Joyce M. Budai

Genetic and temporal relations between formation waters and biogenic methane: Upper Devonian Antrim Shale, Michigan Basin, USA

Abstract Controversy remains regarding how well geochemical criteria can distinguish microbial from thermogenic methane. Natural gas in most conventional deposits has migrated from a source rock to a reservoir, rarely remaining associated with the original or cogenetic formation waters. We investigated an unusual gas reservoir, the Late Devonian Antrim Shale, in which large volumes of variably saline water are coproduced with gas. The Antrim Shale is organic-rich, of relatively low thermal maturity, extensively fractured, and is both source and reservoir for methane that is generated dominantly by microbial activity. This hydrogeologic setting permits integration of chemical and isotopic compositions of coproduced water and gas, providing a unique opportunity to characterize methane generating mechanisms. The well-developed fracture network provides a conduit for gas and water mass transport within the Antrim Shale and allows invasion of meteoric water from overlying aquifers in the glacial drift. Steep regional concentration gradients in chemical and isotopic data are observed for formation waters and gases; dilute waters grade into dense brines (300,000 ppm) over lateral distances of less than 30 km. Radiogenic (14C and 3H) and stable isotope (18O and D) analyses of shallow Antrim Shale formation waters and glacial drift groundwaters indicate recharge times from modern to 20,000 yr bp . Carbon isotope compositions of methane from Antrim Shale wells are typical of the established range for thermogenic or mixed gas (δ13C = −47 to −56‰). However, the unusually high δ13C values of CO2 coproduced with methane (∼+22‰) and dissolved inorganic carbon (DIC) in formation waters (∼+28‰) require bacterial mediation. The δD values of methane and coproduced formation water provide the strongest evidence of bacterial methanogenesis. Methane/[ethane + propane] ratios and δ13C values for ethane indicate: (1) the presence of a thermogenic gas component that increases basinward and (2) progressive bacterial oxidation of ethane as the Antrim Shale subcrop is approached. Multiple episodes of Pleistocene glaciation over northern Michigan appear critical to the development of these gas deposits. Loading of thick ice sheets may have provided hydraulic head that enhanced dilation of preexisting fractures and influx of meteoric water. The physical erosion cycle of repeated glacial advances and retreats exhumed the Antrim Shale around the northern margin of the Michigan Basin, subjecting it to near-surface physiochemical and biochemical processes. The chemical and hydrologic relations demonstrated in the Antrim Shale reservoir suggest a dynamic connection between Pleistocene glacial history of the midcontinent region and development of recoverable, microbially generated natural gas reserves.

Microbial production and modification of gases in sedimentary basins: A geochemical case study from a Devonian shale gas play, Michigan basin

An expanded data set for gases produced from the Antrim Shale, a Devonian black shale in the Michigan basin, United States, has allowed for a detailed examination of the related chemical and isotopic compositional changes in the solid-gas-liquid systems that discriminate between microbial and thermogenic gas origin. In the Antrim Shale, economic microbial gas deposits are located near the basin margins where the shale has a relatively low thermal maturity and fresh water infiltrates the permeable fracture network. The most compelling evidence for microbial generation is the correlation between deuterium in methane and coproduced water. Along the basin margins, there is also a systematic enrichment in 13C of ethane and propane with decreasing concentrations that suggests microbial oxidation of these thermogenic gas components. Microbial oxidation accounts not only for the shift in 13C values for ethane, but also, in part, for the geographic trend in gas composition as ethane and higher chain hydrocarbons are preferentially removed. This oxidation is likely an anaerobic process involving a syntrophic relationship between methanogens and sulfate-reducing bacteria.The results of this study are integrated into a predictive model for microbial gas exploration based on key geochemical indicators that are present in both gas and coproduced water. One unequivocal signature of microbial methanogenesis is the extremely positive carbon isotope values for both the dissolved inorganic carbon in the water and the coproduced CO2 gas. In contrast, the 13C value of methane is of limited use in these reservoirs as the values typically fall between the commonly accepted fields for thermogenic and microbial gas. In addition, the confounding isotopic and compositional overprint of microbial oxidation, increasing the values to typically thermogenic values, may obscure the distinction between methanogenic and thermogenic gas.

Dolomitization of the Madison Group, Wyoming and Utah Overthrust Belt

Two stages of pretectonic dolomitization are recorded in the upper Madison Group of the Wyoming-Utah thrust belt. Marine to hypersaline dolomitization occurred during deposition of upper Madison carbonates and evaporites in peritidal and sabkha settings. This timing is supported by the common association of fine-grained, nonluminescent dolomite with restricted, shallow-water facies, and the relatively enriched isotopic composition of the dolomite (^dgr18O = +2.0 ^pmil; ^dgr13C = +4.0 ^pmil). A second stage of dolomitization began during exposure of the Madison shelf and continued through deposition of the overlying Amsden Formation. Regressions and transgressions during this time caused migration of marine-meteoric mixing zones across the Madison shelf and widespread dolomitization of all depositional facies within the Madison Group. The second stage of dolomitization is volumetrically dominant, has a meteoric isotopic composition (^dgr18O = -2.0 ^pmil; ^dgr13C = +2.0 to +7.0 ^pmil), and in most locations has masked the isotopic record of the first-stage, synsedimentary dolomite. In addition to replacive dolomite, dolomite and calcite cements were also formed during this diagenetic stage. These three components exhibit isotopic variations concomitant with stratigraphic position that reflect spatial gradients in ground water chemistry.

Isotopic exchange during tectonic veining: example from Absaroka sheet in Wyoming overthrust belt

Multiple stages of tectonic veining are preserved in the Mississippian Madison Group within the Absaroka thrust sheet. Three groups of veins have been defined on the basis of structural relationships and stable isotopic character. Group I veins consist of mineralized stylolite and joint surfaces which are oriented normal to both bedding and tectonic transport. Group II veins are filled fractures oriented normal to bedding and parallel to transport. Group III veins occur only in thin bedded limestones of the lower Madison Group and are mineralized bedding-parallel slip surfaces. Calcite twinning strains from each group indicate that, with rare exceptions, all veins formed pretectonically or syntectonically.

Dedolomitization in Tectonic Veins and Stylolites: Evidence for Rapid Fluid Migration During Deformation: ABSTRACT

Jurassic through Tertiary thrust-belt deformation of the Mississippian Madison Group has introduced complex fracturing, stylolitization, and carbonate vein mineralization. Host rocks are dominantly dolostone and dolomitic limestone. Tectonic veins are mineralized first by dolomite and then by multiple calcite phases. Dolomite and some generations of calcite which line veins are highly luminescent, while host-rock dolomite is non-luminescent. Both vein-lining dolomite and host-rock dolomite have been corroded and replaced by subsequent generations of calcite mineralization. These textural relationships suggest that fluids associated with thrust-belt deformation were in part extraformational and had not equilibrated with host-rock dolomite. Because thrust-belt deformation moved from west to east with time, the isotopic composition (18O, 13C) of vein and stylolite mineralization can be used to evaluate fluid migration during deformation. In three sections located along an east-west transect in the southern overthrust belt, calcite vein mineralization displays a wide range of isotopic compositions that are distinctly depleted relative to the host-rock composition. These End_Page 458------------------------------ vein-lining calcites exhibit systematic compositional changes with both time of deformation and with geographic position relative to major thrust faults. These isotopic changes in vein mineralization and pressure-solution products, together with the textural evidence for calcitization of host-rock and vein dolomite, suggest that these rocks were open to allochthonous fluid migration during deformation. End_of_Article - Last_Page 459------------

Diagenesis of Lower Coralline Limestone (Chattian), Maltese Islands: ABSTRACT

The sequence of diagenetic fabrics in the lower Coralline Limestone (Chattian) on the Maltese Islands suggests that there was one phase of early marine cementation followed by at least two periods of phreatic cementation. Cements of marine origin include fine fibrous cement on algal and foraminiferal debris, and clouded syntaxial overgrowths on echinoid fragments. A younger generation of clear overgrowths on echinoid fragments displays luminescent zones that are restricted to the lower Coralline Limestone and lower member of the Globigerina Limestone and can be correlated throughout the Maltese Islands. These clear overgrowths are phreatic and formed both before and during sediment compaction. Subsequent phreatic cementation produced fine to medium-grained, non-luminescin scalenohedral calcite crystals that postdate compaction. The final stage of phreatic cementation consists of fine to medium-grained equant void-filling spar that is non-luminescent. Vertical and lateral distribution of phreatic cements and compacted textures is irregular and discontinuous. In general, well-cemented horizons also show overcompacted textures. The relative timing of these diagenetic features indicates at least two episodes of emergence and meteoric cementation related to the development of freshwater lenses within the lower Coralline Limestone. Erosional and unconformable horizons in the overlying Miocene formations may record times of freshwater alteration corresponding to periods of eustatic lowering of sea level and emergence of the entire central Mediterranean platform. End_of_Article - Last_Page 682------------