Stratal Architecture of the Permian-Carboniferous Lower Unayzah Formation, Subsurface Eastern Central Saudi Arabia Compared with the Juwayl Formation at Outcrop, Wajid Region, Southwest Saudi Arabia

John Melvin* and Arthur K. Norton
Saudi Aramco
*[email protected]

The Juwayl Formation at outcrop and the Unayzah C and B reservoirs in the subsurface of Saudi Arabia are considered stratigraphically equivalent units deposited during the Late Paleozoic Gondwanan glaciation. This is supported by their stratigraphic architecture as well as sparse biostratigraphic data. The Juwayl at outcrop and on satellite imagery commonly occupies elongate features interpreted as glacial paleovalleys. In the subsurface, much of the lower Unayzah, particularly the Unayzah C, is clearly mappable as a paleovalley system: these typically seismically transparent paleovalleys are defined by well control. Significant mid-Carboniferous uplift, with associated widespread erosion and sediment bypass, occurred prior to deposition within these paleovalleys. Evidence exists that an antecedent drainage system controlled paleovalley distribution, as did the Lower Paleozoic subcrop pattern, with preferential erosion of finer grained rocks. The lower Juwayl outcrop consists of high-energy pebbly sandstones and conglomerates deposited in relatively proximal braided rivers. The subsurface Unayzah C comprises sandstones and minor conglomerates, deposited in a relatively distal braided fluvial system. These braided fluvial sandstones represent pro-glacial outwash deposited during times of glacial retreats. In some outcrops, the lower Juwayl paleovalley-fill displays zones of horizontal dislocation and related severe overfolding. Within the Unayzah C sandstones, substantial thicknesses of relatively undeformed sediment are separated by distinct zones of low-angle shear. This deformation is interpreted as the result of glacial re-advances and the formation of glacio-tectonic push moraines. At outcrop the upper Juwayl Formation displays glacio-lacustrine and diamictite facies. In the subsurface, the Unayzah B overlies the Unayzah C and comprises many depositional facies, dominated by glacio-lacustrine sediments, including diamictites. These sediments represent the final meltout phase of the Gondwanan glaciation.

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