Abstract

Water invasions in the Cantarell reservoir (Region Marina Noreste) by an advancing oil-water contact have been related to a natural fracture network (which provides most of the permeability in the field), favoring production of water and gas over oil. In order to quantify the impact of oil extraction on the natural hydraulic mobility of fractured groundwater systems and to provide actualized concepts for the hydrogeological reservoir model, a continuous sampling program with hydrochemical-isotopic analyses (δ²H, ³H, ¹¹B/¹⁰B, δ¹¹B, δ¹³C, ¹⁴C, δ¹⁸_O, and ⁸⁷Sr/⁸⁶Sr) of formation water and core samples was performed in 2008 for the Ku-Maloob-Zaap and Cantarell oil fields, as prototypes for developing and mature reservoirs, respectively. In the Cantarell case, oil extraction causes increasing lateral groundwater flow velocities up to several hundred meters per year, while hydraulic communication between stratified groundwater horizons by vertical fracture networks is calculated to be three to four orders of lower magnitude. In contrast, the Ku-Maloob-Zaap reservoir is still characterized by an unaltered, natural stratification of groundwater horizons, with increasing water salinities towards deepest parts of the reservoir (JSK). Geochemical alteration processes between reservoir host rock and formation water, such as active dolomitization and clay desorption, are likely to increase primary fracture permeability, explaining abrupt changes in current aquifer mobility and pathway reactivity.