New insights into Ordovician oil shales in Hudson Bay Basin - CiteSeerX

BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 56, NO. 4 (DECEMBER, 2008), P. 300–324

New insights into Ordovician oil shales in Hudson Bay Basin: their number, stratigraphic position, and petroleum potential SHUNXIN ZHANG Canada - Nunavut Geoscience Office PO Box 2319, 626 Tumit Plaza, Suite 202 Iqaluit, NU, Canada X0A 0H0 [email protected]

ABSTRACT The Upper Ordovician Bad Cache Rapids and Churchill River groups and Red Head Rapids Formation of the Hudson Bay Basin are 180–300 m thick. These units are dominated by carbonate, but contain thin shales informally named “Boas River shale” and “Sixteen Mile Brook shale”. The stratigraphy and hydrocarbon potential of the shale units are poorly understood. The organic-rich “Boas River shale” and “Sixteen Mile Brook shale” were discovered on Southampton Island about 40 year ago. However, there has been considerable debate surrounding these oil shales, which focused on fundamental issues such as: one, two or three oil shale intervals within the Ordovician sequence, their precise stratigraphic positions, their extension in the Hudson Bay offshore area, and their hydrocarbon potential. Field studies of Upper Ordovician and lowest Silurian strata on Southampton Island, with a focus on the new discovery of the three oil shale intervals in the Cape Donovan area of eastern Southampton Island, “Boas River shale” and “Sixteen Mile Brook shale”, unequivocally demonstrated: 1) the three oil shale intervals in the Cape Donovan area are within the lower Red Head Rapids Formation; and 2) the Cape Donovan lower oil shale interval can be correlated to the “Boas River shale”, and the Cape Donovan middle or upper oil shale interval to the “Sixteen Mile Brook shale”, respectively. Rock-Eval6 data from samples of the three oil shale intervals in the Cape Donovan sections lead to the recognition of Type I–Type II kerogen and much higher yield and TOC than previously reported. Nineteen samples from middle and upper oil shale intervals have average and maximum yields of 136.5 kg HC/tonne and 230 kg HC/tonne, and average and maximum TOC of 20% and 34%; 21 samples from lower oil shale interval have mean and highest yields of 58.5 kg HC/tonne and 112.5 kg HC/tonne, and mean and highest TOC of 9.8% and 17.3%. The three oil shale intervals are recognized in the Hudson Bay offshore area. This is supported by three lines of evidence in the lower Red Head Rapids Formation in the Hudson Bay offshore exploration wells: 1) three prominent positive gamma ray kicks; 2) possible organic-rich fragments found in well cuttings; 3) some reasonably high TOC values (2.29–5.73%) are obtained from several samples within the interval covering the three positive gamma ray kicks by using preferentially picked organic-rich fragments from the well cuttings. RÉSUMÉ Les groupes de Bad Cache Rapids et de Churchill River, et la formation Red Head Rapids, de l’âge de l’Ordovicien supérieur, du bassin de la Baie d’Hudson, présentent une épaisseur de 180–300 m. Ces unités sont dominées par du carbonate, mais contiennent des schistes bitumineux fins, appelés officieusement “Boas River Shale” et “Sixteen Mile Brook Shale”. La stratigraphie et le potentiel d’hydrocarbures des unités de schistes sont encore mal compris. Le “Boas River Shale” et le “Sixteen Mile Brook Shale” sont riches en matières organiques, et ont été découverts sur l’île de Southampton il y a environ 40 ans. Toutefois, ces schistes bitumineux ont donné lieu à d’importants débats sur des questions fondamentales portant sur : un, deux ou trois intervalles de schistes bitumineux à l’intérieur de la séquence de l’Ordovicien, leurs positions stratigraphiques précises, leurs distensions dans la zone hauturière de la Baie d’Hudson, et leurs potentiels d’hydrocarbures. Des études de terrain de l’Ordovicien supérieur et des couches du Silurien inférieur sur l’île de Southampton, ont été entreprises en focalisant sur la nouvelle découverte de trois intervalles de schistes bitumineux dans la zone de Cape Donovan, à l’Est de l’île de Southampton, des schistes de “Boas River Shale” et des schistes de“Sixteen Mile Brook

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Shale”. Elles démontrent sans équivoque que : 1) les trois intervalles de schistes bitumineux présents dans la zone de Cape Donovan sont à l’intérieur de la formation Red Head Rapids ; et : 2) l’intervalle inférieur de schistes bitumineux de Cape Donovan, peuvent être mis respectivement en corrélation avec le “Boas River Shale”, ainsi que l’intervalle moyen ou supérieur de schistes bitumineux de Cape Donovan avec le “Sixteen Mile Brook Shale”. Les données Rock-Eval6 sur des échantillons prélevés sur les trois intervalles de schistes bitumineux provenant de sections de Cape Donovan, conduisent à la reconnaissance de kérogène du Type I–Type II, ainsi qu’à un rendement et à un COT bien plus supérieur que rapporté précédemment. Dix-neuf échantillons prélevés sur des intervalles moyens et supérieurs de schistes bitumineux, présentent une teneur moyenne et des rendements maximum de 136.5 kg HC/tonne et de 230 kg HC/tonne. Ils présentent également une teneur moyenne et un maximum de COT de 20% et 34%. Vingt et un échantillons provenant de l’intervalle inférieur des schistes bitumineux indiquent une moyenne et des rendements supérieurs de 58.5 kg HC/tonne et de112.5 kg HC/tonne, ainsi qu’une moyenne et un COT plus important de 9.8% et de 17.3%. Les trois intervalles de schistes bitumineux sont reconnus dans la zone hauturière de la Baie d’Hudson. Cette reconnaissance s’appuie sur trois lignes d’évidence provenant de la partie inférieure de la formation de Red Head Rapids, dans des puits d’exploration qui se trouvent dans la zone hauturière de la Baie d’Hudson. Elles portent sur : 1) trois protubérances de sursauts de rayons gamma positifs; 2) la possibilité de fragments riches en matières organiques trouvés dans les déblais de forages; 3) quelques valeurs raisonnablement fortes de teneur en COT (2.29–5.73%) sont obtenues à partir de plusieurs échantillons prélevés à l’intérieur de l’intervalle couvrant les trois sursauts de rayons gamma, en utilisant des fragments riches en matières organiques qui ont été préférentiellement sélectionnés dans les déblais de forages. Traduction de Gabrielle Drivet

INTRODUCTION Hudson Bay is a large (1.23 million km2), relatively shallow body of water in northeastern Canada. It is located from 78° to 95°W and from 51° to 70°N, with Southampton Island and other two small islands at its entrance (Fig. 1). During the Paleozoic, Hudson Bay Basin was one of the four principal tectonic elements that formed the Hudson Platform and Southeast Arctic Platform (Hudson Bay Basin in the central, Moose River Basin in the south and Foxe Basin in the north, as well as the grabens underlying the Foxe Channel and Hudson Strait; Sanford and Grant, 1998). The earliest transgression in Hudson Bay Basin started in Edenian, Late Ordovician (Zhang and Banes, 2007). Marine deposits periodically accumulated in Hudson Bay Basin from Late Ordovician to Late Devonian. Breaks in sedimentation have been attributed to both eustatic changes (e.g. the global regression in the latest Ordovician; Zhang and Barnes, 2007) and tectonic movements (e.g. near the end of Early Silurian and in Early Devonian; Sanford and Grant, 1998). The Upper Ordovician succession in Hudson Bay Basin is divided into the Bad Cache Rapids and Churchill River groups and Red Head Rapids Formation. These units are composed of carbonates with small amounts of evaporite and shale. These units are dated Edenian to Richmondian (Zhang and Barnes, 2007). The Silurian rocks include, in ascending order, the Severn River, Ekwan River, Attawapiskat and Kenogami River formations; they are mostly made up of shallow water carbonates, reef facies deposits and shale, and are of Llandovery in age (Zhang and Barnes, 2007). The upper member of Kenogami River Formation is considered as Devonian in age (Norris, 1993), but there is no supporting fossil evidence. The Devonian succession is mainly preserved in the center of the

basin with several hiatuses, and is mostly comprised by halite, clastics and carbonates (Norris, 1993). During the Paleozoic, Southampton Island was located on the north margin of Hudson Bay Basin. The Paleozoic rocks exposed on the island only include those below the Kenogami River Formation (Sanford in Heywood and Sanford, 1976). Within this sequence, two oil shale intervals were discovered during the 1960s and 1970s (Nelson and Johnson, 1966; Sanford in Heywood and Sanford, 1976), and have been considered as potential petroleum source rocks (Macauley, 1986; Macauley et al., 1990). However, there has been a long-lasting debate about the number and stratigraphic position of the oil shales on Southampton Island, as well as their geographic distribution and the petroleum potential. Nelson and Johnson (1966) reported an “oil shale interval” from an outcrop at Sixteen Mile Brook, near the south coast, and in rubble at the East Bay area, near Gore Point on Southampton Island (Figs. 1 and 2). They interpreted that the uppermost 15 m of Ordovician strata included interbedded oil shale and limestone. Trilobite and graptolite fragments found in oil shale rubble at the Sixteen Mile Brook were identified as cf. Pseudogygites latimarginata (Hall, 1847) and Amplexogratus sp. (published by Jackson in 1971), respectively. Thus, the “oil shale interval” was interpreted as Richmondian, Late Ordovician, and stratigraphically located at the top of the Churchill River Group (Nelson and Johnson, 1966). This shale was informally named the “Sixteen Mile Brook shale” (Nelson, 1981). In addition to the Sixteen Mile Brook locality, Nelson and Johnson (1976) discovered 99 occurrences of shale in rubble, extending from Duke of York Bay to Sixteen Mile Brook, and east to the East Bay and Junction Bay areas (Fig. 1). Sanford (in Heywood and Sanford, 1976) discovered another outcrop of oil shale on the Boas River in central

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Fig. 1. Simplified geological map of Southampton Island, modified from Nelson and Johnson (1976) with locations where stratigraphic sections were measured and samples were collected during 2007 field season. The black dots represent outcrops with oil shale intervals, and the white dots represent other localities where sections were measured except for Gore Point: 1. Bad Cache Rapids Group along Fossil Creek; 2. Transition between Churchill River Group and Red Head Rapids Formation at unnamed gully; 3. “bioherms”; 4–6. Bad Cache Rapids Group along Rocky Brook; 7–8. unit 2, Red Head Rapids Formation near junction between Cleveland River and Tungalik Creek; 9–10. Churchill River Group along two creeks southeast of Duke of York Bay; 11. unit 3, Red Head Rapids Formation in Cape Donovan; 12. Bad Cache Rapids and Churchill River groups in Cape Donovan; 13. Ordovician–Silurian boundary section in Cape Donovan. Hudson Bay region showed in the inset map with location of wells drilled during 1960s to 1980s; the wells employed by this study are showed in black dots.

Fig. 2. Historic review of interpretations of the number of oil shale intervals and their stratigraphic positions within the Paleozoic sequence on Southampton Island.

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Southampton Island (Fig. 1). The oil shale at this locality was informally named the “Boas River shale” by Sanford (in Heywood and Sanford, 1976), and later formalized as the Boas River Formation by Sanford and Grant (1990). The 1.5–2 m thick “Boas River shale” was interpreted as being stratigraphically between the Bad Cache Rapids and Churchill River groups (Fig. 2), although the overlying and underlying units are not exposed at the type locality; meanwhile, the “Sixteen Mile Brook shale” was assigned to the Red Head Rapids Formation (Sanford in Heywood and Sanford, 1976). The graptolites Glyptograptus hudsoni Jackson, 1971 and Amplexogratus sp. were found at both the “Boas River shale” and “Sixteen Mile Brook shale” localities; this fossil evidence was employed by Nelson (1981) to conclude that the “Boas River shale” was coeval with the “Sixteen Mile Brook shale” on the top Churchill River Group (Fig. 2). However, in the same publication Nelson (1981) interpreted that the “Boas River shale” is either slightly lower than “Sixteen Mile Brook shale” or coeval with it, and shale rubble in East Bay area is either a lateral extension of “Sixteen Mile Brook shale” or slightly higher within the Red Head Rapids Formation (Fig. 2). Nelson and Johnson (1966, 1976) and Nelson (1981) considered the top of Churchill River Group coincided with the Ordovician–Silurian boundary, and the Red Head Rapids Formation as Lower Silurian. Thus, Nelson (1981) proposed that three distinct oil shale intervals were located in the upper most Churchill River Group and lowest Red Head Rapids Formation, and spanned the Ordovician–Silurian boundary (Fig. 2). In subsequent studies, Dewing et al. (1987) followed Sanford’s (in Heywood and Sanford, 1976) stratigraphic interpretation: two oil shale intervals on the island, “Boas River shale” between Bad Cache Rapids and Churchill River groups, and “Sixteen Mile Brook shale” within Red Head Rapids Formation. Based on conodonts from the “Boas River” and “Sixteen Mile Brook” shales, McCracken and Nowlan (1989) concluded that the “Sixteen Mile Brook shale” is of Richmondian age because of the presence of Aphelognathus cf. A. divergens Sweet, 1979, whereas they considered the “Boas River shale” as being of middle or late Maysvillian in age, considerably older than “Sixteen Mile Brook shale”, based on only one conodont-bearing sample from the “Boas River shale” without Aphelognathus cf. A. divergens. Figure 2 summarizes the previous interpretations of the number and stratigraphic position of the oil shale intervals. The principal differences in interpretation include the number of oil shale intervals (one, or two, or three), and the stratigraphic position of the oil shale intervals (on the top of Churchill River Group or between the Bad Cache Rapids and Churchill River groups). Most authors have followed the interpretation of Sanford (in Heywood and Sanford, 1976) and placed the “Boas River shale” between the Bad Cache Rapids and Churchill River groups (Bolton, 2000; Hamblin, 2008; McCracken, 2000; Morell, 1995; Norris, 1993; Sanford and Grant, 1990, 1998, 2000). However, the number and stratigraphic positions of the oil shale intervals on Southampton Island remains conjectural

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due to: 1) a lack of outcrop overlying and underlying the “Boas River shale” at its type locality; 2) very limited exposure overlying and underlying the “Sixteen Mile Brook shale” at its type locality; 3) a lack of continuous section through the Upper Ordovician and Lower Silurian sequence; and 4) importantly, a lack of thorough study of the sequence. Geochemical studies of the oil shales on Southampton Island were first performed by Macauley (1986). In that study, five and four samples were collected from outcrops at Sixteen Mile Brook and Boas River localities, respectively, plus five from rubble in Gore Point area. The Rock-Eval6 data from these samples indicated that the oil shales: 1) have reasonably high yield and TOC (“Sixteen Mile Brook shale”: average yields of 70 kg/tonne; average TOC — 12%; “Boas River shale”: average yield — 26 kg/tonne; average TOC — 4.5%); 2) contain Type II kerogen; 3) are immature for petroleum generation. Macauley (1986) concluded that the area is not economically attractive. Furthermore, Macauley et al. (1990) used these data and data from Beluga O-23, one of the offshore wells (see Anonymous, 2008), to conclude that petroleum prospects would be limited to the deepest parts of Hudson Bay if the oil shales occur in the offshore area. It is unclear whether the oil shales exist in the Hudson Bay offshore area. Traces of organic rich shale in deep wells in the central part of Hudson Bay indicate they may be widespread (Sanford et al., 1993), but samples from Beluga O-23, Comeault No. 1, Polar Bear C-11 and Narwhal O-58 analysed by Rock-Eval6 pyrolysis failed to find organic-rich intervals (see Anonymous, 2008; Zhang and Dewing, 2008). The objective of this study is to provide new data on the exact number and precise stratigraphic positions of the oil shale intervals within the Upper Ordovician sequence on Southampton Island, their distribution in the Hudson Bay offshore area, and their thermal maturation, based on stratigraphic data collected from outcrops on Southampton Island during the 2007 field season and restudying the limited well materials collected from the offshore area in the early explorations. The current work will shed new light on the prospectivity of the Hudson Bay Basin.

ORDOVICIAN STRATIGRAPHY ON SOUTHAMPTON ISLAND BAD CACHE RAPIDS AND CHURCHILL RIVER GROUPS The Bad Cache Rapids and Churchill River groups were proposed by Nelson (1963, 1964) for Ordovician strata of the Hudson Bay Lowlands in Manitoba. The Bad Cache Rapids and Churchill River groups were first identified on Southampton Island by Nelson and Johnson (1966). The Bad Cache Rapids Group unconformably overlies the Precambrian basement; it is dark grey or brownish grey fossiliferous limestone with thin basal clastic rocks, rich in corals, gastropods, nautiloids, algae, crinoids and trace fossils. The lowest occurrence of conodont Belodina confluens Sweet, 1979 in the Bad Cache Rapids Group is progressively higher from Hudson Bay Lowlands to Hudson Bay


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offshore area (Zhang and Barnes, 2007); this may represent a shallowing-up phase of the group. Because of the patchy outcrops, the thickness of the group is unclear on Southampton Island; the thickness varies insignificantly from Hudson Bay Lowlands (average of 76.7 m) to offshore (average of 82.5 m) (Zhang and Barnes, 2007). The Churchill River Group consists of greenish grey or greyish brown argillaceous limestone, and can be differentiated from the Bad Cache Rapids Group by its dark greenish grey weathered color, argillaceous character and relative lack of macrofossils. The lowest occurrence of Amorphognathus ordovicicus Branson and Mehl, 1933 within the Churchill River Formation becomes progressively higher from Hudson Bay offshore to the Lowlands, which reflects an environmental deepening during a phase of transgression (Zhang and Barnes, 2007). Because of the limited outcrops, the thickness of the formation is unknown on Southampton Island; it is about 103–105 m in Hudson Bay Lowland and offshore area (Zhang and Barnes, 2007). The “Boas River shale” was placed between Bad Cache Rapids and Churchill River groups by Sanford (in Heywood and Sanford, 1976) but without seeing the contact in outcrop. Indeed, no clear contact between Bad Cache Rapids and Churchill River groups was observed in field visits in 2007, but it might be buried by a narrow covered interval in a creek-cut section southeast of Duke of York Bay (Fig. 1). There is no black shale or black shale rubble found at this locality. Conodonts from the Bad Cache Rapids and Churchill River groups record Edenian to lower–middle Richmondian (Upper Ordovician), with the Maysvillian questionably missing (Zhang and Barnes, 2007). RED HEAD RAPIDS FORMATION The Red Head Rapids Formation was named by Nelson (1963) to describe 14 m of unfossiliferous microcrystalline dolomite exposed along the Churchill River in Manitoba. The name was first applied to Southampton Island by Sanford and Norris (1973). The Red Head Rapids Formation was divided into three units in ascending order (Sanford in Heywood and Sanford, 1976): unit 1 laminated beds, unit 2 biostromal beds, and unit 3 biohermal beds. The Red Head Rapids Formation has a distinctive orange-tan color, which is in striking contrast to the grey Bad Cache Rapids and Churchill River groups. Conodonts indicate that the Red Head Rapids Formation has a late Richmondian age, Late Ordovician (Zhang and Barnes, 2007). Sections measured during the course of 2007 field study (Fig. 1, 3) provide a better understanding of the formation, and result in the two new findings within the Red Head Rapids Formation: 1) three oil shale intervals are present in the laminated limestone of unit 1; and 2) a carbonate breccia unit is between the laminated limestone and biostromal units (i.e. between Sanford’s units 1 and 2). The Red Head Rapids Formation is herein divided into four units: unit 1 consists of thin-bedded and laminated argillaceous limestone interbedded with three intervals of oil shale; unit 2 is made of massive, limestone/dolostone breccia; unit 3 is formed by thick-bedded

Fig. 3. Composite stratigraphy of the Red Head Rapids Formation with conodont sample located; the conodont data will be released in a separated paper.

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to massive biostromal dolomitic limestone; and unit 4 is composed of thin bedded limestone with bioherms. The conodont Rhipidognathus symmetricus Branson, Mehl and Branson, 1951 is mainly found in the Red Head Rapids Formation (Zhang and Barnes, 2007), which reflect a restricted, shallow-water, hypersaline condition (Barnes et al., 1973) during the deposition of the Red Head Rapids Formation; the species apparent diachronous migration from the margin to the centre of the basin within the formation probably tracked the terminal Ordovician regression induced by the latest Ordovician Gondwanan glaciation (Barnes et. al., 1995; Zhang et al., 2006; Zhang and Barnes, 2007). Unit 1: Thin-layered and Laminated Argillaceous Limestone Interbedded with Oil Shales Unit 1 is composed of orange–tan weathering, light–medium brown, thin-bedded and finely laminated argillaceous limestone, platy and shaly limestone and dolostone interbedded with thick dolostone and fossiliferous limestone, as well as oil shale at three intervals. The lower oil shale interval (1–1.25 m) is in the lower part of unit 1, whereas the middle (0.3–0.4 m) and upper (0.35–0.5 m) are in the upper part of unit 1. The total measured thickness of unit 1 is 28.1–28.9 m (Fig. 3). Good outcrops of unit 1 were found in two localities. The first is in a small unnamed gully near the south coast, between Sixteen Mile Brook and Rocky Brook (Figs. 1, 4A, 4B). The second is in a deeply cut, partially ice-covered creek in the Cape Donovan area (Figs. 1, 5A–F). It was in the latter locality that three oil shale intervals were found in outcrop (see further). Unit 2: Massive Breccia Limestone Unit 2 is well exposed in the same creek as the three oil shale intervals (unit 1) in the Cape Donovan area, where the base of unit 2 is 5.7 m above the upper oil shale interval (Figs. 3, 6A). Unit 2 is also exposed in a cliff section near the junction of Cleveland River and Tungalik Creek (Figs. 3, 6C). The unit is composed of thick-bedded to massive, brecciated dolostone and limestone with a thickness of 10–15 m. The rock is made up of about 80–90% dolostone and limestone clasts and 10–20% calcareous cements; the clasts are angular and poorly sorted, ranging from less than 1 cm to about 40 cm (Fig. 6B) with abundant porosity. This unit can be a good candidate for reservoir rock. Unit 3: Thick, Massive Biostromal Dolostone and Limestone Unit 3 is equivalent to the unit 2 of Sanford (in Heywood and Sanford, 1976). It is well exposed in the lower reach of the same creek as the previously described unit 1 and 2 in Cape Donovan area. It is even better exposed in another creek about 5 km to the southwest. Unit 3 generally forms conspicuous topographical feature (Fig. 7A), and it is composed of fossiliferous dolostone and limestone and stromatolithic limestone with a total thickness probably greater than 10 m (Figs. 3, 7A). Macrofossils include solitary and colonial corals, brachiopods, and abundant crinoids (Fig. 7B).

Fig. 4. Strata exposed in a gully at 64º02'12.5"N, 83º39'47.9"W, (locality 2 in Fig. 1) near the south coast, between Sixteen Mile Brook and Rocky Brook, about 25 km west of Coral Harbour. A: Transition between Churchill River Group and Red Head rapids Formation. B: The lower unit 1, Red Head Rapids Formation with conodont samples located in place. C: The two very thin layers of black shale near the top of the outcrop; the layer parallel to the red pencil is about 5 mm thick and the layer parallel to the black pencil is less than 5 mm thick.


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Fig. 5. The three oil shale intervals in the unit 1, Red Head Rapids Formation exposed in the same partially ice-covered creek in Cape Donovan area (see locality in Fig. 1). A: Close view of lower oil shale interval in B. B: The lower oil shale interval exposed on the two sides of the creek at 64º45'29.8"N, 82º22'06.7"W. C: Close view of lower oil shale interval in B. D: Close view of upper oil shale interval in E. E: The middle and upper oil shale intervals exposed at 64º45'32.8"N, 82º22'10.0"W in the same creek as lower oil shale interval (A–C). F: Close view of middle oil shale intervals in E.

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Unit 4: Thin-bedded Limestone with Bioherms Unit 4 was previously defined as unit 3 by Sanford (in Heywood and Sanford, 1976). It is composed of light to medium brown, thin-bedded micrite limestone. It is best exposed on a cliff section below the contact between Red Head Rapids and Severn River formations in Cape Donovan area (Figs. 1, 7C, 7E), where about 2 m of thin bedded limestone is exposed. This unit contains a number of mud-dominated biohermal structures, such as a large dome-like algal bioherm about 25 km west of Coral Harbour (Fig. 7D, 7F) that has a diameter of about 400 m. The biohermal structure of the unit 4 can be a good candidate for reservoir of the petroleum system. CORRELATION BETWEEN CAPE DONOVAN OIL SHALES, “BOAS RIVER SHALE” AND “SIXTEEN MILE BROOK SHALE” The sections in Cape Donovan area confirm that three oil shale intervals occur within the Ordovician sequence on Southampton Island and that all three oil shale intervals are within the unit 1 of Red Head Rapids Formation (Fig. 3). The three oil shale intervals exposed in the Cape Donovan area are informally named Cape Donovan lower, middle and upper oil shale intervals herein. As discussed in the introduction, the relationship between the “Boas River shale” and the “Sixteen Mile Brook shale” has remained unclear. The new finding of three oil shale intervals in the Cape Donovan area raise new issues: the relationship between the Cape Donovan three oil shale intervals, “Boas River shale” and “Sixteen Mile Brook shale” — whether they are the same shales, or different ones. THREE OIL SHALE INTERVALS IN UNIT 1, RED HEAD RAPIDS FORMATION The transitional boundary between the Churchill River Group and the lower quarter of unit 1 of Red Head Rapids Formation is well exposed in a small unnamed gully near the south coast, between Sixteen Mile Brook and Rocky Brook, about 25 km west of Coral Harbour (Figs. 1, 3, 4A, 4B). A plateau covered by rubble of dark brown argillaceous limestone and some black shale is adjacent to the gully. Two very thin layers of black shale were found almost at the top of the section (Fig. 4C). The shales are separated by a 15 cm thin-bedded limestone; the shales themselves are only 5 mm thick or less. The very thin black shale layers in the outcrop and black shale rubble on the adjacent plateau occur in an evident stratigraphic context at this locality and represent the lowest stratigraphic occurrence of the oil shale within the Upper Ordovician on Southampton Island. If “Boas River shale” and “Sixteen Mile Brook shale” were the only two shales on the island, the very thin black shale layers and black shale rubble at this locality should represent either of them or both of them, and the stratigraphic position of “Boas River shale” or “Sixteen Mile Brook shale” or both should be within the Red Head Rapids Formation. The rest of the unit 1 is well exposed along a partially icecovered creek in the Cape Donovan area (Figs. 1, 3, 5B, 5E,

Fig. 6. The unit 2, massive breccia limestone, Red Head Rapids Formation. A: Relationship between upper oil shale interval of unit 1 and massive breccia limestone of unit 2 at 64º45'39.8"N, 82º22'30.1"W in Cape Donovan area (see locality in Fig. 1); the lower two short black line segments represent the position of upper oil shale interval with a person pointing the shale position between the two line segments; the upper long black line represents the rough position of the unit 1–2 boundary, Red Head Rapids Formation. B: Close view of breccias and pores within the unit 2, Red Head Rapids Formation at the same locality as A. C: Cliff section with breccia limestone and dolostone of unit 2 near the junction of Cleveland River and Tungalik Creek at 65º01'07.2"N, 84º40'51.6"W (locality 7 in Fig. 1).


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6A). The lowest exposure of unit 1 in the creek is 1–1.25 m thick succession of the lower oil shale interval (Figs. 3, 5A–C). The total thickness of the lower oil shale interval is unknown because its lowest part is ice-covered on one side (Fig. 5A, 5B) and not exposed on the other side (Fig. 5B, 5C). The two very thin layers of black shale exposed at the small unnamed gully near the south coast as discussed above (Fig. 4C) may represent the lowest extent of the lower oil shale interval on the basis of

their occurrence above the transition from Churchill River Group to Red Head Rapids Formation. The exposed lower oil shale interval at Cape Donovan is composed of black and dark brown laminated argillaceous bituminous limestone interbedded with black shale (Figs. 3, 5A–C). The middle oil shale interval is separated from the lower oil shale by 10.1 m of laminated dolomitic limestone (Fig. 3). The upper oil shale interval is separated from the middle by 4.5 m of thin- to thick-bedded

Fig. 7. Unit 3 of thick–massive biostromal dolostone and limestone, and unit 4 of bioherms, Red Head Rapids Formation. A: Massive biostromal dolostone and limestone at 64º44'22.1"N, 82º27'49.1"W (locality 11 in Fig. 1) in a creek southwest of the one where the three oil shale intervals exposed. B: Close view of biostromal limestone. C and E: Distant and close view of Ordovician-Silurian boundary (locality 13 in Fig. 1). D: Distant view of the bioherms at 64º02'36.6"N, 83º49'40.7"W near Sixteen Mile Brook (locality 3 in Fig. 1). F: Close view of the bioherms, showing irregular porosity.

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fossiliferous limestone and minor argillaceous limestone (Figs. 3, 5E). Both middle and upper oil shale intervals are composed of black shale with interbeds of thin layered argillaceous bituminous limestone and platy and shaly limestone. At this location their thicknesses are about 0.3–0.4 m and 0.35–0.45 m, respectively (Figs. 3, 5F, 5D). CAPE DONOVAN LOWER OIL SHALE INTERVAL AND “BOAS RIVER SHALE” At the “Boas River shale” type locality, approximately 3 km south of the Precambrian–Paleozoic boundary near two distinctively shaped hills called The Buttocks, the rocks are poorly exposed (Fig. 8A, 8B). The upper part of the outcrop (light grey color from distant view in Fig. 8A) consists of about 1 m of dark brown laminated argillaceous limestone and black shale rubble. The lower part of the outcrop (dark grey color from distant view in Fig. 8A) consists of about 1 m of interbedded dark brown argillaceous limestone, light brown fossiliferous limestone and black shale (Fig. 8B).

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Detailed studies of both the rubble at the Boas River locality and the well-exposed section of the lower oil shale interval at Cape Donovan allow a correlation between the Cape Donovan lower oil shale interval and the “Boas River shale”. This correlation is supported by the following observations: 1) Paleontology: The trilobite Pseudogygites hudsoni Ludvigsen, 1979 is not only found within the Cape Donavan lower oil shale interval, but also in “Boas River shale” (Fig. 9). This species is unearthed in the dark brown laminated limestone immediately below the unconsolidated layer (Fig. 11) in the upper part of the Cape Donovan lower oil shale interval. It is found together with abundant graptolites Glyptograptus hudsoni and Amplexograptus sp. in the uppermost visible layered limestone at the Boas River locality. Finding the same trilobite species in the in situ rocks at both localities supports the “Boas River shale” being the same age as the Cape Donovan lower oil shale interval. 2) Lithology: A) Orange–tan is typical colour of the Red Head Rapids Formation. The most common rubble at the Boas River locality

Fig. 8. “Boas River shale” and “Sixteen Mile Brook shale” localities. A: Distant view of “Boas River shale” type locality near The Buttocks at 64º22'40.0"N, 84º31'45.5"W (see Fig. 1 for location). B: Close view of deeply eroded “Boas River shale”. C: Distant view of “Sixteen Mile Brook shale” type locality from 64º22'40.0"N, 84º31'35.6"W; the red line indicates the stratigraphic position of the shale interval: D. Close view of the “Sixteen Mile Brook shale”.


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CAPE DONOVAN MIDDLE OR UPPER OIL SHALE INTERVAL AND “SIXTEEN MILE BROOK SHALE” The “Sixteen Mile Brook shale” is exposed discontinuously for about 80 m along the southwest bank of the brook (Figs. 1, 8C). It consists of about 30 cm of black shale interbedded with thinlayered, platy and shaly limestone (Fig. 8D). The rocks below the shale interval include 2–3 m of orange–tan thin-layered and platy limestone (Fig. 8C, 8D), typical rocks of unit 1, Red Head Rapids Formation. In addition, the “Sixteen Mile Brook shale” (Fig. 8D) has a similar thickness to that of the Cape Donovan middle and upper oil shale intervals (35–50 cm) (Fig. 5D–F). These suggest a correlation between “Sixteen Mile Brook shale” and Cape Donovan middle or upper oil shale interval. Therefore, the “Sixteen Mile Brook shale” should be stratigraphically above the “Boas River shale”, and within the unit 1, Red Head Rapids Formation, rather than on the top of Churchill River Group as interpreted by Nelson and Johnson (1966, 1976) and Nelson (1981).

ROCK-EVAL6 PYROLYSIS OF THE CAPE DONOVAN THREE OIL SHALE INTERVALS Fig. 9. Trilobites Pseudogygites hudsoni found in both Cape Donovan lower oil shale interval and “Boas River shale”. A: Specimens in the uppermost visible layered fossiliferous limestone from “Boas River shale”; B: Specimens in the laminated argillaceous limestone from Cape Donovan lower oil shale interval.

is angular, with an orange–tan color (e.g. Fig. 10A), which likely represents part of the Red Head Rapids Formation. B) The dark brown–black, finely laminated argillaceous limestone is the dominant rock type within the Cape Donovan lower shale interval (Figs. 5A–C, 10B), and is the second most common rock type among the rubble at Boas River locality (Fig. 10C). The presence of the dark brown–black argillaceous limestone with distinct fine lamination at both localities adds more evidence in correlating “Boas River shale” with Cape Donovan lower oil shale interval. C) There is an about 10 cm thick layer near the top of Cape Donovan lower oil shale interval containing abundant, 5–20 cm, disc-shaped limestone concretions (Figs. 10B, 11). This is the only stratigraphic level within the entire Upper Ordovician and lowest Silurian where the limestone concretions have been observed in place. Similar limestone concretions are found among the rubble at Boas River locality (Fig. 10D). The paleontological and lithological evidence permits a correlation between the “Boas River shale” and Cape Donovan lower oil shale interval. On this basis, the “Boas River shale” is within the unit 1 of the Red Head Rapids Formation, rather than between Bad Cache Rapids and Churchill River groups as interpreted by Sanford and Norris (1973) and Sanford (in Heywood and Sanford, 1976).

Results of Rock Eval6 pyrolysis analyses for the samples from three oil shale intervals in Cape Donovan are presented in Appendix 1. Forty samples were collected from three outcrops on the southwest bank of the creek in the Cape Donovan area (Fig. 1). Figure 11 shows selected Rock-Eval6 parameters with sample locations within the three oil shale intervals. TMAX, PRODUCTION INDEX AND MATURITY Petroleum generation requires Tmax values of at least 435ºC, depending upon the kerogen type (Peters, 1986). Tmax values from the Cape Donovan three oil shale intervals range from 409ºC to 426ºC (Fig. 11; Appendix 1). Production Index [PI; S1/(S1+S2)] indicates the amount of in situ hydrocarbon generation. A PI of 0.1 is considered to be the minimum value to indicate oil generation. The PI calculated for the Cape Donovan three oil shale intervals ranges from 0.01 to 0.04 (Fig. 11; Appendix 1). Thus, both Tmax and PI from the three oil shale intervals indicate that the oil shales at Cape Donovan are thermally immature. HYDROGEN-OXYGEN INDICES, YIELD RATIO KEROGEN TYPE Hydrogen-Oxygen indices (HI-OI) are used to determine the organic matter type (Peters, 1986). A majority of samples from the three oil shale intervals have HI ranging from 500 to 800, and OI less than 37. On a pseudo-van Krevelan plot (Fig. 12A), they fall in an area between the two curves representing Type I and II kerogen, indicating neither typical Type I, nor typical Type II kerogen. Plotting the samples from three different oil shale intervals separately (Fig. 12B–D) shows that the proportion of Type I kerogen gradually increased up section.

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Macauley (1986, fig. 3) recognized Type II kerogen in 14 samples from outcrops at Sixteen Mile Brook and Boas River localities and from rubble in Gore Point area. There must be reasons why the data collected by Macauley (1986) and present study from the same stratigraphic level in the same area lead to a different recognition of kerogen type. Figure 13 shows the HI-OI from samples within Cape Donovan lower oil shale interval and within “Boas River shale” collected by this study and Macauley (1986). Most “Boas River shale” samples have a HI similar to those from Cape Donovan lower oil shale interval (459–698), but their OI (37–68) is greater than those from Cape Donovan lower oil shale interval (≤37). The data points of “Boas River shale” samples are close to the Type II kerogen line, which is similar to Macauley’s findings (1986, fig. 3). This probably results from significant erosion at Boas River locality, because oxidation tends to remove hydrogen and add oxygen to the kerogen (Durand and Monin, 1980). The ratio of yield to total organic carbon, i.e. (S1+S2) (kg hydrocarbon/tonne) / TOC (Wt %) can be also used for

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kerogen type evaluation. High (>8) and low (3–4) ratios have been correlated with Type I and Type II kerogen, respectively (Macauley and Ball, 1982; Macauley and Snowdon, 1984). Samples from the three oil shale intervals (Fig. 14) indicate the ratios of (S1+S2)/TOC of 5.8, 6.5 and 6.9 within the lower, middle and upper oil shale intervals (Fig. 14B–D), respectively. This also reflects the proportion of Type I kerogen gradually increasing in the mixed Type I–Type II kerogen through time. YIELD, TOTAL ORGANIC CARBON (TOC), HYDROGEN INDEX AND ECONOMIC POTENTIAL 1) 21 samples from the lower oil shale interval have average and maximum yields of 58.5 kg/tonne and 112.5 kg/tonne, and average and maximum TOC of 9.8% and 17.3%; the HI values in most of samples are 500–700 (Fig. 11A). 2) 8 samples from the middle oil shale interval have mean and highest yields of 145.9 kg/tonne and 216.1 kg/tonne, and mean and highest TOC of 22.4% and 34.1%; the HI values in most of samples are 550–800 (Fig. 11B).

Fig. 10. Close view of the rubble at “Boas River shale” type locality and partial outcrop of lower oil shale interval at Cape Donovan locality. A: The most common rubble at “Boas River shale” type locality — angular dolostone and limestone rubble with orange–tan color. B: About 10 cm layer consisting of limestone concretions on the top of the lower oil shale interval at Cape Donovan. C: The second most common rubble at “Boas River shale” locality — argillaceous limestone rubble with very well preserved lamination; the inset figure showing the true nature of the laminated rubble. D: Limestone concretions seen among the rubble at the “Boas River shale” type locality.


Fig. 11. Selected Rock-Eval6 pyrolysis parameters with sample locations in the section of Cape Donovan lower (A), middle (B) and upper (C) oil shale intervals. The thick vertical dashed lines indicate thresholds for organic maturity (Tmax=435; PI=0.1) and good hydrocarbon source rock attributes (TOC>2–4wt%; HI>300). Some of the samples were analyzed twice, represented by both black and white dots. All sample numbers have a prefix C-4576.

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Fig. 12. Modified van Krevelen diagram showing relationship between Hydrogen and Oxygen indices of Cape Donovan three oil shale intervals. A; Total values of three oil shales. B to D: Progressive increase of HI from lower to upper oil shale intervals.


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3) 11 samples from the upper oil shale interval have intermediate and greatest yields of 128.7 kg/tonne and 230.3 kg/tonne, and intermediate and greatest TOC of 18.3% and 31%; the HI values in most of samples are 550–800 (Fig. 11C). All yield, TOC and HI from Cape Donovan three oil shale intervals are significantly higher than those collected from “Boas River shale” and “Sixteen Mile Brook shale” by Macauley (1986).

Possible organic-rich fragments were preferentially picked from the washed well cuttings under a stereo microscope. A small amount of possible organic-rich fragments occur in almost all cutting samples from an interval of 4625–4355 ft in Polar Bear C-11. The weight of possible organic-rich fragments in each sample ranges from 20 mg to 75 mg, which is only 0.06–0.1% of the collected sample. Very few samples from Narwhal O-58 contain tiny amount of the possible organic-rich fragments. Among the preferentially picked possible organic-rich fragments, the most common are limestone fragments with a very small amount of dark sediments attached (Fig. 17A, 17B); more than 95% possible organic-rich fragments are of this type. The less common are fragments with laminae of very thin dark sediments and limestone layers (Fig. 17C). This micro-scale lamination is similar to that in the lower oil shale interval at Cape Donavan and among the rubble at Boas River on Southampton Island. Rare fragments include limestone with a thin black layer attached (Fig. 17E), or a wedge inserted (Fig. 17H); limestone fragments with thin film of black stain (Fig. 17D); limestone fragments with black spots of migrated bitumen (Fig. 17F, 17G) and extremely rare black bitumen fragments (Fig. 17I).

ORDOVICIAN OIL SHALES FROM THE HUDSON BAY OFFSHORE WELLS GAMMA RAY LOGS FROM HUDSON BAY OFFSHORE WELLS Three thermally-immature oil shales are recognized in unit 1 of the Red Head Rapids Formation on Southampton Island. From an exploration point of view, the recognition of source rock in the central segment of Hudson Bay is a critical element given the fact that thermal conditions for the latter area might have allowed the generation of hydrocarbon out of a potential source rock. The gamma ray logs from offshore wells indicate the presence of three positive kicks in the lower–middle Red Head Rapids Formation from four exploration wells in offshore Hudson Bay (Fig. 15). The three positive gamma ray kicks are prominent with position mimicking the three oil shale intervals on Southampton Island (Fig. 15). This may suggest the presence of the three oil shale intervals in the offshore area. However, positive gamma ray kicks can have other origins, such as potash (Schlumberger, 1987). Furthermore, RockEval6 data from cutting samples from Beluga O-23, Polar Bear C-11 and Narwhal O-58, three of the four wells shown in Figure 15, almost did not show any TOC values higher than 0.3% within the three gamma ray kick intervals [Fig. 16; data in Anonymous (2008) and Zhang and Dewing (2008)]. Hence, it has become critical to know whether the three positive gamma ray kicks represent the three oil shale intervals, or something else. SAMPLING FROM POLAR BEAR C-11 AND NARWHAL O-58 Fifty-five cutting samples were collected from the interval of 4625–4355 ft that covers the three gamma ray kicks (4590–4430 ft) in Polar Bear C-11, and 55 cutting samples from the interval of 3645–3375 ft that covers the three gamma ray kicks (3610–3430 ft) in Narwhal O-58. Within the collected intervals in Polar Bear C-11 and Narwhal O-58, a total of 14 and 9 samples were previously analysed by Zhang and Dewing (2008) and most of them had TOC values below 0.3%. The cutting samples were washed using a 250 micron sieve. The weight of clean cuttings of most samples was 35–70 g. The cutting samples are almost pure limestone.

Fig. 13. Modified van Krevelen diagram showing the different Oxygen Index values between Cape Donovan lower oil shale interval and “Boas River shale”.

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TOTAL ORGANIC CARBON (TOC) AND OIL SHALE INTERVALS HUDSON BAY OFFSHORE AREA The possible organic-rich fragments from 23 samples from the interval of 4590–4430 ft in Polar Bear C-11 were analysed by Rock-Eval6 pyrolysis (Fig. 18; Appendix 2). TOC values higher than 2% occur in five samples at three levels; the lower level contains three samples with TOC ranging 3.96–5.73%, the middle and upper levels yield samples with TOC 2.23% and 2.29%, respectively. These three levels can be roughly correlated to the three positive gamma ray kicks, but all are about 10–20 feet below the maximum gamma ray response (Fig. 18). The TOC values collected from Polar Bear C-11 are much lower than those from Southampton Island (Fig. 11; Appendix 1); this is probably because more than 95% of the preferentially picked possible organic-rich fragments from the well are actually limestone that has very small amounts of dark sediments attached (Fig. 17A, 17B). Among the three oil shale intervals on Southampton Island, the lower one yields the lowest TOC, whereas within the three gamma ray kick intervals in Polar Bear C-11, the lower one produces the highest TOC; this is probably because the lower gamma ray kick is the widest among the three in offshore wells, consistent with the fact that

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the lower oil shale interval is the thickest among the three on Southampton Island. Therefore, there is a greater probability of getting more organic-rich fragments from the interval covering the lower positive gamma ray kick than those covering the middle and upper positive gamma ray kicks. The preferentially picked possible organic-rich fragments (Fig. 17A–I) from the well cuttings and the reasonably high TOC values collected from the interval covering the three positive gamma ray kicks (Fig. 18; Appendix 2) support: 1) the existence of the three oil shale intervals in the Hudson Bay offshore area; 2) the correlation between the three positive gamma ray kicks in the Hudson Bay offshore area (Fig. 15) and the three oil shale intervals on Southampton Island (Fig. 5). DISCUSSION Low Tmax Values Opposing High PI Values and Indication of Migrated Hydrocarbon From the Polar Bear C-11, almost all Tmax values collected by this study (average Tmax = 419 for 23 preferentially picked samples; black dots in Fig. 18) are lower than those in Zhang and Dewing (2008) (average Tmax = 434 for 14 randomly picked samples; grey dots in Fig. 18). The data collected from

Fig. 14. Yield ratio of petroleum potential (S1+S2) to total organic carbon (TOC) from Cape Donovan three oil shale intervals. A: For the Cape Donovan three oil shale intervals. B to D: For individual three intervals illustrating the upward increased ratio.


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Fig. 17. Different types of preferentially picked possible organic-rich fragments from Polar Bear C-11. A–B: limestone fragments with very small amount of dark sediments attached (from 4585 ft); C: fragment with laminae of very thin dark sediments and limestone layers (from 4410 ft); D: limestone fragments stained by the thin film of black sediments (4505 ft); E: limestone fragment with thin layer of black shale attached (from 4510 ft); F and G: limestone fragments with bitumen spots (from 4360 ft); H: limestone fragment with black shale wedge inserted (from 4505 ft); I: bitumen fragment (from 4450 ft). All photos are taken by Polarid Digital Microscope Camera 2 under the condition of low sensitivity and Tungster (3200K) color temperature.

all the 23 samples by this study have TOC>0.3% and S2>0.2 mg HC/g rock; therefore, the Tmax values are considered being more reliable than those in Zhang and Dewing (2008). Despite the apparently low maturity, there is indication of hydrocarbon migration, as some organic rich fragments contain bitumen (Fig. 17F, 17G, 17I). The Rock-Eval6 parameter PI is used to characterize the amount of organic matter transformed into free hydrocarbon; a PI>0.1 is commonly taken as the threshold for organic maturity. The three samples from 4615, 4605 and 4600 ft, roughly related to the lower positive gamma ray kick, have high PI values of 0.29, 0.67, and 0.26; another three samples from 4495, 4485 and 4480 ft, correlated with the middle positive gamma ray kick, have reasonably high PI values of 0.15, 0.25 and 0.33 (Fig. 18; Appendix 2). These PI values are much higher than those collected from Cape Donovan three oil shale intervals (all≤0.04; Fig. 11; Appendix 1). Both high Production Index values related to

the prominent lower and middle gamma ray kicks and bitumen fragments found within the three gamma ray kick intervals in the well of Polar Bear C-11 indicate that some in-place petroleum generation might have happened in the Hudson Bay offshore area. The Tmax values vary with a number of factors, such as the operating conditions, heavy-oil accumulations, mineral matrix affects, the type of organic matter (Espitalié, 1986). The erroneously low Tmax values may be the result of suppression caused by either bitumen or particularly reactive kerogen in the samples (Clementz, 1979; Peters, 1986; Paters and Cassa, 1994); it is possible that the Tmax suppression is due to sulphur in the kerogen (Snowdon, 1995). Low Tmax values opposing high PI observed by this study could be caused by either bitumen or sulphur in the kerogen (pyrite and iron-rich minerals are rich in the samples); this needs more data to test.

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Fig. 18. Selected Rock-Eval6 pyrolysis parameters with depth and stratigraphic units for both randomly picked limestone fragments (grey dots; from Zhang and Dewing, 2008) and preferentially picked possible organic-rich fragments (black dots; this study) from the interval covering the three positive gamma ray kicks (the gamma ray log on the right) in well Polar Bear C-11. The formation tops are from Anonymous (2008).

The Reasons for Earlier Failure to Find Organic Matter Rich Intervals in Hudson Bay Offshore Wells Zhang and Dewing (2008) failed to find any TOC value >0.3% in the lower Red Head Rapids Formation from the offshore wells. This may be due to: 1) using randomly picked limestone fragments to collect Rock-Eval6 data gave