A sachet for preparation of a beverage. The sachet has a compact roast and ground coffee cake having first and second cake opposed surfaces extending between to a peripheral cake edge. The first opposed surface is substantially convex and configured for ensuring leaktightness in an extraction chamber that is configured for injecting water and extracting the coffee. The peripheral edge has a peripheral surface that is generally normal to the opposed surfaces. An outer sheet of a flexible material is impermeable to oxygen and water vapor and contains and impermeably seals the cake therein.