Marakushev_subbed.qxp 26/3/09 04:39 Page 37
Formation of Oil and Gas Fields
endogenic nature. In this respect, oils of the vanadium type are crust. The extension regime was replaced by compression, which
particularly productive. In such oils, the V content is as high as prevents the selective migration of hydrogen. This regime promoted
0.2–130 parts per million (ppm), which is two times higher than the concentration of hydrocarbons in fluids and the development of
the content of Ni (0.2–60ppm). Degassing of oil is accompanied alkaline magmatism. The alternation of regimes is reflected in the
by the formation of bitumens (asphaltite, asphalt and others), in development of the crust. The extension regime (regime I) fosters the
which the V and Ni contents increase up to 4,500 and 520ppm, formation of volcanosedimentary and sedimentary depressions,
respectively. The elevated content of these metals in oil is explained whereas the compression regime (regime II) provokes their various
by its enrichment with sulphur: V and Ni concentrations are dislocations (the explosion type included) that are responsible for
550–1,400 and 120–195ppm, respectively, in sulphur-bearing heavy reversed faulting in the crystalline basement of depressions.
oils and malthas. Sulphur-bearing oil is found in the unique Minas
Ragra vanadium deposit (Peru) in Cretaceous rocks. The deposit is The complete cycle of the formation of depressions is recorded in
represented by an asphaltite body (1km long and 8–12m thick) with trap formations by basaltic flows at their bases (regime I) and
a V content of approximately 6%. overlying sequences (regime II), which are usually composed of
carbonaceous sediments, and subalkaline–alkaline rocks at the top
V, Ni and Zn make up a specific (‘petroleum’) association in oils. of the cycle. Basaltic volcanism of normal alkalinity formed the lower
Based on the prevalence of these metals, oils are divided into parts of trap sections, resulting in the formation of depressions,
different geochemical (vanadium, nickel and zinc) types in oilfields simultaneous ascent of the mantle substrate and oceanisation of the
and provinces. The association of these metals in geodes, druses and continental crust. This process is reflected in the formation of the
veins is established in bitumens from diamond-bearing kimberlite secondary oceanic crust around the passive continental margins.
pipes in platforms. Particularly striking is the peak of vanadium,
which is an ‘odd’ element and should be less abundant than its The stage of basaltic eruptions with water–carbon dioxide fluids was
neighbouring ‘even’ elements (according to the Oddo-Harkins rule). followed by the prolonged cessation of volcanism and formation of
However, the V content in asphaltite is substantially higher sedimentary sequences. However, magmatic activity continued in
compared with other trace elements. This is explained by the magma chambers, and the alkalinity of magmas increased gradually
extreme chemical affinity of V with hydrocarbons and its efficient during this stage. The cessation of volcanism reflects the
concentration during all stages of hydrocarbon generation. development of the crustal compression regime (regime I → regime
II), with the corresponding subsidence of magma chambers and an
In geochemical and metallogenic terms, vanadium oil correlates with increase of fluidal pressure and temperature within them.
black shales, in which the average V content (205ppm) is almost two
times higher than in carbon-depleted sedimentary rocks (110ppm). The composition of magmas changed under the influence of the
Sometimes, the V content increases to abnormally high values (a few filtering silica-depleted fluids. They supplied alkaline metals, P, U, Th,
kilograms per ton) and impart metallogenic significance to the black REE and other components to magmas and governed the specifics of
shales. These anomalies are determined by the input of V from deep alkaline magmatism. The removal of silica from magmas by fluids was
zones and indicate a direct relation between the formation of black accompanied by the pyroxene alteration of rocks, thinning of the
shales and oils. This fact is emphasised by the so-called ‘oil shales’. subcrustal mantle substrate and corresponding thickening of the crust
However, in contrast to oil pools, which are located largely in deep and its uplift. Alkaline magmatism at the end of trap formation cycles
zones under significant pressures, oil shales occur at shallow depths was determined by the crustal compression regime, which prevented
and reflect the ascent of oil to near-surface beds of sedimentary the migration of hydrogen from ascending fluid flows. This process
basins. Moreover, low pressure promoted the selective migration of promoted the growth of pressure in magma chambers and restored
hydrogen from oil and the formation of heavy carbonaceous the prevalence of hydrogen over oxygenous components, which were
substances (including shungite) that is typical of black shales. responsible for the water–carbon dioxide affinity of fluids (3H
2
+ CO
= H
2
O + CH
4
). Similar trends are also observed in intrusive rocks of
The remarkable geochemical similarity between black shales and trap depressions that are confined to the sedimentary sequences
vanadium-type oils, which are enriched in many ore metals, can between the lower basaltic and upper alkaline volcanic rocks. Such a
traced through geological history: the most efficient V accumulation distinct tendency is related to the fact that igneous rocks are formed
(average 590ppm) is recorded in Cretaceous black shales. This in the crust due to the replacement of their crustal counterparts
phenomenon correlates with the maximum oil generation accompanied by the removal of a significant share of the excess
mentioned above: 71.2% of oil reserves in Russia are represented by (relative to the magmatic eutectics) components by fluids.
the Cretaceous variety.
6
Such a correlation reflects the relationship
between oil generation and the evolution of the Earth’s core. The In the case of mafic–ultramafic magmatism, the accompanying fluids
Cretaceous period was characterised by a significant decrease in are chemically aggressive to the sialic rocks (sandy–clayey units in trap
inversion of the magnetic field of the core (see Figure 1). This formations). Their leaching under the influence of such fluids
phenomenon is related to intense degassing of the core in the promotes the emplacement of intrusions. For example, the
course of its interaction with the mantle substrate. As mentioned sedimentary sequence (Zhdanov formation) between the lower
above, geodynamic extension of the mantle substrate, which tholeiitic basalt and upper alkali basalt flows in the Pre-Cambrian
stimulates the degassing of the core and selective migration of Pechenga ring trap formation (Kola Peninsula) is only 1.5km thick,
hydrogen from fluids, is accompanied by magmatism with low and although this sequence encloses more than 300 mafic–ultramafic
normal alkalinity associated with destruction of the continental intrusions, including bodies with sulphde Cu–Ni mineralisation.
EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 1
37
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