Developing deep high-resolution concentration and 13C isotope profiles for methane, ethane, and propane

Journal of Petroleum Science and Engineering 170 (2018) 280–290

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Developing deep high-resolution concentration and methane, ethane, and propane

13

C isotope profiles for

T

Karlynne R. Dominatoa, Benjamin J. Rostronb, M. Jim Hendryc, Erin E. Schmelingc, Courtney D. Sandaud, Scott O.C. Mundlea,∗ a

Great Lakes Institute for Environmental Research, University of Windsor, 401 Sunset Ave., Windsor, ON N9B 3P4, Canada Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada c Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2, Canada d Chemistry Matters Inc., Suite 405, 104-1240 Kensington Rd NW, Calgary, AB T2N 3P7, Canada b

A R T I C LE I N FO

A B S T R A C T

Keywords: Methane Isotopes Drilling Profile Gas migration Reservoir

Methods to collect and analyze concentration-depth profiles of dissolved methane (C1), ethane (C2), and propane (C3) and their δ13C-values from directional drilling operations were evaluated. The δ13C values of C1-C3 gases from drill cuttings were consistent; however, deeper zones (> 1200 m BG) were subject to greater variability. The variability for C1-C3 isotope values was attributed to the drilling process, where the samples reflected mixing between different geologic zones. Gases and rock fragments from several formations likely contaminated the samples collected from the underlying intervals. The effects of mixing were observed in deeper zones in the borehole below the ‘kick off’ point for the directional drilling process. The greatest impact was observed for C1 and C2 gases, which was attributed to greater concentrations of these gases in shallow gas-bearing zones. This study demonstrates that mud gas and drill cuttings can generate high-resolution concentration and isotope depth profiles for C1, C2, and C3 gases. However, mixing between zones along the borehole during drilling, an effect that can be exacerbated by directional drilling techniques, can impact the precision and accuracy of the geochemical profiles. The variability in the geochemical data generated from drilling programs can have a significant impact on the effectiveness of these profiles for reservoir characterization, exploration, and in identifying the source(s) of fugitive gases migrating to surface.

1. Introduction The Williston Basin is an important reservoir of unconventional energy in North America underlying 250,000 km2 of Montana and North Dakota in the USA and Saskatchewan and Manitoba in Canada (Fig. 1). Recent production in the basin has come from enhanced oil recovery via CO2 injection in conventional reservoirs from the Midale Member, while the dominant unconventional tight-oil play in the basin is in the underlying Bakken Formation (Emberley et al., 2005; Mayer et al., 2013; Meissner, 1991; Schmoker and Hester, 1983). Vertical migration of fugitive gases, typically dominated by methane (C1) and other low molecular weight, volatile hydrocarbons such as ethane (C2), propane (C3), and butane (C4), are of concern because they can contaminate shallow aquifers, destroy agricultural soils around wellheads, contribute to greenhouse gas emissions, and, in extreme cases, present an asphyxiation/explosion hazard (Baldassare and Laughrey, 1997; Baldassare et al., 2014; Cahill et al., 2017; Osborn et al., 2011; Rowe and Muehlenbachs, 1999). Both molecular and stable isotope (δ13C and ∗

Corresponding author. E-mail address: [email protected] (S.O.C. Mundle).

https://doi.org/10.1016/j.petrol.2018.06.064 Received 12 March 2018; Received in revised form 18 June 2018; Accepted 21 June 2018 Available online 22 June 2018 0920-4105/ © 2018 Elsevier B.V. All rights reserved.

δ2H) compositions of these fugitive gases have been used to determine their origin (Osborn et al., 2011; Rich et al., 1995; Rowe and Muehlenbachs, 1999; Szatkowski et al., 2002; Tilley and Muehlenbachs, 2006). The increased use of advanced oil recovery technology has stimulated interest in developing an improved understanding of the regional baseline geochemistry, new approaches to characterize production and injection zones, and to identify the source zones for fugitive gases migrating to surface (Dawson and Murray, 2011; Hendry et al., 2017a). Baseline characterization of lateral and vertical variability in gas concentrations and their isotope values is needed for the long-term development of an oilfield and to define future environmental concerns (Blanc et al., 2003; McKinney et al., 2007; Jackson et al., 2013; Vidic et al., 2013). Baseline characterizations can be achieved from available access points provided by production, observation, and groundwater wells; however, they often only provide a very limited number of potential sources in many areas. Additionally, collecting gases and/or formation water samples that represent in situ conditions from

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Fig. 1. Location map. Extent of Western Canadian Sedimentary Basin (yellow), Williston Basin (blue) in northwestern USA and western Canada, and location of study area. Location of Weyburn (Hendry et al., 2016, 2017b), southern Saskatchewan (Skoreyko et al., 2014) and west-central Saskatchewan (Szatkowski et al., 2002) sites are indicated. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

measured on the gases released from the cuttings and/or core to the headspace in the jars (Clark et al., 2015; Wiersberg et al., 2015; Hendry et al., 2016, 2017b). However, the cost to collect core samples is often prohibitively high. A key limitation to defining baseline characteristics and using this information to evaluate reservoir continuity (Blanc et al., 2003; McKinney et al., 2007; Milkov et al., 2007), oil/gas genetic information during exploration campaigns (Schowalter and Hess, 1982; Hawroth et al., 1985), and fugitive gas sources is the limited knowledge on the factors that affect the concentrations and isotope values of the dissolved gases from deeper drilling and directional drilling. The factors that affect the reliability of the compositional and isotopic geochemical profiles generated from mud gas, drill cuttings, and core samples collected from discrete depths during drilling have been characterized (Hendry et al., 2016, 2017b). However, limited information is available on the use of these approaches to generate depth compositional and isotopic profiles for C1, C2, and C3 from directional drilling with horizontal segments that accurately reflect in-situ dissolved gas concentrations and their isotopic values. We evaluated the use of mud gas and drill cuttings to characterize the factors that limit achieving high-resolution C1, C2, and C3 concentrations and 13C isotope profiles from the Mississippian (1460 m BG) to surface using directional drilling in the Williston Basin. The results of this work provide new insight that can be used to improve geochemical profiles characterized from deep drilling operations and their use in reservoir characterization, exploration, and identifying source zones for gas migration.

production, observation, source water, and groundwater wells can be problematic. Production and observation wells can have negligible gas concentrations or gases degraded by microbial activity that alter their isotopic values. Slow groundwater recharge rates will prohibit purging the standing water from groundwater wells that can result in a mixed sample of altered and unaltered formation porewater (Wassenaar and Hendry, 1999). This is especially true for characterizing the volatile and reactive components of dissolved hydrocarbon gases, where sample collection is particularly challenging because the pore fluid must remain isolated from the atmosphere at a pressure greater than the total dissolved gas pressure. This limitation combined with the high costs of groundwater and observation well installations (or other formation/ groundwater sampling instrumentation) warrant the investigation of alternate methods to collect and analyze gas concentrations and isotope values during drilling operations. Three main approaches are used to generate geochemical profiles from drilling. One approach targets the direct measurement of hydrocarbon concentrations using field portable gas chromatographs on the gases released from fluids (drill mud) during drilling. Mud gas concentrations are often dependent on the rate of drilling, volume of drill fluid, and amount of background and atmospheric contamination in the mud gas, and they do not directly represent in situ concentrations (Hammerschmidt et al., 2014). Discreet samples of the mud gases can be collected in appropriate gas sampling media (e.g., IsoTubes®) during drilling programs (Mayer et al., 2015; Hendry et al., 2016, 2017a, 2017b; Rowe and Muehlenbachs, 1999; Strauss et al., 2015; Skoreyko et al., 2014). The second approach, often used in place of, or in addition to mud gas profiling, is to collect drill cuttings at specific depth intervals during drilling in appropriate containers (e.g., Isojars®). The third approach is to collect core samples at specific depth intervals during drilling. In the latter two cases, concentrations and isotopic values are

2. Site locations and geology Geochemical profiles were previously reported in Saskatchewan for 281

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are shallow water carbonates that may be subdivided into a lower lithostratigraphic unit informally known as the “Vuggy”, and an upper unit called the “Marly”. Vuggy rocks are predominantly limestone and range from 10 to 22 m in thickness and are highly heterogeneous. A lower interval within the Vuggy contains northeasterly trending shoals with high porosity and permeability that form some of highest quality reservoir in the Weyburn Pool. Upper Vuggy rocks reflect an intershoal depositional environment, are more homogeneous, and have lower porosity and permeability than the lower Vuggy. The Marly is a relatively homogeneous microsucrosic dolostone with higher average porosity and but slightly lower average permeability than the Vuggy. The CO2 used in the miscible flood is currently being injected into the Marly dolostones using both horizontal and vertical injector wells. The sweep of the CO2 extends down into the Vuggy as well as forming a miscible front with Marly oil. 3. Materials and methods 3.1. Drilling

Fig. 2. Regional Saskatchewan.

stratigraphy

and

hydrostratigraphy

of

Geologic logging was performed on a rotary drilled well using a Polycrystalline Diamond Compact (PDC) drill bit with a vertical main portion and two horizontal legs at a confidential location in southern Saskatchewan. Drill cuttings were collected from surface, through the start of deviation (“kick off” point, KOP; 1240 m BG TVD) to the horizontal section of the well (1460 m BG TVD) at the production zone. As a result of the vertical/horizontal combination, samples were collected to 1630 m of measured borehole length (depth) that represented 1460 m of true vertical depth (TVD) below ground surface (BG).

southeastern

3.2. Sample collection and analysis seven locations (shallow drilling) in the Weyburn Study Area (Hendry et al., 2016, 2017a, 2017b), one location (deep drilling) near Estevan, SK (Skoreyko et al., 2014), and one location near Lloydminster, SK (Szatkowski et al., 2002) (Fig. 1). Detailed gas and isotope profiles from the overlying glacial deposits through the Cretaceous shale were collected at two sites, Sites 2 (Hendry et al., 2016) and 5 (Hendry et al., 2017b), to depths of 150 and 200 m below ground surface (BG), respectively, and from the water table through the Mississippian (1600 m BG) at additional sites, Site 6 (Hendry et al., 2017a) and 7 to 1000 and 950 m BG, respectively (Fig. 1). Near Lloydminster, in West-central Saskatchewan, drilling was completed to a depth of approximately 550 m BG (Szatkowski et al., 2002). The stratigraphy in this area consists of Quaternary glacial tills, overlying shale and siltstones of the Colorado and Montana Groups, which overly Cretaceous sandstones, siltstones, and shales of the Mannville Group (Fig. 2). Deep drilling was completed to a depth of approximately 3250 m BG at the Aquistore project site (Skoreyko et al., 2014) in the northern part of the Williston Basin, near the town of Estevan, SK (Fig. 1). The stratigraphy ranges from Quaternary glacial drift at surface to early Ordovician and Cambrian sandstones at depth (Skoreyko et al., 2014). The Weyburn Unit oilfield produces from carbonates of the Mississippian-aged Midale Beds of the Charles Formation that occur at depths ∼1.5 km BG within the northeastern Williston Basin (Fig. 1). The 2800–3000 m thick stratigraphic succession in the area ranges from Middle Cambrian and Early Ordovician sandstones that directly overlie Precambrian basement to Quaternary glacial drift at ground surface. Rocks of the Williston Basin can be subdivided into two general types: deeper Paleozoic strata dominated by carbonates, evaporates, and minor shales; and shallower Mesozoic strata dominated by shales, siltstones, and sandstones. Reservoir rocks of the Weyburn Unit are immediately below a basin-wide angular unconformity (Watrous Formation) that separates the Paleozoic and Mesozoic packages (Mossop and Shetsen, 1994). The Mesozoic Mannville Group comprises a regional aquifer in the Williston Basin (Cody and Hutcheon, 1994). Reservoir rocks of the Midale Beds

Drill cutting samples were collected directly from the shale shaker, washed with de-oxygenated distilled water, and placed immediately into ∼440 cm3 containers (Isojar®) (IsoTech Labs Inc, 2014). Samples were submerged in de-oxygenated water that were flushed with inert gas (argon) prior to sealing. Containers were then transported to a commercial laboratory (Isobrine Solutions Inc.) for analysis. Gas concentrations were measured in the headspace by transferring into an evacuated serum bottle sealed with a butyl-rubber gas-tight septum. The gases in the headspace of the Isojar® were displaced with the same degassed deionized water source that was used on site to submerge the drill cuttings samples. The mud gas collection cylinder, located in the mud tank on the shaker table, was connected to an Agilent 490 gas chromatograph (GC) and an IsoTube® gas-sampling manifold. IsoTube® samples were collected throughout the drilling program at a lower frequency than the Isojars®. IsoTube® and Isojars® samples were shipped to a commercial laboratory (Isobrine Solutions Inc.), where they were analyzed within 60 days. Production gas samples were collected from six offset wells within a 50-km radius of the drilling location. Gas samples were collected at low pressure from wellheads in glass serum bottles sealed with butyl-rubber stoppers for compositional and δ13C analysis. Gases from serum bottles for the drill cuttings and production gases, and gases from the IsoTube® samples were extracted and injected into the septa port of an Agilent 7890B gas chromatograph (GC) equipped with a flame ionization detector (FID) to measure light hydrocarbons and a thermal conductivity detector to measure CO2, O2 and N2. Based on analyses of internal calibration gas standards and on replicate core samples, the accuracy of the analytical method was ± 5%. Carbon isotope values were measured from the serum bottles using a Finnigan MAT252 Isotope Ratio Mass Spectrometer at the University of Alberta. The analytical error estimated for the δ13C values was ± 0.5‰. 3.3. Estimating dissolved gas concentrations C1, C2, and C3 concentrations measured from IsoJars® collected at 282

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Fig. 3. Estimated dissolved concentrations (mg/L) versus true vertical depth (TVD) for C1-C3 gases from drill cuttings. Concentrations were estimated using a linear correlation of dissolved concentrations in core samples collected in IsoJars® and ppmV from cuttings samples collected in IsoJars® from Hendry et al. (2016). Depth presented as TVD not measured depth and are not to scale below the ‘kick off’ point (KOP) for directional drilling.

Site 5 near the study area (18 km) by Hendry et al. (2016) (Hendry et al., 2016) were converted to dissolved concentrations using the method outlined in Kampbell and Vandegrift (1998) (Kampbell and Vandegrift, 1998) and total porosity (nT) and density (ρ) values measured on adjacent core samples. The dissolved C1, C2, and C3 concentrations measured on core samples in the IsoJars® were compared to C1, C2, and C3 concentrations measured on cuttings collected in IsoJars®

(ppmV) at a previously characterized site; the linear relationships (C1 y = 75.8x+954.6, R2 = 0.82, n = 39; C2 y = 104.8x-0.68, R2 = 0.83, n = 38; C3 y = 64.7x-4.5, R2 = 0.79, n = 30; CO2 y = 68.9x-472.65, R2 = 0.82, n = 31) were used to convert C1, C2, and C3 concentrations of cuttings samples from ppmV to dissolved gas concentrations (Hendry et al., 2016).

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4. Results and discussion

4.3. Assessing the impacts of cavings on isotope profiles

4.1. C1, C2, and C3 concentration profiles

Drill cuttings provide a unique opportunity to evaluate the geologic composition of the samples contributing gases for analysis. Rock fragments released from overlying formations (cavings) can mix with the target formation during the drilling process and contaminate cuttings samples. Gases released from different formations in the sample media can impact the precision and accuracy of the geochemical profile. In a directional drilling program, higher rotational forces will exacerbate the release of cavings near the KOP which could adversely impact the gases in the horizontal segment of the borehole. The impacts of cavings were assessed experimentally in the horizontal segment near the base of exploration, where discreet samples with different amounts of cavings were collected from the Midale Member. Fig. 5 shows two cuttings samples collected from the shaker table at measured depths of 1622 m and 1618 m along the horizontal segment, respectively. The sample in Fig. 5 (left) was contaminated with cuttings, whereas Fig. 5 (right) was screened to minimize contamination. A greater effect was observed for C1 and C2 isotope values, where higher concentrations were present in formations near the KOP and deeper zones. The δ13C values were −62.7‰ and −58.7‰ for C1, -44.0‰ and −41.4‰ for C2 for the samples in Fig. 5 left (caving contaminated) and right (representative), respectively. The effects of the gas contributions from cavings were approximated using a two-component mixing relationship that estimated contributions of gases from the Midale Member and from cavings from mixed overlying zones near the KOP (Langmuir et al., 1978). The ‘true’ value for the Midale member was estimated using production zone samples collected from wellheads within a 50-km area of the drilling program, from which end-member values were determined to be −58.0‰ for C1, and -41.0‰ for C2. Averaged isotope values of cavings for the dominant gas zones near the KOP were used to represent the end-member associated with cavings (−65.5‰ for C1 and -45.5‰ for C2). The isotope signatures of C1 and C2 from both caving contaminated samples in Fig. 5 indicated a comparable mixing relationship for both species of gases in each sample. The results of the calculations suggest that roughly 65% of the gases in the sample in Fig. 5 (left) and 8% of gases in Fig. 5 (right) likely evolved from cavings from overlying formations. The percent of gases suggested by the mixing relationship was a good representation of the rock composition in the cuttings samples. This demonstrated that gases released from cavings from overlying formations can impact the isotope values in the geochemical profile associated with underlying zones.

C1, C2, and C3 concentrations followed similar concentration-depth profiles from surface to the base of exploration (Fig. 3). C1 concentrations increased in the shale from 0 mg/L at 75 m BG TVD to 100 mg/L at ∼400 m BG TVD, where it remained constant to the Milk River Formation of the Montana Group (∼600 m BG TVD). There was a sharp increase in gas concentration to 3575 mg/L at the Second White Speckled Shale of the Colorado Group (∼800 m BG TVD). Below the Second White Speckled Shale, C1 concentrations decreased from 3575 mg/L to 300 mg/L in the Mannville Group (∼1000 m BG TVD) and remained constant with depth to the Lower Watrous Formation (1340 m BG TVD), where it decreased slightly through the horizontal segment to the Midale Member (∼1433 m BG TVD) and was variable with measured depth. At ∼1445 m BG TVD there was an increase in C1 concentration to 76.1 mg/L, where it decreases again through the depth of exploration. C2 and C3 concentrations follow a similar profile with depth, but the dissolved concentrations were ∼2 orders of magnitude less than C1 concentrations. The dissolved gas concentrations presented in this approximation are based on the limitations in the porosity data available for the different geological units in this area. In general, large porosity ranges and variability has been reported for these units in the Williston Basin. The accuracy of the dissolved gas concentrations will ultimately reflect any variability the actual porosity in each zone, which in most cases is not available, and contaminant gas released from zones outside of the target zone. We propose that the approximations derived in this study provide the best estimate of dissolved gas concentrations using the techniques and information that are widely available for general use on drilling programs.

4.2. C1, C2, and C3 isotope profiles δ13C values of C1 gas collected from surface to the depth of exploration (1460 m BG TVD) ranged from −75‰ to −52.9‰ (Fig. 4). δ13C values increased through the shale to a value of −61.2‰ at ∼375 m BG TVD. The C1 gases then slowly became more 13C-depleted, reaching a δ13C value of −67.6‰ in the Joli Fou Formation (965 m BG TVD). Below this depth, δ13C values increase to −63.1‰ at 1220 m BG TVD, in the Lower Shaunavon Formation, just above the KOP. From the Lower Shaunavon Formation to the base of the Ratcliffe Member (∼1430 m BG TVD) δ13C values of C1 gas remained relatively constant (−65.4‰ to −63.1‰). From 1430 m BG TVD to the depth of exploration C1 gas became enriched in 13C reaching a maximum value of −52.9‰ within the Midale Member (∼1455 m BG TVD). C2 gas had a smaller range of δ13C values, −53.6‰ to −41.0‰, from the shale to the Midale Member, respectively. Within deeper sections of the shale, and the Milk River Formation C2 gas slowly became more enriched in 13 C reaching a value of −44.5‰ at 695 m BG TVD. From the Colorado Group (∼750 m BG TVD) to the Gravelbourg Formation (∼1245 m BG TVD), just above the KOP, δ13C values of C2 gas stayed relatively constant (−44‰ to −46‰). δ13C values from formations below the KOP to the Ratcliffe Member (∼1410 m BG TVD) were more variable; however, the values were roughly −45‰. Below ∼1430 m BG TVD to the base of exploration, C2 became 13C-enriched reaching a maximum value of −41.0‰ (although there was still a large amount of variability within the δ13C values). C3 gas had the smallest range of δ13C values, where a maximum δ13C value of −39.9‰ was observed within the shale (∼130 m BG TVD), which slowly increased throughout the depth of exploration reaching a value of −33.8‰ in the Midale Member (∼1455 m BG TVD). Similar as to what was observed with δ13C values for C2, there were variable δ13C values for C3 within some formations.

4.4. Mixing relationships for cavings from overlying formations The isotope values collected in the Midale member were influenced by mixing with cavings from overlying zones. Isotope mixing relationships are typically conservative and a mixing model was developed to account for variability in the isotope profile (Langmuir et al., 1978). The glacial till and shale were eliminated from the mixing model because a surface casing was cemented to a depth of 309 m BG TVD (cavings not possible from the surface to 309 m BG). Crossplots of δ13CC1 versus C1 concentrations (Fig. 6a) and δ13C-C2 versus C2 concentrations (Fig. 6b) suggest a two-component mixing relationship does not adequately represent the dataset (Drexler et al., 2013; Inguaggiatoa et al., 2005). To account for the additional complexity observed in the mixing trends, a quaternary mixing relationship was developed that represents mixing of drill cuttings from four distinct formations. The end-members that were identified in the mixing model as the dominant contributors were the Colorado Group (CLD), the Mannville Group (MNVL), the Watrous Formation (WAT), and the Midale Member (MDAL). The mixing relationships between the four contributing formations generated six binary mixing lines resulting from pairs of adjacent endmembers (Fig. 6). The drill cuttings selected to model the mixing relationship used δ13C values for C1 of −64.3‰, −67.8‰, −65.4‰, 284

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Fig. 4. δ13C values for C1-C3 gases versus true vertical depth (TVD) measured from mud gas collected in Isotubes® (open symbols) and drill cuttings collected in Isojars® (closed symbols). Depth presented as TVD not measured depth and are not to scale below the ‘kick off’ point (KOP) for directional drilling.

δ13C values for C1 and C2 from the Midale Member were 13C-enriched relative to the overlying zones. The δ13C-C1 values from the Mannville Group and Watrous Formation were 13C-depleted and shifted the δ13C values from the Midale Member to 13C-depleted values (Fig. 6a). The δ13C-C2 values from the Watrous Formation were 13C-depleted and shifted the δ13C-C2 values from the Midale Member to 13C-depleted values (Fig. 6b). The mixing relationship observed for the individual concentration and isotope values for C1 and C2 were supported by an

and −52.9‰ and δ13C values for C2 of −44.4‰, −44.3‰, −46.5‰, and −41.0‰ from the Colorado Group, the Mannville Group, the Watrous Formation, and the Midale Member, respectively. The Colorado Group had a midrange δ13C value for C1 and C2 relative to underlying zones, as well as high concentrations of C1 and C2 gases. Contributions of these gases shifted the δ13C values of C1 and C2 to 13Cdepleted values if they were 13C-enriched relative to the Colorado Group in samples collected from underlying formations (Fig. 6). The 285

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Fig. 5. Comparison of results for samples collected showing the mixing relationship between gases for cavings and cuttings samples.

BG TVD) revealed that C3 isotope values from the drilling profile were generally within the expected range for this formation (−37.3‰ to −33.8‰). The δ13C values of C1 and C2 for the sample with the lower amount of contamination of cavings, Fig. 5 (right), were consistent with endmember values characterized from Midale Member casing gases in the local area. Whereas the δ13C values of C1 and C2, for the sample with the greater amount of contamination of cavings, Fig. 5 (left), were inconsistent with ‘end-member’ values characterized from Midale Member casing gases in the local area. This suggests that drill cuttings can provide reliable isotope data if samples are chosen in a manner that aims to reduce the amount of contamination from cavings. It also suggests that there will be limitations in sample collection frequency and resolution that can be reliably achieved along the borehole of a drilling program. The results of this study suggest that the release of gases from cavings can have a pronounced effect on the observed isotope values of C1 and C2, with smaller effects on C3 and longer chain alkanes. The mixing relationship between drill cuttings and cavings demonstrated that gas signatures were affected by source rocks from the major shallow gas bearing zone in the area that corresponded with the Colorado Group, as well as cavings from the Mannville Group, and the Watrous Formation. End-members collected from the Midale Member had greater variability compared to the Viking Formation relative to the drilling profile, demonstrating an increased potential for variability from cavings in deeper segments of the borehole. End-members identified directly from the geochemical profile may be impacted by mixing between zones. The quaternary mixing relationship provides an improved assessment of the geochemical signatures of the Colorado and Mannville Groups, and the Watrous Formation that can account for variability in the geochemical profile. However, additional uncertainty in the end-member values defined by the model should be integrated to assess the ‘true’ values for these formations (Fig. 7). These effects were compounded by the relative size of drill cuttings. For the case of relatively smaller rock fragments produced by the Polycrystalline Diamond Compact (PDC) drill bit, the mixing relationship decreased the accuracy

isotope crossplot of these gases, where the majority of the isotope values measured from drill cuttings corresponded to mixing between the four dominant zones contributing cavings to underlying formations (Fig. 7). Low concentrations of C3 in formations near the KOP resulted in a lower impact of this gas as a contaminant for deeper samples. The mixing relationship provides a new approach for qualitative and quantitative evaluation of the variability in the isotope profile generated during drilling programs. 4.5. Assessing the precision and accuracy of the isotope profile The accuracy of the drill cuttings isotope profile was evaluated by comparing the measured values from the drilling program with available end-members characterized from the Viking Formation and Midale Member wellhead gases in the local area. Using the mixing relationship to account for some of the observed variability, the isotope profile generated from the drilling program corresponded well with the wellhead gases. The end-member values of C1 from the Viking Formation (−66.8‰, ∼900 m BG TVD) were in agreement (within 1‰–3‰) with the isotope profile (−65.9‰ to −64.1‰). Comparison with endmembers from the Midale Member (−57.5‰ to −49.1‰, ∼1450 m BG TVD) showed that C1 isotope values from the drilling program (−64.2‰ to −52.9‰) were generally within the expected range for this formation; however, in some cases gases appear 13C-depleted relative to the discreet samples collected from the Midale Member. The end-member value of C2 from discreet downhole samples collected from the Viking Formation (−45.4‰, ∼900 m BG TVD) were in agreement (within 1‰–3‰) with the drilling isotope profile (−45.3‰ to −44.3‰). Comparison with end-members from the Midale Member (−43.4‰ to −42.0‰, ∼1450 m BG TVD) revealed that C2 isotope values from the drilling profile were generally within the expected range for this formation (−45.9‰ to −41.0‰). The end-member value of C3 from discreet downhole samples collected from the Viking Formation (−36.4‰, ∼900 m BG TVD) were in agreement with the drilling isotope profile (−37.7‰ to −36.0‰). Comparison with endmembers from the Midale Member (−37.6‰ to −35.4‰, ∼1450 m 286

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Fig. 6. Plots of (a) δ13C- C1 versus C1 concentrations (mg/L) and (b) δ13C-C2 versus C2 concentrations (mg/L) including a mixing model using the Colorado Group, Mannville Group, Watrous Formation, and Midale Member as end-members. Analytical error bars shown (δ13C = ± 0.5‰ and concentration = ± 5.0%).

4.6. Comparing mud gas and drill cuttings

of the geochemical characterizations for each gas-bearing zone below the KOP of the directional drilling (∼1252 m BG TVD), where the effects of cavings were most pronounced. PDC drill bits produce very small rock fragments with minimal pore space that contained much lower concentrations of gases compared to the larger rock fragments produced by a roller-cone drill bit. This issue was exacerbated in carbonate formations where the micro-fragments from target zones were increasingly difficult to isolate from the drilling mud and larger rock fragments released from shallow segments of the borehole circulating through the system (including cavings).

The C1, C2, and C3 profiles from mud gas profiling yielded a comparable depth trend as the drill cuttings samples. However, the recovered mud gas concentrations were lower than the drill cuttings samples and isotope values were often 13C-depleted relative to the cuttings samples. The lower concentrations in mud gas were attributed to dilution by drilling fluid during hole advancement followed by dilution with atmospheric gases during sample collection (Hendry et al., 2016). 13C-depleted deviations of isotope values between mud gas and formation end-members have been previously linked to shallow gas 287

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Fig. 7. A plot of δ13C-C2 values versus δ13C- C1 values including a mixing model using the Colorado Group, Mannville Group, Watrous Formation, and Midale Member as end-members. Analytical error bars shown (δ13C = ± 0.5‰).

−43.9‰ for C2 gas for the current study). Differences in δ13C values for Sites 2 and 5 (Hendry et al., 2016) and the west-central Saskatchewan profile (Szatkowski et al., 2002) either resulted from variability from sampling and/or reflect regional variability from the reported diffusional isotopic fractionation occurring in the shale. Similar δ13C values for C1 and C2 were observed in shallower segments of the depth profile completed in southern Saskatchewan (−69‰ to −55‰ and −66‰ to −62‰ for C1 gases, −48‰ to −33‰ and −42‰ to −41‰ for C2 gases, in the Milk River and Second White Specks Formations, respectively) during the drilling of the Aquistore well to a depth of 3400 m BG relative to the current study (−66.9‰ to −62.1‰, and −65.2 for C1 gases, −45.8‰ to −44.1‰ and −45.8‰ to 44.4‰ for C2 gases in the Milk River and Second White Specks Formations, respectively). However, increased variability occurred with depth in the Watrous Formation (−62‰ to −55‰ for C1 gas and −41‰ for C2 gas for the southern Saskatchewan site, and −65.4‰ to −62.0‰ for C1 gas and −45.5‰ to −43.8‰ for C2 gas for the current study) and Ratcliffe Member (−55‰ to −45‰ for C1 gas and −42‰ to −33‰ for C2 gas for the southern Saskatchewan site, and −65.2‰ to −61.8‰ for C1 gas and −44.6‰ to −44.0‰ for C2 gas for the current study) (Skoreyko et al., 2014). Although the Aquistore well was a vertical drilling program and may have been less impacted by cavings, the increased variability in depth likely resulted from differences in mixing between zones from the mud gas profile. Regional values reported for comparable formations from different drilling programs were within the same range. The impacts of mixing functions to increase the range of values for each zone, where the variability can result from a combination of experimental error and/or baseline geochemical differences in the region. Improvements in the collection and reporting of geochemical data that accounts for mixed gas signatures will provide a means for high-resolution characterizations of regional geochemistry.

sources mixing with lower concentrations of deeper gases migrating through the drill string (Dawson and Murray, 2011). A similar effect was characterized in the current study with both the mud gas and drill cuttings, which were intensified in the deeper segments of the borehole by contamination with shallow gas sources from zones near the KOP. The gas mixing observed in both the mud gas and drill cuttings were likely impacted by the additional rotational forces required to bend the drill string 90° for the horizontal segment through the Midale Member in the directional drilling process. Avoiding gas mixing from different zones using mud gas approaches will be limited; however, improvements to drill cuttings sample separation, purification, and collection will be capable of providing higher resolution isotope profiles for C1 and C2 gases. 4.7. Regional comparison of geochemical profiles Good agreement was generally observed from δ13C values for C1 and C2 reported for corresponding formations from drilling profiles in Saskatchewan (Szatkowski et al., 2002; Skoreyko et al., 2014; Hendry et al. 2016, 2017a, 2017b). Hendry et al. (2016) reported a mud-gas isotope profile for the till and shale during the drilling of Sites 2 and 5 in the region and reported δ13C values for C1 gases. Within the till, reported values (−83‰ and −80‰ to −77‰ for Site 2 and Site 5, respectively) appear 13C-depleted relative to the current study (−74.8‰ to −52.4‰); however, this zone is biologically active and variability can be expected in the region. Comparable values were observed from the shale (−85‰ to −71‰ and −79‰ to −66‰ for Site 2 and Site 5, respectively, and −75.0‰ to −63.5‰ for the current study), the Colorado Group (−65‰ to −55‰ for C1 gas and −46‰ to −43‰ for C2 gas for the west-central Saskatchewan site, and −65.4‰ to −64.1‰ for C1 gas and −46.2‰ to −44.3‰ for C2 gas for the current study), the Viking Formation (−65‰ to −59‰ for C1 gas and −42‰ to −36‰ for C2 gas for the west-central Saskatchewan site, and −65.9‰ to −64.1‰ for C1 gas and −45.3‰ to −44.3‰ for C2 gas for the current study), and the Mannville Group (−78‰ to −58‰ for C1 gas and −37‰ to −32‰ for C2 gas for the west-central Saskatchewan site, and −67.8‰ to −63.8‰ for C1 gas and −45.5‰ to

4.8. Industrial and environmental significance The relatively low cost and ease of higher density sampling using mud gas and/or drill cuttings can be used for reservoir characterization (Blanc et al., 2003; McKinney et al., 2007), exploration (Schowalter and 288

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which corresponded to the higher concentrations of these gases in shallow gas-bearing zones. Mixing between zones along the borehole can cause artificial 13C-depletion of C1 and C2 isotope values in deeper formations impacting the precision and accuracy of geochemical profiles. The variability in the geochemical data generated from drilling programs can have a significant impact on the effectiveness of these profiles for reservoir characterization, exploration, and in identifying the source(s) of fugitive gases migrating to surface.

Hess, 1982; Hawroth et al., 1985), and baseline characterizations to evaluate gas migration sources from leaking oil/gas infrastructure (Rowe and Muehlenbachs, 1999; Szatkowski et al., 2002; Tilley and Muehlenbachs, 2006; Hendry et al., 2017a). The benefit of these approaches is that they can be integrated with regular operations during well installations over the course of development of an oil/gas field. Variability in the geochemical profiles will limit the effectiveness of these approaches. Continuous monitoring of gas compositions and isotope values used to evaluate reservoir continuity and communication across horizontal segments of a drilling program will be impacted by mixing (mud gas) and cavings (drill cuttings) (Blanc et al., McKinney et al., 2007, 2003; Dawson and Murray, 2011). The mixing will lead to data scatter in the isotope values for thermogenic reservoirs, with 13Cdepleted values arising from mixing with overlying zones decreasing the resolution of the characterization. Exploration campaigns use geochemical profiling during drilling operations to evaluate thermal maturity and bacterial gas contributions using carbon isotope values for C1, C2, and C3 gases (Hawroth et al., 1985; Ellis et al., 2003). Artificially 13 C-depleted values arising from mixing with overlying zones will suppress the thermogenic characteristics of the investigated pay zone, decreasing the perceived commercial value and influencing key strategic decisions for future investment in new areas. Gas migration investigations use geochemical profiles to identify source zones for leaking oil/gas assets. Fugitive gases that migrate from oil and natural gas extraction activities are dominated by C1 with lower concentrations of other gases. High-resolution baseline characterization of spatial and vertical variability in these gas concentrations and their isotopic values are needed to define current and future environmental concerns. Dissolved concentration-depth profiling of C1eC3 gases, and their isotopes from directional drilling can be defined using geochemical techniques dictated by the drilling methods (mud and cuttings) that cannot be obtained using classical monitoring-well technology. The continuous geochemical depth-profiles generated via drilling provide a more complete understanding of gas compositions and isotope values throughout the subsurface than dedicated monitoring wells that are limited to a discreet ‘screened’ interval at a specific depth. Geochemical characterizations coupled to drilling programs for new assets can also provide critical gas concentration and isotopic data needed to address stray gas investigations in exploratory or under-developed oil fields, especially in remote areas. Leaking production wells are an important environmental concern and geochemical profiling, such as presented here, can provide critical insight needed to identify stray gas sources and develop targeted remediation programs. The factors that affect the δ13C values for geochemical profiles produced from drilling operations can lead to observable deviations from the ‘true’ end-member values for the formations, raising important questions regarding the effectiveness of these methods for characterizing the subsurface. The potential for altered isotope values from gases mixing in the drilling mud (mud gas samples) and/or between drill cuttings from gases released from upper segments of the borehole (e.g. cavings) decrease the reliability of this geochemical tool for reservoir characterization, oil/gas exploration campaigns and gas migration investigations; however, high-resolution profiles can be achieved by resolving the distinct contributions of gases originating from different zones in the drilling program.

Acknowledgements This project received industrial funding and additional support was provided by NSERC's Discovery Grant program (SOCM) and an NSERCIRC (grant 184573) to MJH. We would like to thank Serguey Arkadakskiy, Phil Richards, and Laura Smith for their contributions to the field program. References Baldassare, F.J., Laughrey, C.D., 1997. Identifying the sources of stray methane by using geochemical and isotopic fingerprinting. Environ. Geosci. 4 (2), 85–94. Baldassare, F.J., McCaffrey, M.A., Harper, J.A., 2014. A geochemical context for stray gas investigations in the northern Appalachian Basin: implications of analyses of natural gases from Neogene-through Devonian-age strata. Am. Assoc. Petrol. Geol. Bull. 98 (2), 341–372. Blanc, P., Brevière, J., Laran, F., Chauvin, H., Boehm, C., Fréchin, N., Capot, M., Benayoun, A., 2003. Reducing uncertanties in formation evaluation through innovative mud logging techniques. In: Paper presented at the Society of Petroleum Engineers. Cahill, A.G., et al., 2017. Mobility and persistence of methane in groundwater in a controlled-release field experiment. Nat. Geosci. 10 (4), 289. Clark, I.D., Ilin, D., Jackson, R.E., Jensen, M., Kennell, L., Mohammadzadeh, H., Poulain, A., Xing, Y.P., Raven, K.G., 2015. Paleozoic-aged microbial methane in an Ordovician shale and carbonate aquiclude of the Michigan Basin, southwestern Ontario. Org. Geochem. 83–84, 118–126. Cody, J.D., Hutcheon, I.E., 1994. Regional water and gas geochemistry of the Mannville Group and associated horizons, southern Alberta. Bull. Can. Petrol. Geol. 42 (4), 449–464. Dawson, D., Murray, A., 2011. Applications of mud gas isotope logging in petroleum systems analysis. In: Paper Presented at the AAPG Hedberg Research Confrence Natural Gas Geochemistry, Beijing, China. Drexler, J.Z., Paces, J.B., Alpers, C.N., Windham-Myers, L., Neymark, L.A., Bullen, T.D., Taylor, H.E., 2013. 234U/238U and δ87Sr in peat as tracers of paleosalinity in the Sacramento-San Joaquin Delta of California, USA. Appl. Geochem. 40, 164–179. Ellis, L., Brown, A., Schoell, M., Uchytil, S., 2003. Mud gas isotope logging (MGIL) assists in oil and gas drilling operations. Oil Gas J. 101 (23), 2–9. Emberley, S., Hutcheon, I., Shevalier, M., Durocher, K., Mayer, B., Gunter, W.D., Perkins, E.H., 2005. Monitoring of fluid–rock interaction and CO2 storage through produced fluid sampling at the Weyburn CO2-injection enhanced oil recovery site, Saskatchewan, Canada. Appl. Geochem. 20 (6), 1131–1157. Hammerschmidt, S.B., Wiersberg, T., Heuer, V.B., Wendt, J., Erzinger, J., Kopf, A., 2014. Real-time drilling mud gas monitoring for qualitative evaluation of hydrocarbon gas composition during deep sea drilling in the Nankai Trough Kumano Basin. Geochem. Trans. 15 (15). Hawroth, J.H., Sellens, M., Whittaker, A., 1985. Interpretation of hydrocarbon shows using like C1-C5 hydrocarbon gases from mud-log data. AAPG (Am. Assoc. Pet. Geol.) Bull. 69, 1305–1310. Hendry, M.J., Barbour, S.L., Schmeling, E.E., Mundle, S.O.C., Huang, M., 2016. Fate and transport of dissolved methane and ethane in cretaceous shales of the Williston Basin, Canada. Water Resour. Res. 52 (8), 6440–6450. Hendry, M.J., Schmeling, E.E., Barbour, S.L., Huang, M., Mundle, S.O.C., 2017a. Fate and transport of shale-derived, biogenic methane. Sci. Rep. 7 (4881), 1–9. Hendry, M.J., Barbour, S.L., Schmeling, E.E., Mundle, S.O.C., 2017b. Measuring concentrations of dissolved methane and ethane and the 13C of methane in shale and till. Groundwater 55 (1), 119–128. IsoTech Labs Inc, 2014. Procedure for taking cuttings samples in IsoJars®. Retrieved from. http://www.isotechlabs.com/customersupport/samplingprocedures/IsoJarSM.pdf. Inguaggiatoa, S., Martin-Del Pozzob, A.L., Aguayob, A., Capassoa, G., Favaraa, R., 2005. Isotopic, chemical and dissolved gas constraints on spring water from Popocatepetl volcano (Mexico): evidence of gas–water interaction between magmatic component and shallow fluids. J. Volcanol. Geoth. Res. 141, 91–108. Jackson, R.B., Vengosh, A., Darrah, T.H., Warner, N.R., Down, A., Poreda, R.J., Osborn, S.G., Zhao, K., Karr, J.D., 2013. Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction. Proc. Natl. Acad. Sci. Unit. States Am. 110 (28), 11250–11255. Kampbell, D.H., Vandegrift, S.A., 1998. Analysis of dissolved methane, ethane, and ethylene in ground water by a standard gas chromatographic technique. J. Chromatogr. Sci. 36 (5), 253–256. Langmuir, C.H., Vocke, R.D., Hanson, G.N., Hart, S.R., 1978. A general mixing equation with applications to Icelandic basalts. Earth Planet Sci. Lett. 37 (3), 390–392.

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