Isotopes (¹³C and ¹⁸O) Geochemistry of Lower Triassic Montney Formation, Northeastern British Columbia, Western Canada

Edwin I. Egbobawaye

Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, Canada.

DOI: 10.4236/ns.2017.910034                

Abstract

Oxygen isotope (δ¹⁸O) serves as paleothermometer, and provides paleotemperature for carbonates. δ¹⁸O signature was used to estimate the temperature of fractionation of dolomite and calcite in Montney Formation, empirically calculated to have precipitated, between approximately 13°C to ±33°C during Triassic time in northeastern British Columbia, Western Canada Sedimentary Basin (WCSB). Measurements of stable isotopes (δ¹³C and δ¹⁸O) fractionation, supported by quantitative X-ray diffraction evidence, and whole-rock geochemical characterization of the Triassic Montney Formation indicates the presence of calcite, dolomite, magnesium, carbon and other elements. Results from isotopic signature obtained from bulk calcite and bulk dolomite from this study indicates depleted δ¹³C_PDB (-2.18‰ to -8.46‰) and depleted δ¹⁸O_PDB (-3.54‰ to -16.15‰), which is interpreted in relation to oxidation of organic matter during diagenesis. Diagenetic modification of dolomitized very fine-grained, silty-sandstone of the Montney Formation may have occurred in stages of progressive oxidation and reduction reactions involving chemical elements such as Fe, which manifest in mineral form as pyrite, particularly, during early burial diagenesis. Such mineralogical changes evident in this study from petrography and SEM, includes cementation, authigenic quartz overgrowth and mineral replacement involving calcite and dolomite, which are typical of diagenesis. High concentration of chemical elements in the Montney Formation -Ca and Mg indicates dolomitization. It is interpreted herein, that calcite may have been precipitated into the interstitial pore space of the intergranular matrix of very fine-grained silty-sandstone of the Montney Formation as cement by a complex mechanism resulting in the interlocking of grains.

Keywords

Isotopes, Stable Isotopes, ¹³C and ¹⁸O, Isotope Geochemistry, Montney Formation, Geochemistry, Chemical Element, Mineralogy, Tight Gas Reservoir, British Columbia, Western Canada Sedimentary Basin (WCSB), Triassic, Subsurface Geology

1. Introduction

The application of stable isotope geochemistry in the study of sedimentary rocks has increasingly become an integral part of sedimentary geology. In particular, isotopic composition of sediments is important in interpreting diagenesis resulting from dolomitazation, and differentiating sources of organic matter [ 1 - 3 ], classification of kerogen types [ 4 ], and correlating crude oils with source rocks [ 5 - 7 ]. [ 8 ] first originated and formulated the idea of using thermometer to measure the variations with temperature of fractionation factors in isotopic exchange equilibria, particularly, in relation to the oxygen isotopes in the system. Subsequently, [ 9 ] showcased that oxygen isotope (δ¹⁸O) in sedimentary carbonates can serve as a paleothermometer, and can be used to estimate the temperature at which carbonate was formed. The concept of using oxygen isotope as paleothermometer was developed on the premise that calcium carbonates precipitated by organisms is in isotopic equilibrium with the seawater in which the organisms grow [ 8 - 10 ].

This study of the Montney Formation in Fort St. John area (T86N, R23W and T74N, R13W), northeastern British Columbia (Figure 1) utilized stable isotopes (¹³C and ¹⁸O), whole-rock geochemistry (Table 1) and mineralogical composition (Table 2) to interpret dolomitization of the Montney Formation because they are established methods for studying dolomitization and diagenesis in carbonates [ 11 , 12 ].

This research reported herein, primarily focus on using stable isotope (δ¹⁸O) to determine the paleotempeterature of precipitation of dolomite in the Triassic Montney Formation.

2. Geological Settings

The Montney Formation is the basal stratigraphic unit of Triassic succession in the subsurface of western Canada [ 12 - 14 ]. It rests, unconformably in most areas, upon carbonate or mixed siliciclastic-car- bonate strata of Carboniferous to Permian age [ 15 - 22 ]. The succession was deposited in a west-facing, arcuate extensional basin on the western margin of Pangaea [ 23 - 27 ]. The Montney Formation consists of siltstone, very fine-grained sandstone, bioclastic packstone/grainstone (coquina, in Alberta) [ 19 , 20 ], interlaminated, interbedded, dolomitic silty-sandstone [ 12 - 14 , 22 ] and shale. The Triassic Montney Formation is separated by an unconformity from the underlying Permian Belloy Formation (Figure 2).The unconformity along the Permian-Triassic boundary has been interpreted by [ 15 , 20 , 24 ] to be related to a global eustatic sea level fall. The global eustatic fall was related to the amalgamation of Pangaea Supercontinent, and was followed by a protracted Late Permian transgression that continued into the Triassic period [ 24 ]. The transgression was accompanied by anoxic conditions that induced profound environmental change [ 26 - 28 ], and may have severely increased levels of greenhouse gases [ 26 - 27 ]. These were the primary factors that contributed to the Late Permian-Triassic extinction crises, the largest extinction episode in geologic history [ 25 , 27 ].

The paleoclimate reconstruction suggests that the paleoclimate may have ranged from sub-tropical to temperate [ 23 - 28 ]. The region has been interpreted to be arid during the Triassic, and was dominated by winds from the west [ 18 , 28 ].

The WCSB forms a northeasterly tapering wedge of sedimentary rocks with thickness of more than 6000 meters, which extends southwest from the Canadian Shield into the Cordilleran foreland thrust belt [ 15 , 29 ]. The Cordilleran of the WCSB provides the evidence that the origin and development of the basin was associated with tectonic activity [ 15 , 30 ]. Later epeirogenic events resulted in subsidence that created the basin for sediment accumulation, which were attributed to the effects of contemporaneous episodes of orogenic deformation in the Cordillera [ 29 , 31 ]; this is interpreted to be post Triassic, especially due to mountain influences [ 15 ].

Table 1. Whole-rock geochemistry showing Montney Formation chemical element concentrations.

Table 2. Quantitative x-ray diffraction (XRD) analyses showing whole-rock mineralogy of the Montney Formation, British Columbia, Western Canada. Data source: B.C. Oil and Gas Commission.

Figure 1. Location map of study area showing Montney Formation isopach contour map in northeastern British Columbia, Western Canada Sedimentary Basin (WCSB).

3. Method of Study

The laboratory experiment and procedure for isotopic analyses in this study was performed in the isotope laboratory of Prof. Karlis Muehlenbachs in the Department of Earth and Atmospheric Sciences, University of Alberta (Figure 3). The extracted isotopic composition was analyzed using the Finningan-MAT 252 Mass Spectrometer at the University of Alberta.

Stable isotope analysis for calcite and dolomite (¹³C/¹²C and ¹⁸O/¹⁶O) involves arrays of mechanics, namely;

1) Samples were grinded into uniform grains (powder form) using the pulverizing shatter box-machine for homogeneity of samples in order to provide a uniform surface area for acid reaction with samples following the method of [ 32 ], and samples were allowed to dry in air;

2) Samples were measured ~40 - 50 mg per sample and 3 ml of anhydrous phosphoric acid (H₃PO₄) were measured into each glass reaction vessel and evacuated overnight on a vacuum line to remove atmospheric components (gas) from the samples;

3) The samples were reacted with anhydrous phosphoric acid (H₃PO₄) at 25˚C for one hour. The reaction is expressed chemically as:

${\text{3CaCO}}_{\text{3}}+{\text{2H}}_{\text{3}}{\text{PO}}_{\text{4}}\to {\text{3H}}_{\text{2}}\text{O}+{\text{Ca}}_{\text{3}}{\left({\text{PO}}_{\text{4}}\right)}_{\text{2}}$

4) Following the method of [ 33 ], CO₂ was evolved after one-hour time from the reaction of acid with

Figure 2. Type log of the Triassic Montney Formation [ 12 ], northeastern British Columbia, Canada.

Figure 3. (a) Exemplifies Montney Formation core where samples were obtained for isotopes (¹³C and ¹⁸O) analyses; (b) Shows research scientist at the University of Alberta’s Isotope Geochemistry Laboratory; loaded samples into glass reaction vessel to evacuate atmospheric components (gas) from the samples on a vacuum line; (c) Shows a magnified version of samples on a vacuum line from (b).

calcite in the sample;

5) This CO₂ was purified by distillation through a dry ice trap, condensed in a sample collection tube immersed in liquid nitrogen, and analyzed for calcite δ¹³C and δ¹⁸O values;

6) The CO₂ gas formed between the first and the fourth hour from the time of reaction was pumped out into a collection vessel as CO₂ for calcite to avoid contamination;

7) The vessel was then placed in a hot water bath at 25˚C and the reaction was left in that condition for 72 hours;

8) The CO₂ that formed during the remainder reaction was extracted in a similar process and analyzed for δ¹³C and δ¹⁸O of the dolomite component.

All analyses follow the standard method of [ 34 ]. The δ value is conventionally defined by [ 35 ], using the following expression:

$\delta =\left(\frac{R_{\text{sample}}}{R_{\text{standard}}}-1\right)\times 1000$ (1)

where R = ¹³C/¹²C or ¹⁸O/¹⁶O. The standard for carbonate is PDB [ 36 ], and that for water is SMOW [ 37 ]. The results derived from the analysis are shown in Table 3.

Table 3. Carbon and oxygen isotope data for bulk calcite cement and bulk dolomite analyzed from the Montney Formation, northeastern British Columbia, Canada.

4. Results from This Study

4.1. Stable Isotope Geochemistry-Description of Data

Carbon and oxygen isotopes (δ¹³C_PDB and δ¹⁸O_PDB) were analyzed in order to determine the bulk calcite and bulk dolomite (δ¹³C and δ¹⁸O) isotopic signature of the Montney Formation (Table 3). The data in Table 3 and Figure 4 show in general, depletion in isotopic composition (δ¹³C_PDB and δ¹⁸O_PDB) of both calcite and dolomite in the host lithology (very fine-grained silty-sandstone). The bulk calcite isotopic compositions of the Montney sediments range from (δ¹³C_PDB −2.8‰ to −6.10‰, and δ¹⁸O_PDB −3.66‰ to −16.15‰), and bulk dolomite isotopic composition values range from (δ¹³C_PDB ?2.71‰ to ?8.46‰, and δ¹⁸O_PDB −3.66‰ to −7.19‰) (Figure 4). High values (−13.50‰ to −16.15‰) occur in intervals that probably

(a) (b)

Figure 4. Isotopic data plot for the Montney Formation. (a) Bar graph showing calculated δ¹³C, δ¹⁸O, and SMOW; (b) Displays δ¹³C, δ¹⁸O (PDB) fields.

have higher organic matter content or the presence of hydrocarbon. [ 36 , 37 ] show that high negative δ¹³_PDB depleted values greater than (−10) are associated with methane gas in the Formation. Within the study area, the high methane (gas) is probably the cause of high negative isotopic values (−13.50‰ to −16.15‰) seen in some of the analyzed samples from Montney Formation. According to a recent report by [ 38 ], in British Columbia, the Montney Formation host substantial hydrocarbon reserve with estimated volume of natural gas (271TCF), Liquified Natural Gas (LNG = 12,647 million barrels) and oil reserve (29 million barrels).

4.2. The Calculation of the Temperature of Fractionation of Calcite Was Rendered Using the Equation below [ 35 ]

${10}^3\ln {\alpha }_{\text{Calcite-water}}=2.78\times {10}^6{T}^{-2}-2.89$ (2)

There is a relationship:

${10}^3\ln {\alpha }_{\text{Calcite-water}}\approx {\delta }^{18}{\text{O}}_{\text{calcite}}-{\delta }^{18}{\text{O}}_{\text{water}}$ (3)

Therefore:

${\delta }^{18}{\text{O}}_{\text{calcite}}-{\delta }^{18}{\text{O}}_{\text{water}}=2.78\times {10}^6{T}^{-2}-2.89$ (4)

Using the δ¹⁸O_PDB range of bulk calcite (−2 to −7.19) assuming the δ¹⁸O_SMOW value of pore water is between −2‰ and −7‰ (based on laboratory experiment from this study, which shows depleted δ¹⁸O values; see Table 3).

The resultant expression by substituting into Equation (4) gives the following:

$24.91{\left(\text{SMOW}\right)}_{\text{calcite}}-\left(-2\right){\delta }^{18}=2.78\times {10}^6{T}^{-2}-2.89$ (5)

On solving Equation (5), gives:

$T=305.43\text{K}$

The temperature in degrees Celsius (˚C) is 32.42˚C. Therefore, paleotemperature of precipitation of calcite during the Lower Triassic Period in the study area is ~32˚C.

4.3. For the Calculation of the Temperature of Fractionation of Dolomite, the Equation below Is Used [ 39 ]

${10}^3\ln {\alpha }_{\text{Calcite-water}}=3.2\times {10}^6{T}^{-2}-3.3$ (6)

Therefore:

${\delta }^{18}\text{O}{\left(\text{SMOW}\right)}_{\text{dolomite}}-{\delta }^{18}{\text{O}}_{\text{water}}=3.2\times {10}^6{T}^{-2}-3.3$ (7)

Using the δ¹⁸O PDB range of bulk dolomite (−5 to −6.79) assuming the δ¹⁸O (SMOW) value of pore water is between −5‰ and −7‰ (based on laboratory experiment which show a depleted δ¹⁸O values).

By substituting into Equation (7), Equation (8) below is obtained:

$23.99\left(\text{SMOW}\right)-\left(-6.79\right){\delta }^{18}{\text{O}}_{\text{water}}=3.2\times {10}^6{T}^{-2}-3.3$ (8)

The value of temperature $T$ , which satisfies Equation (8) is: 306.4 K (33.3˚C). Therefore, $T={33.3}^{\circ }\text{C}$ is the paleotemperature of precipitation of dolomite during the Lower Triassic Period in the present study. However, one of the samples analyzed in this study show paleotemperature of 13˚C.

4.4. Interpretation of Isotopic Signature

The values of the result from δ¹³C_PDB bulk calcite (Table 3) show depletion in the isotopic composition (δ¹³C_PDB −2.1‰ to −8.46‰). The negative δ¹³C_PDB values are indicative of pore-water derived from seawater and dissolution of metastable carbonate in conjunction with organic matter decomposition by bacteria in sulfate reducing environment. The total organic carbon (TOC) of the Montney Formation is a result of high nutrient rich sediment source, rapid sedimentation, and preservation of organic matter [ 40 ] in oxygen-depleted, anoxic depositional environment [ 41 - 44 ]. This phenomenon explains the biasing of carbon isotope towards a very low (negative δ¹³C_PDB) discerned from the Montney Formation. The anoxic condition generate high alkalinity, which increases the total dissolved carbon that causes calcite to precipitate from pore-water, thereby biased towards light δ¹³C_PDB values during early diagenesis [ 35 ].

The depleted δ¹⁸O_PDB of bulk calcite values range from (δ¹⁸O_PDB −3.54‰ to −16.15‰) and the δ¹⁸O_SMOW range between 22.48‰ and 27.42‰ (Table 3, which is within the fresh water range) reported by [ 44 ] as indication of mixing of marine pore-water and meteoric groundwater during authigenic calcite precipitation. Applying the δ¹⁸O_PDB range of the bulk calcite (δ¹⁸O_PDB −3.54‰ to −16.15‰), the calcite fractionation equation ${10}^3\ln {\alpha }_{\text{calcite-water}}=2.78\times {10}^6{T}^{-2}-2.89$ [ 35 , 44 ] and assuming that the δ¹⁸O_SMOW values of pore-water is between −2 to −7.19, the paleotempertaure under which the calcite have precipitated is interpreted to have occurred between approximately 13˚C to ±33˚C (from analyzed samples; see calculation above). This interpretation is consistent with a warm paleotemperature reported for the Triassic period of western Canada [ 23 , 45 ].

The bulk dolomite isotopic values (δ¹³C_PDB −2.71‰ to −8.46‰) provide information on the origin and the precipitation of the dolomite. The very low (negative) values of δ¹³C_PDB from the Montney sediment indicate depleted δ¹³C_PDB (−2.71 to −8.46). The interpretation for the negative values (light bulk-dolomite δ¹³C_PDB) indicates that biogenic CO₂ significantly contributed to the total dissolved inorganic carbon [ 46 ]. The evidence of biogenic CO₂ contribution from isotopic signature is supported by the total organic carbon (TOC) content based on source-rock kerogen (Table 2) from the Montney Formation. The organic carbon content coupled with the depleted bulk dolomite (δ¹³C_PDB −3.66‰ to −16.15‰) of the Montney Formation jointly indicate an anoxic, extremely poor oxidation environment where anaerobic sulfate reduction characteristic of early stage zone of methanogenesis occurs. The δ¹³C_PDB has been used as a proxy of upwelling intensity because upwelling waters are ¹³C depleted [ 11 ]. [ 47 ] assert that the upwelling isotopic effect might be compensated by the effect of planktonic blooms induced by the nutrient enrichment of upwelled water. The present of apatite mineral (Table 2) in the Montney Formation indicates upwelling of nutrient rich waters, which implies the presence of dissolved carbon in anaerobic condition.

The biasing towards light (warm) bulk dolomite δ¹⁸O_PDB results show depleted in isotopic composition (δ¹⁸O_PDB −2.71‰ to −8.46‰) indicates the present of meteoric water in the pore-water during precipitation of dolomite (Muehlembachs, 2011, personal communication). Further interpretation for the light, depleted isotopic values (δ¹⁸O_PDB −2.71‰ to −8.46‰) suggest the formation within, or modification by meteoric water, or under elevated temperatures [ 38 ]. Applying the bulk calcite dolomite values of δ¹⁸O_PDB −5.51‰ to −6.97‰, the dolomite-water fractionation equation 10³lnα_{dolomite-water} = 3.2 × 10⁶ T⁻² − 3.3 [ 38 ], assuming the pore-water δ¹⁸O_SMOW of −5.51‰ to −6.97‰, the dolomite was precipitated in temperatures ~33˚C. According to [ 48 ], oxygen isotopic analysis of marine carbonates gives at best, the estimate of temperatures at which the carbonate was deposited. The paleotemperature suggests that the Montney Formation has only encounter eodiagenetic realm of diagenetic stage.

On a geological time frame, the oxygen isotope composition of seawater is controlled by exchange of oxygen with silicate rocks [ 11 , 49 - 51 ]. Unaltered silicate rocks are enriched in δ¹⁸O relative to seawater by ~5.7‰ [ 11 ]. The isotopic composition of seawater is controlled by kinetic steady-states, reflecting major influxes of continental input, principally, the dissolved load of rivers; oceanic crust/seawater exchange at mid-oceanic ridges [ 52 ] and removal of chemical species via sedimentation [ 53 ].

High temperature interactions between seawater and rocks that occur during hydrothermal circulation are at axial mid-ocean ridges drive the isotopic composition of seawater towards increasing δ¹⁸O that of the rock [ 50 ]. Low temperature interactions such as those that occur in off axis vent systems and during continental weathering, drive seawater isotopic composition towards low δ¹⁸O [ 50 ], perhaps, such phenomenon may have implications for the low δ¹⁸O values found in the Montney Formation.

5. Discussions

5.1. Carbon and Oxygen Isotope Geochemistry

Important observation from isotopic studies of the Montney Formation sediments (very fine-grained, dolomitic silty-shale) shows that paleotemperatures of dolomitization is ~±33˚C. Such low temperature is characteristic of shallow burial, thus, suggests that the Montney Formation has only undergone eogenetic (early) phase of diagenesis and diagenetic process. The drive for warm temperature during the Triassic period have been reported to be associated with increase in CO₂ content in the atmosphere, which led to a global temperature increase that heralded most of the Permian-Triassic period [ 54 ].

The extraction of CO₂ during isotopic lab analyses for bulk calcite and bulk dolomite reveal that although the sediments are believed to be dolomitized, some of the analyzed samples show no evidence of calcite and in some cases, dolomite where not found (and therefore, not extracted). This observation was confirmed by utilizing X-ray diffraction (XRD) analyses, which explicitly confirms that dolomite and calcite where both found in most samples, but, in other samples, calcite was not present with dolomite and verse versa. Interpretation for this important discovery in the Montney Formation samples can be surmise in relation to the mode of substitution of Mg and Ca in carbonate rocks.

Thermodynamically, dolomite is stable in most natural solutions at earth surface conditions, and a thermodynamic drive exists for the conversion of calcite to dolomite [ 55 ]. Characteristically, most natural dolomite exhibits some degree of mixing of calcium and magnesium between cation layers [ 55 , 56 ]. The phenomenon of the present/absent of calcite/dolomite in the Montney Formation proves that dolomites commonly depart from stoichiometric composition of an excess of calcium, which is accommodated in the magnesium layers [ 56 , 57 , 59 ]. Other explanations for the absent of calcite in some of the Montney Formation samples may be due to the fact that several cations, principally, Fe, Sr, Na, and Mn, substitute for calcite in many dolomites [ 58 ]. Significant amount of Fe, Sr, Na, and Mn in this study (Figure 5) support such possibility of substitution for calcite [ 58 ].

Overall, stable isotope geochemistry and results of this study leads to two important conclusions: 1) isotopic composition (δ¹³C and δ¹⁸O) of the Montney Formation in the study area serves as a paleothermometer, and was used to constrain the temperature at which the carbonate (dolomite and calcite) associated with the Montney Formation was formed at seawater temperature ±33˚C. Mineral precipitation at low temperatures are enriched in ¹⁸O while minerals formed at high temperatures show less ¹⁸O enrichment [ 11 ]; and 2) dolomitized Montney Formation has mainly undergone eogenetic stage of diagenesis. The δ¹⁸O SMOW_calcite values of (−3.66‰ to −16.15‰), and that of δ¹⁸O_dolomite range from −2.71‰ to −8.46‰ indicate some oxidation of organic matter during diagenesis. This interpretation conforms with

(a) (b) (c) (d)
(e) (f)

Figure 5. Shows the variation pattern of concentration of trace elements. (a) Illustrates the composition of dolomite, in which there is higher concentration of magnesium (Mg) relative to the calcite (Ca) component. The graph pattern shows good correlation between Mg and Ca; (b) Shows major elements, potassium (K) and aluminum (Al). The concentration of K is very high because of the clay mineral and the organic richness of the Montney Formation sediments. The Al concentration is related to the clay mineralogy and partly has affinity to organic matter. The graph pattern of K and Al correlates very well; (c) Shows alkaline earth metals―Strontium (Sr) and Rubidium (Rb) concentration; (d) Illustrates the concentration of Iron (Fe) and Manganese (Mn). Fe and Mn are both related to diagenesis. The concentration of Fe is very high compared to the concentration of Mn. This indicates that Fe has more dominating diagenetic influence in the Montney Formation. Evidence from thin-section petrography (Figure 6(d)) shows replacement of organic matter by pyrite; (e) Shows the relationship between radioactive elements―Uranium (U) and Thorium (Th). These elements are particularly related to the clay mineralogy and organic components of the Montney Formation sediments; (f) Shows the relationship between calcite (Ca) and Potassium (K). Evidently, Ca has enormously high concentration due to the stoichiometric co-existence with dolomite. Adapted from [ 12 ].

isotopic signature of depleted δ¹⁸O_SMOW of [ 35 ] with respect to calcite-water fractionation equation, and dolomite-water fractionation of [ 38 ].

5.2. Dolomitization of the Montney Formation

Dolomite is a rhombohedra carbonate with the ideal formula CaMg(CO₃)₂, in which calcium and magnesium occupy preferred sites [ 38 ]. [ 59 , 60 ] used hydrothermal experiments extrapolated to low temperatures to demonstrate that calcite and dolomite are essentially ideal in composition at 25˚C. Thus, any double carbonate crystal of Ca and Mg at 25˚C is not essentially pure dolomite, and is either metastable or unstable with respect to calcite [ 38 ]. This relationship is evident in XRD analysis (Table 2), which supports the co-existence of dolomite and calcite.

Isotopic signature obtained from bulk calcite and dolomite results from this study indicates depleted (δ¹³C_PDB −2.71‰ to −8.46‰) and (δ¹⁸O_PDB −2.71‰ to −8.46‰), which is interpreted in relation to the oxidation of organic matter during diagenesis. Diagenetic modification of the very fine-grained, silty-sandstone of the Montney Formation may have occurred in stages of progressive oxidation and reduction reactions involving chemical element such as Fe, which manifest in mineral form as pyrite, particularly, during early burial diagenesis [ 61 ], or late stage diagenesis [ 62 ]. Mineralogical changes in the form of cementation and mineral replacement involving calcite and dolomite are typical of diagenesis [ 61 ], and are evident in the Montney Formation based on petrographic study and SEM.

Oxidation and reduction reaction mechanisms explains the modification of sediments, shortly after burial, prior to lithification or compaction during which fluids are ejected into the depositional interface [ 63 , 64 ]. This phenomenon drives the oxidation and reduction processes involving Fe, sulfur, and carbon [ 61 ]. The significant amounts of organic matter (TOC) in the Montney Formation essentially make these elements principal reactants. The carbon compound of the organic matter content is the most rapidly oxidized and consequently contributing energy to drive the Fe into the ferrous state, thereby causing fixation of sulfur as pyrite [ 61 ]. Because of the present of organic matter in the Montney sediments, pyrite occurs as scattered “clots” in some of the samples examined. This occurrence is evident in thin-section petrography (Figure 6). Pyrite is related to post-depositional emplacement [ 65 ] caused by the dissolution of organic matter due to diagenesis.

High concentration of chemical elements in the Montney Formation, particularly, Ca and Mg indicate dolomitization. It is interpreted herein, that calcite may have been precipitated into the interstitial pore space of the intergranular matrix of the very fine-grained silty-sandstone of the Montney Formation as cement by a complex mechanism resulting in the interlocking of grains, welded together by calcite cement [ 66 ]. Evidence of grain interlocking is revealed by SEM image showing authigenic quartz overgrowth (Figure 6).

It is established through mineralogical composition in this study (Table 2) that the Montney Formation is quartz rich and contains clay minerals as well, including unstable mineral such as feldspar (Table 2). This sort of compositional mixture of quart, clay and feldspar minerals may have resulted in the decomposition of feldspar along the clay-quartz boundary due to processes involving hydrolysis in the course of diagenesis [ 61 ].

The depositional environment interpreted for the Montney Formation is proximal to distal offshore marine setting [ 14 , 22 ]. In the marine environment, several weathering and transformation are prevalent. As a result, there exist an exchange of cations, in which the positions of clay minerals are changed, thereby resulting in the substitution of Mg for Ca [ 39 , 40 , 60 ]. In the marine environment chlorite and illite minerals are formed by fixation of Mg and K in montmorillonite or degraded illite delivered into distal settings due to continental denudation resulting from fluvial processes [ 40 ]. The present of illite and palygorskite clay mineral (Figure 6) support the evidence of diagenesis in the Montney Formation formed from ionic solutions in extreme conditions of temperature and pressure [ 40 ]. Dolomite in the Montney Formation appears as detrital in thin-section petrography (Figure 7). Such allogenic (non in-situ) dolomite and calcite may have only played a role (minimal) in the Montney Formation diagenesis, compared to sediments that are mainly composed of biogenic carbonate formed completely from aragonite and calcite [ 40 ]. Thus, authigenic calcite may have played dominant role in the vast diagenetic phenomenon in the Montney Formation.

6. Conclusions

Results obtained from isotopes (¹³C and ¹⁸O) from this study indicate depleted δ¹³C_PDB (−2.71‰ to

Figure 6. Microphotographs showing SEM and thin-section petrography of the Montney Formation illustrating the mineralogical composition. (a) Dolomite and quartz matrix. The quartz exhibits conchoidal fracture; (b) Shows magnification of plate C. Authigenic quartz overgrowth is illustrated in plate-B as evidence of grain dissolution due to diagenesis, and subsequent mineral precipitation (calcite) in the form of calcite cement that welded the grains together; (c) Shows quartz and illite with the authigenic quartz overgrowth; (d) Illustrates pyritized, dolomitized, organic carbon rich siltstone. Pyrite replaces organic matter in plate-D during decomposition of organic matter due to diagenensis; (e) Illustration of a very well formed palygorskite clay; a form of clay mineral growth, formed from the transformation of dickite to illite, and further transformation of illite to palygorskite due to severe temperature and pressure in a diagenetic regime. Adapted from [ 12 ].

−8.46‰) and it is related to oxidation of organic matter during diagenesis. Diagenetic modification of the Montney Formation (very fine-grained, silty-sandstone) may have occurred in stages of progressive oxidation and reduction reactions involving chemical element such as Fe, which manifest in mineral form as pyrite, particularly, during early burial diagenesis, or late stage diagenesis [ 67 ]. Mineralogical changes in the form of cementation and mineral replacement involving calcite and dolomite are typical of diagenesis. Based on isotopic signature and paleotemperature calculations, the calcite and dolomite of the Montney Formation may have formed at temperatures ranging from ~13˚C to ±33˚C, which is consistent with a warm, arid paleoclimate reported for the Triassic time [ 23 , 30 ].

Figure 7. Photomicrographs showing dolomitic silty-sandstone. (a) Shows detrital dolomite resembles ripple; (b) shows detrital dolomite admixed siltstone [ 12 ]; (c) shows mica, quartz, dolomite, and detrital grains; red arrow = Quartz; yellow arrow = Dolomite; white arrow = Mica.

Acknowledgement

Profoundly, I extend my appreciation to Professor Karlis Muehlenbachs for his mentorship and several stimulating discussions on carbonates and isotope geochemistry during the time I spent in his isotope lab at the University of Alberta. A big thank-you to Professors Murray Gingras and J-P Zonneveld for their support during my studies at the University of Alberta. The University of Alberta’s Faculty of Graduate Studies and Research is greatly appreciated for awarding scholarship that supported this work. Laboratory Technician, Levner Olga is appreciated for all the support she offered during the lab work analyses. Geoscience B.C. is highly appreciated for awarding scholarship in unconventional oil and gas reservoir research that supported this work in British Columbia. My thanks goes to Dr. Lucky Anetor for reviewing this manuscript and for his timely feedback.

Conflicts of Interest

The authors declare no conflicts of interest.

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