GREENSAND RESERVOIRS IN SILICICLASTIC SHORELINE SYSTEMS: FACIES MODELS FOR HYDROCARBON EXPLORATION

Abstract

Greensand reservoirs are particularly significant in siliciclastic shoreline systems. Formation of autochthonous glauconite is restricted almost exclusively to the rising limb of the relative sea level (RSL) curve. Depositional environments that are typically developed in response to a rise in RSL are estuaries, barrier bars and shoreface sands. In this setting, the greensands are diachronous and exploration for hydrocarbon reservoirs should ideally be based on a sequence stratigraphic framework. These deposits are characterised by a number of significant surfaces that differ greatly in their origin, geographic extent and chronostratigraphic significance, and their thickness depends on the rate of RSL rise and sediment supply. The identification of significant surfaces bounding or contained within depositional sequences is critical, especially where drillhole samples are either unreliable or not available for geochemical and palynological analysis. Abrupt physical and chemical changes often occur on or across these surfaces, e.g. porosity, permeability and the presence of Fe-rich authigenic minerals such as glauconite and siderite. Greensands deposited in estuarine, barrier bar and shoreforce environments have excellent oil and gas potentail. Early Cretaceous greensand reservoirs from the North West Shelf are best developed where they overlie ravinement surfaces. Recognition of greensands within estuarine, barrier bar and shoreface environments is dependent on the correct identification of facies and the subsequent interpretation of the bounding discontinuities.