Chemostratigraphy, tectonic setting and provenance of the Cambro-Ordovician clastic deposits of the subsurface Algerian Sahara

Chemostratigraphy, tectonic setting and provenance of the Cambro-Ordovician clastic deposits of the subsurface Algerian Sahara

Journal of African Earth Sciences 55 (2009) 158–174 Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: ww...

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Journal of African Earth Sciences 55 (2009) 158–174

Contents lists available at ScienceDirect

Journal of African Earth Sciences journal homepage:

Chemostratigraphy, tectonic setting and provenance of the Cambro-Ordovician clastic deposits of the subsurface Algerian Sahara Nordine Sabaou a, Hamid Ait-Salem b, Reda Samy Zazoun b,* a b

E.ON Ruhrgas UK North Sea Ltd., 29 Wilton Road, London SW1V 1JZ, United Kingdom Centre de Recherche et Développement-SONATRACH, Avenue du 1er Novembre, Boumerdès 35000, Algeria

a r t i c l e

i n f o

Article history: Received 25 March 2008 Received in revised form 21 March 2009 Accepted 9 April 2009 Available online 5 May 2009 Keywords: Chemostratigraphy Sandstone Cambro-Ordovician Reservoirs Fluvial transport Tectonic Algeria

a b s t r a c t The Cambro-Ordovician sandstones are one of the main oil reservoirs in Algeria. They are, in the most part a braided fluvial system and the reservoir is subdivided into four lithozones (R3, R2, Ra, Ri) based on grain size and wireline logs signatures. Geochemical analysis has been applied to these units with no or very poor biostratigraphic control for more detailed and accurate stratigraphy and correlations. Several Wells are selected from the northern part of the Saharan Platform. Geochemical data have been acquired from cores with 20 elements being determined. A new nomenclature of sequences or subunits, based on chemostratigraphy and sedimentology, is established from base to top for the Cambrian and the Lower Ordovician. The majority of sandstones have an average SiO2 content between 86.4 wt% and 96.8 wt%, i.e. quartz-rich. Cambro-Ordovician sandstones are therefore quartz arenites in their majority. The evolution of the two ratios SiO2/Al2O3 and K2O/SiO2, which are significant variables for differentiating between the Cambro-Ordovician units, display an increase in the maturity of sandstones from the base to the top. Ferro-magnesian content and trace elements such as Ni, Co and Cr show that a significant contribution from volcanic rocks is unlikely. Mineralogical maturity of the analysed rocks along with the occurrence of zircon is consistent with a felsic plutonic or reworked sedimentary source rocks. Sediments were derived mainly from deeply weathered cratonic landmasses or recycling sediments. A passive margin origin for the Cambro-Ordovician sandstones is indicated by the discriminant plot of K2O/Na2O vs. SiO2. The maturity of these sandstones is characteristic of cratonic environments, where sedimentary recycling is an important process. The tectonic setting and the provenance of Cambro-Ordovician clastic deposits conform to a passive margin setting. However, sequence isopachs and sedimentary architectures indicate that the Hassi Messaoud area was locally unstable during the Cambrian. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The tectonic evolution of the Algerian Sahara involved multiple periods of extension and compression ranging from the Pan-African orogeny (Bertrand and Caby, 1978), to the Alpine tectonic event (Follot, 1952; Boudjema, 1987; Makhous and Galushkin, 2003; Mitra and Leslie, 2003; Fabre, 1988, 2005). During most of the Early Palaeozoic, the Saharan Palaeozoic basins were part of a large, inter-connected North African shelf system that was in a sagging stage after an Infracambrian extensional phase (Craig et al., 2004). At the end of the Palaeozoic, collision between the Gondwana and Laurasian plates resulted in uplift of North Africa. Widespread erosion followed, resulting in the formation of the Hercynian unconformity (Follot, 1952; Underdown and Redfern, 2007), which cut deep into the Palaeozoic section (Haddoum et al., 2001; Zazoun, 2001; Mitra and Leslie, 2003). A number of * Corresponding author. E-mail addresses: [email protected] (N. Sabaou), [email protected] (R.S. Zazoun). 1464-343X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2009.04.006

previous studies relating to the tectonic setting of the Cambro-Ordovician clastic deposits have been carried out (Debyser et al., 1965; Beuf et al., 1971; Rognon et al., 1972; Fabre, 2005). The Cambro-Ordovician sequence is one of the main reservoir intervals in Algeria. It was deposited either directly on igneous and metamorphic basement or, in some cases, onto sedimentary rocks of Precambrian age. The Cambro-Ordovician tends to be barren of fossils and, as a result, precise biostratigraphic determinations are impossible. Early workers in the area tried to integrate outcrop and borehole lithostratigraphy to help understand the geology of the newly discovered oil reservoirs (Gevin, 1960; Legrand, 1962; Legrand and Nabos, 1962; Freulon, 1964; L’Homer, 1967), while the first major synthesis of the Cambro-Ordovician was undertaken by Beuf et al. (1971). Later work on the sedimentology and reservoir geology of the Cambro-Ordovician of the super giant Hassi Messaoud oil field and the surrounding Saharan Platform (Balducchi and Pommier, 1970; Aliev et al., 1971; Whiteman, 1971; Ali, 1973; Chiarelli, 1978). The most recent publications have used palynology and sequence stratigraphic approaches to try and correlate the Cambro-Ordovician sequences (Legrand, 1985; Fekirine


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and Abdellah, 1998; Vecoli et al., 1995; Vecoli and Playford, 1997; Vecoli et al., 1999; Vecoli, 2000; Carr, 2002; Ghienne et al., 2007). The first biostratigraphic evidence of Middle to Late Cambrian sedimentation in the Algerian subsurface resulted from the palynological analysis of pre-Ordovician clastic sequences penetrated by borehole AMG-1 in northwestern Algeria (Vecoli et al., 2008). Despite the efforts of many workers, the common correlation scheme of the Cambro-Ordovician in Algeria is based on a subdivision into four informal lithozones which are from base to top: R3, R2, Ra (anisotropic reservoirs-supposed Lower Cambrian) and Ri (isotropic reservoir-supposed Ordovician). These subdivisions are based on grain size sorting and wireline log signatures.

tion of its application and utility in a variety of circumstances are given in Pearce and Jarvis (1995), Roser et al. (1996), Preston et al. (1998) and Pearce et al. (1999). Applications of chemostratigraphic techniques to correlate the Cambro-Ordovician of Algeria have been undertaken at the Centre of Research and Development of Sonatrach since 1993. This paper describes the results of a chemostratigraphic study of the Cambro-Ordovician sequence in seven wells from oil and gas fields in Algeria (Hassi Messaoud, Hassi R’Mel, Ain Romana and Hassi Touila). The location of these fields is shown in Fig. 1. Previous studies on sandstones (Al-Gailani, 1980; Bhatia, 1983; Bhatia and Crook, 1986; Ehrenberg and Siring, 1992; Roser et al., 1996; Ratcliffe et al., 2004) but very few concentrated on its use in hydrocarbon exploration and production (Ratcliffe et al., 2002; Pearce et al., 2005). The common subdivision of the Cambro-Ordovician in Algeria into R3, R2, Ra and Ri units is difficult to use for a productive field. Chemostratigraphy provides a complementary tool along side sedimentology and petrography. According to Wright et al. (2006), hydrocarbons reservoirs within low accommodation fluvial depositional environments are notoriously difficult to understand in a rigorous stratigraphic framework (Ratcliffe et al., 2004, 2006). However, in order to exploit these reservoirs it is essential to have high resolution stratigraphic correlations

Chemostratigraphy involves the application of whole rock geochemical data to the characterization and correlation of sediments in petroleum basins. The geochemistry of sediments is highly variable and is sensitive to subtle changes in composition (Wright et al., 2006). Inorganic geochemistry can be useful when applied to sequences with very poor biostratigraphic control. The technique has been extensively employed by the petroleum industry, but only a few studies have been published. Descrip-





Melghir Trough

Tilghemt Dome


+ ++ + + +++







el gu Ba El de




Hoggar Tuareg Shield

50 Km


ur de




-E lB




E ae l B oh io ig d h) Ar ch id


gu m A


IDje rane - Az (Pa zaz - M laeo 'Z high ab - A ) rch


West African Craton (Reguibat Shield)

Main Faults




400 Km

Fig. 1. Regional map showing location of study area (after unpublished Sonatrach reports, modified).



Main Faults

Hassi R'Mel Hassi Messaoud








Gassi Touil




a s A t l u t hO S o ug



++ + + +

s ux la tea At n t Pla a h n r Hig o ha r Sa F

s s rè Au ntain u Mo


Te l l i a n A t l a s



El Agreb



0° Algiers


Hassi Messaoud Ridge

HASSI TOUILA Well To-1 + + + +


+ ++









Telemzane Arch (Palaeohigh)



1.1. Objectives and purpose



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and these are expected to allow a better understanding of the reservoir distribution and architecture. 2. Geological framework The Saharan Platform constitutes a broad intra-cratonic basin in North Africa. It is bounded to the north by the Atlas Mountains and to the southwest and south by the West African Craton and the Hoggar (Tuareg) Shield (Fig. 1). The Hassi Messaoud field is a giant Cambrian oil field situated in the north central of the Saharan Platform on a NE–SW oriented basement arch, which is the northern extension of the Amguid-El Biod Arch (Boote et al., 1998). This Arch is flanked by deep N–S to NE–SW Pan-African fractures, which extend in to the Hoggar basement (Tuareg Shield) (Fig. 1). In addition, E–W trending faults were also identified (Guehria et al., 2005). Pan-African tectonics ended in the Late-Precambrian followed by a regional subsidence, a local intracratonic rifting and a synchroneous erosion period (Coward and Ries, 2003). Isopachs for Cambrian strata mimic the basement structures and presumably define areas of Upper Pan-African rifting. The Upper Pan-African and Cambrian faulting favoured the development of half-grabens. There was a pulse of minor inversion during the Ordovician, with further extensional rifting in the Silurian. Presumably the rifting was initially associated with collapse of the Pan-African Mountain belt in Late Precambrian–Cambrian, causing perturbations in the mantle lithosphere (Coward and Ries, 2003). The Hassi R’Mel field is a giant Triassic gas field situated northwest of Hassi Messaoud. Hassi R’Mel lies on the culmination of the broad regional Tilghemt Dome, which is the northern extension of Idjerane-Azzaz-M’Zab Arch and the western part of the Talemzane Arch. Ain Romana and Hassi Touila are small Cambro-Ordovician oil accumulations situated east of Hassi Messaoud in the Berkine

Basin. The Cambro-Ordovician lithological succession that is found in all four fields is described below with reference to the commonly used lithostratigraphic zonation. This zonation is summarised in Fig. 2, together with the chemostratigraphic zonation discussed in this paper. The supposed Cambrian comprises medium to coarse-grained sandstones and clays. – The R3 sandstones at the base are coarse-grained, poorly bedded and very poorly sorted with angular quartz grains; diagenetic illite is present which is progressively replaced by kaolinite towards the top of the interval. Quartz and siderite cements are present in varying amounts. The thickness varies from 150 m in the Hassi R’Mel Field to 250 m at Hassi Messaoud. Sedimentary structures, such as sets of planar cross bedding, indicate that Unit R3 represents alluvial sediments. It was deposited on porphyritic granites (e.g. in Wells Md-2 and Om-1 in Hassi Messaoud and Well Hr-4 in Hassi R’Mel), or on red/purple shales of possible Early Cambrian age (e.g. in Well Omg-57, Hassi Messaoud) (Fabre, 2005). Geochronometric determinations carried out by Mobil Field Research Laboratory on the granites gave an age Precambrian–Early Cambrian (560 ± 25 Ma). – The R2 sandstones overlie the R3 unit. They are medium to coarse-grained, poorly sorted, well bedded, with subangular to rounded quartz grains and are 30 m to 100 m thick. Illite is the dominant cement. Partially dissolved feldspar is common and contributed to the formation of diagenetic illite (Djarnia and Fekirine, 1998). Sedimentary structures consist of planar–tabular cross bedding and trough cross-beddings with minor ripple cross-laminations. – The overlying Ra unit is mainly composed of quartz arenite sandstones with the appearance of Skolithos towards the top. Grain size is finer than in R2, sorting is better, and quartz grains are subrounded to rounded (Fig. 3) (Bessa, 2004). Quartz overgrowth is the main cement, and illite and kaolinite are abundant. The average thickness ranges between 20 m and 100 m. Major sedimentary structures include planar–tabular cross bedding and trough cross bedding. Ripple cross-laminations are also important and more marine influence is observed at the top with occurrence of Skolithos. This unit is considered as a transitional facies from continental to shallow marine. This marine influence presages a widespread transgression over the Saharan Platform during the Early Ordovician. Unit Ra constitutes the principal reservoir in Hassi Messaoud and Ain Romana region. In the absence of fractures, this reservoir would be classified as being tight. Instead, the Cambrian Ra is considered to be a naturally fractured reservoir (Guehria et al., 2005). – Unit Ri, considered to be Ordovician, unconformably overlies the Cambrian deposits. It consists of poorly bedded, massive and structureless quartzites at the base, passing upwards into quartz arenites interbedded with shales towards the top. The sandstones are well sorted and the quartz grains well rounded, silica (quartz and chalcedony) being the main cement with minor illite and kaolinite (Djarnia and Fekirine, 1998; Bessa, 2004). The abundance of Skolithos burrows and the occurrence of invertebrates such as lingula in these rocks suggest a shallow marine depositional environment. A summary of the sedimentological and mineralogical characteristics of the Cambro-Ordovician succession is given in Table 1.

3. Geochemistry of the Cambro-Ordovician sandstones 3.1. Analytical methods and data acquisition

Fig. 2. General stratigraphic column of the Cambro-Ordovician.

Chemostratigraphy, or chemical stratigraphy, involves the characterization and correlation of strata using major and trace


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Fig. 3. (A, B, C and D): photomicrographs showing the common modes of occurrence of quartz cement in Hassi Messaoud sandstones (After Bessa, 2004). The micrograph (A) illustrates syntaxial overgrowths (black arrows). Note well-developed dust-rims (black arrows) (Well OMJ 822, 3382.50 m, LN 4). (B) Micrograph in same field of view as (A) showing crystallographic continuity between host grain and cement micrograph (Well OMJ 41, 3439.50, LN 4). (C) Micrograph of small euhedral prismatic quartz crystals projecting into moldic macroporosity created by the dissolution of unstable frame work grain (Well OMJ 822, 3424.50 m, LN 4). (D) Micrograph illustrates quartz cement in partially healed fracture (Well OMJ 422-3413.50 m, LN 4) (E, F, G and H): photomicrographs illustrating at the pore level, quartz cement distribution and abundance is controlled by the nature of substrate grains and the presence of clays cements (After Bessa, 2004). monocrystalline quartz grains [M] have well developed overgrowths (black arrows) (photomicrographs E and F) (Well OMJ223-3461.50 m, LP and LN 40). Whereas polycrystalline quartz [P], has complete poorly developed overgrowths center (micrograph G) (Well OMJ41, 3421 m, LN 40). (H) In samples contain illite, kaolinite or dickite cement quartz overgrowths terminate when they reach the kaolin cement or illite (black arrows), whereas in clay free pores quartz cementation continued until the pores were completely occluded (white arrows) (Well OMJ-24, 3479 m, LN 40).

Table 1 Depositional environments, sedimentological and mineralogical characteristics of the Cambro-Ordovician sandstones. Units


Sedimentological characteristics


Geochemical characteristics

Depositional environment




Kaolinite, illite, rare chlorite. Quartz and illite rich-sands, kaolinite, and rare chlorite. Very rare feldspars Quartz and kaolinite rich-sands. Illite and rare chlorite. Rare feldspars

Very high SiO2/Al2O3, very low K2O/ SiO2 in G1. High Cr content and low TiO2 content in G1. It is the opposite in G2 Very high SiO2/Al2O3, very low K2O/ SiO2. High Cr content. Low TiO2 content, except for B1

Upper shoreface to lower shoreface




Illite rich-sands and kaolinite, feldspars

High K2O/SiO2, low SiO2/Al2O3. High Cr content. TiO2 is slightly lower

Braided-fluvial deposits with minor marine influence Fluvio-deltaic deposits



G2-Quartzitic sandstones and siltstones interbedded with shales, Skolithos and bioturbation. Well-sorted. G1-Massive and compact fine-grained quartzites, wellsorted, structureless, abundant Skolithos Quartz arenites interbedded with thin shales (1– 20 cm). Cross-bedding and Skolithos in the upper sequences. Erosive base and several disconformities. Poorly-sorted Argillaceous medium to coarse-grained quartz-arenites interbedded with thin layers of shales. Cross-bedding and no Skolithos. Poorly-sorted Subarkosic and argillaceous coarse-grained sandstones interbedded with thin layers of shales. Cross-bedding and no Skolithos. Poorly-sorted

Illite rich-sands and minor kaolinite, siderite, feldspars are important at the base

High K2O/SiO2, low SiO2/Al2O3 and Cr content. TiO2 is slightly higher

Fluvio-deltaic deposits

element geochemistry (Ratcliffe et al., 2004). Our study involved in seven wells from four oil and gas fields. Four wells are from Hassi Messaoud (Wells Md-28, Md-81, Md-72 and Md-64), one from Hassi R’Mel (Well Hr-4), one from Ain Romana (Well Anr1) and one from Hassi Touila (Well To-1). Major and trace element contents were determined using inductively-coupled plasma and atomic emission spectroscopy (ICP–AES). This analytical technique has been utilised to acquire data for 10 major elements (SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O and P2O5) and 10 trace elements (Ba, Co, Cr, Cu, Ni, Pb, Zn, Sr, V, Li), although not all trace elements were analysed in all of the samples (Table 2). The major element data are expressed as weight% oxides and the trace element data are expressed as (lg g1 (ppm). The number of samples analysed for each well ranges between 23 and 163. Samples of 200–300 g were collected from cores of the CambroOrdovician sandstones and 0.25–1 g was then taken from these samples for analyses. For example, heavily carbonate-cemented samples contain much higher CaO than uncemented intervals

and sandstones, which contain small mudclasts might yield an anomalous Al2O3 content. In order to concentrate on regional geochemical trends, samples of this nature were excluded. All analyses were carried out at the Department of Geochemistry of the Centre of Research and Development of Sonatrach. 3.2. Geochemistry Table 2 displays average chemical values and standard deviation for the samples analysed. The stratigraphical distributions of elements allow to distinguish several sequences and boundaries within the Cambro-Ordovician (Figs. 4 and 5). Lithostratigraphic boundaries have been identified from the chemical logs, and a new nomenclature of sequences has been established. The R3 unit has a high K2O/SiO2 ratio due to illite and feldspar presence (Table 1). The R2 unit with the same characteristics as the R3 except the grain size decreases toward the top of the unit. The Ra is defined by sequences B, C, D, E, and F. The B and the C sequences are cyclic and


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Table 2 Average geochemical composition and standard deviation of the Cambro-Ordovician sandstones. Hassi R’Mel

Hassi Messaoud Well Hr-4

Number of samples Major elements (wt%) SiO2 TiO2 Al2O3 Fe2O3* MnO MgO CaO Na2O K2O P2O5

Well Md-28


91.3 0.03 2.85 1.38 0.009 0.1 0.12 2.06 2.1 nd

±Std 5 0.02 3 1.13 0.02 0.15 0.52 2 1.28 nd

Trace elements (ppm) Ba Co Cr Cu Ni Pb Zn Sr V Li

nd nd nd nd 33 nd nd 57 8 nd

nd nd nd nd 52 nd nd 50 9 nd

SiO2/Al2O3 Cr/Ni

1.64 nd


Well Md-64


Well Md-72


95.4 nd 1.89 0.36 0.028 0.035 0.05 1.84 0.47 nd

±Std 4 nd 1.8 0.53 0.08 0.08 0.06 3 0.52 nd

nd 16 82 6 20 nd nd 68 20 25

nd 11 65 4 10 nd nd 142 15 77

1.86 4.78

Well Md-81


95.9 nd 1.7 0.62 0.02 0.04 0.11 0.32 0.23 nd

±Std 2.6 nd 2.6 0.36 0.02 0.03 0.21 0.37 0.24 nd

nd 5 146 17 11 nd nd 14 16 8

nd 7 121 123 5 nd nd 53 13 14

1.97 9.71


96.8 nd 1.12 1.07 0.074 0.08 0.11 0.59 0.23 nd

±Std 3 nd 2.4 0.8 0.01 0.13 0.07 1.2 0.46 nd

nd 75 617 63 98 nd nd 23 11 nd

nd 61 412 32 17 nd nd 23 25 nd

2.18 6.11

86.4 0.41 8.15 0.96 0.004 0.29 0.17 2.43 1.6 nd

±Std 7.8 0.47 6.25 1.78 0.01 0.48 0.46 2.84 1.79 nd

nd 26 223 7 26 nd nd 75 38 58

nd 10 241 6 20 nd nd 131 39 112

1.15 11.44

Ain Romana

Hassi Touila

Well Anr-1

Well To-1



90.9 0.21 2.25 1.11 0.014 0.16 0.21 0.25 1.51 nd

±Std 3.5 0.2 1.3 0.7 0.01 0.1 0.1 0.09 1 nd

92.5 0.13 2.77 1.28 0.02 0.14 0.39 0.21 1.84 0.73

±Std 5.7 0.2 2.9 0.7 0.01 0.08 0.1 0.1 1.6 0.3

nd nd 341 139 505 nd nd 0.04 61 nd

nd nd 185.6 246 1030 nd nd 0.42 71.7 nd

252 453 269 65 307 191 97 14 72 nd

167 337 103 67 197 54 84 19 68 nd

1.73 18.34

1.67 10.14

Fe2 O3 : total iron; nd: not determined; Std: Standard deviation.

Fig. 4. Geochemical profiles: determination of unconformities/disconformities (A) and sequence boundaries (B); examples from Hassi Messaoud (Md-81) and Hassi R’Mel (Hr-4).

characterised essentially by a very high SiO2 and low K2O content. This is due to quartz coating. The abundance of kaolinite in this

unit is the result of K-feldspar dissolution. The G1 and G2 sequences define the Ri. The G1 is a quartz arenite with a low K2O

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content whereas the G2 argillaceous sandstones display higher K2O and lower SiO2 content. The sedimentological and geochemical


characteristics of the defined Cambro-Ordovician sequences are summarised in Table 1.

Fig. 5. Major and trace elements geochemical profiles from Ain Romana (Well Anr-1).

Fig. 6. Characterization of sandstones maturity using the evolution of K2O/Na2O, K2O/SiO2 and SiO2/Al2O3 ratios with depth. Example from Hassi R’Mel (Well Hr-4).


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3.3. Major elements Quartz arenites are defined as having log (SiO2/Al2O3) greater than 1.5 (Pettijohn et al., 1972; Herron, 1988). SiO2 is mainly in quartz whereas Al2O3 is traditionally linked to clay minerals. The majority of the sandstones analysed have an average SiO2 content between 86.4 wt% and 96.8 wt%, i.e. quartz-rich (Table 2). Silica (quartz) enrichment reflects mineralogical maturity and can be represented by SiO2/Al2O3 (Fig. 6); the upper sequences are very mature especially sequence G1 which comprises a highly siliceous quartzite. The ratio increases as quartz is progressively concentrated at the expense of less resistant phases during transport, weathering and recycling (Maynard et al., 1982; Roser and Korsch, 1986). A decrease in SiO2 values (Well Hr-4 and Md-81) is probably caused by pervasive carbonate cement (calcite, siderite and dolomite). The tops of unconformities/disconformities are generally marked by a decrease in Al2O3 percentages, followed by a sharp increase due to the concentration of alumino-silicates in the beds (Al-Gailani, 1980). This is clearly seen at the unconformity between the Cambrian and the Ordovician (Figs. 4 and 5), and possibly presents at the interpreted disconformities at the tops of R3 and R2. Al2O3 displays a negative correlation with SiO2 in all wells, i.e. r = 0.88 in Well Md-72, r = 0.85 in Well Md-64 and r = 0.79 in Well Hr-4 (Fig. 7). The ratio K2O/Na2O reflects the proportion

of potassic feldspar to plagioclase as well as the compositions of the feldspars themselves, and decrease in the percentage of unstable sodium feldspar can be taken to reflect mineralogical maturity. The K2O/SiO2 ratio typically shows an evolution from arkosic (sequence A through sequence A5) to quartzitic deposits (sequence B1 through sequence G2) (Fig. 6). The petrographic analyses confirm this upward increase in maturity, showing a depletion of Kfeldspar and total disappearance of plagioclase from base towards top, a change in clay mineralogy (illite to kaolinite), and an increase in roundness of grains. This effect, is however, overprinted and accentuated by diagenetic processes that result in an increase in silica cement towards the top. The increased maturity of the sandstones in the upper sequences of the Cambro-Ordovician is well represented by the evolution of the two ratios SiO2/Al2O3 and K2O/Na2O. These are the most significant variables for differentiating between the different Cambro-Ordovician units. Very low contents of Fe2 O3 þ MgO, which range between 0.4 wt% and 1.48 wt% together with low TiO2 (0.03 wt% to 0.41 wt%) show that there is unlikely to be a significant contribution from volcanic rocks. TiO2 is commonly represented by minerals like rutile and leucoxene, which have been identified in thin sections, but the lower positive correlation between TiO2 and Al2O3 (r = 0.53 both in Well Md-81 and Hr-4), suggests that an important amount of TiO2 is incorporated in clay fractions and

Fig. 7. Negative correlation between SiO2 and Al2O3.

Fig. 8. Positive correlation between TiO2 and Al2O3.

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associated phases and, therefore, that TiO2 has been concentrated during weathering processes rather than in detrital heavy mineral grains (Fig. 8). The chemical index of alteration (CIA) was suggested by Nesbitt and Young (1982, 1984) as a method to quantify the degree of weathering to which rocks have been subjected and is used as a general guide to the degree of weathering in the provenance regions. This criterion is based on the formula:

CIA ¼ ½Al2 O3 =ðAl2 O3 þ CaO þ Na2 O þ K2 OÞ  100½1 where CaO* represents CaO in silicate minerals only. CIA values range from 50 or less for fresh rocks (unweathered source areas) and 76–100 for sediments derived from intensively weathered rocks and composed of secondary minerals such as kaolinite. The CIA was calculated for five wells from Hassi Messaoud (Md-28, Md-81), Hassi R’Mel (Hr-4), Ain Romana (Anr-1), and Hassi Touila (To-1) and is plotted vs. depths (Fig. 9). The graphs portray an evolution of CIA content from base to top. Upper sequences (B, C, D, E, and F & G) from wells Md-28 and Md-81 are extremely weathered with an average value of about 70. The same package of sequences from Anr-1, To-1 and Hr-4 are less weathered with less than 60 as


an average value. Sequence B contains the lowest values of CIA in the upper sequences. For the lower sequences (A, A1, A2, A3, A4, A5) the values are very low (50 for Anr-1 and 40 for Hr-4). Ternary diagrams are used in order to compare the effects of weathering and sorting (Fig. 10). The bulk compositions are plotted and related to the mineralogy of sandstones, showing molar proportions, Al2O3– (CaO* + Na2O)–K2O, where CaO* represents silicates only. Nesbitt and Young (1982, 1984, 1996) and Nesbitt et al. (1996) used this diagram to predict weathering trends. Bulk compositions of sandstones from Well Md-28 portray a slope parallel to a predicted weathering trend from the Na2O + CaO* apex towards the Al2O3 apex. Well Md-81 show more intensely weathered samples, which are plotted towards the Al2O3 Apex. This reflects the preponderance of aluminous clay minerals within the sandstones. Sediments from Hr-4 are less weathered and tend towards a K-feldspar pole rather than a plagioclase pole. The bulk compositions from Wells Anr-1 and To-1 are closer to the K-feldspar pole. There are regional variations in chemical compositions of the Cambro-Ordovician sandstones between the eastern and the western regions. The general trend of the Cambro-Ordovician bulk compositions is indicative of the dominance of K-feldpars and illite as secondary minerals rather

Fig. 9. Evolution of Chemical Index of Alteration (CIA) in Cambro-Ordovician sandstones.


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Fig. 10. Al2O3–Na2O + CaO–K2O diagram from Nesbitt and Young (1982, 1984), showing the trend defined by analyses of samples from Cambro-Ordovician sandstones. Ka = kaolinite, Cl = chlorite, Gi = gibbsite, Sm = smectite, IL = illite, Pl = plagioclase, Ks = K-feldspar.

than plagioclases and confirms also that the sandstones contain an appreciable amount of aluminous phases. Plagioclase is altered to kaolin primarily, and K-feldspar to illite, and perhaps to kaolinite or other clay minerals (Grant, 1963; Nesbitt and Young, 1984, 1989; Bessa, 2004). 3.4. Trace elements Trace elements (Vanadium, Nickel, Copper, Chromium and Boron) have been used to characterise the evolution of the continental crust, and to analyse provenance and tectonic setting in clastic sediments (Van De Kamp et al., 1976, 1994; Taylor and McLennan; 1985; Bhatia and Crook, 1986; Bock et al., 1998). Trace elements analysed for this study and their average concentrations are given in Table 2. An increase in Ni concentration towards sequence boundaries may be due to enrichment in pyrite. Pyrite is well represented throughout the wells both in sandstones and mudstones. The occurrence of pyrite as framboids cubes minerals or cement is of post-depositional origin. The variation in Ni is probably more controlled by the pyrite content than by volcanogenic source materials. The same variation of Ni, Co and Cr with depth is seen at Hassi R’Mel, Hassi Messaoud and Ain Romana (Fig. 11). These three elements are highest in the eastern part of the area (Ain Romana and Hassi Touila). The Cr and Co contents in this region may suggest an acidic weathered material with mafic influences; Cr could also be associated with disseminated magnetite. Ni and Co contents correlate well. The range of the coefficient of correlation is from 0.52 at Md-64 to a more significant 0.83 at Md-28 (Fig. 12). On the geochemical profiles, they follow the same trend (Fig. 11). As mafic minerals are the least stable in the weathering and transport processes and largely go into solution, some of these elements are found in the clay minerals (Van De Kamp et al., 1976, 1994). The nature of the positive correlation between Ba and K2O (r = 0.6) in Well To-1 is not clear, it may prove that Ba is associated with feldspar component or is the product of drilling additives in cores. It may also be linked to barite and clay minerals.

4. Classification of sandstones The significance of silica, alumina, oxides, alkali and magnesia was discussed by Pettijohn et al. (1972), Blatt et al. (1980) and Herron (1988) in order to geochemically classify sandstones and shales. A scattergram from Herron (1988) is used for the Cambro-Ordovician sandstones (Fig. 13). Herron (1988) distinguished subtle compositional classes such as subarkoses from arkoses and sublitharenites from litharenites. Herron (1988) showed that

this system provides more accurate classification of sandstones than the graph log (K2O/Na2O) vs. (SiO2/Al2O3) with compositional fields, proposed by Pettijohn et al. (1972). A comparative study between the two methods was done by Lindsey (1999).Two wells were chosen to assess the value of this technique, Ain Romana (Anr-1) and Hassi Messaoud (Md-72). The whole analysis were plotted in scattergram log (SiO2/Al2O3) vs. log (Fe2 O3 =K2 O). The diagram apparently demonstrates more mature quartzarenites in Well Anr-1 than in Well Md-72; however this may be due to the presence of unusually high amounts of clay and siderite cements or other constituents, which could lead to an anomalous bulk composition and therefore plotting in unexpected compositional fields such as wacke and litharenite. The other wells were also plotted in the same diagram and they show the same trend. The evolution from subarkoses at the base to mature quartzarenites at the top is confirmed by petrography. Both K-feldpars and plagioclase tend to disappear upsection. Likewise the roundness of quartz grains and other heavy minerals increases and the clay minerals are dominated by kaolinite.

5. Sequences and depositional environment Seventeen depositional sequences have been identified using petrography, sedimentology and chemostratigraphy, correlation was achieved over 600 km from Hassi R’Mel to Hassi Touila (Fig. 14). The oldest sequences are represented by very poorly sorted medium to coarse-grained sandstones; they are from base to top A, A1, A2, A3, A4 and A5. When compared to overlying sequences, these sandstones are characterised by high K2O/Na2O and K2O/SiO2 and low SiO2/Al2O3. The geochemical composition is reflected by their mineralogy, which is composed mainly of quartz and leached feldspar, with illite as cement. Several major unconformities/disconformities were recognised within these sequences. At Hassi R’Mel the sediments overly porphyric granite, weathered at its top to saprolite. This Pan-African unconformity is observed throughout the Saharan Platform. These granites were dated as Precambrian/Early Cambrian (560 ± 25 Ma) by Mobil Research Laboratory, and overlying strata are thought to be Cambrian or younger in age, although devoid of fauna and flora. The depositional environment is thought to be a braided fluvial system, for the majority of the sequences. The upper part of the Cambrian is better known as it represents the main reservoir in the Cambrian petroleum system. All wells reach this reservoir and it has been extensively cored. Based on sedimentology and geochemistry, nine depositional sequences were recognized. These are, from base to top, B1, B2, C1, C2, C3, C4, D, E, and F. These sandstones are

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Fig. 11. Evolution of Ni, Co and Cr with depth in four wells of Hassi R’Mel (Hr-4), Hassi Messaoud (Md-64), Ain Romana (Anr-1) and Hassi Touila (To-1).

medium to fine-grained, sometimes coarse-grained. They are poorly sorted, with quartz as the dominant mineral. Leached feldspars are present but are less important than in the underlying sequences. Clay mineral cements are predominantly kaolinite with subordinate illite. The geochemistry is different from the underlying sequences and associated with mineralogical changes. Chemical content in SiO2, Al2O3 and K2O are characteristic. Thus SiO2/ Al2O3 is very high as consequence of the sandstones being

quartz-rich. The cement is dominated by quartz coating or kaolinite. The low content in K2O/SiO2 is related to a decrease in K-feldspar and illite cement. The sequences B1 and B2 show a fining-upwards trend, and consist of coarse-grained channelized sandstones, interpreted as braided-fluvial deposits. Sequences C1 to sequence F show cyclicity, an increase in interbedded shales, less coarse-grained sandstones, less feldspar as well as the occurrence of Skolithos, which


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Fig. 12. Positive correlations between Ni and Co in Hassi Messaoud and Hassi Touila Wells.

Skolithos and intense bioturbation; it is interpreted as a lower shoreface. Geochemically, the G2 sandstones contain greater amounts of Al2O3, K2O and MgO than sequence G1. The thickness of this sequence is variable due to erosion during the Hercynian orogeny. 6. Tectonic setting and provenance

Fig. 13. Classification of the Cambro-Ordovician sandstones. Scattergram of log (Fe2 O3 /K2O) vs. log (SiO2/Al2O3) from Herron (1988). Fe2 O3 = total iron.

is evidence of marine influence. The irregular distributio and frequency of Skolithos within the sequences and from well to well suggest diachronous regressive–transgressive cycles. The C1 to F sequences are interpreted as a transitional facies of alternating foreshore marine and braided-fluvial deposits. Chemostratigraphy shows an increase in Al2O3 and K2O comparing to the sequence B1; this is a consequence of an increase in shale content due to marine influence. The variation of thickness is subtle, some cycles of sequence C do not occur in some wells. Minor discontinuities are common. The top of this set of sequences is marked by regression over the whole area. The overlying, probably Tremadocian sequence began by a major marine transgression over the North of Sahara (Fabre, 1988); the deposits are represented by a shallow marine facies with two sequences identified. Sequence G1, is characterised by structureless fine-grained quartz arenites with subrounded to rounded grains, a proliferation of Skolithos and very small amounts of clay minerals. Sequence G1 is considered to be of upper shoreface origin. Important amounts of quartz and quartz coating induce high content of SiO2. The overlying sequence, G2, is composed of fine-grained sandstones with interbedded shales,

Schwab (1975), Bhatia (1983), Blatt et al. (1980), Bhatia and Crook (1986) and Roser and Korsch (1986, 1988) have related sandstone geochemistry to specific tectonic setting. Even though diagenesis may alter the original chemistry, changes are themselves related to plate tectonic environment (Siever, 1979), and bulk composition should still reflect tectonic setting and so enable development of chemically-based discriminants to supplement a petrographic approach (Roser and Korsch, 1986). The ratio K2O/ Na2O has been used to constrain tectonic settings of sedimentary basins (Roser and Korsch, 1986). The binary diagram K2O/Na2O vs. SiO2 is used for all wells. Results from Well Md-72 are presented as an example (Fig. 15). A passive margin origin for the Cambro-Ordovician sandstones is indicated by the discriminant plot of K2O/Na2O vs. SiO2. The maturity of these sandstones is characteristic of cratonic environments, where sedimentary recycling is an important process. As expected the sandstones have higher SiO2 and correspondingly lower Al2O3. SiO2/Al2O3 ratios of the sandstones range between 1.15 and 2.18. According to Badalini et al. (2002), during the Cambrian, North Africa and Arabia underwent extension and intracratonic subsidence that resulted in the deposition of sediments in basins across Morocco, Algeria, Libya, Oman and Saudi Arabia. The Thickness of the Cambrian sediments also increases in the Anti-Atlas and Atlas. Thicknesses of several kilometers are reported from the Atlas, but some of this may be tectonic thickening rather than stratigraphically controlled. According to Frostick and Steel (1993), on a large scale the general tectonic setting controls the size, shape, orientation and structural evolution of a basin. In order to better investigate the structural context of the Cambro-Ordovician clastic deposits. Thickness contour maps (isopach maps) were constructed for each sequence. For that purpose the southern Hassi Messaoud oil field region was selected because of its dense well

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Fig. 14. Stratigraphic correlation of the Cambro-Ordovician throughout the northern Saharan platform.

Fig. 15. Tectonic discrimination diagram of Roser and Korsch (1986) for CambroOrdovician sandstones of the Saharan Platform, an example from Hassi Messaoud (Well Md-72).

bore network and the availability of wireline logs. Thus, two isopach maps are selected (Fig. 16B). The thickness of the sequence (Fig. 17) and the sedimentary architecture (fluvial channels, geometry of non-depositional areas and the Skolithos extension limit), indicate a passive margin setting rather than a transtensional tectonic event (Ait-Salem et al., 1999). However, at the end of the Cambrian, a discontinuity in sedimentation occurred and the sequence contains evidence of deposition under unstable tectonic conditions and the subtle deformations can be observed across the Sahara (Fabre, 1988). In fact, the comparison of the isopach maps of sequence B (B1 + B2) (Fig. 16A) and sequence F (Fig. 16B) suggests a structural inversion with depocenter converted to in a palohigh. This feature cannot be explained without tectonic instability not that the Hassi Messaoud field is located at

the prolongation of a major NE–SW palohigh (Amguid-El Biod arch) (Fig. 1). In fact, the Phanerozoic reactivation of the 4°300 E Pan-African shear zone controlled the development of Amguid-El Biod arch and the Cambrian rocks thicken onto the crest of this arch (Beuf et al., 1971; Boudjema, 1987) and probably, the basement fault reactivation is the main cause for the distribution and drainage of the sediments (Jennette et al., 1991). According to Coward and Ries (2003), during the Cambrian many of the Upper Pan-African rift systems were reactivated. Volcanics rocks, interbedded within the Cambrian sandstones, have been cross-cut by a number of wells (Md-2 and Md-172) (Figs. 16A and 17). Fabre (2005) documented volcanic rocks in the Ra lithozone in Hassi Messaoud. During the early Ordovician, the relatively uniform thickness of the Tremadocian suggests that the region was dominated by post-rift subsidence (Coward and Ries, 2003). Burke et al. (2003) suggested that accommodation space for deposition of the Cambro-Ordovician (520–440 Ma) sandstone resulted from thermal subsidence in the aftermath of late Neoproterozoic (post-orogenic) rifting. Mineralogical maturity of the analysed rocks along with the occurrence of zircon (Ait-Salem et al., 1999) is consistent with felsic plutonic or reworked sedimentary source rocks. Igneous and metamorphic Pan-African hinterland is believed to lie beyond the actual Hoggar shield to the South (Bennacef et al., 1971; Beuf et al., 1971) and to have contributed largely to the filling of the northern Saharan basins. Braided river systems probably transported clastic material into the basin from relatively stable source areas. 7. Discussion The Cambro-Ordovician sandstones were deposited throughout North Africa and Arabia. The sandstones have a widespread


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Fig. 16. Isopachs and palaeogeographic maps of Cambrian sequences. (A) Sequence B (contour line interval is 10 m). Three main features are apparent: the western high forms an East–West ridge, an other NE–SW palaeo-high and the eastern depocenter (see discussion in text). (B) Sequence F (contour line interval is 1 m). Two main features are apparent: the south-eastern high forms an NW–SE ridge, and the depocenters (see discussion in text).

sheet-like distribution, uniformity of composition and are largely barren of fossils. Our chemostratigraphy demonstrates that sandstone units can be subdivided into several sequences or sub-units for more accurate correlations. The sandstones are mainly quartz arenites. This indicates an intensive recycling event within these sandstones especially for the youngest sequences where marine

incursions influenced the clastic deposits. The sandstones contain mainly quartz, less important amount of K-feldspar, illite and kaolinite or quartz overgrowth as cement. Lack of ferro-magnesian minerals is evidence of a felsic source-rock contribution in a relatively-stable intra-cratonic basin. The typology of zircons which represent the main heavy minerals, support the felsic source rock




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Md-115 Md-10 3294,70m





Md-14 3332m


0m 5






















C3 volcanic rocks

Disconformity 2


Ra C2

3399,8m (69,8m)

A3 ity




3414m (87m)



o isc





3433,1m (117,6m)



3387,3m (92,8m)



Disconformity 1





on isc

R2 A5


A1 (R3)



Depth: 3224,4m

Gamma Ray Thickness: (134,4m) TSE: Transgressive Surface of Erosion


3413,2m (102,2m)


A2 100 3425,4m (93,4m)

Md-172 Md-147 Md-94 Md-115 Md-10


5 Km

Md-88 Vertical Exageration: X365

Fig. 17. The cross section shows the correlation of the Cambro-Ordovician sequences. Note the lack of B1 and B2 sequences at the Md-115, Md-10 and Md-94 wells.

hypothesis (Ait-Salem et al., 1999). The D2 unconformity is clearly associated with a break in major element composition in Well-HR4 (Fig. 6). However, in well Anr-1 the geochemical break is clearly associated with B1/B2 boundary (Fig. 5) above the D2 unconformity that is associated with any geochemical feature. In this case most of the chemical elements at the disconformity did not respond. This case in not uncommon in chemostratigraphic studies where a major event in some locations is not sometimes geochemically well represented. The D2 unconformity (R2/Ra transition) it is associated with the rejuvenation of remote eroding areas as the same sedimentological character and units are observed in the Cambrian outcrops of the Tassili and the Hoggar mountains (Fabre, 1988, 2005). Deposition and formation architecture were controlled by regional subsidence which was very low, accommodation space which increases gradually in relation to sea level rise, tectonic setting and palaeoclimate. This abundant supply of fluvial clastic materials at the base was delivered to rifted and sags basins, becoming high energy shallow marine with some fluvial influences at the top. Fluvial dispersal currents allowed a redistribution of this clastic supply forming widespread sheet-like sandbodies. Regional studies over Gondwana show similarities between the Cambro-Ordovician of North Saharan Platform and the rest of North African and Arabian Cambro-Ordovician (Klitzsch, 1981; Millson et al., 1996; Burke, 2000; Avigad et al., 2005). The final assembly of western Gondwana which resulted in the construction of huge PanAfrican chains (or Trans-Saharan Megabelt) in parts of Algeria, Mali and Niger between 750 and 520 Ma through the collision of more than 20 terranes between the West African and East Sahara cratons (Kennedy, 1964; Fabre, 1988; Black et al., 1994; Burke, 2000; Azzouni-Sekkal et al., 2003; Caby, 2003; Craig et al., 2004). According to Avigad et al. (2003), the Pan-African orogeny was followed by continental-scale uplift, erosion, the formation of intramountain ba-

sins and rifting and the development of an extensive peneplain that can be traced from Morocco to Oman. The petrographic and geochemical compositions of the CambroOrdovician deposits support the idea that the original material was more felsic than mafic. Granites and metamorphic rocks such as gneiss are the main contributors as well as pre-existing sedimentary rocks. The erosion products of these crystalline rocks are dominated by coarse clastic materials. Granitic sources tend to produce illitic and kaolinitic clays. Granitic sources yield detritus rich in potassium feldspar, whereas mafic sources are dominated by plagioclase (Wedepohl, 1978). The primary deposits of the CambroOrdovician over a widespread peneplaned surface are considered as first-cycle clastic input (sensu Cox and Lowe, 1995). Avigad et al. (2005) attribute the mature, first-cycle origin of the North African Cambro-Ordovician siliciclastic sections to intensive chemical weathering of the Pan-African continental basement in a warm-humid climate that prevailed over north Gondwana from the end of Neoproterozoic to the (pre-glacial) Ordovician. In our study, this is reflected by the geochemistry and the maturity of the sands up stratigraphy. The Cambrian palaeogeographic reconstitutions tend to place North Africa at rather high latitudes – this area was under an extensive ice sheet by the late Ordovician (Avigad et al., 2005). The reconstitutions suggest that around 545 Ma and 514 Ma, the Algerian Sahara lay in even higher latitudes at 40–50°S (Scotese et al., 1979; Dalziel, 1997; Scotese, 1997; Meert and Lieberman, 2004). According to Avigad et al. (2005), if correct, this would imply that intensive chemical weathering produced quartz-rich sandstones across the entire north Gondwana margin, regardless of palaeolatitude. The primary deposits of the Cambro-Ordovician are characterised by coarse-grained sandstones and more K-feldspar than the upper sequences. They display high content of Al2O3 and K2O.


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The upper sequences are characterised by reworked and recycled sediments, less coarse in grain size and more mature with high content of SiO2 and low content of K2O. The reworking of sediments involves high-energy environments such as shallow marine which destroys feldspar and induce quartz-rich sand (Bessa, 2004) These sandstones are dominated by monocrystalline quartz rather than polycrystalline quartz or other primary silicates (Bessa, 2004). According to Rajchl (1999), the study of syn-sedimentary deformations proved very useful for discriminating between the allogenic (tectonic-driven) and autogenic (sedimentation-driven) influences on depositional style. Probably, the sediment recycling occurred through successive tectonic uplift phases and its relation with eustacy-driven lowstand. 8. Conclusion The Cambro-Ordovician of the Saharan platform is one of the most important reservoirs in Algeria. Our chemostratigraphy study allowed to determine several depositional sequences within the previously-assessed lithostratigraphic subdivision and to perform more accurate regional correlations. The study demonstrates that correlations over several hundreds of kilometers can be achieved and that the unconformities/disconformities can be identified and traced over large distances. The geochemical variations from well to well are minor; the differences is the degree of diagenesis and weathering. The mineralogy assemblages are in general, the same as well as the source-area composition and the tectonic setting for the sandstones. The stratigraphic evolution from the base to the top reflects the change in mineralogy and geochemistry. The basal sandstones are illite-dominant with important amount of K-feldspars and high content in K2O and Al2O3 whereas the upper sequences are quartz-rich with kaolinite, high SiO2, low Al2O3 and K2O. Cambro-Ordovician mature quartz arenites are typical deposits of large intra-cratonic basins with gentle tectonic, generally slow subsidence rates and low accommodation space. The first sequences at the base are rapidly followed by recycled and reworked sediments towards the top as sea level rises with time. The tectonic setting and the provenance of the Cambro-Ordovician clastic deposits conform to a passive margin setting. However, sequence isopachs and sedimentary architectures indicate that the Hassi Messaoud area is remarkably unstable during the Cambrian. Acknowledgements We wish to thank Centre of Research and Development – Sonatrach for providing data and especially all the staff of mineral geochemistry department. The authors acknowledge with thanks extensive review comments and suggestions by P. Eriksson and two anonymous referees on previous versions of the manuscript. References Ait-Salem, H., Sabaou, N., Zazoun, R.S., 1999. Tectono-sedimentary evolution and paleogeography of silico-clastic deposits in the Cambrian of Hassi-Messaoud Oil Field (Algerian Sahara). Sciences et Technologies des Hydrocarbures 1 (1), 27– 33. Al-Gailani, M.B., 1980. Geochemical identification of unconformities using semiquantitative X-ray fluorescence analysis. Journal of Sedimentary Petrology 50 (4), 1261–1270. Ali, O., 1973. Stratigraphy of Lower Triassic sandstone of northwest Algerian Sahara, Algeria. The American Association of Petroleum Geologists Bulletin 57, 528– 540. Aliev, M., A Laoussine, N., Avrov, V., Aleksine, G., Barouline, G., Iakovlev, B., Korj, M., Kouvikine, J., Makarov, V., Medvedev, E., Mkrtchiane, O., Moustafinov, R., Oriev, L., Oroudjeva, D., Oulmi, M., Sa, A., 1971. Structures géologiques et perspectives en pétrole et en gaz du Sahara algérien. Tome 1. Sonatrach, Espagne. Dépôt légal: M, 1497/1972, 275 p.

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