Heath Formation of Central Montana: A New Tight Oil Resource Play?

Graham McClave

The Mississippian Heath Formation is recognized as the key source rock for the Heath-Tyler-Amsden (!) petroleum system in the Big Snowy trough of central Montana which has produced more than 100 million bbl of oil to date from conventional traps. Thickness of the Heath varies regionally from 100-400ft, and its present depth is at 3,000-5,500 ft within the central region of the trough. It is composed of various marine lithologies including mostly black micritic shale interbedded with thin fossiliferous limestone and dolomite stringers, and minor gypsum/anhydrite occurrences. The most organic-rich zone of the Heath, the Cox Ranch Shale Member, ranges from 5-60ft in thickness, and is composed of finely laminated micritic black shale. Adjacent to this organic zone is the Forestgrove Member which is composed of interbedded limestone, dolomite and shale. The carbonate beds in this interval are thin (2-5ft thick), but generally become thicker to the south. Rock-Eval pyrolysis data for the Heath shales indicate very good source rock character. Within the Cox Ranch section, %TOC ranges from 0.5-26% and averages 9% with S2 peak values averaging 20 mg HC/g rock. Kerogen type analyses point mainly to Type II/Type II-S kerogen, while Tmax suggests oil window maturity with values ranging from 420 to 449 deg C. Maps of the Rock-Eval parameters indicate a salient zone of oil maturity in the south-central region of the study area. Burial history and thermal maturity models from southern wells show evident oil window expulsion from the Heath with the critical moment occurring in the Early Tertiary. Oil generation from the Heath shales is clearly evident; however, the Heath's potential as a tight oil resource play is contingent upon the ability to develop effective permeability within the thin carbonate interbeds. If permeability issues can be overcome, the upside to this play is strong with various estimates of recoverable resources ranging from 500 million to 4 billion barrels of oil.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012