Fossil Fuel Resource History - Milton Ranch Area
By Karen Porter
Petroleum resources
Oil exploration across the Central Montana Uplift has a long history, beginning in the 1920s. There have been notable successes and puzzling failures, due in large part to the complex geologic history of the Uplift. Specific conditions are necessary for oil to originate, migrate, or accumulate, and each time the geologic setting is altered those conditions may be enhanced or compromised or terminated. The central Montana Uplift, as a major block of earth’s crust, experienced a number of downwarps and uplifts over a long period of geologic time, which in turn influenced the patterns of erosion and deposition across the Uplift through time. As a result, geologists must first try to understand this history before imagining where the oil-productive beds will be in the subsurface.
Oil exploration was most active on and adjacent to the Central Montana Uplift in the 1940s through 1970s. The early efforts were driven primarily by the nation’s need for large supplies of petroleum in the conduct of WWII, followed by continuing need to supply a growing post-war nation. Throughout those decades, exploration was conducted primarily by detailed surface mapping and rudimentary seismic methods.
On the south side of the Uplift in northern Golden Valley and Musselshell Counties, exploration began with the discovery of oil in the Mississippian Heath Formation at Devil’s Basin Field in 1919. Devil’s Basin Anticline is a large, elongated dome-like structure that can easily be observed in the field – or just driving up US Highway 87. It is one of many "sheepherder anticlines" recognized across the Uplift by their obvious surface structural expression. That is, geologists in the field saw that the orientation (strike and dip) of rock units on the ground defined a closed anticline or a dome-like feature, recognizable even when the anticline or dome has been extensively eroded. In more recent decades, and continuing today, exploration has depended on more sophisticated analytical tools, primarily 3D seismic technology, but even more on the ability of geologists to interpret the subsurface relationships of rock units through time. A primary example of the complexity encountered is the Pennsylvanian Tyler Formation play involving oil accumulation in ancient channel sandstones. For success in the Tyler, both stratigraphic and structural data must be evaluated.
Two currently producing oil fields are located on or immediately adjacent to the Milton Ranch. Both are typical sheepherder anticlines. Gage Dome Field, on the south flank of the Bull Mountains Basin (Fig. 2), was discovered in 1943, and as of November 2013 has produced 686,464 barrels (Bbls) of oil from the Amsden Dolomite (Pennsylvanian), with 2 wells still in production. Big Wall Field, located just north of the ranch, on the Central Montana Uplift (Fig. 2), was discovered in 1948, and as of November 2013, has produced 8,723,175 barrels (Bbls) of oil from the Tyler Formation (Mississippian-Pennsylvanian) and Amsden Formation (Pennsylvanian), with 12 wells still in production. The wells still producing in these two fields are shut-in (not pumping) many days of the year and probably are being operated as "stripper" wells, that is, producing less than 10 barrels of oil per day. Table 1 summarizes the petroleum production (primarily oil) from fields on or near the Milton Ranch property.
Coal resources
Coal beds are associated with onshore coastal plain and deltaic environments governed by stable, low-energy, fresh-water conditions over a long time period. Although vegetation may be dense along a coastal shoreline (such as a coastal mangrove swamp), the shoreline is too unstable and subject to erosion to permit thick layers of matted vegetation to form. Studies of modern peat-forming environments show us that only where the depositional environment is quiet and stable over a long time period can vegetative matter accumulate and compress into peat. Estimates vary but generally six to ten inches of peat are required to form one inch of coal.
In the Western Interior of North America, both the Upper Cretaceous and lower Tertiary (Paleocene) stratigraphic records contain significant coal deposits. Of these, the Paleocene deposits are by far the most significant.
The Upper Cretaceous Eagle and Judith River Formations (Fig. 4) record two periods when large deltas formed along the western margin of the Cretaceous Interior seaway – that seaway that extended from the Arctic to the Gulf of Mexico. The huge sediment influx reflected the newly rising Rocky Mountains to the west. These deltas prograded far out into the seaway. Behind the fronts of these deltas depositional conditions varied from high-energy stream channels carrying sediment to the delta front to low-energy settings including extensive swamps. Within these swamps, enough peat accumulated to form coal beds upon burial and compaction by overlying sedimentary layers. In both the Eagle and Judith River Formations, the coal beds are at the top of the formation, finally flooded over by the advance of the next seaway (the Claggett and the Bearpaw seas, respectively) (Fig. 4). These Cretaceous coals have never been found in significant volume or with adequate lateral continuity to be economic. However, early settlers in north-central Montana commonly used Judith River coal for domestic heating, and this practice may have been more common than is recorded.
At the very end of Cretaceous time, as the Western Interior basin was filling in and the seaway was draining away, deltaic outbuilding from the seaway’s western margin occurred again, this time for good. A regional deltaic system extended from western Wyoming northeastward into eastern Montana and the Dakotas – today’s Powder River Basin -- depositing the delta-front (marine) Fox Hills Sandstone and overlying delta-plain Hell Creek Formations (Fig 4). Coal beds at the base of the Hell Creek (top of the Fox Hills) are well developed in the region, but do not extend westward into central Montana where these two formations have been mapped as one unit, the Lance Formation (Kl) (Figs 2, 4).
The subtropical, swampy coastal plain advanced across the region uninterrupted into the early Tertiary (Paleocene), recorded by the Fort Union Formation (Fig. 4). Coal beds in the Tongue River Sandstone, upper member of the Fort Union Formation, are a significant economic resource for Montana and Wyoming, centered in the Powder River Basin. These sub-bituminous, low-sulphur coals are ten’s of feet thick, reflecting long periods of quiet, fresh-water growing conditions for Paleocene vegetation. They are interbedded with the sandstones of channels that coursed across the coastal-deltaic plain. As with the coals of the Fox Hills-Hell Creek interval, Tongue River coals do not extend westward into central Montana in anything but thin, discontinuous non-economic beds and stringers.
*Possibly in reference to the favorable elevations from which sheep herders could view their flocks.
Oil exploration across the Central Montana Uplift has a long history, beginning in the 1920s. There have been notable successes and puzzling failures, due in large part to the complex geologic history of the Uplift. Specific conditions are necessary for oil to originate, migrate, or accumulate, and each time the geologic setting is altered those conditions may be enhanced or compromised or terminated. The central Montana Uplift, as a major block of earth’s crust, experienced a number of downwarps and uplifts over a long period of geologic time, which in turn influenced the patterns of erosion and deposition across the Uplift through time. As a result, geologists must first try to understand this history before imagining where the oil-productive beds will be in the subsurface.
Oil exploration was most active on and adjacent to the Central Montana Uplift in the 1940s through 1970s. The early efforts were driven primarily by the nation’s need for large supplies of petroleum in the conduct of WWII, followed by continuing need to supply a growing post-war nation. Throughout those decades, exploration was conducted primarily by detailed surface mapping and rudimentary seismic methods.
On the south side of the Uplift in northern Golden Valley and Musselshell Counties, exploration began with the discovery of oil in the Mississippian Heath Formation at Devil’s Basin Field in 1919. Devil’s Basin Anticline is a large, elongated dome-like structure that can easily be observed in the field – or just driving up US Highway 87. It is one of many "sheepherder anticlines" recognized across the Uplift by their obvious surface structural expression. That is, geologists in the field saw that the orientation (strike and dip) of rock units on the ground defined a closed anticline or a dome-like feature, recognizable even when the anticline or dome has been extensively eroded. In more recent decades, and continuing today, exploration has depended on more sophisticated analytical tools, primarily 3D seismic technology, but even more on the ability of geologists to interpret the subsurface relationships of rock units through time. A primary example of the complexity encountered is the Pennsylvanian Tyler Formation play involving oil accumulation in ancient channel sandstones. For success in the Tyler, both stratigraphic and structural data must be evaluated.
Two currently producing oil fields are located on or immediately adjacent to the Milton Ranch. Both are typical sheepherder anticlines. Gage Dome Field, on the south flank of the Bull Mountains Basin (Fig. 2), was discovered in 1943, and as of November 2013 has produced 686,464 barrels (Bbls) of oil from the Amsden Dolomite (Pennsylvanian), with 2 wells still in production. Big Wall Field, located just north of the ranch, on the Central Montana Uplift (Fig. 2), was discovered in 1948, and as of November 2013, has produced 8,723,175 barrels (Bbls) of oil from the Tyler Formation (Mississippian-Pennsylvanian) and Amsden Formation (Pennsylvanian), with 12 wells still in production. The wells still producing in these two fields are shut-in (not pumping) many days of the year and probably are being operated as "stripper" wells, that is, producing less than 10 barrels of oil per day. Table 1 summarizes the petroleum production (primarily oil) from fields on or near the Milton Ranch property.
Coal resources
Coal beds are associated with onshore coastal plain and deltaic environments governed by stable, low-energy, fresh-water conditions over a long time period. Although vegetation may be dense along a coastal shoreline (such as a coastal mangrove swamp), the shoreline is too unstable and subject to erosion to permit thick layers of matted vegetation to form. Studies of modern peat-forming environments show us that only where the depositional environment is quiet and stable over a long time period can vegetative matter accumulate and compress into peat. Estimates vary but generally six to ten inches of peat are required to form one inch of coal.
In the Western Interior of North America, both the Upper Cretaceous and lower Tertiary (Paleocene) stratigraphic records contain significant coal deposits. Of these, the Paleocene deposits are by far the most significant.
The Upper Cretaceous Eagle and Judith River Formations (Fig. 4) record two periods when large deltas formed along the western margin of the Cretaceous Interior seaway – that seaway that extended from the Arctic to the Gulf of Mexico. The huge sediment influx reflected the newly rising Rocky Mountains to the west. These deltas prograded far out into the seaway. Behind the fronts of these deltas depositional conditions varied from high-energy stream channels carrying sediment to the delta front to low-energy settings including extensive swamps. Within these swamps, enough peat accumulated to form coal beds upon burial and compaction by overlying sedimentary layers. In both the Eagle and Judith River Formations, the coal beds are at the top of the formation, finally flooded over by the advance of the next seaway (the Claggett and the Bearpaw seas, respectively) (Fig. 4). These Cretaceous coals have never been found in significant volume or with adequate lateral continuity to be economic. However, early settlers in north-central Montana commonly used Judith River coal for domestic heating, and this practice may have been more common than is recorded.
At the very end of Cretaceous time, as the Western Interior basin was filling in and the seaway was draining away, deltaic outbuilding from the seaway’s western margin occurred again, this time for good. A regional deltaic system extended from western Wyoming northeastward into eastern Montana and the Dakotas – today’s Powder River Basin -- depositing the delta-front (marine) Fox Hills Sandstone and overlying delta-plain Hell Creek Formations (Fig 4). Coal beds at the base of the Hell Creek (top of the Fox Hills) are well developed in the region, but do not extend westward into central Montana where these two formations have been mapped as one unit, the Lance Formation (Kl) (Figs 2, 4).
The subtropical, swampy coastal plain advanced across the region uninterrupted into the early Tertiary (Paleocene), recorded by the Fort Union Formation (Fig. 4). Coal beds in the Tongue River Sandstone, upper member of the Fort Union Formation, are a significant economic resource for Montana and Wyoming, centered in the Powder River Basin. These sub-bituminous, low-sulphur coals are ten’s of feet thick, reflecting long periods of quiet, fresh-water growing conditions for Paleocene vegetation. They are interbedded with the sandstones of channels that coursed across the coastal-deltaic plain. As with the coals of the Fox Hills-Hell Creek interval, Tongue River coals do not extend westward into central Montana in anything but thin, discontinuous non-economic beds and stringers.
*Possibly in reference to the favorable elevations from which sheep herders could view their flocks.