Viability assessment

If we suspect that hydrocarbons might be present in an area, we commission an exploration well to obtain further information. These are also called wildcat wells. Using various tools, we are able to analyze the formations in great detail to establish whether or not it will be worthwhile producing.

First of all, we need to establish where the hydrocarbons may have come from – the source rock – and analyze the organic content, known as kerogen. Then, we look at the reservoir formation – that is, the rock into which the oil or gas has migrated and where it accumulates in a geological trap of some kind.

The most important aspects of a reservoir formation are its porosity and permeability. Most rocks, in particular sandstones and conglomerates, have at least some porosity. If enough pores are present, the pores are large enough and the pores are interconnected so that fluids flow through them (i.e., the rock is permeable), then the rock is a potential petroleum reservoir. With sandstones, a permeability of 18% or more is usually needed for an economic oil reservoir. Gas flows easier than oil, so as little as 12% permeability may be enough for a gas reservoir. Porosity and permeability are important, but a petroleum reservoir needs to contain hydrocarbons as well. In water-bearing reservoirs, the pores are filled entirely with a salty solution called formation water, but in oil- and gas-bearing reservoirs, some oil or gas is present as well. A general rule of thumb is that 40% or more of the pore fluids must be hydrocarbons (i.e., the water saturation is less than 60%) in order for the reservoir to be economic enough to produce. If the water content is greater, then the oil tends to stay behind and the reservoir produces only water. These types of reservoirs are said to be ‘wet’. If the water saturation is less, then the reservoir may be ‘productive’. Whether or not it will be ‘economic’ to produce and make any money will depend not only on the rate of production, but also on how long the well will produce, in combination with many other factors, not least of which is market price.

Task 2. Answer the questions:

Задание 2. Ответьте на вопросы:

1. What are some of the basics of petroleum geology?

2. What two factors are mentioned in the text?

3. What is the difference between an economic and an uneconomic reservoir?

Task 3. Read this description of the three main types of sedimentary rock and complete the table below:

Задание 3. Прочитайте описание трех основных типов осадочных пород и заполните таблицу, данную ниже:

Sandstone is composed primarily of sand grains that have been naturally cemented together. It is rough to the touch and can be white to buff to dark in colour. Sandstones are commonly deposited on beaches, river channels or dunes. Sandstone is a common reservoir rock for gas and oil and is the most important reservoir rock in North America.

Shale is composed of clay-sized particles and is the most common sedimentary rock. It is relatively soft and the colour ranges from green and grey to black, depending on the organic content. Darker shales are common source rocks for oil and gas, commonly deposited on river flood plains, and on the bottom of oceans, lakes or lagoons. Some shales are now being treated as reservoirs in North America.

Limestone is composed of calcite mineral grains that range in size from very fine to large sparkling crystals. The rock is commonly white or grey in colour. Limestone is a common reservoir rock and is typical in desert areas, such as the oil and gas fields of the Middle East. An organic-rich, dark-coloured limestone can also be a source rock for oil and gas.

Many sedimentary rocks are a combination of these three types, and mixtures are described as either sandy, shaly or limey (calcerous). For example, sandy shale is a shale with a high sandstone composition. Coal is also increasingly being exploited as a gas-bearing reservoir, having originally been a source rock, as in the southern North Sea.

	Source, reservoir rock or both?	Typical colour(s)	Typical location(s)
Sandstone
Shale
Limestone

Task 4. Fill in the blanks in the sentences. In two of the sentences, the word is used twice – once singular and once plural. Translate the sentences into Russian:

Задание 4. Заполните пропуски словами, данными в рамочке. В двух предложениях одно из слов употребляется дважды, один раз в единственном числе, второй – во множественном. Переведите предложения на русский язык:

maturation, migration, reservoir, seal, source, trap

1 Evaluation of the _____ uses the methods of geochemistry to quantify the nature of organic-rich rocks which contain the precursors to hydrocarbons, such that the type and quality of expelled hydrocarbon can be assessed.

2 The _____ is a porous and permeable lithological unit or set of units that the hydrocarbon reserves. Analysis of _____ at the simplest level requires an assessment of their porosity (to calculate the volume of in situ hydrocarbons) and their permeability (to calculate how easily hydrocarbons will flow out of them).

3 The _____ , or cap rock, is a unit with low permeability that impedes the escapes of hydrocarbons from the reservoir rock. Common _____ include evaporites chalks and shales.

4 The _____ is the stratigraphic or structural feature that ensures the juxtaposition of reservoir and seal, such that hydrocarbons remain in the subsurface rather than escaping (due to their natural buoyancy) and being lost.

5 Analysis of _____ involves assessing the thermal history of the source rock in order to make predictions of the amount and timing of hydrocarbon generation and expulsion.

6 Finally, careful studies of _____ reveal information on how hydrocarbons move from source to reservoir and help quantify the source (or kitchen) of hydrocarbons in a particular area.

Task 5. Complete this table using the different forms of each word:

Задание 5. Заполните таблицу недостающими формами:

Verb	Adjective	Noun
		sediment
	porous
permeate
		migration
	saturated
accumulate
		fracturing
expel

Task 6. Read about geographical features where subsurface oil and gas reservoirs can often be found and decide whether operations in such environments would be onshore drilling or offshore drilling:

Задание 6. Прочитайте геологическую характеристику и решите, будет ли бурение наземным или морским:

а A basin is a large area with thick sedimentary rocks, covered by shallow tropical water. In some cases over time, the basin has been filled by sediment and become dry land.

b Reefs are mounds of shells separated from the land by a body of water known as a lagoon. Some ancient reefs are now landlocked, created when the sea evaporated.

с A coastal plain is a strip of land adjacent to the ocean

d Shorelines or beaches are long narrow deposits of well-sorted sand.

e Sand dunes are formed by wind in both desert and coastal environments.

f A trench is a deep channel in the seabed.

g A ridge is a submarine range of mountains, including volcanoes, which sometimes appear above the surface of the ocean.

h A delta is a mass of sediments deposited by a river flowing into a large body of water, such as a lake or ocean.

i A continental shelf is a shallow platform extending into the ocean from the beach.

j A continental slope is steeper than a shelf and lies between the shelf and the ocean bed.

k A submarine canyon is a narrow eroded channel through a shelf or slope.

Task 7. Retell the text «Viability assessment».

Задание 7. Перескажите текст «Viability assessment».

Практическое занятие 18

Petroleum Traps

Нефтяные ловушки

structural trap структурная ловушка

stratigraphic trap стратиграфическая ловушка

reservoir rock порода-коллектор

fault трещина

fold складка

syncline синклиналь

anticline антиклиналь, антиклинальная складка

impenetrable rock непроницаемая порода

angular unconformity угловое несогласие в напластовании

limestone известняк

shale сланец

truncation перерыв, усечение

pinch-out выклиниваться, сжимать

salt dome соляной купол

Task 1. Read and translate the text:

Задание 1. Прочитайте и переведите текст: