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Effect of diagenesis on clay mineralogy and organic matter in the Tunisian southern subsurface |
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| Authors: | Samir Mefteh Mounir Medhioub Elhoucine Essefi Fakher Jamoussi |
| Institution: | 1. CNRSM, Faculté des Sciences de Sfax (FSS), Département des Sciences de la Terre, Laboratoire de Valorisation des Matériaux Utiles, Route de Soukra, Km 3.5, BP 802, 3018, Sfax, Tunisia 2. CNRSM, Technopole de Borj Cedria, Laboratoire de Valorisation des Matériaux Utiles, BP 273, 8020, Soliman, Tunisia 3. UR: Dynamique Sédimentaire et Environnement (DSE) (Code 03/UR/10-03), école Nationale des Ingénieurs de Sfax (ENIS), Sfax, Tunisia |
| 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. |
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