ABSTRACT

The Mississippian Heath Formation is the primary contributing source rock of the Central Montana Platform, sourcing virtually all oil production from the Pennsylvanian/ Mississippian Tyler Formation (over 80 million barrels produced) and the Pennsylvanian Amsden Formation (over 10 million barrels produced). Most past authors assumed this relationship based on the Heath Formation sediments in subcrop overlain by Tyler Formation sediments, and the obvious visible source rock qualities of the Heath sediments. Modern geochemical analysis confirms that the Tyler-Amsden oils match characteristics of the Heath rocks. This geochemical study induded oils from virtually all Tyler and Amsden fields and rock samples induding core or cuttings from twelve wells plus outcrop samples.

Two other families of oils suggest two additional source rocks, but the identity of these sources remains unknown. The high gravity exotic oil produced at the Cat Creek Field (27,000,000 barrels, 52° APD has baffled industry for decades. Its source is still not identified, despite sampling the entire stratigraphic column. A third family of oil has been produced at Crooked Creek and Mason Lake Fields to the south (less than 1 million barrels of oil, 46° API). This group produces from similarly-aged reservoir rocks (Lower Cretaceous sands) and is also high gravity (46° APD, but clearly has no geochemical affiliation with the Cat Creek Field oils. The Crooked Creek-Mason Lake oil is very mature, has high wax content, and has very few geochemical fossils.

Additional prospective areas in central Montana remain. One key to exploration is to study the distribution of the Heath Formation, map areas where its thermal maturity has reached the oil window, and compare these areas to those which have reservoir potential in the Tyler and Amsden Formations. In addition, the Mowry Formation has excellent potential as a source rock if it can be located where sediments have matured to the oil window.