Fracture-fill calcite as a record of microbial methanogenesis and fluid migration: a case study from the Devonian Antrim Shale, Michigan Basin

The Devonian Antrim Shale is an organic-rich, naturally fractured black shale in the Michigan Basin that serves as both a source and reservoir for natural gas. A well-developed network of major, through-going vertical fractures controls reservoir-scale permeability in the Antrim Shale. Many fractures are open, but some are partially sealed by calcite cements that retain isotopic evidence of widespread microbial methanogenesis. Fracture filling calcite displays an unusually broad spectrum of δ 13 C values (+34 to −41‰ PDB), suggesting that both aerobic and anaerobic bacterial processes were active in the reservoir. Calcites with high δ 13 C values (>+15‰) record cementation of fractures from dissolved inorganic carbon (DIC) generated during bacterial methanogenesis. Calcites with low δ 13 C values (<−32‰) are solely associated with outcrop samples and record methane oxidation during cement precipitation. Fracture-fill calcite with δ 13 C values between −10 and −30‰ can be attributed to variable organic matter oxidation pathways, methane oxidation, and carbonate rock buffering. Identification of 13 C-rich calcite provides unambiguous evidence of biogenic methane generation and may be used to identify gas deposits in other sedimentary basins. It is likely that repeated glacial advances and retreats exposed the Antrim Shale at the basin margin, enhanced meteoric recharge into the shallow part of the fractured reservoir, and initiated multiple episodes of bacterial methanogenesis and methanotrophic activity that were recorded in fracture-fill cements. The δ 18 O values in both formation waters and calcite cements increase with depth in the basin (−12 to −4‰ SMOW, and +21 to +27‰ PDB, respectively). Most fracture-fill cements from outcrop samples have δ 13 C values between −41 and −15‰ PDB. In contrast, most cement in cores have δ 13 C values between +15 and +34‰ PDB. Radiocarbon and 230 Th dating of fracture-fill calcite indicates that the calcite formed between 33 and 390 ka, well within the Pleistocene Epoch.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75720/1/j.1468-8123.2002.00036.x.pd

Microbial generation of economic accumulations of methane within a shallow organic-rich shale

ALTHOUGH methane of bacterial origin is ubiquitous in marine and feshwater sediments, economic accumulations of bacterial gases occur mainly at depths of several kilometres in Tertiary basins that had high sedimentation rates(1'2). Here we present an integration of geochemical and isotopic data from gas and water extracted from the Upper Devonian Antrim shale, along the northern margin of the Michigan basin, which demonstrates that significant volumes of bacterial gas have been generated in organic-rich shales at depths of less than 600 metres. The Antrim shale is mainly a self-sourced reservoir, in contrast to conventional gas deposits that have migrated from a source to a reservoir, and has become one of the most actively exploited gas reservoirs(3) in the United States. The gas-forming processes operating at shallow depths in the Antrim shale are noe unique(4) and an understanding of these processes should lead to the identification acid development of other economic, non-conventional gas deposits around the world.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62808/1/383155a0.pd