Stratigraphy and source rock analyses of the Heath Formation in Fergus, Garfield, Golden Valley, Musselshell, Petroleum, and Rosebud counties, central Montana

Abstract

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 an estimated 137 million bbl of oil to date from conventional traps. Due to the limited amount of conventional production to date within this petroleum system over the region's 90 year history of development, and estimates that 60% of total generated oil still remains in place within the source rock, the Heath is well suited for being assessed as a resource play. This research project is designed around addressing this possibility. Specifically it seeks to gain a thorough understanding of the Heath's potential as a resource play, and does this by meeting two primary objectives: 1) conducting a petroleum system analysis of regional stratigraphy in the Big Snowy Trough of central Montana with an emphasis toward identifying the Mississippian Heath Formation's contribution to this system, and 2) performing a source rock assessment of the Heath Formation. The results drawn from meeting these objectives lead to establishing the view of the Heath as its own stand-alone petroleum system that can potentially serve as a resource play target. The stratigraphic analysis yielded the following observations. Thickness of the Heath varies regionally from 150-450 ft, and its present depth ranges from outcropping at the surface to occurring at 6,000 ft total vertical depth, while averaging around 3,000-6,000 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 unit, ranges from 5-25 ft in thickness, and is composed of finely-laminated micritic black shale. Adjacent to this organic zone is the informally defined Heath B carbonate unit (from this study) which is composed of interbedded limestone, dolomite and shale. The carbonate beds within this interval are thin (2-5 ft thick), but generally are more developed and more prevalent in the southern region of the study area. Less organic-rich shale intervals occur abundantly in the Upper and Lower Heath units that are informally defined in this study. The source rock analysis provided the following results. Programmed pyrolysis, TOC, and vitrinite reflectance data for the Heath shales indicate very good source rock character. Within the Cox Ranch section, TOC wt. % 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 I/Type II kerogen, while Tmax suggests marginal maturity to oil window maturity with values ranging from 420 to 449 deg C. Maps of the thermal maturity 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 indicate oil expulsion from the Heath with the critical moment occurring in the Early Tertiary. The results of the stratigraphic and source rock analyses of the Heath combined clearly indicate the presence of the necessary lithologic elements needed for comprising a stand-alone petroleum system. They also exhibit the presence of system processes that signal a once active petroleum system which has subsequently been uplifted and is no longer active. Thus, the Heath's overall character suggests that it should be considered a viable candidate as a resource play target, especially when taking into consideration the result of the oil-generation-based volumetrics analysis conducted in this study. Estimates of the Heath's production potential could be 1.37 billion bbls of recoverable oil through unconventional development. In addition, thermal maturity results and reservoir facies trends indicate a highly prospective zone for this type of play in the south-central region within the study area. Based on an overall assessment, this study establishes the Heath Formation as a prospective target for resource play development. Given these results, the time has come for industry to test the Heath's viability as an oil producer.