Geometry, spatial arrangement, and connectivity of grain-dominated, storm-event deposits in outcrop analogue of Late Jurassic Arab‐D reservoir, Saudi Arabia
The Late Jurassic Arab‐D reservoir, composed of the Arab-D Member of the Arab Formation and upper part of the underlying Jubaila Formation, is highly prolific in several supergiant oil and gas fields in the Middle East. An outcrop analogue of equivalent age in central Saudi Arabia shows depositional facies and stratigraphic architecture that are similar to those inferred in the subsurface. This analogue has been studied using a high-resolution digital outcrop model integrated with measured sections, in order to understand and quantify facies relationships in storm-dominated, shallow-marine carbonates.
Outcrops of the lower to middle Arab-D reservoir reveal a succession of interbedded muddy and grainy rocks that occur as a series of thin (0.5-1 m) fining-upward cycles. Cycles typically comprise a coarse-grained grainstone-to-rudstone lower part that contains muddy intraclasts and, locally, stromatoporoid and coral fragments, which fines upward into a wackestone cap. The finer portions of these cycles are bioturbated, and swaley cross-stratification occurs locally in both mud‐ and grain-dominated beds. Cycles are separated by sharp-to-erosional bases of varying relief, which cause cycle thickness to vary laterally. Locally, 1-3 m thick chaotically bedded conglomeratic intervals containing overturned stromatoporoid and coral clasts up to 1 m in diameter infill scours with steep-to-vertical walls that incise several meters into underlying deposits.
The fining-upward cycles are interpreted to result from storm events that locally scoured and reworked sediments. The occurrence of swaley cross-stratification suggests deposition below fair weather wave base but above storm wave base. Larger storm events produced steep-sided scours that were filled by conglomeratic debris transported offshore from shallower water settings.
Storm-event deposits vary laterally and vertically in their geometry, spacing and connectivity. Few coarse-grained beds extend across outcrop (<1 km) but instead pinch out laterally. Their lateral extent and degree of vertical amalgamation is controlled by erosional relief and paleotopography at bed boundaries. Conglomeratic scour fills show symmetrical and asymmetrical cross-sectional profiles, implying 3D variation in scour geometry and orientation. The heterogeneity observed in these outcrops has implications for the identification and correlation of reservoir flow units between wells, and for the effective properties of the flow units.