When water in the pores of a reservoir rock is replaced by hydrocarbon, the acoustic impedance universally reduces. When the reservoir becomes more porous, the impedance reduces further. When net-to-gross ratio (for most reservoirs) increases, the average impedance of the reservoir reduces further still. Thus low impedance is prospective. If the prospective reservoir is encased in higher-impedance rock, this produces more contrast and thus a seismic bright spot. If the prospective reservoir is encased in lower-impedance rock, this produces less contrast and thus a seismic dim spot.