vennin2003

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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
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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
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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.
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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
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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).
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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).
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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
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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.
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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.
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E. Vennin et al.
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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
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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).
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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
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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
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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
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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.
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Revised typescript accepted 9 June 2002.