ROCK EVAL PYROLYSIS

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 300C and the free hydrocarbons are volatilized and measured as the S1 peak (detected by FID). The temperature is then increased from 300 to 550C (at 25C/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 300-390C 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:

S1 = the amount of free hydrocarbons (gas and oil) in the sample (in milligrams of hydrocarbon per gram of rock). If S1 >1 mg/g, it may be indicative of an oil show. S1 normally increases with depth. Contamination of samples by drilling fluids and mud can give an abnormally high value for S1.
S2 = the amount of hydrocarbons generated through thermal cracking of nonvolatile organic matter. S2 is an indication of the quantity of hydrocarbons that the rock has the potential of producing should burial and maturation continue. This parameter normally decreases with burial depths >1 km.
S3 = the amount of CO2 (in milligrams CO2 per gram of rock) produced during pyrolysis of kerogen. S3 is an indication of the amount of oxygen in the kerogen and is used to calculate the oxygen index (see below). Contamination of the samples should be suspected if abnormally high S3 values are obtained. High concentrations of carbonates that break down at lower temperatures than 390C will also cause higher S3 values than expected.
Tmax = the temperature at which the maximum release of hydrocarbons from cracking of kerogen occurs during pyrolysis (top of S2 peak). Tmax is an indication of the stage of maturation of the organic matter.

The RE II apparatus can also be used to determine the TOC of the sample by oxidizing (in an oxidation oven kept at 600C) 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:

HI = hydrogen index (HI = [100 x S2]/TOC). HI is a parameter used to characterize the origin of organic matter. Marine organisms and algae, in general, are composed of lipid- and protein-rich organic matter, where the ratio of H to C is higher than in the carbohydrate-rich constituents of land plants. HI typically ranges from ~100 to 600 in geological samples.
OI = oxygen index (OI = [100 x S3]/TOC). OI is a parameter that correlates with the ratio of O to C, which is high for polysacharride-rich remains of land plants and inert organic material (residual organic matter) encountered as background in marine sediments. OI values range from near 0 to ~150.
PI = production index (PI = S1/[S1 + S2]). PI is used to characterize the evolution level of the organic matter.
PC = pyrolyzable carbon (PC = 0.083 x [S1 + S2]). PC corresponds to carbon content of hydrocarbons volatilized and pyrolyzed during the analysis.

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 = 400-430C represents immature organic matter; Tmax = 435-450C represents mature or oil zone; Tmax > 450C 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.

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