Volumes of petroleum (oil, condensate, NGL and gas) can be measured and quantified in multiple ways. Quantities can be expressed by mass (weight), volume and energy content. Different units can be used for the different quantities, for example cubic metres and barrels, both of which express volume.
In Norway, we generally use the metric system (International System of Units), but older American (British) units are also frequently used in the petroleum industry.
When performing conversions, for example from volume to energy content, there is no exact conversion factor and you must know/make assumptions about the properties of the substance you are converting. Assumptions regarding energy content per cubic metre gas and weight per volume unit NGL are examples of this.
Oil and gas volumes are often stated in standard cubic metres (Sm3), and for precise indication of volumes, the temperature and pressure at which they apply must also be stated. The standard conditions are 15 °C and normal atmospheric pressure (1013.25 hPa).
To add up amounts of different petroleum products (oil, gas, NGL and condensate), they must first be converted to a uniform quantity and unit. The most common method is using standard cubic metres of oil equivalents (abbreviated as Sm3 o.e.).
When performing conversions, the Norwegian Offshore Directorate uses a volumetric conversion of NGL to liquid and an energy-based (but not exact) conversion factor for gas, based on typical properties on the Norwegian shelf. The properties of oil, gas and NGL vary from field to field and over time. However, in the resource accounts and other comparisons where oil equivalents are used, a common and constant conversion factor is used for all fields and discoveries.
The following factors are used for conversions to oil equivalents
| 1 Sm3 oil | = | 1.0 Sm3 o.e. |
| 1 Sm3 condensate | = | 1.0 Sm3 o.e. |
| 1000 Sm3 gas | = | 1.0 Sm3 o.e. |
| 1 Sm3 NGL | = | 1.0 Sm3 o.e. |
| 1 tonne NGL | = | 1.9 Sm3 o.e. |
Conversion between different units that are used for gas
| 1 Sm3 | = | 35.315 SCF (scf) | SCF - Standard Cubic Feet |
| 1 SCF | = | 0.028317 Sm3 | |
| 1 Sm3 | ≈ | 40 MJ = 11.111 kWh | Energy content varies; 40 MJ is used as standard |
| 1 Sm3 | ≈ | 37913 BTU | BTU - British Thermal Unit |
Conversion factors for liquid (oil, condensate and NGL)
| 1 Sm3 | = | 6.2898 barrels | Barrel (bbl) or Stock Tank Barrel (STB) |
| 1 bbl | = | 0.1590 Sm3 | |
| 1 Sm3 | ≈ | 0.84 toe | Tonne of oil equivalent (at a density of 840 kg/Sm3) |
| 1000 Sm3/y | ≈ | 17.23 bbl/d | Production rate (17.18 bbl/d in leap years) |
| 1000 bbl/d | ≈ | 58035 Sm3/y | Production rate (58194 Sm3/y in leap year) |
Energy content for idealised/normalised products
| 1 Sm3 natural gas | ≈ | 40 MJ | Normalised sales gas |
| 1 Sm3 crude oil | ≈ | 36000 MJ | Varies |
Conversion factors between different energy units
| MJ | kWh | BTU | ||
| MJ | Megajoule | 1 | 0.2778 | 947.81 |
| kWh | Kilowatt hour | 3.6 | 1 | 3412.13 |
| BTU | British Thermal Unit | 0.001055 | 0.0002931 | 1 |