OR/14/055 Technical information

Linley, K A. 2014. User Guide for vitrinite reflectance data. British Geological Survey Internal Report, OR/14/055.

Definitions

Vitrinite reflectance - a method for identifying the maximum temperature history of sediments in sedimentary basins and in the context of this dataset an indicator of potential hydrocarbon resources. VR can be calibrated to indicate the maturity of hydrocarbon source rocks and indicates whether a rock has been heated enough to produce oil, oil and gas or gas only, Figure 1 gives a schematic illustration of this.

Figure 1    Vitrinite reflectance (Ro) values indicating hydrocarbon type.
(based on droplet diagram first presented by W Dow in the Journal of Geochemical Exploration 1977).

Vitrinite reflectance measures the percentage of incident light reflected from a polished surface of vitrinite and is presented in units %R_o, the measured percentage of reflected light from a sample which is immersed in oil (%R_o = % reflectance in oil).

Maturity is described in four classes based on the vitrinite reflectance:

Under mature — source rocks have not yet been exposed to sufficient heat for thermal generation of oil or gas.

Oil window - onset of oil generation is correlated with VR of 0.5–0.6 % R_o and termination of oil generation between 0.85–1.1 % R_o.

Gas window-onset of the gas window is associated with values of 1.0–1.3% R_o and terminates around 3.0% R_o

Over mature — spent rocks which have entered the gas window and have already generated oil or gas and as a result are depleted and have therefore exhausted all hydrogen necessary for further oil or gas generation.

It should be noted that these generation windows vary between different source rocks and with different kerogens (mixtures or organic chemical compounds). The temperature ranges associated with the vitrinite reflectance values are indicated in Figure 2.

Figure 2    Maturity zones.

Scale

The vitrinite reflectance data is produced as point data from boreholes whose locations are given to the nearest 10 metres, identified by an easting and northing. The depth at which samples were taken for analysis is recorded to two decimal places.

Field descriptions

Table 1    Attribute table field descriptions

Field Name	Field Type	Description
WID	String	Well identification number
Well Name	Sting	Nationally registered well name in the BGS Borehole database SOBI (Single Onshore Borehole Index)
Easting	Long	Six figure national grid reference
Northing	Long	Six figure national grid reference
Depth	Float	Depth in meters at which the sample was taken
VR	Float	Vitrinite Reflectance value (%) represents the level of maturity of the source rock sample
Maturity	String	Level of maturity of the potential source rock; four categories identified: under mature, over mature, oil window, gas window
Lex Code	String	Formation level BGS lexicon code for rock sample used
Lex Desc	String	Rock description as provided in the BGS lexicon
Version	String	Version of data, updated when revisions to data take place

Creation of the dataset

Boreholes for which vitrinite reflectance analyses have been carried out were identified in the Single Onshore Borehole Index (SOBI). These locations along with their VR data were extracted and are contained in the accompanying dataset.

Dataset history

The British Geological Survey has been carrying out basin analysis and unconventional hydrocarbon research. This dataset draws on the interpretations gathered during this research and provides the first access to the vitrinite reflectance data held by BGS.

BGS is committed to ongoing development of its data products and it is expected that this dataset will continue to develop and that further boreholes and interpretations will be included in later versions.

Coverage

Data coverage is currently sparse; data distribution is based on boreholes which have interpreted and has measurable vitrinite reflectance.

Figure 3    Data coverage of vitrinite reflectance.

Data format

The vitrinite reflectance dataset has been created as asset of point locations (easting and northing) and are available in a range of GIS formats, including ArcGIS (.shp), ArcInfo Coverages and MapInfo (.tab). More specialised formats may be available but may incur additional processing costs.

Limitations

The Vitrinite Reflectance dataset is produced as point data from a number of strategic boreholes whose location is given to the nearest 10 metres.

The VR data is based on, and limited to, an interpretation of the records in possession of the British Geological Survey at the time the data was released.

A high VR value is not necessarily an indicator of shale gas or oil potential. Other factors such as total organic content must be assessed.

Vitrinite reflectance has been created as a set of point data, and is available in a range of GIS formats, including ArcGIS (.shp), ArcInfo Coverages and MapInfo (.tab). More specialised formats may be available but may incur additional processing costs.