ABSTRACT

The Mississippian Upper and Lower Tyler Formations of central Montana are alluvial valley fill and estuarine deltaic sequences that mimic modern clastic models. The Recent Brazos River and the Trinity River delta, depositional sequences from the Texas Gulf Coast, help predict sand distribution. Tyler deposition was restricted to parts of the Central Montana Trough, a Proterozoic-generated aulacogen which extended westward from the Williston Basin to Missoula, Montana. This rift formed a Mississippian basin about 60 miles wide by 400 miles long (97 by 644 km).

The Upper and Lower Tyler Formations are westward draining Mississippian (Late Chesterian) alluvial valley fill and estuarine deltaic depositional systems. They consist of a number of river valleys that were funneled into the topographic low of the aulacogen. Rift-controlled estuarine euxenic limestones and shales below, and interbedded with, the Tyler provide thermally mature, petroleum-rich source rocks.

The modern Brazos River valley of southeast Texas is an excellent analog for Tyler alluvial valley fill. Individual Tyler river valleys are approximately 6 mi (10 km) wide, with valley fill about 300 ft (92 m) thick. Sand bodies are generally in meandering patterns; point bars fine upward and are continuous lateral ribbons about 3,000 ft (915 m) wide and up to 80 ft (24 m) thick. Grain size ranges from basal conglomerates fining upward to silts and clays. Sand bodies of the Tyler exhibit reduced porosity due to diagenesis. Tyler net sand reservoirs generally average 1,200 to 1,500 ft (366 to 458 m) wide and 10 to 50 ft (3 to 15 m) thick. Small estuarine deltas also occur, primarily in the Upper Tyler. The modern bayhead Trinity birdfoot delta is a good analog for Tyler deltas.