Hydrological phenomena related to elevation head on the Arabian Plate

Abstract

Arabian Plate geology consists of exposed basement in the west overlain by a Phanerozoic sequence in the east that reaches 10 km in thickness and incorporates the Zagros fold-and-thrust belt on the northeast margin of the plate. The Phanerozoic wedge results from Neogene to Recent plate tilt and consequent uplift and erosion, exposing most of the Phanerozoic formations at outcrop, becoming progressively older from the east coast of Saudi Arabia, to the Arabian Shield at an altitude of ∼900 m. Reservoirs in this Phanerozoic sequence that have top and base seals but are laterally extensive and open to the land surface have active hydrodynamics, with flow driven up to 1000 km northeastward into the basin by elevation head, discharging into the Arabian Gulf via topseal failures deep in the basin. Deep wells and seismic reflection data originally acquired for hydrocarbon exploration have been repurposed to investigate the basin-scale hydrogeology. This data has revealed previously undescribed phenomena including intra-reservoir haloclines separating fresh and saline water layers more than 1000 m below sea level, and quantified regional hydraulic gradients and an area of over 60,000 km2 where the vadose zone is up to 450 in thickness. Hydraulic gradients have been measured by wells at ∼1 m/km in several different reservoirs.