The Antrim Shale: Structural and Stratigraphic Influences on Gas Production

Abstract

The Antrim Shale of the Michigan basin is one of the most actively drilled gas plays in the United States. Core analysis, geologic mapping, and core to log correlations of a 9 mi{sup 2} study area in the middle of the present play have defined geologic influences on the location and productivity of Antrim reservoirs. Application of these factors in the design of exploration and development strategies could improve gas recovery from the Antrim Shale. The lower section of the Antrim Shale, containing the present producing horizons, is composed of four lithologies that subdivide the Antrim into facies and parasequences based upon their mineralogy and textural characteristics. The black shales of the producing horizons are characterized by high but variable quartz contents and an extremely fine-grained matrix of muscovite and clays. The black shales are surrounded by two types of gray shale, differentiated by amount and form of carbonates, and a green shale. The type of shale bounding the productive, organic-rich black shales may affect stimulation strategies and their effectiveness. These black shales average 10% but can be as high as 20% TOC by weight. The organic contents impart a distinctive signature to gamma ray logs that enabled isopach, lithofacies,more » and structural mapping of the Antrim. Correlated with available production data, the maps reveal distinct trends suggesting that well performance is influenced by both structural and stratigraphic controls.« less