sachse et al.2011-petroleum source rocks of the tarfaya basin and adjacent areas, morocco

8/19/2019 Sachse Et Al.2011-Petroleum Source Rocks of the Tarfaya Basin and Adjacent Areas, Morocco

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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.

210 V.F. Sachse et al. / Organic Geochemistry 42 (2011) 209–227


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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.

V.F. Sachse et al. / Organic Geochemistry 42 (2011) 209–227 219


12/19

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


13/19

Maturity information derived from typical sterane ratios shows

clear immaturity for all samples, i.e. the 20S/(20S + 20R) values

are all


14/19

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.



15/19


16/19

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


711 Cenomanian/Turonian 2638.0010N 1158.7370W 0.34 11.04 89.15 0.27 0.17







733 Cenomanian/Turonian 2655.1440N 1212.0120W 0.41 2.19 0.04 14.86 0.75 0.26





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



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







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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


764 Lower Cretaceous 2840.5690N 1107.4570W 0.26 6.37 0.44 50.85



767 Lower Cretaceous 2840.6220N 1107.3410W 0.42 5.16 0.11 39.53



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



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






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


713 Middle Devonian 2648.9230N 1148.6780W 0.06 1.63 13.01



743 Upper Ordovician 2634.8880N 1125.6110W 0.03 1.32 10.69







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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