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2012-03-14 / Columns / Oil and Gas Report

Oil & Gas Report

Crude Oil occurs mostly updip in Eagle Ford Shale
Leon Zabava
Staff Writer

Crude oil is one of the components found in the Eagle Ford Shale. We had previously talked about natural gas and then, condensate. Wells drilled in the more northerly, updip, portion of the Eagle Ford Shale, at shallower depths, will produce mostly crude oil.

Petroleum or crude oil is a fossil fuel that is formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rock and undergo intense heat and pressure. It is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the earth’s surface.

Drilling for crude oil comes after the studies of structural geology, sedimentary basin analysis, reservoir characterization (mainly in terms of porosity) and permeable structures. It is refined and separated, most easily by boiling point, into a large number of consumer products, from petrol to kerosene to asphalt and chemical reagents used to make plastics and pharmaceuticals. Petroleum is used in manufacturing a wide variety of materials and is estimated that the world consumes about 88 million barrels each day.

In the 19th century, the term petroleum was frequently used to refer to mineral oils produced by distillation from mined organic solids such as cannel coal (and later shale oil), and refined oils produced from them; in the United Kingdom, storage (and later transport) of these oils were regulated by a series of Petroleum Acts, from the Petroleum Act, 1862.

In its strictest sense, petroleum includes only crude oil, but in common usage it includes all liquid, gaseous, and solid hydrocarbons. Under surface pressure and temperature conditions, lighter hydrocarbons such as methane, ethane, propane and butane occur as gases, while pentane and heavier ones are in the form of liquids or solids. However, in an underground oil reservoir the proportions of gas, liquid and solid depend on subsurface conditions and on the phase diagram of the petroleum mixure.

An oil well produces predominantly crude oil, with some natural gas dissolved in it. Because the pressure is lower at the surface than underground, some of the gas will come out of solution and be recovered (or burned) as associated gas or solution gas. A gas well produces predominantly natural gas. However, because the underground temperature and pressure are higher than at the surface, the gas may contain heavier hydrocarbons such as pentane, hexane, and heptane in the gaseous state. At surface conditions these will condense out of the gas to form natural gas condensate, often shortened to condensate. Condensate resembles petrol in appearance and is similar in composition to some volatile light crude oils.

The proportion of light hydrocarbons in the petroleum mixture varies greatly among different fields, ranging from as much as 97% by weight in the lighter oils to as little as 50% in the heavier oils and bitumens.

Wikipedia is a good source for information.



The hydrocarbons in crude oil are mostly alkanes, cycloalkanes and various aromatic hydrocarbons while the other organic compounds contain nitrogen, oxygen and sulfur, and trace amounts of metals such as iron, nickel, copper and vanadium. The exact molecular composition varies widely from formation to formation but the proportion of chemical elements vary over fairly narrow limits.

Four different types of hydrocarbon molecules appear in crude oil. The relative percentage of each varies from oil to oil, determining the properties of each oil.

Crude oil varies greatly in appearance depending on its composition. It is usually black or dark brown (although it may be yellowish, reddish or even greenish). In the reservoir it is usually found in association with natural gas, which being lighter forms a gas cap over the petroleum, and saline water which, being heavier than most of crude oil, generally sinks beneath it. Crude oil may also be found in semisolid form mixed with sand and water, as in the Athabasca oil sands in Canada, where it is usually referred to as crude bitumen. In Canada, bitumen is considered a sticky, black, tar-like form of crude oil which is so thick and heavy that it must be heated or diluted before it will flow. Venezuela also has large amounts of oil in the Orinoco oil sands, although the hydrocarbons trapped in them are more fluid than in Canada and are usually called extra heavy oil. These oil sands resources are called unconventional oil to distinguish them from oil which can be extracted using traditional oil well methods. Between them, Canada and Venezuela contain an estimated 3.6 trillion barrels of bitumen and extra heavy oil, about twice the volume of the world’s reserves of conventional oil.

Petroleum is used mostly, by volume, for producing fuel oil and petrol, both important “primary energy” sources. By volume, 84% of the hyrdocarbons present in petroleum are converted into energy-rich fuels (petroleumbased fuels), including petrol, diesel, jet, heating, and other fuel oils, and liquefied petroleum gas.

Due to its high energy density, easy transportability and relative abundance, oil has become the world’s most important source of energy since the mid-1950s.

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