Fate and Transport of Shale-derived, Biogenic Methane

Natural gas extraction from unconventional shale gas reservoirs is the subject of considerable public debate, with a key concern being the impact of leaking fugitive natural gases on shallow potable groundwater resources. Baseline data regarding the distribution, fate, and transport of these gases and their isotopes through natural formations prior to development are lacking. Here, we define the migration and fate of CH4 and δ13C-CH4 from an early-generation bacterial gas play in the Cretaceous of the Williston Basin, Canada to the water table. Our results show the CH4 is generated at depth and diffuses as a conservative species through the overlying shale. We also show that the diffusive fractionation of δ13C-CH4 (following glaciation) can complicate fugitive gas interpretations. The sensitivity of the δ13C-CH4 profile to glacial timing suggests it may be a valuable tracer for characterizing the timing of geologic changes that control transport of CH4 (and other solutes) and distinguishing between CH4 that rapidly migrates upward through a well annulus or other conduit and CH4 that diffuses upwards naturally. Results of this study were used to provide recommendations for designing baseline investigations.

the Pierre Shale Fm (between 600 and 0 m BTS) at Sites 6 and 7. In the case of the CH4 and Clprofiles, the lower and upper boundaries were held constant at 280 and 180 g m -3 and 6200 and 4000 g m -3 , respectively. The lower and upper boundary conditions for 12 C-CH4 and 13 C-CH4 were set at 276.9 and 3.1 g m -3 (-69.5‰ for δ 13 C-CH4) and 0 and 0 g m -3 , respectively. The initial CH4 and Clconcentrations were assigned concentrations of 0 and 19,400 g m -3 (roughly sea water) and the initial concentrations for both stable isotopes of C were set at 0 g m -3 . The three models were run for 72 Ma (age of the Pierre Shale Fm) and resulted in near steady-state concentrations throughout the Pierre Shale Fm prior to glaciation.

Phase 2: Defining Glacial Profiles
The impact of Sutherland and Saskatoon Gr glacial periods on the final (present-day) profiles for CH4, Cl -, and δ 13 C-CH4 at both Sites 2 and 5 was assessed by simulating nine scenarios that represented the probable ranges in depositional times for the Sutherland Gr (i.e., 0.6, 1.0, and 1.6 Ma) and Saskatoon Gr (i.e., 0.1, 0.3, and 0.6 Ma) tills on CH4, Cl -, and δ 13 C-CH4 profiles. Based on our geologic logs, the measured thicknesses of the tills (9 m of Sutherland Gr till and 45 m of Saskatoon Gr till) were deposited in two sequential glacial periods at Site 5. Due to the lack of Sutherland Gr till at Site 2, we assumed that 9 m of Sutherland Gr till (i.e., the same thickness as the Sutherland Gr till at Site 5) was initially deposited but completely eroded prior to deposition of 36 m of Saskatoon Gr till.
The impact of glacial deposition on the upper boundary concentrations over geologic time is unknown. As a result, we were required to make several simplifying assumptions. The upper boundaries for both CH4 and Clat Site 2 were located at the top of the Pierre Shale Fm based on the present-day CH4 and Clconcentration profiles across the till-shale interface. The CH4 and Clconcentrations were assigned values of 95 and 2,200 g m -3 , respectively, at the onset of the Sutherland Gr and 6.8 and 240 g m -3 at the onset of the Saskatoon Gr. The upper boundary concentrations at the time of the deposition of the Saskatoon Gr till were based on current measurements; however, because of a lack of information, the Clconcentration at the time of Sutherland Gr till deposition was determined based on the quality of the fit to the observed data in the subsequent phases of the simulation and the CH4 concentration calculated using the linear equation between Cland CH4. At Site 5, the upper boundary conditions for CH4 and Clat the time of deposition of the Sutherland Gr till was fixed at 95 and 2200 g m -3 , respectively, consistent with Site 2. The simulated profiles at the end of Sutherland Gr till deposition period were used to define the initial profile for the simulation of Saskatoon Gr till deposition. The upper boundary conditions for CH4 and Clwere fixed at 3.5 and 66 g m -3 during Saskatoon Gr till deposition (based on present-day measurements). Based on their present-day elevations of Sutherland and Saskatoon Gr tills, the upper boundary conditions for CH4 and Clat Site 5 were set at 45 and 13 m BG during the deposition of Sutherland and Saskatoon Gr tills, respectively.
The initial concentrations in both tills were set at 0 g m -3 for both CH4 and Cl -. These assumptions are supported by low background concentrations of Cland CH4 (about 20 and 0 g m -3 , respectively) measured in thick, recently deposited (Battleford Fm; Table S1) till 17,48 and believed to reflect the concentrations in tills immediately after till deposition 16 .
In the case of δ 13 C-CH4, the initial δ 13 C-CH4 profile was assumed to be constant at -69.5‰ prior to glaciation, consistent with the presence of near steady-state CH4 diffusion and the underlying δ 13 C-CH4 boundary. Constant concentrations of 93.993 g m -3 for 12 C-CH4 and 1.007 g m -3 for 13 C-CH4 (a δ 13 C-CH4 of -102.5‰) were set as upper boundary conditions at the top of the Sutherland Gr till during the deposition of the Sutherland till at Sites 2 and 5. The simulated profiles of 12 C-and 13 C-CH4 at the end of Sutherland Gr till deposition were used as initial concentrations to simulate Saskatoon Gr till deposition. Based on present-day measurements of the total CH4 concentration and the value of δ 13 C-CH4 at the top of Saskatoon till for both sites, upper boundary concentrations of 12 C-and 13 C-CH4 assigned to the top of Saskatoon till were 6.68 and 0.07 g m -3 (a δ 13 C-CH4 of -85‰) at Site 2 and 3.46 and 0.04 g m -3 (a δ 13 C-CH4 of -85‰) at Site 5, respectively. The concentrations for total CH4 used in the simulations were those used in the CH4 simulations. Fig. S1 Calculated CH4 concentrations through the Quaternary deposits and Cretaceous shales from Isojars ® at Sites 2 (solid blue diamonds) and 5 (solid green squares) and mud gas logs at Site 6 (solid red circles). The estimated CH4 solubility profile based on Site 5 Isojars ® and cuttings samples collected in IsoJars® at Site 7 is represented by the solid blue line based on in vitro gas concentrations corrected for in situ temperatures and pressures. Dashed lines represent calculated CH4 gas solubility profiles based on gas concentrations of 10, 25, 50, and 100% CH4 (left to right). The CH4 gas solubilities were corrected for in situ temperature and pressure.  . Geology and dissolved CH4 concentrations through the Quaternary deposits and Cretaceous shales at Sites 2, 5, 6, and 7. Isojar ® data from Sites 2 and 5 are shown as solid blue diamonds and open grey stars, respectively, and calibrated mud gas data from Sites 2, 5, and 6 as dashed black line, solid black line, and solid red circles, respectively.. The best-fit 1-D diffusive modeling results (1.6 Ma and 0.3 Ma for Sutherland and Saskatoon Group tills, respectively) are presented as solid lines (Sites 2 and 6) and dashed lines (Site 5). Figure S4. Geology and Clconcentrations through the Quaternary deposits and Cretaceous shales at Sites 2, 5, 6, and 7. Squeezed core data from Sites 2 and 5 are shown as solid blue diamonds and open grey stars, respectively, and calibrated mud gas data from Sites 2, 5, and 6 as dashed black line, solid black line, and solid red circles, respectively. Measured Clconcentrations from the base of the Pierre Fm to the Joli Fou Fm from a drill site located 250 km SE of the study area 22 are presented as solid black circles. respectively. The best-fit 1-D diffusive modeling results (1.6 Ma and 0.3 Ma for Sutherland and Saskatoon Group tills, respectively) are presented as solid lines (Sites 2 and 6) and dashed lines (Site 5). Figure S5. Geology and δ 13 C-CH4 values through the Quaternary deposits and Cretaceous shales at Sites 2, 5, 6, and 7. IsoTube® data from Sites 2 and 7 are presented as solid blue diamonds and green crosses, respectively, and mud gas data from Sites 5 and 6 as open grey stars and solid red circles, respectively. The best-fit 1-D diffusive modeling results (1.6 Ma and 0.3 Ma for Sutherland and Saskatoon Group tills, respectively) are presented as solid lines (Sites 2 and 6) and dashed lines (Site 5).