Sedimentology and stratigraphic evolution of fluvial–tidal transition reservoirs: an outcrop analog for the hydrocarbon-bearing Bahariya Formation, Western Desert, Egypt

Abstract

The deposits of the tidal–fluvial transition zone are one of the most significant and complicated components of marginal marine systems. Sedimentological studies of these deposits are necessary due to their heterogeneous nature, which is controlled by competing tidal and fluvial parameters. Outcrop studies are required to understand the architecture, sedimentology, and evolution of tidal–fluvial deposits. The Cenomanian upper unit of the Bahariya Formation in the northern part of the Western Desert of Egypt is a tide-dominated fluvio-estuarine deposit sourced from crystalline basement and Early Cretaceous siliciclastic sedimentary rocks that lie to the southeast and south. Based on sedimentary facies analysis and paleocurrent data, the upper Bahariya Formation is composed of six main architectural elements: 1) river-dominated, tide-influenced point bar, 2) tide-dominated, river-influenced point bar, 3) floodplain, 4) crevasse splay, 5) crevasse channel, and 6) mud plug. These elements are stacked in a multistory tidal–fluvial channel complex and associated depositional elements. The reconstructed paleochannels trend from southeast to northwest, and migrated to the east and southeast. The relative contribution of fluvial processes decreased upwards through the stacked stories, with a corresponding increase in the contribution of tidal processes that were associated with transgression. An understanding of the architecture and sedimentology of the tidal–fluvial transition from outcrop successions allows the improved characterization of tidal–fluvial point-bar reservoirs and associated elements.