Secondary target refers to other possible reservoirs that may be above or below the primary target, which are usually thought to be not as attractive for various reasons as the primary target, possibly because they are thinner, of variable quality, are of low poroperm qualities, etc. Primary target usually refers to the most important reservoir which an O&G company hopes will contain hydrocarbons when drilling. In real life secondary targets can turn out to be better than primary, or both targets can be good or both bad