Geology Reference
In-Depth Information
Fig. 18.3
Characterization of kerogen type on a Van-Krevelen
equivalent diagram (Espitalie et al.
1977
), using Rock-Eval Hydrogen
and Oxygen indexes (HI and HO).
Bold lines
show the evolution path
of kerogen types with thermal maturation (along the direction shown
by the
arrows
). Kerogen type I: rich in aliphatic chains and poor in
aromatics, derived from algal lipids or enriched by microbial activity,
typical of lacustrine organic-matter; type II: richer in aromatics,
usually related to marine organic matter deposited on a reducing
environment but also formed by a combination algae (type I) and
terrestrial (Type III) organic matter, Type III: aromatics and oxy-
genated functions, derived from terrestrial higher plants (Tissot and
Welte
1978
). (
a
) RMCA samples, analyzed by RWTH Aachen
(Sachse et al.
2012
), with two samples of Kipala shales from
Kadima (
2007
).
(
b
)
JNOC samples
(JNOC
1984
). Data from
Tables
18.1
and
18.2
sandstones and contain very little quantity of organic matter
(C
org
ΒΌ
Similarly, samples of the Loia Group (Cretaceous) are rich
in C
org
in the Samba well (Couches 4 of Cahen et al.
1959
),
but very poor in the Dekese well (Couches C of Cahen et al.
1960
) and in the region of Kananga (JNOC samples). Those
from the Samba well display also high Hydrogen Indexes
(~800mgHCequivalen /gC
org
) and medium Oxygen
Indexes (50-60 mg CO2/gC
org
).
Sulphur content, microscopic observations, n-alcane
and biomarker patterns (Sachse et al.
2012
) show that
brown-grey shales of the Loia and Stanleyville Groups
are composed predominantly of algae-derived aquatic
organic matter and small amounts of terrestrial higher
plant material. The range of composition of the
Stanleyville samples analysed by JNOC (Fig.
18.3b
)
might therefore reflect various proportions of the end-
members (type I algae-derived and type III terrestrial
higher plant-derived) rather than the presence of marine
type II organic matter.
0.43 %, Table
18.3
).Themostorganic-richfaciesis
the brown-grey shales (termed bituminous shales by Passau
1923
), found in outcrops and boreholes from the Kisangani-
Ubundu portion of the Congo River (Lualaba). Both the
RMCA and the JNOC samples from this lithology are
equally rich in C
org
, with ultra-high Hydrogen Index
(900-1,000 mg HC equivalent/g C
org
) and a moderate to
low Oxygen Index (30-70 mg CO2/g C
org
). Some of the
JNOC samples have anomalously high HI values (
1,000),
which likely reflects an analytical problem in isolating the
inorganic from the organic carbon. Limestones are less rich
in C
org
(ca. 1-2 %), but still with relatively high Hydrogen
Index (500-600 mg HC equivalent/g C
org
). Mudstones and
sandstones are poor to very poor in C
org
. On the Van
Krevelen-equivalent HI-OI diagram (Fig.
18.3
), the samples
of the Stanleyville Group display a range of compositions,
from type I (algae) to type III (higher plant-derived) kerogen.
>
Search WWH ::
Custom Search