In the course of development there will be some surprises, and there will modify the earlier decisions.

The Nature of an oil field with a single large pool is quite different from one with many pools and from one with many pools in a faulted structure, and the economics also differ. A single large pool may be developed with relatively few wells spaced several kilometers apart if the field permeability is good, or a few wells if the oil column is thick and the field area relatively small, whereas the same volume of oil in several faulted reservoirs may require wells at close spacing for each reservoir in each fault block. Part of the development planning of such fields with multiple reservoirs involves siting wells for multiple completions on more than one reservoir (dual completions, even triple completions). It is in the nature of oil fields that those in transgressive sequences tend to be large single-pool accumulations and those in regressive sequences tend to be multiple-pool accumulations.

Text 9

gradient – ухил, схил; скат; що рівномірно підвищується (знижується)

saturate – насичувати (-ся)

saline – соляний; сіль

juxtaposition – співставлення

connate – вроджений

calcarenite – вапнисте утворення

marl – мергель б вапнякова глина

commensurately – відповідно

The reservoir, consists of porous and permeable rock – usually sedimentary rock, but some are of volcanic and other igneous rocks – of which the pores and fissures are filled with water and oil, water and gas, or (rarely) water, oil and gas. The fluids and solids are at elevated temperatures, relative to the surface, depending on the depth and the geothermal gradient. Geothermal gradients very widely, commonly between 25 and 40°C/km, so the temperature in a typical reservoir at a depth of 2000 m will be between 65°and 95°C. The fluids and the solids are also at elevated pressures, depending generally on the depth and the pressure gradients due to the weight of the overlying materials.

The accumulation is bounded above by a relatively fine – grained material such as a mudstone – or, more precisely, by a material with small pores. The criterion here is that the capillary injection pressure required for the oil or gas to penetrate the cap rock is greater than that existing in the reservoir fluid. The accumulation is usually bounded below by a material of small pores, and, within the reservoir rock unit, by the oil/water or gas/water contact below which the pores are entirely saturated with water that is usually saline. This interface between the petroleum and the water is horizontal or nearly horizontal. The accumulation is sometimes bounded laterally by a fault which may itself be a barrier to further migration on account of the fine – grained fault gouge in the fault plane, or which may bring fine – grained material with small pores into juxtaposition with the reservoir across the fault. Within the reservoir pore spaces, both water and petroleum exist. Reservoir engineers call this «connate» water.

Text 10

deficient – нестача, відсутність

catalyst - каталізатор

aliphatic – аліфатичний

naphthenic – нафтеновій

terrestrial – континентальний

alter – змінюватися

devoid – позбавлений, вільний

prevail – переважати, бути поширеним

tissue – павутина, сплетіння

bark – кора

spore – спора

Petroleum is a product of the diagenesis of fundamental organic compounds in organic matter that accumulated with fine-grained sediment in a low-energy environment deficient in oxygen.

The energy of petroleum in its source rock is greater than that it will have when it reaches the accumulation. Each path of migration from source to accumulation is a path of continuously decreasing energy during migration. The energy is derived largely from the compaction of the petroleum source rock – usually a mudstone, but some may be fine-grained carbonates.

Petroleum exists as a separate phase by the end of primary migration, from source to a permeable carrier bed. Capillary forces then retard migration, and prevent it when the energy of the migrating petroleum is insufficient.

Primary migration may be stratigraphically upwards or downwards, depending on the direction of decreasing energy. Both normally occur in a compacting mudstone that is intercalated between sandstones or other permeable units. The surface dividing upwards and downward migration within the mudstone is a perfect physical and chemical barrier to migration.

Secondary migration is lateral within porous and permeable rock units, generally towards the land of the time. Petroleum accumulates when it arrives in a position in which there is insufficient energy to move it further.