Origin of sulfate-rich fluids in the Early Triassic Montney Formation, Western Canadian Sedimentary Basin

Journal Pre-proof Origin of sulfate-rich fluids in the early Triassic Montney Formation, Western Canadian Sedimentary Basin Mastaneh H. Liseroudi, Omid H. Ardakani, Hamed Sanei, Per K. Pedersen, Richard A. Stern, James M. Wood PII:

S0264-8172(20)30019-2

DOI:

https://doi.org/10.1016/j.marpetgeo.2020.104236

Reference:

JMPG 104236

To appear in:

Marine and Petroleum Geology

Received Date: 16 November 2019 Revised Date:

17 December 2019

Accepted Date: 10 January 2020

Please cite this article as: Liseroudi, M.H., Ardakani, O.H., Sanei, H., Pedersen, P.K., Stern, R.A., Wood, J.M., Origin of sulfate-rich fluids in the early Triassic Montney Formation, Western Canadian Sedimentary Basin, Marine and Petroleum Geology (2020), doi: https://doi.org/10.1016/ j.marpetgeo.2020.104236. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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Origin of Sulfate-rich Fluids in the Early Triassic Montney

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Formation, Western Canadian Sedimentary Basin

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Mastaneh H. Liseroudia*, Omid H. Ardakanib,a, Hamed Saneic, Per K. Pedersena, Richard A. Sternd, James

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M. Woode

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a

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*

Department of Geoscience, University of Calgary, 2500 University Drive NW Calgary, AB T2N 1N4, Canada Geological Survey of Canada, 3303 33rd St. NW Calgary, AB T2L 2A7, Canada c Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, building 1671, 223, 8000 Aarhus C, Denmark d Canadian Centre for Isotopic Microanalysis, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada e Calaber1 Resources, Calgary, Alberta, Canada b

Corresponding author: Mastaneh H. [email protected]

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Authors email address: Dr. Omid H. Ardakani: [email protected] Dr. Hamed Sanei: [email protected] Dr. Per K. Pedersen: [email protected] Dr. Richard Stern: [email protected] Dr. James M. Wood: [email protected]

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Highlights:

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•

Regional distribution of two generations of anhydrite in the Montney Formation.

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•

The late anhydrite and barite cement precipitated from Devonian-sourced brines.

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•

Similar fluid source for the late anhydrite/barite and fracture-filling anhydrite.

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•

Migration of sulfate-rich hydrothermal fluids through deep faults in Alberta.

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2

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Abstract

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This study investigates diagenetic and geochemical processes that control regional

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distribution and formation of sulfate minerals (i.e., anhydrite and barite) in the Early Triassic

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Montney Formation in the Western Canadian Sedimentary Basin. The generation of H2S in

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hydrocarbon reservoirs is often associated with the dissolution of sulfate minerals, as a major

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source of sulfate required for sulfate-reducing reactions. The formation of pervasive late

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diagenetic anhydrite and barite in the high H2S zone of the Montney Formation is therefore

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contrary to the normal paragenetic sequence of sour gas reservoirs.

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Petrographic observations revealed early and late anhydrite and barite cement. The early

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fine-crystalline anhydrite cement is dominant in northeastern British Columbia (low H2S

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zone), while the late-stage coarse-crystalline cement and fracture/vug-filling anhydrite are

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dominant in Alberta (high H2S zone). The bulk isotopic values (δ34S: +2.9 to +24.7‰ V-

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CDT, δ18O: -11.2 to +15.7‰ V-SMOW) suggest that sulfate-rich fluids originated mainly

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from modified Triassic connate water was the origin of early anhydrite. In contrast, the SIMS

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isotopic values of late anhydrite (δ34S: +18.5 to +37‰ V-CDT, δ18O: +12 to +22‰ V-

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SMOW) and barite cement (δ34S: +23.3 to +39‰ V-CDT, δ18O: +13.2 to +18.7‰ V-

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SMOW) as well as fracture/vug-filling anhydrite (δ34S: +23.5 to +24.7‰ V-CDT, δ18O:

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+13.3 to +14.7‰ V-SMOW) from Alberta represents a mixed isotopic signature of Triassic

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connate water and contribution of dissolved sulfate-rich fluids derived from dissolution of

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Devonian evaporites.

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The

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Sr/86Sr isotope ratios of the fracture/vug-filling anhydrite (0.7092 to 0.7102) are

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highly radiogenic suggesting extensive water/rock interactions between sulfate-rich fluids

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and siliciclastic and basement rocks. The similar isotopic composition of the late

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anhydrite/barite and fracture/vug-filling anhydrite in western Alberta with Devonian

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evaporites isotopic signature, and the highly radiogenic

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sulfate-bearing fluids were mainly originated from underlying Devonian evaporites and

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migrated upwards through deep-seated faults/fractures to the Montney Formation.

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Sr/86Sr ratio further supports

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Keywords: Anhydrite, barite, diagenesis, stable isotopes, strontium isotopes, Secondary Ion

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Mass Spectrometry (SIMS), Devonian evaporites

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1. Introduction

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The Early Triassic Montney Formation is a major siltstone dominated unconventional tight

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gas and hydrocarbon liquids play in the Western Canadian Sedimentary Basin (WCSB; Fig. 1).

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The estimated natural gas, oil, and liquid condensate reserves of the Montney Formation are 450

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Trillion Cubic Feet (TCF), 1125, and 1.7 Million Barrels (MMBBL), respectively (Natural

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Energy Board, 2013; USGS, 2018).

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Hydrogen sulfide (H2S) concentration in the Triassic natural gas reservoirs of the WCSB

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ranges from less than 1% up to 29% with regionally isolated high and low distribution zones

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(Kirste et al., 1997; Desrocher et al., 2004). The H2S concentrations of these reservoirs generally

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increase westward with burial depth (Desrocher et al., 2004). In the current study, the maximum

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H2S concentration of 18% is reported from the western Alberta section of the Montney

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Formation (high H2S zone; Fig. 2). The high concentrations of H2S in gas-producing wells from

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the Montney Formation in western Alberta have raised concerns due to the economic impact of

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extraction and processing of natural gas and its adverse environmental impacts.

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Three major mechanisms have been proposed for H2S formation in hydrocarbon reservoirs: (i)

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thermal chemical alteration (TCA) of organic sulfur compounds in kerogen or oil; (ii) microbial

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sulfate reduction (MSR); and (iii) thermochemical sulfate reduction (TSR) (Orr, 1977; Worden

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and Smalley, 1996; Machel, 2001; Bottrell and Newton, 2006; Kelemen et al., 2008, Sim et al.,

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2011 among many others). Kerogen maturation and thermal cracking of oil result in

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decomposition of their bound organic sulfur through the TCA process leading to the formation of

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minor amounts of H2S (< 3%, Orr, 1977). During MSR and TSR, sulfate-rich fluids, including

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seawater, buried seawater (connate water), evaporative brines and/or dissolved sulfate sourced 4

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from the dissolution of sulfate minerals (mainly anhydrite and gypsum) (Machel, 2001; Worden

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and Smalley, 1996 ) react with organic matter and/or hydrocarbons. This reaction results in the

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reduction of dissolved sulfate and leads to the formation of H2S, carbonate minerals, CO2,

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elemental sulfur, and water (Machel, 2001) as summarized in equation (1):

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SO42- + organic matter/hydrocarbon → carbonate mineral (s) + H2S + H2O ± S0 ± CO2 (1).

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Regardless of H2S generation mechanisms, sulfate-rich fluids derived from the dissolution of

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evaporite minerals, specifically anhydrite and gypsum, are the major ingredient in the H2S

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formation process. In most sour gas reservoirs worldwide, H2S forms at the expense of anhydrite

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available in the matrix of the reservoir (Hutcheon et al., 1995; Machel et al., 1995; Worden and

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Smalley, 1996; Heydari 1997; Cai et al., 2004; Jenden et al., 2015). However, in the Montney

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Formation anhydrite pervasively precipitated as a late-stage pore- and fracture-filling cement

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within the high H2S concentration zone. The presence of late anhydrite and barite cement in the

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high H2S concentration zone of the Montney Formation in western Alberta (Fig. 2) suggests that

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the dissolved sulfate required for H2S generation may have been introduced to the Montney

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Formation contributed not only to H2S generation but also to the precipitation of the late sulfate

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mineral cement in the study area.

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Previous studies on the source of H2S in the Montney Formation have mainly focused on the

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stable isotope geochemistry of hydrogen sulfide (Kirste et al., 1997; Desrocher et al., 2004). In

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these studies, dissolved sulfate, as one of the major ingredients for H2S generation was

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considered to have originated from the overlying Upper Triassic Charlie Lake Formation and

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was reduced through TSR to produce H2S in the Montney Formation. However, the role of

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diagenetic processes in the formation of dissolved sulfate leading to the generation of H2S was

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overlooked. 5

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The objective of the current study is to address the major origin(s) of sulfate-rich fluids as one

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of the major ingredients of H2S generation in the Montney Formation. The major mechanism(s)

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responsible for H2S generation and H2S potential timing relative to the major diagenetic

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phases/events in the Montney Formation will be addressed in a following contribution using bulk

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and SIMS sulfur isotope analysis of different pyrite generations, and its relationship to H2S

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sulfur isotope composition in the study area. Transmitted light petrography, scanning electron

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microscopy, energy dispersive X-ray spectroscopy (SEM/EDX) imaging, and bulk and in-situ

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secondary ion mass spectrometry (SIMS) sulfur and oxygen isotope analysis, as well as

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strontium isotope analysis of a regional Montney Formation core sample set were used to

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identify the source(s) of sulfate-rich fluids and sulfate minerals in the Montney Formation. To

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the best of our knowledge, this study is the first to report and explain the concurrence of

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pervasive late diagenetic anhydrite and barite cement and high H2S concentration in natural tight

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gas reservoirs. The results of this study provide important conclusions for the future economic

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development of the Montney natural gas play and can also be applied to other hydrocarbon

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reservoirs worldwide featuring the simultaneous presence of sulfate minerals and H2S gas.

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2. Geological Setting

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2.1. Structural Framework

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Deposition of the Early Triassic Montney Formation in western Alberta and northeastern

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British Columbia (BC) occurred in the Peace River Embayment (PRE) of the WCSB (Barclay et

127

al., 1990; Fig. 1a). Formation of the PRE was initiated by the complete burial of the uplifted

128

Peace River Arch (PRA) by the Late Devonian cyclic carbonate-clastic and carbonate-evaporite

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deposits of the Winterburn Group and Wabamun Formation (O’Connell, 1994). Subsequent

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subsidence of the PRA led to the development of an extensive network of high-angle normal 6

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faults known as the Dawson Creek Graben Complex (DCGC) during Carboniferous to Permian

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that ultimately resulted in the formation of the broad arcuate PRE (Barclay et al., 1990;

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O’Connell, 1994).

134

Triassic sediments, including the Montney Formation, mostly accumulated within the DCGC,

135

although they extended over the initial PRE towards the north and south (Barclay et al., 1990;

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Davies et al., 2018). In the Peace River region, faults of the DCGC or Carboniferous faults are

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underlain by precursor Precambrian basement faults (Hope et al., 1999; Fig. 3). Extensional

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faulting that occurred in the PRA has been interpreted to be accommodated by brittle faulting of

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basement material, which led to the formation of an upward propagating deformed zone of

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fractured, folded and faulted sediments in the overlying ductile sedimentary strata (Hope et al.,

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1999).

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Seismic interpretation of structural and tectonic elements of the PRA region demonstrates that

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extensional normal faulting has reoccurred in this region, from the Devonian until at least the

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Triassic (Hope et al., 1999) (Fig. 3). Some of the DCGC-associated faults even extend into the

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overlying Cretaceous rocks (Mei, 2009). Loading of compressional thrust faults associated with

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the Jurassic Colombian and Cretaceous Laramide orogenies gave also rise to the reactivation of

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underlying faults and fractures in the PRA region (O’Connell et al., 1990).

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2.2. Montney Formation

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The Early Triassic Montney Formation of the WCSB consists of mixed siliciclastic-carbonate

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depositional environments. Lithologically, it is comprised of predominant dolomitic siltstone

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with a lesser component of very fine-grained sandstone, rarely fine-grained sandstone, and

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bioclastic packstone and grainstone (Davies et al., 1997; Davies et al., 2018; Zonneveld and

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Moslow, 2018). The Montney Formation was deposited along the western margin of the Pangea 7

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Supercontinent in a collisional retro-foreland basin setting (Rohais et al., 2018). The main

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depocenter of the Montney Formation with up to 320 m thickness is located to the northeast of

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the Cordilleran deformation belt in the WCSB (Kuppe et al., 2012; Wood, 2013). Toward the

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north and east, it thins to an eroded zero edge (Edwards et al., 1994).

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Sedimentologically, the Montney Formation accumulated on a clastic ramp succession,

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comprised of shoreface to offshore, turbidite and channel deposits (Edwards et al., 1994; Davies

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et al., 2018; Zonneveld and Moslow, 2018). In the Peace River region, the Montney Formation

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uncomfortably overlies the Permian Belloy Formation and is overlain conformably by the

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phosphatic organic-rich shales of the Doig Formation (Davies et al., 1997; Davies et al., 2018;

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Fig. 1b).

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In western Alberta where the high H2S concentration zone occurs, the Montney Formation is

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underlain by thick Devonian carbonate-evaporite deposits (Fig. 3). The main evaporitic

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carbonate units underlying the Montney Formation in the study area are: anhydrite sheets of the

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Chinchaga Formation and basin-wide evaporites of the Muskeg Formation, both of the lower to

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middle Devonian Elk Point Group (Meijer Drees, 1994), the Fort Vermilion Formation of the

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middle to upper Devonian Beaverhill Lake Group (Oldale and Munday, 1994), and the upper

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Devonian Woodbend and Winterburn groups (Switzer et al., 1994; Fig. 3).

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Paleogeographically, Triassic-age sedimentation in the WCSB occurred in a mid-latitudinal,

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west coast setting with hot and seasonally arid climate conditions represented by restricted

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fluvial sedimentation, increased aeolian-sourced influx and formation of meso-saline to

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hypersaline environments throughout most of this time (Davies, 1997 ; Zonneveld and Moslow,

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2018). The occurrence of these hypersaline environments is regarded as a source of sulfate in the

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pore fluids of Triassic sediments, including the Montney Formation, and resulted in the 8

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generation of syn-depositional to very early diagenetic (pre-burial) anhydrite cement (Davies,

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1997; Davies et al., 1997).

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3. Sampling and Methodology

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3.1. Sampling

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Hundred-ten core samples from twelve wells located in northeastern BC and western Alberta

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(Fig. 1) were selected for transmitted light petrography, bulk, and in-situ sulfur and oxygen

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isotope, and strontium isotope analysis of major sulfate minerals, in the Montney Formation

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(Fig. 1, Table 1). Ninety thin sections were prepared and half-stained with Alizarin Red S plus

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potassium ferricyanide, and sodium cobaltinitrite to distinguish carbonate phases and potassium

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feldspar, respectively. Detailed petrography was carried out on these thin sections, in order to

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identify the rock texture, mineralogy, and paragenetic sequence, using a Zeiss Axio Scope A1

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microscope at the Geological Survey of Canada-Calgary. Due to the fine-grained nature of the

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studied samples, nine samples were selected for scanning electron microscopy (SEM) and energy

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dispersive X-ray spectroscopy (EDXS) analyses, using FEI Quanta 250 FEG instrument

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equipped with a Bruker Quantax EDS at the Instrumentation Facility for Analytical Electron

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Microscopy (IFFAEM) lab at the University of Calgary.

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3.2. Isotope Ratio Mass Spectrometry (IRMS)

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Fifty-four bulk rock samples from eleven wells in which different anhydrite cement phases

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were identified by petrography and SEM observations (section 4.1.1) were crushed by a swing

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mill grinder at the University of Calgary. Five more fracture- and vug-filling anhydrite samples

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from the Montney Formation and six fracture- and vug-filling anhydrite samples from the

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Middle Triassic Doing and Halfway formations in western Alberta were also individually micro-

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drilled. All samples were analyzed for sulfur (δ34S) and oxygen (δ18O) isotopes (Table 2a-c),

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using isotope ratio mass spectrometry at the Isotope Science Laboratory (ISL) of the University

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of Calgary.

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Approximately 2-5 g of bulk anhydrite-bearing samples and 100 mg of fracture- and vug-

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filling samples were washed with De-ionized Milli-Q water heated up to 80°C, the temperature

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above which the solubility of anhydrite declines significantly (Blount and Dickson, 1969; Li and

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Duan, 2011). Washing anhydrite-bearing samples in water will also significantly minimize pyrite

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dissolution (very low solubility in water with PKs = 16.4 ± 1.2, Davison, 1991) and the effect of

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pyrite oxidation during sample preparation. The solution basically containing dissolved sulfate

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from the dissolution of water-soluble sulfate (anhydrite) was then filtered through a 0.45 µm

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millipore filter.

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To remove any remaining dissolved inorganic carbon (DIC), the filtrate was then acidified by

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adding 0.8 ml of 12N HCl and allowed to react for half an hour. The dissolved sulfate extracted

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from anhydrite was converted to BaSO4 by adding 5 ml of 10% BaCl2 solution to the filtrate,

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then boiled to concentrate BaSO4 in the solution by reducing the volume. The final precipitate

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was re-filtered through a 0.45 µm millipore filter, washed and air-dried (Hall et al., 1988).

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Although the final BaSO4 precipitate was not weighted but qualitatively between 5 to 20 mg of

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BaSO4 was yielded. The BaSO4 precipitate was packed in tin cups for δ34S analyses and silver

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cups for δ18O analyses. The δ34S value of BaSO4 was measured by thermal decomposition in a

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Carbo-Erba NA 1500 elemental analyzer in sulfur mode coupled to a continuous flow Thermo

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Delta Finnigan PLUS XL mass spectrometer. The δ18O value of BaSO4 was determined by thermal

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conversion in a High-Temperature Heka oxygen analyzer coupled to a continuous flow Thermo

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Delta Finnigan PLUSXL mass spectrometer. 10

The

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S/32S and

18

O/16O ratios were measured on the liberated SO2 and CO gases,

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respectively. Results are reported in per mil (‰) notation relative to the internationally accepted

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standard Canyon Diablo Troilite (‰V-CDT) for δ34S and Standard Mean Ocean Water (‰V-

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SMOW) for δ18O. Calibration ranges were defined and normalized based on sulfur standards of

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NBS 127 (+21.1 ± 0.4‰), IAEA S05 (+0.5 ± 0.2‰), and IAEA S06 (-34.1 ± 0.2‰) for BaSO4.

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For oxygen isotope measurements of BaSO4, calibration ranges were defined and normalized

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using oxygen standards of NBS 127 (+8.6 ± 0.4‰), IAEA S05 (+12.0 ± 0.2‰), and IAEA S06 (-

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11.3 ± 0.2‰). QA/QC (BaSO4, ISL internal standard) was analyzed within each sequence of

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unknowns (∼ 25 unknowns) every 5th sample. This provided a measure of the accuracy and

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precision of each sequence analyzed by the instrument (EA-IRMS). Precision and accuracy as 1

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sigma of (n =10) lab standards for δ34S and δ18O of BaSO4 are equal to 0.3 ‰ and 0.5 ‰,

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respectively.

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3.3. Thermal Ionization Mass Spectrometry (TIMS)

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The

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Sr/86Sr isotope ratios were measured in four selected micro-drilled fracture- and vug-

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filling anhydrite samples using a Thermo-Fisher Scientific Triton Thermal Ionization Mass

237

Spectrometer (TIMS) at the Isotope Science Laboratory (ISL) of the University of Calgary. For

238

87

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added to the samples. The samples were then centrifuged and the nitric acid was decanted for ion

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exchange after eight days of leaching. Prior to the TIMS analysis, the strontium in the samples

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was isolated using EiChrom Sr resin. The samples were then loaded onto a single rhenium

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filament using a Ta2O5 activator solution. Ten blocks of 15 cycles were measured (for a total of

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150 ratios/sample) with matrix rotation of the amplifiers between the blocks. The measured

Sr/86Sr isotope ratio analysis, 5-10 mg of samples were weighed and 0.5 ml of 1M HNO3 was

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Sr/86Sr ratio was normalized to the accepted

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Sr⁄86Sr ratio of 8.375209. The typical relative

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uncertainty of the 87Sr/86Sr isotope amount ratio is 20 ppm (2SD).

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3.4. Secondary Ion Mass Spectrometry (SIMS)

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Secondary ion mass spectrometry (SIMS) mount preparation and SIMS were carried out at the

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Canadian Centre for Isotopic Microanalysis (CCIM), University of Alberta. Regions of interest

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(ROI) from six anhydrite- and barite-bearing polished thin sections from four representative

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wells in western Alberta were cored using diamond bits with inside diameters ranging from 1.5 –

251

2 mm. The ROIs (41 in total) were arranged along with pre-polished fragments of CCIM

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anhydrite and barite reference material (RM) S0431 and S0327, respectively, and cast in epoxy

253

to form a single mount (M1500). All material was contained within the inner 12 mm diameter,

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where spatially correlated analytical biases are minimized.

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Following casting, the mounts were polished lightly with diamond compounds on rotary

256

equipment to create a uniformly flat surface, cleaned with a lab soap solution and de-ionized

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H2O, and then coated with 20 nm of high-purity Au prior to scanning electron microscopy

258

(SEM). Detailed SEM characterization with backscattered electrons was carried out on each

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ROI and sub-areas utilizing a Zeiss EVO MA15 instrument at beam conditions of 20kV and 3 –

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4 nA. A further 80 nm of Au was subsequently deposited on the mounts prior to SIMS analysis.

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Sulfur isotope ratios (34S/32S) were determined in anhydrite (151 spots) and barite (30 spots)

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using the IMS-1280 multi-collector ion microprobe at the CCIM (Table 3, Fig. 7b-c). Primary

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beam conditions utilized 20 keV133Cs+ ions focused to 15 µm for anhydrite and barite,

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employing beam currents of 1.5 nA. The normal incidence of electron gun was utilized for

265

analysis of anhydrite and barite. Negative secondary ions were extracted through 10 kV to the

266

grounded secondary column (Transfer section). Conditions for the Transfer section included an

12

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entrance slit width of 80 µm for S-isotopes and 122 µm for O-isotopes, field aperture of 5 × 5

268

mm, and a field aperture-to-sample magnification of 100×. Automated tuning of the secondary

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ions in the transfer section preceded each analysis. The energy slit was fully open. Both 32S- and

270

34 -

S were analyzed simultaneously in Faraday cups (L’2 and FC2 using 1011 Ω

271

amplifier circuits) at mass resolutions of ~2000 and 3500, respectively. Count rates for 32S- and

272

34 -

273

over a 60 s total counting interval for each analysis. The analytical protocol interspersed

274

analyses of unknowns with the anhydrite and barite RMs in a 4:1 ratio (anhydrite S0431 with

275

δ34SV-CDT = +10.87 ±0.3‰, and barite S0327 with δ34SV-CDT = +22.0 ± 0.3‰, where the sulfates

276

are scaled to NBS-127 = +21.1‰; R. Stern, unpublished data).

S ranged from 0.6 – 1.5 × 108 counts/s and 3.0 – 6.0 × 106 counts/s, respectively, determined

Instrumental mass fractionation (IMF) for

277

34 - 32 -

S / S was determined from utilizing all the

278

replicate analyses of the RMs for each session (IP18032A, B; IP18033), after correction for

279

systematic within-session drift in IMF. The standard deviation of the within-session

280

ratios for anhydrite was ± 0.13‰, and for barite was ± 0.11‰. Final uncertainties in δ34SV-CDT

281

are reported at 95% confidence level (2σ) and propagate within-spot counting errors, between-

282

spot errors to account for geometric effects (blanket uncertainty of ± 0.10‰ applied), and

283

between-session error (≤ ± 0.02‰) that accounts for uncertainty in the mean IMF for the session.

284

The total uncertainties average ± 0.30‰ (2σ) per spot for anhydrite and ± 0.25‰ for barite. The

285

34

34 - 32 -

S/ S

S/32S value of VCDT utilized for normalization was 0.0441626 (Ding et al., 2001).

286

Oxygen isotope ratios (18O/16O) were determined in anhydrite (151 spots) and barite (30

287

spots) subsequent to the S-isotope measurements directly within the previous S-isotope spots

288

(Table 3, Fig. 7b-c). Both

289

using 1010 Ω amplifier, and H’2 with 1011 Ω) at mass resolutions of 1900 and 2250, respectively.

16

O- and

18

O- were analyzed simultaneously in Faraday cups (L’2

13

16

O- and

18

O- were ~1.0 × 109 and ~ 2 × 106 counts/s, respectively,

290

Mean count rates for

291

determined over a 90s counting interval.

292

unknowns with the anhydrite and barite RMs (S0431 anhydrite with δ18OVSMOW = +16.74

293

±0.1‰, S0327 with δ18OV-SMOW = +11.0 ±0.1‰, R. Stern, unpublished data) in a 4:1 ratio.

The analytical protocol interspersed analyses of

294

The standard deviation of 18O-/16O- ratios for S0431 anhydrite was ±0.11‰, and ±0.10‰ for

295

S0327 barite. Final uncertainties are reported at 95% confidence level (2σ) and propagate

296

within-spot counting errors, between-spot errors to account for geometric effects (±0.10‰), and

297

between-session error that accounts for uncertainty in the mean IMF for the session. The total

298

uncertainties in δ18OV-SMOW average about ±0.3‰ (2σ) per spot for both minerals.

299

4. Results

300 301

4.1. Petrographic observations - main sulfur-bearing phases 4.1.1. Anhydrite

302

Anhydrite occurs in almost all examined dolomitic siltstone samples in the study area with

303

variable occurrence and abundance in northeastern BC and western Alberta. The volumetric

304

occurrence of anhydrite is minimal (~2%) in northeastern BC and significantly increases up to

305

15% in some western Alberta samples. Anhydrite is found in various forms, including dissolved

306

residue of anhydrite crystals, blocky (vug-filling, nodule), poikilotopic pore-filling and fracture-

307

filling cement, and replacive crystals (Fig. 4a-f).

308

The first phase of anhydrite (approx. 8 to 24 µm in size), which is less preserved due to

309

dissolution by later silica cement occurs as a minute corroded vestige of anhydrite crystals in

310

authigenic quartz crystals and quartz overgrowths (Fig. 4a) suggesting an early diagenetic origin

311

for this anhydrite phase in the Montney Formation (pre-authigenic quartz anhydrite of Davies, et

14

312

al., 1997). This form of anhydrite is predominant in the studied samples from northeastern BC

313

(Fig. 4a).

314

The second phase of anhydrite occurs as replacive, blocky (vug-filling), poikilotopic and

315

fracture-filling cement with a slightly mottled appearance (Figs. 4b-f & 5a-b). The replacive

316

variety of anhydrite is most commonly formed as irregular to lath-like tabular crystals (approx.

317

20 to 50 µm in size) and mostly replaces dolomite and calcite crystals (Fig. 4b). Replacive

318

anhydrite is seen in both parts of the study area; however, its abundance is significantly higher in

319

western Alberta. Blocky anhydrite generally filled sporadic vugs and nodules, most commonly as

320

single anhydrite crystals (Figs. 4c & 5b).

321

The poikilotopic pore-filling anhydrite makes up approximately 10-15% of the rock volume

322

in some western Alberta samples and has encompassed quartz, dolomite, and K-, and Na-

323

feldspars, suggesting a later diagenetic origin (Fig. 4d). The poikilotopic anhydrite is also

324

enclosed by large euhedral pyrite clusters (Fig. 4e) that possibly formed at the latest stage of

325

diagenesis (Section 4.1.2; Fig. 6).

326

Fracture-filling anhydrite is only observed in cores from western Alberta (e.g., high H2S

327

zone) with approximate width and length of 0.3 to 2.5 cm and 3 to 30 cm, respectively (Figs. 5a).

328

Poikilotopic anhydrite cement has considerably formed in the close contact of the fractures and

329

the host rock and extends laterally through the host rock (Figs 4f).

330

4.1.2. Pyrite

331

Two dominant forms of authigenic pyrite were observed in the studied samples with higher

332

abundance in northeastern BC as framboidal and euhedral crystals (Figs. 4e & 4g-h). Spherical

333

framboids (single or poly-framboids) and loosely aggregated framboids are commonly

334

comprised of both cubic and octahedral pyrite microcrystals (Fig. 4g). Both varieties of

15

335

framboidal pyrite generally occur in intergranular/crystalline pore spaces, noticeably surrounding

336

detrital grains and their associated overgrowth or pore-filling cement, and typically in close

337

vicinity of organic matter (i.e., solid bitumen; Sanei et al., 2015). This form of pyrite is more

338

abundant in northeastern BC.

339

The second form of pyrite occurs as small single euhedral crystals (cubic pyrite) and variably

340

sized euhedral to anhedral pyrite clusters. Similar to pyrite framboids, cubic pyrite crystals are

341

generally scattered in the available pore spaces encasing detrital grains and early-stage

342

authigenic mineral assemblages. Mostly observed in western Alberta, larger individual euhedral

343

or anhedral clustered crystals typically enclose minute inclusions of dolomite, quartz, feldspars,

344

and anhydrite, as well as framboids (Figs. 4e & 4h), demonstrating a very late diagenetic origin

345

(Fig. 6).

346

4.1.3. Barite

347

In comparison to anhydrite, trace amounts of barite were observed in only three wells in

348

western Alberta as disseminated pore-filling cement enclosing quartz, dolomite, K- and Na-

349

feldspar and framboidal and euhedral pyrite crystals (Fig. 4i). Establishing a direct paragenetic

350

relationship between anhydrite and barite is not possible, as they are rarely observed in the same

351

sample or same location. In most studied samples, however, barite encloses the latest euhedral

352

pyrite crystals and clusters (Fig. 4j), which themselves postdate the late diagenetic anhydrite

353

phase. This textural relationship demonstrates that barite formed later than anhydrite at the very

354

late stage of diagenesis. Figure 6 illustrates the interpreted paragenetic sequence for the Montney

355

Formation, with anhydrite and barite paragenetic relationships with other diagenetic elements

356

shown in grey bars.

357

16

358 359

4.2. Isotope Geochemistry 4.2.1. Bulk sulfur and oxygen isotope composition of sulfate fraction (Anhydrite)

360

The measured δ34S and δ18O values of bulk-rock sulfate fraction (predominantly anhydrite)

361

across the study area are presented in Table 2a-b. The δ34S and δ18O values of all samples are

362

variable, ranging from +2.9 to +24.7‰ (V-CDT) and -11.2 to +15.7‰ (V-SMOW), respectively

363

(Fig. 7a). The δ34S value of anhydrite in northeastern BC (low H2S zone) ranges from +4.9 to

364

+22.9‰ (V-CDT) and its δ18O value varies from -11.2 to +0.3‰ (V-SMOW). The δ34S value of

365

anhydrite samples from western Alberta (high H2S zone) varies from +2.9 to +24.6‰ (V-CDT),

366

which overlaps the range in anhydrite from northeastern BC. The δ18O values of anhydrite

367

samples from western Alberta (high H2S zone) show a wider range of variation from -10.5 to

368

+15.7‰ (V-SMOW) (Fig. 7a). The northeastern BC samples show less isotopic variation in

369

comparison to the western Alberta samples with most of the samples (approximately 80%)

370

having lower values: δ34S from +4.9 to +15.9‰ (V-CDT) and δ18O from -11.2 to -1.2‰ (V-

371

SMOW).

372

The δ34S and δ18O values of fracture- and vug-filling anhydrite cement in western Alberta

373

vary from +23.5 to +24.7‰ (V-CDT) and +13.3 to +14.7‰ (V-SMOW), respectively. The

374

isotopic composition of fracture- and vug-filling anhydrite samples from the Middle Triassic

375

Doig and Halfway formations in western Alberta is also similar to the δ34S and δ18O values of

376

fracture- and vug-filling anhydrite cement of the Montney Formation ranging from +21.1 to

377

+23.7 ‰ (V-CDT) and +12.6 to +15.2 (V-SMOW), respectively (Table 2c, Fig. 7a).

378

4.2.2. SIMS- sulfur and oxygen isotope composition of anhydrite and barite

17

379

The δ34S and δ18O values of individual anhydrite and barite cement crystals were measured

380

using SIMS. The SIMS analysis data are presented in Table 3. SIMS-analyzed anhydrite is

381

predominantly the late pore-filling poikilotopic type. Sulfur and oxygen isotope composition of

382

early anhydrite (dominant in northeastern BC) could not be measured due to its ultra-fine grain

383

size. All the measured spots in anhydrite and barite from western Alberta have positive δ34S and

384

δ O values (Table 3; Fig. 7a). The δ S and δ O values of late poikilotopic anhydrite in western

385

Alberta vary from +18.5 to +37‰ (V-CDT) and +12 to +22‰ (V-SMOW), respectively (Fig. 7a

386

and Fig. 7b). The δ34S values of the majority of anhydrite samples (98%) fall in the range of

387

+18.5 to +33‰ (V-CDT).

18

34

18

388

Barite exhibits a narrower range in terms of both δ34S and δ18O relative to anhydrite (Fig. 7a).

389

Except for two spots with δ34S values of +39‰, the remaining measured spots in barite show

390

δ S values ranging from +23.3 to +33.6‰ (V-CDT) and δ O values varying from +13.2 to

391

+18.7‰ (V-SMOW) (Fig. 7a & Fig. 7c).

392 393

34

18

4.2.3. 87Sr/86Sr isotope composition of anhydrite The

87

Sr/86Sr ratios of four fracture- and vug-filling anhydrite samples from western Alberta

394

range from 0.7092 to 0.7102 (Table 2b, Fig. 9), which is pretty higher than the assumed Triassic

395

seawater 87Sr/86Sr isotope ratios (0.7073 to 0.7082, Veizer et al., 1999).

396

5. Discussion

397

5.1. Anhydrite and barite occurrence and its relationship to the high H2S zone

398

The diagenetic evolution of the Montney Formation in northeastern BC and west-central

399

Alberta has been previously investigated by Davies et al., (1997) and Vaisblat et al., (2017). The

400

dominant diagenetic phases reported in these studies were calcite, dolomite, K-feldspar, quartz,

18

401

and pyrite. Davies et al., (1997) reported the occurrence of anhydrite as an early cement in the

402

Montney Formation in west-central Alberta, however, in the current study this type of anhydrite

403

was mainly observed in northeastern BC (i.e., low H2S zone). According to the present study, the

404

anhydrite abundance and textural variation are significantly higher in western Alberta (i.e., high

405

H2S zone) (Fig. 4b-f).

406

Among all four types of anhydrite, including replacive, blocky (vug-filling), fracture-filling

407

and poikilotopic types, the latter is the most abundant type in the studied samples. This type of

408

anhydrite has poikilotopically enclosed detrital grains and their associated overgrowth or pore-

409

filling cement, including quartz, potassium- and sodium-feldspar, and dolomite (Fig. 4d & 4f).

410

This petrographic evidence is in good agreement with the petrographic and SEM-CL

411

observations corroborating that the Montney siltstone has undergone early diagenetic

412

cementation. The early-stage cementation processes impeded the development of mechanical and

413

chemical compaction and significant porosity loss during progressive burial. This argument is

414

mainly verified by the dominant planar or tangential and minimal local interpenetrating

415

grain/crystal contacts and a dearth of bend/crushed mica flakes or fractured grains in the studied

416

samples.

417

These textural relationships suggest that the key phases of anhydrite formation postdate the

418

major cementation phases in the Montney Formation and are late diagenetic in origin.

419

Poikilotopic anhydrite also occurs in the vicinity of the fractures and vugs filled with anhydrite

420

suggesting lateral migration of the same sulfate-rich fluid(s) in the host Montney Formation (Fig.

421

4f). The occurrence of these anhydrite varieties has not been reported in the previous studies on

422

the diagenetic history of the Montney Formation.

19

423

Barite has been reported as an early diagenetic phase in the west-central Alberta section

424

(Davies et al., 1997) and as a late fracture-filling cement mostly in the BC section of the

425

Montney Formation (James Wood, personal communication). In the current study, minor barite

426

cement is present postdating the late anhydrite cement (Fig. 6), dominantly in western Alberta

427

(i.e., high H2S zone).

428

5.2. Implications for the origin of sulfate

429

5.2.1. Early anhydrite δ34S and δ18O signature (Bulk rock analysis)

430

Equilibrium isotopic exchange reactions and kinetically controlled dissimilatory sulfate

431

reduction are two major processes that control sulfur isotope fractionation (e.g., Holser and

432

Kaplan 1966; Ohmoto and Rye, 1979; Seal et al., 2000; Canfield, 2001; Hoefs, 2018). The

433

equilibrium sulfur isotope fractionation between dissolved sulfate and sulfate minerals is

434

negligible and varies from 0 to 1.65‰ (Thode and Monster, 1965; Paytan et al., 1998).

435

Therefore, due to minimal fractionation, the δ34S value of sulfate minerals approximately reflects

436

the δ34S value of the parent fluid (i.e., coeval seawater) which precipitated the sulfate mineral

437

(e.g., Goldhaber and Kaplan, 1974; Claypool et al., 1980; Seal et al., 2000; Hoefs, 2018). The

438

sulfate oxygen isotope composition of seawater is controlled by the dynamic balance of input,

439

output and partial re-equilibration that occurs in the sulfate oxygen cycle. However, when sulfate

440

ion is formed, the change in its oxygen isotope ratio from the parent fluid would be insignificant

441

due to its highly stable nature (Holser et al., 1979). Therefore, the oxygen isotopic composition

442

of sulfate minerals, generally, represents the oxygen isotopic value of sulfate in the coeval

443

seawater, which leads to only a few per mil enrichment of

444

3.6‰) in the precipitated sulfate minerals (Holser et al., 1979; Claypool et al., 1980).

20

18

O (∆18Oevaporite-dissolved sulfate ≈ 3.5 to

445

Carbonate-associated sulfate (CAS) is thought to reflect the sulfur and oxygen isotopic

446

composition of sulfate in the contemporaneous seawater (Kampschulte and Strauss, 2004;

447

Bottrell and Newton, 2006: Rennie and Turchyn, 2014; Algeo et al., 2015, and references

448

therein). Unlike evaporite minerals, carbonate rocks and minerals are globally pervasive and

449

continuous over the geological record. Therefore, the paleo-proxies extracted from these rocks

450

have the advantages of continuity and potentially higher temporal resolution than evaporite

451

minerals proxies (Newton et al., 2004; Bottrell and Newton, 2006; Rennie and Turchyn, 2014).

452

The δ34S values of the majority of studied samples from both western Alberta and

453

northeastern BC (+2.9 to +24.7‰ V-CDT) are consistent and exhibit similar δ34S values to the

454

estimated CAS and evaporites precipitated from Triassic seawater (+10 to +26‰ V-CDT;

455

Claypool et al., 1980; Kampschulte and Strauss, 2004; Algeo et al., 2015) and also the Devonian

456

evaporites (+17 to +34‰ V-CDT; Claypool et al., 1980; Machel, 1985; Fu, 2005; Fig. 7a).

457

However, the mean calculated sulfur isotope fractionation between these samples and the

458

estimated Triassic seawater dissolved sulfate (Claypool et al., 1980; Kampschulte and Strauss,

459

2004; Algeo et al., 2015) is negligible (∆34Sanhydrite-dissolved sulfate (Triassic) ≈ +0.7‰) in comparison to

460

the Devonian evaporites (∆34Sanhydrite-dissolved sulfate (Devonian evaporites) ≈ +9‰; Claypool et al., 1980;

461

Machel, 1985; Fu, 2005). This might suggest that the anhydrite cement in these samples was

462

presumably precipitated from Montney marine connate water with similar isotopic composition

463

to Triassic seawater sulfate (Fig. 7a).

464

Nevertheless, the measured δ34S values of bulk anhydrite (+2.9 to +24.7‰ V-CDT) in the 34

465

present study are less enriched in

S than the CAS sulfur isotope signature of the Jesmond

466

carbonate section of the Cache Creek Terrane in western Canada (+28.6 to +41.0‰ V-CDT),

467

which is assumed to be representative of the Early Triassic Panthalassic Ocean sulfur isotope 21

468

signature (Stebbins et al., 2018). Furthermore, most of these anhydrite samples, especially in

469

western Alberta, are more enriched in δ34S than the upper Triassic evaporites of the Alberta sub-

470

basin in the WCSB (Charlie Lake Formation, Claypool et al., 1980, Fig 7a). This calls into

471

question the assumption that sulfate in the Montney Formation is sourced from the dissolution of

472

overlying upper Triassic evaporites (Desrocher et al., 2004).

473

As shown in Figure 7a, the δ18O values of bulk anhydrite samples display a wide range of

474

values (-11.2 to +15.7‰ V-SMOW). The mean oxygen isotope fractionation between these

475

samples and the assumed Triassic seawater dissolved sulfate (mean δ18OTriassic

476

+8‰; Claypool et al., 1980) is -5.9‰. They exhibit even larger fractionation (-6.4‰) relative to

477

the presumed oxygen isotope composition of upper Triassic seawater dissolved sulfate in the

478

Alberta sub-basin (mean δ18Odissolved sulfate-Alberta ≈ +8.5‰; Claypool et al., 1980). There is also a

479

significant difference between the measured δ18O of studied bulk anhydrite and the CAS δ18O

480

values of the Early Triassic Jesmond Section in western Canada (Stebbins et al., 2018). The

481

Jesmond carbonate section has noticeably higher CAS δ18O values (+20.4 to +23.9‰; Stebbins

482

et al., 2018) than our studied anhydrite from the Montney Formation in both western Alberta and

483

British Columbia.

dissolved sulfate

≈

484

The sulfur isotope composition of the studied samples falls within the overall δ34S range of

485

Triassic seawater sulfate globally (Fig. 7a; Kampschulte and Strauss, 2004; Algeo et al., 2015).,

486

suggesting the sulfur isotope signature of Triassic seawater for the Montney anhydrite samples.

487

Conversely, both δ34S and δ18O values of the Montney bulk anhydrite samples substantially

488

diverge from sulfur and oxygen isotope composition of Triassic seawater sulfate in the WCSB

489

(Claypool et al., 1980; Stebbins et al., 2018; Fig. 7a). This may suggest that the isotopic

490

signature acquired from anhydrite samples are not characteristic of initial Triassic marine 22

491

connate water (i.e., Triassic seawater) sulfate at the time of the Montney deposition. It is

492

potentially indicative of more modified Triassic seawater through different early to late

493

diagenetic processes.

494

Nearly one-third of the bulk anhydrite samples measured in this study (30%) have isotopically

495

lighter sulfur than the assumed Triassic seawater sulfate, but the majority (70%) have lighter

496

oxygen isotope signature respective to the estimated Triassic seawater sulfate composition

497

(Claypool et al., 1980; Fig. 7a). This isotopic signature is particularly common in the samples

498

with a higher abundance of early diagenetic anhydrite confirmed by petrographic and SEM

499

observations in both western Alberta and northeastern BC. Two main processes are likely

500

responsible for the isotopic composition of fluids that precipitated early-stage anhydrite cement

501

in the Montney Formation; (1) sulfide oxidation and, (2) water/rock interaction with formation

502

waters and brines in the basin.

503

Aqueous sulfate resulting from sulfide oxidation (either pyrite or dissolved sulfide, including

504

H2S, HS- and S2-) has variable sulfur and oxygen isotopic composition, but generally, its δ34S and

505

δ18O are depleted relative to marine sulfate composition (Claypool et al., 1980; Van Stempvoort

506

and Krouse, 1994; Bottrell and Newton, 2006, and references therein). The sulfate-sulfide sulfur

507

isotope fractionation associated with the sulfide oxidation process is negligible but generally

508

produces sulfate with an isotopically lighter sulfur isotope sourced from depleted oxidized

509

sulfide (Claypool et al., 1980; Taylor and Wheeler, 1994; Canfield, 2001).

510

The oxygen atoms incorporated into the sulfate molecule during sulfide oxidation process

511

commonly derives from either atmospheric origin or ambient water, which in turn is controlled

512

by the redox conditions and the biological activity (Lloyd, 1967; Balci et al., 2007). The δ18O of

513

sulfate formed through sulfide oxidation typically ranges from -10 to +2‰ (Claypool et al., 23

514

1980; Van Stempvoort and Krouse, 1994; Krouse and Mayer, 2000) dominated mostly by

515

ambient water-derived oxygen atoms of meteoric or seawater origin or even modified by

516

evaporation processes (Taylor and Wheeler, 1994; Bottrell and Newton, 2006).

517

The early diagenetic settings are dominated by oxidation process of dissolved sulfide, where

518

MSR-induced H2S gets re-oxidized (Jorgensen, 1982, Canfield and Teske, 1996) and produces

519

sulfate, elemental sulfur, and other sulfur intermediates with more depleted sulfur and oxygen

520

isotope signature (Thamdrup et al., 1994; Krouse and Mayer, 2000; Poser et al., 2014). The

521

early-stage anhydrite in the Montney Formation from both western Alberta and northeastern BC

522

may have been sourced from isotopically depleted sulfate initially produced through the sulfide

523

oxidation process during early diagenesis, which led to depleted 34S- and 18O- isotope signatures

524

of these type of anhydrite in comparison to Triassic seawater.

525

It is noteworthy that the different pyrite phases observed in the current study are in pristine

526

condition with no evidence of pyrite oxidation (Figs. 4e & 4g-h). Hence, an in-situ source of

527

dissolved sulfate derived from the oxidation of existing pyrite within the Montney Formation can

528

be excluded. Consequently, the dissolved sulfate generated through sulfide oxidation might have

529

been present from an earlier sulfide oxidation event (e.g. MSR-induced H2S phase) in the

530

Montney Formation.

531

The isotopic signature of early anhydrite from both western Alberta and northeastern BC is

532

also similar to the isotopic composition of dissolved sulfate in formation waters and brines in the

533

Alberta sub-basin of the WCSB (Hitchon and Friedman, 1969; Connolly et al., 1990). This is

534

mostly the case for the δ18O values of the majority of early anhydrite cement (-11.2 to +6.8‰ V-

535

SMOW; Fig. 7a) which are in the range of the δ18O values of dissolved sulfate in the Devonian

536

to Carboniferous or even Triassic formation waters and brines (-15.9 to +10.7‰ SMOW; 24

537

Hitchon and Friedman, 1969; Connolly et al., 1990; Simpson, 1999). It appears that the early

538

anhydrite cement precipitated from sulfate-rich fluids interacted with formation waters and

539

brines in the basin, particularly with underlying Devonian to Carboniferous strata (discussed

540

below in section 5.2.2).

541

In summary, sulfate-rich fluids that precipitated the early anhydrite cement may have

542

originated from Triassic marine connate waters. However, they have likely been modified

543

through mixing with formation waters and brines in the WCSB and also mixing with the

544

dissolved sulfate sourced from the sulfide oxidation process during early diagenesis.

545

5.2.2. Late diagenetic anhydrite and barite δ34S and δ18O signature (Bulk rock and SIMS

546

analysis)

547

The late diagenetic anhydrite and barite cement show two distinctive sulfur and oxygen

548

isotopic signatures when compared with the assumed sulfur and oxygen isotope composition of

549

Triassic seawater dissolved sulfate reported by different authors in the literature (Fig. 7a). The

550

bulk and SIMS δ34S and δ18O values of a large number of anhydrite and barite samples from

551

western Alberta (i.e., high H2S zone) (δ34S: +16 to +34‰ V-CDT and δ18O: +10 to +22‰ V-

552

SMOW) are significantly enriched relative to the assumed global Triassic seawater dissolved

553

sulfate and upper Triassic evaporites of the Alberta sub-basin in the WCSB (Claypool et al.,

554

1980), showing more similarity to the Devonian evaporites isotopic composition (Claypool et al.,

555

1980; Machel, 1985; Fu, 2005, Fig. 7a). However, some other samples display bulk and SIMS

556

δ34S values falling within the range of assumed sulfur isotope composition of Triassic seawater

557

(+10 to +26‰ V-CDT; Claypool et al., 1980; Kampschulte & Strauss, 2004; Algeo et al., 2015;

558

Fig. 7a). Furthermore, the bulk δ18O values of a limited number of anhydrite samples from

25

559

western Alberta are in accordance with the estimated δ18O signature of global Triassic seawater

560

sulfate (Claypool et al., 1980).

561

The isotopic signature of sulfate minerals in the western Alberta section of the Montney

562

Formation (both bulk and SIMS values) suggests that at least two major sulfate-rich fluids were

563

involved in the precipitation of late anhydrite and barite cement. The trend shown in Fig. 7a

564

generally overlaps with both sulfur and oxygen isotope composition of Triassic seawater sulfate

565

(Claypool et al., 1980; Kampschulte & Strauss, 2004; Algeo et al., 2015) and Devonian

566

evaporites of the Alberta sub-basin (Claypool et al., 1980; Machel, 1985; Fu, 2005) with more

567

contribution of sulfate from Devonian evaporites. The δ34S and δ18O values of fracture- and vug-

568

filling anhydrite from the Middle Triassic Doig and Halfway formations (Fig.7a) also exhibit

569

more affinity to the isotopic composition of Devonian evaporites than Triassic evaporites of the

570

Alberta sub-basin (Claypool et al., 1980). The similar δ34S and δ18O values of all fracture- and

571

vug-filling anhydrite in the Montney, Doing and Halfway formations, almost all SIMS anhydrite

572

and barite samples, and a considerable amount of bulk poikilotopic anhydrite cement from the

573

Montney Formation in western Alberta, and sulfate from dissolution of underlying Devonian

574

evaporites (Fig. 7a), suggest significant contribution of sulfate-rich fluids derived from the

575

Devonian evaporites.

576

The mean concentration of dissolved sulfate and total dissolved solids (TDS) in the formation

577

waters of the WCSB are 410 mg/l (2-3910 mg/l; Hitchon et al., 1971, 10-1280 mg/l; Connolly et

578

al., 1990) and 80000 mg/l (4000-235000 mg/l; Connolly et al., 1990), respectively. In contrast,

579

the Devonian formation waters themselves with a significant amount of evaporite deposits, have

580

significantly higher mean dissolved sulfate value of 946 mg/l (276-3910 mg/l; Hitchon et al.,

581

1971), and mean TDS value of 125,000 mg/l (93000-235000 mg/l; Connolly et al., 1990) or even 26

582

300000 mg/l (Grasby and Chen, 2005) in the WCSB. The salinity of Devonian MgCl2–CaCl2–

583

NaCl brines in the WCSB is three to eight times higher than that of modern seawater or modified

584

seawater (Al-Aasm, 2003; Davies and Smith, 2006).

585

The occurrence of thick Devonian evaporite strata underneath the Montney Formation (Fig. 3)

586

is, therefore, most likely a potential source of sulfate-rich fluids for precipitation of late anhydrite

587

and barite cement and fracture-filling anhydrite in western Alberta. The occurrence of extensive

588

fault systems, including Precambrian basement, Paleozoic extensional, and Jurassic and

589

Cretaceous compressional faults in the subsurface of the Peace River region with several

590

episodes of reactivation through Devonian to Cretaceous is well documented (Barclay et al.,

591

1990; Hope et al., 1999; O’Connell et al., 1990; Mei, 2009). These fault and fracture networks

592

likely played a key role as conduits for sulfate-rich fluids that resulted from the dissolution of

593

Devonian evaporites as a source of excess sulfate for precipitation of late-stage anhydrite and

594

barite cement and fracture-filling anhydrite in western Alberta. The results of a recent study on

595

the geochemical and sulfur isotopic evolution of flowback and produced waters from the

596

Montney Formation (Osselin et al., 2019) showed that the isotopic composition of the Montney

597

Formation produced waters became similar to the sulfur and oxygen isotopic composition of

598

Devonian evaporites (Fig. 7a). This further supports the significant involvement of brines

599

originated from the dissolution of Devonian evaporites in the Montney Formation diagenesis.

600

Petrographic examinations of the current investigation indicate that the Montney Formation in

601

the study area is texturally heterogeneous, even at a small scale, with highly cemented coarse-

602

grained laminated siltstone interbedded with fine-grained siltstone(Fig. 4b-f). This heterogeneity

603

is reflected in the depth profile of sulfur and oxygen isotopic composition of closely spaced

604

samples in a single cored-well (Fig. 8). The porous coarse-grained laminae with late anhydrite 27

605

cement have δ34S and δ18O values similar to the isotopic composition of Devonian evaporites

606

(Fig. 8). The less porous fine-grained laminae with early anhydrite cement on the other hand

607

exhibit δ34S and δ18O values of both fluid end members, suggesting the mixing of two fluid

608

sources, i.e. modified Triassic formation water and brines that originated from the dissolution of

609

Devonian evaporites (Fig. 7a). Lateral migration of diagenetic fluids through faults and fractures

610

into the more porous Montney intervals may have contributed to this vertical variation and

611

precipitation of late anhydrite cement in the vicinity of the fault/fracture networks.

612

5.3. Structurally-controlled hydrothermal activity in the study area

613

The extensive occurrence of hydrothermal dolomite reservoirs in the Devonian and

614

Mississippian carbonate reservoirs of the WCSB is attributed to tectonically driven hydrothermal

615

activities (Al-Aasm, 2003; Davies and Smith, 2006). Fluid-inclusion studies indicate that

616

dolomite precipitated from hot (100-180 °C) brines with average salinity between 12-25 wt. %

617

(NaCl eqv.) (Al-Aasm, 2003; Davies and Smith, 2006). The brines generally have MgCl2-CaCl2-

618

NaCl-H2O composition suggestive of post-evaporative residual brines (Spencer, 1987) that were

619

variably modified by interaction with siliciclastic and basement rocks (Al-Aasm, 2003; Davies

620

and Smith, 2006).

621

Upward movement of saline fluids was facilitated by thermal and structural drives (Davies

622

and Smith, 2006). Hydrothermal fluid flow is mostly associated with the extensional and

623

transtentional tectonic setting of the WCSB characterized by elevated heat flow (Davies and

624

Smith, 2006). The occurrence of higher heat flow in northwestern parts of the WCSB, including

625

western Alberta and the high H2S zone, has also been confirmed by Majorowicz (2018). Upward

626

migration of hydrothermal fluids, facilitated by the extension of fault and fracture networks, in

627

the Devonian of the WCSB is previously considered to have terminated in the middle or at the 28

628

top of limestone reservoirs due to the occurrence of internal and top shale or tight limestone seals

629

(Davies and Smith, 2006).

630

However, faulting and fracturing in western and northwestern Alberta is proven to have

631

propagated upward and formed a broader zone of deformation, matching well with the

632

extensional forced folding model (Withjack et al., 1990; Hope et al., 1999). Basement and

633

Paleozoic extensional and Mesozoic (Jurassic and Cretaceous) compressional faults in the PRA

634

region have repeatedly been active from Devonian to Triassic time (Hope et al., 1999), or even to

635

Cretaceous time (O’Connel et al., 1990; Mei, 2009) (Fig. 3), and likely controlled upward later

636

migration of hydrothermal fluids in this region.

637

Furthermore, the PRA, which was a topographic high with progressive uplift from

638

Precambrian to Devonian (Edwards et al., 1994, O′Connell, 1994), has locally brought the

639

Precambrian basement in close proximity of the Carboniferous and Triassic sedimentary cover.

640

This resulted in an easier transfer of basement- and Devonian-derived hydrothermal fluids up to

641

the Triassic Montney Formation via existing fault/fractures networks.

642

The 87Sr/86Sr isotope composition of the fracture- and vug-filling anhydrite (0.7092 to 0.7102)

643

is significantly more radiogenic than the estimated values of coeval Triassic seawater (0.7073 to

644

0.7082, Veizer et al., 1999; Fig. 9). These results also demonstrate that diagenetic modification

645

of the Montney Formation in western Alberta might have occurred by hydrothermally-affected

646

residual evaporated seawater (Spencer, 1987) interacted with Precambrian basement and/or other

647

siliciclastic strata in the basin (Al-Aasm, 2003; Davies and Smith, 2006) through deep-rooted

648

faults and fractures (Hope et al., 1999). This further confirms the occurrence of extensive

649

water/rock interactions in the basin and the Montney Formation.

29

650

All these studies corroborate the importance of hydrothermal diagenesis/mineralization in

651

the PRA region of the WCSB, water/rock interactions and the recurrent activation of subsurface

652

fault networks at least up to the Triassic in western and northwestern Alberta, the H2S

653

concentration hotspot zone. The occurrence of local high porous intervals around the

654

fault/fracture networks also enabled hydrothermal fluids to migrate laterally and create late-stage

655

anhydrite and barite cement in western Alberta.

656

Our results indicate that sulfate minerals are significant diagenetic minerals in the Montney

657

Formation, more specifically in western Alberta (i.e., high H2S zone). The type, timing,

658

abundance, isotopic signature, and spatial occurrence of sulfate minerals in the study area

659

suggest that the upward migration of sulfate-rich hydrothermal fluids sourced from the

660

dissolution of Devonian evaporites into the Montney Formation was channeled through available

661

fault/fracture networks in the study area (Fig. 3).

662

6. Conclusions

663

This study presents petrographic observations, and bulk and in-situ SIMS sulfur and oxygen

664

stable and strontium isotope geochemistry of diagenetic sulfate minerals in the Montney

665

Formation in northeastern BC and western Alberta. The results show the occurrence of

666

regionally distributed early and late diagenetic anhydrite and barite cement throughout the study

667

area. The early diagenetic anhydrite is predominant in northeastern BC (the zone of low H2S

668

concentration), while late-stage anhydrite and barite cement are dominant in the zone of high

669

H2S concentration in western Alberta.

670

The wide range of δ34S and δ18O values of anhydrite and barite suggests that two different

671

sulfate-bearing fluid end-members contributed to sulfate mineral precipitation in the Montney

672

Formation. Variable mixing and water/rock interaction led to the wide variation in the sulfur and 30

87

Sr/86Sr isotope

673

oxygen isotopic signature of anhydrite and barite phases and radiogenic

674

signature of anhydrite. The early diagenetic anhydrite exhibits less enriched or even depleted

675

sulfur and oxygen isotope composition than the assumed Triassic seawater sulfate. This suggests

676

that they precipitated from Triassic formation/pore water, which was modified through

677

interaction with sulfate sourced from sulfide oxidation or formation waters/brines in the basin.

678

The isotopic signature of the late-stage anhydrite (both poikilotopic and fracture- and vug-

679

filling cement) and barite demonstrate the contribution of an extra-formational source of sulfate-

680

rich fluids in precipitation of late-stage sulfate minerals in the Montney Formation. Similar

681

isotopic composition of late diagenetic sulfate minerals with Devonian evaporites further

682

suggests the dominant involvement of sulfate-rich fluids originated from the dissolution of

683

WCSB Devonian evaporites and mixing with connate water of Triassic seawater origin.

684

Enrichment of the late fracture- and vug-filling anhydrite phases in

685

contribution of deep basinal brines modified by interaction with siliciclastic and basement rocks

686

in their precipitation. The Devonian-sourced sulfate-rich hydrothermal brines migrated upward

687

to the Montney Formation through extensive deep-seated fault/fracture networks in the

688

subsurface of western Alberta. This was likely the major source of sulfate surplus in the system,

689

which led to the formation of late-stage anhydrite and barite in the Montney Formation.

690

Acknowledgments

87

Sr further confirms the

691

We wish to thank Drs. I. Al-Aasm (Associate Editor) for handling the manuscript, K. Azmy

692

and an anonymous reviewer for their constructive comments. This work was supported by

693

Natural Resources Canada's Geoscience for New Energy Supply (GNES) program and industry

694

sponsors of the Tight Oil Consortium (TOC). The cores and thin sections for this study were

695

provided by Ovintiv (formerly Encana Corporation). This ongoing support is gratefully 31

696

acknowledged. The authors wish to thank Drs. D. Lavoie and S. Grasby of the Geological

697

Survey of Canada for their constructive comments on the earlier versions of the manuscript. The

698

authors would like also to thank Dr. C. Debuhr for SEM imaging and EDXS analyses and Mr. S.

699

Taylor and the staff of the Isotope Science Lab at the University of Calgary for sulfur and

700

oxygen isotope analyses, and Drs. M. Wieser and K. Miller from the Department of Physics and

701

Astronomy of the University of Calgary for strontium isotope analyses. We also thank Mr. R.

702

Dokken from the Canadian Center for Isotopic Microanalysis (CCIM) of the University of

703

Alberta for SIMS mount preparation, and SEM/BSE imaging.

704

705

706

707

708

709

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Withjack, M.O., Olson, J., Peterson, E., 1990. Experimental models of extensional forced folds. AAPG Bulletin 74, 1038-1054. https://doi.org/10.1306/0C9B23FD-1710-11D78645000102C1865D. Wood, J., 2013. Water distribution in the Montney tight gas play of the Western Canadian sedimentary basin: significance for resource evaluation. SPE Reservoir Evaluation & Engineering. SPE-161824-PA http://dx.doi.org/10.2118/161824-PA. Worden, R.H., Smalley, P.C., 1996. H2S-producing reactions in deep carbonate gas reservoirs: Khuff Formation, Abu Dhabi. Chemical Geology 133, 157–171. https://doi.org/10.1016/S0009-2541(96)00074-5. Zonneveld, J.P., and Moslow, T.F., 2018. Palaeogeographic setting, lithostratigraphy, and sedimentary framework of the Lower Triassic Montney Formation of western Alberta and northeastern British Columbia. In: Euzen, T., Moslow, T.F., Caplan, M., (Eds.), The Montney Play: Deposition to Development. Special Volume, Bulletin of Canadian Petroleum Geology 66, 93-127.

960

38

961

Figures Caption

962

Figure 1. (a) Montney subcrop map showing the location of the study area and high and low H2S

963

zones (Modified from Edwards et al., 1994). (b) Stratigraphic column of Permian, Triassic and

964

Jurassic strata in the WCSB (Modified from Davies et al., 1997).

965

Figure 2. Distribution map of H2S in western Alberta and northeastern BC of the study area in

966

Fig. 1a (orange rectangle). H2S distribution data are from Ovintiv (formerly Encana

967

Corporation). The location of studied wells is shown by numbered blue circles. The location of

968

Precambrian basement and Paleozoic extensional faults in and around the Dawson Creek Graben

969

Complex (DCGC) is compiled from Henderson et al., (1994); Richards et al., (1994); Davies at

970

al., (1997); and Mei, (2009). Abbreviations used for faults names are RF: Rycroft Fault, TF:

971

Teepee Fault, BF: Blueberry Fault, GF: Gordondale Fault, SHF: Saddle Hills Fault, PCF: Pouce

972

Coupe Fault, and KF: Kilkerran Fault.

973

Figure 3. Conceptual cross-section of the study area in western Alberta and northeastern BC

974

(located on Fig. 1) showing the location of the extensional faults and hydrothermal fluid flow

975

path in western Alberta. The stratigraphy is compiled from the Geological Atlas of the Western

976

Canadian Sedimentary Basin (Mossop and Shetsen: eds. and comps., 1994), and Davies et al.,

977

(1997). The location of extensional faults (Precambrian basement faults and DCGC) and their

978

continuation up to the Triassic (Montney Fm.) in the Peace River Region are compiled from

979

Edwards and Brown, (1994), and Hope et al., (1999). The Montney Formation in western

980

Alberta is underlain by thick evaporitic units of the Devonian Elk Point Group, Beaverhill Lake

981

Group, Woodbend and Winterburn groups (Meijer Drees, 1994; Oldale and Munday, 1994;

982

Switzer et al., 1994).

39

983

Figure 4. (a) Photomicrograph of authigenic quartz crystals (A-Qtz) enclosing minute remnants

984

of anhydrite (Anh) and pyrite crystals (Py), (UWI: A-036-G/093-P-01/0, 3753.33 m, XPL). b)

985

Photomicrograph of irregular to lath-shaped replacive anhydrite replacing calcite (Cal) crystal at

986

different regions of the crystal, (UWI: 14-13-078-16W6/0, 2286.42 m. c) Photomicrograph of

987

blocky anhydrite cement (Anh), filling a vug as a single anhydrite crystal in the sample (UWI:

988

06-14-078-11W6, 2198.86 m, XPL). d) False-color backscattered electron (BSE) image of

989

poikilotopic anhydrite cement (Anh) enclosing quartz, feldspar, and dolomite detrital grains and

990

authigenic cement (UWI: 01-32-070-09W6/0, 2549.03 m). e) False-color backscattered electron

991

(BSE) image of large euhedral pyrite cluster (Py-C) enclosing late anhydrite cement (Anh)

992

(UWI: 16-29-069-10W6/0, 2868.72 m). f) Photomicrographs of fracture-filling anhydrite and

993

pore-filling poikilotopic anhydrite cement formed in the vicinity of the fracture (UWI: 01-32-

994

070-09W6/0, 2547.33 m). g) Backscattered electron (BSE) image of framboidal pyrite cluster

995

(polyframboids-Py-F) formed in the pore spaces (UWI: 01-32-070-09W6/0, 2531.94 m). h)

996

False-color backscattered electron (BSE) image of pyrite cluster (Py-C) with dolomite (Dol),

997

quartz (Qtz), and K-feldspar (K-F) inclusions (UWI: 16-29-069-10W6/0, 2877.05 m). i) False-

998

color backscattered electron (BSE) image of late pore filling barite (BRT) cement filling

999

available pore spaces and enclosing dolomite, K- and Na-Feldspar, and quartz in this sample

1000

(UWI: 04-19077-10W6/0, 2140.40 m). j) False-color backscattered electron (BSE) image of

1001

barite (BRT) cement enclosing pyrite (Py) crystals (UWI: 13-22-070-08W6, 2418.41 m).

1002

(Abbreviations: Anh: anhydrite, Na-F: sodium feldspar, K-F: potassium feldspar, Dol: dolomite,

1003

Qtz: quartz, Cal: calcite, Py-C: pyrite cluster, Py-F: pyrite-framboid, BRT: Barite, Py: pyrite),

1004

Montney Formation-northeastern BC and western Alberta.

40

1005

Figure 5. (a) Core photo of the fracture-filling anhydrite cement shown in Fig. 4f (UWI: 01-32-

1006

070-09W6/0, 2547.33 m). b) Core photo of the vug-filling anhydrite cement in western Alberta

1007

(UWI: 13-22-070-08W6, 2424.23 m).

1008

Figure 6. The paragenetic sequence of the main diagenetic phases observed in the Montney

1009

Formation.

1010

Figure 7. (a) Bulk rock and SIMS δ34S and δ18O values of anhydrite and barite from high and

1011

low H2S zones of the Montney Formation. Larger box shows estimated δ34S and δ18O values of

1012

Devonian evaporites in the Alberta Basin (Claypool et al., 1980; Machel, 1985, Fu, 2005). The

1013

range of δ34S and δ18O values of global Triassic evaporites and Triassic evaporites of the Alberta

1014

sub-basin (Charlie Lake Formation) has also been shown by smaller boxes (Claypool et al.,

1015

1980). The δ34S and δ18O values of the Middle Triassic Doing and Halfway fracture- and vug-

1016

filling anhydrite samples are shown by orange triangle for comparison. 7b) Backscattered

1017

electron (BSE) image of a representative anhydrite sample with SIMS δ34S and δ18O values

1018

(UWI: 01-32-070-09W6/0, 2549.03 m, western Alberta). 7c) Backscattered electron (BSE)

1019

image of a representative barite sample with SIMS δ34S and δ18O values (UWI: 13-22-070-

1020

08W6, 2418.41 m, western Alberta). The values are in standard δ-notation relative to V-CDT for

1021

sulfur and V-SMOW for oxygen. The δ34S and δ18O values are in blue and yellow, respectively.

1022

SIMS spots are not to scale. (Abbreviations: ANH: anhydrite, BRT: Barite, Py: pyrite).

1023

Figure 8. Depth profile for δ34S and δ18O values of well “ECA HZ ELM 13-22-70-8” (well # 34-

1024

this study) in the high H2S zone of the Montney Formation, western Alberta.

1025

Figure 9. Secular 87Sr/86Sr curve of Cretaceous to Cambrian seawater (Veizer et al., 1999) and 87Sr/86Sr

1026

ratios of fracture- and vug-filling anhydrite phases from the Early Triassic Montney Formation. 41

1027

Tables Caption

1028

Table 1. Well locations and depths of the studied Montney Formation samples.

1029

Table 2. (a) Bulk rock δ34S and δ18O results of the sulfate fraction (anhydrite) in selected

1030

samples from both high and low H2S concentration zones in the study area. b) Bulk rock δ34S

1031

and δ18O results of five fracture- and vug-filling anhydrite samples and

1032

of four same samples from the Montney Formation in western Alberta. c) Bulk rock δ34S and

1033

δ18O results of six fracture- and vug-filling anhydrite samples from the Middle Triassic Doing

1034

and Halfway formations in western Alberta.

1035

Table 3. SIMS δ34S and δ18O results of anhydrite and barite in selected samples from the high

1036

H2S concentration zone in western Alberta.

42

87

Sr/86Sr isotope ratios

Figures: Fig. 1 a-b

a

b

1

Fig. 2

2

Fig. 3

3

Fig. 4

a

b

c

d

e

f

4

5

g

h

i

j

6

Fig. 5

a

b

7

Fig. 6

8

Fig. 7a-c 45 High H2S Zone (ANH-SIMS)

a

High H2S Zone (BRT-SIMS)

40

Low H2S Zone (Bulk Sulfate Fraction)

Devonian Evaporites

35 High H2S Zone (Bulk Sulfate Fraction)

δ34Ssulfate (‰V-CDT)

30

Fracture- & Vug-Filling Anhydrite-High H2S Zone Fracture- & Vug-Filling Anhydrite-High H2S ZoneHalfway/Doig

25

20

Triassic Evaporites

15

Triassic Evaporites- Alberta (Charlie Lake Fm.)

10

5

0 -15

-10

-5

0

5

10

δ18Osulfate (‰V-SMOW)

b

c

9

15

20

25

Fig. 8 δ 34Ssulfate (‰V-CDT) 0

5

10

15

20

25

30

2410

2420

2430

Depth (m)

2440

2450

2460

2470

2480

2490 sulfur

oxygen

2500 -5

0

5

10

δ18Osulfate (‰V-SMOW)

10

15

20

Fig. 9

0.7110

Sr Curve

0.7100

Fracture- & Vug-Filling Anhydrite

87Sr/86Sr

0.7090

0.7080

0.7070 Triassic 0.7060 50

100 150 200 250 300 350 400 450 500 550

Age (Ma)

11

Tables Table 1 UWI∗ ∗

Well Name

Well #

Depth from

Depth to

(this study)

(m)

(m)

Location

01-32-070-09W6/0

ECA HZ ELM 1-32-70-9

1

2531.94

2604.20

W Alberta

A-036-G/093-P-01/0

ECA HZ KELLY B-035-G/093-P-01

2

3718.70

3771.28

NE British Columbia

C-081-J/093-P-07/0

ECA ET AL HZ SUNDOWN C-081-J/093-P-07

6

3323.67

3342.43

NE British Columbia

B-052-I/093-P-06/2

ECA CRP HZ SUNDOWN B-052-I/093-P-06

21

4008.59

4038.36

NE British Columbia

14-13-078-16W6/0

ECA CRP SUNRISE 14-13-078-16

28

2283.97

2306.07

NE British Columbia

04-19-077-10W6/0

GREY WOLF ET AL PCOUPES 4-19-77-10

31

2130.90

2140.40

W Alberta

13-22-070-08W6

ECA HZ ELM 13-22-70-8

34

2412.21

2493.19

W Alberta

16-29-069-10W6/0

ECA 102 ELM 16-29-69-10

50

2839.03

2877.05

W Alberta

C-086-H/093-P-07/0

COPOL SUNDOWN C-086-H/093-P-07

51

3552.30

3665.00

NE British Columbia

07-34-078-11W6/0

ECA PCOUPES 7-34-78-11

58

2094.39

2097.73

W Alberta

06-14-078-11W6/0

ECA PCOUPES 6-14-78-11

59

2188.35

2216.24

W Alberta

60

2222.35

2237.83

W Alberta

15-30-077-10W6/0 GREY WOLF ET AL PCOUPES 15-30-77-10 ∗Unique Well Identifier

12

Table 2a

Well # (this study)

6

Low H2S Zone

2

21

Depth (m)

δ34S (‰ V-CDT) (Anhydrite)

δ18O (‰ V-SMOW) (Anhydrite)

3323.67

5.3

-4.7

3334.31

4.9

-3.1

3342.43

7.0

-11.2

3718.70

15.1

-6.8

3736.52

21.8

-6.5

3753.33

21.5

3764.78

22.9

-1.2

4010.29

13.6

-10.7

4028.53

10.7

-9.4

2283.97

14.0

-1.6

2288.61

18.9

-10.2

2295.54

15.9

-9.9

2299.95

12.8

0.3

2306.07

14.7

-9.9

2130.90

13.7

-10.5

2133.03

15.7

-9.7

2135.63

16.9

-10.1

2137.45

19.0

-9.8

2145.06

19.9

-10.5

2140.40

16.7

-10.4

2222.35

20.3

6.1

2227.60

16.7

3.5

2237.83

21.1

12.2

2232.29

21.4

4.1

2194.81

22.6

14.4

2198.86

20.9

10.2 5.2

28

31

High H2S Zone

60

59

58

34

2214.10

21.5

2094.39

12.9

2107.46

21.1

11.2

2097.73

14.6

-5.3

2413.58

23.9

14.5

2416.74

10.1

-4.2

2418.41

24.4

12.7

2424.99

21.9

14.7

2429.71

3.6

0.9

2434.31

2.9

-1.7

13

1

50

2436.39

7.6

0.9

2446.25

23.7

12.7

2452.94

16.6

-0.6

2461.97

15.5

-0.5

2469.13

12.7

-2.4

2474.41

14.0

-0.6

2478.88

13.4

-1.9

2484.88

14.7

-2.3

2493.19

10.8

-1.0

2413.6

24.6

15.7

2424.23

18.3

8.1

2450.79

24.3

12.6

2594.08

20.2

8.9

2540.56

22.9

14.1

2839.03

19.6

8.7

2851.76

15.3

2.6

2868.72

21.4

6.8

2877.05

23.2

12.6

Well # (this study)

Depth (m)

δ34S (‰ V-CDT) (Fracture- and vug-filling anhydrite)

δ18O (‰ V-SMOW) (Fracture- and vug-filling anhydrite)

1

2546.11 2547.00 2547.33 2839.53 2844.12

24.7 24.0 23.5 23.5 24.7

14.4 13.8 13.3 14.7 14.0

50

87 Sr/86Sr (Fracture- and vug-filling anhydrite) 0.7094552

0.7102264 0.7091732 0.7094272

Table 2c

High H2S Zone

High H2S Zone

Table 2b

Well # (this study) 59

Formation

Doig

59

Halfway

Depth (m)

δ34S (‰ V-CDT) (Fracture- and vug-filling anhydrite)

δ18O (‰ V-SMOW) (Fracture- and vug-filling anhydrite)

1911.85 1914.64 1918.3 1919.98 1924.65 1911.4

22.0 22.9 21.1 23.7 23.4 23.0

13.3 14.9 12.6 15.2 14.3 14.7

14

Table 3

Well # (this study) 1

Depth (m)

2549.03

δ34S (‰ V-CDT) (Anhydrite) 18.5

δ18O (‰ V-SMOW) (Anhydrite) 14.8

23.4

17.3

22.9

17.3

20.0

15.7

24.1

18.2

21.2

16.1

24.9

16.0

23.4

17.8

22.2

17.0

26.4

16.3

22.6

17.5

31.3

17.6

22.0

16.8

24.4

16.2

20.7

15.7

21.9

16.5

22.3

16.7

23.1

17.5

21.1

15.6

22.6

16.1

20.1

15.4

29.5

15.9

21.9

16.4

20.2

15.2

20.3

15.5

25.1

16.3

19.2

13.9

20.5

15.4

20.1

15.3

23.2

15.8

26.8

15.3

24.3

13.8

20.3

15.2

20.5

15.3

20.6

15.1

21.1

15.5

20.5

15.2

15

50

2868.72

2877.05

20.4

15.6

20.4

15.2

20.7

15.8

19.3

15.0

19.8

15.5

20.3

15.0

20.0

15.9

20.2

15.3

20.4

15.4

19.5

14.9

19.4

15.2

20.4

15.3

19.9

15.2

24.3

18.3

19.8

15.2

20.2

15.4

20.0

14.9

20.2

14.5

24.9

15.8

20.0

14.5

19.7

15.5

20.5

15.0

20.2

15.4

21.8

15.0

19.8

15.0

34.8

17.1

20.1

14.9

20.0

15.2

19.6

14.7

25.7

15.9

19.8

15.1

20.5

15.0

19.5

14.6

20.2

15.4

23.3

17.9

21.8

16.9

37.0

18.0

20.8

16.5

35.2

17.6

33.0

20.5

20.9

15.9

33.1

19.0

19.4

14.6

16

34

2413.58

19.5

14.7

19.6

15.4

19.3

14.8

20.6

16.1

18.6

16.3

20.4

16.1

20.4

16.4

20.6

16.1

20.6

15.7

20.9

16.1

21.1

16.7

19.6

15.0

19.9

15.5

19.8

15.5

27.1

19.0

26.9

18.8

25.9

15.8

26.0

16.6

24.9

18.6

26.0

19.1

25.6

19.4

25.3

19.3

25.1

19.5

24.5

18.8

25.5

18.6

25.3

18.9

25.6

17.5

25.1

20.7

25.6

22.1

25.9

19.0

25.8

18.8

26.1

18.1

24.8

17.9

26.8

18.4

25.7

19.7

25.5

19.2

24.8

18.9

26.0

19.3

26.3

20.8

26.2

20.8

24.5

18.6

25.6

20.5

25.1

19.3

17

2418.41

26.5

18.8

25.2

18.2

25.7

18.6

25.8

21.7

26.2

20.1

25.4

20.3

25.5

19.8

24.8

19.0

24.5

18.2

19.2

14.5

31.4

16.3

32.6

17.4

20.1

14.6

31.7

16.3

26.8

15.9

18.9

13.9

22.7

12.1

19.1

14.0

19.7

14.5

19.1

14.7

19.2

14.2

19.3

14.3

18.9

14.6

20.0

15.0

20.8

15.1

19.3

14.4

19.1

14.3

19.6

14.2

18

Well # (this study)

31

50

34

Depth (m)

2140.4

2868.72

2418.41

δ34S (‰ V-CDT) (Barite) 39.0

δ18O (‰ V-SMOW) (Barite) 18.2

33.6

18.6

28.8

15.5

25.8

18.1

25.5

18.3

29.9

13.6

24.2

17.3

24.1

17.6

23.3

13.5

23.7

13.2

23.5

13.3

23.7

16.8

24.0

17.7

24.1

16.9

26.8

17.7

26.4

17.8

26.4

17.9

26.4

17.3

26.4

17.6

26.0

17.4

26.1

17.1

26.7

17.4

26.4

17.6

26.3

17.6

26.7

18.0

26.5

17.6

27.3

18.2

27.2

17.7

27.9

18.7

19

The contribution of authors for this study is as follows: • Mastaneh H. Liseroudi (corresponding author) • Analysis and interpretation of data (Petrographic and SEM/EDXS observations, stable and strontium isotope geochemistry) • Drafting the article and revising it critically for important intellectual content • Omid H. Ardakani • The conception and design of the study • Assistance with the interpretation of data (stable and strontium isotope geochemistry) • Revising the manuscript critically for important intellectual content • Hamed Sanei • Providing funding for the study • Revising part of the manuscript critically for important intellectual content • Per K. Pedersen • Providing funding for the study • Revising the manuscript critically for important intellectual content • Richard Stern • Secondary Ion Mass Spectrometry (SIMS) measurement of the studied samples • Revising the manuscript critically for important intellectual content • James M. Wood • Providing the samples for this study • Revising the manuscript critically for important intellectual content

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: