DIAGENESIS AND FORMATION WATER CHEMISTRY OF TRIASSIC RESERVOIR SANDSTONES FROM SOUTHERN TUNISIA

The fluvial Triassic reservoir subarkoses and arkoses (2409.5-2519.45 m) of the El Borma oilfield, southern Tunisia, were subjected to cemen tation by haematite, anatase, infiltrated clays, kaolinite and K-felds par at shallow burial depths from meteoric waters. Subsequently, basin al brines controlled the diagenetic evolution of the sandstones and re sulted initially in the precipitation of quartz overgrowths, magnesian siderite, minor ferroan magnesite and anhydrite. The enrichment of si derite in C-12 isotope (delta(13)C(PDB) = -14.5 to -9 parts per thousa nd) results from derivation of carbon from the thermal decarboxylation of organic matter. During further burial, the precipitation of dickit e and pervasive transformation of kaolinite into dickite occurred, fol lowed by the formation of microcrystalline K-feldspar and quartz, chlo rite and illite, prior to the emplacement of oil. Present day formatio n waters are Na-Ca-Cl brines evolved by the evaporation of seawater an d water/mineral interaction and are in equilibrium with the deep buria l (less than or equal to 3.1 km) minerals. These waters are suggested to be derived from the underlying Silurian and Devonian dolomitic muds tones.