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Analysis
and interpretation of environment
sequence
models in
Hassi R’Mel Field in Algeria*
R. Baouche1, A. Nedjari2, and R. Chaouchi3
Search and Discovery Article #50188 (2009)
Posted May 15, 20099
*Revised from manuscript prepared for presentation at 2008 AAPG International Conference and Exhibition, Cape Town, South Africa, October 26-29, 2008.
1Laboratory of the Physical Earth, University of Boumerdes, Boumerdes, Algeria ([email protected])
2University of Sciences and Technology–Sedimentary Basins–FTSGAT, Algiers ([email protected])
3Laboratory of Geology of the Sedimentary Basins, University of Boumerdes, Boumerdes, Algeria ([email protected])
With a surface area of 400,000 km2, the Triassic Province in Algeria represents a vast Saharan territory in which significant hydrocarbon layers are exploited at the Triassic and Cambrian–Ordovician levels. The Saharan Triassic consists of sediments of varied continental environments; namely, fluvial, floodplain, lake, sebkha, and wind. At the top of Formation I, the lower series of the Triassic, there are intercalations of volcanic rocks represented by dolerites. Sedimentation interspersed with periods of nondeposition is thought to have occurred during the period of the development of the Triassic continental; resulting in the development of more or less intense pedogenesis, along with associated sediments. These episodes of pedogenesis, significantly influenced by climate, are the source of important discontinuities, which can be used for sequential subdivision.
Formation
II is characterized by the eruptive units at the base of the lower member IIa
and they consist primarily of dolerites. The top of that unit is defined
primarily by a channel in a fine sandy fill. Member IIb is characterized by
fluvial facies of channels, changing upward into a complex of playas or
evaporites. This complex is characterized, in comparison of neutron/density
curves, by a negative polarity at the bottom, with a broad separation of the
curves. Formation III is characterized primarily by an evaporite facies, of
sebkha-type halite, with weak gamma ray (lower than 10 API). The sonic
value is relatively constant--around 70 µs/ft.
The various expressions of pedogenesis have been well
documented; they include marmorization slits of desiccation features, root
traces, polyhedron structures (slickenslides), ferruginizations, and Liesegang
rings. In the Saharan Triassic there are also expressions of silcrete, gypscrete,
dolocrete, and calcrete. The suggested models of electrosequences are varied at
Hassi R’Mel, with several models representing sequences-second, third, and
fourth order. The models recognized in wells at Hassi R’Mel field are
representative primarily of units filled with siliciclastic, eruptive sediments,
and evaporite-type saliferous shale deposits. The top of stratigraphic sequences
is represented by discontinuities, modifications related to pedogenesis and not
characterized in the old models. This work therefore attempts to refine the
above models by using a
well
-
log
-based modeling approach (gamma ray, neutron,
sonic, density, and resistivity) to examine the evolution of successions, along
with facies
analysis
in Hassi R’Mel field.
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The Triassic formations of Hassi R’Mel field
In Algeria have been the subject of several studies (Nedjari et al.,
2002) including facies and sequential
Description of the Hassi R’Mel Area
The Hassi R’Mel field is located in Algeria,
between 33°15 and 33°45 North, 2°55 and 3°0 East, approximately 50 km
south of Laghouat (Figure 1). Elevation is of about 760 m, and the field
extends 80 km north–south and 60 km east–west (Figure 2; Magloire, 1970;
Courel L. et al., 2000). The gas discovery with condensate in
Wells and Wells of South Hassi R’Mel used in this study are: Hrs-2, Hrs-4, Hrs-7, Hrs-8, Hrs-9, Hr-6b, Hr-8, Hr-10, Hr-11, Hr-12, and Hr-162. All wells were the subject of correlations, with emphasis on the pedogenetic products (paleosols) as defined by the University of FTSGAT (Aît Ouali R. et al., 1996). Information utilized is:
a) Reports/ratios of the layers: work completed by the geologists of
Sonatrach in connection with b) Other sources of information such as the that collected from the available bibliographies (Zeggane F., Chikhi Z., USTHB, 2000).
c) The database includes the reports and
The second stage, aimed at establishing correlations between the various vertical sections established in stage one includes:
Facies and Core
In South Hassi R’Mel, alternating
fine-to-coarse continental terrigenous facies with some interbedded
dolomites were observed (Figure 3). Eight main facies are described: one
clay facies, three sandstone facies, three dolomite facies, and dolerite
(Table 1). In a) Clean sandstones and argillaceous sandstones at the base. b) Argillaceous matrix, absence of major silicification. c) Importance of the sandstones compared to clays in the series T1 – T2. The fluvial series is characterized primarily by the volume of sand they contain and the associated natural resources (e.g., water, oil).
d) Red clays (Photo-1 of the Hrs-2 e) Salt (saliferous) is present in rather massive layers, of pink colour, with interbeds of clay, grey black, locally dolomitic; characterizing an evaporitic laguna, under hot and even arid climate; Clays with anhydrite, grey black with dark or green grey, injected anhydrite, characterizing sedimentation at the edge of the evaporitic lagoon, where water is almost permanent, but under influence of siliciclastics derived from areas with flat relief (Photo – 3. Figure 3 [Zeggane F and Chikhi Z, USTHB, 2000]).
Characterization of Environments
The studies carried out by the students of
FSTGAT (USTHB) on the Triassic sequences showed that sedimentation is
very complex but made of various continental-type environments, ranging
from alluvial fan to sebkha, with some features of a fluvial cone and a
playa. Each environment can be characterized by a standard
1. The evaporite
2. The fluvial
3. The meander
4. The
5. The volcanic
6. The alluvial fan
The combination of the various · The deposits of channels show fining-up (bell-shaped signature) or uniform grain size (cylindrical type signature) (Bourquin et al, 1996) (Figure 5). · The deposits of bars within some channel are usually coarsening-up, whereas the saliferous units and/or shale-saliferous units are of the cylindrical type.
Sedimentation at Hassi R’Mel during the Triassic
Sedimentation for the Southern Triassic in the area of Hassi R’Mel was
continental because of the climate and the structural context.
Sedimentation began with fluvial deposition (braid, meander, and
floodplain), along with the saliferous deposits under an arid and hot
climate. The succession of the various
formations was interrupted by episodes of volcanic eruptions during a
rifting phase that affected the entire Saharan platform (Nedjari et al.,
2002). Several models of sequences (Figure 5) of various scales, have
been recognized, in particular second, third, and fourth order.
The facies The interpretation of spacing (Rhob-Nphi) between the two curves leads us to introduce the concept of polarity of electrofacies, which one can distinguish as follows: · Electrofacies with positive polarity, where the density (Rhob) curve is on the right of the neutron (Nphi) curve, can indicate the presence of clay, anhydrite, or dolomite. The type of lithology is then distinguished by the values from the density (Rhob), Pef, and GR. · Electrofacies has negative polarity, where the density (Rhob) curve is to the left of the neutron (Nphi) curve, indicating the presence of sandy facies, gas, or halite. · The third electrofacies has a neutral polarity, where the density (Rhob) curve overlies the neutron (Nphi) curve, indicating sandy facies.
Fluvial system Fluvial bars of channel meanders (Hrs-12) have a negative polarity with a rather broad spacing depending, however, on the content of matrix clay; they are characterized by the following: 13<GR<45 (API), 58<DT<8 2 (µs/ft), 2.28<Rhob<2.60 (g/cc) (2, Figure 5).
The sandy bars of the type found in the braid are present primarily in
Formation II (Hrs-7) (2, 3, Figure 5), which is generally sandy with
intercalation of thin beds of clay. The sandy units show
decreasing grain size upward, as
In the
floodplain, sedimentation is represented primarily by clays. Traces of
roots as
Lacustrine Environments Lake environments are characteristic primarily on the level of the wells ( 4,
Figure
5. They have a negative polarity, with a great spacing neutron/density
that is connected to the matrix clay fraction and whose
Environments of Evaporites (Sebkha) Sebkha facies correspond to evaporites of the coastal plain and are halite, according to cross-plot of neutron vs density (1, Figure 5). The sequences observed reflect marine brine incursions and then evaporation (Bourquin et al., 1996). There appears to be a clay–salt–clay succession. These units are dated Rhaetian and they are widespread. The halite beds exhibit a negative polarity, with a great spacing neutron/density. Representative values are: 5<GR<50 (API), 68<Dt<84 (µs/ft), –0.009<Nphi<0.06a; resistivities are greater than 6000 ohm/m.
Wind
deposits or wind-blown particles are not easily detectable by the
Results and One can identify the dolerite associated with paleosols at the base of Formation II; this is representative of the major discontinuity with the top of Formation I. Shown in Figure 9 are:
·
Significant thickness variations; at Hrs-4 · Formation I has considerable shale in Hrs-10; yet this unit probably has good reservoirs qualities.
·
The thickness of the evaporite series decreases from
Hrs-10 toward Hrs-4 but increases toward the Hrs-11
In · Red marmorization rusts at 2200m; · Gypscrete at 2158 m; · Desiccation feature filled with crystalline quartz at 2151.5 m; · Argilane at the coast at 2144.5 m; · Burst quartz at 2153 m; · Pedogenetic nodules at 2150 and 2158 m; · Brecciation at 2157.5 m; and · Dolerite at 2157.5 m.
Detailed Formation I is characteristic of meander-type fluvial deposit, distal where a fine and argillaceous material dominates. Formation II was formed in a fluvial system that was predominantly braided, and it is represented in all the reservoirs of Hassi R’Mel. These reservoirs are developed in a complex way; yet they present a tabular geometry in layers with wide extent, limited by argillaceous beds (primarily vertical seals) and by Formation III of the shale-saliferous series (S4), which in turn is defined at its top by the reference mark “D2”, which is dated “Lias.” The base of the Triassic is the Hercynian discordance. In Hrs-9, paleosols aid in definition of the formations, respectively with representative values as follows:: Formation III: 24<GR<75; 9<Rt<160; 2.50<Rhob<2.75; 0.30<Phin<0.08; 57<Dt<93. Formation II: 33<GR<95; 0.14<Phin<0.33; 2.33<Rhob<2.60; 70<DT<87. Formation I: 40<GR<80; 4<Rt<13; 2.57<Rhob<2.46; 0.16<Phin<0.20; 75<Dt<86.
The authors would like to thank Professor Ahmed Nedjari for providing
the data set used for this research study and correcting this
manuscript. We remain grateful to Digital Formation for their software
support and to Geosciences Rennes1 for technical advice during the
training course studies. Our gratitude goes to Bourquin Sylvie
(Sedimentary basins of Geosciences Rennes1) for his assistance during
the
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