Regional Variations in Formation Water Salinity, Hollin and Napo Formations (Cretaceous), Oriente Basin, Ecuador

Resumen

ABSTRACT Salinity concentrations of formation waters in Cretaceous reservoirs of the Oriente basin of Ecuador range between 500 and 130,000 ppm NaCl equivalent. These values conform to salinity gradients detected within and vertically between stratigraphically discrete sandstones. Well logs and drill-stem test data document a consistent pattern of increasing salinity from east to west within individual sandstones and vertically from deeper to shallower sandstones. A model describes the hydrodynamic flow of meteoric water entering the basin through elevated and exposed section to the west, with expulsion occurring along the basin’s eastern shallow rim. The outcropping Hollin sandstone at the base of the Cretaceous section provided the only passage through which meteoric water could have initially entered the buried section. This freshwater-bearing sandstone is the source for fresh water within the younger Cretaceous sandstones of the Napo Formation. Connate fluids associated with these younger sandstones were increasingly displaced vertically and to the east where the entire Cretaceous section thins and the sandstones merge into a single column. This zone of merger is the principal vertical pathway for fluid communication between older and younger sandstones and explains the lateral and vertical gradients observed from salinity maps and electric-log profiles.