Sequential Stratigraphy of Outcropping Strata Equivalent to Arab-D Reservoir, Wadi Nisah, Saudi Arabia

Abstract

Two outcrops in Wadi Nisah of Central Saudi Arabia expose sedimentary successions of the Jubaila and Arab formations which are stratigraphic equivalents of the Arab-D reservoir in Ghawar field. These carbonate exposures consist of lithofacies and stacking patterns similar to the succession found in Arab-D reservoir. Nineteen vertical sedimentary cycles were identified by collectively evaluating disconformable relationships, textural characteristics, and lithofacies assemblages. These, in turn, define 3 depositional cycle types: (1) stromatoporoid, (2) skeletal bank, and (3) thinning-upward. The lateral distribution of lithofacies within the stromatoporoid cycle shows that stromatoporoid grainstones form organic buildups flanked by burrowed muddominated carbonates. Stromatoporoid banks locally exhibit thickness increases of about 3 meters (10 feet) over a horizontal distance of 500 meters (1, 700 feet). Conversely, the five foot-thick skeletal grainstone capping the skeletal bank cycle forms a continuous sheet exceeding 2 kilometers (1.2 miles) in width. Thinly bedded carbonate sand units within the thinning-upward cycle exhibit pronounced lateral discontinuity. Beds pinch out against topographic highs formed by stromatoporoid buildups over distances of less than a hundred meters (328 feet). Results of the outcrop stratigraphic analysis enable visualization of lateral and vertical stratigraphic relationships and potential fluid-flow pathways. This study demonstrates that outcropping cycles and their lithofacies components have several important implications for ongoing subsurface reservoir characterization and modeling of the Arab-D. Firstly, cycle definition identifies stratigraphic units whose bounding surfaces describe envelopes that constrain the interwell distribution of lithofacies. Secondly, the lithofacies define geometries for the interwell distribution of petrophysical characteristics. Thirdly, the understanding of lateral relationships of these carbonates aids in reservoir simulation modeling.