GEOCHEMISTRY OF FORMATION WATERS AND HYDRODYNAMIC EVOLUTION OF A YOUNG AND RESTRICTED SEDIMENTARY BASIN (MAHAKAM DELTA BASIN, INDONESIA)

Chemical and isotopic data on formation waters of oil-fields from two parallel anticline axes of the Mahakam Delta Basin provide information about the present-day mass transfers in the sedimentary sequence of t his basin. Depletions in Ca, Sr and K, enrichment in Rb, and the Sr/Ca ratios in the waters are related to illitization of smectite and prec ipitation of carbonate minerals, as well as dissolution of K-feldspar and precipitation of albite. These short-lasting processes seem to hav e been more pronounced in the deeper segments of the proximal anticlin e. The Sr, oxygen and hydrogen isotopic data emphasize occurrence of l ateral migrations of the formation waters within permeable units from Borneo Island to the proximal anticline under continental influence, a nd further to the distal anticline under marine influence. Description of a combined process including illitization of smectite, precipitati on of carbonate minerals, dissolution of K-feldspar and precipitation of albitic feldspar, during lateral migrations of the formation waters was only possible because of the young age and the restricted volume of the basin. In addition, the chemical signatures in the formation wa ters were not obscured by water-rock interactions during long-distance migrations that occur systematically in large sedimentary basins.