OR/14/056 Technical information

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Cooke, I L. 2014. User Guide Total Organic Carbon (TOC) data. British Geological Survey Internal Report, OR/14/056.

Definitions

Total organic carbon - a measure of the dry weight per cent of organic carbon within hydrocarbon source rocks. Hydrocarbons, including natural gas (principally methane, ethane and propane), may be generated by the heating of organic carbon through burial over geological time.

S1 - the value of free oil and gas (hydrocarbons) that are volatilised from the rock sample without cracking the kerogen during the first stage of heating at 300ºC. S1 represents how many milligrams of free hydrocarbons can be thermally distilled out of one gram of the rock sample. S1 values are used to derive the Production Index (PI) and petroleum potential of a rock sample.

S2 - the value of hydrocarbons that are generated from the sample during the second programmed heating stage of pyrolysis. These result from the cracking of heavy hydrocarbons and from the thermal breakdown of kerogen. S2 represents the amount of residual hydrocarbons in milligrams per gram of rock.

The S2 value gives an indication of the potential amount of hydrocarbons that the source rock many produce if thermal maturation continues. Generally the following values apply when assessing source rock quality:

None <2 mg/g
Poor 2 to 3 mg/g
Fair 3 to 5 mg/g
Good 5 to 10 mg/g
Very good >10 mg/g

S2 values are used to derive the Hydrogen Index (HI), Production Index (PI) and petroleum potential of a rock sample.

Production Index (PI) - derived from the relationship between hydrocarbons generated during the first and second stages of pyrolysis. It is defined as S1/ (S1+S2). It is used to characterise the evolution of the organic matter as PI tends to gradually increase with depth in fine-grained rocks.

S3 - represents the CO2 that is emitted from the thermal cracking of kerogen, expressed in milligrams per gram of rock. S3 values are used to derive the Oxygen Index (OI) of a rock sample.

Oxygen Index (OI) - derived from the ratio of CO2 to TOC. It is defined as 100 x S3/TOC. The OI relates to the amount of oxygen contained in the kerogen and can be useful in tracking kerogen maturation or type.

P1 - represents the volume of bitumen (PPM) that was extracted from a sample during the older (now obsolete) analysis method of Mini-Pyrolysis.

P2 - represents the volume of pyrolysate (PPM) that was extracted from a sample during the older (now obsolete) analysis method of Mini-Pyrolysis.

Hydrogen Index (HI) - an indicator of the unoxidised hydrogen in a sample which gives an indication of kerogen maturity (MG_HC refers to milligrams of hydrocarbons — the unit in which the Hydrogen Index is reported).

High values (>0.15) indicate good oil and gas generation potential and Low values (<0.15) indicate good gas generation potential.

Kerogen types can also be inferred from this index.

TMAX - the temperature (ºC) at which the rock sample reaches its maximum rate of hydrocarbon generation.

Analysis techniques

Borehole samples have been analysed using one of two pyrolysis techniques. The process of thermal maturation can be simulated in the laboratory. The process involves heating a sample under controlled conditions and measuring the oil like material released from the rock. Using this technique the potential richness of immature sediments can be evaluated. Previously mini-pyrolysis and pyrolysis was used but this method has been superseded by the use of rock-eval pyrolysis.

Mini-pyrolysis and pyrolysis is an older analytical method. It is broadly similar to that of rock-eval pyrolysis with bitumen (P1) and pyrolysate (P2) the end products of the analysis, quoted in Parts per Million (PPM) the use of this analytical method has largely been replaced by the use of rock-eval pyrolyis.

Rock-eval pyrolysis is a more modern method for determining the hydrocarbons generation potential, organic type and thermal maturity of a source rock. A grounded, homogenised sample is heated in stages at set temperatures to obtain the amount of hydrocarbons (S1 and S2 peaks) and CO2 content (S3 peak). The temperature at which S2 reaches its maximum rate of hydrocarbon generation is known as Tmax.

Field descriptions

Table 1    Attribute table field descriptions
Field Name Field Type Description
Well Name Text Nationally registered well name in the BGS Borehole database Single Onshore Borehole Index.
Reference Text Published reference from which the analyses values were obtained. All samples were taken from BGS boreholes
Depth M Numerical Sample depth in metres
TOC Numerical Dry weight % of organic carbon

-3 value in this field indicates data not available

Easting Numerical Six figure National Grid reference
Northing Numerical Six figure National Grid reference
S1 Numerical Value of free oil and gas (hydrocarbons) that are volatilised from the rock sample without cracking the kerogen during the first stage of heating at 300ºC. (mg/g)

-3 value in this field indicates data not available

S2 Numerical Volatilisation of higher carbon number hydrocarbon compounds and cracking of non-volatile organic matter, 300–600°C (mg/g)

-3 value in this field indicates data not available

S3 Numerical CO2 trapped in 300–390°C (mg CO2/g)

-3 value in this field indicates data not available

TMAX Numerical Temperature of maximum kerogen pyrolysate yield (S2) (°C)

-3 value in this field indicates data not available

P1 Numerical The amount of thermal bitumen released by heating a rock sample in helium up to 350°C (parts per million)

-3 value in this field indicates data not available

P2 Numerical The amount of pyrolysate released by heating a rock sample in helium from 350–550°C (parts per million)

-3 value in this field indicates data not available

HI-MG-HC Text Hydrogen Index = yield of S2 or P2 relative to the TOC (S2/TOC or P2/TOC) (mg of hydrocarbon per g)
Version Text Version of data, updated when revisions to data take place.

Creation of the dataset

BGS borehole data was extracted from the BGS Single Onshore Borehole Index (SOBI) where TOC samples were validated with published information by a BGS Energy specialist.

Dataset history

This is the first approved BGS dataset containing Total Organic Content further data is to be released as it becomes available.

Scale

The Total Organic Content dataset is produced as point data from a number of strategic boreholes.

Borehole locations are given to the nearest 10 metres and down borehole readings have been taken at various depths recorded in metres to two decimal places.

The boreholes are identified by an easting and northing together with field observations taken from quarries and geologically relevant outcrops.

Coverage

The Total Organic Content dataset is taken from a number of strategic boreholes around the UK, together with field samples from quarries and geologically relevant outcrops (see Figure 1).

Figure 1    Sample sites included in the Total Organic Content dataset.

Currently 36 locations are represented with a total of 572 downhole interpretations. Most boreholes include multiple interpretations from different sample depths.

Data format

The Total Organic Carbon dataset has been created as point data and is available in a range of GIS formats, including ArcGIS (.shp) and MapInfo (.tab). More specialised formats may be available but may incur additional processing costs.

Limitations

Total Organic Carbon is based on, and limited to, an interpretation of the records in the possession of The British Geological Survey at the time the dataset was created.

A high TOC value is not necessarily an indicator of shale gas or oil potential.

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

Where combined with field observations taken from quarries and geologically relevant outcrops the location may be more approximate.

Borehole sample readings have been taken at various depths recorded in metres to two decimal places.