Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 A 3D outcrop analogue model for Ypresian nummulitic carbonate reservoirs: Jebel Ousselat, northern Tunisia E. Vennin1, F. S. P. van Buchem2, P. Joseph2, F. Gaumet2, Marc Sonnenfeld3, M. Rebelle4, H. Fakhfakh-Ben Jemia5 and H. Zijlstra6 1 Musée National d’Histoire Naturelle, 43 rue Buﬀon, 75005 Paris, France (e-mail: [email protected]) 2 Institut Français du Pétrole, Rueil-Malmaison, France 3 TotalFinaElf, Pau, France (present address: Reservoir.com, Greenwood Village, Colorado 80.111, USA) 4 TotalFinaElf, Pau, France 5 ETAP, Tunis, Tunisia 6 University of Utrecht, Netherlands ABSTRACT: A three-dimensional high resolution sequence stratigraphic model of an Ypresian nummulitic carbonate ramp and organic-rich basin is presented based on outcrops in Central Tunisia. The sedimentation pattern is influenced by the interplay of diﬀerent orders of variations in eustatic sea-level (third to fifth order), the pre-existing palaeotopography, and probably some synsedimentary tectonism (diﬀerential subsidence). Time-equivalent rocks deposited in a comparable structural and depositional setting along the northern margin of the African plate are hydrocarbon bearing (Tunisia and Libya). This example may thus serve as an outcrop analogue for this petroleum system, providing valuable information on the subseismic-scale distribution pattern, geometries and heterogeneities of both the reservoir and source rock facies. The studied outcrops cover an area of 10 by 20 km where present-day valleys provide three-dimensional access to the rocks. In addition, the transition from the inner/mid-ramp, with nummulitic reservoir facies, to the carbonate source rocks in the basin is exposed in continuous outcrops. This transition takes place in about 3 km, a distance generally beyond the resolution of well spacing. Based on the physical tracing of beds, and the recognition of three orders of depositional sequences (third to fifth) a high resolution time framework is constructed. The accumulation of large nummulites (best reservoir facies) is stratigraphically controlled, and occurs in the transgressive phases of the landward-stepping fourth order cycles (overall transgression). Carbonate production was at that time so high that aggrading geometries are observed during these transgressive pulses. Our observations show that size, morphology and reproduction strategy of foraminiferal assemblages and, particularly, nummulites and Discocyclina, are related to changes in water depth and, consequently, in accommodation space. A regional east–west cross-section shows significant thickness variations of the Ypresian succession that were probably controlled by synsedimentary diﬀerential subsidence. The detailed, sub-seismic-scale, geometrical information on stratal patterns and lateral facies change are quantified, and used in a 3D numerical stochastic modelling (HERESIM) of this petroleum system. KEYWORDS: benthonic foraminifera, nummulites, palaeoecology, event stratigraphy, Tunisia, carbonate reservoir INTRODUCTION The Ypresian nummulitic limestones form important carbonate reservoirs in northern Africa (Anz & Ellouz 1985) with proven reserves in Tunisia (Ashtart, Sidi El Itayian fields and numerous smaller occurrences) and in Libya (e.g. Bourri Field). In the Tunisian fields, the reservoirs occur in shallow-water carbonate Petroleum Geoscience, Vol. 9 2003, pp. 145–161 facies of the El Garia Formation, which is dominantly composed of the large benthic Foraminifera species Nummulites and Discocyclina, while the source rock facies occur in the timeequivalent, interfingering basinal facies of the Bou Dabous Formation (Fig. 1; Bishop 1988). One of the main problems encountered in the production and exploration of these Ypresian plays are the sub-seismic-scale, lateral facies variations 1354-0793/03/$15.00 2003 EAGE/Geological Society of London Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 146 E. Vennin et al. Fig. 1. (a) Map of the regional distribution of the El Garia facies in Tunisia (modified from Moody 1987). (b) Geological map of the Jebel Ousselat area with the position of the logged outcrop sections and the transects (modified from the geological map 1/200 000, Kairouan). and the strong local tectonic control on the palaeogeography and thus the facies distribution (e.g. Bishop 1988; Racey et al. 2001). Time-equivalent strata of both reservoir and source rock facies outcrop in several locations in Central Tunisia (Fig. 2; Bishop 1988). Comte & Lehmann (1974) and Fournié (1975) showed the strong lateral variations in thickness of the Ypresian rocks, reflecting tectonic control of the sedimentation pattern Fig. 2. Chrono-lithostratigraphic scheme of the Ypresian formations in Tunisia (after Moody 1987; Bishop 1988; Bailey et al. 1989; and Racey et al. 2001). by block faulting along NW–SE trends. General depositional models and descriptions of the variability in sedimentary facies have been presented in a number of publications (e.g. Fournié 1975; Moody 1987; Fakhfakh- Ben Jemia 1989; Loucks et al. 1998; Racey et al. 2001). There exists, however, no study documenting the evolution of the sedimentary facies and stratal geometries in a high resolution time framework of depositional sequences. The creation of a time framework allows one to address stratigraphic architecture topics such as the relationship between granulometry (i.e. Nummulites of diﬀerent sizes), depositional processes and resulting stratal geometries in a steep carbonate ramp system and, from an ecological point of view, the timing of the proliferation of large benthic Foraminifera during depositional sequences of diﬀerent orders (third to fifth order). The Ypresian outcrops in Jebel Ousselat, west of Kairouan, are unique in that they expose a basin to mid-ramp transition along continuous, three-dimensional outcrops in an area of 10 km by 20 km. In addition, in an E–W transect of 50 km length, from Es Sfeia, to Jebel Ousselat and El Garia (Fig. 1), the influence of synsedimentary tectonics on the palaeotopography can be demonstrated. From the point of view of both the tectonic setting and the depositional environment these outcrops may thus represent a very valuable analogue for the Ypresian oil fields in the El Garia Formation. The more proximal Ypresian evaporitic facies, outcropping SW of Jebel Ousselat in Jebel Cherahil, are not considered in this study. The purpose of this paper is: (i) to construct a high resolution sequence stratigraphic framework at the third- to fifth-order scale for the Ypresian rocks of the Jebel Ousselat area; (ii) to document lateral facies change, palaeoecological evolution and stratal geometries within these sequences and quantify dimensions and heterogeneities of the reservoir facies; (iii) to define the relative influence of synsedimentary tectonics Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 A 3D outcrop analogue model for carbonate reservoirs and eustatic sea-level fluctuations on the sedimentation pattern; and (iv) to construct a 3D stochastic reservoir model (HERESIM) of this steep nummulitic carbonate ramp system. GEOLOGICAL SETTING From the Jurassic to the Lower Miocene several large basins developed along the Tethyian margin in Tunisia in response to a general distentional E–W history (Bishop 1988). During the Eocene, the palaeogeography was also strongly controlled by synsedimentary tectonic movements that induced the development of NE–SW orientated topographic highs and lows (Rigane et al. 1994). The Lower Eocene succession corresponds to the Metlaoui Group, comprising the shallow water, nummulitic El Garia Formation, which is underlain by the phosphatic Chouabine Formation, and overlain by the argillaceous Cherahil Formation (Fig. 2). The nummulitic limestones of the El Garia Formation occupy a broad zone that extends from the Gulf of Gabes in the southeast through central Tunisia towards the northwest border with Algeria (Fig. 2; Bishop 1988). The time-equivalent basinal facies consists of the Bou Dabous Formation which contains abundant globigerinid planktonic Foraminifera and is rich in organic matter (Bishop 1988). The proximal Faid and Ain Merhotta formations consist of sabkha facies and lagoonal gastropod facies, respectively (Moody 1987). PREVIOUS MODELS The El Garia Formation is of Upper Ypresian age, and probably represents a duration of about 3 Ma (Bishop 1988; Racey et al. 2001). The Bou Dabous Formation spans the entire Ypresian. Diﬀerent depositional models have been presented for the Metlaoui Group. The outcrops in Jebel Ousselat were first studied for their facies and depositional environment by Fakhfakh-Ben Jemia (1989). Moody et al. (1988) and Loucks et al. (1998) have proposed a facies model for the El Garia Formation in a ramp setting based on subsurface and outcrop observations. They proposed an epiphytic way of life for the nummulites in a facies belt rich in sea grasses. The association of algae and nummulites is, however, neither observed in the subsurface (e.g. Hasdruhal field, Racey et al. 2001), nor in the Ousselat outcrops (this study). The depositional model proposed by Racey et al. (2001) for the Ashtart and Bourri oil fields corresponds to nummulite production on structural highs and depocentres in the lows around them (Anz & Ellouz 1985; Hmidi & Sadras 1991). MATERIALS AND METHODS This study is based on 15 measured outcrop sections (total 1760 m) and semi-quantitative microfacies analysis of 500 thin sections. In the Bou Dabous section 23 samples have been analysed with a Rock-Eval II to determine the organic matter content (TOC), carbonate content, hydrogen index (HI), oxygen index (OI) and Tmax. Measurements were carried out at the Organic Geochemistry Laboratory of the Institut Français du Pétrole in Rueil-Malmaison. The Bou Dabbous type section has been logged with an outcrop spectral gamma-ray tool. This tool provides one-minute measurements of total gamma-ray count, %K, ppm of Th and ppm of U. A portable, suspended, four-channel, natural gamma-ray spectrometer from the Canadian company Exploranium was used. High-resolution sequence stratigraphy is used to unravel the fine-scale stratigraphic architecture of the sedimentary system. 147 This approach has found widespread application in siliciclastic systems (e.g. van Wagoner et al. 1989; Wilgus et al. 1989; Homewood et al. 1992) and, more recently, also in shallow water carbonates (e.g. Goldhammer et al. 1990; Pomar 1991; Loucks & Sarg 1993; Kerans & Tinker 1997; Homewood & Eberli 2000; van Buchem et al. 2002). The subdivision of depositional sequences into five orders, which fall into a general time framework, is followed here (Haq et al. 1988; Vail et al. 1991). The sequence orders that are of relevance to this study are: third order (0.5–3 Ma), fourth order, also referred to as high frequency cycles, para-sequences or genetic sequences (0.5–0.08 Ma) and fifth order (0.08–0.02 Ma). The methodology is summarized in four steps. It is, however, stressed that sequence definition, in particular in carbonate systems, is very much a feedback process between the diﬀerent steps. + The first step is the detailed description of bedding pattern, texture, lithological composition and faunal content (macroscopic field observations complemented with thin section analysis), the identification of significant surfaces and the interpretation of depositional environment and sedimentary processes. The abundant large benthic Foraminifera found in the studied area provide a relatively independent way of determining neritic palaeobathymetry (Hottinger 1997). + The second step is the 1D sequence analysis. The trends of increase or decrease in the accommodation/sediment supply ratio are based on both palaeobathymetric interpretations, the preservation of sedimentary structures (sets and co-sets), and significant surfaces. + In the third step, a time-based correlation scheme is established using the higher frequency, smaller-scale cycles of change in accommodation (or accommodation relative to sediment supply). The main constraints for these correlations are (a) the stratal geometries provided by the continuous outcrops, (b) the hierarchy of the depositional sequences as defined in each section and (c) biostratigraphic or other independent time control such as isotope stratigraphy. + The fourth step is the construction of the high-resolution sequence stratigraphic model. The time framework allows the definition of spatial and temporal relationships between diﬀerent facies types and the volumes of sediment involved (sediment flux). Based on the environmental or bathymetric changes across limiting surfaces, the landward- or seawardstepping character of the depositional sequences is determined. In this way a three-dimensional, dynamically evolving, depositional model is defined. The resulting model can subsequently be tested, and possibly refined or changed, as additional outcrop sections (or wells), and geochemical, palaeontological and mineralogical observations yield more data. For the three-dimensional reservoir simulation use has been made of the in-house developed IFP program HERESIM. This is a stochastic modelling program generating equiprobable 3D images of the reservoirs based on an algorithm using the truncated Gaussian random function (Matheron et al. 1987). Main geostatistical parameters are the vertical and horizontal proportion curves and the variograms. The simulation algorithm uses a Gaussian random function computed with the previous factorized exponential variogram (stationary cases). By using a variable function for the thresholds that truncate the Gaussian function, it is possible to simulate lateral facies changes throughout depositional environments (Doligez et al. 1999). The non-stationary simulation method is chosen here in order to respect lateral facies changes from the coastal plain domain to the basin domain. Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 148 E. Vennin et al. Fig. 3. Sedimentary facies (F1 to F12) and environmental interpretation of a steep carbonate ramp system. All facies are carbonate dominated except for the facies F12, which is quartz dominant. FACIES Fournié (1975) recognized 17 microfacies within the Metlaoui Group and Moody et al. (1988) divided the Ypresian carbonates into 25 lithofacies. Rigane (1991) proposed three major lithofacies for the Metlaoui Formation: Bou Dabous facies, El Garia facies and the transitional Ousselat facies. This last facies, composed of nummulitic debris, is considered to be the lateral equivalent of the Bou Dabous ‘limestones’ (Bailey et al. 1989; Moody & Grant 1989) and corresponds to the Ousselat Member (Moody & Grant 1989; Loucks et al. 1998). In this study, which deals only with the El Garia and Bou Dabous formations, twelve sedimentary facies are distinguished (Fig. 3). Their position along a bathymetric profile is presented in Figure 4, together with some microfacies illustrations. All facies are carbonate dominated, except for facies 12 which may contain up to 70% quartz sand. The general setting is interpreted as a steep carbonate ramp system, subdivided into basin, outer ramp, mid-ramp and inner ramp environment (Fig. 4) following the terminology of Burchette & Wright (1992). RAMP DEPOSTIONAL ENVIRONMENTS The main ramp environments are briefly described in the following section. The palaeobathymetry of each facies depends on both faunal associations and sedimentological features, and is relative with respect to the other facies rather than absolute. Basin environment: two facies (F1 and F2) are distinguished, which are arranged in limestone–shale couplets. The limestones (F2) vary in thickness from 0.1 to 1 m and are composed of Foraminifera and nummulithoclasts floating in micritic matrix. The shales (F1) are structureless and contain a diverse biota including nautiloids, sponges, planktonic Foraminifera (globigerinid forams), vertebrate debris and organic matter. The organic matter concentration varies between 0.1 and 3.5% TOC, is of marine Type II and immature (HI between 400 and 600; Tmax <435). These facies have a good source-rock potential and correspond to the Bou Dabous Formation. Outer ramp environment: three facies (F3 to F5) are associated in this environment and are composed mainly of nummulithoclastic (F3) and echinoderm packstones (F4) and rare nummulitic floatstones (F5). The outcrops vary from wellbedded limestones in the relatively distal position to metre-scale bedded limestones in proximal areas. Well-sorted nummulithoclastic debris comprises 60 to 90% of whole rock. Hummucky cross-stratifications correspond to episodic reworked fossil deposits and show the influence of storminduced activity. These nummulithoclastic packestones are well described by Moody & Grant (1989) and interpreted as ramp or slope margin deposits (the Ousselat member described by Moody & Grant 1989). Mid-ramp environment: five facies are distinguished (F6 to F10) by their faunistic association which are arranged, from distal to proximal environments, as follows: (a) flat discocyclines in a nummulithoclastic matrix (F6); (b) elongate and flat nummulite forms in a nummulithoclastic matrix (F7); (c) Discocyclina– nummulite associations (F9); (d) big and lenticular nummulite Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 A 3D outcrop analogue model for carbonate reservoirs 149 Fig. 4. Depositional model of a steep carbonate ramp system with main microfacies illustrations: F2, wackestone of planktonic Foraminifera and nummulithoclasts; F3, nummulithoclast-rich packstones; F4, echinoderms and nummulithoclast packstones; F5, small nummulitic floatstones; F6, Discocyclina floatstones; F8, small nummulitic rudstones; F10, big nummulitic rudstones; F11, grainstones composed of coarse nummulithoclasts; F12, calcarenitic facies. (Facies F1, F7 and F9 are not illustrated in this figure and are described in Fig. 3.) forms (F10); and (e) small nummulites associated with thick and rounded nummulitic forms (F8). This facies corresponds to the shoal or middle ramp foraminiferal bioaccumulations of Moody & Grant (1989). Inner ramp environment: two facies (F11 and F12), occur in a proximal shoal shoreface environment as: (a) grainstones composed exclusively of coarse nummulithoclasts (F11) with bedding patterns and sedimentary structures typical for high energy conditions (Kef el Guitoune sections); and (b) as calcarenites arranged in cross-bedded channel deposits of coastal plain origin (F12) in the more eastern and proximal area (El Sfeia and Bgour sections). DEPOSITIONALLY SIGNIFICANT STRATIGRAPHIC ATTRIBUTES Four types of special surface are distinguished (Figs 5 and 6): + Maximum flooding surfaces (MFS) are characterized by an increase of the clay content, the presence of glauconite, and a landward shift of the facies belts, expressed by the change from small nummulite facies (F8) to fine nummulithoclastic wackestone facies (F2 and F3) in the distal domain (outer ramp), and from big and lenticular nummulite floatstones (F7) to flat Discocyclina facies (F6) in the proximal domain (mid-ramp). Typical condensation features in the distal part of the carbonate shelf are the accumulation of glauconite, phosphate, vertebrate debris and shark teeth. Faunal elements, indicating a deepening of the environment, include nautiloids, planktonic Foraminifera and abundant echinoids. Seven maximum flooding surfaces (MFS 0 to VI) have been recognized in the Jebel Ousselat outcrops (Figs 5 and 6). + Hardground surfaces are marked by intense bioturbation and iron crusts and are interpreted as temporary starvation of sediment associated with the small-scale sequence boundaries (Fig. 5). + Bioturbated layers vary in thickness from 0.1 to 3 m (Kef El Guitoune and El Sfeia outcrops; Fig. 5). Trace fossils are mainly Planolites, Thalassinoides and Teichichnus. These layers are associated with surfaces which mark a temporary halt to sedimentation both in the El Garia and in El Sfeia outcrops. + Transgressive ravinement surfaces indicate erosional or non-erosional marine flooding events. They occur at a small scale in depositional sequences but can also be followed across outcrops of kilometre scale. In the distal area, they are attributed to storm-induced processes, whereas in proximal areas ravinement surfaces are characterized by an abrupt deepening of the sedimentary environment (Figs 5 and 8). Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 150 E. Vennin et al. Fig. 5. Kef El Guitoune 2 outcrop section, of the proximal El Garia facies with semiquantitative microfacies analysis and sequence stratigraphic subdivision. One large-scale sequence (third order), six medium-scale sequences (fourth order from I to VI) and small-scale sequences have been identified in the Kef El Guitoune outcrop. Good potential reservoir facies are represented in green. Significant surfaces are identified for the six medium-scale sequences (dotted line for maximum flooding surfaces: MFSI to VI; HG, hardground surfaces; Tr, transgressive surfaces). HIGH-RESOLUTION SEQUENCE STRATIGRAPHY In this section, first representative examples of a proximal section (Kef El Guitoune 2, Fig. 5) and a distal section (Bou Dabous type section, Fig. 6) are presented. This is followed by a presentation of the correlation scheme, based on three transects (Figs 7 and 8A, B). Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 A 3D outcrop analogue model for carbonate reservoirs 151 Fig. 6. Bou Dabous outcrop section, with the distal globigerinid facies of the Bou Dabous Formation, Rock-Eval analyses and outcrop gamma-ray readings. The organic mater (TOC varies between 0.1% and 3.1%) presents a marine Type II origin (HI 400 to 600) and is found interbedded with argillaceous deposits. (I to V correspond to fourth-order sequences; dotted line for maximum flooding surfaces: MFS0 to VI). The Kef El Guitoune 2 section (proximal location) In the proximal El Garia Formation a large variety of facies is observed (Fig. 5). Based on the evolution of their texture, faunal content and significant surfaces, diﬀerent orders of sequences are defined. At the large scale, an overall shallowing-deepening trend is observed. At the base, facies consist of nummulithoclastic and planktonic Foraminifera-rich mudstones and wackestones, indicative of the outer ramp environment, followed by small nummulites and nummulithoclastic wackestones and packstones, interpreted as the external part of the mid-ramp. Shallowest water conditions occur at 168 m (boundary between medium-scale sequences III and IV in Fig. 5), as indicated by the presence of a grainstone composed of coarse nummulithoclasts and lowangle to parallel-laminations. The overall transgression starts with an aggradational phase, when a package of about 50 m of large nummulite and Discocyclina packstone to rudstone accumulated in the mid-ramp environment. The increasing abundance of Discocyclina is interpreted as a deepening of the sedimentary environment. At the top of the section outer ramp conditions return again, with the abundant presence of planktonic Foraminifera in decimetre-bedded mudstones and wackestones. In the outcrop photo of Figure 7 the bedding pattern evolution of the Kef El Guitoune section is visible, with the massive cliﬀ-forming bed of the nummulite accumulation in sequence IV. Six medium-scale sequences are distinguished (Fig. 5). During the overall regressive phase (sequences I to III), they are dominated by the shallowing-up trends (red triangles in Fig. 5), and they increase in thickness. Facies are dominated by small nummulites and nummulithoclasts. During the overall transgressive phase (sequences IV to VI), the medium-scale sequences are organized in a strikingly diﬀerent way: during the early rises in sea-level, aggradation occurs (green rectangles in Fig. 5) with the accumulation of the large Nummulites and Discocyclina (sequences IV and V; 50 m and 30 m, respectively). The thickness of these sequences, dominated by the deepeningupward trends (blue triangles in Fig. 5), decreases upward and indicates the landward-stepping trend during the overall transgression. The best reservoir quality is found in facies 6 to 10, which are nummulitic packstones and grainstones deposited in the mid-ramp environment (Fig. 5). These facies tend to accumulate during the transgressive phase (aggradation) of the medium-scale sequences during the large-scale transgression. This is a significant observation, since it determines the timing and distribution of the best reservoir facies, and allows the construction of a predictive model. The Bou Dabous section (distal location) The distal section is the type section of the Bou Dabous Formation (Moody 1987). Much less variation in facies occurs Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 152 E. Vennin et al. here, and the sediment is organized in limestone/shale couplets, with common to abundant planktonic Foraminifera (globigerinids) and varying amounts of fine nummulithoclastic debris. Limestone beds, composed of scarce small Nummulites and abundant nummulitic clasts, tend to vary in thickness, and thicker beds are interpreted as reflecting a higher supply of material from the nearby ramp, corresponding to progradational phases. Maximum progradation occurred during the highstand of sequence III, and this boundary is placed on top of the thickest and most resistant bed of this section (Fig. 6). Outcrop gamma-ray measurements show relatively highest values in sequences I, II and the lower part of III. This is probably a reflection of the concentration of organic matter in this interval, which varies between 1% and 3.1% TOC. The organic matter is of marine Type II origin, immature and locally slightly altered (HI between 400 and 650; Tmax <435, average around 428). Measurements were carried out in the slightly shaly interbeds, which show a CaCO3 concentration varying between 75% and 90%. Sequence stratigraphic correlation Three transects have been constructed to show the significant variations in thickness in the studied area. Transect 1 (Fig. 7) has a N–S orientation and shows a transition from mid-ramp to basin in a high accommodation location. Transect 2 (Fig. 8A) is also orientated N–S, and shows an inner ramp to basin transition in a low accommodation location (local high). Transect 3 (Fig. 8B) has an E–W orientation, and provides a more regional picture, including the highs at Jebel Es Sfeia and Kef El Garia (Fig. 1b). The three-dimensional correlations in Jebel Ousselat are greatly helped by the ability to trace surfaces over the entire area. Examples of such surfaces are: an unconformable glauconite layer at the base of the studied succession, marking the Paleocene–Eocene boundary; a nautiloid-rich horizon at the base of sequence 0 in the eastern part of Jebel Ousselat (Transect 2); and the MFS of sequence IV, rich in vertebrates, which is mappable between the eastern and western sides of the Jebel. The studied succession covers most of the Ypresian stage (Fig. 2) and has an approximate duration of 4 Ma. In this interval 1.5 long-term sequences have been defined, which classifies them as third order. Eight medium-scale sequences are distinguished (fourth order), which themselves are composed of small-scale sequences (fifth order). Large-scale sequences (third order). The base of the studied interval is a glauconitic bed, which marks the unconformable contact with the underlaying El Haria Formation of Paleocene age (Bishop 1988; Saint-Marc 1992). This bed is exposed in five sections: in the Kef El Guitoune 1 and Bou Dabous sections of Transect 1, in the Sidi-El-Rahbi section of Transect 2, and in the El Sfeia and Kef El Garia sections of Transect 3. The top of the studied interval is determined by present-day erosion, and varies from section to section (sequence V to sequence VII). At the large scale, the influence of both variations in relative sea-level and diﬀerential subsidence can be seen. The facies succession shows a long-term deepening– shallowing evolution, followed by a second deepening at the top. The first transgressive phase is represented by glauconitic marls in the Jebel Ousselat area, followed by the basinal, planktonic-rich limestone–shale couplets and corresponds to the Bou Dabous Formation. Some 40 km to the south, in Jebel Cherahil, this transgressive phase is glauconite rich, phosphatic and contains reworked anhydrite pebbles. It corresponds there to the Chouabine Formation. The regressive phase is charac- terized by a shallowing-up trend and the progradation of the nummulitic ramp system (sequences I to III), the El Garia Formation. The facies show an overall evolution from nummulithoclastic to small nummulite facies in the Kef El Guitoune outcrop (Transect 1; Fig. 7) and big nummulites associated with oyster nummolithoclastic grainstones in a more proximal area (Bgour sections, Transect 2; Fig. 8A). The upper boundary is marked by a bioturbated surface covered by an iron crust. The second transgressive phase shows a deepening-upward trend and a stepwise landward retreat of the ramp system (sequences IV to VII). During the early part of the overall transgression (sequence IV), thin and big Nummulites and Discocyclina accumulated in the entire Jebel (up to 50 m thick; Figs 7 and 8). This aggradational trend marks the first deepening pulse of the El Garia Formation with the appearance of deeper water faunal associations of Nummulites/Discocyclina in the mid-ramp, and nautiloids/planktonic foraminifers in the outer ramp. The overall transgression is pulsed with at least four medium-scale sequences. The strongest transgressive pulse, the MFS of sequence VI, is marked by a widespread developed condensed surface (e.g. presence of shark teeth and phosphate) and the biggest landward shift of the facies belts (Fig. 7). The thickness and facies variations illustrated in Transect 3 (Fig. 8B) are due to antecedent topographic relief, and probably also to synsedimentary tectonics in the form of diﬀerential subsidence, further enhancing the relief of palaeohighs and troughs. Evidence for pre-existing palaeogeographic relief during the deposition of the El Garia is demonstrated by the fact that the El Garia Formation overlies progressively older stata starting with the earliest Ypresian Chouabine Formation, followed by the Paleocene El Haria Formation and eventually various Cretaceous units (Fournié 1975; Racey et al. 2001). The presence of the basal glauconite bed in both the troughs and the highs (Kef El Garia and El Sfeia) shows that initially water was deep over the entire area. Facies then shallowed up rapidly on the highs, which display condensed, but complete successions. These latter may be a result of diﬀerential subsidence. Clear indications of relative sea-level control of the sedimentation pattern are the widespread occurrence in all sections of the big nummulite facies in the transgressive part of sequence IV, when accommodation was created. The distribution of the siliciclastic fraction, sourced from Jebel Es Sfeia, also shows a direct link with the relative sea-level variations, showing a maximum dispersal during the maximum regressive phase (sequences II and III; Fig. 8B), and a gradual landward retreat during the overall transgressive phase (sequence IV to VI in Bgour 1 and Jebel Es Sfeia; Fig. 8). Another example of thickness contrast is the Bou Dabous section, which measures a total thickness of 190 m, about half the thickness of the Kef El Guitoune-1 section of 390 m thick. This is probably due to its location on the same palaeohigh as the Jebel Es Sfeia section. Medium-scale sequences (fourth order). Eight medium-scale sequences are distinguished and correlated over the Jebel Ousselat area. Sequence 0 is the least well defined, since many of the sections are incomplete at the base, and sequence definition in the pelagic domain is very diﬃcult, due to the absence of good criteria. This sequence thus includes all the sediment that is found between the basal unconformity, marked by the glauconite marls, and the base of sequence I. This may represent very little sediment, such as in Kef El Garia and Es Sfeia, or a very thick succession of pelagic facies, such as in Kef El Guitoune 1, where about 100 m of sediment were measured for this sequence. At the MFS a concentration of nautiloids and A 3D outcrop analogue model for carbonate reservoirs Fig. 7. Transect 1: western part of the Jebel Ousselat, NE–NW and N–S orientation with outcrop panorama photo. This transect, in a high accommodation location, shows the depositional evolution and geometry of the El Garia Formation with a dominant retrogradational trend towards the top of the El Garia Formation. Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 153 154 Fig. 8. (A) Transect 2: eastern part of the Jebel Ousselat, N–S orientation. This transect in a low accommodation location shows the depositional evolution and geometry of the El Garia Formation with an aggradational trend in the upper part of the El Garia Formation. (B) Regional Transect 3, E–W orientation. This transect provides a more regional picture and presents the depositional evolution and geometry of the El Garia Formation. Initial palaeotopographic relief (highs at Jebel Es Sfeia and Kef El Garia) influenced the thickness and facies variations. Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 E. Vennin et al. Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 A 3D outcrop analogue model for carbonate reservoirs 155 Fig. 9. Illustration of small-scale sequences: (A) in a third-order progradation from basin to inner ramp, showing both asymmetrical and symmetrical sequences. These sequences are bounded preferentially by sequence boundaries represented by iron-rich hardground surface or erosive surfaces; (B) in a third-order retrogradation from basin to inner ramp showing symmetrical sequences. These sequences are bounded by maximum flooding surfaces which represented the best surfaces recorded in a retrogradational trend. Foraminifera is found. The top of the sequence corresponds to a locally erosive discontinuity (Krouma Souda to Bgour sections, Fig. 8A). Sequences I, II and III mark the progradation phase of the mid-ramp system, with the deposition of the nummolithoclastic sediments and small nummulites in Transect 1, and the further progradation of the inner ramp in Transect 2. Subaerial exposures not far from the eastern part of the studied area provided at this time an influx of quartz sand (Bgour and El Sfeia sections; Fig. 8). In the quartz sand-rich facies sigmoidal cross-bedding indicative of tidal deposits is preserved (El Sfeia, Bgour-1; facies 12, Fig. 8B). The base of the sequence is characterized by an erosion surface and its top by a widespread and sharp erosive surface (Figs 7 and 8). The base of sequence IV corresponds to an important transgressive surface marked by glauconite (Kef El Guitoune sections). The early transgression is characterized by an aggradational trend composed of Nummulite–Discocyclina lithosomes (40 m thick; Kef El Guitoune 2), while in the temporarily starved basin intensely bioturbated horizons (Kef El Guitoune 1) and phosphate nodules (Bou Dabous) were formed. This aggradational part is overlain by a flooding surface, marking an important shift of the facies belts in the landward direction. The highstand is indicated by the progradation of the mid-ramp (Transects 1–3, Figs 7 and 8) and the occurrence of nummulites in a coarse nummulitic clast matrix and the decrease of Discocyclina. During sequences V to VII, which are only preserved in the western part of the Jebel Ousselat (Transect 1), the system continues to retrograde with the deposition of outer ramp, decimetre-scale bedded mudstones and carbonate/ marl couplets. The thickness of these sequences decreases upward and facies (F2 to F5) exhibit a low content of Foraminifera and are rich in micrite matrix and thin nummulithoclastic debris. Small-scale depositional sequences (fifth order). Small-scale sequences are observed at diﬀerent scales: the individual decimetre- to metre-scale beds, forming, for example, the limestone–marl couplets in the external domain (see photo in Fig. 6), fall in the range of the duration of Milankovich cycles. They often group together in metre- to decametre-thick packages, in which case they represent the shorter fifth-order scale sequences. An example of the facies change occurring in the small-scale sequences along the bathymetric profile is illustrated in Figure 9 for a distal outcrop (Kef El Guitoune-1 section). Figure 9A shows three small-scale sequences along a bathymetric profile, from basin to inner ramp, in an overall thirdorder progradational trend. These exhibit a shallowing-upward evolution in the proximal area (Bgour-1) and composite deepening–shallowing cycles in the more distal settings. Characteristic is the vertical increase of nummulites recording a gradual transition to high energy conditions. Nummulithoclastic grainstones are found at the top. The upper boundary of these small-scale sequences is formed by an iron-rich hardground surface or an erosive surface (inner ramp). Figure 9B shows three small-scale sequences of an overall third-order retrogradational trend. They represent composite shallowing–deepening cycles and are bounded by maximum flooding surfaces. Maximum flooding surfaces are associated with marls and mudstones (F1 and F2) in the distal area and by wackestones to packstones (rich in planktonic Foraminifera, F3) in the more proximal area. The shallow water facies consists of high energy, nummulitic packstones, F8 to F10, to nummulithoclast-rich packstones, F7. The deepening trend is illustrated in the diﬀerent environments by the gradual evolution of nummulite-rich floatstones to bioturbated nummulithoclast-rich packstones (F4 to F6). Small-scale sequences are best developed and preserved in the open marine, low-energy environments (e.g. the Bou Dabous and Kef El Guitoune-1 sections, Figs 5 and 6). In the Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 156 E. Vennin et al. Fig. 10. Nummulite form distribution in a sequence context: at a medium-scale sequence in third-order transgression, and in third-order regression. Thin and lenticular tests are concentrated in a transgressive trend, whereas thick and ovoid tests are best developed in a regressive trend. Smaller nummulites indicate increasing accommodation and are observed during flooding of medium-scale sequences. Biggest nummulites are observed both in proximal and distal areas, and in high (Transect 1) and low (Transect 2) accommodation settings. Megalospheric forms (A-form) are concentrated during the initial retrogradation trend, whereas microspheric forms dominated during times of deepening and shallowing due, respectively, to an increase and decrease in accommodation. proximal position, the presence of numerous erosional surfaces testifies to higher energy, and here the sequences are less complete due to amalgamation (Transects 2 and 3; Fig. 8). NUMMULITE PALAEOECOLOGY AND STRATIGRAPHIC DISTRIBUTION Ypresian rocks are typically dominated by the benthic Foraminifera Nummulites in the entire NeoTethys domain, e.g. Spain (Jiminez & Rey 1988), Tunisia (Bishop 1988), Egypt (Aigner 1984) and Oman (Racey et al. 2001). Nummulites have attracted much attention in the literature and have been studied for their way of life (e.g. Arni 1967; Blondeau 1972; Arni & Lanterno 1976; Aigner 1984; Moody 1987; Buxton & Pedley 1989), their use as bathymetric indicators (e.g. Hallock 1981; Hottinger 1997), their mode of deposition (e.g. Aigner 1984, 1985; Racey 2001) and their reservoir characteristics (Moody et al. 1988; Bishop 1988; Macaulay et al. 2001). Here we will pay special attention to their use as indicators of bathymetry. Nummulites are dimorphic species with alternative sexual and asexual reproduction phases (Hottinger 1997). In ecosystems with high ecological stress, asexual reproduction is expected. In contrast, sexual reproduction commonly takes place in a zone of optimal living conditions with greatest population density. The microspheric or megalospheric nature of nummulites is easily recognizable in thin section. The megalospheric form (A-form sensu Aigner 1984; generated by asexual reproduction) appears preferentially in shallow, more restricted areas, whereas the microspheric form (B-form sensu Aigner 1984; generated by sexual reproduction) is located in open marine areas associated with planktonic Foraminifera. Bigger microspheric nummulites are formed in zones of ‘optimal’ living conditions. Morphological change in shell shape can be considered as either an evolutionary adaptation or as a phenotypic response to environmental conditions (Hottinger 1997). Hence, some variations in shape may be indicative of change in accommodation space as well. One of the interesting aspects of this study is that it is now possible to analyse the distribution of the diﬀerent nummulites (morphology, etc.) in a stratigraphic context, since the high resolution time framework was constructed largely independent of the faunal content, by physically tracing beds from the distal to the proximal domain, the recognition of sedimentary structures and the geometrical reconstruction of the basin to ramp transition. Hence the palaeoecological significance of shallow benthic foraminiferal assemblages can be demonstrated here in an objective manner. Morphology, size and reproductive characters of nummulites seem to be indicators of change in water depth and, consequently, in accommodation rate at a medium-scale sequence stratigraphic pattern (Fig. 10). With regard to the morphology, field observations made on Eocene deposits independent of a sequence stratigraphic context demonstrated the correlation of nummulite populations of thin lenticular tests with times of increasing water depth (increase of accommodation), preferentially under high accommodation rate. In contrast, the population of thick ovoid-shaped tests corresponds to times of decreasing water depth (decreasing accommodation space). The same morphological variation correlated with change in water depth is applied to Discocyclina. Loucks et al. (1998) have identified robust and inflated tests of Discocyclina as being deposited in shallower waters than the nummulitic-rich facies. However, Henson (1950) and Ghose (1977) record the genus Discocyclina as occurring in deeper waters than Nummulites. Robust, ovoid forms of this genus are thought to have occupied fore-reef palaeoenvironments, while larger, flatter forms are thought to have occupied deeper, quieter waters (Racey et al. 2001) as confirmed by the field observations made in Tunisian deposits (Figs 5 and 10). With regard to the size, small nummulites seem to indicate time of flooding and increase in water depth, whereas larger nummulites can be dominant in both the proximal and distal environment and accumulated under both low and high accommodation rates (Fig. 10). In addition, turbidity, in combination with terrigenous input, inhibited the abundance and size of the nummulites. Outcrops in the eastern part of Jebel Ousselat (Transect 2) show an increase in terrigenous influx associated with both a decreasing size and decreasing abundance of the nummmulites (Bgour-1 and -2; Fig. 8A). Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 A 3D outcrop analogue model for carbonate reservoirs 157 Fig. 11. (A) Eustatic and (B) palaeotopographic/tectonic control on the distribution and heterogeneities of the reservoir units respectively in N–S (model of Transect 1) and E–W orientations (model of Transect 3). With regard to the reproductive characteristics, the megalospheric forms of nummulites (asexual reproduction; A-form) are abundant at the base of the transgressive trend and top of the regressive trend in zones of ecological stress under restricted conditions. Microspheric forms (sexual reproduction; B-form) dominate, in association with Discocyclina, in lithosomes in times of deepening water, due to increasing accommodation, while in regressive trends they are less abundant (Fig. 10). Jimenez & Rey (1988) have also documented a change in nummulite population with respect to short- and medium-term variation in accommodation. They have argued that lateral variations occur in contemporaneous nummulite populations that are related to environmental conditions. They show that microspheric forms produced larger size ranges and larger absolute sizes during times of deepening (increasing accommodation). IMPLICATIONS FOR THE PETROLEUM SYSTEM The high resolution sequence stratigraphic model has immediate implications for the Ypresian petroleum system. It provides a geological model that is predictive at the sub-seismic scale with respect to the distribution, geometries and heterogeneities of the reservoir, source rock and intra-formational seal facies. This model forms the basis for a three-dimensional stochastic reservoir characterization. Predictive geological model The most valuable information provided by the outcrops is the insight into geometries and lateral facies changes that cannot be resolved on seismic lines (sub-seismic scale), nor with log correlations. In particular, in transition zones, where facies changes occur, this information is critical. In Jebel Ousselat the lateral change from the inner/mid-ramp to the basin takes place in less than 3 km (outcrop photo in Fig. 7). Looking in closer detail, the pattern is more complicated, but, when put in a stratigraphic context, it appears that the diﬀerent orders of depositional sequences control, in an orderly manner, the distribution, geometries and size of the potential reservoir facies (Fig. 11). Generally speaking, during the overall regression reservoir facies are average, with a dominance of the nummulithoclasts and the small nummulites. During overall transgression, good potential reservoir facies, in the form of thick packages of nummulite rudstone to floatstone, are deposited (e.g. 50 m in sequence IV, 30 m in sequence V; Fig. 5). With respect to their spatial distribution, these good reservoir units occur in a general landward-stepping trend, and have a lateral continuity at the scale of hundreds of metres. Vertically they are separated by more muddy, transgressive deposits, which may disappear in the proximal domain (Fig. 11A). Apart from this control by changes in sea-level, a topographical control on the spatial distribution can also be demonstrated. At Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 158 E. Vennin et al. the regional scale, the eastern sections (Bgour-2 and El Sfeia sections), and in the west at Kef el Garia, a sedimentation pattern typical for palaeohighs developed, with condensed sections due to lack of accommodation space (Figs 8B and 11B). Here, an amalgamated stack of large nummulite facies beds is found at the top of sequence III and in sequence IV (Fig. 8). Two polarities thus exist: a N–S Ypresian ramp/basin polarity, which is dominantly controlled by sea-level changes, and an E–W polarity due to the tectonically induced lows and highs (both a pre-Eocene inherited palaeotopography and, possibly, some synsedimentary diﬀerential subsidence). Heterogeneities in the form of intraformational seals have also been distinguished. In a seaward direction they correspond to the phases of maximum flooding, when argillaceous mudstone facies were deposited on the ramp. In a landward direction, they correspond to cemented sandstone facies (Bgour and El Sfeia sections), and cemented grainstone facies (Transect 1, Kef El Guitoune-2 and Chara sections). The boundary between sequences III and IV, which marks the maximum regression, may have been exposed in the proximal domain. Possible related dissolution and subsequent dolomitization may have modified the property of these sediments. Good source rock facies were deposited during sequences I to IV in the adjacent basin (for the upper part of the section no data are available). The organic matter is found in the argillaceous interbeds, and varies between 0.1% and 3.1% TOC. It is of marine Type II origin (HI 400 to 600) and, in the studied outcrops, still immature (Tmax <435). Marine origin of the organic matter found in the Bou Dabous Formation is also proposed by Bishop (1988) and Racey et al. (2001). With a carbonate content varying between 75% and 90%, it classifies as a carbonate source rock. Three-dimensional stochastic modelling The three-dimensional dataset has been used for stochastic modelling with HERESIM, an in-house developed IFP program. This program is designed to respect sequence stratigraphic constraints in the modelling procedure and to honour the well data and their spatial variability. A non-stationary simulation method has been used here, in order to respect lateral facies changes from the coastal plain domain (El Sfeia area) to the basin domain (Djebel Djebil area). Main geostatistical parameters are the vertical and horizontal proportion curves and the variograms. Vertical proportion curves represent the percentage of each lithofacies at a given level in the lithostratigraphic unit, while horizontal proportion curves show the spatial distribution of the percentages of all lithofacies. The variograms are distance-dependent mathematical functions which characterize the spatial correlation of a given property or function, in order to determine the mean size of sedimentary bodies or the distance among themselves. The vertical variograms are calculated along the vertical direction of the gridded wells. The horizontal variograms are calculated layer by layer in a given direction. From these experimental variograms, a variogram model (factorized exponential model) is chosen by a fitting step. In this study, we calculate a horizontal variogram of 3500 m, which corresponds to the distance of lateral facies change from the mid-ramp to the basin in Jebel Ousselat. The database comprises 19 vertical sedimentological sections (14 outcrop sections and 5 virtual sections) interpreted in lithofacies and 24 surfaces which correspond to both the top of nummulite bodies and maximum flooding surfaces, many of which can be physically followed in the outcrops. The addition of a number of virtual outcrops allows the constraint of both surface interpolation and interpreted geometrical pattern (over- all progradation and then aggradation/retrogradation). These virtual outcrop sections do not play a part in the litho-unit simulation. Ten litho-units are simulated with the help of a proportional layering and a grid size of 250 250 0.5 m for a total volume of 25 10 0.4 km. Specific 3D matrix of proportions are built by zones with the external constraints of the geological model (Fig. 11). A matrix of vertical proportions is a 3D grid informed with local vertical proportion curves computed from one or several locations (Fig. 12A). In this study, a proportion matrix is computed by the method of areas for each given litho-unit. This method is based on the digitization of zones of cells that belong to the same depositional environment (cell dimension: Dx=Dy=1000 m). The architecture of this ramp system consists of nummulitic deposits which alternate with open marine fine-grained deposits, formed during the high-frequency fluctuations of relative sea-level. Generally speaking, the vertical proportion matrix belongs to three main types that underline the large-scale sequence architecture (Fig. 12A). The overall regressive phase is highlighted by the progradation-type matrix. It corresponds to a vast platform domain, dominated by nummulithoclast-rich floatstones, with important siliciclastic input from the southeastern coastal plain. The aggradation-type matrix, showing an abrupt decrease of clastics and a platform setting dominated by nummilitic rudstones, is recorded just above the general turnaround surface (sequence IV; Figs 11 and 12A). The third one is the retrogradation-type matrix (open ramps with nummuliticrich rudstones without clastics) during the general transgressive phase (Fig. 12A). The lithofacies simulation (Fig. 12B) illustrates in three dimensions the extension of nummulitic reservoir units and the distribution of heterogeneities, and helps to constrain subsurface reservoir models. Reservoir facies are mainly floatstones during the overall regression (Fig. 12B). Their reservoir potential is average, because of the reduced grain size of nummulithoclasts and nummulites. The best reservoir units consist of 30 to 50 m thick packages of nummulitic rudstones, which are preferentially deposited during the overall transgression (sequences IV, V, VI; Fig. 11). Based on the lithofacies simulation a simplified reservoir quality simulation (Fig. 12C) is carried out, using the semiquantitative analysis of porosity (Fig. 5). The best reservoir potential is attributed to the nummulitic rudstones with a maximum inter-granular porosity value of 15%. Best reservoirs are located in the platform margin (close to Kef El Guitoune 2, Chara 2, Darklate and Krouma Souda) and occur in a general landward-stepping trend (Fig. 12C). These are characterized by: (1) the thicknesses of porous facies, which are mostly connected in this site; (2) the low heterogeneities within porous nummulitic units, notably during the aggradation/retrogradation phase, highlighting a stratigraphic control upon reservoir quality. Towards the platform and the basin, reservoir potential decreases because of the interbedding with tight clastic-rich grainstones and tight marls/fine-grained limestones, respectively (Fig. 12C). CONCLUSIONS The main conclusions of this study can be listed as follows. 1. A hierarchical organization of three orders of depositional sequence (third to fifth) has been demonstrated for this steep carbonate ramp depositional system. A distinct overall transgressive–regressive–transgressive pattern at the thirdorder scale has been distinguished. In Jebel Ousselat four seaward-stepping and four landward-stepping medium-scale sequences have been studied and mapped in detail. Each one Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 A 3D outcrop analogue model for carbonate reservoirs 159 Fig. 12. (A) Variograms, HERESIM facies modelling based on the quantification of lithotypes from basin to inner ramp. (Variogram Azimuth: N95 to N140; Horizontal Variogram range in the Azimuth direction: 3500 m; Horizontal Variogram range in the perpendicular direction: 3500 m; Vertical variogram range: 5 m). (B) 3D block diagram, HERESIM modelling: depositional facies, illustrating the geometrical and lithofacies simulation of the nummulitic ramp system from El Sfeia platform to Djebel Djebil basin. (C) 3D block diagram, HERESIM modelling: potential reservoir facies deduced from the lithofacies simulation. of them is composed of small-scale (fifth-) order sequences, which are best developed and preserved in open marine low-energy environments, and tend to be amalgamated in the proximal sections (inner ramp setting). 2. The controlling factors on the sedimentation pattern and geometries are: (a) relative sea-level variations, at diﬀerent scales: third order of eustatic/tectonic origin, fourth order and fifth order of eustatic/climatic origin (Milankovitch type Downloaded from http://pg.lyellcollection.org/ at University of St Andrews on November 30, 2014 160 E. Vennin et al. cycles); (b) pre-Ypresian relict palaeotopography; (c) some synsedimentary diﬀerential subsidence during the Ypresian along NNW–SSE orientated fault trends, possibly controlled by salt tectonics; and (d) input of siliciclastic sands from local high (Es Sfeia area, not in El Garia area). 3. Size, morphology and reproductive characters of nummulites seem to be directly related to changes in water depth (accommodation). In a medium-scale sequence context (in third-order transgression (A) and regression (B), respectively), there is a direct relationship between water depth and morphology of nummulites: thin and lenticular nummulite tests are concentrated in transgressive trends, whereas thick and ovoid tests are best developed in regressive trends. There is a direct relationship between water depth and size of nummulites: smaller nummulites indicate increasing accommodation and are observed during flooding of medium-scale sequences. The largest nummulites are observed both in proximal and distal areas, and in high (Transect 1) and low (Transect 2) accommodation settings. There is a direct relationship between water depth and reproductive characters: megalospheric forms (A-form) are concentrated during the initial retrogradation trend, whereas microspheric forms dominated during times of deepening and shallowing due, respectively, to increase and decrease in accommodation. Accumulation of nummulites and discocyclines increases during the aggradational and transgressive systems tracts of the medium-scale sequences, in the overall, third-order transgression. 4. This study provides a three-dimensional outcrop analogue for Ypresian nummulitic carbonate reservoirs and petroleum systems in the subsurface of northern Africa. It is predictive and provides quantitative information at the sub-seismic scale with regard to the distribution, size and heterogeneities of the reservoir, seal and source rock facies. Good potential reservoir facies are found in the nummulitic accumulations. These decametre-thick packages were preferentially deposited during the transgressive phases of the medium-scale sequences during overall transgression. Heterogeneities in the form of intraformational seals are created by: (i) interfingering in the distal domain with basinal marls; (ii) interfingering in the proximal domain with cemented and compacted nummulithoclastic grainstones. The presence of source rocks in time-equivalent, interfingering, basinal sediments is confirmed (Bou Dabous). This paper summarizes the results of several field work campaigns carried out between 1992 and 1997 in Jebel Ousselat by IFP and Elf geologists. In 1992, P. Homewood (Elf) and F. van Buchem (IFP) inspected the outcrops guided by SEREPT geologists M. Majoub and S. Kharbachi. In 1993 a first field study was undertaken by an IFP team consisting of F. van Buchem, O. Point (ENSPM) and A. van der Pijl (ENSPM), with logistic support from SEREPT. A larger field campaign was organized in 1997 by IFP and ELF with logistical support of ETAP, in the context of ARTEP project ‘Quantification des corps reservoirs carbonates’ (C. Pabian-Goyheneche, project leader). The field team consisted of F. van Buchem, P. Joseph and O. Lerat (IFP), E. Vennin, M. Rebelle, M. Sonnenfeld, B. Balvay, A. Dubois, N. Laarif, R. Metlahi (Elf), H. Fakh-Fakh-Ben Jemia, C. El-Mahersi (ETAP), C.-A. Hassler (Total) and H. Zijlstra (University of Utrecht). We acknowledge all the above-mentioned geologists for their various contributions to the study presented here. COPREP, IFP, Elf (Pau), Total and GDF are acknowledged for their permission to publish. Useful and constructive reviews were provided by anonymous referees. REFERENCES Aigner, T. 1984. Biofabrics as dynamic indicators in nummulite accumulations. Journal of Sedimentary Petrolology, 55, 131–134. Aigner, T. 1985. Biofabrics as dynamic indicators in nummulite accumulations – Reply. Journal of Sedimentary Petrolology, 59, 320. Anz, J.M. & Ellouz, M. 1985. Development and operation of the El Garia reservoir oﬀshore Tunisia. Journal of Petroleum Technology (ETAP), 481–487. Arni, P. 1967. A comprehensive graph of the essential diagnostics of the nummulites. Micropaleontology, 13(1), 41–54. Arni, P. & Lanterno, E. 1972. Considérations paléoécologiques et interprétation des calcaires de l’Éocène du Véronais. Arch. 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