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Oil families in Mannville Group reservoirs of southwestern Alberta, Western Canada sedimentary basin
Org. Geochem. Vol. 29, No. 1-3, pp. 769 784, 1998 © 1998 ElsevierScienceLtd. All rights reserved Printed in Great Britain PII: S0146-6380(98)00100-4 0146-6380/98/$- see front matter
Pergamon
Oil families in Mannville Group reservoirs of southwestern Alberta, Western Canada sedimentary basin F. A. KARAVAS', C. L. R I E D I G E R ~*, M. G. F O W L E R 2 and L. R. S N O W D O N 2 ~Department of Geology and Geophysics, The University of Calgary, 2500 University Dr. NW, Calgary, Alberta, Canada T2N 1N4 and 2Geological Survey of Canada (Calgary), 3303 33rd Street NW, Calgary, Alberta, Canada T2L 2A7
Abstrac~Lower Cretaceous oils in southern Alberta have been assessed using column chromatography, sulphur analysis, gas chromatography (GC) and gas chromatography-mass spectrometry (GCMS). These oils can be divided into four oil families, E, Q, EQ and F, on the basis of biomarker characteristics. Family E oils are characterized by low pristane/phytane ratios and a C35 homohopane prominence. Family Q oils are identified by the presence of so-called "Q compounds" (Q/R ratio > 0.5) and have high C27 diasterane to regular sterane ratios. Family EQ oils are a mixture of Family E and Family Q oils, with biomarker characteristics intermediate between these two families and low abundance of "Q compounds" (Q/R ratio < 0.5). Family F oils have high saturate/aromatic ratios, high pristane/phytane ratios (> 1.2) and no C35 homohopane prominence. Oil-source rock correlation to available source rock data suggest that Family E oils are derived from the Upper Devonian-Mississippian Exshaw Formation and Family Q oils correlate to the Lower Cretaceous Ostracode Zone. Family F oils are likely derived from shales of the Upper Jurassic Rierdon Formation, however additional geochemistry data from this potential source rock are required to confirm this interpretation. © 1998 Published by Elsevier Science Ltd. All rights reserved Key words--oil oil, oil source rock correlation, Mannville Group, Western Canada Basin
INTRODUCTION The Lower Cretaceous Mannville Group of Alberta, Western Canada Sedimentary Basin, contains substantial reserves of conventional crude oil and heavy oil/tar sands bitumens. Conventional crude oil reserves of the Mannville Group account for 2.0 x 109 m 3 or 23% of initial oil in place in Alberta. The source for the tar sands bitumens and heavy oils of Alberta, has been of much debate in the literature (Moshier and Waples, 1985; Brooks et al., 1988; Riediger, 1994), resulting in the suggestion of many source rocks and no clear conclusions (Leenheer, 1984; du Rouchet, 1985; Allan and Creaney, 1991; Creaney and Allan, 1992; Creaney et al., 1994). A few studies have reported on the source(s) of the conventional crude oil reserves of the Mannville Group in southern Alberta (Deroo et al., 1977; Creaney and Allan, 1990, 1992; Dolson et al., 1993; Creaney et al., 1994; Riediger et al., 1996, 1997a,b). Deroo et al. (1977) published the first report on the geochemistry and origin of Mannville conventional oils throughout Alberta. They noted that oils pooled in Mississippian, Jurassic and Lower Cretaceous reservoirs have similar geochemical characteristics and suggested that these oils were derived from organic-rich source rocks within the *To whom correspondence should be addressed.
Mannville Group. Creaney and Allan (1990, 1992) suggested that Mannville conventional oils are derived from the Devonian Mississippian Exshaw Formation and the Jurassic Nordegg Member and Fernie shales and that their distribution is related to the location of the Mississippian and Jurassic subcrop edges below the Mannville Group. Dolson et al. (1993) examined the geochemistry of the Bakken/Exshaw interval in southwestern Alberta and northern Montana, which they suggested was the source for oils found in Lower Cretaceous reservoirs of the Sweetgrass Arch area (e.g. Cutbank field in Montana). Riediger et al. (1995) identified two oil families in Lower Cretaceous reservoirs of the Provost oil field, one derived from the Upper Devonian Duvernay Formation and a second oil family representing a mixed oil from the Lower Cretaceous Ostracode Zone and the Upper Devonian Mississippian Exshaw Formation. The occurrence of a unique family of oils from the Manyberries field of southeastern Alberta with an unknown source was discussed by Stevenson and Riediger (1997). Recently, Riediger et al. (1997b) noted the presence of three distinct oil families in southern Alberta, based on detailed biomarker analysis of over 150 oils. The source rocks of these oil families include the Upper DevonianMississippian Exshaw Formation, the Lower Cretaceous Ostracode Zone and the Upper Jurassic Fernie Group. Based on these preceding studies, it 769
770
F.A. Karavas et
is evident that petroleum systems of the Lower Cretaceous Mannville Group of southern and central Alberta are complex, with contributions from at least five different source rocks, if not more. This paper provides a detailed description of the geochemical characteristics of four oil families in southern Alberta in the area bounded by Townships 13 to 20 and Ranges 18 to 27, west of the fourth meridian (Fig. 1). The geochemical characteristics of the oils are used to~'determine the source rock origin of each oil family by comparison with available source rock extract data. GEOLOGICAL
SETTING
The Lower Cretaceous Mannville Group of Alberta was deposited as an eastward thinning clastic wedge in a tectonic foredeep, bounded to the west by the rising Cordillera and to the east by the Precambrian Shield (Strobl, 1988). The Mannville Group was deposited on an irregular and incised erosional surface, which truncates tilted Paleozoic and older Mesozoic strata. Mannville sediments represent a third order sequence, with the transgressive systems tract (TST) of the Lower Mannville separated from the highstand systems tract (HST) of the Upper Mannville surface by a maximum
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flooding surface in the Ostracode Zone (Cant, 1996). The Lower Cretaceous Mannville Group in the study area unconformably overlies Upper Paleozoic to Jurassic strata and is unconformably overlain by sandstones and mudstones of the Colorado Group. The stratigraphic nomenclature within the Mannville Group is complex due to the lithological diversity of the unit. This has led to differing opinions on stratigraphic terminology (James, 1985; Hayes, 1986; Wood and Hopkins, 1992) and the designation of many informal names for the various oil producing horizons (Fig. 2). These include the Lower Mannville Sunburst and Cutbank members and the Upper Mannville Glauconitic member. Petroleum reservoirs within these units comprise a complex stratigraphy of sheet sandstones and incised valley fill deposits (Farshori and Hopkins, 1989; Wood and Hopkins, 1992). METHODS Fifty-one oil samples from ten oil fields in southern Alberta (Armada, Carmangay, Farrow, Gladys, Jumpbush, Little Bow, Long Coulee, Majorville, Queenstown and Shouldice fields; Fig. 1; Table 1) were collected from reservoirs of the
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Oil families in Mannville Group reservoirs
771
Glauconitic, Sunburst and Cutbank members of the Mannville Group (Fig. 2). A LECO SC32 Sulphur Analyzer was used to analyze the sulphur content of each oil. Each oil sample was fractionated by liquid column chromatography according to Snowdon et al. (1987). Gasoline range, C]5+ saturated hydrocarbon and biomarker analyses were performed by gas chromatography (GC) and gas chromatographymass spectrometry (GC-MS) as outlined by Fowler et al. (1995).
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Mannville oils in the area of this study are separated into four oil families, designated E, Q, EQ and F, on the basis of gross composition, C15+saturated hydrocarbon analysis and biomarker characteristics.
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Gross composition
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Fig. 2. Lower Cretaceous stratigraphy in southern and central Alberta showing Jurassic and Lower Cretaceous reservoir units (light shading) in relation to source rock intervals (dark shading).
The API gravity, sulphur content and gross composition of each oil are given in Table 1. Family E oils are characterized by a low saturate to aromatic ratio (ranging from 0.45 to 1.18; Table 1), high resin and asphaltene content (greater than 14%), sulphur contents that range from 1 to 4wt.% (Fig. 3) and API gravity for most oils ranging from
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Fig. 3. Bivariate plot of the saturate/aromatic ratio vs. weight percent sulphur for each oil family. See key for oil family designations.
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Family E
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Fig. 4. Representative CI5 + saturate fraction gas chromatograms for each oil family in the study area. (a) Family E (sample 9); (b) Family Q (sample 24); (c) Family EQ (sample 29); (d) Family F (sample 46). Pr = pristane; Ph = phytane; 20, 25 indicate C2o and C25 n-alkanes, respectively. 17 to 32 ° (Table 1). Family Q oils have a higher saturate to aromatic ratio relative to Family E oils, ranging from 1.2 to 1.7, variable sulphur contents (0.5 to 1.12wt.%) and API gravity of 31 to 35 ° (Table 1). The gross composition of Family EQ oils is similar to the Family Q oils (Table 1). The saturate to aromatic ratio ranges from 1.11 to 1.56, sulphur content ranges from 0.8 to 2.14 wt.% and API gravity of Family EQ oils ranges from 33 to 35 ° (Table 1; Fig. 3). Family F oils are characterized by the highest saturate to aromatic ratio of the four families (Fig. 3), from 1.26 to 2.51, low sulphur contents (less than 1 wt.%) and API gravity ranging from 32 to 38 ° (Table 1).
C15-}-saturate fraction analysis Representative Cjs+saturate fraction gas chromatograms (SFGC) for each oil family are shown in Fig. 4. Pristane/phytane and pristane/nCl7 ratios are given in Table 1 for each oil. The saturate fraction gas chromatograms for all four oil families exhibit abundant n-alkanes in the C~5+range. A slight even n-alkane preference is
exhibited by Family E and EQ oils whereas Family Q oils do not indicate any carbon preference and Family F oils show a slight odd n-alkane preference (Fig. 4). Family E oils have the lowest pristane/phytane ratios of all four oil families, generally less than 1.2 (Fig. 5). Several of these oils exhibit a prominent unresolved complex mixture in the SFGC suggesting they have been biodegraded. Pristane/ phytane ratios range from 1.06 to 1.43 for Family Q oils and from 1.ll to 1.73 for Family EQ oils (Fig. 5). Family F oils have the highest pristane/ phytane values ranging from 1.18 to 2.50. The range in pristane/phytane ratios within each oil family may be due to facies variations within the source rock for Family Q, EQ and F oils and to biodegradation for Family E oils. Biomarker analysis
The biomarker distributions of the oils provide key evidence supporting the presence of four families of oils in the study area. Selected peak height ratios from GC-MS data for the four oil families are provided in Table 2.
774
F.A. Karavas et al. KEY O
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PRISTANEIPHYTAN E
Fig. 5. Bivariate plot of pristane/phytane vs. the saturate/aromatic hydrocarbon ratio for each oil family. See key for oil family designations.
Terpane distributions show considerable variation between the four oil families (Fig. 6, Tables 2 and 3). Tricyclic terpanes are prominent in Family E, Q and EQ oils and are low in abundance in Family F oils. C23 tricyclic terpane/C30 hopane ratios for Family F oils are less than 0.25 with the exception of one sample with a ratio of 0.59 (see Table 2). C24 tetracyclic/C26 tricyclic terpane ratio is greater than 1 for Family Q, EQ and F oils and ranges from 0.67 to 1.42 for Family E oils. C29/C30 and T J T m ratios are less than or equal to one for all the oils. There is a C35 homohopane prominence in Family E, Q and EQ oils (C35/C34 ratio greater than 1) but no C35 prominence in Family F oils (Figs 6 and 7). Sterane distributions also show some variations among the oil families (Figs 8 and 9; Table 2). Oil families Q and EQ tend to have higher relative abundances of C21 steranes compared to Family E and Family F oils. C27 diasterane/Cz7 regular sterane ratios are greater than 2.00 for Family Q and EQ oils, less than 2.00 for Family E oils and range from 1.56 to 3.34 for Family F oils (Fig. 10). The C290~0~20S/C290~0~0~20R ratio of the oils ranges from 0.60 to 1.00.
The distribution of C27:C28:C29 o(flfl (20S + 20R) steranes also indicates some variation among the oil families (Figs 9 and 10; Table 2). Family E, Q and EQ are characterized by C29 > C27 > C28 distribution (Fig. 9), whereas most Family F oils have a C27~C28 < C29 distribution (from Glauconitic member reservoirs). A C29 > C27 > C28 distribution is observed for Family F oils from Cutbank reservoirs, however, the C27/C2~ ratios for all of the Family F oils (less than 1.57) are generally lower than those observed for Family E, Q and EQ oils (Table 2, Fig. 10). It is also noted that C30 4-desmethyl steranes are present on the m/z 217 fragmentograms of all four oil families indicating they are all from Devonian and younger sources (Moldowan et al., 1985, 1990). So-called "Q compounds" are unusual polycyclic (tri-, tetra-, penta-, hexa- and heptacyclic) alkane and aromatic hydrocarbons (Li et al., 1996, 1997). These compounds are useful as a correlation parameter, as their presence indicates a contribution from the Ostracode zone, the only unit to date in which they are known to occur (Riediger et al., 1997a). "Q compounds" have major fragment ions o f m / z 191,217, 218 and 259 (e.g. Figs 6(b) and (c),
Oil families in M a n n v i l l e G r o u p reservoirs
775
Table 2. Oil family designations and biomarker data for each oil sample Terpanes Sample
Family
23/30H 24T/26t
1 E 2 E 3 E 4 E 5 E 6 E 7 E 8 E 9 E 10 E 1t E 12 E 13 E 14 E 15 E 16 E 17 E 18 E 19 E 20 E 21 E 22 E 23 Q 24 Q 25 Q 26 EQ 27 EQ 28 EQ 29 EQ 30 EQ 31 EQ 32 EQ 33 EQ 34 EQ 35 EQ 36 F 37 F 38 F 39 F 40 F 41 F 42 F 43 F 44 F 45 F 46 F 47 F 48 F 49 F 50 F 51 FQ Rierdon extract
0.28 0.62 0.53 0.15 0.28 0.35 0.17 0.15 0.45 0.32 0.48 0.55 0.59 0.65 0.44 0.52 0.53 0.66 0.40 0.64 0.44 0.36 0.47 0.66 1.42 0.74 0.50 0.54 0.60 0.86 0.60 0.79 0.47 0.51 0.28 0.18 0.12 0.16 0.23 0.24 0.59 0.12 0.06 0.10 0.05 0.22 0.09 0.20 0.16 0.24 0.34 0.09
1.07 1.33 1.24 0.67 0.78 0.76 0.94 0.91 0.97 0.93 0.83 0.75 0.77 0.76 0.78 0.80 0.92 1.07 0.76 1.62 1.16 1.29 1.27 1.17 1.39 1.48 1.43 1.33 1.33 1.52 1.17 1.45 1.38 1.50 1.44 1.54 1.18 1.33 1.27 1.33 1.69 1.05 1.40 1.14 1.75 1.42 1.67 1.45 1.40 1.29 1.41 26.00
Steranes
ts/tm
29/30
35/34
0.58 0.70 0.60 0.44 0.47 0.49 0.43 0.48 0.48 0.54 0.46 0.48 0.52 0.50 0.46 0.50 0.55 0.53 0.50 0.56 0.50 0.60 0.70 0.64 1.05 0.70 0.61 0.65 0.63 0.65 0.68 0.69 0.87 0.57 0.52 0.69 0.73 0.48 0.71 0.74 0.76 0.67 0.56 0.54 0.53 0.65 0.60 0.82 0.70 0.67 0.65 0.33
0.55 0.76 0.69 0.65 0.75 0.79 0.60 0.59 0.65 0.62 0.69 0.72 0.73 0.72 0.64 0.64 0.64 0.70 0.64 0.73 0.74 0.58 0.87 0.79 0.85 0.76 0.65 0.78 0.81 0.83 0.89 0.86 0.48 0.66 0.49 0.49 0.49 0.66 0.55 0.51 0.63 0.48 0.42 0.46 0.39 0.58 0.38 0.48 0.47 0.51 0.98 0.46
1.00 1.00 1.14 0.89 1.07 I. 14 1.00 0.94 1.34 1.27 1.33 1.31 1.33 1.25 1.41 1.28 1.11 1.13 1.42 1.05 1.00 1.16 1.19 1.10 1.13 1.22 1.06 1.10 1.22 1.24 1.09 1.21 1.00 1.00 0.95 0.63 0.75 0.71 0.73 0.71 0.67 0.95 0.58 0.92 0.58 0.69 0.65 1.00 0.88 0.89 0.73 0.41
21/29~R dia/reg 0.66 0.89 1.08 0.31 0.37 0.48 0.39 0.41 0.98 0.61 0.97 0.97 0.96 1.07 0.93 1.03 0.97 1.02 0.85 1.40 0.77 0.92 1.00 1.32 3.09 1.57 1.41 1.19 t.26 1.70 1.15 1.83 0.94 1.55 0.78 0.39 0.37 0.45 0.80 0.91 1.64 0.38 0.22 0.31 0.30 0.85 0.54 0.57 0.58 0.81 0.66 0.66
1.38 1.61 1.98 0.75 0.92 1.06 0.74 1.07 1.17 1.34 1.29 1.16 1.24 1.33 1.27 1.33 1.60 1.52 1.22 2.54 1.76 1.74 2.10 3.36 3.88 2.43 2.20 2.54 2.25 2.65 2.15 3.14 2.89 2.45 2.10 1.72 1.96 1.84 3.06 3.34 3.22 2.35 1.56 1.74 1.73 1.89 1.96 2.20 2.26 2.64 1.72 0.43
29s/r
27
28
29
C27/28
28/29
0.76 0.75 0.83 0.68 0.79 0.87 0.77 0.74 0.79 0.76 0.80 0.82 0.81 0.82 0.77 0.76 0.76 0.80 0.78 0.84 0.69 0.68 0.75 0.74 0.83 0.84 0.93 0.89 0.80 0.82 0.78 0.83 0.60 0.79 0.82 0.75 0.74 0.73 0.73 0.79 1.00 0.78 0.69 0.73 0.70 0.84 0.83 0.75 0.76 0.74 0.81 0.63
30 31 32 30 33 36 28 29 33 33 34 34 34 35 34 34 35 34 32 33 31 33 27 30 32 32 32 29 34 32 34 33 29 32 27 26 27 26 28 30 35 23 24 23 23 30 27 31 28 30 27 25
21 23 20 21 22 22 21 22 20 21 20 21 20 21 21 22 22 22 22 21 21 20 22 22 23 22 22 25 21 23 20 21 21 22 22 24 22 22 26 24 22 26 25 23 24 22 27 20 25 23 21 22
49 45 48 49 46 42 51 49 46 46 46 45 45 44 46 45 43 45 46 46 48 47 51 48 44 46 47 47 46 44 46 46 50 46 51 50 51 52 46 46 42 51 51 54 53 48 47 49 48 46 52 53
1.38 1.35 1.56 1.41 1.50 1.59 1.30 1.31 1.63 1.55 1.74 1.63 1.68 1.71 1.64 1.54 1.61 1.58 1.48 1.56 1.52 1.63 1.22 1.38 1.39 1.48 1.44 1.16 1.63 1.39 1.72 1.57 1.42 1.44 1.23 1.07 1.21 1.16 1.08 1.24 1.57 0.89 0.98 1.02 0.99 1.36 1.01 1.57 1.12 1.30 1.31 1.14
0.44 0.52 0.43 0.43 0.47 0.53 0.42 0.45 0.44 0.45 0.42 0.47 0.45 0.46 0.45 0.49 0.50 0.48 0.47 0.47 0.43 0.43 0.43 0.46 0.53 0.48 0.47 0.53 0.45 0.52 0.44 0.45 0.41 0.48 0.44 0.48 0.43 0.42 0.56 0.53 0.53 0.52 0.49 0.42 0.44 0.46 0.57 0.41 0.52 0.50 0.40 0.42
Q/R
1.35 4.35 3.66 0.57
0.47
Terpanes: 23/30H = C23tricyclic terpane/C30hopane; 24T/26t = C24tetracyclic terpane/C26tricyclic terpane; Ts = C27 18~(H)-trisnorneohopane; Tm = C27 17~(H)-trisnorhopane; 29/30 - C29norhopane/C30hopane; C35/C34=C35/C34 homohopane; Steranes: dia/reg = C27 13/~(H),17~(H)-diasterane (20S)/Cz7 5~(H),14e(H),17~(H)-sterane (20R); S = C29 5e(H),14e(H),17e(H)-sterane (20S); R = C29 5~(H),14~(H),17x(H)-sterane (20R); C27,C28,C29=C27,C28,C29 5e(H),14/:~(H),17/~(H)-sterane ((20S + 20R)/2) (respectively); Q - Q compounds (m/z 217); R - C29 5~(H),14~(H),17e(H)-sterane (20R). 8(b)
and
cantly
(c) a n d
after
analytical
C35
9(b)
and
technique).
Their
presence elution
using
is d e t e r m i n e d
by anomalous
fragmentograms
t h e o i l s is v a r i a b l e ratio
of "Q
Family
(c); T a b l e trations
2). S a m p l e s
of "Q
whereas
in Family
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trations
times
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EQ (Q/R
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designated
designated 26
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o n t h e m/z 2 1 7 DISCUSSION
present in by the
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Family
Table
our
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compound"
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homohopane
greater
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Thermal maturity Biomarker determining
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the relative degree of thermal
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used parameters and
Ts/Tm a r e
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s u c h a s C29 often
used
to
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Oil families in Mannville Group reservoirs Table 3. Peak identificationsfor terpanes (m/z 191;Figure 5), and steranes (m/z 217; Figure 6; m/z 218; Figure 7) used in selected ratios (Table2)
777
towards the north with the least mature oils in Little Bow field and the most mature Family E oils in Jumpbush field (Fig. 11). Family Q oils are found along the northern edge of the study area in Townships 19 and 20. All Family Q oils are of relatively similar maturity (C298/R of 0.74 to 0.83; Table 2) and exhibit a slight increase in maturity towards the east from Gladys field towards Queenstown field (Fig. 11). Family EQ oils are also found in the northern portion of the study area (Townships 18 to 20 and Ranges 18 to 23 W4). Family EQ oils increase in maturity towards the east from Shouldice field to Jumpbush field. Family F oils are of relatively similar maturity (Table 2), with no clear maturity trend. T~/Tm ratios are useful for determining relative thermal maturity within a particular oil family (e.g. Fig. 12). Ts/Tm ratios of families E, Q and EQ give maturity trends that are similar to those exhibited by the C298/R ratio. However, Ts/Tm ratios for Family F oils exhibit a trend in maturity whereas the C298/R ratios did not. There is an overall increase in thermal maturity from Farrow field southward to Carmangay field, based on TjTm ratios. An increase in thermal maturity with depth is also indicated by the Ts/Tm ratios for oils from Long Coulee oil field. Oils in the Glauconitic member (Upper Mannville) reservoirs are less mature
l c23 tricyclicterpane 2 C24 tetracyclicterpane 3 C26 tricyclicterpane 4 18~(H)-trisnorneohopane (Ts) 5 17~(H)-trisnorhopane (Tm) 6 17ct(H),21 fl(H)-norhopane 7 17~(H)-21~(H)-hopane (C30) 8 22(S) and 22(R) 17~(H),21fi(H)-tetrakishomohopane(C34) 9 22(S) and 22(R) 17~(H),21/3(H)-pentakishomohopane(C35) a C2~ sterane b 13~(H),17~(H)-cholestane(20R) c 5~(H),14~(H),l 7~(H)-cholestane(20R) d 24-ethyl-5~(H),14~(H),17~(H)-cholestane (20R) e 24-ethyl-5~(H),14~(H),l 7c~(H)-cholestane(20S) Q, Q* Q compounds r C2v 5~(H),14fl(H),17~(H)-cholestane(20Sand 20R) s C2s 5~(H),14/:t(H),17/3(H)-methylcholestane(20Sand 20R) t C29 5~(H),14/:t(H),17/:t(H)-ethylcholestane(20Sand 20R) u C30 5~(H),14p(H),17fl(H)-propylcholestane(20S and 20R)
assess the thermal maturity of oil (Waples and Machihara, 1990; Requejo, 1992). Relative maturity of the oils can be described by the C29S/R ratio for each oil Family. Family E, EQ and F oils exhibit a larger range in maturity than do Family Q oils (Table 2). Family E oils occur in the southeastern portion of the study area (Townships 14 to 17 and Ranges 18 to 21 W4; Fig. 11). These oils increase in overall maturity
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than oils in the Cutbank Mannville) reservoirs.
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Oil-source rock correlation
Petroleum systems of the Lower Cretaceous Mannville Group in southern Alberta are more complex than previously described in the literature (e.g. Deroo et al., 1977; Creaney and Allan, 1990). Through comparison of the geochemistry of the oils to available source rock information from previous studies, oil-source rock correlations are proposed for each oil family. The Family E oils display biomarker characteristics that are very similar to the Cutbank oils from Montana described by Dolson et al. (1993). The Cutbank oils were suggested as having an Exshaw source. An Exshaw Formation rock extract exhibits a C24 tetracyclic/C26 tricyclic ratio < 1.0, C23 tricyclic terpane/C30 hopane ratio > 0.40, C35 homohopane prominence and a C 2 9 > C 2 7 > C 2 8 sterane distribution (Dolson et al., 1993) and the Exshaw is thus considered as the likely source rock for the Family E oils.
Family E oils are also somewhat similar to the Family C.~w ("Lodgepole source") oils from southwestern Saskatchewan (Osadetz et al., 1994), although the Family C,w oils are less mature than the Family E oils. There are three possible explanations to account for these similarities. Family E and C,w oils are thermal maturity variations within the same family and were all derived from (A) mature Lodgepole Formation sources in North Dakota, or (B) mature Exshaw Formation sources in western Alberta, or (C) the source rock kerogen for the Family E and C~,~ oils are very similar. Derivation of Family E oils from the Lodgepole Formation in the central Williston Basin is unlikely for several reasons. First, Lodgepole-derived oils would have had to migrate in excess of 600 km from source to trap in southern Alberta, moving across the Sweet Grass Arch and then down-dip into the southern Alberta reservoirs. In addition, one would expect to see more mature oils (i.e. Family E) closer to the source whereas the opposite is observed. More geochemical data from the potential source rocks is required to decide whether scen-
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ario (B) or (C) is the most likely explanation for the similarities between the Family E and Csw oils. Family Q oils have biomarker characteristics indicating an Ostracode Zone source (Riediger et al., 1997a). Ostracode Zone rock extracts display considerable variability in their biomarker distributions (e.g. Pr/Ph ranging from 1.2 to 5.4), however, they show sterane distributions where C29 > C27 and C28 and some samples have high diasterane to regular sterane contents (Riediger et al., 1997a). The Ostracode Zone is also the only source rock from the Western Canada Sedimentary Basin known to contain the unusual polycyclic alkane and aromatic hydrocarbons referred to as "Q compounds" (Riediger et al., 1997a). The relatively high concentrations of "Q compounds" in the Family Q oils is considered a clear indication of an Ostracode Zone source for these oils. Family EQ oils represent a mixture of Family Q and Family E oils. The Family EQ oils contain "Q compounds" in low concentrations (Q/R less than 0.5; Table 2) suggesting some contribution from the Ostracode Zone. Many of the biomarker characteristics of these oils are similar to Family E oils and suggest that they have received a significant contribution from the Upper Devonian-Mississippian Exshaw Formation (e.g. C35 homohopane prominence). The distribution of Family EQ oils in the study area is consistent with mixing of Family Q oils from the west and Family E oils from the south (Fig. 11). Mixed Ostracode Zone and Exshaw Formation oils are present in Lower Cretaceous reservoirs in the Provost field (Riediger et al., 1995), some 200 km structurally updip, to the northeast of the study area. Provost oils are correlated to the Family EQ oils described here (i.e. in Jumpbush, Queenstown and Shouldice fields), implying significant southwest to northeast migration of this oil family. Migration almost certainly occurred within the Lower Cretaceous Mannville Group itself, presenting the possibility of entrapment of some of this oil along the migration fairway. The biomarker characteristics of Family F oils are not readily correlated with any previously documented source rock in the Western Canada Sedimentary Basin. Riediger et al. (1996, 1997b) suggested that the source for oils in the Long Coulee oil field were from a Mesozoic source rock, younger than middle Jurassic in age and suggested shales within the Upper Jurassic Fernie Group as potential sources for these oils. Limited geochemical data suggest that shales of the Upper Jurassic Rierdon Formation (Fernie Group equivalent; Fig. 2) in southern Alberta may be the source rock for Family F oils. The Rierdon shales have TOC values up to 13% and HI values as high as 342 mg/ gTOC. An extract from the Rierdon Formation (Table 2) shows a low abundance of tricyclic ter-
panes, no C35 homohopane prominence and a sterane distribution of C27 < C28 < C29, characteristics which are consistent with the biomarker distributions of the Family F oils. Other biomarker characteristics of the Rierdon extract (e.g. low diasterane/regular sterane ratio and Ts/Tm ratios relative to the Family F oils) may reflect thermal maturity variations. Nonetheless, additional rock extracts from the Rierdon and other shaly horizons within the Jurassic and Lower Cretaceous interval are required in order to demonstrate the source of the Family F oils. Family F oils occur structurally downdip (west) of the Jurassic (Rierdon) subcrop edge (Fig. 12). The Rierdon shales may have acted as the source for the Family F oils, but they are also an important regional seal, preventing upward migration of Family E oils into Lower Cretaceous reservoirs lying above the Rierdon. As noted above, Family EQ oils, representing a mix of Family E and Family Q oils, are common within the study area. Very little mixing of other oil families has been observed, with exception of one oil from the Farrow oil field (sample 51), which displays biomarker characteristics suggesting mixing of Family F and Family Q oils. CONCLUSIONS Four oil families are present in Lower Cretaceous reservoirs of southern Alberta. Family E oils are characterized by low saturate/aromatic ratios, low Pr/Ph ratios and a C35 homohopane prominence and are likely derived from the Upper DevonianMississippian Exshaw Formation. Family Q oils are characterized by the presence of Q compounds and are derived from the Lower Cretaceous Ostracode Zone. Family EQ oils have characteristics suggesting mixing of Family E and Q oils. Family F oils are characterized by high saturate/aromatic ratios, Pr/Ph ratios > 1.2, low C27/C28 sterane ratios and are possibly derived from shales of the Upper Jurassic Rierdon Formation. Additional biomarker data from the Rierdon Formation are required to confirm this preliminary correlation. Thermal maturity of Family E oils increases from the south of the study area towards the northeast. Family Q and EQ oils increase in maturity from the west towards the eastern portion of the study area. Family F oils increase in maturity from Farrow field southward to Carmangay field and increase with depth in the Long Coulee field. Acknowledgements--We would like to thank Sneh Achal
and Marg Northcott of the Organic Geochemistry Labs, Geological Survey of Canada (Calgary) for expert technical assistance. The study was supported by NSERC Individual Research and New Faculty Support Grants to C. L. Riediger. The Canadian Society of Petroleum Geologists and the following companies are also thanked
Oil families in Mannville Group reservoirs for their support of the southern Alberta Oil Study: Bow Valley Energy, Canadian Hunter Exploration, Morrison Petroleums, Numac Energy, Ocelot Energy, PanCanadian Petroleums, Petrel Roberston, Pinnacle Resources, Poco Petroleums and Talisman Energy. Steve Creaney and Erdem Idiz are thanked for their constructive reviews of the manuscript. REFERENCES
Allan, J. and Creaney, S. (1991) Oil families of the Western Canada Basin. Bulletin of Canadian Petroleum Geology 39, 107-122. Brooks, P. W., Fowler, M. G. and Macqueen, R. W. (1988) Biological marker and conventional organic geochemistry of oil sands/heavy oils, Western Canada Basin. Organic Geochemistry 12, 529-536. Cant, D. J. (1996) Sedimentological and sequence stratigraphic organization of a foreland clastic wedge, Mannville Group, Western Canada Basin. Journal of Sedimentary Research 66, 1137-1147. Creaney, S. and Allan, J. (1990) Hydrocarbon generation and migration in the Western Canada Sedimentary Basin. In Classic Petroleum Provinces, ed. J. Brooks. Geological Society of London Special Publication 50, 189-202. Creaney, S. and Allan, J. (1992) Hydrocarbon systems in the Foreland basin of western Canada, Chapter 10. In Foreland Basins and Fold Belts, ed. R. W. Macqueen and D. A. Leckie. AAPG Memoir 55, 279-307. Creaney, S., Allan, J., Cole, K., Fowler, M., Brooks, P., Osadetz, K., Macqueen, R. W., Snowdon, L. and Riediger, C. L. (1994) Hydrocarbon generation and migration in the Western Canada Sedimentary Basin, Ch. 32. In Geological Atlas of the Western Canada Sedimentary Basin, ed. G. D. Mossop and I. Shetson, pp. 455-468. Canadian Society of Petroleum Geologists and Alberta Research Council, Calgary. du Rouchet, J. (1985) The origin and migration paths of hydrocarbons accumulated in the Lower Cretaceous "giant" tar accumulations of Alberta-II. Journal of Petroleum Geology 8, 101-114. Deroo, G., Powell, T. G., Tissot, B. and McCrossan, R. G. (1977) The origin and migration of petroleum in the Western Canada Sedimentary Basin, Alberta. Geological Survey of Canada Bulletin 262, 136. Dolson, J., Piombino, J., Franklin, M. and Harwood, R. (1993) Devonian oil in Mississippian and Mesozoic reservoirs--unconformity controls on migration and accumulation, Sweetgrass Arch, Montana. The Mountain Geologist 30, 125-146. Farshori, M. Z. and Hopkins, J. C. (1989) Sedimentology and petroleum geology of fluvial and shoreline deposits of the Lower Cretaceous Sunburst sandstone member, Mannville Group, southern Alberta. Bulletin of Canadian Petroleum Geology 37, 371-388. Fowler, M. G., Hamblin, A. P., Hawkins, D., Stasiuk, L. D. and Knight, I. (1995) Petroleum geochemistry and hydrocarbon potential of Cambrian and Ordovician rocks of western Newfoundland. Bulletin of Canadian Petroleum Geology 43, 187-213. Hayes, B. J. R. (1986) Stratigraphy of the basal Cretaceous Lower Mannville Formation, southern Alberta and north-central Montana. Bulletin of Canadian Petroleum Geology 34, 30~[8. James, D. P. (1985) Stratigraphy, sedimentology and diagenesis of Upper Jurassic and Lower Cretaceous (Mannville) strata, southwestern Alberta. Unpublished Ph.D. thesis, Oxford University, 3 volumes. Leenheer, M. J. (1984) Mississippian Bakken and equivalent formations as source rocks in the Western Canadian Basin. Organic Geochemistry 6, 521 532.
783
Li, M., Riediger, C. L., Fowler, M. G., Snowdon, L. R. and Abrajano, T. A. (1996) Unusual polycyclic alkanes in Lower Cretaceous Ostracode Sediments and related oils of the Western Canada Sedimentary Basin. Organic Geochemistry 25, 199-209. Li, M., Riediger, C. L., Fowler, M. G. and Snowdon, L. R. (1997) Unusual polycyclic aromatic hydrocarbons in Lower Cretaceous Ostracode Sediments and related oils of the Western Canada Sedimentary Basin. Organic Geochemistry 27, 439-448. Moldowan, J. M., Seifert, W. K. and Gallegos, E. J. (1985) Relationship between petroleum composition and depositional environment of source rocks. AAPG Bulletin 69, 1255-1268. Moldowan, J. M., Fago, F. J., Lee, C. Y., Jacobson, S. R., Watt, D. S., Slongui, N. E., Jeganathan, A. and Young, D. C. (1990) Sedimentary 24-n-propylcholestanes, molecular fossils diagnostic of marine algae. Science 247, 309 312. Moshier, S. O. and Waples, D. W. (1985) Quantative Evaluation of Lower Cretaceous Mannville Group as source rock for Alberta's oil sands. AAPG Bulletin 69, 161-172. Osadetz, K. G., Snowdon, L. R. and Brooks, P. W. (1994) Oil families in Canadian Williston Basin (southwestern Saskatchewan). Bulletin of Canadian Petroleum Geology 42, 155-177. Requejo, A. G. (1992) Quantitative analysis of triterpane and sterane biomarkers: methodology and applications in molecular maturity studies. In Biological Markers in Sediments and Petroleum, ed. J. M. Moldowan, P. Albrecht and R. P. Philp, pp. 222-240. Prentice Hall, Englewood Cliffs, NJ. Riediger, C. L. (1994) Migration of "Nordegg" oil in the Western Canada Basin. How much and how far? Bulletin of Canadian Petroleum Geology 42, 63-73. Riediger, C. L., Cody, J. D. and Fowler, M. G. (1996) Origin and alteration of Mannville oils, southern Alberta: Preliminary investigations. In Program and Abstracts, CSPG Conference, Calgary, Alberta. Riediger, C. L., Fowler, M. G. and Snowdon, L. R. (1997a) Organic Geochemistry of the Lower Cretaceous Ostracode Zone, a brackish/non-marine source for some Lower Mannville oils in southeastern Alberta. In Petroleum Geology of the Cretaceous Mannville Group, Western Canada, ed. S. G. Pemberton and D. P. James, pp. 93 102. Canadian Society of Petroleum Geologists Memoir 18. Riediger, C. L., Karavas, F. K., Fowler, M. G. and Snowdon, L. R. (1997b) Oil-oil and oil-source rock correlations in southern Alberta, Western Canada Sedimentary Basin: the key to understanding Mannville petroleum systems. In Sedimentary Events, Hydrocarbon Systems, Program with Abstracts, ed. B. Beauchamp, pp. 238. CSPG-SEPM Joint Convention, Calgary, Alberta, Canada. Riediger, C. L., Snowdon, L. R., Fowler, M. G., MacDonald, R. W. and Sherwin, M. (1995) Prospecting for light-medium gravity crude oils in the heavy oil belt of eastern Alberta. The Lower Mannville Petroleum system, Provost area, Alberta, Canada. In Organic Geochemistry: Developments and Applications to Energy, Climate, Environment and Human History, ed. J. O. Grimalt and C. Dorronsoro, pp. 454-456. European Association of Organic Geochemists, Donostia-San Sebastian, The Basque Country. Snowdon, L. R., Brooks, P. W., Williams, G. K. and Goodarzi, F. (1987) Correlation of the Canol Formation source rock with oil from Norman Wells. Organic' Geochemistry 11, 529-548. Stevenson, P. M. and Riediger, C. L. (1997) Oil-oil and oil-source rock correlation for the Manyberries and
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Medicine Hat oil fields; implications for oil migration. In Sedimentary Events, Hydrocarbon Systems, Program with Abstracts, ed. B. Beauchamp, pp. 265. CSPGSEPM Joint Convention, Calgary, Alberta, Canada. Strobl, R. S. (1988) The effects of sea-level fluctuations on prograding shorelinesand estuarine valley-fill sequences in the Glauconitic member, Medicine River field and adjacent areas. In Sequences, Sedimentology: Smface and Subsurface, ed. D. P. James and D. A. Leckie, Canadian Society of Petroleum Geologists, Memoir 15, 221-236.
Waples, D. W. and Machihara, T. (1990) Application of sterane and triterpane biomarkers in petroleum exploration. Bulletin of Canadian Petroleum Geology 38, 357380. Wood, J. M. and Hopkins, J. C. (1992) Traps associated with paleovalleys and interfluves in an unconformity bounded sequence: lower Cretaceous Glauconitic Member, Southern, Alberta, Canada. AAPG Bulletin 76, 904 926.
Pergamon
Oil families in Mannville Group reservoirs of southwestern Alberta, Western Canada sedimentary basin F. A. KARAVAS', C. L. R I E D I G E R ~*, M. G. F O W L E R 2 and L. R. S N O W D O N 2 ~Department of Geology and Geophysics, The University of Calgary, 2500 University Dr. NW, Calgary, Alberta, Canada T2N 1N4 and 2Geological Survey of Canada (Calgary), 3303 33rd Street NW, Calgary, Alberta, Canada T2L 2A7
Abstrac~Lower Cretaceous oils in southern Alberta have been assessed using column chromatography, sulphur analysis, gas chromatography (GC) and gas chromatography-mass spectrometry (GCMS). These oils can be divided into four oil families, E, Q, EQ and F, on the basis of biomarker characteristics. Family E oils are characterized by low pristane/phytane ratios and a C35 homohopane prominence. Family Q oils are identified by the presence of so-called "Q compounds" (Q/R ratio > 0.5) and have high C27 diasterane to regular sterane ratios. Family EQ oils are a mixture of Family E and Family Q oils, with biomarker characteristics intermediate between these two families and low abundance of "Q compounds" (Q/R ratio < 0.5). Family F oils have high saturate/aromatic ratios, high pristane/phytane ratios (> 1.2) and no C35 homohopane prominence. Oil-source rock correlation to available source rock data suggest that Family E oils are derived from the Upper Devonian-Mississippian Exshaw Formation and Family Q oils correlate to the Lower Cretaceous Ostracode Zone. Family F oils are likely derived from shales of the Upper Jurassic Rierdon Formation, however additional geochemistry data from this potential source rock are required to confirm this interpretation. © 1998 Published by Elsevier Science Ltd. All rights reserved Key words--oil oil, oil source rock correlation, Mannville Group, Western Canada Basin
INTRODUCTION The Lower Cretaceous Mannville Group of Alberta, Western Canada Sedimentary Basin, contains substantial reserves of conventional crude oil and heavy oil/tar sands bitumens. Conventional crude oil reserves of the Mannville Group account for 2.0 x 109 m 3 or 23% of initial oil in place in Alberta. The source for the tar sands bitumens and heavy oils of Alberta, has been of much debate in the literature (Moshier and Waples, 1985; Brooks et al., 1988; Riediger, 1994), resulting in the suggestion of many source rocks and no clear conclusions (Leenheer, 1984; du Rouchet, 1985; Allan and Creaney, 1991; Creaney and Allan, 1992; Creaney et al., 1994). A few studies have reported on the source(s) of the conventional crude oil reserves of the Mannville Group in southern Alberta (Deroo et al., 1977; Creaney and Allan, 1990, 1992; Dolson et al., 1993; Creaney et al., 1994; Riediger et al., 1996, 1997a,b). Deroo et al. (1977) published the first report on the geochemistry and origin of Mannville conventional oils throughout Alberta. They noted that oils pooled in Mississippian, Jurassic and Lower Cretaceous reservoirs have similar geochemical characteristics and suggested that these oils were derived from organic-rich source rocks within the *To whom correspondence should be addressed.
Mannville Group. Creaney and Allan (1990, 1992) suggested that Mannville conventional oils are derived from the Devonian Mississippian Exshaw Formation and the Jurassic Nordegg Member and Fernie shales and that their distribution is related to the location of the Mississippian and Jurassic subcrop edges below the Mannville Group. Dolson et al. (1993) examined the geochemistry of the Bakken/Exshaw interval in southwestern Alberta and northern Montana, which they suggested was the source for oils found in Lower Cretaceous reservoirs of the Sweetgrass Arch area (e.g. Cutbank field in Montana). Riediger et al. (1995) identified two oil families in Lower Cretaceous reservoirs of the Provost oil field, one derived from the Upper Devonian Duvernay Formation and a second oil family representing a mixed oil from the Lower Cretaceous Ostracode Zone and the Upper Devonian Mississippian Exshaw Formation. The occurrence of a unique family of oils from the Manyberries field of southeastern Alberta with an unknown source was discussed by Stevenson and Riediger (1997). Recently, Riediger et al. (1997b) noted the presence of three distinct oil families in southern Alberta, based on detailed biomarker analysis of over 150 oils. The source rocks of these oil families include the Upper DevonianMississippian Exshaw Formation, the Lower Cretaceous Ostracode Zone and the Upper Jurassic Fernie Group. Based on these preceding studies, it 769
770
F.A. Karavas et
is evident that petroleum systems of the Lower Cretaceous Mannville Group of southern and central Alberta are complex, with contributions from at least five different source rocks, if not more. This paper provides a detailed description of the geochemical characteristics of four oil families in southern Alberta in the area bounded by Townships 13 to 20 and Ranges 18 to 27, west of the fourth meridian (Fig. 1). The geochemical characteristics of the oils are used to~'determine the source rock origin of each oil family by comparison with available source rock extract data. GEOLOGICAL
SETTING
The Lower Cretaceous Mannville Group of Alberta was deposited as an eastward thinning clastic wedge in a tectonic foredeep, bounded to the west by the rising Cordillera and to the east by the Precambrian Shield (Strobl, 1988). The Mannville Group was deposited on an irregular and incised erosional surface, which truncates tilted Paleozoic and older Mesozoic strata. Mannville sediments represent a third order sequence, with the transgressive systems tract (TST) of the Lower Mannville separated from the highstand systems tract (HST) of the Upper Mannville surface by a maximum
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flooding surface in the Ostracode Zone (Cant, 1996). The Lower Cretaceous Mannville Group in the study area unconformably overlies Upper Paleozoic to Jurassic strata and is unconformably overlain by sandstones and mudstones of the Colorado Group. The stratigraphic nomenclature within the Mannville Group is complex due to the lithological diversity of the unit. This has led to differing opinions on stratigraphic terminology (James, 1985; Hayes, 1986; Wood and Hopkins, 1992) and the designation of many informal names for the various oil producing horizons (Fig. 2). These include the Lower Mannville Sunburst and Cutbank members and the Upper Mannville Glauconitic member. Petroleum reservoirs within these units comprise a complex stratigraphy of sheet sandstones and incised valley fill deposits (Farshori and Hopkins, 1989; Wood and Hopkins, 1992). METHODS Fifty-one oil samples from ten oil fields in southern Alberta (Armada, Carmangay, Farrow, Gladys, Jumpbush, Little Bow, Long Coulee, Majorville, Queenstown and Shouldice fields; Fig. 1; Table 1) were collected from reservoirs of the
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Glauconitic, Sunburst and Cutbank members of the Mannville Group (Fig. 2). A LECO SC32 Sulphur Analyzer was used to analyze the sulphur content of each oil. Each oil sample was fractionated by liquid column chromatography according to Snowdon et al. (1987). Gasoline range, C]5+ saturated hydrocarbon and biomarker analyses were performed by gas chromatography (GC) and gas chromatographymass spectrometry (GC-MS) as outlined by Fowler et al. (1995).
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Mannville oils in the area of this study are separated into four oil families, designated E, Q, EQ and F, on the basis of gross composition, C15+saturated hydrocarbon analysis and biomarker characteristics.
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Gross composition
li
Fig. 2. Lower Cretaceous stratigraphy in southern and central Alberta showing Jurassic and Lower Cretaceous reservoir units (light shading) in relation to source rock intervals (dark shading).
The API gravity, sulphur content and gross composition of each oil are given in Table 1. Family E oils are characterized by a low saturate to aromatic ratio (ranging from 0.45 to 1.18; Table 1), high resin and asphaltene content (greater than 14%), sulphur contents that range from 1 to 4wt.% (Fig. 3) and API gravity for most oils ranging from
®
4.00
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@
KEY
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3.00 --
O
FAMILYE
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FAMILYQ
•
FAMILYEQ
0®
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et
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•
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1.00 O
÷~ O
0.00 0.00
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I
o
I
2.00 SATUPJ~TE/AROMATIC
1.00
%0
I
I 3.00
Fig. 3. Bivariate plot of the saturate/aromatic ratio vs. weight percent sulphur for each oil family. See key for oil family designations.
772
F . A . Karavas et al.
. . . .
5
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Oil families in Mannville Group reservoirs
773
Family E
Family EQ
2O
20
._=
!
Family O
Family F 2O
20
i
/
25 D.
25
1
een
Time (min)
Time (min)
Fig. 4. Representative CI5 + saturate fraction gas chromatograms for each oil family in the study area. (a) Family E (sample 9); (b) Family Q (sample 24); (c) Family EQ (sample 29); (d) Family F (sample 46). Pr = pristane; Ph = phytane; 20, 25 indicate C2o and C25 n-alkanes, respectively. 17 to 32 ° (Table 1). Family Q oils have a higher saturate to aromatic ratio relative to Family E oils, ranging from 1.2 to 1.7, variable sulphur contents (0.5 to 1.12wt.%) and API gravity of 31 to 35 ° (Table 1). The gross composition of Family EQ oils is similar to the Family Q oils (Table 1). The saturate to aromatic ratio ranges from 1.11 to 1.56, sulphur content ranges from 0.8 to 2.14 wt.% and API gravity of Family EQ oils ranges from 33 to 35 ° (Table 1; Fig. 3). Family F oils are characterized by the highest saturate to aromatic ratio of the four families (Fig. 3), from 1.26 to 2.51, low sulphur contents (less than 1 wt.%) and API gravity ranging from 32 to 38 ° (Table 1).
C15-}-saturate fraction analysis Representative Cjs+saturate fraction gas chromatograms (SFGC) for each oil family are shown in Fig. 4. Pristane/phytane and pristane/nCl7 ratios are given in Table 1 for each oil. The saturate fraction gas chromatograms for all four oil families exhibit abundant n-alkanes in the C~5+range. A slight even n-alkane preference is
exhibited by Family E and EQ oils whereas Family Q oils do not indicate any carbon preference and Family F oils show a slight odd n-alkane preference (Fig. 4). Family E oils have the lowest pristane/phytane ratios of all four oil families, generally less than 1.2 (Fig. 5). Several of these oils exhibit a prominent unresolved complex mixture in the SFGC suggesting they have been biodegraded. Pristane/ phytane ratios range from 1.06 to 1.43 for Family Q oils and from 1.ll to 1.73 for Family EQ oils (Fig. 5). Family F oils have the highest pristane/ phytane values ranging from 1.18 to 2.50. The range in pristane/phytane ratios within each oil family may be due to facies variations within the source rock for Family Q, EQ and F oils and to biodegradation for Family E oils. Biomarker analysis
The biomarker distributions of the oils provide key evidence supporting the presence of four families of oils in the study area. Selected peak height ratios from GC-MS data for the four oil families are provided in Table 2.
774
F.A. Karavas et al. KEY O
FAMILY E
3.00 --
FAMILY Q •
FAMILY EQ FAMILY F
O O <> o
O
2.00
O
OO OO
I
eA+°+
w
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0.50
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% •
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1.00
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1.50
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i
2.50
PRISTANEIPHYTAN E
Fig. 5. Bivariate plot of pristane/phytane vs. the saturate/aromatic hydrocarbon ratio for each oil family. See key for oil family designations.
Terpane distributions show considerable variation between the four oil families (Fig. 6, Tables 2 and 3). Tricyclic terpanes are prominent in Family E, Q and EQ oils and are low in abundance in Family F oils. C23 tricyclic terpane/C30 hopane ratios for Family F oils are less than 0.25 with the exception of one sample with a ratio of 0.59 (see Table 2). C24 tetracyclic/C26 tricyclic terpane ratio is greater than 1 for Family Q, EQ and F oils and ranges from 0.67 to 1.42 for Family E oils. C29/C30 and T J T m ratios are less than or equal to one for all the oils. There is a C35 homohopane prominence in Family E, Q and EQ oils (C35/C34 ratio greater than 1) but no C35 prominence in Family F oils (Figs 6 and 7). Sterane distributions also show some variations among the oil families (Figs 8 and 9; Table 2). Oil families Q and EQ tend to have higher relative abundances of C21 steranes compared to Family E and Family F oils. C27 diasterane/Cz7 regular sterane ratios are greater than 2.00 for Family Q and EQ oils, less than 2.00 for Family E oils and range from 1.56 to 3.34 for Family F oils (Fig. 10). The C290~0~20S/C290~0~0~20R ratio of the oils ranges from 0.60 to 1.00.
The distribution of C27:C28:C29 o(flfl (20S + 20R) steranes also indicates some variation among the oil families (Figs 9 and 10; Table 2). Family E, Q and EQ are characterized by C29 > C27 > C28 distribution (Fig. 9), whereas most Family F oils have a C27~C28 < C29 distribution (from Glauconitic member reservoirs). A C29 > C27 > C28 distribution is observed for Family F oils from Cutbank reservoirs, however, the C27/C2~ ratios for all of the Family F oils (less than 1.57) are generally lower than those observed for Family E, Q and EQ oils (Table 2, Fig. 10). It is also noted that C30 4-desmethyl steranes are present on the m/z 217 fragmentograms of all four oil families indicating they are all from Devonian and younger sources (Moldowan et al., 1985, 1990). So-called "Q compounds" are unusual polycyclic (tri-, tetra-, penta-, hexa- and heptacyclic) alkane and aromatic hydrocarbons (Li et al., 1996, 1997). These compounds are useful as a correlation parameter, as their presence indicates a contribution from the Ostracode zone, the only unit to date in which they are known to occur (Riediger et al., 1997a). "Q compounds" have major fragment ions o f m / z 191,217, 218 and 259 (e.g. Figs 6(b) and (c),
Oil families in M a n n v i l l e G r o u p reservoirs
775
Table 2. Oil family designations and biomarker data for each oil sample Terpanes Sample
Family
23/30H 24T/26t
1 E 2 E 3 E 4 E 5 E 6 E 7 E 8 E 9 E 10 E 1t E 12 E 13 E 14 E 15 E 16 E 17 E 18 E 19 E 20 E 21 E 22 E 23 Q 24 Q 25 Q 26 EQ 27 EQ 28 EQ 29 EQ 30 EQ 31 EQ 32 EQ 33 EQ 34 EQ 35 EQ 36 F 37 F 38 F 39 F 40 F 41 F 42 F 43 F 44 F 45 F 46 F 47 F 48 F 49 F 50 F 51 FQ Rierdon extract
0.28 0.62 0.53 0.15 0.28 0.35 0.17 0.15 0.45 0.32 0.48 0.55 0.59 0.65 0.44 0.52 0.53 0.66 0.40 0.64 0.44 0.36 0.47 0.66 1.42 0.74 0.50 0.54 0.60 0.86 0.60 0.79 0.47 0.51 0.28 0.18 0.12 0.16 0.23 0.24 0.59 0.12 0.06 0.10 0.05 0.22 0.09 0.20 0.16 0.24 0.34 0.09
1.07 1.33 1.24 0.67 0.78 0.76 0.94 0.91 0.97 0.93 0.83 0.75 0.77 0.76 0.78 0.80 0.92 1.07 0.76 1.62 1.16 1.29 1.27 1.17 1.39 1.48 1.43 1.33 1.33 1.52 1.17 1.45 1.38 1.50 1.44 1.54 1.18 1.33 1.27 1.33 1.69 1.05 1.40 1.14 1.75 1.42 1.67 1.45 1.40 1.29 1.41 26.00
Steranes
ts/tm
29/30
35/34
0.58 0.70 0.60 0.44 0.47 0.49 0.43 0.48 0.48 0.54 0.46 0.48 0.52 0.50 0.46 0.50 0.55 0.53 0.50 0.56 0.50 0.60 0.70 0.64 1.05 0.70 0.61 0.65 0.63 0.65 0.68 0.69 0.87 0.57 0.52 0.69 0.73 0.48 0.71 0.74 0.76 0.67 0.56 0.54 0.53 0.65 0.60 0.82 0.70 0.67 0.65 0.33
0.55 0.76 0.69 0.65 0.75 0.79 0.60 0.59 0.65 0.62 0.69 0.72 0.73 0.72 0.64 0.64 0.64 0.70 0.64 0.73 0.74 0.58 0.87 0.79 0.85 0.76 0.65 0.78 0.81 0.83 0.89 0.86 0.48 0.66 0.49 0.49 0.49 0.66 0.55 0.51 0.63 0.48 0.42 0.46 0.39 0.58 0.38 0.48 0.47 0.51 0.98 0.46
1.00 1.00 1.14 0.89 1.07 I. 14 1.00 0.94 1.34 1.27 1.33 1.31 1.33 1.25 1.41 1.28 1.11 1.13 1.42 1.05 1.00 1.16 1.19 1.10 1.13 1.22 1.06 1.10 1.22 1.24 1.09 1.21 1.00 1.00 0.95 0.63 0.75 0.71 0.73 0.71 0.67 0.95 0.58 0.92 0.58 0.69 0.65 1.00 0.88 0.89 0.73 0.41
21/29~R dia/reg 0.66 0.89 1.08 0.31 0.37 0.48 0.39 0.41 0.98 0.61 0.97 0.97 0.96 1.07 0.93 1.03 0.97 1.02 0.85 1.40 0.77 0.92 1.00 1.32 3.09 1.57 1.41 1.19 t.26 1.70 1.15 1.83 0.94 1.55 0.78 0.39 0.37 0.45 0.80 0.91 1.64 0.38 0.22 0.31 0.30 0.85 0.54 0.57 0.58 0.81 0.66 0.66
1.38 1.61 1.98 0.75 0.92 1.06 0.74 1.07 1.17 1.34 1.29 1.16 1.24 1.33 1.27 1.33 1.60 1.52 1.22 2.54 1.76 1.74 2.10 3.36 3.88 2.43 2.20 2.54 2.25 2.65 2.15 3.14 2.89 2.45 2.10 1.72 1.96 1.84 3.06 3.34 3.22 2.35 1.56 1.74 1.73 1.89 1.96 2.20 2.26 2.64 1.72 0.43
29s/r
27
28
29
C27/28
28/29
0.76 0.75 0.83 0.68 0.79 0.87 0.77 0.74 0.79 0.76 0.80 0.82 0.81 0.82 0.77 0.76 0.76 0.80 0.78 0.84 0.69 0.68 0.75 0.74 0.83 0.84 0.93 0.89 0.80 0.82 0.78 0.83 0.60 0.79 0.82 0.75 0.74 0.73 0.73 0.79 1.00 0.78 0.69 0.73 0.70 0.84 0.83 0.75 0.76 0.74 0.81 0.63
30 31 32 30 33 36 28 29 33 33 34 34 34 35 34 34 35 34 32 33 31 33 27 30 32 32 32 29 34 32 34 33 29 32 27 26 27 26 28 30 35 23 24 23 23 30 27 31 28 30 27 25
21 23 20 21 22 22 21 22 20 21 20 21 20 21 21 22 22 22 22 21 21 20 22 22 23 22 22 25 21 23 20 21 21 22 22 24 22 22 26 24 22 26 25 23 24 22 27 20 25 23 21 22
49 45 48 49 46 42 51 49 46 46 46 45 45 44 46 45 43 45 46 46 48 47 51 48 44 46 47 47 46 44 46 46 50 46 51 50 51 52 46 46 42 51 51 54 53 48 47 49 48 46 52 53
1.38 1.35 1.56 1.41 1.50 1.59 1.30 1.31 1.63 1.55 1.74 1.63 1.68 1.71 1.64 1.54 1.61 1.58 1.48 1.56 1.52 1.63 1.22 1.38 1.39 1.48 1.44 1.16 1.63 1.39 1.72 1.57 1.42 1.44 1.23 1.07 1.21 1.16 1.08 1.24 1.57 0.89 0.98 1.02 0.99 1.36 1.01 1.57 1.12 1.30 1.31 1.14
0.44 0.52 0.43 0.43 0.47 0.53 0.42 0.45 0.44 0.45 0.42 0.47 0.45 0.46 0.45 0.49 0.50 0.48 0.47 0.47 0.43 0.43 0.43 0.46 0.53 0.48 0.47 0.53 0.45 0.52 0.44 0.45 0.41 0.48 0.44 0.48 0.43 0.42 0.56 0.53 0.53 0.52 0.49 0.42 0.44 0.46 0.57 0.41 0.52 0.50 0.40 0.42
Q/R
1.35 4.35 3.66 0.57
0.47
Terpanes: 23/30H = C23tricyclic terpane/C30hopane; 24T/26t = C24tetracyclic terpane/C26tricyclic terpane; Ts = C27 18~(H)-trisnorneohopane; Tm = C27 17~(H)-trisnorhopane; 29/30 - C29norhopane/C30hopane; C35/C34=C35/C34 homohopane; Steranes: dia/reg = C27 13/~(H),17~(H)-diasterane (20S)/Cz7 5~(H),14e(H),17~(H)-sterane (20R); S = C29 5e(H),14e(H),17e(H)-sterane (20S); R = C29 5~(H),14~(H),17x(H)-sterane (20R); C27,C28,C29=C27,C28,C29 5e(H),14/:~(H),17/~(H)-sterane ((20S + 20R)/2) (respectively); Q - Q compounds (m/z 217); R - C29 5~(H),14~(H),17e(H)-sterane (20R). 8(b)
and
cantly
(c) a n d
after
analytical
C35
9(b)
and
technique).
Their
presence elution
using
is d e t e r m i n e d
by anomalous
fragmentograms
t h e o i l s is v a r i a b l e ratio
of "Q
Family
(c); T a b l e trations
2). S a m p l e s
of "Q
whereas
in Family
EQ
trations
times
(39 min),
EQ (Q/R
and
are
samples (Q/R
designated
designated 26
and
29
of 0.57 and
as Family
as
Family
have
lower
0.47; Table
Q
oils,
concen-
2) a n d
are
EQ.
o n t h e m/z 2 1 7 DISCUSSION
present in by the
sterane
for
oils (Fig. 8(b) and
23 to 26 have higher concen-
compounds"
2)
b y Q * in F i g . 8 ( b )
to C29c~20R
Family
Table
our
and can be characterized
some
signifi-
using
of "Q compounds"
compound"
Q and
peaks
(indicated
a n d (c)). T h e a m o u n t
elute
(45 min
shorter
oils, analyzed mass
(c). T h e y
homohopane
greater
than
0.5;
Thermal maturity Biomarker determining
parameters
can
be
very
useful
the relative degree of thermal
o f a n oil. C o m m o n l y ~0~20S/C290~0~20R
used parameters and
Ts/Tm a r e
in
maturity
s u c h a s C29 often
used
to
776
F . A . K a r a v a s et al.
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Oil families in Mannville Group reservoirs Table 3. Peak identificationsfor terpanes (m/z 191;Figure 5), and steranes (m/z 217; Figure 6; m/z 218; Figure 7) used in selected ratios (Table2)
777
towards the north with the least mature oils in Little Bow field and the most mature Family E oils in Jumpbush field (Fig. 11). Family Q oils are found along the northern edge of the study area in Townships 19 and 20. All Family Q oils are of relatively similar maturity (C298/R of 0.74 to 0.83; Table 2) and exhibit a slight increase in maturity towards the east from Gladys field towards Queenstown field (Fig. 11). Family EQ oils are also found in the northern portion of the study area (Townships 18 to 20 and Ranges 18 to 23 W4). Family EQ oils increase in maturity towards the east from Shouldice field to Jumpbush field. Family F oils are of relatively similar maturity (Table 2), with no clear maturity trend. T~/Tm ratios are useful for determining relative thermal maturity within a particular oil family (e.g. Fig. 12). Ts/Tm ratios of families E, Q and EQ give maturity trends that are similar to those exhibited by the C298/R ratio. However, Ts/Tm ratios for Family F oils exhibit a trend in maturity whereas the C298/R ratios did not. There is an overall increase in thermal maturity from Farrow field southward to Carmangay field, based on TjTm ratios. An increase in thermal maturity with depth is also indicated by the Ts/Tm ratios for oils from Long Coulee oil field. Oils in the Glauconitic member (Upper Mannville) reservoirs are less mature
l c23 tricyclicterpane 2 C24 tetracyclicterpane 3 C26 tricyclicterpane 4 18~(H)-trisnorneohopane (Ts) 5 17~(H)-trisnorhopane (Tm) 6 17ct(H),21 fl(H)-norhopane 7 17~(H)-21~(H)-hopane (C30) 8 22(S) and 22(R) 17~(H),21fi(H)-tetrakishomohopane(C34) 9 22(S) and 22(R) 17~(H),21/3(H)-pentakishomohopane(C35) a C2~ sterane b 13~(H),17~(H)-cholestane(20R) c 5~(H),14~(H),l 7~(H)-cholestane(20R) d 24-ethyl-5~(H),14~(H),17~(H)-cholestane (20R) e 24-ethyl-5~(H),14~(H),l 7c~(H)-cholestane(20S) Q, Q* Q compounds r C2v 5~(H),14fl(H),17~(H)-cholestane(20Sand 20R) s C2s 5~(H),14/:t(H),17/3(H)-methylcholestane(20Sand 20R) t C29 5~(H),14/:t(H),17/:t(H)-ethylcholestane(20Sand 20R) u C30 5~(H),14p(H),17fl(H)-propylcholestane(20S and 20R)
assess the thermal maturity of oil (Waples and Machihara, 1990; Requejo, 1992). Relative maturity of the oils can be described by the C29S/R ratio for each oil Family. Family E, EQ and F oils exhibit a larger range in maturity than do Family Q oils (Table 2). Family E oils occur in the southeastern portion of the study area (Townships 14 to 17 and Ranges 18 to 21 W4; Fig. 11). These oils increase in overall maturity
1.60 - KEY
• O' I,g
FAMILYE FAMILYQ FAMILYEQ FAMILYF
O°
0
• •
1.20 @
o A-I-
o
0
•
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h
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•
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O
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0.40
'
0.20
I
0.40
~
I
'
0.60
I
J
0.80
1.00
029 NORHOPANE/C30 HOPANE
Fig. 7. Bivariate plot of C29 norhopane/C30 hopane vs. C35/C34 homohopane ratios. See key for oil family designations.
778
F.A.
et al.
Karavas
. C~ ~
e'~
"0
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e~
E
A
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Oil families in Mannville Group reservoirs
779
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.=
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,,,, ~E ,-r
O Ill
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.~
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E
E
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7
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780
F.A. Karavas et al. KEY
@ ÷
1.80
FAMILY E FAMILY Q FAMILY EQ
I
<>
@
FAMILY F
Q•
1.60 •O O
m 1.4o
÷
W I-ffl
•
• /
u 1.20-
°
/
/
t
O
O
O
O
1.00
O O
0.80
0.00
1.00
2.00
3.00
4.00
DIASTERANE/REGULAR STERANE
Fig. 10. Bivariate plot of C27/C28 sterane vs. Czv diasterane/regular sterane ratios. Family F oils have lower C27/C2~ratios. See key for oil family designations.
than oils in the Cutbank Mannville) reservoirs.
member
(Lower
Oil-source rock correlation
Petroleum systems of the Lower Cretaceous Mannville Group in southern Alberta are more complex than previously described in the literature (e.g. Deroo et al., 1977; Creaney and Allan, 1990). Through comparison of the geochemistry of the oils to available source rock information from previous studies, oil-source rock correlations are proposed for each oil family. The Family E oils display biomarker characteristics that are very similar to the Cutbank oils from Montana described by Dolson et al. (1993). The Cutbank oils were suggested as having an Exshaw source. An Exshaw Formation rock extract exhibits a C24 tetracyclic/C26 tricyclic ratio < 1.0, C23 tricyclic terpane/C30 hopane ratio > 0.40, C35 homohopane prominence and a C 2 9 > C 2 7 > C 2 8 sterane distribution (Dolson et al., 1993) and the Exshaw is thus considered as the likely source rock for the Family E oils.
Family E oils are also somewhat similar to the Family C.~w ("Lodgepole source") oils from southwestern Saskatchewan (Osadetz et al., 1994), although the Family C,w oils are less mature than the Family E oils. There are three possible explanations to account for these similarities. Family E and C,w oils are thermal maturity variations within the same family and were all derived from (A) mature Lodgepole Formation sources in North Dakota, or (B) mature Exshaw Formation sources in western Alberta, or (C) the source rock kerogen for the Family E and C~,~ oils are very similar. Derivation of Family E oils from the Lodgepole Formation in the central Williston Basin is unlikely for several reasons. First, Lodgepole-derived oils would have had to migrate in excess of 600 km from source to trap in southern Alberta, moving across the Sweet Grass Arch and then down-dip into the southern Alberta reservoirs. In addition, one would expect to see more mature oils (i.e. Family E) closer to the source whereas the opposite is observed. More geochemical data from the potential source rocks is required to decide whether scen-
Oil families in Mannville Group reservoirs
R26W4
R24W4
R22W4
781
R20W4
....
~ ~ ~::,.....
R18W4 "
F ~,: ,~:~,:;~~ , ~
I,~f1~11I!!!
i=
,.
t~!:~s,:o.
.,.............................
i
,,; ,~,,~..................
~i~:~f i~MaJ(: 'villa i i4"Z:
Absent ' ~ :: :::::;::::::~S~I~:'
~,,~
!:;i;:;:;:;i;;:;~::~;;:%:;:; ;~,k;;;,;::~':;::;;::;:;%: ~
LEGEND
~ , ~ . ~
- - - Rierdon Subcrop edge •
Family Family Family Family
[]
[] o
~ ~;~££ ~I~:~F~ %~
.,m.
E oils Q oils E Q oils F oils
~::::~:~:;::::: :,~
i
~,~
....::X:~:~i:~
:,
:
:~~,,~,~,~
:~ ~ : ~ ' ~ "
..........
:~ ;~:~ :~
~:~
::::::::::::::::::::: +'~,~,::~:~:~:~:~::~ :::~:................:~ =~J ~ , ; ; ~ 1 ~ .................~"...........................................................
ii;,,
SCALE 0
15 km
Fig. 11. Map showing the study area and oil field designations. Dashed line indicates approximate location of Jurassic subcrop edge. Distribution of oil Families is indicated in relation to the oil fields sampled. 4.00 -mnCretsing thermal aturity
4. I.g Z
O
3.00
I,LI
@
I-0q
•
•
Iv
o-S O
8
2.00
•
•
O @•A~O
,?, Z ILl I--
KEY
f°
<
m
1.00 1 /
/
4"
O •~.
Family E Family Q
•
Family EQ Family F
L !
0 100 ~
-
i
0.20
-
I .......... ~i
0.40
~
--]-
....
I
0.60
]
0.80
I
I
1.00
I
]
1.20
TslTm Fig. 12. Bivariate plot of Ts/T,~,,1 vs. diasterane/regular sterane ratios, showing relative increase in thermal maturity of the oil families. See key for oil family designations.
782
F.A. Karavas et al.
ario (B) or (C) is the most likely explanation for the similarities between the Family E and Csw oils. Family Q oils have biomarker characteristics indicating an Ostracode Zone source (Riediger et al., 1997a). Ostracode Zone rock extracts display considerable variability in their biomarker distributions (e.g. Pr/Ph ranging from 1.2 to 5.4), however, they show sterane distributions where C29 > C27 and C28 and some samples have high diasterane to regular sterane contents (Riediger et al., 1997a). The Ostracode Zone is also the only source rock from the Western Canada Sedimentary Basin known to contain the unusual polycyclic alkane and aromatic hydrocarbons referred to as "Q compounds" (Riediger et al., 1997a). The relatively high concentrations of "Q compounds" in the Family Q oils is considered a clear indication of an Ostracode Zone source for these oils. Family EQ oils represent a mixture of Family Q and Family E oils. The Family EQ oils contain "Q compounds" in low concentrations (Q/R less than 0.5; Table 2) suggesting some contribution from the Ostracode Zone. Many of the biomarker characteristics of these oils are similar to Family E oils and suggest that they have received a significant contribution from the Upper Devonian-Mississippian Exshaw Formation (e.g. C35 homohopane prominence). The distribution of Family EQ oils in the study area is consistent with mixing of Family Q oils from the west and Family E oils from the south (Fig. 11). Mixed Ostracode Zone and Exshaw Formation oils are present in Lower Cretaceous reservoirs in the Provost field (Riediger et al., 1995), some 200 km structurally updip, to the northeast of the study area. Provost oils are correlated to the Family EQ oils described here (i.e. in Jumpbush, Queenstown and Shouldice fields), implying significant southwest to northeast migration of this oil family. Migration almost certainly occurred within the Lower Cretaceous Mannville Group itself, presenting the possibility of entrapment of some of this oil along the migration fairway. The biomarker characteristics of Family F oils are not readily correlated with any previously documented source rock in the Western Canada Sedimentary Basin. Riediger et al. (1996, 1997b) suggested that the source for oils in the Long Coulee oil field were from a Mesozoic source rock, younger than middle Jurassic in age and suggested shales within the Upper Jurassic Fernie Group as potential sources for these oils. Limited geochemical data suggest that shales of the Upper Jurassic Rierdon Formation (Fernie Group equivalent; Fig. 2) in southern Alberta may be the source rock for Family F oils. The Rierdon shales have TOC values up to 13% and HI values as high as 342 mg/ gTOC. An extract from the Rierdon Formation (Table 2) shows a low abundance of tricyclic ter-
panes, no C35 homohopane prominence and a sterane distribution of C27 < C28 < C29, characteristics which are consistent with the biomarker distributions of the Family F oils. Other biomarker characteristics of the Rierdon extract (e.g. low diasterane/regular sterane ratio and Ts/Tm ratios relative to the Family F oils) may reflect thermal maturity variations. Nonetheless, additional rock extracts from the Rierdon and other shaly horizons within the Jurassic and Lower Cretaceous interval are required in order to demonstrate the source of the Family F oils. Family F oils occur structurally downdip (west) of the Jurassic (Rierdon) subcrop edge (Fig. 12). The Rierdon shales may have acted as the source for the Family F oils, but they are also an important regional seal, preventing upward migration of Family E oils into Lower Cretaceous reservoirs lying above the Rierdon. As noted above, Family EQ oils, representing a mix of Family E and Family Q oils, are common within the study area. Very little mixing of other oil families has been observed, with exception of one oil from the Farrow oil field (sample 51), which displays biomarker characteristics suggesting mixing of Family F and Family Q oils. CONCLUSIONS Four oil families are present in Lower Cretaceous reservoirs of southern Alberta. Family E oils are characterized by low saturate/aromatic ratios, low Pr/Ph ratios and a C35 homohopane prominence and are likely derived from the Upper DevonianMississippian Exshaw Formation. Family Q oils are characterized by the presence of Q compounds and are derived from the Lower Cretaceous Ostracode Zone. Family EQ oils have characteristics suggesting mixing of Family E and Q oils. Family F oils are characterized by high saturate/aromatic ratios, Pr/Ph ratios > 1.2, low C27/C28 sterane ratios and are possibly derived from shales of the Upper Jurassic Rierdon Formation. Additional biomarker data from the Rierdon Formation are required to confirm this preliminary correlation. Thermal maturity of Family E oils increases from the south of the study area towards the northeast. Family Q and EQ oils increase in maturity from the west towards the eastern portion of the study area. Family F oils increase in maturity from Farrow field southward to Carmangay field and increase with depth in the Long Coulee field. Acknowledgements--We would like to thank Sneh Achal
and Marg Northcott of the Organic Geochemistry Labs, Geological Survey of Canada (Calgary) for expert technical assistance. The study was supported by NSERC Individual Research and New Faculty Support Grants to C. L. Riediger. The Canadian Society of Petroleum Geologists and the following companies are also thanked
Oil families in Mannville Group reservoirs for their support of the southern Alberta Oil Study: Bow Valley Energy, Canadian Hunter Exploration, Morrison Petroleums, Numac Energy, Ocelot Energy, PanCanadian Petroleums, Petrel Roberston, Pinnacle Resources, Poco Petroleums and Talisman Energy. Steve Creaney and Erdem Idiz are thanked for their constructive reviews of the manuscript. REFERENCES
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