Whole-Rock Geochemistry and Heavy Mineral Analysis As Petroleum Exploration Tools in the Bowser and Sustut Basins, British Columbia, Canada

BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 55, NO. 4 (DECEMBER, 2007), P. 320–336

Whole-rock geochemistry and heavy mineral analysis as petroleum exploration tools in the Bowser and Sustut basins, British Columbia, Canada

K.T. RATCLIFFE A.C. MORTON D.H. RITCEY1 C.A. EVENCHICK Chemostrat Inc. HM Research Associates Geological Survey of Canada Geological Survey of Canada 5850 San Felipe Woodhouse Eaves, Leics United 625 Robson Street 625 Robson Street Suite 500 Kingdom and Department of Vancouver, BC V6B 5J3 Vancouver, BC V6B 5J3 Houston, TX 77057 Geology and Petroleum Geology Canada Canada USA University of Aberdeen United Kingdom

ABSTRACT

Principles of chemostratigraphic characterization and correlation employing whole-rock inorganic chemical data and heavy mineral grain counts are applied to the frontier Bowser and Sustut basins. Methodologies commonly used with well samples in mature petroleum provinces can be applied to field samples, providing a vital and practical link between the earliest frontier investigations and more advanced hydrocarbon exploration. The major stratigraphic divisions of the basins, the Bowser Lake and Sustut groups, have markedly different indications of sedimentary provenance from heavy mineral analysis, and are readily differentiated geochemically. Variations in key elements are related directly to the provenance indications identified by heavy minerals. Lithofacies assemblages within the Bowser Lake Group also display contrasting provenance signatures. Sandstones and conglomerates from marine facies (Ritchie-Alger, Todagin, and Muskaboo Creek assemblages) are differentiated from those of the deltaic and nonmarine units (Eaglenest, Skelhorne, Groundhog-Gunanoot and Jenkins Creek assemblages)

by higher Fe2O3 and MgO contents, which may relate to increased glauconite contents in marine units. Sandstones and conglomerates from the deltaic to nonmarine units are separated with less certainty by heavy mineral contents and element concentrations or ratios.

RÉSUMÉ

Des principes de caractérisation chimiostratigraphique et de corrélation, utilisant des bases de données compositionnelles de roche totale et des comptes des grains de minéraux lourds, sont appliqués dans les régions pionnières des bassins de Bowser et de Sustut. Les méthodologies communément employées sur des échantillons de puits dans les provinces matures en pétrole, peuvent s’appliquer sur des échantillons prélevés sur le terrain, fournissant un lien vital et pratique entre les recherches pionnières les plus anciennes et les explorations d’hydrocarbures les plus avancées. Les divisions stratigraphiques majeures des bassins, ceux des groupes de Bowser Lake et de Sustut, présentent des indications nettement différentes de provenance sédimentaire à partir d’analyses de minéraux lourds, et se différencient aisément sur le plan géochimique. Des variations d’éléments clés sont reliées directement aux indications de provenance, identifiées par les minéraux lourds. Les assemblages de lithofaciès, à l’intérieur du groupe de Bowser Lake , montrent des signatures de provenance contrastée. Les grès et les conglomérats, provenant de faciès marins (assemblages de Ritchie-Alger, Todagin, et de Muskaboo Creek), se différencient de ceux des unités deltaïques et non marines (assemblages de Eaglenest, Skelhorne,

Groundhog-Gunanoot et de Jenkins Creek) par de hautes teneurs en Fe2O3 et en MgO, qui peuvent être liées à des augmentations de teneur en glauconite dans les unités marines. Les grès et les conglomérats, provenant des unités deltaïques à non marines, sont séparés avec moins de certitude par des teneurs en minéraux lourds et des concentrations ou des fractions d’éléments.

Traduction de Gabrielle Drivet

1 Now with Diamondex Resources Ltd., P.O. Box 11584, 1410 - 650 West Georgia Street, Vancouver, BC V6B 4N8 Canada

320 GEOCHEMISTRY AND HEAVY MINERAL ANALYSIS AS EXPLORATION TOOLS 321

INTRODUCTION provenance, and may permit geochemical modeling of aspects such as facies variations and clastic mineralogy. Chemical and CHEMOSTRATIGRAPHIC RATIONALE mineralogical compositions may also provide information The application of whole-rock geochemical data to solving about spatial or stratigraphic variation in potential reservoir correlation problems is commonly referred to as chemostrati- quality, which will aid in prioritizing regions or units in a fron- graphy, or chemical stratigraphy. Strictly, however, chemo- tier setting. stratigraphic characterization is the zonation of a sequence in Chemostratigraphy, as with many stratigraphic techniques, terms of its chemical characteristics, whereas chemostrati- is largely a subjective and interpretative exercise. Within any graphic correlation is the extension of this zonation from one sedimentary succession, the large number of variables that geographic location to another. Chemostratigraphy in this affect element concentrations means that not all samples will paper involves the characterization of strata using variations in fall within the “typical” range of compositions defined for that their major and trace element concentrations. The technique is unit. A successful chemostratigraphic study therefore requires a extensively used in the oil industry to define chemostrati- dataset sufficiently large to avoid undue influence of outliers. graphic correlation frameworks between well-bore sections Outliers may need to be disregarded for the overall characteri- (Ehrenberg and Siring, 1992; Racey et al., 1995; Preston et al., zation, but they cannot be ignored entirely and should be taken 1998; Pearce et al., 1999; Wray, 1999; Pearce et al., 2005a; as an indication of the limitations of the dataset. Alternatively, 2005b; Ratcliffe et al., 2004; Ratcliffe et al., 2006). Although they may be viewed as potential indicators of stratigraphic the technique is becoming increasingly accepted as a strati- issues not otherwise realized. graphic tool in the petroleum industry, most petroleum-related A definitive geochemical study of the Bowser and Sustut chemostratigraphic studies are in mature hydrocarbon basins is beyond the scope of this paper. Rather, the aims of this provinces with numerous well penetrations, rather than frontier work are to demonstrate the applicability of chemostratigraphy scenarios where well data are limited or non-existent. to a frontier setting, and to investigate some implications of the This paper applies chemostratigraphic methodologies to a chemical and mineralogical compositions. suite of field samples from frontier basins. The approach in mature petroleum settings is generally to use well cuttings that, GEOLOGICAL SETTING by their depth-related nature, carry an inherent stratigraphic The area covered in this paper is the northern two-thirds of the component (unless disrupted by faults and folds), that is absent Bowser and Sustut basins, in north-central British Columbia, for the geographically widespread samples in this study. Canada (Fig. 1). These are frontier petroleum exploration Additionally, chemostratigraphic correlation involves the basins where much new information pertinent to petroleum extension of a geochemical characterization from one geo- systems has recently been developed and released (e.g. graphic area to another (e.g. one well to another), which is not Evenchick et al., 2005a, and references therein). the case with field samples. Rather, in the situation of field- The Bowser and Sustut basins together occupy an area of based samples, the stratigraphic assignment of each field sam- more than 62,000 square kilometres in the intermontane mor- ple is used to build a chemostratigraphic characterization for phogeological belt of the Canadian Cordillera (Fig. 1). The each map unit. Using the methods and general approach Bowser Basin is the depocentre of the Bowser Lake Group, a employed by authors such as Pearce et al. (2005b) and Ratcliffe late Middle Jurassic to early Cretaceous clastic overlap assem- et al. (2006), the elemental data from field samples can be used blage with a total thickness probably in excess of 5000 m to develop a chemical “fingerprint” for each lithostratigraphic (Evenchick and Thorkelson, 2005). Clastic deposition was ini- unit. Once this characterization is accomplished, it can be used tiated by accretion of the Stikine Terrane (Stikinia) to North to assign any sample to one of the pre-defined units. America (Gabrielse, 1991; Ricketts et al., 1992). Strata were Classification of cuttings samples which, by their nature, lack deposited upon volcanic arc units of Stikinia, and clasts were many of the sedimentary features used to assign field samples substantially derived from the accretionary complex and to a unit, provides an important link between frontier investi- oceanic units of the Cache Creek terrane to the east. (Eisbacher, gations and more advanced petroleum basin exploration. Lack 1974a, 1981; Evenchick and Thorkelson, 2005, and references of definitive features in cuttings samples is exacerbated when therein). fixed cutter (or PDC) bits are used and the entire fabric of the Within the study area, the Bowser Lake Group has been sub- rock is destroyed. Use of these drill bits has no deleterious divided into seven lithofacies assemblages and one defined for- effect on the whole rock geochemistry. In general, the chemical mation (Evenchick and Thorkelson, 2005; Evenchick et al., “fingerprint” of a unit may be made up of the absolute concen- 2005b). Subdivisions are the Ritchie-Alger assemblage (sub- tration of particular elements, certain element ratios, or a dis- marine fan), Todagin assemblage (marine slope), Muskaboo tinctive trend through the stratigraphic thickness of the unit. Creek assemblage (marine shelf to shoreface), Eaglenest, In addition to characterization and classification of units, the Skelhorne, and Groundhog-Gunanoot assemblages (3 distinct large dataset routinely obtained for a chemostratigraphic appli- deltaic successions), Jenkins Creek assemblage (nonmarine), cation (47 element concentrations for each sample in this and the fluvial-alluvial Devils Claw Formation. Across the study) can provide important information about sediment northern two-thirds of the basin, the Bowser Lake Group is an 322 K.T. RATCLIFFE, A.C. MORTON, D.H. RITCEY and C.A. EVENCHICK overall regressive sequence with shallow marine and deltaic facies belts migrating to the south and southwest through time (Evenchick et al., 2001). The assemblages interfinger laterally, and are substantially overlapping in age (Fig. 2). In addition to these depositional patterns, the present outcrop distribution of these assemblages is further controlled by fold and thrust structures in the Skeena Fold Belt (e.g. Evenchick, 1991; Evenchick and Thorkelson, 2005). The mid- to Late Cretaceous Sustut Basin extends along much of the northeast side of the Bowser Basin. In part, this is a foreland basin, developed in response to fold and thrust deformation within the Bowser Basin (Evenchick, 1991). Fluvial, alluvial, and lacustrine units of the Sustut Group have a total thickness of at least 2000 m, and are divided into the lower Tango Creek Formation and the overlying, more con- glomeratic Brothers Peak Formation. On the southwest side of the Sustut Basin, strata unconformably overlie deformed Bowser Lake Group and Stikinia strata, and on the northeast side they directly overlie Stikinia. Early in its depositional history, the Sustut Group had eastern sources, including metamorphic and plutonic rocks of the Omineca Belt (Fig. 1), followed in time by a western source largely derived from the Bowser Lake Group (Eisbacher, 1974b). Distinct sources for the Bowser Lake and Sustut groups have been recognized from clast compositions and paleocurrents (Eisbacher, 1974a, b, 1981), and detrital zircon ages (McNicoll et al., 2005).

STUDY METHODS AND DATA SET

This study provides an initial application of chemostratigraphic Fig. 1. Location of the Bowser and Sustut basins within the tools to a large, relatively little-studied sedimentary basin. Canadian Cordillera, showing morphogeological belts and the general- Outcrop samples were selected from all stratigraphic units of ized distribution of Cache Creek and Stikine terranes. Modified from Wheeler and McFeely, 1991. the Bowser Lake and Sustut groups, with broad geographic coverage and representation from different grain sizes. The sample suite comprises 36 coarse siltstones to shales (considered together as siltstones/claystones), 71 sandstones (mainly fine to medium arenites), and 51 conglomerates (chert granule to chert pebble conglomerates). Analysed conglomerates generally have a sandy matrix, relatively few clasts greater than 5 mm in diameter, and mean grain size less than 1 mm (Reichenbach et al., 2006). Sample locations are shown on Figure 3, and the number and type of samples associated with each stratigraphic unit is summarized in Table 1. Few samples are tightly constrained in age, but available age information suggests the sample set covers much of the known age range of the units. However, Upper Jurassic strata cover a much larger portion of the Bowser Basin than Middle Jurassic strata (Evenchick et al., 2001). Therefore, sampling is biased toward the younger portions of units in the Bowser Lake Group. Whole-rock compositional data for ten major elements (expressed as oxides) and thirty-seven trace elements (including REEs) was acquired by ICP-OES (inductively-coupled plasma- optical emission spectrometry) and ICP-MS (inductively- coupled plasma - mass spectrometry). Sample preparation and analytical procedures are as outlined in Jarvis and Jarvis (1995) Fig. 2. Approximate ages and relationships of units in the Bowser and Pearce et al. (1999). Samples were prepared by crushing Lake and Sustut groups in the northern Bowser and Sustut basins. GEOCHEMISTRY AND HEAVY MINERAL ANALYSIS AS EXPLORATION TOOLS 323 approximately 250 g of rock, followed by alkali fusion of a identification and counting using a polarizing microscope. homogenized 0.25 g powdered subsample. Precision for major Mineral proportions were estimated by counting up to 200 non- element data is generally below 2%, whereas precision is below opaque detrital grains. approximately 3% for the high abundance trace element data Geochemical and heavy mineral datasets have been pub- determined by ICP-OES, and below approximately 5% for lished in Ritcey et al. (2005), and petrographic data and elemental data acquired by ICP-MS. Standard rock reference descriptions for the same sample suite are in Reichenbach et al. materials (SRM’s) were analyzed along with the field samples, (2006). and used to apply drift corrections. Heavy mineral analyses were carried out on a subset of SEDIMENT PROVENANCE thirty sandstone and sandy conglomerates from this suite. Samples were disaggregated without grinding, sieved, HEAVY MINERAL PROVENANCE INDICATORS cleaned, and dried. Heavy minerals were separated from the Heavy mineral analyses were carried out primarily to identify 63–125 micron fraction in bromoform with a measured specific contrasts or variations in heavy mineral suites that can be gravity of 2.8, and mounted under Canada Balsam for optical related to sediment provenance. Additionally, since many

Fig. 3. Location map for samples in this study. 324 K.T. RATCLIFFE, A.C. MORTON, D.H. RITCEY and C.A. EVENCHICK heavy minerals have distinctive concentrations of certain trace and within the Sustut and Bowser Lake groups. Although many elements, the heavy mineral data can be used to better under- of these variations suggest variations in sediment provenance, stand the factors controlling geochemical variations used for heavy mineral suites are not entirely controlled by source rock chemostratigraphic characterization and correlation. mineralogy. Other processes, principally weathering, hydrody- Heavy mineral abundances for all 30 samples in Figure 4 namics, and diagenesis, may overprint the original provenance show markedly different mineral assemblages, both between signal (Morton and Hallsworth, 1999). Such effects can be

Table 1. Distribution of samples by stratigraphic unit and lithology.

Fig. 4. Heavy mineral grain counts, normalized to 100% total abundance. GEOCHEMISTRY AND HEAVY MINERAL ANALYSIS AS EXPLORATION TOOLS 325 counteracted by examining ratios or indices of stable minerals chrome spinel plus zircon, and CGi as % chrome spinel in total with similar densities, because these indices are not affected by garnet plus chrome spinel (after Morton and Hallsworth, 1994). changes in hydraulic conditions during sedimentation or by Sustut Group samples commonly contain abundant epidote diagenesis (Morton and Hallsworth, 1994). Table 2 contains and almost ubiquitously lack chrome spinel (Fig. 4). Although calculated indices for provenance-sensitive heavy minerals. epidote is abundant in the Sustut Group overall, it is relatively The index ATi is defined as % apatite in total apatite plus tour- unstable during burial diagenesis (Morton and Hallsworth, maline, GZi as % garnet in total garnet plus zircon, RZi as % 1999), and is therefore not used as a provenance indicator.

TiO2 minerals in total TiO2 minerals plus zircon, RuZi as % Sustut Group samples show substantial variation in garnet:zir- rutile in total rutile plus zircon, CZi as % chrome spinel in total con (GZi) and rutile:zircon (RuZi) (Table 2) resulting from the

Table 2. Provenance-sensitive heavy mineral indices, determined on the 63–125 micron fraction of sandstone samples. ATi = % apatite in total apatite plus tourmaline, GZi = % garnet in total garnet

plus zircon, RZi = % TiO2 minerals in total TiO2 minerals plus zircon, RuZi = % rutile in total rutile plus zircon, CZi = % chrome spinel in total chrome spinel plus zircon, CGi = % chrome spinel in total garnet plus chrome spinel (after Morton and Hallsworth, 1994). Blanks signify lack of accurate data due to poor recovery of relevant heavy minerals. 326 K.T. RATCLIFFE, A.C. MORTON, D.H. RITCEY and C.A. EVENCHICK interplay of two end-member provenances, one characterized acid igneous provenance. On the CZi vs. CGi plot (Fig. 5), by high GZi and high RuZi, the other by low GZi and low Jenkins Creek samples plot with high CGi, but low to moder- RuZi. The high GZi, high RuZi component (Table 2) is proba- ate CZi indices supporting the suggestion of an acid igneous bly indicative of a metasedimentary source, which could also sediment source, with minor supply from mafic-ultramafic have supplied the abundant epidote. The low GZi, low RuZi rocks. Samples from the Groundhog-Gunanoot assemblage end member is probably an acid igneous component, likely contain a mixture of chrome spinel, zircon and apatite, sug- including allanite-bearing granitoids in part. The variations in gesting mixed provenance from mafic-ultramafic and acid ATi may indicate variable involvement of a tourmaline-rich igneous sources. This is supported by their position relative to source, probably within the Omineca Belt. other BLG samples on Figure 5. Most samples from the Skelhorne, Eaglenest, Muskaboo Creek, Todagin, and Ritchie-Alger assemblages are substantially dominated by chrome spinel, suggesting a strong mafic- PROVENANCE INDICATIONS FROM ultramafic component. The high chrome spinel:zircon ratios WHOLE ROCK GEOCHEMISTRY (CZi) and chrome spinel:garnet ratios (CGi) of these samples Concentration of each element in the full analytical suite is plot as a tight cluster on Figure 5. Pyroxenes are expected from influenced by the distribution of minerals, which in turn reflects an ultramafic source, but are probably lacking due to their low changes in a variety of geological conditions, such as sediment stability during burial diagenesis (Morton and Hallsworth, provenance, facies, syndepositional weathering, basin redox 1999). Samples from the Skelhorne assemblage exhibit a range conditions, and diagenesis. A complete chemostratigraphic of heavy mineral indices similar to most other BLG sandstones framework identifies geological controls on the key elements and conglomerates, but they have notably lower total counts of that are employed. Establishing the links between sediment geo- heavy minerals (data in Reichenbach et al., 2006). chemistry and mineralogy can only be fully achieved when The Jenkins Creek assemblage is dominated by zircon and X-ray diffraction, thin section petrography, and scanning apatite, with some chrome spinel, indicative of a predominantly electron microscopy studies are carried out on each sample geochemically analyzed. In practice, this type of detailed mineralogical analysis is seldom carried out in an oil field setting, particularly when dealing with cuttings samples, which means that the mineralogical affinities of elements can only be generalized. This approach is adopted herein. There are marked variations in the heavy mineral species present in sandstones and conglomerates of the various stratigraphic units, and many of these heavy minerals have distinctive major and trace element compositions that exert strong influences on the whole rock geochemistry. Profiles of selected trace element concentrations and ratios in Figure 6 (akin to profiles commonly presented for wellbore studies) display geochemical variations between and within the Sustut and Bowser Lake groups. Zr and Ce are markedly enriched in the Sustut Group relative to the Bowser Lake Group and Cr and Ni are both relatively depleted. Comparison of element concentrations to heavy mineral distribution shows that broadly Zr values are high where zir-

con (ZrSiO4) is abundant. However, discrepancies exist between the detailed distribution of Zr and zircon. For example, the Ritchie-Alger Assemblage has higher Zr contents than the Todagin and Muskaboo Creek assemblages, yet has the lowest zircon contents. This discrepancy probably reflects the size of the zircons. Heavy mineral analysis for this study was carried out on the 63–125 micron size fraction, yet as discussed below, it is known that airfall zircons are present in parts of the Bowser Lake Group. These airfall zircons are likely to be accounted for in the whole rock geochemistry, but not the heavy mineral analysis, resulting in apparent discrepancies between the two data sets. The mineral association of high Ce in the Sustut Group is more enigmatic, but the heavy mineral allanite has a general formula Fig. 5. Key heavy mineral indices (CGI vs. CZi) plotted as a binary 2+ 3+ diagram to demonstrate provenance variations in Bowser Lake and of (Ca, REE, Th)2(Fe , Fe Al)3Si3O12OH and is a probable Sustut groups. Double-headed arrows indicate possible mixing lines source of the high Ce values within the Sustut Group. Cr values between provenance types. GEOCHEMISTRY AND HEAVY MINERAL ANALYSIS AS EXPLORATION TOOLS 327

2+ 3+ Heavy mineral analysis is expected to be a more sensitive are high where Cr-spinel [(Mg Fe )O(CrAl Fe )2O3] is abundant, implying this mineral exerts a strong influence on the indicator of sediment provenance than whole rock geochem- concentrations of this element. Ni is a common substitution in istry, due to the multiple and complex mineralogical controls Cr-spinel, potentially accounting for its high concentrations in on major trace element distributions, but it is ordinarily carried the Bowser Lake Group sediments. The high Ni may also be res- out only on coarse grained samples. Once the geochemical ident in chlorite, the product of pyroxene alteration. In either modeling of provenance has been calibrated with heavy min- interpretation, the high Ni values indicate a mafic-ultramafic eral data, it may be possible to extend the interpretations to sediment provenance. For the sandstones and sandy conglomer- finer-grained lithologies. This can be vitally important since ates, simple comparisons of geochemical composition and heavy fine-grained rocks commonly constitute a high proportion of mineral distribution identify the Zr/Cr and Ce/Cr ratios displayed units encountered in many basins. Profiles of provenance- on Figure 6 as provenance indicators, such that high values of the sensitive elements (as determined by calibrating sandstone ratios indicate acid igneous provenance and low values a strong whole rock data with heavy mineral data) are shown in Figure 7 mafic influence on the sediment provenance. for siltstone/claystone samples. There is a strong influence of

Fig. 6. Comparisons of selected trace element distributions and heavy mineral abundances for sandstones. Vertical position on profiles does not imply relative age of samples. Pie charts for the heavy mineral data are averages calculated for each assemblage. 328 K.T. RATCLIFFE, A.C. MORTON, D.H. RITCEY and C.A. EVENCHICK provenance, probably in the form of silt-sized heavy mineral values due to calcite cements within an otherwise uncemented grains, specifically the relative significance of acid igneous and sandstone is a valid stratigraphic characterization, but offers mafic igneous sources. Such an approach must be tempered by little potential for stratigraphic correlation. The choice of the realization that certain provenance indicators for sand- which elements to use for chemostratigraphic characterization stones, e.g. Cr and Ni, are chemical constituents of some clay is initially an entirely pragmatic one. Elements that display minerals, and cannot be used to recognize provenance signa- systematic variations between stratigraphic units are selected tures for true claystones. However, in practice, true claystones as potential key chemostratigraphic elements and ratios with zero silt content are uncommon, and the profiles in between these elements calculated to enhance the often subtle Figure 7 closely reflect those of Figure 6. A similar close rela- variations in absolute concentrations. Statistical techniques tionship between heavy mineral-related elements in sandstones such as multivariate analysis and principal component analy- and claystones was also noted by Ratcliffe et al. (2006), further sis are commonly employed to identify elements that can supporting the generalization that provenance indicators can be serve as discriminators. modeled from typical fine grained lithologies. In well-bore sections, systematic trends in element concentrations are generally readily identified, since in structurally simple sequences, changes in element concentrations with CHEMOSTRATIGRAPHIC CHARACTERIZATION depth are directly related to stratigraphy. For field samples A strict definition of a chemostratigraphic characterization is from widespread geographic locations, it is imperative that a “the definition of stratigraphic units based on changes in stratigraphic framework is in place prior to attempting chemical composition”. However, in a practical sense, it is chemostratigraphic characterization. Once the key chemostrati- important to develop characterizations that can be used for graphic elements are identified, element ratios are devised that stratigraphic correlation. A stratigraphic characterization emphasize more subtle variations between the stratigraphic made, for instance, on the definition of a unit with high CaO units, enabling chemostratigraphic characterizations that have

Fig. 7. Provenance-sensitive elements in siltstone/claystone samples. Vertical position on profiles does not imply relative age of samples. GEOCHEMISTRY AND HEAVY MINERAL ANALYSIS AS EXPLORATION TOOLS 329 potential as correlation tools. Only then is consideration given to the likely controls on these key elements and ratios. The primary control on element distributions in any sequence is lithology. Each of the major lithological groups (siltstones/claystones, sandstones, and conglomerates) is expected to have its own geochemical characteristics, such that compositional contrasts between claystones and sandstones within a single unit may be far greater than the variations between units for one lithology (Ratcliffe et al., 2006). Siltstones/claystones, sandstones and conglomerates of the Sustut Group are all geochemically distinct from their counter- parts in the Bowser Lake Group. This is evident from the chemical logs displayed on Figures 6 and 7 and can also be expressed with binary and ternary diagrams (Fig. 8). Once dis- criminatory diagrams have been erected, they can be used as classification tools to characterize samples of unknown stratigraphic affinity, including well cuttings samples. Variables used here to differentiate the Sustut and Bowser Lake groups are Zr, Cr and Ce, elements whose distribution has been shown to be related to changes in sediment provenance in the discussions above. Key geochemical features and potential geological interpretations are summarized in Table 3. Sandstones and conglomerates from the Bowser Lake Group have geochemical characteristics that can be related to the different lithostratigraphic assemblages. There are insufficient siltstone/claystone samples to make meaningful chemostratigraphic characterizations for these finer grained lithologies. The primary geochemical characterization of the Bowser Lake Group samples is related to changes in the depositional environments of the assemblages. Sandstones and conglomerates from the marine units (Ritchie-Alger, Todagin and Muskaboo Creek assemblages) can be readily differentiated from the deltaic to nonmarine units (Eaglenest, Skelhorne, Groundhog-Gunanoot, Jenkins Creek assemblages and Devils Claw Formation) using binary and ternary diagrams (Fig. 9). Marine assemblage sandstones and conglomerates have generally higher values of

Fe2O3 and MgO than the nonmarine samples. Although the mineralogical significance of this can only be surmised without detailed X-ray diffraction or petrographic data, it most probably relates to the amount of Fe-Mg minerals, which could be carbonates (e.g. siderite or ferroan dolomite) or Al silicates

(e.g. chlorite, mica or glauconite). The increase in Fe2O3 and MgO corresponds to a broad change from nonmarine to marine facies, which could see an increase in glauconite, pointing toward the high element values being related to high glauconite. However, influences of other minerals cannot be ruled out without extensive mineralogical data. Existing XRD and point-count data for these samples (Reichenbach et al., 2006) show the highest average chlorite and total clay contents are in the Ritchie-Alger assemblage, but the sparse dataset, which only sporadically reports glauconite, does not indicate a clear distinction between marine and nonmarine units in terms of any Fig. 8. Graphical plots differentiating the Bowser Lake Group from one Fe-Mg mineral. the Sustut Group. A) and B) sandstone and conglomerate data; C) silt- Sandstones and conglomerates from the deltaic and non- stone-claystone data. marine assemblages are characterized and differentiated in 330 K.T. RATCLIFFE, A.C. MORTON, D.H. RITCEY and C.A. EVENCHICK

Figures 10 and 11. Data for one sandstone and one conglom- overall linear relationship between Na2O and Al2O3, strongly erate from the fluvial-alluvial Devils Claw Formation are suggesting that Na2O is controlled by more than one mineral. included in this series of plots: in most cases the sample There are no evaporite sequences in these units, suggesting points lie close to those for the fully nonmarine Jenkins Creek that the Na2O/Al2O3 ratio is related to clay minerals and to assemblage. These two units are distinguished from the strati- plagioclase. A higher value of this ratio indicates higher rela- graphically lower assemblages by their high Zr and low Cr tive plagioclase content, thereby implying that the Eaglenest contents. The Groundhog-Gunanoot and Skelhorne deltaic assemblage has distinctly lower plagioclase contents than the assemblages are most clearly separated by the Zr vs. Cr other deltaic assemblages. binary plots, whereas the Eaglenest assemblage is chemically distinguished from other units by low Na O values for both 2 IMPLICATIONS FOR RESERVOIR QUALITY sandstones and conglomerates (Fig. 10). Variations in the Zr and Cr values likely reflect changes in sediment provenance, While the primary aim of chemostratigraphic studies in petro- albeit more subtle signatures than those which result in the leum provinces is to provide stratigraphic correlation schemes, strong geochemical contrasts between Bowser Lake and the whole rock geochemical data can also be used to make

Sustut groups. Controlling factors for Na2O values cannot be interpretations regarding reservoir quality. For this study, deal- unambiguously determined, however, Na2O is most com- ing with limited data in a frontier basin, only very broad gen- monly related to clay minerals, evaporites or feldspar (pla- eralizations about potential reservoir quality can be made. gioclase). On the binary diagrams of Figure 10, there is no Although these are generalizations, they do provide insights as

Table 3. Summary of geochemical and provenance indicators for sandstones and conglomerates from subunits of the Bowser Lake and Sustut groups. GEOCHEMISTRY AND HEAVY MINERAL ANALYSIS AS EXPLORATION TOOLS 331

Fig. 9. Binary and ternary diagrams used to show characterization and differentiation of marine from nonmarine units of the Bowser Lake Group. Upper two graphs are constructed from sandstone data, lower two graphs are constructed from conglomerate data. 332 K.T. RATCLIFFE, A.C. MORTON, D.H. RITCEY and C.A. EVENCHICK to where, within a large, poorly explored area, the probability less clay content than those deposited in marine environments. of good reservoir development is greatest. As such, the gross change in SiO2 : Al2O3 ratio can be used as The SiO2/Al2O3 ratio in a siliciclastic sediment is a measure a proxy for quartz vs. clay contents and therefore reservoir of the amount of quartz vs. Al-silicates. Generally, sandstones quality. Furthermore, high Zr concentrations in sandstones, if with high quartz contents are likely to have greater potential directly related to detrital zircon and not airfall zircon, suggest reservoir quality. Most sandstones of the Eaglenest, Skelhorne deposition in relatively high energy environments with miner- and Groundhog-Gunanoot assemblages have generally higher alogically and texturally mature sediments, i.e. more likely to

SiO2 and lower Al2O3 values than those from the marine be better reservoirs. Therefore, it can be tentatively suggested Muskaboo Creek, Todagin and Ritchie-Alger assemblages that of the Eaglenest, Skelhorne and Groundhog-Gunanoot (Fig. 12). Sandstones from the nonmarine Jenkins Creek assemblages, the Eaglenest assemblage, with its high Zr values, assemblage display a wide range of SiO2 : Al2O3 ratios. For the may have the best reservoir properties. entire Bowser Lake Group sandstone and conglomerate Sandstones and conglomerates from the Sustut Group have dataset, SiO2 and Al2O3 have a correlation coefficient of SiO2 / Al2O3 values that are generally higher than those from approximately minus 0.60. The most probable interpretation of the deltaic units of the Bowser Lake Group (Fig. 12). this overall trend is that the fluvial and deltaic sandstones have Additionally, using the Zr/Cr ratio values as a proxy for acid

Fig. 10. Binary diagrams constructed to characterize Eaglenest, Skelhorne Groundhog-Gunanoot, and Jenkins Creek assemblages, and Devils Claw Formation: A) using sandstone data; B) using conglomerate data. GEOCHEMISTRY AND HEAVY MINERAL ANALYSIS AS EXPLORATION TOOLS 333

igneous vs. mafic sedimentary provenance, the Bowser Lake DISCUSSION Group is indicated to have higher mafic mineral content relative to the Sustut Group. Generally, mafic minerals are delete- The Bowser and Sustut basins offer a frontier petroleum explo- rious to reservoir quality as they are relatively unstable, ration scenario, for which energy resource studies have identified degrading into clay minerals. Although these parameters do not significant petroleum potential (e.g. Evenchick et al., 2005a; take into account important variations in diagenetic history that Stasiuk et al., 2005; Osadetz et al., 2004), yet as of late 2006, obviously affect reservoir quality, they do suggest that in a only two conventional hydrocarbon exploration wells have been gross sense and on a basin-wide scale, the Sustut Group could drilled [Dome et al. a-3-J/104-A-6 British Columbia Ministry of be expected to have better overall reservoir potential than the Energy Mines and Petroleum (BCMEM) well File WA#2529, Bowser Lake Group and better reservoir quality may be and c-62-G/104-A-6 BCMEM well File WA#3215]. Chemo- expected in the nonmarine lithostratigraphic assemblages of stratigraphy has been applied to this basin scenario to test the Bowser Lake Group than in the marine units of that group. whether whole rock geochemical data can be used to characterize the main lithostratigraphic units, and to investigate provenance indications and potential influences on reservoir quality. Contrasts in whole rock composition and provenance-sensitive heavy mineral abundances and ratios identified in this study corroborate the findings of previous work that indicated distinct sediment sources for the Bowser Lake and Sustut groups. The two groups are readily differentiated geochemically, providing a first order chemostratigraphic characterization. In addition to the geochemical and mineralogical indicators presented here and the clast studies of previous workers (Eisbacher, 1974a, b; Evenchick and Thorkelson, 2005, and references therein), contrasting sources for the Bowser Lake and Sustut groups are also shown by detrital zircon geochronology, which identifies North American Precambrian contributions in the Sustut Group that are absent in Bowser Lake Group samples (McNicoll et al., 2005). Based largely on the predominance of chert grains in the Bowser Lake Group, Eisbacher (1974a; 1981) inferred sources in the Cache Creek terrane, and this source terrane was confirmed by radio- larian fossil ages from chert clasts in the Bowser Lake Group (Currie, 1984; Evenchick and Thorkelson, 2005, and references therein). In addition to abundant chert, the Cache Creek Terrane contains alpine ultramafic bodies, and Cookenboo (1993) determined an alpine peridotite source for detrital chrome spinel in the Bowser Lake Group from microprobe analyses of detrital chrome spinel. Eisbacher (1981), Green (1992) and Cookenboo (1993) inferred Mesozoic arc terranes as contributors of volcanic material noted in Bowser Lake Group clasts. The predominance of accreted oceanic and arc terranes as sediment sources for the Bowser Lake Group is also supported by neodymium isotopic compositions of sedimentary rocks from the northwestern Bowser Basin (Samson et al., 1989). In contrast to older units of the Bowser Lake Group, Cookenboo (1993) identified a metamorphic clastic component in the Devils Claw Formation. Lithostratigraphic assemblages within the Bowser Lake Group are diachronous interfingering units representing facies belts that occupied different positions in the basin over time. As such, the assemblages are expected to have similar sediment sources, yet some provenance contrasts are identified for the largely coeval Ritchie-Alger, Todagin, and Muskaboo Creek Fig. 11. Ternary diagrams constructed to characterize Eaglenest, Skelhorne, Groundhog-Gunanoot, and Jenkins Creek assemblages, assemblages. The observed distinctions suggest contributions and the Devils Claw Formation: A) using sandstone data; B) using con- from sources other than those identified by a simple model of glomerate data. Symbols are as in Figure 10. detrital transport and deposition from hinterland sources washed 334 K.T. RATCLIFFE, A.C. MORTON, D.H. RITCEY and C.A. EVENCHICK progressively through proximal (shelf and shoreface) to distal based on lithologies, small-scale sedimentary structures, con- (deeper marine) environments. Some of the contrast may be tained fossils, and macroscopic features such as nature and explained by the likelihood that most Todagin assemblage sam- scale of cyclicity. Most of these features will not be recogniza- ples are probably somewhat older than the majority of Ritchie- ble in cuttings samples, particularly if future exploration Alger and Muskaboo Creek samples. McNicoll et al. (2005) employs fixed cutter (or PDC) bits as is most often the case in proposed airfall deposition of volcanic material as a significant modern hydrocarbon wells. However, the whole rock geo- contributor to sedimentary sequences within the Bowser Basin, chemistry will be preserved in the cuttings samples and there- based on U-Pb geochronology of detrital zircons from siltstones fore, by applying the criteria discussed above, the nonmarine and sandstones having well-constrained paleontological ages. assemblages of the Bowser Lake Group should be geochemi- The U-Pb age of the dominant detrital zircon population is cally identified from cuttings samples. equivalent, within uncertainty, to the depositional age of the rocks (McNicoll et al., 2005), although volcanic flows and vol- CONCLUSIONS caniclastic rocks in the Bowser Basin are restricted to Early Oxfordian, and are present only in the south and southeast part Whole rock geochemistry and heavy mineral analysis are poten- of the basin. Contemporaneous volcanism, south, and possibly tially very effective tools in frontier basin exploration. An impor- west, of the basin depocentre, may have been a significant sed- tant general conclusion from this introductory study is that iment source that contributed an acid igneous provenance sig- mineralogical and geochemical variations do exist in the Bowser nature to portions of the Bowser Lake Group. and Sustut basins, allowing for the application of a chemostrati- In addition to the first-order differentiation of the Bowser graphic approach. The first-order characterization identified here Lake and Sustut groups, geochemical characterization of is the predominantly mafic provenance of the Bowser Lake marine vs. nonmarine assemblages of the Bower Lake Group is Group contrasted with the acid igneous and metasedimentary possible using relatively simple variables such as Fe2O3, MgO provenance of the Sustut Group. This contrast is evident from and SiO2/Al2O3. It is also possible to chemostratigraphically heavy minerals and several element ratios. Chemostratigraphic differentiate the deltaic lithostratigraphic assemblages characterization separates sandstones and conglomerates of the (Eaglenest, Skelhorne, Groundhog-Gunanoot) and the non Bowser Lake Group into marine and deltaic-nonmarine deposi- marine Jenkins Creek assemblage and Devils Claw Formation. tional settings, and further characterzation allows geochemical The differentiation of these deltaic and nonmarine units can be differentiation of each deltaic and nonmarine assemblage. With considered as a third order characterization and it is this differ- this type of characterization developed from field samples that entiation that has most potential application in future explo- have confidently been assigned to a lithostratigraphic unit, it ration. Differentiation of the lithostratigraphic assemblages is is possible to assign samples of unknown affinity, including

Fig. 12. SiO2 vs. Al2O3 binary diagram for all sandstone and conglomerate samples. Double-headed arrow indicates a silt- or clay-dilution trend. GEOCHEMISTRY AND HEAVY MINERAL ANALYSIS AS EXPLORATION TOOLS 335 cuttings samples from hydrocarbon wells, to a chemostrati- Evenchick, C.A. 1991. Geometry, evolution, and tectonic framework of graphic, and therefore lithostratigraphic, unit. the Skeena Fold Belt, north-central British Columbia. Tectonics, v. 10, Ratios of SiO to Al O and provenance indications for p. 527–546. 2 2 3 ______and Thorkelson, D.J. 2005. Geology of the Spatsizi River map area, sandstones and conglomerates indicate the Sustut Group is north-central British Columbia. Geological Survey of Canada, Bulletin expected to have generally better hydrocarbon reservoir poten- 577, 276 p. tial than the Bowser Lake Group. Limitations of the current ______, Ferri, F., Mustard, P.S., McMechan, M.E., Ritcey, D., McNicoll, dataset make this a very broad generalization, and geographic V.J., Osadetz, K.G., O’Sullivan, P.B., Stasiuk, L.D., Wilson, N.S.F., and stratigraphic variations in quality are extremely likely. Poulton, T.P., Lowe, C., Enkin, R.J., Waldron, J., Snyder, D.B., Turner, R.J.W., Nowlan, G. and Boddy, M. 2005a. Highlights of recent research in The provenance implications of this study are in general the Bowser and Sustut basins project, British Columbia. Geological Survey agreement with previous work. Heavy minerals and whole rock of Canada, Current Research 2005-A1, 11 p. geochemistry identify multiple sources for the Bowser Lake ______, Mustard, P.S., Woodsworth, G.J. and Ferri, F. 2005b. Compilation Group, and this information will be of considerable importance of geology of Bowser and Sustut basins draped on shaded relief map, north- to models and concepts of basin development. Therefore, central British Columbia. Geological Survey of Canada, Open File 4638, scale 1:500 000. acquiring geochemical and heavy mineral data at an early stage ______, Poulton, T.P., Tipper, H.W. and Braidek, I. 2001. Fossils and facies of basin investigation is highly desirable not only for the prag- of the northern two-thirds of the Bowser Basin, northern British Columbia; matic issues of unit identification and correlation, but also as Geological Survey of Canada, Open File 3956. tool for understanding large-scale basin processes. Gabrielse, H. 1991. Late Paleozoic and Mesozoic terrane interactions in north- central British Columbia. Canadian Journal of Earth Sciences, v. 28, p. 947–957. ACKNOWLEDGMENTS Green, G.M. 1992. Detailed sedimentology of the Bowser Lake Group, northern Bowser basin, north-central British Columbia. MSc. thesis, Carleton This study was initiated by a request from EnCana Corporation University, Ottawa, Ontario, 197 p. to the Geological Survey of Canada to provide a suite of field Jarvis, I. and Jarvis, K. E. 1995. Plasma spectrometry in earth sciences: tech- samples for study. The support of EnCana Corporation in pro- niques, applications and future trends. In: Plasma Spectrometry in Earth Sciences. I. Jarvis. and K.E. Jarvis (eds.). Chemical Geology, v. 95, p. 1–33. viding geochemical and mineralogical data for public release is McNicoll, V.J., Evenchick, C.A. and Mustard, P.S. 2005. Provenance studies gratefully acknowledged by the authors. on the depositional histories of the Bowser and Sustut basins, and their Dr. Brian Zaitlin, formerly of EnCana Corporation, was a implications for tectonic evolution of the northern Canadian Cordillera. major driving force behind the project, and provided many GAC-MAC-CSPG-CSSS Joint Meeting, Halifax, Nova Scotia, Abstracts insightful comments to early discussions leading to the prepa- Volume 30, p. 133. Morton, A. C. and Hallsworth, C.R. 1994. Identifying provenance-specific fea- ration of this paper. Dr. Tim Pearce of Chemostrat Ltd also sup- tures of detrital heavy mineral assemblages in sandstones. Sedimentary plied helpful input into the chemostratigraphic discussions Geology, v. 90, p. 241–256. throughout preparation of the manuscript. The acquisition and ______and ______1999. 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