ABSTRACT: The Otsego County (Michigan) Antrim Shale Gas Field: Unconventional Reservoir, Unconventional Trapping Mechanism

MANESS, TIMOTHY R., Maness Petroleum Corporation, Mt. Pleasant, MI, DONALD J. BAILEY, Consulting Petroleum Geologist, Tomball, TX, and DAVID J. MANESS, Maness Petroleum Corporation, Mt. Pleasant, MI

A large gas accumulation (probably several TCF) within the Antrim Shale centered in Otsego County, Michigan, while geographically widespread, is a single field. The gas field, although considered unconventional, is governed by a set of critical factors common to all hydrocarbon accumulations: hydrocarbon source, reservoir, seal, and trapping mechanism. It is widely recognized that the Antrim Shale serves as both source and reservoir and that permeability is provided by natural fractures. The two other factors, seal and trapping mechanism, have largely been ignored. Yet only through an understanding of all the critical factors is it apparent why gas is found in Otsego field.

Gas is trapped in Otsego field by hydrodynamic flow and water block. Updip and north of Otsego field, the Antrim Shale subcrops beneath glacial drift. In the subcrop area, fresh water in the overlying drift enters the highly fractured and permeable Antrim Shale and flows downdip to the south. While absolute permeability of the reservoir on the updip edge is apparently better than downdip, higher water saturation in the updip fractures decreases the relative permeability to gas and suppresses gas flow, resulting in water block. The combined effects of hydrodynamic flow downdip and water block effectively serve as seal. Contrary to intuitive thinking, gas is trapped by a transition from less permeable reservoir downdip to more permeable reservoir updip.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)