Impact of the Hercynian Orogeny on Hydrocarbon Accumulations in Eastern Saudi Arabia

Faqira, Mohammad, Martin Rademakers, Abulkader Afifi, Saudi Aramco, Dhahran, Saudi Arabia

Recent well and seismic data confirm that most Saudi oil and gas accumulations are located in structures that were initiated during the Carboniferous Hercynian Orogeny. The widespread Hercynian deformation formed two kinds of structures (1) a broad regional arch oriented NNE, and (2) narrow north and Northeast trending basement horsts. These uplifts underwent erosion of pre-Carboniferous rocks, and syntectonic deposition of the continen­tal Unayzah Formation clastics along their flanks. The Hercynian Orogeny exerted significant impact on Paleozoic oil and gas plays. The extent of the Hercynian deformation and erosion controlled the hydrocarbon accumulations in the Devonian Jauf, Permo-Carboniferous Unayzah and Permian Khuff reservoirs. The Hercynian contributes to the Paleozoic oil and gas plays by (1) focusing hydrocarbon migration along the eastern and southern edges of the Central Arabian Arch, (2) providing top and lateral seals for the Devonian Jauf truncation play along the edges of Hercynican uplifts where the Permian Unayzah is absent, and (3) channeling hydrocarbons through major Hercynian faults. The reactivation of Hercynian faults during the first and second Alpine Orogenies developed migration pathways from the Silurian source rocks to the Upper Paleozoic reservoirs. Furthermore, in areas where the Hercynian unconformity has eroded into the Cambro-Ordovician section, the Unayzah reser­voir was subjected to high pressure- brines from the Cambro- Ordovician, which may have displaced hydrocarbons from these traps.