Managing clastic reservoir heterogeneity I: Sedimentology and sequence stratigraphy of the Late Triassic Minjur Sandstone at the Khashm al Khalta type locality, Central Saudi Arabia

The Minjur Sandstone (Bramkamp and Steinecke inArkell, 1952; Powers et al., 1966; Powers, 1968) is the uppermost formation of the Permian–Triassic Buraydah Group (Vaslet, 1987), which from the base up, comprises the Khuff Formation (Middle Permian–Early Triassic), the Sudair Shale (Early Triassic), the Jilh Formation (Middle–Late Triassic) and the Minjur Sandstone (latest Triassic). This group has been extensively described in outcrop by Le Nindre et al. (1987, 1990), and is also referred to in GeoArabia publications concerning the Khuff Formation and Permian/Triassic boundary (Vaslet et al., 2005; Vachard et al., 2005; Crasquin-Soleau et al., 2006, Angiolini et al., 2006; Nicora et al., 2006; Berthelin et al., 2006, Chirat et al., 2006) and in the Explanatory Notes to the BRGM-DMMR 1:250,000-scale geological maps of the Ba’qa (Vaslet et al., 1987), Qibah (Robelin et al., 1994), Buraydah (Manivit et al., 1986), Al Faydah (Vaslet et al., 1985a), Ad Dawadimi (Delfour et al., 1982) Ar Rayn (Vaslet et al.,1983a, b), Wadi al Mulayh (Manivit et al., 1985), Sulayyimah (Vaslet et al., 1985b) and Wadi Tathlith (Kellogg et al., 1986) quadrangles. Nevertheless, the amount of literature published on the Triassic formations overlying the Khuff Formation is very limited, particularly concerning the Minjur Sandstone, even though this is one of the main aquifers supplying water to the city of Riyadh (Sogreah, 1967, 1968; Mott MacDonald, 1975, Williams, 1982; El-Sharif, 1985; Al-Saleh, 1992; GTZ, 2010).

The Minjur Sandstone is a complex assemblage of sandstone and shale with minor evaporites that was assigned by Le Nindre (1987) to a dominantly fluvial-deltaic system. The type section at Khashm al Khalta – Khashm al Minjur exhibits a prominent sandstone cliff (ca. 300 m) capped by the Marrat Formation and lower Dhruma Formation (Figure 1). A detailed lithostratigraphic section covering the Minjur Sandstone and underlying Jilh Formation at 23°31′N, 46°00′E to 23°36′N, 46°13’E was measured by Vaslet and others in October 1980 in the Wadi ar Rayn Quadrangle (Vaslet et al., 1983a,b). This locality had been chosen for a more detailed study of the vertical and horizontal spatial variations of the reservoir sand bodies.

The current field survey involved collecting 3-D information on the Minjur facies and sedimentary bodies (shape and dimensions) to be used for numerical modelling in relation to the geological storage of CO₂ in complex heterogeneous formations. This was helped by the thickness of the outcrop (300 m) and its low scree coverage, as well as by previous studies at Khashm al Khalta, which provide reliable information for the reservoir characterization task work (Figure 2). In the present paper we examine the sedimentological characterization of the Minjur Sandstone as an outcrop analogue of the Triassic reservoir in the French Paris Basin, which is a major hydrocarbon/CO₂/geothermal target—a further paper will be devoted to the 3-D modelling of the depositional environments determined from outcrop data, and to reservoir simulations of different sandstone distributions derived from the sedimentary conceptual model.

The lithology is described using the 2009 Munsell color chart (Munsell Color, 2009; http://www.hq.nasa.gov/alsj/GeoRockClrChart.pdf) and its designated color names, such as ‘moderate orange pink’ which corresponds to the chart’s color code 10R 7/4.

The fieldwork involved sedimentological mapping of a 5 square kilometer area from November 2008 to March 2009 using a GPS to log the observation points. This georeferenced network of observation points provided a dataset that was subsequently mapped using ArcGIS 9.3 and a georeferenced high-resolution (1 m) Ikonos color imagery base map (Ikonos basemap ©Spot Image). The resulting document (Figure 3) is a depositional environment map of 12 classes grading from sabkha to tidal mudflat to fluvial channel; it gives an indication of the lateral and vertical evolution of the depositional environments and the impact on reservoir connectivity. In addition to the observation points, 50 outcrop sections were measured and used for the facies descriptions and lateral correlation of the depositional environments. Spectral gamma-ray measurements with a Fugro GR320 portable spectrometer on 10 of the sections provided a more quantitative analysis of the clay and mineral content. Radioelement determinations for ⁴⁰K, ^eU and ^eTh, show a low concentration of ⁴⁰K in all the facies other than the sandstone with potassium feldspar, and a generally five times higher ^eTh spectrum in the paleosol (20 ppm) than in the sandstone facies (4–5 ppm); this latter result was helpful for the lateral correlation of the depositional environments and determined reference horizons for the mapping work. The same trend was observed with the ^eU spectrum (ca. 6 ppm in the Paleosol, 2 ppm in the sandstone facies).

Le Nindre et al. (1990), taking into account the time period commonly accepted for the deposition of the Minjur Sandstone (15 My), considered the formation to be a second-order sequence comprising a basal unit of meander-type deposits that was flooded by a transgressive system tract with ophiurids and dolomitic siltstone, and an upper unit of braided type deposits.

In the present study, based on our data from mapping the sedimentary units, the type section of the Minjur Sandstone has been split into nine depositional sequences. Although the lack of chronological evidence at this scale impedes assignation of a time basis for the sequence order, an average duration of 2 My is assumed for the deposition of each sequence, compatible with a third-order ranking. Also, to be able to locate the deposits more precisely within the sequences, we use the terms ‘lowstand’, ‘highstand’ and ‘transgressive’ system tract. The sequence boundaries are placed at erosional surfaces.

The base of Sequence 1 consists of thin, laminated, silty sandstone with desiccation cracks (Figure 4) and pulverulent residual gypsum and anhydrite, containing thin interlayers (between 30 and 50 cm) of pale-yellowish structureless sandstone. The beds contain voids from the dissolution of anhydrite that formed in the sediment, and scattered narrow channel structures (200 m wide, 3 m deep) with pink, well-rounded, well-sorted, fine- to medium-grained sandstone are observed (Figure 5). The main sedimentary structure is trough cross-bedding grouped into vertical decametric cosets; paleocurrent analysis indicates a northeasterly flow direction (Figure 6). The sand body also contains local interbeds of small quartz gravel (5 mm).

Thin laminated carbonate siltstone (Figure 7) with asymmetrical current ripples (N145° paleocurrent; Figure 8) rests on the above succession. It is cut by a 200-m-wide, 3–4-m-deep channel structure with trough cross-bedded, pale yellowish orange, well-sorted, fine-grained sandstone. The troughs show abundant mud drapes and the sediment is disrupted by Skolithos ichnofacies. Paleocurrent measurements indicate a diverging flow (N215°, N170°, N255°, N150°).

A 4-m-thick succession of laminated silty-sandstone with interlayered residual gypsum and thin (3 cm) layers of quartz gravel (up to 0.5 mm) overlies the bioturbated sandstone body (Figure 9).

The laminated fine-grained succession with gypsum and anhydrite nodules is suggestive of a coastal plain environment drained by scattered, narrow, distal fluvial channels flowing to the northeast (well-sorted fine-grained sandstone). The muddy episode can be interpreted as tidal-influenced sediments deposited in a tidal mudflat (carbonate siltstone) drained by small tidal creeks flowing mainly to the south.

The Sequence 2 boundary (SB2) is marked by a 30-m-wide, 8-m-thick ribbon of dark ferruginous, microconglomeratic to medium-grained sandstone. The base of this sandstone body exhibits microconglomeratic planar bedding, typical of an upper flow regime (Figure 10; Miall, 1978), grading vertically to coarse planar trough cross-bedding (southerly paleocurrent). Abundant wood (trunks up to 2 m long) and leaves are impressed in the sandstone (Figure 11), which is marked by a ferruginous diagenesis resulting from the lithological contrast with the underlying fine-grained deposits. As described by Le Nindre et al. (1990) “the hydroxides may be deposited at the grain surfaces, or in the pores where they corrode the quartz grains”. A similar sandstone body is observed 5 km south of the Khashm al Khalta location, where paleocurrent analysis indicates a mainly northeasterly flow.

The sandstone body is overlain by a 1-m-thick white, thinly laminated, clayey and carbonate mudstone embedded with a thick (10 m) moderate orange pink, well-sorted, fine-grained sandstone ridge (Figure 12). Large-scale trough cross-bedding (1 m high, 3 m long, 10° dip) reflects a N170° paleocurrent. Flaser bedding and asymmetrical current ripples can be seen on the top of this solitary ridge, as well as Skolithos ichnofacies. The mud deposits enclosing the ridge have been eroded by the wadis.

Overlying the ridge is moderate orange pink, well-sorted, fine-grained sandstone. Its main sedimentary structures are trough cross-bedding grouped into one decimeter vertical cosets, and 1-m-long gullies stacked laterally to other megaripples. The stacked deposits are 1 m thick with widespread, but predominantly westerly, paleocurrent directions in the ripples (Figure 13). The megaripples are overlapped by thin, laminated, white carbonate siltstone (50 cm thick) that in turn is capped by 1–2 m of pale-olive siltstone interlayered with a void-pulverulent sandstone bed (30 cm). The laterally equivalent deposits lying to the north within the outcrops comprise pale yellowish orange, fine- to medium-grained sandstone filling channel structures with a basal microconglomeratic lag.

Sequence 2 ends with varicolored unstructured siltstone containing an embedded 5-m-thick body of greyish yellow, poorly sorted, poorly cemented, coarse- to medium-grained sandstone. Fining-upward trough cross-bedding exhibits N080° paleocurrent directions (Figure 14).

A 5-m-thick clayey silty-sandstone with intense pedogenesis (iron staining) and abundant varied size (6 to 10 cm) calcite concretions with a mean diameter of 2 cm (Figure 15) overlies the previous sandstone body and ends Sequence 2.

The lower part of Sequence 2 is interpreted as a coastal plain (see interpretation for Sequence 1) with low-sinuosity fluvial channel deposits (straight paleocurrent, very coarse upper-flow type deposits, and plant imprints). The muddy facies and sand-ridge deposits above the channel suggest a marine transgression with the development of a tidal inlet (Figure 16), as indicated by the mud drapes, broad paleocurrent directions and Skolithos ichnofacies. The inlet was bounded laterally by a salt marsh (varicolored siltstone) cut by small coastal channels. The coarse-grained sandstone body and associated siltstone in the upper part of Sequence 2 reflect part of a river system within a coastal plain grading to a floodplain capped by a soil showing deep pedogenesis and roots.

Sequence 3 starts with a lower unit consisting of silty claystone (8 m) with channels of trough cross-bedded, moderate yellow, coarse- to medium-grained sandstone. The upper part consists of white thinly laminated siltstone (2 m) with small channels of moderate orange pink sandstone. The paleocurrent directions are divergent around N230°. Some of the channel structures are filled by white fine-grained siltstone (Figure 17). Many flaser beds (Figure 18) and worm burrows can be seen in the uppermost part of the fill deposits. The main sedimentary structure is trough cross-bedding with interlayered mud drapes (Figure 19). This unit is truncated by the overlying sequence (SB4).

This thin sequence is interpreted as mainly shallow marine, initiated at the base by a coastal plain of varicolored fine-grained siltstone cut by coastal channels similar to those described in Sequence 2. The fine-grained sediment, with an important concentration of silt (ca. 30%), bioturbations and flasers, suggests conditions of intertidal deposition. The channel structures depict tidal channels incised in a mudflat similar to those of sequences 1 and 2 and filled with cross-stratified sand when active and by laminated silt when abandoned.

The sequence boundary (SB4) must have been extremely erosive in view of the fact that Sequence 3 has completely disappeared in the north of the study area. The deposits associated with SB4 are deep dusky red coarse-grained sandstone with basal microconglomeratic sandstone and soft rounded clasts of silty-sandstone wrenched from the upper part of Sequence 3. The sandstone shows trough cross-bedding grouped into vertical 30-cm cosets (Figure 20); paleocurrent studies indicate a unidirectional N045° flow. This sedimentary body (ca. 8 m thick) is exposed over the entire study area and is overlain by a complex system of interfingered sandstone and siltstone. The sandstone here is fine- to medium-grained, well-sorted and pale yellowish with local microconglomeratic levels; its main sedimentary structure, observed in the south of the study area, is tabular inclined cross-bedding (epsilon crossbedding) with varied azimuths (Figure 21). Thinly laminated fine-grained silty-sandstone, the thickness of which increases upward, is intercalated near the top of each sandstone sheet (Figure 22). The stacks of tabular inclined cross-beds are draped by alternating thin (10 cm) beds of fine-grained sandstone and white laminated siltstone (Figure 23). These deposits become subhorizontal at the base of the stacked epsilon cross-bedded sandstone.

A local wedge-shaped coarse-grained sandstone body (10 m thick) overlies the tabular cross-bedded sandstone. The contact is marked by soft reworked pebbles of white siltstone embedded in moderate orange pink fine-grained sandstone (Figure 24). The fining upward sediment grades from pebbles to coarse-medium grains with reverse graded structures in large trough cross-beds (Figure 25) to mixed white to very pale green siltstone and pale-red fine-grained sandstone. Paleocurrent analysis indicates a large dispersion with no dominant azimuth.

The thick sandstone wedge is covered by varicolored silty-sandstone grading to greyish yellow limy-siltstone and moderate yellow dolomitic siltstone in thin (10 cm) beds (Figure 26). These form an 8-m-thick succession in which one observes some moderate-pink, well-sorted, fine-grained sandstone in channel-scour structures. White siltstone with flaser bedding drapes the top of the sandstone. Intense bioturbation (Teichichnus ichnofacies) is noted in the muddiest facies (Figure 27).

The siltstone is overlain by an 5 to 8-m-thick sandstone bar with a flat, non-erosive base (Figure 28). It is pale yellowish orange, fine grained, well sorted and homogeneous with trough cross-bedding. Many recumbent beds are seen in the middle of the sandstone bar, possibly due to steep foresets disrupted by a shear between a winged current and a rough substratum. The paleocurrent directions show alternating N000° and N180° azimuths.

The topset of the sequence is a 3-m-thick assemblage of pale-olive clay and siltstone with sand sheets (50 cm) cemented by gypsum or with gypsum dissolution molds in outcrop.

Sequence 4 (Figure 29) is a complex unit, which plays a major role in our understanding of the architecture of the Minjur Sandstone because it separates the lower and upper Minjur. Above SB4, the low-sinuosity fluvial deposits are overlain by a floodplain and associated meander deposits in which abandoned channels can be seen. This evolution of the sedimentary style may be linked to increasing accommodation space. The coarse-grained, reverse-graded sandstone body is interpreted as grainflow deposits similar to the ‘hyperpycnal flow deposits’ of Lamb et al. (2010) deposited on steep sedimentary slopes (i.e. lateral accretion surfaces) in a shallow-marine environment. It is capped by silty-carbonate facies corresponding to the shallow-marine transgressive event of Le Nindre (1990) and in which one finds small tidal-creek deposits with bioturbations in the low-energy areas. The sandstone bar with recumbent bedding (Allen, 1984, v. II, p. 386) and opposing paleocurrent directions is interpreted as an estuarine mouth bar marking an increased sediment supply compared to the restricted hypersaline carbonate episode. Finally, the topset of the sequence and its gypsum content indicate a coastal plain environment similar to those described in sequences 1 and 2.

SB5 is deeply incised into Sequence 4 in the north of the study area. The base is marked by coarsegrained sandstone with trough cross-bedding (Figure 30) and steep-angle planar cross-bedding grading to large-scale low-angle coarse-grained cross-bedding (2 m long, 10° dip) suggestive of vigorous hydrodynamic flow (Miall, 1978). The foresets denote northeasterly paleocurrent directions. This coarse-grained sandstone body can be followed in outcrop over the entire area, ranging in thickness from approximately 10 m in the north to a few meters in the south.

An interfingered complex of sandstone bodies and silty-sandstone overlies the coarse-grained deposits. The base of the sandstone bodies is a microconglomeratic lag onlapped by trough crossbedded, pale yellowish orange, well-sorted, fine-to medium-grained sandstone; the upper part shows epsilon cross-bedded, fine-grained sandstone alternating with white clayey siltstone on top of the bedding planes. As with Sequence 4, fine-grained deposits of alternating pale-olive siltstone and pale yellowish orange sandstone beds (10 cm) drape the uppermost inclined tabular cross-beds. The sediment mass enclosing the sand bodies (three superimposed bars with a total thickness of 15 m) is composed of laminated silty-sandstone with interlayered thin, homogeneous and unstructured, fine-grained sandstone beds (10–20 cm).

This silty-sandstone succession grades to green clay and varicolored laminated siltstone in which channel scours were filled by trough cross-bedded, well-sorted, fine-grained sandstone. The top of the sequence is marked by common mud drapes between the troughs and by flaser bedding.

The sequence boundary and associated unidirectional coarse-grained sediments are interpreted as braided/low-sinuosity deposits. They are overlain by meander deposits (well developed preserved point bar) and associated floodplain deposits. The meander stage is capped by varicolored siltstone associated with a mud-enriched fine-grained channelled body interpreted as a tidal mudflat drained by small interconnected tidal creeks; the common mud drapes and flaser bedding at the top of the sequence reflect a strict current control. This marks the transgressive stage of Sequence 5. The Sequence 5 highstand systems tract (HST) was eroded and truncated by SB6.

The lower part of Sequence 6 is marked by a thick (12 m) bar made up of pale yellowish orange, poorly sorted, coarse- to medium-grained sandstone (Figure 31). Large-scale low-angle cross-bedding (> 1 m long, 15°), in 30 cm to 1 m vertical cosets, forms the foot of a cliff and grades to trough crossbedding at the top of the bar. Abundant quartz gravel and white silty-sandstone extraclasts highlight the base of the fining upward foresets. The lateral extension of this bar exceeds the study area and can be followed over several kilometers. The uppermost part of the sandstone bar is eroded by a 6-m-deep channel scour filled with microconglomeratic basal lag, coarse-grained sandstone with trough cross-bedding (paleocurrent N100°) and interbedded laminated siltstone and fine-grained sandstone at the top of the sedimentary body.

A 10-m-thick complex of thin white to varicolored clayey siltstone alternates with moderate pink, well-sorted, unstructured fine-grained sandstone (Figure 32). Intense pedogenesis affects some of the thin siltstone layers and gives way to disrupted multicolored siltstone.

The white/varicolored silty-sandstone unit is overlain by a 3-m-thick assemblage of very pale green siltstone with abundant residual gypsum whose alteration has totally disrupted the sediment. Some soft reworked pebbles of white siltstone are seen in the basal part of channel scours filled with moderate-pink, well-sorted, fine-grained sandstone with mud drapes in the sigmoidal ripples. The trough cross-beds indicate a N160° paleocurrent direction.

Sequence 6 contrasts sharply with the previous sequences in terms of deposit dimensions. The massive (12 m thick, several kilometers long) basal sand bar system rests on a very coarse lag at the sequence boundary, which appears relatively flat throughout the study area. Its sedimentary structures are typical of an upper flow regime (large-scale, low-angle cross-bedding; Miall, 1978). The structural characteristics are interpreted as reflecting a proximal depositional environment, such as braided rivers to alluvial fans, whilst the flat SB6 surface suggests a large-scale erosional episode extending over tens of kilometers. The appearance of smaller scale coarse-grained channel sediments and the development of a thick complex of interbedded siltstone and sandstone are interpreted as reflecting a period of increasingly available space with braided/low-sinuosity to meandering rivers and floodplain deposition.

The green shale and fine-grained muddy sandstone at the top of Sequence 6 are interpreted as protected shallow-marine deposits in a supra- to intertidal environment. The green shale reflects a restricted hypersaline environment drained by tidal creeks (sandstone lenses).

The flat surface of SB7 is overlain by a 15-m-thick sandstone bar comprising coarse- to medium-grained sandstone at the base with suspended clasts of quartz gravel (5 mm to 5 cm) and rounded extraclasts (up to 30 cm diameter) of pale yellowish orange medium-grained sandstone and white siltstone (Figure 33). The main sedimentary structure is intermediate between large-scale, low-angle cross-bedding (1.5 m long, < 10°), and planar bedding. The sediment grades upwards to medium-grained sandstone with trough, planar and locally overturned cross-bedding (Figure 34), with the uppermost part of the bar being marked by thin horizontal and wavy laminations of fine-grained sandstone and siltstone. This first bar is truncated by a second sandstone bar showing similar features, i.e. medium-grained sandstone at the base with some coarser occurrences and trough, planar and overturned cross-bedding, becoming horizontal in the uppermost part of the bar system and passing to sandstone and siltstone beds. A 4-m-thick channel of red, unsorted, coarse-grained sandstone with large-scale low-angle and trough cross-bedding (N355° paleocurrent directions) incises the top of this depositional unit.

The stacked sandstone bars are covered by a 10-m-thick alternation of laminated white to varicolored pedogenic siltstone and moderate orange pink, well-sorted, fine-grained sandstone.

The two thick sandstone bars (Figure 35), characterized near the base by large (5 mm to 30 cm) extraclasts floating in a medium-grained sandstone matrix, are interpreted as debris flow type deposits. Their sedimentary structures (large-scale low-angle trough cross-bedding grading to trough, planar and overturned cross-bedding) are typical of high-velocity currents and suggest vigorous hydrodynamic conditions such as are found in a proximal alluvial fan system. The subsequent deposition of alternating fine-grained sediments may mark a backstepping of the facies belts from an alluvial fan system to a meander plain system characterized by a floodplain (white pedogenic siltstone) with interlayered sand sheets deposited by river floodwaters (i.e. overbank deposits).

SB8 is overlain by an 8-m-thick sandstone body comprising a basal residual lag of 20 cm passing to coarse-grained sandstone with scattered quartz gravel (2–5 cm) in decimeter-thick beds. The lower part of this sandstone bar contains rounded reworked extraclasts (10 cm diameter) of fine-grained sandstone and white silty-sandstone. The upper part of the bar exhibits trough cross-bedding (N000° paleocurrent direction), overturned foresets and 6 m of interlayered fine-grained sandstone (20 cm thick beds) and pedogenic siltstone cut by a 3-m-thick channel body of poorly sorted, coarse-to medium-grained sandstone. The uppermost part of Sequence 8 spans the deposition of a body of well-sorted fine-grained or clayey sandstone. Some thin tangential laminations with mud drapes were observed.

Sequence 8 is basically a repetition of sequences 6 and 7 (Figure 36). The basal sandstone bar was deposited in a proximal depositional environment (braided/low-sinuosity rivers) that passed to a more meander-type floodplain with increasing available space. The top of the sequence is marked by clay- and siltstone-enriched fine-grained sandstone reminiscent of the descriptions of the tidal channels within the lower sequences, and is interpreted as a regression of the facies belt from a braided to a meander and shallow-marine intertidal environment (Figure 37)

Sequence 9 at the top of the Minjur Sandstone, just below the Jurassic Marrat Formation, is exposed in a steep 30 m cliff (Figure 38). It comprises thick massive sand bars with abundant gravel beds and trough and large-scale low-angle cross-bedding. Analysis of the paleocurrent direction in the trough cross-beds indicates N000° directions. Three to four bars can be distinguished, separated from one another by erosive surfaces. They all display similar characteristics and are interpreted as braided-type deposits.

The Minjur Sandstone at the Khashm al Khalta type locality shows two depositional trends: a lower Minjur trend (sequences 1 to 4) of mainly fine-grained deposits with scattered sandstone bodies, and an upper Minjur trend (sequences 5 to 9) of mainly coarse-grained sandstone forming massive sand bars 10–20 m thick and extending laterally over several kilometers beyond the study area. To analyze the formation’s lateral variations, vertical sections were studied to the north (Khashm al Jufayr) and south (Khashm al Birk) of the type locality (Figure 39).

The Khashm al Jufayr section (23°59′N, 46°10′E; Figure 40) of the Minjur Sandstone is 30–40 m thick, although the lower boundary with the Jilh Formation is not visible. The base of the section exposes calcareous siltstone with orthogonal joints (Figure 41) and a strong reaction to HCL. This is overlain by a thick alternation of (1) siltstone containing residual gypsum and cut by small channel structures (< 1 m) with coarse-grained to conglomeratic sandstone, and (2) bioturbated or trough cross-bedded fine-grained sandstone with mud drapes.

A thick coarsening upward bar (10 m) of well-sorted, mud-enriched, fine-grained sandstone (Figure 42) overlies this interlayered assemblage. Many of the trough cross-beds are draped with siltstone and show recumbent figures, probably due to a shear brought about by a rough substratum and increasing current velocity. Trough cross-bedded medium-grained sandstone with abundant soft rounded siltstone pebbles cover the top of this bar; they indicate N045° paleocurrent directions. The top of the coarsening upward sand bar is incised by an erosive coarse-grained sandstone body with trough cross-bedding reflecting N045° paleocurrent directions (Figure 43). A ferruginous diagenesis coloring the sandstone very dark red may have been due to a grain-size contrast between the overlying medium-grained sandstone and the coarse-grained sandstone localizing the fluid migration at the interface.

A 10-m-thick scree cone covers the dark-red sandstone, making it impossible to estimate the sandstone’s thickness, and is itself overlain by a 3-m-thick body of poorly sorted coarse-grained sandstone with trough cross-bedding in sets 5–50 cm thick. This is followed by homogeneous, well-sorted and intensely bioturbated (worm burrows), moderate reddish orange fine-grained sandstone that shows low-angle (< 10°) trough cross-bedding and is iron encrusted on top.

Six meters of pale olive green siltstone with a high residual gypsum content onlap the bioturbated sandstone body from the first beds of the Marrat Formation.

The lower section is interpreted as an infratidal environment (calcareous siltstone) grading to a supratidal coastal plain drained by coastal channels under a continental (coarse grained) or tidal (fine grained) influence. The first sandstone bar is interpreted as a tidal or estuarine bar in reference to the trough cross-bedding with siltstone drapes, the recumbent stratification, the sorting and the grain size. Its uppermost part appears to have been dominated by fluvial processes, with a N045° influx of coarse material. Thin cosets with trough cross-bedding in the sandstone body reflect deposition in a shallow fluvial channel. The homogeneity, grain size (fine grained), sorting (well sorted) and bedforms (thin large-scale troughs and bioturbation) of the last sandstone body suggest tidal influences.

The Minjur Sandstone at Khashm al Birk is exposed in a 100 m section (23°03′N, 46°14′E; Figure 44). As in the Khashm al Jufayr section, the lower contact with the Jilh Formation is not seen. The base of the section (Figure 45) is marked by trough cross-bedded, moderate-red, poorly sorted, coarse-grained sandstone (N040° paleocurrent directions) grading to interbeds of sorted fine-grained sandstone and white laminated siltstone. This assemblage was deposited in tabular stacked beds with a N270° inclination and is overlain by 30 m of laminated, pale-olive, locally multicolored, siltstone and residual gypsum (Figure 46). Two types of sandstone body are found within these greenish sediments:

1. thin beds (30 cm) of homogeneous, weakly consolidated, fine-grained sandstone with small ripples;

2. channel deposits of well-sorted, fine-grained sandstone with mud-draped sigmoidal ripples and bioturbations (Teichichnus ichnofacies). Some of these channel deposits are greyish purple in color.

A first sandstone bar (Figure 47), 12 m thick, incises the underlying siltstone-dominated unit. It is characterized by large-scale trough cross-bedding (ca. 2 m long, 40 cm thick) highlighted by quartz gravel and soft white siltstone pebbles. Quartz gravel is also found as (1) scattered grains in a medium-grained matrix, and (2) localized layers (up to 30 cm thick). Large-scale low-angle cross-bedding alternates regularly with trough and planar cross-bedding. The main paleocurrent direction trend is N030°. A second sandstone bar, 13 m thick, erodes the previous one and shows the same features.

These sandstone bodies are overlain by 10 m of reddish laminated siltstone with interbeds (50–100 cm thick) of homogeneous fine-grained sandstone. The top of this sedimentary body is eroded by a sharp discordant conglomeratic lag that is followed by 10 m of very pale orange, fining upward sandstone: unsorted, coarse- to medium-grained sandstone with large-scale trough cross-bedding and many microconglomeratic layers of quartz gravel (< 5 cm), grading upward to horizontal laminated fine-grained sandstone. A 10-m-thick assemblage of white to varicolored siltstone with thin interbeds of fine-grained sandstone covers the pale-orange sandstone sequence.

The upper Minjur Sandstone in the Khashm al Birk section comprises a 15-m-thick sandstone body identical to the thick underlying sandstone bars described above, and is capped by pale-olive siltstone with residual gypsum of the Marrat Formation.

The lower Minjur (ca. 40 m thick) consists of a siltstone unit with residual gypsum and thin scattered sandstone lenses. It is interpreted as a supratidal coastal plain cut by coastal and infratidal channels and is equivalent to sequences 1 to 4 of the Khashm al Khalta type location, although differing slightly in that there is no sustainable development of shallow-marine environments.

The deposition of the thick coarse-grained sandstone bodies marks a change in the sediment supply with the development of proximal braided/low sinuous rivers grading to a meander plain (alternating siltstone and sandstone beds) as available space increased. It is the equivalent of sequences 5 to 9 of the Khashm al Khalta type section.

In summarizing the sedimentological characteristics of the Minjur Sandstone one can identify several typical paleoenvironments and sedimentary bodies which, taken together, depict the formation’s context and temporal evolution fairly clearly. The paleoenvironments alternate between proximal shallow marine (intertidal) and proximal fluvial (alluvial fan); the fluvial systems generally exhibit paleocurrent directions ranging from N350° to N090° (except for the incised river at the base of Sequence 2 which is southward), while the tidal systems are commonly characterized by current directions of N150° to N230°. The following depositional environments are distinguished from the bottom upwards:

1. Coastal plain (sequences 1 and 2): A network of decametric to pluri-decametric wide channels related to an adjacent pre-evaporitic context and filled with trough cross-bedded, pink, well-sorted, medium-grained sandstone.

2. Sabkha deposit (Sequence 1): Laminated silty-clayey-siltstone and medium-grained sandstone in unstructured decimetric beds with dissolution voids and residual gypsum.

3. Salt marsh: Red-brown silty claystone with rootlet staining.

4. Sub- to intertidal complex (Sequence 1): Pink fine-grained bioturbated sandstone with current ripples and white clay drapes, incised by decametric channels showing varied flow directions.

5. Incised river (base of Sequence 2): Black coarse- to micro-conglomeratic ferruginous sandstone with unidirectional low-sinuosity trough cross bedding.

6. Lagoon/Brackish swamp (Sequence 2): Tabular deposits of mottled white silty-claystone with purple-violet dots/spots/staining.

7. Tidal inlet and Tidal channel (Sequence 2): Decametric to pluri-decametric ridges of pink medium-to fine-grained sandstone isolated in a muddy matrix or grouped into multidirectional ridges of megaripples with ‘cut-back’ reactivation ripples.

8. Inter- to supratidal flat capping the tidal channel succession: Cream-colored laminated siltstone with a partly calcareous matrix, passing to silty claystone with rootlet imprints.

9. Flood plain paleosol (more clayey with deep roots in Sequence 2, more sandy and mottled in Sequence 4): Silty and sandy mottled claystone with violet and purple root staining.

10. Coastal plain channels (Sequence 3): Coarse- to medium-grained sandstone in small isolated decameter-wide channel in clayey siltstone.

11. Tidal sand flat and channels (Sequence 3): Meter-thick brown to pink sandstone with megaripples (some sigmoidal) and varied current directions (generally opposite to the main fluvial current).

12. Tidal mudflat (sequences 3 and 4): Pale-green laminated siltstone with sand lenses, flasers and channel fill:

13. Braided river (Sequence 4): Plurimetric thick and hectometric wide red sandstone sheets with low sinuosity trough cross-bedding, steep foresets and an erosional base.

14. Fluvial lateral accretion bar (stacked in Sequence 4, well-developed and imbricated in Sequence 5): Plurimetric thick and pluridecametric wide bars of cross-bedded coarse- to medium-grained sandstone draped by thin beds of fine-grained sandstone and clayey siltstone deposited by lateral accretion perpendicular to the main current direction on the bar shoulder.

15. Delta-front grain-flow (‘hyperpycnal’) deposit (Sequence 4): Wedge of coarse- to fine-grained sandstone with large clinoforms and reverse grading passing downstream to brown fine-grained sandstone with white silty-clay drapes and disrupted bedding.

16. Tidal flat, main marine flooding (Sequence 4): Silty claystone, bioclastic dolomitic siltstone, bioturbated (trails and burrows) yellow, fine- to medium-grained sandstone and metric-size channels with tangential megaripples.

17. Estuarine mouth bars (Sequence 4): Fine- to medium-grained sandstone with recumbent and overturned cross-stratification (Allen, 1984, Chapter II, p. 386.) caused by drag process in water-saturated sand.

18. Alluvial fan with debris flow passing to braided stream (thickening and coarsening upward from Sequence 6 to 9): Strong imbricated bar system (pluridecametric [ca. 20 m] in thickness, plurihectometric in extent) of conglomeratic to coarse-grained sandstone with decametric debris (extraclasts) near the base and planar, locally overturned, cross-bedding.

19. Temporary inter- to supratidal interruption of the fluvial influx (tops of sequences 6 and 8): Decametric wide channels of well-sorted fine-grained sandstone with tangential megaripples and clay drapes of varied current direction, clayey fine-grained sandstone, green laminated siltstone separating the fluvial bars.

The base of the Minjur Sandstone at the Khashm al Khalta location was, in the past, mapped as of the top of the Jilh cuesta (J3), but there are several arguments to suggest that the first sequence (Sequence 1) described from this point should rather be assigned to the Jilh Formation. For example:

The description of the Jilh Formation near the Khashm al Khalta location (23°38′N) provided by Le Nindre et al. (1990, p. 173 and 176) shows similarities with the description of Sequence 1 of the Minjur Sandstone.

• Unit J2 of the Jilh Formation consists of white fine-grained sandstone with trough cross-bedding in greenish gypsum shale. Rare carbonate horizons may be observed, consisting of sandy algal dolomite. This reflects a coastal environment with lagoons and alluvial bodies.

• Unit J3 is a thick complex of prograding alluvial sand deposits interlayered in its middle by rare gypsum shale and dolomite. The top of J3 contains greyish yellow, shaly and gypsum-rich, fine-grained sandstone grading vertically to sandy dolomite with algae. The environment is deltaic with algal flats.

In the Wadi ar Rayn Quadrangle (Vaslet et al. 1983a,b) an equivalent outcrop of Sequence 1, located 11 km north of the study area, was previously mapped as Jilh unit J3 (Figure 48).

On the geological map of the Al Faydah Quadrangle (Vaslet et al., 1985a, explanatory notes), the base of the Minjur Sandstone is identical to the base of Sequence 2 at the type locality. The authors describe the contact as “conformable on the Jilh Formation with black, conglomeratic, coarse-grained, cross-bedded sandstone having a ferruginous cement […] and contains silicified plants debris”.

Farther north, in the Shaqra Quadrangle (Vaslet et al., 1988) “the basal part of the Minjur Sandstone consists of conglomeratic cross-bedded sandstone with ferruginous cement and quartz gravel and with pebbles as large as 2 cm in diameter. The succession continues with graded sequences of conglomeratic sandstone, medium- to fine-grained ferruginized sandstone, and white to violet pedogenic silty claystone (paleosol)”.

To the south of Wadi ar Rimah, in the Buraydah Quadrangle (Manivit et al., 1986), the Minjur Sandstone consists of conglomeratic sandstone with a ferruginous cement and silicified tree trunks.[…] To the north of Wadi ar Rimah, in the vicinity of Al Barud (26°50′N), the bottom part of the Minjur Sandstone consists of silicified conglomeratic sandstone (3.0 m) containing abundant petrified tree trunks (5 m long and 50 cm in diameter).

In conclusion, the base of Sequence 2 in these localities is well identified as the base of the Minjur Sandstone. Consequently, Sequence 1 is likely to represent the top of Unit J3 as the last filling stage of the Jilh Formation; we suggest that it be considered as the highstand systems tract (HST) of the Jilh Formation.

The Minjur Sandstone clearly exhibits two assemblages when compared to the Arabian Platform Cycle Chart for the Cambrian to Triassic period (Haq and Qahtani, 2005).

• The basal sequence boundary (SB) at 215 Ma is clearly the base of Sequence 2 (this study) in view of its equivalence to the facies described by Vaslet et al. (1985a) as the base of the Minjur Sandstone in the Al Faydah Quadrangle, although in a revised position at Khashm al Khalta.

• The lowstand systems tract (LST) overlying the basal unconformity corresponds to the dark coarse-grained ferruginous sandstone with numerous plant debris, which locally incises the underlying tidal deposits.

• The transgressive systems tract (TST), dominated by the alternation of continental clastic influx and more marked marine transgression cycles, represents Sequence 4, the top of which defines the limit between the lower and upper Minjur. It grades from well-developed tidal parasequences to thin tabular aggrading restricted-lagoon deposits and correlates with MFS Tr80 at 211 Ma.

• The upper assemblage is dominated by coarse clastic discharges and corresponds to (1) a sea-level ‘still stand’ visible from the reconstructed coastal onlap curve (long HST), and (2) an erosional period due to tectonic activity (ca. 202 Ma).

Sharland et al. (2004) place the Lower Minjur (sequences 2 to 3) as Lacinian (Early Norian) consistent with the age given by the conodont Metapolygnathus abneptis abneptis (Huckriede) (Vrielinck, 1984, 1986, inManivit et al., 1986 p. 18 of the Buraydah Quadrangle explanatory notes). It overlies a Lower Norian unconformity, like the Mulussa Clastics in Syria and the Baluti Formation in Iraq, and also the basal Minjur Sandstone in Abu Dhabi.

MFS Tr80, marked by the peak of carbonate deposition, corresponds to the Middle Norian transgression (215 Ma, Early Alaunian) recorded in Sequence 4. It is fairly well correlated with Tr80 in Kuwait where pollen described from a time-equivalent 50 m (160 feet) dolomite within the upper Middle Minjur Formation (Khan, 1989) supports a Norian age for this assignment (Sharland et al., 2001, p. 198). The major regression (sequences 6–9) occurred during the Rhaetian, e.g. the Mulussa Formation (Syria), the Hussainiyat Formation dolomite (Iraq), the Mafraq? Formation (Oman), and was followed by the major pre-Marrat hiatus.

Further discussion concerning the age of the Jilh Formation and Jilh/Minjur boundary are outside the scope of this paper.

Study of the Permian–Triassic on the Arabian Platform (Le Nindre, 1987; Le Nindre et al., 1990) shows that the Upper Triassic (Jilh Formation and Minjur Sandstone) apparently developed its maximum thickness and maximum clastic discharge in a topographic depression interpreted as a fluvio-tidal inlet/outlet, which may be the result of a subsiding substratum weakness (Figure 49). Local steep and thick occurrences of deltaic clastic wedges in the Jilh Formation at Jabal al Arid (23°38′N), contrasting with thin carbonate and siltstone deposits laterally at Khabra Halwah (23°59′N), argue for a fault-related mechanism (Le Nindre et al., 1990, p. 207; Le Nindre et al., 2003, p. 281).

Deep-seated tectonic lineaments in the Arabian Plate (e.g. Central Arabian Arch and Dibdiba Trough) have existed since Permian times (Ziegler, 2001) and may have controlled the depocenters and facies distribution of the Minjur Sandstone. The Late Triassic is known to have been tectonically active, especially during a Neo-Tethyan extensional phase (Le Métour et al., 1995) at which time most of the Triassic sediments were deposited (Ziegler, 2001). Late Triassic extensional tectonic events have also been recorded in France’s Southeast Basin (Brunet, 1994; Poli, 1998).

Le Nindre et al. (2003) describe the Norian to Rhaetian progradation of the Minjur Sandstone as the result of a high terrigenous influx that controlled the subsidence mainly by sediment loading. The Middleton subsidence model (Middleton 1980), fitted by the authors on the experimental subsidence curves, shows uplift during the Late Triassic (Figure 50). This event is likely to give rise to hinterland erosion and a sediment supply that accumulated in a depression centered on the Khashm al Khalta location (Figure 49). This sediment overweight was the main subsidence driver.

Using previously published data combined with field observations, the sedimentary dynamics of the Minjur Sandstone can be established as follows (Figure 51):

• The lower Minjur: Sequence 1 (Upper Jilh Sequence) to Sequence 4 consists mainly of shaly deposits with little terrigenous sediment in any of the three studied locations. At the type locality, it is marked by an incised valley at the base of Sequence 2; the created accommodation space enabled a dominant aggradation process with tabular and clayey deposits. The relative highstand at the scale of this sequence determined the first point bar progradation up to space saturation (paleosol). The sea-level rises then culminated with the major maximum flooding that separates the lower and upper Minjur. This early period more-or-less spans the Norian. The landscape and the platform structure can be imagined as quite a flat area with tidal inlets, where the delta system could spread broadly far northward over the platform and onto the Central Arabian Arch, as in the lower Minjur reconstruction by Ziegler (2001) from other contributions.

• In contrast, the upper Minjur is dominated by massive thick sandstone bodies extending over several kilometers and showing very proximal sedimentary features. The transition between lower and upper Minjur is sharp, suggesting an abrupt “equilibrial” modification. The TST of Sequence 4 is, for example, a reliable analogue: (a) its base consists of shaly sediment, with the deposition of fine-grained carbonate material (‘passive sedimentation’), that grades abruptly to (b) an active estuary with an important sediment supply (estuarine mouth bars, tidal creeks) which marks a sedimentary pulse in turn overlain by a shaly coastal-plain succession.

• To summarize: (1) the basal TST of Sequence 4 overruns a passive sedimentation with the deposition of suspended fine-grained particles and precipitated carbonate minerals. The upper TST of Sequence 4 is marked by a pulse of sediment supply enabling the development of an active sand-rich estuary. Once the pulse terminated, passive sedimentation was re-established, spanning the deposition of gypsiferous shale.

The architecture of the overlying sequences followed the same mechanism, which we ascribe to tectonics, uplift and subsidence (onset of Mediterranean rifting as of the Rhaetian [Cozzi and Hardier, 2003; Roži et al., 2008]). It modified the upstream-downstream relationships, with a flexured platform —the upstream part supplying the clastics and the downstream part becoming more marine, with the upper Minjur alluvial fan marking the hinge between the two.

On a larger scale, the reconstruction by Ziegler (2001) is fairly realistic in its depiction of the change between the two stages. The denomination of ‘alluvial fan’, however, although fully pertinent for the upper Minjur does not agree with our observations of the lower Minjur (Figures 52 and 53). To complete this detailed reconstruction, some explanatory sketches were drawn (Figures 54 and 55).

The tectonic and eustatic aspects at platform scale are illustrated by the tectonostratigraphic megasequence (TMS) (Sharland et al., 2001): Mid Permian to Early Jurassic (255–182 Ma) - Opening of the Neo-Tethys, northeast extension, and passive margin post-rift thermal subsidence. The tectonic aspects are illustrated by the setting of the Late Triassic hinterland in the context of the Neo-Tethys opening, with the Arabian Plate supplying clastics to the Tethysian trough (page 88). The eustatic aspects are illustrated by the chronostratigraphic chart of this period (page 91). However, Sharland et al. (op. cit.) chose Iran for representing the Norian–Rhaetian evolution where, in the absence of a clastic influx, carbonates and evaporites reflect a lowstand at the end of the Triassic; this fall in base level could also be responsible for erosion of the Arabian Plate. Moreover, this sea-level fall induced a hiatus of more than 15 million years before onlap of the Marrat Formation during the Early Jurassic transgression. Note that in Figure 3.22 in Sharland et al. (2001), MFS Tr80 correctly reflects the flooding event that we identified in the middle of the Minjur Sandstone (Sequence 4) according to its conodont biostratigraphic age (Vrielinck, 1984, 1986 inLe Nindre et al., 1990, p. 267-269).

The Minjur Sandstone appears to be much more heterogeneous than previously described. Eight sequences (Sequence 1 being assigned to the upper Jilh) are recognized, grouped into two distinct units (lower and upper Minjur) separated by a maximum flooding surface (Figure 56).

• The lower Minjur is predominantly shaly with scattered narrow sandstone bodies, which suggests a low sediment supply. Upper Jilh Sequence 1 and Minjur sequences 2 and 3 are interpreted as forming a thick coastal plain with interbeds of intertidal and more continental deposits.

• The upper Minjur, in contrast, is characterized by thick coarse-grained sediments with a horizontal extension of several kilometers. The architecture of the sequences 5 to 9 indicates depositional environments ranging from a proximal alluvial fan system to a large fluvial plain to a shallow marine supra- and intertidal environment.

The Minjur Sandstone thus represents a fluvio-deltaic system with very different sedimentary dynamics between the lower and upper units.

• The lower Minjur, with its low sediment supply and regular interbeds of intertidal deposits, is dominated by relative sea-level variations. Its small-scale variations led to the deposition of small dispersed sandstone bodies, resulting in sharp decreases in connectivity.

• The upper Minjur reveals a massive sediment supply mainly through pulses at the base of each sequence; the SBs consist of thick, coarse-grained sandstone deposits grading to a floodplain environment characterized by a sharp decrease of the sand content (meter-thick interbeds). The uppermost facies of the sequences retrograde up to the level of intertidal/tidal gullies and tidal mudflats. These features suggest a tectonic control which, through hinterland uplift or basin subsidence, results in a relative drop in sea level and increased erosion of the reliefs to provide sediment to the system.

Le Nindre et al., (1990) describe the Minjur Sandstone as being weakly affected by diagenetic processes. The well-preserved porous networks and the thick well-connected sandstone deposits extending over tens of kilometers in the upper Minjur, suggest excellent reservoir performance.

Integrating these detailed outcrop study results with the recent Saudi Aramco work by Jones and Hooker (2010) on subsurface cores and well logs could provide a more comprehensive picture and a better understanding of this very interesting depositional system and its implications for reservoir studies.

The aim of this problem-oriented study was to construct a spatial distribution model of the sedimentary bodies for reservoir simulation. Our field results demonstrate that: (1) lithological heterogeneity is much greater that can be imagined merely from subsurface cores; (2) field mapping is a good approach for a quantitative analysis of size, geometry, internal heterogeneity and connectivity of effective reservoir bodies; and (3) a good understanding of the logic of the sedimentary processes is essential for predicting reservoir response to external solicitation (e.g. production/injection fluids). The results from this study of the Minjur Sandstone have provided the basis for static numerical geological modelling and fluid-flow simulation in terms of reservoir heterogeneities.

The results presented here are based on a BRGM research project into reservoir heterogeneities and were presented as a poster display at the GEO 2010 Conference, Bahrain, 2010. The work was carried out in the framework of a PhD thesis supported by Ademe (French Environment and Energy Management Agency) and the Carnot Institutes. It was made possible through an agreement between the Saudi Geological Survey (SGS) and BRGM. Dr Zohair A. Nawab and Sami S. Maddah of the SGS are gratefully acknowledged for providing the full participation, encouragement, assistance and support of the SGS. The authors would like thank the anonymous reviewers for their valuable editorial comments, Pascal Audigane for advice regarding reservoir engineering and Sir Patrick Skipwith for his precious help on the English writing. The final poster design and layout by GeoArabia’s Graphic Designer Nestor “Niño” A. Buhay was very much appreciated.

Benoît Issautier is a PhD. student at the French Geological survey (BRGM) in collaboration with the University of Provence and the Saudi Geological survey. He is working on the impact of heterogeneities on CO₂ geological storage and his research projects focus on the reservoir characterization of heterogeneous formation and the impact of depositional as well as stacking patterns on the pressure field and reservoir performances during gas injection at industrial scale. Benoît received his Master degree from the University of Nice-Sophia Antipolis in 2008 with a specialty in structural and reservoir geology.

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Yves-Michel Le Nindre has been contributing to Middle East geology since 1979 and has more than 10 years of experience in the geological mapping of the Phanerozoic rocks of Saudi Arabia. He received his Doctorate of Sciences from the University of Paris in 1987 with a dissertation on the sedimentation and geodynamics of Central Arabia from the Permian to the Cretaceous. Yves-Michel was involved in many research and consulting projects in France and abroad (Bolivia, Morocco, Tunisia, Kuwait, Iran, Oman, Saudi Arabia, Ethiopia, India), for sedimentary basin analysis and modelling, specially in hydrogeology, with the Bureau de Recherches Géologiques et Minières until 2000. Since then he has been involved in international projects for CO₂ storage, working with EU state members and CSLF countries (Canada, China, Russia). As a Sedimentologist, Yves-Michel works in France on present-day littoral integrated management. He is a member of the EAGE, ASF and, with BRGM, of the European Network of Excellence for CO₂ geological storage (CO₂GeoNet).

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Abdullah M.S. Memesh joined in 2006 the Saudi Geological Survey (SGS), Jeddah, as head of the Phanerozoic rocks mapping and Paleontology Division. He has more than 10 years of experience in the geological mapping of the Phanerozoic rocks. He received his BSc from King Abdul Aziz University, Jeddah, in 1993. From 1993–1999 Abdullah was a geologist with the French Geological Survey (BRGM). From 1999–2006, he was an exploration Geologist for the Saudi Arabian Mining Company (MA’ADEN) phosphate Project. He authored and co-authored 1:250,000-scale geologic maps.

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Saleh M. Dini is Head of the Phanerozoic rocks Mapping Unit at the Saudi Geological Survey (SGS), Jeddah. He has about 30 years of experience in the geology of the phosphate exploration, resource evaluation and geological mapping of Phanerozoic rocks particularly of northern Saudi Arabia. Saleh received his BSc from King Abdulaziz University, Jeddah, in 1980. From 1980–1986, Saleh joined Riofinex Limited. He was involved in the discovery of significant phosphate resources in Umm Wu’al area. From 1987–2000 Saleh was a staff geologist in the U.S. Geological Survey (USGS). Saleh has been involved in several research projects in the prefeasibility, resource assessment and feasibility studies of the Al Jalamid, Al Khabra and Umm Wu’al phosphate deposits in northwestern Saudi Arabia. Since 2001 Saleh has been a staff geologist in the Saudi Geological Survey (SGS). He authored and co-authored nine 1:250,000-scale published geologic maps and 14 written reports about Saudi phosphate deposits.

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Sophie Viseur is a numerical geologist. She received her Ph.D. from the Nancy School of Geology in 2001. Her primary interests are in geostatistics for channel simulations and their application to hydrocarbon exploration and production. For recent years, she has worked in developing methods for the integration of outcrop data (Lidar) and geological concepts into 3-D carbonate architecture models. Sophie is currently working at GSRC Laboratory, University of Provence, France.

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