Depositional Facies and Environments of the Devonian Jauf Formation in the Shedgum Area, Northeast Ghawar, Saudi Arabia
Alsalhi, Raed A.*1; Melvin, John X.1
(1) Reservoir Characterization, Saudi Aramco, Dhahran, Saudi Arabia.
The subsurface Jauf Formation (Early Devonian: Emsian) of Saudi Arabia is a significant gas reservoir along the north eastern flanks of the giant Ghawar structure. It is informally subdivided into lower, middle and upper Jauf intervals, based on extensive core studies. The lower Jauf interval is characterized by aggradational cycles of middle to upper shoreface deposits dominated by the following facies types: variably bioturbated flat to low angle laminated sandstone, bioturbated and locally dewatered argillaceous sandstone, and ripple laminated sandstone. The overlying middle Jauf interval is mostly characterized by vertically stacked upward-fining packages of tidal inlet to lagoonal bay deposits dominated by the following facies: cross bedded sandstone with argillaceous and carbonaceous laminations, clay-draped tangentially cross bedded sandstone, poorly sorted laminated sandstone with mudclasts, and flat laminated sandstone and siltstone bundles, and rarely occurring sandstones and mudstones showing inclined heterolithic stratification (IHS). These facies represent tidal inlet, tidal channel and bar complex, lagoon, and washover fan deposits. The uppermost part of the middle Jauf interval contains a variably thick (5-13 ft) palynologically defined carbonaceous mudstone unit overlying sediments of interbedded sandstone and mudstone, and highly deformed sandstone and siltstone. Based on the presence of a very low diversity (monospecific?) assemblage of Leiosphaerid palynomorphs, this mudstone unit is interpreted as representing deposition within restricted bays. The overlying upper Jauf interval records a major shift in depositional style, characterized by the vertically stacked successions of upward-fining sandstone dominated fluvial deposits, including cross bedded sandstone with common mudclasts, ripple laminated sandstone, and rare silty mudstones.
Tentative sequence stratigraphic interpretations suggest that the marine shoreline deposits of the lower Jauf interval represent aggradational transgressive to early highstand deposits of a composite sequence upon which have prograded the tidal dominated late highstand deposits of the middle Jauf interval. The middle Jauf interval contains a laterally extensive restricted bay mudstone unit, previously considered to represent a maximum flooding event. The upper Jauf interval forms the basal unit of the overlying composite sequence that is dominated by fluvial sandstones.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain