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

Abstract

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 d 13 C values (þ34 to À41% PDB), suggesting that both aerobic and anaerobic bacterial processes were active in the reservoir. Calcites with high d 13 C values (>þ15%) record cementation of fractures from dissolved inorganic carbon (DIC) generated during bacterial methanogenesis. Calcites with low d 13 C values (<À32%) are solely associated with outcrop samples and record methane oxidation during cement precipitation. Fracture-fill calcite with d 13 C values between À10 and À30% 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 d 18 O values in both formation waters and calcite cements increase with depth in the basin (À12 to À4% SMOW, and þ21 to þ27% PDB, respectively). Most fracture-fill cements from outcrop samples have d 13 C values between À41 and À15% PDB. In contrast, most cement in cores have d 13 C values between þ15 and þ34% 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.

The biogenic methane potential of European gas shale analogues: Results from incubation experiments and thermodynamic modelling

“…Calcites with high δ 13 C values (N+15‰) record cementation of fractures from dissolved inorganic carbon (DIC) generated during bacterial methanogenesis. Such 13 C-rich calcite is clearly indicative of biogenic methane generation (Budai et al, 2002), and resembles the δ 13 C values of fracture-filled calcite in mature Posidonia Shale (Fig. 9).…”

“…Posidonia Shale: (a) Stable carbon isotopes of carbonate in fractures for different thermal maturities(Jochum, 1988), and (b) comparison with features in the Antrim Shale(Budai et al, 2002).…”

Potential evidence of fossilised Neoproterozoic deep life: SEM observations on calcite veins from Oppaminda Creek, Arkaroola, South Australia

“…Mineral veins, as were investigated here, are particularly prospective, since they represent former sites that provided space for organisms to live in, as well as elevated fluid percolation to bring the necessary nutrients (e.g. Hofmann and Farmer 2000;Budai et al 2002). Mineral precipitates inside veins also form a stable matrix to encapsulate microbes (Cady et al 2003) and enable their preservation over geologic time.…”

“…Finally, microbial activity can also drive mineral precipitation inside rocks (e.g. Hofmann and Farmer 2000;Budai et al 2002).…”

Diagenetic mineralization in Pennsylvanian coals from Indiana, USA: 13C/12C and 18O/16O implications for cleat origin and coalbed methane generation

“…The expanded area corresponds to the gray-shaded counties in the index map of Indiana (top left). ; Shieh and Suter, 1979;Anderson et al, 1980; Spears and Caswell, 1986; Daniels and Altaner, 1990;Ambers, 1993;Daniels et al, 1996;Budai et al, 2002;Pitman et al, 2003;among others).…”