Mechanism of overpressure variations in the Permian dune fields of Saudi Arabia

Overpressures in a Permian clastic formation that rests on the Hercynian unconformity in Saudi Arabia are described and interpreted. This formation includes aeolian dune fields that reach 10,000 km2 in area. The formation is part of a basin-scale pressure seal that separates pore fluid pressure systems above and below the Hercynian unconformity. Pore fluid overpressure variations range from hundreds to thousands of pounds per square inch within and between the dune fields respectively. The steps in overpressure are arranged in nonrandom regional spatial trends that consistently increase to the east and south, but the reason for that pattern is not immediately obvious. The overpressures are analyzed and explained here using three-dimensional seismic attributes, well data, tectonostratigraphy, and conceptual models of dune field architecture. The overpressure originates from a Silurian hydrocarbon gas-generating source rock that is present below all the dune fields. The lateral variations in overpressure are accounted for by the hydrogeology of the section between this source rock and the Permian dune fields, featuring vertical pressure transfer, combined with the architecture and properties of the formation that contains the dune fields. The relatively porous Devonian section, present only where the Paleozoic is least eroded below the Hercynian unconformity, acts as a pressure sink, draining westwards where it connects with open reservoirs that are normally pressured due to Cenozoic regional tilt, erosion, and exposure. At a finer scale, pressure domains within each dune field, defined by overpressure variations in the order of hundreds of pounds per square inch, are accounted for by eastwards climbing geometry of interdune fine-grained baffles.