Application and Seismic Recognition of a New Permian-Carboniferous Exploration Paradigm: Unayzah C Glacial Paleovalleys, Ghawar field, Eastern Saudi Arabia

Ronald A. Sprague*, A. Kent Norton, and George S. Adcock
Saudi Aramco
*[email protected]

The Unayzah C reservoir is one of the significant gas exploration and development targets in Saudi Arabia. Recent extensive core-description work tied to regional mapping of wireline log signatures has documented the existence of relatively narrow, deeply incised linear features, which overlie the widespread Hercynian unconformity across portions of the Ghawar field. Core evidence shows that these linear features are typically filled with stacked sets of tightly-cemented to semi-friable glacio-fluvial channel sands, often separated by relatively-thin brittle and ductile shear deformation zones, which were recently interpreted as evidence of glacial overthrusting and moraine formation. These rocks are frequently, though not always, overlain by glacio-lacustrine Unayzah B and/or fluvio-eolian Unayzah A sediments. In the Hawiyah area of Ghawar field, these rocks were previously assigned to the uppermost part of the Devonian Jauf Formation despite their lack of distinctive palynomorphs or other age-datable material. This assignment was purely based of their stratigraphic position above the known Jauf reservoir sandstones. However, cores tied to wireline logs from recent Hawiyah wells showed that these rocks exhibit typical Unayzah C sedimentology and wireline-log signatures; namely stacked, variable-thickness sections of low gamma-ray and spiky high-resistivity profiles. Pressure profiles from these intervals also indicated that they appear unrelated to nearby Jauf production. Recent development mapping in the Jauf reservoir showed that this overlying Unayzah C trend extends northward into the ‘Uthmaniyah area of Ghawar field. Several west-east 3-D seismic acoustic impedance profiles through central ‘Uthmaniyah exhibited valley-shaped, low-impedance (gas-charged?) zones. These zones represent a significant, hitherto unsuspected in-field exploration and development target.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain