LITHOLOGY OF NATURAL RESERVOIRS
Types of natural reservoirs and hydrocarbon traps
Oil and gas is one of the most important properties of sedimentary strata. They include certain lithologic-stratigraphic complexes, which differ in regional oil and gas content within a vast territory. The main factors determining the formation of regional oil and gas bearing complexes are :
1) the accumulation of organic matter and its enclosing sediments in a sub-aqual environment with an anaerobic geochemical environment against a background of relatively stable deflection of the sedimentation basin;
2) the absence of the possibility of penetration of this stratum into the zone of active water exchange and aeration in the subsequent phases of development of ascending movements;
3) the presence in the complex of rocks characterized by favorable reservoir properties;
4) the presence in the complex of a series of poorly permeable rocks - tires to ensure the safety of deposits.
Thus, regional oil and gas complex - is a natural system consisting of a set of rocks, the conditions of accumulation and further transformation of which are characterized by favorable geological, geochemical, hydrogeological, tectonic and other factors that caused the emergence, development and the completion of processes of regional oil and gas formation and oil and gas accumulation .
The regional oil and gas complex consists of three parts:
• oil and gas producing oil or gas generating layer;
• oil and gas bearing strata represented by reservoirs containing oil and gas accumulations;
• its overlapping, weakly permeable cover-layer, ensuring the preservation of hydrocarbon accumulations.
The last two components create natural reservoirs - natural receptacles for oil, gas and water, within which these fluids can circulate.
The shape of the natural reservoir is determined by the ratio of the reservoir to its poorly permeable rocks. This principle distinguishes reservoir, massive and lithologically limited natural reservoirs .
Reservoir is a collector that has a wide area distribution (hundreds and thousands of square kilometers), characterized by low power (from fractions to tens of meters) and limited in the roof and the bottom by poorly permeable rocks. Movement of fluids along the reservoir - lateral along the reservoir.
There are 3 options for the structure of the reservoir (Figure 85):
a) the reservoir is of uniform composition and well-maintained in terms of area and power;
b) reservoir, regionally sustained on large areas, wedges to the vaults of individual uplifts
c) the reservoir is represented by several layers interconnected, having an uneven roof and a sole.
The massive reservoir is a thick (a few hundred meters) thickness of permeable rocks, overlapped from above and bounded from the sides by poorly permeable rocks (Figure 86).
Fig. 85. Varieties of reservoir:
1 - collector: 2 - screening thickness
Fig. 86. Scheme of the structure of a massive reservoir. Species:
1 - reef building;
2 - organogenic-detrital limestones: 3 - clayey limestones
4 - dolomites:
5 - marls: 6 - argillites
A massive reservoir is usually confined to a structural, erosive or biogenic protrusion. In the thickness of reservoir layers there may be impermeable interlayers, but all the layers of permeable rocks are reported, representing a single hydrodynamically bound thickness. Collectors
can be represented by terrigenous, carbonate rocks. The movement of fluids in massive tanks is vertical and lateral.
A frequent case of a massive natural reservoir is a fossil reef, which is a reef building buried beneath a thick layer of young sediments.
Natural reservoirs, cytologically limited , are surrounded on almost all sides by impenetrable rocks. An example of such a natural reservoir is a lens of sandstones in clays (Figure 87).
In addition, they include all areas of increased porosity and permeability, which can occur in different rocks for various reasons (for example, zones of crushing, leaching, etc.). This type of reservoir is a closed isolated system with limited circulation of fluids.
Fig. 87. Lithologically bounded natural reservoir: a-scheme: 6-section of the Pokrovsky oil field (Volga-Ural NGP). Species: 1 sand; 2-non-saturated sandstones; 3-clay; 4 - limestones
The shape, size, porosity and permeability of reservoir rocks determine its capacity, t. c. energy reserve. In reservoir reservoirs, it is the largest, since fluids that create pressure, leak from a huge area.
A part of a natural reservoir in which an accumulation of oil or gas can form is called a trap. Schematically, the formation of the trap occurs as follows [13, 22].
The rocks of the natural reservoir are saturated with primary sedimentation or infiltration infiltration (atmospheric) waters. Oil and natural gas in relation to sedimentation water are later formations. Once in a free state in a natural reservoir filled with water, oil or gas tend to occupy the highest position in it. They move upward, pushing the water out, until they reach the roof of the reservoir.
Oil (or gas) from point A (or B) can move to point A, but can not move from point A to point A (or B). At point A, oil
(or gas) will be delayed (screened), that is, it will fill the trap (Figure 88).
Fig. 88. Scheme of possible displacements and screening of oil (or gas) in a natural reservoir : a - in the case of a lithologic screen: b - in an anticlinal curved formation
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