ABSTRACT: Geological Controls on Natural Gas Production in the Antrim Shale of the Michigan Basin

Timothy M. Dellapenna

The Antrim Shale (Devonian-Mississippian) is a prolific natural gas reservoir with distinctive source rock and reservoir characteristics. The Antrim is composed of two distinctive lithofacies: a black shale lithofacies having total organic carbon (TOC) values as high as 12% and a gray-green shale lithofacies having TOC values generally below 0.5%. The black shale facies is the primary source bed and reservoir for natural gas. Based on Rock-Eval data, the kerogen in the Antrim Shale is only marginally mature with respect to generation of liquid hydrocarbons. T_max is in the range of 435°C, vitrinite reflectance values are 0.41, and thermal alteration index (TAI) is 2. These data indicate a maximum burial temperature of approximately 60°C in Otsego Count , Michigan, the main play area. Although the thermal maturation level in this area is very low, the gas produced from the Antrim is internally sourced and thermogenic; this interpretation is based on the presence of trace amounts of high-gravity oil and a thermogenic isotopic signature of produced gas samples. The black shale facies in the Antrim contains significant authigenic silica and is more susceptible to fracturing compared to the carbonate-cemented gray-green shales facies. Extremely low gas permeability values (0.1 md) indicates that fractures serve as conduits for delivery of gas to the borehole. Most of the gas must be stored in the shale matrix porosity because total gas volume produced far exceeds fracture reservoir volume.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990