Pleistocene recharge to midcontinent basins: effects on salinity structure and microbial gas generation

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Abstract

The hydrogeochemistry of saline-meteoric water interface zones in sedimentary basins is important in constraining the fluid migration history, chemical evolution of basinal brines, and physical stability of saline formation waters during episodes of freshwater recharge. This is especially germane for interior cratonic basins, such as the Michigan and Illinois basins. Although there are large differences in formation water salinity and hydrostratigraphy in these basins, both are relatively quiescent tectonically and have experienced repeated cycles of glaciation during the Pleistocene. Exploration for unconventional microbial gas deposits, which began in the upper Devonian-age Antrim Shale at the northern margin of the Michigan Basin, has recently extended into the age-equivalent New Albany Shale of the neighboring Illinois Basin, providing access to heretofore unavailable fluid samples. These reveal an extensive regional recharge system that has profoundly changed the salinity structure and induced significant biogeochemical modification of formation water elemental and isotope geochemistry.

New-formation water and gas samples were obtained from Devonian-Mississippian strata in the Illinois Basin. These included exploration wells in the New Albany Shale, an organic-rich black shale of upper Devonian age, and formation waters from over- and underlying regional aquifer systems (Siluro-Devonian and Mississippian age). The hydrostratigraphic relations of major aquifers and aquitards along the eastern margin of the Illinois Basin critically influenced fluid migration into the New Albany Shale. The New Albany Shale formation water chemistry indicates significant invasion of meteoric water, with δD values as low as −46.05‰, into the shale. The carbon stable isotope system (δ13C values as high as 29.4‰), coupled with δ18O, δD, and alkalinity of formation waters (alkalinity ≤24.08 meq/kg), identifies the presence of microbial gas associated with meteoric recharge. Regional geochemical patterns identify the underlying Siluro-Devonian carbonate aquifer system as the major conduit for freshwater recharge into the fractured New Albany Shale reservoirs. Recharge from overlying Mississippian carbonates is only significant in the southernmost portion of the basin margin where carbonates directly overlie the New Albany Shale.

Recharge of dilute waters (Cl <1000 mM) into the Siluro-Devonian section has suppressed formation water salinity to depths as great as 1 km across the entire eastern Illinois Basin margin. Taken together with salinity and stable isotope patterns in age-equivalent Michigan Basin formation waters, they suggest a regional impact of recharge of δ18O- and δD-depleted fluids related to Pleistocene glaciation. Devonian black shales at both basin margins have been affected by recharge and produced significant volumes of microbial methane. This recharge is also manifested in different salinity gradients in the two basins because of their large differences in original formation water salinity. Given the relatively quiet tectonic history and subdued current topography in the midcontinent region, it is likely that repeated cycles of glacial meltwater invasion across this region have induced a strong disequilibrium pattern in fluid salinity and produced a unique class of unconventional shale-hosted gas deposits.

Introduction

The midcontinent region of the United States is characterized by a number of intracratonic sedimentary basins (Fig. 1). Many of these basins contain fractured black shales, which could provide preferred recharge paths for meteoric waters related to Pleistocene deglaciation. Loading and unloading of thick glacial ice sheets may have enhanced natural fractures in basin sediment and increased hydraulic conductivity Boulton and Caban 1995, Walter et al 1996. Importantly, Pleistocene glacial recharge to the Late Devonian Antrim Shale in the Michigan Basin has critically aided generation of microbial gas deposits along the shallow basin margins Martini et al 1996, Martini et al 1998(Fig. 2). Geochemical evidence for microbial methanogenesis includes the unusually high carbon isotope values (+20 to +32‰ PDB) for dissolved inorganic carbon (DIC) in Antrim Shale fluids and the covariance of δD values for methane and δD values for formation waters. In the Illinois Basin, the highly fractured New Albany Shale is reported to contain major accumulations of gas, analogous to the age-equivalent Antrim and Ohio shales (Appalachian Basin). This study investigates the role of Pleistocene glaciation on dilute water influx to the eastern margin of the Illinois Basin and subsequent microbial activity within the Late Devonian, organic-rich, New Albany Shale.

The infiltration of glacial meltwaters into the Illinois Basin is well documented and compelling; however, the majority of researchers focused on aquifers along the western margin of the basin Bond 1972, Siegel and Mandle 1984, Siegel 1989, Siegel 1991, Stueber and Walter 1994. Siegel (1989) identified regional invasion of Pleistocene waters into the Cambro-Ordovician aquifers in Wisconsin through southern Illinois. Other units, such as the Siluro-Devonian, Mississippian, and Pennsylvanian aquifers have also been influenced by meteoric water invasion Stueber and Walter 1991, Stueber et al 1993. However, the Devonian shales on the western margin are much less organic-rich and have a higher carbonate content, making them unlikely targets for gas exploration and drilling. Panno and Bourcier (1990) suggested that mixing of glacial meltwater with basinal brine along the southern margins of the Illinois and Michigan basins could be responsible for the formation of karst features. Water recharged from the basal melting of glaciers and associated overburden pressure could also flush brines from underlying aquifers, leading to saline discharges.

Although many workers have published data on Illinois Basin fluids Meents et al 1952, Clayton et al 1966, Keller 1983, few have analyzed carbon system parameters or stable isotopes that are essential to understanding the evolution of fluids and gases in basins Clayton et al 1966, Walter et al 1990, Stueber and Walter 1991, Stueber and Walter 1994, Stueber et al 1993, Demir and Seyler 1999. This study collected and analyzed fluid and gas samples from New Albany Shale wells along the eastern margin of the Illinois Basin where recent exploration enabled sampling over a relatively large geographic area. Importantly, prolific hydrocarbon exploration in stratigraphically adjacent Mississippian and Devonian strata also permitted collection of fluids from these regional aquifer systems. Formation waters were fully characterized for elemental and stable isotopic compositions, with special focus on carbon system parameters and carbon isotope compositions of gas and dissolved inorganic carbon. The hydrogeochemistry of the New Albany Shale will be compared with the age-equivalent Antrim Shale in northern Michigan. Further comparisons will be made between fluid migration pathways in the Illinois Basin vs. those in the Michigan Basin and the overall regional impact of Pleistocene recharge in reorganizing formation water salinity structures.

Section snippets

Hydrogeologic framework

The Illinois Basin is an intracratonic sedimentary basin located in the midcontinent, United States (Fig. 1). The basin is underlain by Precambrian basement rocks, part of the eastern granite-rhyolitic province (Schmus et al., 1996). The structure of the basement rocks is reflected in the overlying Paleozoic strata, which dip gently basinward, 6 to 14 m/km (Zuppann et al., 1988). The basin is structurally confined by the Kankakee Arch to the north and the Cincinnati Arch to the east, separating

Methods and results

This study spanned two major collection efforts: one during exploration and development of the New Albany Shale (1997 to 1998) and a later concerted effort to collect formation waters from over- and underlying aquifers (1999). In addition, petrophysical data (well logs, TOC vs. depth, total gas content), and water and gas analyses were provided by industrial sources. Formation water and gas samples from the New Albany Shale were collected and analyzed for elemental and isotope geochemistry (65

Chloride content

The New Albany Shale contains formation waters of variable salinity associated with mixed microbial and thermogenic gas. The formation water salinity pattern (Fig. 6a) along the eastern margin of the Illinois Basin shows that relatively dilute waters (Cl <300 mM) near the shale subcrop penetrate to significant depths in the basin. In southwestern Indiana, the chloride content increases rapidly from <300 to >2000 mM basinward. The major axis of freshwater incursion into the shale appears to

Implications

Repeated Pleistocene glacial advances and retreats likely played a key role in recharge of dilute waters into the New Albany Shale and underlying carbonates. Studies have shown that basal melting of continental ice sheets in the lower ablation zone could have provided adequate meltwater and hydrostatic head for recharge of dilute waters to basin margins Siegel and Mandle 1984, Boulton and Caban 1995, Piotrowski 1997. Glacially driven groundwater flow was likely to have been parallel to the

Acknowledgements

is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research (PRF grant 35927-LMW). Additional support was provided by the Gas Research Institute under contract 5094. We thank K. L. Shelton, G. Garven, and D. I. Siegel for manuscript review. Members of the New Albany Shale GRI Consortium provided access to key water and gas samples as well as to timely production information. We also thank the Illinois Basin oil and

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