
Leaching of the granite–gneiss layer of the crust by the ascending
abyssal fluid flows, rather than its down-warping, played the main Localisation of oil and gas pools in sedimentary basins is traditionally
role in the formation of sedimentary and volcanosedimentary (trap) attributed to various structural and lithological traps. However, the
sequences in the platformal and shelf depressions.
7
This is evident significance of such concepts faded after the discovery of giant oil
from the so-called inversed relief of depressions. Their formation is pools, which pose the problem of space for their formation. This
accompanied by the counter-uplift of the mantle substrate due to issue is similar to the problem of space for intrusions, which was
the emplacement of ultramafic intrusions that replace the crustal solved by the discovery of magmatic replacement of intruded rocks
basaltic layer up to the point of its complete transformation, for and large-scale removal of crustal material by fluids. As discussed
example in the spacious basin (more than 20km deep) in the above, removal of crustal material also governs the formation of
southern Caspian Sea. The associated leaching of the granite layer depressions under the influence of fluids that leach the crustal
and formation of depressions is fostered by the percolation of granite layer and promote its replacement by sedimentary rocks.
aggressive transmagmatic fluids through the ultramafic magmas that Subsequently, processes of leaching also embrace the sedimentary
replace the crustal basaltic layer. This scenario of depression rocks of depressions and their basements. These processes correlate
evolution in the continental crust is a universal mechanism that to various dislocations. In oil-productive depressions, hydrocarbon
governs the formation of both regional depressions (e.g. marginal accumulation is preceded by the hydrothermal acidic leaching that
sea basins) and local volcanosedimentary and sedimentary basins in promotes the decompaction of rocks and provides the subsequent
their framing, such as the Sea of Okhotsk. The Okhotsk depression localisation of oil pools therein. Due to leaching, quartz grains
is related to thinning of the continental crust and simultaneous uplift acquire the spherical shape that gives an impression of their
of the mantle substrate accompanied by its compaction, which has roundness. Similar processes are responsible for the formation of
been traced down to a depth of 4–5km. Uplift of the mantle mud volcanoes. All of them control the localisation of oil and gas
substrate was accompanied by its expansion, which fostered the pools in sedimentary basins. The dome-shaped anticlinal structures
centrifugal spreading of stress and compression in peripheral areas are interesting in this respect. Their arches frequently host mud
of the sea. These processes created favourable settings for the volcanoes and, sometimes, oil pools at deep levels, complicated by
concentration of hydrocarbons in the peripheral local depressions the roots of these volcanoes in some places. The relationship
and conjugate magma chambers. The consequent block between mud volcanism and hydrocarbon generation is evident from
displacements and folding controlled the localisation of oil and gas the methane composition of the accompanying gases (39–96vol%).
pools in depressions. Thus, the involvement of mantle uplift in the The activity of mud volcanoes illustrates removal of the residual
general process of destruction and oceanisation of the continental undissolved sedimentary rocks by fluids and creation of space for oil
crust played a key role in the formation and realisation of the accumulations. The role of abyssal leaching is evident from the
hydrocarbon potential in marginal seas. This process was responsible localisation of oil pools in upthrown blocks of the basement in
for the centrifugal tectonics that prevented the loss of hydrogen by sedimentary depressions.
the abyssal magma chambers associated with peripheral depressions
and promoted the accumulation of hydrocarbons (3H
2
+ CO = H
2
O The generation of oil in the upthrown basement of depressions
+ CH
4
). The harmonic system of oil-bearing structures thus formed reaches large scales that are incomparable to the limited
along the periphery of marginal seas was successively destructed by accumulation of oil in sedimentary rocks. The White Tiger field on
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EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 1
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