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Petroleum source rocks of the Tarfaya Basin and adjacent areas, Morocco
Victoria F. Sachse a, Ralf Littke a,⇑, Sabine Heim a, Oliver Kluth b, Jürgen Schober b, Lahcen Boutib c,Haddou Jabour c, Ferdinand Perssen d, Sven Sindern e
a Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen University, D-52056 Aachen, Germanyb RWE Dea AG, D-22297 Hamburg, Germanyc National Office of Hydrocarbons and Mining, Rabat 10050, Moroccod GeoForschungs-Zentrum Potsdam, D-14473 Potsdam, Germanye Institute of Mineralogy and Economic Geology, RWTH Aachen University, D-52056 Aachen, Germany
a r t i c l e i n f o
Article history:
Received 25 August 2010
Received in revised form 9 December 2010
Accepted 11 December 2010
Available online 22 December 2010
a b s t r a c t
This work characterizes the source rock potential of the Tarfaya Basin and enabled us to reconstruct its
geochemical history. Outcrop samples covering different stratigraphic intervals, plus the northwestern
part of the Zag/Tindouf Basin (Bas Draa area), were analyzed for total organic carbon (Corg) and total inor-
ganic carbon contents and total sulfur content. Rock–Eval analysis and vitrinite reflectance measure-
ments were performed on 56 samples chosen on the basis Corg content. A set of 45 samples were
extracted and non-aromatic hydrocarbons were analyzed by way of gas chromatography-flame ioniza-
tion detection (GC-FID) and GC-mass spectrometry (GC-MS). Non-isothermal open system pyrolysis at
different heating rates was applied to obtain kinetic parameters for modelling petroleum generation from
four different source rocks.
High quality petroleum source rocks with high Corg content and hydrogen index (HI) values were found
for samples of Eocene, Coniacian, Turonian and Cenomanian age. Most samples were carbonate rich and
organic/sulfur values were high to moderate. Various maturity parameters indicated immature or possi-
bly early mature organic matter. Based on organic geochemical and petrological data, the organic matter
is of marine/aquatic origin (Cenomanian) or a mixture of aquatic and terrigenous material (Eocene). The
Early Cretaceous interval did not contain high quality source rocks, but indications of petroleum impreg-
nation were found.
2010 Elsevier Ltd. All rights reserved.
1. Introduction
For a number of years, exploration interest has focussed on-
shore in western Morocco and offshore on the Atlantic margin
( Jarvis et al., 1999). Several companies drilled wells in the on-
and offshore parts of the Tarfaya Basin, such as the offshore wells
MO-1 to MO-8, Cap Juby-1, Puerto Cansado-1 and El Hamra-1
(Leine, 1986). Oil shows in Mauritania and in the northern Moroccan
basins emphasize the possible economic relevance of activepetroleum systems along the north-west African continental
margin. Numerous studies focussed on Cenomanian/Turonian
sediments and their depositional environment (e.g. Lüning et al.,
2004; Kolonic et al., 2002; Kuhnt et al., 2001, 2005).
The main objective of this study was to determine source rock
parameters for different sedimentary sequences in the Tarfaya
Basin and adjacent areas, with a focus on Upper Cretaceous and
Eocene units. We aimed to assess the depositional environment of
these source rocks and provide implications for future petroleum
exploration and resource assessment in Moroccan coastal regions.
A detailed overview of organic matter (OM) type, quantity, and
quality is presented to improve the overall organic geochemical
information available for the Tarfaya Basin. For this purpose, out-
crop samples covering various stratigraphic units of the basin were
collected. Because of the absence of Jurassic outcrops within the
basin, samples of this age were sampled north of Agadir, the out-
crop location nearest to the basin. In addition, we analyzed some
Palaeozoic sediments, which occur in the deep subsurface. BecausePaleozoic sequences are not exposed or are not present in the
Tarfaya Basin, a few sediments of this age were sampled along
the northern flank of the adjacent Tindouf Basin.
2. Tarfaya Basin – geological background
Tarfaya Basin is an Atlantic margin basin in the southernmost
part of Morocco, stretching over 1000 km along the western mar-
gin of the Sahara (Fig. 1). It is bounded by the Mauritanide thrust
belt (Adrar Souttouf, Dhoul, Zemmour) and the Precambrian
Reguibat Arch to the southeast and by the Paleozoic outcrops of
the Anti-Atlas and the Tindouf Basin to the northeast (Fig. 1). The
0146-6380/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.orggeochem.2010.12.004
⇑ Corresponding author.
E-mail address: [email protected] (R. Littke).
Organic Geochemistry 42 (2011) 209–227
Contents lists available at ScienceDirect
Organic Geochemistry
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / o r g g e o c h e m
http://dx.doi.org/10.1016/j.orggeochem.2010.12.004mailto:[email protected]://dx.doi.org/10.1016/j.orggeochem.2010.12.004http://www.sciencedirect.com/science/journal/01466380http://www.elsevier.com/locate/orggeochemhttp://www.elsevier.com/locate/orggeochemhttp://www.sciencedirect.com/science/journal/01466380http://dx.doi.org/10.1016/j.orggeochem.2010.12.004mailto:[email protected]://dx.doi.org/10.1016/j.orggeochem.2010.12.004
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basement consists of the Precambrian North-West African Craton
and Paleozoic rocks discordantly overlain by Mesozoic sedimentsthat reach a local maximum thickness of 12 km (Kolonic et al.,
2002). The geological and stratigraphic structure of the basin have
been investigated in detail using well and seismic data. The more
recent onshore part of the basin is characterized by syn-rift
structures in terms of grabens and half grabens overlain by
almost undeformed Jurassic to Cretaceous formations that form a
westward dipping monocline.
As a result of early rifting of the central and southern North
Atlantic during the Permian and a marine incursion into this rift
system during the Triassic, a major phase of evaporite deposition
occurred in the region (Heyman, 1989). The resulting salt province
extends along the northwestern coast of Morocco and salt diapirs
are important structural elements in the offshore area. Major
transgression occurred in the Jurassic, coupled with high subsi-dence rates that were compensated by thick terrigenous clastic se-
quences as well as carbonate platform buildups in the shallower
water shelf area, with a transition into a thin, basinal sequence.
The 700 m of Jurassic sands, silts and shales are preserved on
Fuerteventura, indicating transport over the shelf into the deep
water environment ( Jarvis et al., 1999). The end of the Jurassic is
marked by a sea level fall and sub-aerial exposure of the shelf
Jurassic sequence, as well as karstification and presence of red
beds, which were drilled in wells Cap Juby-1, MO-2 and MO-8.
Early Cretaceous (Aptian–Albian) sedimentation was mainly
deltaic, most likely with sediment input from the Tindouf Basin
in the northeast and the African Craton in the southeast. Major
transgressions then occurred during the Cretaceous, mainly Mid
to Late Cretaceous (Albian/Cenomanian, Cenomanian/Turonian,Santonian/Campanian), which initiated deposition of >800 m of
laminated biogenic sediments of Cenomanian to Santonian age in
the Tarfaya Basin (Kolonic et al., 2002; Kuhnt et al., 2001, 2005).The sediments are rich in calcareous nanoplankton, dispersed
biogenic silica, planktonic foraminifera, and organic matter (OM).
Sedimentation rates exceeded 0.1 cm/ky for deep areas (DSDP
367; Nzoussi-Mbassani et al., 2003) to ca. 1.1–1.7 cm/ky at Tarfaya
and around 10–22 cm/ky for continental shelves (Kolonic et al.,
2002; Nzoussi-Mbassani et al., 2003). The depositional environ-
ment was nutrient-rich as a result of an open shelf upwelling sys-
tem. During the Cenomanian/Turonian anoxic event, organic
carbon rich black shales were deposited (Lüning et al., 2004;
Kolonic et al., 2002; Kuhnt et al., 2001, 2005).
An erosional unconformity truncates all of the Paleocene, Upper
Cretaceous and part of the Lower Cretaceous at the shelf edge. The
erosion probably took place in Santonian to Paleocene times.
Eocene and Oligocene units are overlain by a thicker Miocenesequence (max. thickness 1 km; Davison, 2005). More detailed
geological descriptions are given by e.g. Choubert et al. (1966,
1972), Wiedmann et al. (1978), von Rad and Arthur (1979), von
Rad and Einsele (1980), Ranke et al. (1982), Heyman (1989),
Davison (2005) and Michard et al. (2008).
3. Methods
Samples were collected from outcrops of Albian, Cenomanian,
Turonian, Coniacian, Santonian, Campanian and Eocene formations
within the Tarfaya Basin, as well as Palaeozoic units (Ordovician,
Devonian, and Carboniferous) in the northern flank of the Tindouf
Basin.
Total inorganic carbon (Cinorg) and organic carbon (Corg) weremeasured with a LECO RC-412 carbon analyzer via IR absorption
Fig. 1. Location of study area.
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in a two stage measurement process (Corg between 350 C and
520 C; Cinorg between 520 C and 1050 C). With this method Corgand Cinorg could be determined in a single run without prior
removal of carbonate via acid treatment. Total carbon (Ctotal) con-
centration was determined using Ctotal = Cinorg + Corg. The CaCO3proportion (%) was calculated using CaCO3 = Cinorg 8.333. This
leads to a slight overestimation of carbonate content if dolomite
is present instead of calcite. Microscopically, however, no dolomiterhomboeders were detected. Additionally, total sulfur concentra-
tion (TS) was measured using a Leco S 200 sulfur analyser (preci-
sion is
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Inc.). The generated bulk petroleum formation curves served as in-
put for the bulk kinetic model consisting of an activation energy
distribution and a single pre-exponential factor. The pyrolysis
products were transferred via a constant He flow (50 ml/min)
and detected via FID. Low heating rates were used to avoid heat
transfer problems that might influence the product evolution
curves and, consequently, the geological predictions (Schenk and
Dieckman, 2004). The discrete activation energy distribution witha single frequency factor was determined using the Kinetics 2000
software (Burnham et al., 1987).
4. Results
4.1. Elemental analysis
Highest Corg values were found for the Eocene, Coniacian and
Cenomanian/Turonian samples, with values up to 17% (Appendix).
For the Eocene and Cenomanian/Turonian samples, there was a
tendency for the highest Corg values to occur in outcrops close tothe coast [Outcrop 1 (Eocene) and Outcrop 2 for Cenomanian sam-
ples]. The Cinorg content and thus the CaCO3 content was generally
Table 1
Overview of Rock–Eval pyrolysis data of selected samples from the Tarfaya Basin.
Sample Age/outcrop Corg (%) S 1 (mg/g rock) S 2 (mg/g rock) S 3 (mg/g rock) T max (C) HI (mg/g Corg) OI (mg CO2/g Corg) PI (S 1/S 1 + S 2)
681 Eocene 0.6 0.08 0.53 1.86 412 87 311 0.13
682 Eocene 1.0 0.14 1.65 3.86 409 164 383 0.08
683 Eocene 3.29 1.2 11.52 12.68 423 350 385 0.09
684 Eocene 1.96 0.81 5.17 5.57 418 263 284 0.14
685 Eocene 2.29 0.95 10.64 21.58 417 465 944 0.08
686 Eocene (Outcrop 1) 7.2 2.34 29.15 7.38 425 405 103 0.07
687 Eocene 2.75 1.69 9.35 24.49 408 340 891 0.15
688 Eocene 1.68 0.65 8.31 2.32 427 494 138 0.07
690 Eocene 0.5 0.04 0.06 3.86 – 12 766 0.4
705 Eocene 3.54 0.48 13.51 2.91 423 382 82 0.03
706 Eocene 3.34 0.51 16.42 3.6 420 492 108 0.03
707 Eocene 4.47 2.45 31.04 1.92 411 694 43 0.07
674 Campanian 0.8 0.09 0.47 2.28 429 59 285 0.16
675 Campanian 1.66 0.22 5.56 6.99 422 335 421 0.04
676 Campanian (Outcrop 4) 2.05 0.39 7.82 9.76 428 382 477 0.05
692 Campanian 0.37 0.02 – 0.2 – – 54 –
723 Santonian 0.36 0.09 0.01 0.89 – 3 246 0.9
726 Santonian 0.51 0.08 0.1 1.46 – 19 286 0.44
728 Santonain 0.31 0.02 0.04 0.96 – 13 311 0.33
693 Coniacian 6.1 1.78 37.24 5.24 408 610 86 0.05
694 Coniacian 2.31 1.15 12.08 10.64 414 522 460 0.09
695 Coniacian 5.21 1.72 38.49 21.84 409 738 419 0.04
696 Coniacian (Outcrop 3) 6.99 2.04 43.98 3.07 417 629 44 0.04
698 Coniacian 1.09 0.14 4.95 1.61 413 454 148 0.03
699 Coniacian 7.02 1.72 44.07 2.93 413 628 42 0.04700 Coniacian 4.33 0.85 24.03 3.77 412 554 87 0.03
701 Coniacian 5.63 2.11 35.1 5.15 409 624 92 0.06
702 Coniacian 4.58 1.57 24.01 25.13 410 524 549 0.06
703 Coniacian 0.61 0.06 0.05 1.97 – 8 323 0.55
704 Coniacian 0.75 0.07 0.12 – – 16 – 0.37
705 Coniacian 1.56 0.8 6.63 1.05 417 426 67 0.11
770 Turonian 8.54 2.54 50.68 2.68 413 594 31 0.05
771 Turonian 5.86 1.73 31.88 3.27 410 544 56 0.05
772 Turonian 7.79 2.2 49.01 4.6 417 629 59 0.04
773 Turonian 4.8 1.2 25.49 5.67 417 530 118 0.04
774 Turonian 5.06 1.41 30.57 2.72 415 604 54 0.04
775 Turonian 7.35 1.84 40.28 5.82 410 548 79 0.04
776 Turonian 3.9 0.97 18.81 8.2 419 482 210 0.05
668 Cenomanian 14.48 4.12 98.15 2.53 412 678 17 0.04
669 Cenomanian 11.32 2.99 78.23 4.84 418 691 43 0.04
670 Cenomanian 15.57 3.06 104.92 4.43 417 674 28 0.03
671 Cenomanian (Outcrop 2) 16.79 4.46 122.6 4.9 422 730 29 0.04672 Cenomanian 10.05 2.12 71.79 5.62 412 714 33 0.03
673 Cenomanian 8.29 2.57 64.47 5.62 411 778 68 0.04
711 Cenomanian 0.34 0.06 0.01 1.51 – 3 447 0.86
722 Cenomanian 0.32 0.02 – 2.17 – – 682 –
733 Cenomanian 0.41 0.11 0.17 0.71 – 42 173 0.39
779 Cenomanian 0.38 0.07 0.04 1.4 – 11 370 0.64
794 Albian 0.38 0.16 0.39 0.51 415 103 135 0.29
750 Albian 0.5 0.04 0.06 – – 12 – 0.4
751 Albian 0.31 0.03 0.15 0.27 – 49 88 0.17
752 Albian 0.38 0.09 1.27 0.72 – 335 190 0.07
753 Albian 0.51 0.09 0.39 1.48 419 76 289 0.19
754 Albian 0.57 0.06 0.4 0.94 – 70 166 0.13
755 Albian 0.65 0.04 0.09 0.84 – 14 130 0.31
759 Albian 0.41 0.06 0.04 2.36 – 10 570 0.6
718 Lower Cretaceous 0.32 0.03 – 0.88 – – 272 –
719 Lower Cretaceous 0.41 0.64 0.62 1.8 – 150 435 0.51
658 Carboniferous 0.5 0.03 0.08 0.28 – 16 56 0.27
649 Upper Devonian 0.38 0.16 0.39 0.51 415 103 135 0.29
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high for the Cenomanian/Turonian samples and variable or low for
the Eocene and Aptian samples, respectively.
Coniacian samples were selected from only one outcrop area
(Outcrop 3) and had Corg up to 7% and CaCO3 content >50%. Corgcontents of
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Cinorg/Corg patterns defined clear trend lines for the Carbonifer-
ous, Devonian, and Ordovician samples: with Corg increasing with
increasing, CaCO3 content. A different pattern was obvious for
the Lower Cretaceous and Jurassic samples: despite low Corg values,
the CaCO3 values were high for some samples.
Sulfur content was highly variable. Highest values – up to 2.8%
– occurred in the Cenomanian/Turonian. Nevertheless, because of
high Corg values, the respective TS/Corg values were only moderateto low, ranging between 0.1 and 0.24. Moderate to high sulfur con-
tents also occur in the Eocene and the Coniacian (up to 2%). High
TS/Corg values seemed to be characteristic of the Eocene, with val-
ues up to 0.6 (Fig. 2a).
In the Santonian and Campanian samples, the sulfur content
was quite low, with values
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land-derived eroded material in the Tarfaya Basin during Turonian
and Cenomanian times. A higher amount of vitrinite/vitrinite-like
particles could be observed in samples of Eocene and Campanian
samples. The Santonian samples revealed a high amount of
opaque black particles, which represent strongly oxidized and
re-sedimented terrestrial OM. Coniacian samples revealed a highamount of liptinite, but also vitrinite/vitrinite-like particles. The
Albian samples contained predominantly vitrinite.
Characteristic microscopic observations for selected samples
are shown in Fig. 5 and listed in Table 2.
4.4. Molecular composition
Biomarker ratios were used to characterize the depositional
environment and the maturation range of potential source rocks.
The biomarkers evaluated were tri- and pentacyclic triterpanes,
as well as steranes (cf. Peters and Moldowan, 1991), with a focus
on evaluation of peak area ratios from tri- and pentacyclic terpanes
measured from the m/ z 191 trace and steranes measured from the
m/ z 217 trace (Table 3).The n-and iso-alkane patterns (Fig. 6), except for the Albian and
some of the Eocene samples, showed a clear dominance of short
chain (C15–C19) vs. long chain n-alkanes (C27–C31). In the Eocene
samples (e.g., 695), long and short chain n-alkanes occurred in
similar concentration. In contrast, the Albian sediments showed a
different pattern, characterized by a clear predominance of long
chain n-alkanes.
The abundance of pristane (Pr) and phytane (Ph) relative to the
n-alkanes was moderate to high, especially in Coniacian, Turonian
and Cenomanian samples. Samples of the Eocene, Santonian and
Albian samples showed a dominance of C17 over Pr and of C18 over
Ph (Fig. 7). In particular, the Cenomanian samples had a dominance
of Pr vs. n-C17. Pr/Ph for the Eocene samples revealed values up to
1.4; similar values were also given by Albian samples with valuesup to 1.2. Lowest values were given by Campanian samples, which
were 1 for nearly all samples(Table 4). Eocene, Campanian, Coniacian, and Santonian samples
had a wide range of values. A narrow range was given for the
Turonian (1.03–1.44) and Cenomanian (1.03–1.16) samples,
whereas highest values were for the Albian (1.85–2.85). The
OEP showed a similar pattern to the CPI: Eocene and Campanian
samples showed a wide variation. Samples of the Coniacian
(0.58–6.30), Turonian (1.94–4.7) and Cenomanian (1.13–2.01), as
well as the Albian ones (2.17–3.13) showed a narrower range
for the OEP values.
Tricyclic terpanes in the range C20–C28 were nearly absent from
all samples. Pentacyclic terpanes of the hopane series from C27 to
C35 were dominated by C30 hopane as well as C31 to C33 hopanes.
The 17a(H)21b(H) and 17b(H)21b(H) isomers dominated the m/ z
191 fragmentograms. In particular, C31–C33 hopanes were presentin the Coniacian, Cenomanian and Turonian samples, but were
not present or only at low concentration in the Eocene and Albian
samples. The norhopane (C29)/hopane (C30) ratio was
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5. Discussion
5.1. General sediment composition and depositional environment
TS was measured to provide insight into the depositional envi-
ronment and in particular the intensity of bacterial sulfate reduc-
tion (Berner, 1970, 1984). Under anoxic conditions dissolved
sulfate is reduced to H2S, which reacts with iron minerals to form
iron sulfides. TS vs. Corg ratio reflects the intensity of microbial sul-
fate reduction in OM decomposition, giving a qualitative indicationof the redox status of the environment of deposition. Berner (1970,
1984) found an empirical relationship between S content and C orgcontent, which is typical of most marine sediments deposited un-
der aerobic bottom waters (Fig. 2a).
Moderate or high TS content and TS/Corg values (Fig. 2a) are
characteristic of nearly all of our Eocene and Upper Cretaceous
samples. These moderate to high ratio values indicate, in general,
strong bacterial sulfate reduction. Very high TS/Corg values indicate
that more OM is consumed via sulfate reduction than under nor-
mal marine conditions (Berner, 1984). Visual analysis indicates sig-
nificant amounts of pyrite in the Eocene, Coniacian, Turonian and
Cenomanian samples, derived from bacterial sulfate reduction
and OM oxidation. This indicates that sufficient iron was available
to fix most of the sulfur in the form of iron sulfide. Littke et al.(1991a) and Lückge et al. (1996) showed that the consumption of
part of the metabolized OM during early diagenesis greatly influ-
ences the OM quality in shallow marine sediments. The bulk of
the Cretaceous and Eocene samples shows high Corg and TS con-
tents, but only moderate TS/Corg values. This might indicate that
these samples were deposited under high productivity conditions,
with bottom waters that were not anoxic, but only partly reduced
in molecular oxygen. Thus, sulfur uptake into the sediments oc-
curred only in the freshly deposited sediment, probably at a fewdecimetres below the sediment/sea water interface. In completely
anoxic waters, sulfide precipitation and sulfur uptake into OM
already starts within the water column, leading to high TS/Corgvalues (Sinninghe Damsté and Köster, 1998).
Interestingly, very high values also are typical of the Albian, but
at low Corg and low HI values. The latter indicate only poor preser-
vation of the primary OM, i.e. no anoxic bottom water. Another
explanation for the high TS/Corg values in these sediments would
be petroleum impregnation followed by (bacterial) sulfate reduc-
tion and petroleum oxidation. However, allochtonous solid bitu-
men was not observed in the samples in great quantity. Further
studies would be necessary to test this hypothesis of petroleum
impregnation, which is supported by high PI values for the Albian.
Further insight is provided by sulfur/iron values (Fig. 2b). Most
Eocene and Campanian samples plot close to the pyrite line, which
implies that essentially all iron is fixed in pyrite. Nevertheless, for
these samples, iron availability was not limiting with respect to
pyrite formation. Some Eocene and Campanian samples even have
excess iron, which could be fixed in minerals such as smectite,
chlorite and other clay minerals, or magnetite. In contrast, the
Cenomanian and Turonian samples clearly have excess sulfur
(Fig. 2b). This observation implies that not all the sulfur is fixed
in pyrite, as a result of limited availability of reactive iron. Most
of the excess sulfur was probably incorporated into OM.
V and Ni are common in marine carbonate source rocks
(Barwise, 1990). In our samples, the highest V/(V + Ni) values occured
in the Eocene, Cenomanian and Turonian samples, up to 0.89
(Fig. 3a, b), indicating a more marine than terrestrial milieu
(Barwise, 1990). An increase in V/(V + Ni) is often interpreted asan indication of anoxic bottom water conditions (Barwise, 1990;
Sundararaman et al., 1991). Most of our Eocene samples have
Ni/V values
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Turonian define trends, showing that, with an increase in silicate,
the content of OM also increases. This tendency is explained as a
consequence of a higher nutrient supply when the supply of clastic
material increased in otherwise carbonate-dominated environ-
ments (Stein and Littke, 1990; Thurow et al., 1992).
The Albian samples represent another system, with a high
proportion of silicate and a low amount of OM. Here, silicate-rich,
carbonate-lean sediments may represent times in which
bioproduction was significantly reduced; therefore, both OM and
carbonate were depleted. In the case of the Eocene, there is no clear
trend because of the occurrence of several siliciclastic and several
carbonate dominated samples. In these sediments, OM productiv-
ity increased with increasing nutrient supply, usually in combina-tion with a higher input of siliciclastics (Fig. 9). In contrast, most of
the siliciclastic Eocene sediments showed an increase in OM con-
tent with increasing carbonate content, probably reflecting the in-
put of algal/phytoplankton biomass, which was almost absent from
the pure siliciclastic sediments. Deposition may have occurred
when bioproduction was much reduced, so carbonate and OM
were depleted. Oxidation within these sediments further contrib-
uted to organic carbon loss.
Short chain n-alkanes (
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sedimentary OM. Most of the samples had Pr/Ph values 1 for
Eocene samples indicate a higher oxygen content in bottom
waters during deposition. The same holds true for Albian samples,
which showed a wider variation in Pr/Ph. Coniacian, Turonian and
Cenomanian samples exhibited a predominance of Ph. Santonian
samples showed a wider spread of values, possibly indicating a
variation in the depositional environment (Table 4).
Eocene OM in our samples comprises a mixture of marine and
terrestrially derived material. The dominance of the short chain
n-alkanes supports the assumption of mainly marine, aquatic OM
in the case of the Turonian and Cenomanian samples. In contrast,
the n-alkane pattern of Albian sediments indicates a predominance
of terrestrial OM, with little input of algae/phytoplankton material
(Fig. 6).
Trycyclic terpanes in the range C20–C28 are thought to represent
evidence for a contribution from higher plant material (Tissot and
Welte, 1984) but were only minor compounds close to the detec-
tion limit. The occurrence of hopanes reflects the contribution of
prokaryotic membranes in bacteria and blue-green algae (Tissot
and Welte, 1984; Moldowan et al., 1985). The high contribution
of C31–C33 hopanes in Coniacian, Turonian and Cenomanian
(Fig. 8) samples is an indicator for deposition under marine (shelf)
conditions because the C31–C33 homohopanes are derived from
bacteriohopanetetrol. Homohopanes were only detected in small
amounts in Eocene and Albian samples. Low homohopane index
values suggest that anoxic conditions did not occur during deposi-
tion, but suboxic to dysoxic bottom water may have existed (Peters
and Moldowan, 1991; Tyson and Pearson, 1991).High concentrations of C29 norhopane, as a result of preferential
preservation in the presence of sulfur, are frequently observed in
sediments with limited iron availability, such as carbonates (Blanc
and Connan, 1992). Neohop-13(18)-ene was detected in all sam-
ples, except from the Eocene (Fig. 8; Table 3), which is typical of
OM rich sediments (Greiner et al., 1977). It is assumed that such
components are derived from bacteria dwelling at or below the
chemocline, and that they may be used as indicators of water
bodies which were stratified in the past. Gammacerane, indicative
of a stratified water column, often accompanying hypersalinity,
was not detected.
The dominance of C29 steranes, especially in Eocene sample 705,
leads to the conclusion of a mainly terrestrial OM contribution.
Samples from the Cenomanian and Turonian mainly showed an in-
put of the C28 steranes associated with marine OM (Fig. 11). C27steranes were dominant in samples from the Coniacian, Turonian,
and Cenomanian, but not from the Eocene: This biomarker is most
probably derived from various types of marine algae.
5.2. Petroleum potential and maturity
The generative potential of sedimentary rocks is defined by Corgcontent and hydrogen richness, which is a function of kerogen type
and preservation. Sedimentary rocks with Rock–Eval S 2 values >5
and 10 mg HC/g rock are considered as having a good and very
good source potential, respectively (Peters, 1986). Given the rela-
tively low maturity of the Morrocan outcrop samples, the mea-
sured Corg and pyrolysate yields provide a close estimate of original generative potential.
The Eocene samples exhibited a wide variation in HI and OI val-
ues, with the lowest HI values correlating with samples lower in
Corg. The data fluctuation may be caused by the presence of re-
sedimented organic particles (in samples with S 2 < 5 mgHC/g rock)
and/or the moderate quality of amorphous OM even within the
Fig. 7. Pr/nC17 vs. Ph/nC18 for selected samples from various stratigraphic units in
the Tarfaya Basin (after Shanmungam, 1985).
Table 4
Hopane and sterane biomarker parameters.
Sample Age Hopanes Steranes n-and iso-
alkanes
22S/
(22S + 22R)
Ts/
(Ts + Tm)
Ts/
C3017aHopaneC29Hopane/
(C29Hopane + C29Ts)
17b,21aHopane/17a,21bHopane
Steranes/
17aHopanesC29aaa(S)/C29aaa(S + R)
20S/
(20S+ 20R)
Pr/
Ph
CPI OEP
683 Eocene – 0.41 – – – – – – 1.17 1.97 0.21688 Eocene – 0.16 – – – – – – – – –
690 Eocene – 0.11 – – – – –– – 0.78 1.18 1.83
705 Eocene 0.45 0.48 0.33 – 0.40 3.01 0.20 0.20 0.77 1.07 1.73
707 Eocene 0.55 0.44 – – – 3.86 0.23 0.24 1.33 1.19 2.09
728 Santonian – 0.74 – – – – – – 0.05 0.92 1.13
693 Coniacian 0.64 0.52 0.17 0.37 0.41 3.09 0.29 0.29 0.50 1.00 2.14
695 Coniacian 0.49 0.71 0.1 0.27 0.48 2.86 0.33 0.33 0.52 1.15 –
696 Coniacian 0.38 0.56 – – – 9.00 0.34 0.33 0.97 0.64 0.58
698 Coniacian – 0.86 – – – – 0.34 0.34 0.50 1.11 1.54
700 Coniacian 0.48 0.47 0.16 0.65 0.57 1.39 0.32 0.31 0.40 1.07 2.38
770 Turonian – 0.47 0.24 – 0.52 16.76 0.38 0.37 0.46 1.03 1.94
773 Turonian – 0.56 0.18 – 0.44 38.32 0.46 0.46 0.46 1.02 1.87
774 Turonian 0.51 0.53 0.17 – 0.50 1.49 0.37 0.37 0.39 1.11 4.10
775 Turonian 0.45 0.45 0.16 – 0.46 1.9 0.40 0.39 0.42 1.44 4.70
668 Cenomanian 0.47 0.78 0.21 0.45 0.49 3.92 0.41 0.41 0.44 – –
669 Cenomanian 0.57 0.58 0.14 0.41 0.54 5.07 0.47 0.47 0.58 – –
671 Cenomanian 0.55 0.40 0.05 0.37 0.53 3.19 0.50 0.50 0.50 1.03 1.45
755 Albian – 0.39 0.30 0.30 – – – – 1.22 2.17 3.13
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intervals with moderate Corg values. Similarly, higher HI values
(>300 mg HC/g rock) for the majority of the samples may reflect
better preservation of marine derived amorphous OM or a smaller
supply of terrestrial or inert OM (Farrimond et al., 1990). The ker-
ogen in Eocene samples is mainly Type II. Incident light microscopy
results indicate a high abundance of algal material (Fig. 5) as well
as some inertinite and re-sedimented vitrinite, consistent with a
Type II classification. The Rock–Eval maturity indicators, T max and
PI, showed an overall immature or early mature stage for the Eo-cene sediments on the coastal area of the Tarfaya Basin.
The Campanian samples were characterized by Type III kerogen,
based on the HI and OI values (Fig. 4a). This is supported by micro-
scopic observations, which revealed a high proportion of vitrinite
or vitrinite-like particles, T max and PI identifying this unit as imma-
ture with respect to oil generation.
According to the method of Langford and Blanc-Valleron (1990)
using a plotof Corg vs. S 2 (Fig. 4b), the OM of the Santonian samples
was allocated to Type IV kerogen, which implies no potential for oil
generation. The T max and PI values identify this unit as immaturewith respect to oil generation.
Fig. 8. Hopane (A–E) andsterane (F–J) distributions of selected samples from theTarfaya Basin (A andF, Eocene; B andG, Coniacian; C andH, Turonian; D andI, Cenomanian;
E and J, Albian). For peak identifications see Table 3.
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HI values for the Coniacian samples were moderate to high,
accompanied by low OI values and high Corg contents. Based on
these data and the petrological observations, the OM was allocated
to oil-prone Type II kerogen, with some outlier samples that
plotted between the trend lines for oil prone Type I and Type II
kerogens (Fig. 4a). T max values and vitrinite reflectance values
reveal an immature source rock.
Type I/II kerogen with a high potential for oil generation is pres-
ent in Cenomanian/Turonian samples. Plots of HI vs. OI and HI vs.
T max supported the latter observation (Fig. 4a and b; Table 1). Per-
sistently low T max values and a vitrinite reflectance of ca. 0.3, con-
firmed the immature stage (Table 2). Fluorescence microscopy
supports these data, because strong fluorescence was observed
(see Leine, 1986).The Albian samples contain Type III kerogen, having only a low
potential for oil generation, with low HI and S 2 values (Fig. 4b and
c) supporting the latter observation. The PI values for the Albian
samples were up to 0.6, indicating active petroleum migration
and sediment impregnation.
All the Lower Cretaceous, Jurassic, Carboniferous, and Devonian
samples had very low potential for oil or gas generation, based on
OM quantity and quality (Table 1; Appendix).
CPI and the OEP values support the above conclusion about
immature to early mature OM, except for some of the Eocene sam-
ples (Table 4). This is consistent with the Pr/n-C17 and Ph/n-C18 val-
ues, which are generally dominated by Pr and Ph ( Fig. 7). Only in
the cases of the Albian and Eocene samples did n-C17 and n-C18show a dominance, potentially indicating a more mature level.
Maturity as expressed by hopane isomerisation ratios [22S/
(22S + 22R)] of nearly all samples showed that the OM is immature
to marginally mature, as expressed in Cenomanian samples (Fig. 10a,
Table 4). The Ts/Tm ratio [18a(H)-22,29,30-trisnorneohopane(Ts)/17a(H)-22,29,30-trisnorhopane (Tm), Table 4] is dependent onboth thermal maturation and source lithology. During catagenesis,
Tm is less stable than Ts, so the ratio of Ts to Tm should increasewith maturity. This is also the case for the C29Ts/(C29 hopane +
C29Ts) equivalent. In our samples, both ratios show similar values
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Maturity information derived from typical sterane ratios shows
clear immaturity for all samples, i.e. the 20S/(20S + 20R) values
are all
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Fig. 13. Calculated generation rate (normalized in %; black lines;) vs. temperature for a geological heating rate of 3 C/Ma and calculatedvitrinite reflectance (thin black lines)
of selected samples from the Tarfaya Basin.
Fig. 14. Calculated predictions of kerogen transformation ratio (black lines) vs. temperature for a geological heating rate of 3 C/Ma and calculated vitrinite reflectance (thinblack lines) of selected samples from the Tarfaya Basin.
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Appendix A (continued)
Sample Age Outcrop Lat. Long. Corg (%) Ctotal (%) S (%) CaCO3 (%) Fe (%) V/(Ni+V)
694 Coniacian 2759.8640N 1207.2300W 2.31 11.30 0.93 74.86
695 Coniacian 2759.8650N 1207.2030W 5.21 10.81 46.61
696 Coniacian 3 2759.8640N 1207.1890W 6.99 14.00 1.53 58.39
697 Coniacian 2759.864‘N 1207.1890W 0.00
698 Coniacian 2759.865‘N 12
07.180
0
W 1.09 10.96 0.35 82.21699 Coniacian 2759.8620N 1207.1720W 7.02 11.89 1.25 40.59
700 Coniacian 2759.8590N 1207.1670W 4.33 10.08 47.86
701 Coniacian 2759.8580N 1207.1650W 5.63 12.24 55.08
702 Coniacian 2759.8620N 1207.1580W 4.58 12.48 65.80
703 Coniacian 2759.858‘N 1207.1520W 0.61 10.01 0.12 78.30
704 Coniacian 2759.8560N 1207.1470W 0.75 6.87 51.05
679 Coniacian 2650.5830N 1303.9260W 1.56 2.26 2.14 5.84
770 Turonian 2812.1610N 1146.8920W 8.54 12.14 2.01 30.03 0.89 0.75
771 Turonian 2812.1600N 1146.8900W 5.86 12.15 1.05 52.43
772 Turonian 2812.1670N 1146.8930W 7.79 15.10 1.14 60.89 0.28 0.62
773 Turonian 2812.1700N 1146.8930W 4.81 12.03 60.11
774 Turonian 2812.1710N 1146.9000W 5.06 12.88 0.85 65.11 0.19 0.54
775 Turonian 2812.1700N 1146.9020W 7.35 13.02 47.20
776 Turonian 2812.1720N 1146.9010W 3.90 12.38 0.49 70.63 0.26 0.57
709 Cenomanian/Turonian 2637.9920N 1158.7210W 0.05 11.27 93.44
710 Cenomanian/Turonian 2637.9960N 1158.7310W 0.08 12.04 99.62
711 Cenomanian/Turonian 2638.0010N 1158.7370W 0.34 11.04 89.15 0.27 0.17
720 Cenomanian/Turonian 2634.6620N 1214.7920W 0.12 11.97 98.69
721 Cenomanian/Turonian 2634.6810N 1214.7900W 0.07 9.49 78.45
722 Cenomanian/Turonian 2634.6810N 1214.6970W 0.32 10.28 82.98
730 Cenomanian/Turonian 2655.1750N 1212.0430W 0.06 11.08 91.84
731 Cenomanian/Turonian 2655.1780N 1212.0170W 0.18 12.06 98.95
732 Cenomanian/Turonian 2655.1440N 1212.0110W 0.24 10.85 88.38
733 Cenomanian/Turonian 2655.1440N 1212.0120W 0.41 2.19 0.04 14.86 0.75 0.26
734 Cenomanian/Turonian 2655.4930N 1212.5670W 0.26 9.19 74.39
735 Cenomanian/Turonian 2655.5100N 1212.5830W 0.29 9.93 80.30 0.38 0.26
736 Cenomanian/Turonian 2655.5470N 1212.5950W 0.27 11.18 90.88 0.14 0.28
777 Cenomanian/Turonian 2809.7520N 1138.2480W 0.07 0.09 0.14
778 Cenomanian/Turonian 2809.6180N 1138.2950W 0.12 9.61 0.05 79.13779 Cenomanian/Turonian 2809.5910N 1138.3150W 0.38 0.85 3.93
654 Cenomanian/Turonian 2802.4290N 916.1760W 0.04 0.05 0.11
655 Cenomanian/Turonian 2802.4100N 916.4340W 0.03 0.29 2.17
668 Cenomanian 2800.2750N 1228.6810W 14.48 17.21 22.74
669 Cenomanian 2800.2750N 1228.6760W 11.32 16.04 1.7 39.32 0.16 0.48
670 Cenomanian 2800.2740N 1228.6540W 15.57 18.79 2.33 26.82
671 Cenomanian 2 2800.2790N 1228.6450W 16.79 19.89 2.84 25.82
672 Cenomanian 2800.2850N 1228.6330W 10.05 17.50 2.16 62.06 0.11 0.53
673 Cenomanian 2800.2820N 1228.6200W 8.29 15.50 1.34 60.07 0.38 0.72
749 Albian 2817.1670N 1132.2910W 0.06 0.09 0.18
750 Albian 2817.4910N 1132.2880W 0.50 3.04 0.1 21.15
751 Albian 2817.6340N 1132.5920W 0.31 0.47 1.38
752 Albian 2817.6130N 1132.6330W 0.38 0.49 0.92
753 Albian 2817.6110N 1132.6250W 0.51 0.76 0.77 2.02754 Albian 2817.6110N 1132.6260W 0.57 0.67 1.34 0.82
755 Albian 2817.6120N 1132.6230W 0.65 0.87 0.73 1.87
756 Albian 2817.6090N 1132.6230W 0.28 0.30 0.16
757 Albian 2817.6070N 1132.6270W 0.33 2.81 0.52 20.63
758 Albian 2817.6130N 1132.6190W 0.27 3.68 0.08 28.45
759 Albian 2817.6090N 1132.6170W 0.41 2.53 17.59
667 Cretaceous 2806.3120N 918.5800W 0.04 0.11 0.64
708 Cretaceous 2637.7040N 1152.7870W 0.04 0.08 0.40
714 Lower Cretaceous 2701.8390N 1200.2750W 0.13 11.94 98.41
715 Lower Cretaceous 2701.8610N 1200.2790W 0.22 10.04 81.83
716 Lower Cretaceous 2701.8700N 1200.2870W 0.13 11.83 97.47
717 Lower Cretaceous 2707.2640N 1152.7540W 0.33 9.96 80.25
718 Lower Cretaceous 2707.2970N 1152.7350W 0.11 0.19 0.67
224 V.F. Sachse et al. / Organic Geochemistry 42 (2011) 209–227
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Appendix A (continued)
Sample Age Outcrop Lat. Long. Corg (%) Ctotal (%) S (%) CaCO3 (%) Fe (%) V/(Ni+V)
719 Lower Cretaceous 2707.3130N 1152.7520W 0.41 1.40 8.17
737 Lower Cretaceous 2707.1310N 1134.5710W 0.02 0.03 0.11
738 Lower Cretaceous 2707.1150N 1134.5950W 0.03 0.04 0.09
760 Lower Cretaceous 2816.3020N 1130.2650W 0.26 0.47 1.75
761 Lower Cretaceous 28
16.2690
N 11
30.2490
W 0.03 0.04 0.06762 Lower Cretaceous 2816.2710N 1130.2490W 0.05 0.05 0.02
763 Lower Cretaceous 2816.3190N 1130.2520W 0.38 3.33 24.56
764 Lower Cretaceous 2840.5690N 1107.4570W 0.26 6.37 0.44 50.85
765 Lower Cretaceous 2840.6350N 1107.3410W 0.24 4.85 38.44
766 Lower Cretaceous 2840.6220N 1107.3410W 0.16 3.41 27.10
767 Lower Cretaceous 2840.6220N 1107.3410W 0.42 5.16 0.11 39.53
768 Lower Cretaceous 2840.6160N 1107.3410W 0.39 5.66 43.88
769 Lower Cretaceous 2840.6000N 1107.3490W 0.42 1.86 12.02
780 Jurassic 3033.7630N 932.670W 0.07 9.31 76.95
781 Jurassic 3033.9620N 932.2530W 0.06 11.51 0.05 95.37
782 Jurassic 3035.3250N 931.8750W 0.09 7.70 63.39
783 Jurassic 3035.6070N 931.6660W 0.13 11.39 93.81
784 Jurassic 3035.8760N 930.5590W 0.06 7.53 0.10 62.24
785 Jurassic 3036.0070N 929.5350W 0.06 7.49 61.93
786 Jurassic 3036.1730N 929.6630W 0.12 4.28 34.69
787 Jurassic 3037.3530N 930.3200W
788 Jurassic 3037.7690N 930.6670W 0.04 6.16 0.07 50.91
789 Jurassic 3038.8860N 931.1760W 0.02 3.74 30.99
790 Jurassic 3039.4420N 928.9880W 0.08 8.62 71.13
791 Jurassic 3040.3430N 928.9400W 0.04 11.72 0.16 97.31
650 Namurian 2821.9250N 921.6310W 0.04 0.12 0.68
651 Late Namurian 2809.6740N 918.4700W 0.05 0.37 2.71
652 Late Namurian 2809.6740N 918.4700W 0.03 0.05 0.10
653 Late Namurian 2809.6740N 918.4700W 0.01 0.04 0.19
658 Vise 2822.9130N 929.3890W 0.50 0.61 0.94
644 Upper Carboniferous. Vise 2824.1060N 923.9400W 0.04 7.82 0.08 64.84
645 Upper Carboniferous. Vise 2824.1060N 923.9400W 0.05 11.84 0.14 98.25
646 Upper Carboniferous. Vise 2824.1060N 923.9400W 0.03 0.06 0.27
647 Lower Vise 2825.0450N 924.5980W 0.06 0.61 4.55648 Lower Vise 2831.4440N 924.0440W 0.13 0.17 0.34
641 Late Famennian/Tournai 2827.5290N 921.6890W 0.07 0.21 1.10
642 Late Famennian/Tournai 2827.5290N 921.6890W 0.02 0.57 4.59
643 Late Famennian/Tournai 2827.5290N 921.6890W 0.01 2.00 16.52
712 Devonian 2649.3140N 1146.9010W 0.04 11.88 98.59
739 Devonian 2701.4770N 1133.5360W 0.11 0.16 0.37
649 Upper Devonian 2831.4300N 924.0050W 0.38 0.46 1.55 0.69
659 Upper Devonian 2837.5090N 924.0200W 0.06 10.42 86.33
660 Upper Devonian 2837.5090N 924.0200W 0.06 0.12 0.54
661 Upper Devonian 2652.3900N 952.1390W 0.04 0.11 0.56
662 Upper Devonian 2652.3900N 952.1390W 0.02 0.07 0.44
663 Upper Devonian 2652.3840N 952.1480W 0.02 0.05 0.25
664 Upper Devonian 2652.4730N 952.2490W 0.05 0.11 0.48
665 Upper Devonian 2652.5060N 952.2340W 0.03 0.14 0.97666 Upper Devonian 2652.5060N 952.2340W 0.05 0.25 1.68
740 Upper Devonian 2635.0650N 1126.0560W 0.16 0.70 4.47
713 Middle Devonian 2648.9230N 1148.6780W 0.06 1.63 13.01
741 Middle Devonian 2634.0920N 1125.4170W 0.05 10.54 87.42
742 Middle Devonian 2634.1670N 1125.4170W 0.03 11.25 93.43
743 Upper Ordovician 2634.8880N 1125.6110W 0.03 1.32 10.69
744 Upper Ordovician 2635.1790N 1126.2800W 0.09 0.92 6.90
745 Upper Ordovician 2638.2200N 1126.6410W 0.04 1.22 9.79
746 Upper Ordovician 2640.8430N 1128.6740W 0.16 4.23 33.94
747 Upper Ordovician 2641.8470N 1132.2640W 0.04 0.06 0.17
748 Upper Ordovician 2643.3260N 1136.5530W 0.06 0.10 0.35
636 Upper Ordovician 2843.5980N 927.9040W 0.05 0.07 0.18
637 Lower Ordovcian 2849.7430N 931.6180W 0.05 0.08 0.21
(continued on next page)
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Associate Editor —C.C. Walters
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Appendix A (continued)
Sample Age Outcrop Lat. Long. Corg (%) Ctotal (%) S (%) CaCO3 (%) Fe (%) V/(Ni+V)
638 Lower Ordovcian 2849.7430N 931.6180W 0.05 0.12 0.56
639 Lower Ordovcian 2849.7430N 931.6180W 0.15 0.25 0.82
640 Lower Ordovcian 2843.6680N 927.9640W 0.01 0.02 0.04
656 Lower Ordovcian 2901.7270N 854.0640W 0.11 1.19 9.02
657 Lower Ordovcian 2901.727
0
N 854.064
0
W 0.12 0.99 7.25
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