200 Peter Stitle and Graham Shields
The oxidation of organic carbon and reduction of sulphate will lead to the
production of COz and H_~O and the formation of carbonic acid. The presence of
relatively large quantities of pyrite in the investigated samples suggest that
sufficient sulfur atoms were indeed available in order for such reactions to take
place. The carbonic acid makes further dissolution of alkali feldspar and the new
formation of chlorite and illite possible. It does not only help in dissolution
reactions but also in the subsequent silicification of the alkali feldspar (Fig. 7.3C;
see also Chap. 6. ! ). The REE patterns of the clay fractions with the pronounced
Eu (europium) anomaly are typical of feldspar and confirm the close relationship
between illite and feldspar (Fig. 7. t0). As feldspars, oil and sulfides coexisted, we
can assume that the process of feldspar dissolution and illite-chlorite
crystallization took place in situ and without the migration of fluid phases across
large distances. Isotopic exchange with other silicate phases from neighbouring
rock associations can therefore be excluded (Stille et al. 1993).
As both the organic carbon and the sulfides are depleted in REE (their
concentrations are tn the ppt-range), it can be assumed that the Nd isotopic
composition of the newly formed clay minerals were controlled by the
replacement of the feldspars only. Provided these detrital feldspars possessed
nearly identical, initial isotopic ratios, this means to say that they were derived
from the same source area, then the subsequently formed illites will have
incorporated the same isotopic composition. These clay minerals define isochrons,
whose gradients correspond to the age of crystallization and whose initial isotopic
composition would reflect that of the detritat feldspars at the time of diagenesis.
This isotopic composition corresponds to that of the fluid phase in which illite and
chlorite formed. Nd isotopic homogenization of this sort is likely to be important
on a small-scale (hand specimen scale) but unlikely to be important in more
sizeable rock successions.
The oil coexisted in isotopic equilibrium with the diagenetic fluids (aqueous
formation waters). The mechanism that led to isotopic equilibrium is difficult to
understand as oil and aqueous solutions are not able to mix under normal
conditions. It is conceivable that diffusion played an important role. However, it is
more likely that the isotopic composition of the REE-poor hydrocarbons were
overprinted by the REE rich, acidic, watery solutions, which had already taken
part tn the etching and replacement of the feldspars. This second phase may have
coexisted within the oil without the necessity for mixing. In addition it might be
possible that the originally REE-poor hydrocarbons mainly contain REE adsorbed
from the diagenetic fluid phases. Under these conditions, identical isotopic
compositions may arise in both the oil and in the authigenic, silicate mineral
phases. If we can identify all possible parent rocks, we can compare the isotopic
compositions of authigenic mineral phases in these rocks and the oil in order to
determine the source of the oil.
Whole rocks can only reflect the isotopic composition of the oil if they consist
exclusively of authigenic mineral phases, which formed under the conditions just
described. The whole rock sample with the highest potassium and 6rganic carbon
The oxidation of organic carbon and reduction of sulphate will lead to the
production of COz and H_~O and the formation of carbonic acid. The presence of
relatively large quantities of pyrite in the investigated samples suggest that
sufficient sulfur atoms were indeed available in order for such reactions to take
place. The carbonic acid makes further dissolution of alkali feldspar and the new
formation of chlorite and illite possible. It does not only help in dissolution
reactions but also in the subsequent silicification of the alkali feldspar (Fig. 7.3C;
see also Chap. 6. ! ). The REE patterns of the clay fractions with the pronounced
Eu (europium) anomaly are typical of feldspar and confirm the close relationship
between illite and feldspar (Fig. 7. t0). As feldspars, oil and sulfides coexisted, we
can assume that the process of feldspar dissolution and illite-chlorite
crystallization took place in situ and without the migration of fluid phases across
large distances. Isotopic exchange with other silicate phases from neighbouring
rock associations can therefore be excluded (Stille et al. 1993).
As both the organic carbon and the sulfides are depleted in REE (their
concentrations are tn the ppt-range), it can be assumed that the Nd isotopic
composition of the newly formed clay minerals were controlled by the
replacement of the feldspars only. Provided these detrital feldspars possessed
nearly identical, initial isotopic ratios, this means to say that they were derived
from the same source area, then the subsequently formed illites will have
incorporated the same isotopic composition. These clay minerals define isochrons,
whose gradients correspond to the age of crystallization and whose initial isotopic
composition would reflect that of the detritat feldspars at the time of diagenesis.
This isotopic composition corresponds to that of the fluid phase in which illite and
chlorite formed. Nd isotopic homogenization of this sort is likely to be important
on a small-scale (hand specimen scale) but unlikely to be important in more
sizeable rock successions.
The oil coexisted in isotopic equilibrium with the diagenetic fluids (aqueous
formation waters). The mechanism that led to isotopic equilibrium is difficult to
understand as oil and aqueous solutions are not able to mix under normal
conditions. It is conceivable that diffusion played an important role. However, it is
more likely that the isotopic composition of the REE-poor hydrocarbons were
overprinted by the REE rich, acidic, watery solutions, which had already taken
part tn the etching and replacement of the feldspars. This second phase may have
coexisted within the oil without the necessity for mixing. In addition it might be
possible that the originally REE-poor hydrocarbons mainly contain REE adsorbed
from the diagenetic fluid phases. Under these conditions, identical isotopic
compositions may arise in both the oil and in the authigenic, silicate mineral
phases. If we can identify all possible parent rocks, we can compare the isotopic
compositions of authigenic mineral phases in these rocks and the oil in order to
determine the source of the oil.
Whole rocks can only reflect the isotopic composition of the oil if they consist
exclusively of authigenic mineral phases, which formed under the conditions just
described. The whole rock sample with the highest potassium and 6rganic carbon