Geometric attributes of reservoir elements in a modern, low accomodation, tide-dominated delta

Abstract

The influence of wave, tide and fluvial processes on shorelines varies considerably in and between coastal systems; this can result in the development of architecturally complex, mixed-process systems. Of these, tide-dominated systems produce some of the most heterogeneous deposits. The arrangement of reservoir elements generated by wave and fluvial processes in such tide-dominated systems can be, to some degree, systematic and predictable. This research details a modern, tide-dominated, fluvial-influenced, wave-affected coastal system. It presents geometric attributes for reservoir elements that can be used to improve the construction of 3D reservoir models of these depositional environments. The Mitchell River is the largest fluvial system, discharging into the low accommodation setting of the Gulf of Carpentaria. Its Holocene delta extends to more than 500 km2. Eleven types of depositional elements (n = 3,100) were mapped across the delta plain: 286 km2 of tidal, 133 km2 of fluvial and 101 km2 of wave elements make up the delta surface. Fluvially and wave-formed reservoir elements form systematic arrangements across the system. More than 75% of wave elements are aligned inside 45° of the shoreline and these are generally crescentic (asymmetric) or linear in shape. Fluvial elements are aligned either perpendicular to the shoreline, or alongshore, because they are trapped behind wave-formed, shore parallel features. Separate wave and fluvial reservoir element datasets demonstrate convincing, though distinctly different, length-to-width relationships; wave-formed elements are much longer than fluvial-formed elements, relative to their widths. Despite pronounced heterogeneity in the distribution of these depositional elements across the delta surface, these relationships suggest their distribution is, to some degree, predictable. Analysis of the connectivity of adjacent sandbody elements suggests the largest connected sandbody is significant and extends to more than 90 km2.