Page 32 - Drilling Technology in Nontechnical Language

Chapter 2 – OIL AND GAS RESERVOIR FORMATION 23

temperatures generating lighter (and more valuable) hydrocarbons. If the
temperature of the rock becomes too high (above 260°C), the organic
material, and therefore the oil-generating potential, is destroyed.

Petroleum is comprised of carbon (83%) and hydrogen (13%),
sometimes with small amounts of sulfur (up to 2%), nitrogen (0.5%), and
oxygen (0.5%). Hydrocarbons (composed of carbon and hydrogen only)
make up over 90% of most crude oils. The hydrocarbons in crude oils vary
in molecular size and molecular type, with the heavy crudes comprising
more large molecules and the light crudes comprising smaller and more
volatile molecules.
Rocks that produce hydrocarbons from organic matter buried within
the rock pore spaces are known as source rocks. The most common
organic-rich sedimentary rock, thought to be the source rock for most oil
and gas, is shale. Many types of shale are black and are often referred to
as black shale. The black color comes primarily from its organic content.
Black shale may contain 1%–3% organic matter, whereas green or
gray shale has only about 0.5% organic matter. Organic-rich black shale
is relatively common in many areas of the world (e.g., the Kimmeridge
Clay Formation of the North Sea, which forms the foundation rock for the
Humber Bridge and is an important source rock). Of the organic matter
deposited in the earth’s crust, only about 2% becomes petroleum. Of this
2%, only about 0.5% (that is, 1 part in 10,000, or 0.01% of the original

organic matter) finds its way into a commercially exploitable reservoir.
Thus the potentially exploitable petroleum deposits come from only a tiny
fraction of all the organic matter that is deposited within the earth’s crust.

Petroleum generation is a very inefficient process.
Coal comes from the deposits of woody plant remains. Peat and
poor-quality “brown” coal has been subjected to lower temperatures and
pressures, while high-quality black coal (e.g., anthracite) has been buried
deeper and exposed to higher temperatures. As the woody remains convert
to coal, hydrocarbon gas (methane) is generated. This can migrate upwards
to form a gas-only reservoir (no associated oil). This methane can also
remain trapped within the coal seams, where it presents a serious danger
to the miners extracting the coal.

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