The Heath-Tyler(!) petroleum system, located in the Big Snowy trough in central Montana, has produced about 100 million bbl of oil from over 25 fields in an area covering 800-1200 km². This petroleum system is relatively simple in that it has a source rock, the Heath Formation of Mississippian age, overlain by a sandstone reservoir rock, the Pennsylvanian Tyler Formation. The lower part of the Heath Formation was deposited in a deep neurotic (outer shelf) environment which grades upward into a shallow, restricted, nearshore marine environment. The organic-rich mudstones and carbonates of the upper part of the Heath Formation have good to excellent source rock characteristics. Reservoir rocks within the overlying lower Tyler Formation are lenticular channel sand tones, probably resulting of a limited a.dlsfluviolacustrine, deltaic phase of erosion and deposition that incised into the underlying Heath source rock when it was uplifted and deformed prior to deposition of the Tyler Formation.

By using stable carbon isotopic and biological marker data, the moderate API gravity, low sulfur oils recovered from the Tyler correlate to the Heath Formation, an oil-prone source rock. Burial history-thermal modeling and measured thermal maturity results show an increase in maturity from west to east with maximum maturity occurring with maximum burial just preceding uplift and erosion during the Laramide orogeny in early Tertiary (40 Ma). The pod of active source rock is estimated to cover 800 km², and the thermally most mature source rock is found in pre-Laramide synclines. The generation-migration-accumulation phase occurred in early Tertiary (52-40 Ma), but subsequent oil remigration occurred as a result of Laramide deformation.

The volume of in-place hydrocarbons is estimated to be 38% of the volume of generated hydrocarbons. This high efficiency is due to the juxtaposition of the source rock and reservoir rock. Only limited volumes of oil are yet to be discovered.