Page 161 - Geology and Geochemistry of Oil and Gas

130 DISPERSED ORGANIC MATTER

physical techniques for studying the dispersed organic matter are complex or require
expensive, sophisticated equipment.
A technique that became very common over the last 10 to 15 years is a pyrolysis
technique introduced by Espitalie and others in 1971, which is called RockEval. It is
based on heating the kerogen sample to 550 1C, following a predetermined heating
program. The first peak (S 1 ) of the curve corresponds to the free and adsorbed
hydrocarbons evaporated at a moderate heating to 200 1C–250 1C. The second peak
(S 2 ) reflects hydrocarbons and other similar components generated at a higher tem-
perature due to pyrolysis of the insoluble portion of kerogen. Finally, the third peak
(S 3 ) reflects CO 2 and water.
Tissot and Welte (1981, p. 413) suggested that the value of S 1 represents the
fraction of initial genetic potential that has been transformed into hydrocarbons.
The S 2 value is the residual potential. The total S 1 +S 2 (usually expressed in kg/ton)
indicates the total genetic potential. When S 1 +S 2 is less than 2 kg/t, the rock is not
considered to be a source rock, whereas when S 1 +S 2 is equal 2 to 6 kg/t, the rock is
considered to be a source rock with a moderate hydrocarbon generating potential. If
S 1 +S 2 is greater than 6 kg/t, the rock has a high hydrocarbon generating potential.
A kerogen type is determined by two characteristic parameters: (a) the hydrogen
index (S 2 /C org ) and (b) the oxygen index (S 3 /C org ). There is a good correlation
between the hydrogen index and the H/C org atomic ratio, as well as between the
oxygen index and the O/C org atomic ratio.
It is recommended to use the ratio [S 1 /(S 1 +S 2 )] (transformation index) and
the temperature for determination of the organic matter maturity. It was found
that the transformation index does not depend on the type of organic matter. On
the other hand, the type of kerogen depends on the temperature, especially during
diagenesis and early catagenesis. Tissot and Welte (1981) stated that the temper-
ature is lower for the continental kerogen (type III) and higher for the marine or
lacustrine kerogen (type I and type II). Yet, the temperature is almost the same for
different kerogen types in the maximum oil generation zone and later in the gas
generation zone.
To characterize the components of S 2 peak, gas-chromatography is utilized. It was
found that the source of chemofossils (long-chained normal alkanes) in the kerogen
of modern deposits is higher plants. Modern deposits are dominated by the mol-
ecules with odd numbers of carbon atoms. With depth, new alkanes with shorter
carbon chains are generated, and the odd/even ratio inequality levels off.
It was proposed to use the ratio [odd n-alkanes/even n-alkanes] or [odd n-alkanes/
(even n-alkanes+odd n-alkanes)] to characterize the degree of kerogen transforma-
tion or its maturity. The limitation here is the dependence of these parameters on the
kerogen type and the transformation conditions.
Another frequently used parameter is the isoprenoid distribution. It was shown
that the pristane/phytane ratio changes in the process of the thermal evolution of
coals. This ratio, therefore, can be used to characterize the Type III kerogen. This
has not been proved, however, for the kerogen of Types I and II.
Trofimuk et al. (1982) opposed the use of the above-mentioned parameters for the
identification of source rocks and determination of their potential. Trofimuk et al.