A method is disclosed for determining a component of the response of a reservoir to tidal forces (such as the amplitude of variation or delay time) in a hydrocarbons reservoir. That method comprises measuring a variable responsive to tidal forces within the reservoir over a measurement time period, determining a theoretical earth-tide for the reservoir over that measurement time period, and determining the component of the response to tidal forces by comparing the variable measurements and the theoretical earth-tide determinations. Examples of variables that are responsive to the tidal forces are fluid level in a borehole, pressure, and borehole stress. The measurement time period should be sufficiently long so that both a maximum and a minimum variable measurement are achieved during that time period (e.g., fourteen or twenty-four hours).