Rock Eval pyrolysis is used to identify the type and maturity of organic matter and to detect petroleum potential in sediments. Rock Eval pyrolysis is done using the Delsi-Nermag Rock Eval II Plus TOC module. Samples chosen to be measured on the Rock Eval are usually subsampled from the freeze-dried material previously crushed for analyses on the coulometer and CNS.
The Rock Eval (RE) pyrolysis method consists of a programmed temperature heating (in a pyrolysis oven) in an inert atmosphere (helium) of a small sample (~100 mg) to quantitatively and selectively determine (1) the free hydrocarbons contained in the sample and (2) the hydrocarbon- and oxygen-containing compounds (CO2) that are volatilized during the cracking of the unextractable organic matter in the sample (kerogen).
The pyrolysis oven temperature program is as follows: for 3 min, the oven is kept isothermally at 300°C and the free hydrocarbons are volatilized and measured as the S1 peak (detected by FID). The temperature is then increased from 300° to 550°C (at 25°C/min). This is the phase of volatilization of the very heavy hydrocarbons compounds (>C40) as well as the cracking of nonvolatile organic matter. The hydrocarbons released from this thermal cracking are measured as the S2 peak (by FID). The temperature at which S2 reaches its maximum depends on the nature and maturity of the kerogen and is called Tmax. The CO2 issued from kerogen cracking is trapped in the 300°-390°C range. The trap is heated, and CO2 is released and detected on a TCD during the cooling of the pyrolysis oven (S3 peak).
In summary, the four basic parameters obtained by pyrolysis (Fig. F4; from Tissot and Welte, 1984) are as follows:
The RE II apparatus can also be used to determine the TOC of the sample by oxidizing (in an oxidation oven kept at 600°C) the organic matter remaining in the sample after pyrolysis (residual organic carbon). The TOC is then determined by adding the residual organic carbon detected to the pyrolyzed organic carbon, which in turn is measured from the hydrocarbon compounds issuing from pyrolysis.
The type and maturity of organic matter in petroleum source rocks can be characterized from Rock Eval pyrolysis data (see Fig. F5; from Emeis and Kvenvolden, 1986. This figure has been adapted for marine sediments cored by ODP.) using the following parameters:
Maturation of the organic matter can be estimated by (1) the location of HI and OI on Figure F5 (the arrows point toward increasing maturation) and (2) Tmax range. Tmax = 400°-430°C represents immature organic matter; Tmax = 435°-450°C represents mature or oil zone; Tmax > 450°C represents the overmature zone.
Rock Eval pyrolysis is not normally used to make real-time drilling decisions because of the lengthy sample preparation, running, and interpretation time.