Page 24 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

the pipeline and measure energy content in kJ/kg (also called calorific value
or Wobbe index).

3.1.3 Condensates
While the ethane, propane, butane, and pentanes must be removed from
natural gas, this does not mean that they are all 'waste products'. In fact,
associated hydrocarbons, known as 'natural gas liquids' (NGL) can be very
valuable by-products of natural gas processing. NGLs include ethane,
propane, butane, iso-butane, and natural gasoline. These are sold
separately and have a variety of different uses such as raw materials for oil
refineries or petrochemical plants, as sources of energy, and for enhancing
oil recovery in oil wells. Condensates are also useful as diluents for heavy
crude, see below.

3.2 The reservoir
The oil and gas bearing structure is typically of porous rock such as
sandstone or washed out limestone. The sand might have been laid down as
desert sand dunes or seafloor. Oil and gas deposits form as organic material
(tiny plants and animals) deposited in earlier geological periods, typically 100
to 200 million years ago, under, over or with the sand or silt, are transformed
by high temperature and pressure into hydrocarbons.

Anticline Fault Salt dome

Gas Porous rock
Oil Impermeable rock
Fossil water in porous reservoir rock

For an oil reservoir to form, porous rock needs to be covered by a non-
porous layer such as salt, shale, chalk or mud rock that can prevent the
hydrocarbons from leaking out of the structure. As rock structures become

22