vennin2003

A 3D outcrop analogue model for Ypresian nummulitic carbonate
reservoirs: Jebel Ousselat, northern Tunisia
E. Vennin
1
, F. S. P. van Buchem
2
, P. Joseph
2
, F. Gaumet
2
, Marc Sonnenfeld
3
,
M. Rebelle
4
, H. Fakhfakh-Ben Jemia
5
and H. Zijlstra
6
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 sub-
seismic-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 control-
led, 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 obser-
vations 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
facies of the El Garia Formation, which is dominantly com-
posed of the large benthic Foraminifera species Nummulites and
Discocyclina, while the source rock facies occur in the time-
equivalent, 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
Petroleum Geoscience, Vol. 92003, pp. 145–161 1354-0793/03/$15.00 2003 EAGE/Geological Society of London
at University of St Andrews on November 30, 2014http://pg.lyellcollection.org/Downloaded from

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
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 palaeo-
topography 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
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).
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).
E. Vennin et al.146
at University of St Andrews on November 30, 2014http://pg.lyellcollection.org/Downloaded from

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 develop-
ment 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 pro-
posed 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), oxy-
gen index (OI) and T
max
. 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.
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, how-
ever, stressed that sequence definition, in particular in carbon-
ate 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 (macro-
scopic 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 estab-
lished using the higher frequency, smaller-scale cycles of
change in accommodation (or accommodation relative to
sediment supply). The main constraints for these corre-
lations are (a) the stratal geometries provided by the continu-
ous 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 seaward-
stepping character of the depositional sequences is deter-
mined. 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 algor-
ithm 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.
A 3D outcrop analogue model for carbonate reservoirs 147
at University of St Andrews on November 30, 2014http://pg.lyellcollection.org/Downloaded from

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 litho-
facies 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; T
max
<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 nummu-
lithoclastic (F3) and echinoderm packstones (F4) and rare
nummulitic floatstones (F5). The outcrops vary from well-
bedded limestones in the relatively distal position to metre-scale
bedded limestones in proximal areas. Well-sorted nummu-
lithoclastic debris comprises 60 to 90% of whole rock.
Hummucky cross-stratifications correspond to episodic
reworked fossil deposits and show the influence of storm-
induced 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
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.
E. Vennin et al.148
at University of St Andrews on November 30, 2014http://pg.lyellcollection.org/Downloaded from

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 com-
posed exclusively of coarse nummulithoclasts (F11) with bed-
ding 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 nummu-
lithoclastic 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).
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.)
A 3D outcrop analogue model for carbonate reservoirs 149
at University of St Andrews on November 30, 2014http://pg.lyellcollection.org/Downloaded from