A method and apparatus is provided for detecting and categorizing subsurface hydrocarbon reservoirs. Air samples are collected and analyzed by counting hydrocarbon ions, such as methane ions and counting at least one higher order hydrocarbon ions, such as propane, pentane or hexane. The methane ions and at least one higher order hydrocarbon ions are associated with location co-ordinates to form a first raw data set and second raw data set. The first and second raw data sets are analyzed and processed to produce hydrocarbon footprints. The hydrocarbon footprints are superpositioned with other available geological information and subsurface formations of interest are identified.