A likely liquids-rich resource play targeting the Silurian age A-1 Carbonate is fast developing in the Michigan Basin. Some have compared the A-1 Carbonate to the middle member of the Bakken shale at prolific Parshall Field in North Dakota. The emerging Michigan play appears to be developing in an area with abundant shows from the A-1, including a well drilled by Gulf Oil in 1941, which tested rates as high as 12 MMcf/d and 100 Bc/d.

The A-1 Carbonate is the lowermost Salina carbonate cycle which is often observed to be in lateral contact with upper-most Silurian pinnacle reef strata. This relationship has allowed the A-1 to be the dominant source for hydrocarbons produced from Niagaran pinnacle reefs in the Michigan Basin. Excellent discussions of Silurian source-rock potential can be found in Gardner and Bray, 1984, and Obermajer, et al, 2000.

Maturation data is limited, but the Mobil-Jelinek well, located in Shiawassee County (also straddling the rift and the Howell anticline) had a reported Ro (equivalent) value of 1.1 for the A-1. This may indicate it is within the oil window of the A-1 Carbonate play.

The area includes multiple stacked pays and high-potential intervals ranging from lower Mississippian to Ordovician in age. Reservoirs are dominantly limestones and dolomites with primary and secondary porosity, including hydrothermally altered rocks that are often associated with the many faults that cross the area. Prolific hydrocarbon-bearing pinnacle reefs, and possible biostromes of Silurian age are also present in the area.

Deeper Ordovician hydrothermally altered micritic limestone in the Trenton and Black River formations, as well as St. Peter and Prairie du Chien sandstone reservoirs productive on closed structures, also make up historical targets.

The structural fabric of the south-central portion of the Michigan Basin is dominated by the underlying Midcontinent rift and the Howell Anticline. The rift, which nearly bifurcates the basin, appears to have also played an important part in maturation of sediments and in the control of sedimentation. Collingwood and Utica depositional patterns appear to be influenced by this Precambrian rift. Encana's first Collingwood well was located within the rift in northern Missaukee County.

Michigan Basin is enjoying a new flush of interest. (Source: Tim Gognat)

Recently, Devon Energy and Sinclair leased large acreage positions south and southeast of the center of the Michigan Basin. Sinclair Oil & Gas Co. has drilled two horizontal interreef Brown Niagaran wells, and Devon has permitted a horizontal Silurian A-1 Carbonate well. Auburn Operating LLC also drilled at least two stratigraphic tests in the vicinity of Devon's acreage, but results are not published. The Sinclair wells are gas productive with small amounts of oil and condensate. They are located in Livingston and St. Clair Counties; the Livingston County well is located along the trend of the Howell Anticline. This acreage appears to fall within the vicinity of the Silurian pinnacle reef trend in Michigan.

In its July 1, 2011 investor presentation, Devon reported leasing 300,000 acres west and southwest of Sagninaw Bay. The company indicates that its targets are the A-1 Carbonate and the Utica (Collingwood) shale. In late July, Devon permitted a horizontal well to test the A-1 Carbonate. The location is on trend with a fault system that extends to the southeast to the site of the Gulf Oil-Bateson well, which encountered overpressured A-1 gas and oil in 1941.

Contact the author Tim Gognat at tgognat@globalgeodata.net

References are available on request.