Hydrogeochemical Evaluation of the Las Piedras (Pliocene-Miocene) Aquifers in the Jusepin-El Furrial Area, Venezuela Eastern Basin

The Venezuela Eastern Basin (VEB) is subject of extensive petroleum exploitation in the last three decades, and where El Furrial Field is one of the most important producing fields. Initially to prevent pressure dropping and for additional oil recovery, EOR methods of water injection alternated with miscible gas has been used for more than 10 years where water injection rates reached values up to 450 MBWPD in 2006. Assuring a reliable water source for this project was critical. Since mid’s 1980 in the Jusepin-El Furrial area, PDVSA identified this potential groundwater source in thick (max. 1370m) and porous (34%) Pliocene-Miocene sandy deltaic to shallow-marine deposits of Las Piedras Fm., occurring in the subsurface at variable depth (300-1200m) due to folding generated by the tectonic activity in the VEB.

Las Piedras Fm. is a confined multilayered aquifer system, and this hydrogeochemical study was oriented to construct a geochemical model able to (1) identify the main aquifer’s hydrochemical facies, (2) identify the source of the waters feeding the aquifers, (3) delineate the geochemical process controlling the water’s chemical composition, and (4) evaluate the recharge potential in the Guarapiche’s river basin, using the chloride mass balance method. The constructed model was based on the data from chemical analysis and physicochemical parameters for different water wells from 5 different yearly surveys available.

The aquifer’s water is brackish-type (TDS 1000-3000mg/L) forming a dominant Na-HCO3(Cl) hydrochemical facies. The molar ratio Na+/Cl- reveals the groundwater’s meteoric origin, and the modeling indicates that the chemical composition observed is caused by a combination of processes occurring from direct recharge zones, mainly Las Piedras rocks outcropping in the foothills of the Serraníadel Interior, through the subsurface. This processes are silicates and carbonates weathering, ionic exchange, source in-situ of CO₂(g) and mixing with deeper saline NaCl-rich waters. The model also reveals an estimated recharge of at least 10% of the yearly rainfall, equivalent to 735 MBWPD. This value is higher than the actual extraction rates. With these results, several technical contributions and recommendations were achieve towards an improvement of PDVSA Policy related to Efficient Management of Aquifers Assets in the Venezuelan Eastern Region, and to evaluate impacts with potential rate increases of water withdrawal from the Las Piedras aquifers.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California