JOURNAL GEOLOGICAL SOCIETY OF INDIA SAMIR MEFTEH AND OTHERS Vol.83, February 2014, pp.198-210
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Effect of Diagenesis on Clay Mineralogy and Organic Matter in the Tunisian Southern Subsurface SAMIR MEFTEHa,b, MOUNIR MEDHIOUBa,b, ELHOUCINE ESSEFIc and FAKHER JAMOUSSIb a
Laboratoire de Valorisation des Matériaux Utiles, CNRSM, Faculté des Sciences de Sfax (FSS), Département des Sciences de la Terre, Route de Soukra, Km 3.5, BP 802, 3018 Sfax, Tunisia b Laboratoire de Valorisation des Matériaux Utiles, CNRSM, Technopole de Borj Cedria, BP 273, 8020, Soliman, Tunisia c UR: Dynamique Sédimentaire et Environnement (DSE) (Code 03/UR/10-03), École Nationale des Ingénieurs de Sfax (ENIS), Tunisia Email:
[email protected]
Abstract: This study aims to follow the effect of the diagenetic transformations on the clayey fraction and the organic matter of the Tunisian southern sub-surface. 61 samples from oil well named NWA-1 were recuperated for series of analyses. This study follows a comparative approach between the mineralogical, geochemical and petrographic studies. To discuss results from a statistical viewpoint, the Principal Component Analysis (PCA) was applied in order to find out any correlation between different components. The associated minerals quartz, feldspar, calcite, pyrite, anhydrite, gypsum, dolomite and olivine are also detected. These associated minerals remove by-products by the illitization reaction. The Index of Crystallinity (IC) of illite shows that, except some anomalies, the studied samples are between the epizone and the anchizone. Downward, samples show the effect of diagenetic processes and weak signs of low-grade metamorphism. As regards to the organic matter, values of Tmax range between 333°C and 463°C. On the other hand, potential hydrocarbon compounds (S2) show low values compared to those of (S1); but they maintain a similar variability from 0.63 to 21.12. SEM observations and X-ray microanalyses supported the formation of authigenic micro-quartz. The PCA of clay minerals, chemical components, and the depth shows three different populations. Feldspar, chlorite and quartz make up a population positively correlated with the depth. The second population seems to be indifferent to depth variation; it is made up of two sub-populations: the population of illite, gypsum and anhydrite, which is obtained by a counter clock rotation of depth population; and the population of pyrite, kaolinite, olivine phyllosilicate, which is obtained by an anticlockwise rotation of depth population. Third, the population of smectite, calcite and dolomite is inversely proportional to the depth variation. On the other hand, the PCA of TOC, Tmax, HI, S1, S2 and the depth make up a homogenous statistical population following the depth evolution. Keywords: Mineralogy, Geochemistry, Petrography, Clay, Illite, Kaolinite, Organic matter, Burial diagenesis, Tunisia.
INTRODUCTION
On the basis of an investigation of the mineralogical composition of recent marine sediments, the diagenesis of clay minerals was first studied by Dietz (1941). In this study, it was proven that marine water rich in potassium causes the illitisation of montmorillonite by the continental erosion. Then, Grim (1958) went so far to say that the diagenesis of clay minerals is not part of the sedimentary cycle. Rather, it is a succession of crystallochemical transformations of minerals under the influence of the depositional environment. Hence, it is a particular mechanism that should be distinguished from metamorphism. Bradley (1961) considered the diagenesis of clay minerals as positive and
negative transformations (agradation and degradation) affecting the clay networks. These transformations are enhanced by a downward increase in temperature and pressure. From then onwards, the diagenesis of clay minerals has become a well defined mechanism, in which crystal lattices can change independently of the geological context. Consequently, clay minerals have been considered as entities sensitive to the thermal and mechanical constraints and may be used as markers of diagenetic evolution (Ahmad and Majid, 2010; Devaraju et al., 2010; Jha et al., 2012). Nonetheless, distinguishing the effects of diagenesis from variations in primary composition of clays is a challenging problem (Mefteh et al., 2012).
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EFFECT OF DIAGENESIS ON CLAY MINERALOGY AND ORGANIC MATTER, IN TUNISIAN SOUTHERN SUBSURFACE 199
Similarly, the transformation of the organic matter by diagenetic processes implies the placement of several complex mechanisms under the influence of the microbiological and/or physicochemical processes (the thermal cracking of the kerogen under an increasing pressure and temperature) (Montacer, 1984; Durand, 1987; Vandenbroucke and Largeau, 2007; Suárez-Ruiz et al., 2012, and references therein). Indeed, the progressive increase in the temperature and pressure due to burial is the principal factor of the diagenetic evolution of the organic matter. This evolution is the result of an increase of the internal agitation of molecules. This agitation generates the breakage of chemical bonds; even the strong bonds could not be stable due to increase of temperature and pressure. Such evolution will result in hydrocarbon formation. Up to the limit of metamorphism, the organic matter will have a homogeneous chemical composition and a quasi stable carbonaceous solid (Pelet, 1980). Accordingly, the organic markers also record the diagenetic history according to a thermal and chemical irreversible kinetics. In the geological context, these hydrocarbons may migrate toward sub-surface reservoirs or towards the surface by means of fluids or without any agent of transport (Essefi, 2009; Essefi et al., 2013). Hence, clay minerals and organic matter are both recorders of diagenetic processes. Furthermore, their evolution are by no means separated; since by-products of organic processes may be used for the clay mineral reactions and vice versa (Suárez-Ruiz et al., 2012). Thus, the study of one of them may be useful to infer the degree of evolution
of the other. Added to the effect of the maturation of the parental material, the diagenesis of the organic matter and clay minerals also controls the quality (porosity and permeability) of oil reservoirs. At the early stage, primary porosity of the rock is reduced by compaction and cementation through mechanical processes. Whereas, the later stage of diagenesis resulted in the generation of a secondary porosity through chemical processes (Saikia et al., 2011). Having this twofold effect on the parental material and the quality of reservoir, the study of diagenesis may be useful as tool for petroleum exploration (Peltonen et al., 2009) to locate petroleum parental materials and to evaluate the quality of potential reservoirs. In this context, our integrated study is to follow the concomitant evolutions of clay minerals and organic matter along an oil well crossing the Tunisian southern sub-surface from the Silurian to the Cretaceous (Fig. 1). STUDY AREA AND GEOLOGICAL SETTINGS
The studied well, which is named as NWA-1, is located in the Saharian platform, namely in the topographic map No.139 (Fig. 1). It crosses the Tunisian southern subsurface from the Silurian to the Cretaceous (Table 1). A detailed description of the identified formations and previous works dealing with their ages and their sedimentary contents was discussed by Mefteh (2009). Figure 2 is a synthetic log showing the chronostratigraphy, the identified formations, and depths of the recuperated samples. In this paper, this
Fig.1. Geographical location of NWA-1 well. JOUR.GEOL.SOC.INDIA, VOL.83, FEB. 2014
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SAMIR MEFTEH AND OTHERS Table 1. Collected samples from NWA-1 well: chronology, lithology, and depths of samples Age
Lithology
XRD (Depth m)
Rock-Eval (Depth m)
Cretaceous (0-870 m)
Green to gray claystone; White anhydrite; Gray to pink dolomitic limestone.
130, 220, 260, 290, 390,460, 630
130, 510
Jurassic (870-1500 m)
Alternation of claystone, siltstone, and sandstone, with intercalations of dolomitic limestone.
920, 970, 980, 1100, 1110, 1140, 1160, 1240, 1250, 1290, 1320, 1340, 1450,
880, 1100, 1290, 1470, 1500
Triassic (1500-1934 m)
Consists of evaporitic deposits (anhydrite and halite with intercalations of claystone and sandstone.
1470 1550, 1560, 1570, 1590, 1660, 1700, 1725, 1920
1580, 1630, 1650, 1725, 1920
Carboniferous (1934-2049 m)
Claystone: dark gray black, non calcareous, silty in part, pyritic, micaceous.
1945, 1950, 1990, 2010
Devonian (2049-3250 m)
Light to dark gray sandstone, occasionally white; clayey non-calcareous siltstone, moderately hard; Limestone: occasionally cream white, dolomitic, argillaceous moderatly hard; Claystone: with traces of pyrite.
2060, 2245, 2310, 2795,
Silurian (3250-3970 m)
Gray silty claystone: rare red brown; Sandstone quartz white, clean translucent very finemedium, with argillaceous cement.
3498, 3502, 3552, 3562, 3580
distinction in terms of formations and ages is meant to be made in the background; because our line of thinking will be in terms of depth evolution, without giving any special focus on the regional formations. METHODS
This study follows a comparative approach between the mineralogical, geochemical, and petrographic studies of clay minerals and organic matter. The mineralogical study was carried out by the X-RayDiffraction (XRD) of the clay minerals. A total of 61 samples were analyzed for whole-rock (bulk) and clay fraction (
