--> New Insights & Perspectives on the Effects of Structural Reactivation on the Upper Devonian Antrim Shale, Michigan Basin

AAPG Eastern Section Meeting

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New Insights & Perspectives on the Effects of Structural Reactivation on the Upper Devonian Antrim Shale, Michigan Basin

Abstract

The Upper Devonian Antrim Shale of the Michigan Basin has proven significant economic viability with a total cumulative gas production of >3.43 TCF. Over 11,000 Antrim wells have been completed with 9,181 of those wells being online as of January, 2016. Spatially, production volumes and the chemical composition of both natural gas and formation waters vary throughout the Antrim Shale. Previous studies have attributed variable natural gas compositions to mixing, bacterial alteration and migration whereas formation waters indicated the mixing of brines from deeper formations with freshwater recharge. Recent studies of noble gas signatures suggest that the source of natural gas in the Antrim Shale has migrated vertically from deeper formations. Variability in the natural gas and formation waters composition were attributed to microbial methanogenesis of thermogenic gas due to the influx of meltwater proximal to the Antrim subcrop following the Wisconsin glaciation. Currently, there is limited knowledge on the mechanism that has induced migration and the controls on the distribution of thermogenically derived hydrocarbons. Evaluation of the spatial distribution of the specific gravity and chloride of formations waters and gas composition suggest the occurrence of localized thermogenic hydrocarbon (e.g. C2+ - ethane) anomalies. The preservation of thermogenic components is attributed to isolation from meltwater invasion or recent migration of hydrocarbons due to neotectonic influences. Localized thermogenic hydrocarbon anomalies were observed spatially proximal to the subcrop suggesting the occurrence of structural conduits that have enabled the migration of dense saline brines as well as thermogenically derived hydrocarbons. Evaluation of structural contour and derivative models suggest that these younger structural lineaments extend vertically through the Traverse Limestone as well as the Sunbury Shale, the lower and upper stratigraphic boundaries of the Antrim Shale, respectively. It is proposed that these structural lineaments overlie deep seated basement faults enclosed within a regional transtensional pull-apart subbasin. Subsequent reactivation within the Michigan Basin is proposed to have induced movement along the deeper pull-apart system extending vertically through the Sunbury Shale. Overall, this study provides new insights and a conceptual model for the potential structural mechanism that controls the occurrence and distribution of thermogenically derived hydrocarbons.