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Fluvio-aeolian Interactions at a Sand-Sea Margin

Abstract

The Wahiba Sands are a coastal dune field of about 12,500 Km2 in northeast Oman. The evolution of the Wahibas has occurred in distinct stages defined by changes in fluvial and wind processes over the last 230,000 years. These multiple growth (and erosion) stages have been driven by glacially-controlled fluctuations in the strength and direction of the monsoonal wind system, in the activity of major drainages, and in sea level. The dune field has been characterized by barchanoid and linear dunes of various sizes that dominate at various times. Our study area is in the northern Wahiba Sands along Wadi Batha, where we focus on the interaction of modern wind and water processes along the wadi, and the resulting sedimentary facies. In this area, northward migrating linear megadunes and smaller barchan and linear dunes encounter the south and eastward directed ephemeral flows in Wadi Batha. Using modern weather records, we describe the seasonal wind energy distribution for Oman and the Wahiba Sand Sea. Our sand trap measurements and field observations confirm that there is currently a geologically significant northward drift of sand in the northern “High Sands” of the Wahiba and onto the course of ephemeral Wadi Batha. The sand eroded from dunes and other eolian deposits is recycled into fluvial deposits. Our trenches revealed that these basic process regimes have created complex facies variants that are intermixed at fine (trench) scale within the floodplain of the wadi. Eolian geobodies extending across Wadi Batha are elongate parallel to the dominant current direction (southeast). Dunes formed on these geobodies are oriented with respect to the wind from the south with variable dune types producing surprising crossbedding patterns in the resultant geobodies that are of interest from the standpoint of petroleum reservoir modeling. It is likely that the small scale intercalation of eolian and fluvial sands, with differences in crossbedding and texture, would, if preserved, create distinctive small scale flow units in petroleum reservoirs. Preliminary injector-producer simulation models based on our trenches suggests that inefficient sweep of hydrocarbons may occur in a reservoirs with lithologies and architecture similar to those seen in our trenches. This is due to inherent permeability contrasts at bed and lamination scale, and the small size of geobodies that comprise the petroleum reservoir.