Antrim Shale Depositional Environments, Diagenesis, and Source Rock Potential in Michigan Basin

Timothy M. Dellapenna

The Antrim Shale in the Michigan basin is a significant reservoir of in-situ natural gas. Presently it is being drilled in only a few northern counties of Michigan, but may be producible basin-wide. Several authors have suggested that the reservoir resulted from fracturing. Two processes may have caused these fractures: (1) framework collapse due to volume reduction associated with the conversion of organic matter to gas, and (2) the loss of interstitial water resulting from the conversion of smectite to illite. It has been suggested that the sediments are thermally immature and that the natural gas is biogenic in origin.

The Antrim Shale is a facies equivalent of other Mid-Continent Black Shales that are documented to have been deposited in an oxygen-stratified basin, with three zones, aerobic, dysaerobic, and anaerobic. Water depth and wave energy apparently controlled the amount of oxygen in the depositional environment and thus the preservation of organic matter in the sediments. If anaerobic conditions existed then bioturbation of the sediments was low and the preservability of organic matter was high. The organic content of the shales, and the potential production of natural gas, is related to environment of deposition. By understanding the facies relationships, the environment of deposition, and the vertical and areal distribution of these facies, nature of the fracture systems, and the maturati n level of the hydrocarbons, a better exploration program can be developed for the exploitation of the Antrim Shale gas reservoirs.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.