Delineating compositional variabilities among crude oils from Central Montana, USA, using light hydrocarbon and biomarker characteristics

Organic Geochemistry 33 (2002) 1343–1359 www.elsevier.com/locate/orggeochem

Delineating compositional variabilities among crude oils from Central Montana, USA, using light hydrocarbon and biomarker characteristics Mark Obermajera,*, Kirk G. Osadetza, Martin G. Fowlera, James Sillimanb, William B. Hansenc, M. Clarkd a

Geological Survey of Canada, 3303-33rd Street NW, Calgary, AB, Canada T2L 2A7 b Texas A&M University, Corpus Christi, TX, USA c Jireh Consulting Services, PO Box 3572, Great Falls, MT 59403, USA d Beartooth Oil & Gas Co., PO Box 2564, Billings, MT 59403, USA

Abstract Three compositionally distinctive groups of oils identified in central Montana by biomarker analyses are also recognized by the unique compositions of their light hydrocarbon (gasoline range) fraction. The majority of oils produced from Paleozoic pools (Pennsylvanian Tyler–Amsden interval) group into one broad category based on the distribution of C20–C40 biomarkers. These oils not only have the lowest Paraffin Indices and relative concentrations of normal heptane, but are readily distinguishable from the other compositional groups by using selected ‘‘Mango’’ parameters. However, the biomarker-based subdivision of this group into at least two sub-families is not reflected in the gasoline range fraction, suggesting little effect of source rock host lithology on the distribution of C5–C8 hydrocarbons. Oils occurring predominantly in Jurassic–Cretaceous reservoirs display different biomarker and gasoline range characteristics, including Paraffin Indices, K1 parameter and relative concentrations of C7 compounds, and are classified in two separate compositional categories. In contrast to oils from the Tyler–Amsden interval, the oils produced from the Mesozoic strata are amongst the most mature oils in the study area. The unique biomarker/light hydrocarbon signatures are likely due to different source organic matter. Secondary alteration of oil due to biodegradation and migration, although recognized, appears less significant. The results indicate the overall usefulness of gasoline range compositions in delineating compositional affinities of crude oils in central Montana, clearly suggesting that the oils found in Paleozoic and Mesozoic reservoirs belong to different petroleum systems. Crown Copyright # 2002 Published by Elsevier Science Ltd. All rights reserved.

1. Introduction Central Montana (Fig. 1) is a small but prolific petroleum province. It is one of the oldest hydrocarbon producing regions in Montana, with the first oil discovery in 1919 (Manley and Kim, 1985) and commercial oil production established in 1920 (Fanshawe, 1985). By the end of 1999, the total cumulative oil production in

* Corresponding author. E-mail address: [email protected] (M. Obermajer).

central Montana exceeded 24106 m3 (150 MM barrels), with almost 75% of the total coming from the Pennsylvanian Tyler–Amsden interval. The annual production in this region reached over 600,000 barrels in 1999 (source—Montana Oil and Gas Annual Review 1999). Most of the limited number of organic geochemical studies conducted in central Montana focussed on the most prolific Mississippian–Pennsylvanian interval. A close geographical and geological co-occurrence of oil accumulation in the Tyler Formation and petroliferous limestone of the underlying Mississippian Heath Formation led Kranzler (1966) to the conclusion that the

0146-6380/02/$ - see front matter Crown Copyright # 2002 Published by Elsevier Science Ltd. All rights reserved. PII: S0146-6380(02)00118-3

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Heath Formation was the most likely source of the oil found in Tyler reservoirs. The excellent petroleum source potential of the Heath Formation was geochemically documented by Rinaldi (1988), who also determined that oils in the Tyler Formation belong to a single oil family. Integrating results of the geochemical oil–source rock correlation study with local geology, Rinaldi (1988) defined the Heath Formation as a primary source of these oils. A more complete characterization of the so-called Heath–Tyler(!) petroleum system, with conventional biomarker-based correlation supported by stable carbon isotope data and a generation–migration– accumulation model for the Tyler oil, was given by Cole and Drozd (1994). Aram (1993) extended the Heath Formation source model to include virtually all oil produced from the Tyler and Amsden formations in central Montana. That study, although concentrating mostly on the characterization of possible petroleum source rocks, also identified three oil families, based on oil gravity, viscosity, shape of chromatograms and abundance of biomarkers (no specific compositional details on the geochemistry of oils were provided). Aram (1993) indicated that high gravity oils from the Cat Creek and Crooked Creek– Mason Lake fields could be distinguished from the so called ‘‘Tyler–Amsden’’ oil family. These oils showed no geochemical affinities with the Paleozoic oils and were grouped into two separate families, Cat Creek and Mason Lake–Crooked Creek families. The sources of these two other oil families were not recognized, although it was suggested that these oils might have originated from multiple source rocks. The purpose of this study is to geochemically examine and classify a suite of crude oil samples from central Montana. A number of standard biomarker parameters, including n-alkanes, isoprenoids, terpenoids and steroids, used to identify the genetic affinities of the analyzed oils is supported by the distribution of lower molecular weight (C5–C8) hydrocarbon compounds. The application of light hydrocarbons can be extremely useful for geochemical correlations that include high gravity crude oil, such as those from Cat Creek or Mason Lake fields, where higher molecular weight biomarkers occur in relatively low concentrations. This technique is relatively rapid and inexpensive (Thompson, 1983; Mango, 1990; ten Haven, 1996), and it has been successfully applied as a supplementary component in geochemical correlations of crude oils in the adjacent Williston Basin (Jarvie and Walker, 1997; Obermajer et al., 2000; Jarvie, 2001). Although no source rock material was available during the present study, possible oil–source rock relationships, based on the geochemistry of oils, are also briefly discussed. These geochemical analyses will allow a better understanding of the petroleum systems in central Montana providing a framework for appraising the future hydrocarbon potential of this region.

2. Geological setting A large East–West elongate geological structure, known as the Central Montana trough (Fig. 1), persisted over much of Paleozoic and Mesozoic in central Montana. Initiated in the Precambrian as a rift feature, it has undergone a very complex tectonic history with numerous episodic movements and faulting. During reactivation and tectonism in the Late Paleozoic the Big Snowy trough formed as a more active northern segment (Lageson, 1985), separating the Central Montana trough into two parallel features (Fig. 1). In the eastern portion of the trough close to the western limits of the adjacent Williston Basin, an uplift area persisted (Central Montana or Big Snowy Uplift—Lageson, 1985, Rinaldi, 1988). The central Montana oil province that occurs in this area can be subdivided into tectonic regimes: a more stable Bull Mountains basin in the South and a more active Montana aulacogen (Central Montana platform) in the North (Shepard, 1993; Luebking et al., 2001). The stratigraphy of central Montana is well established. The bedrock consists of a variety of predominantly sedimentary rock, mostly of Mississippian to Cretaceous age. While carbonate, evaporite and fine clastic sedimentation dominated in the Paleozoic, Mesozoic strata are dominated by clastic rocks. Economic accumulations of oil are found in several stratigraphic units, including the Pennsylvanian Tyler– Amsden, Jurassic Ellis–Morrison and Lower Cretaceous Cat Creek intervals (Fig. 2). Oil fields in central Montana occur along four linear trends of anticlines forming reservoirs that are believed to be fluvial in origin based on their geometry and sedimentological characteristics (Rinaldi, 1988). Both, stratigraphic and structural traps are present in central Montana, although the former type (lenticular sand bodies) is a more common trapping mechanism. The Madison Group carbonates were the first extensive late Paleozoic rocks series to be deposited in central Montana. Later in the Mississippian period, tectonism re-shaped this area by uplifting the Alberta and Wyoming shelves (Cole and Drozd, 1994). Erosion of these shelves supplied sedimentary material for deposition of the Kibbey Formation at the base of the Big Snowy Group. The Upper Mississippian Heath Formation, thought to be the primary source rock in central Montana, occurs at the top of the Big Snowy Group (Kranzler, 1966) and is composed of interbedded limestone and dark gray to black shale (indicative of euxinic marine conditions), mudstone, gypsum (representing evaporitic environments), and a few coal layers (originating from coastal swamps) deposited during the Upper Mississippian period (Rinaldi, 1988). Unconformably overlying the Heath Formation is the Pennsylvanian Tyler Formation—a major oil reservoir. Its base consists of

M. Obermajer et al. / Organic Geochemistry 33 (2002) 1343–1359

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Fig. 1. Main geological elements in central Montana, geographic distribution of oil samples and stratigraphic relationship in the study area (cross-section simplified from Peterson and MacCary, 1987).

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are overlain by the carbonates of the Amsden Group and clastics of the Tensleep Formation. Marine transgressions occurred during the Middle Jurassic and Lower Cretaceous resulting mainly in deposition of a variety of clastic rocks. The Jurassic strata include marine rocks of the Ellis Group overlain by continental deposits of the Morrison Formation. The Cretaceous section consists of the Kootenai-Dakota clastic rocks overlain by more extensive deposits of the Colorado and Montana groups. Commercial quantities of oil were found in some of these strata (Swift and 1st Cat Creek sandstone). Much of this interval was removed by additional mountain-building and erosion that occurred during the Middle Jurassic and from the Late Cretaceous through the Paleocene (Laramide orogeny). Bed thinning and unconformities are common in this interval. These last two tectonic episodes decreased the size of the trough and further modified its dimensions. Moreover, a structural inversion occurred during the Laramide orogeny (Kranzler, 1966).

3. Samples and experimental

Fig. 2. Generalized stratigraphy of the Mississippian to Cretaceous strata in central Montana.

sandstone overlain by sequences of mudstones, sandstones, and minor coals. Tyler Formation oil accumulations occur at the up dip edge of the basal sandstone layer where it is in direct contact with the underlying Heath Formation limestone (Rinaldi, 1988). Tyler strata

Sixteen crude oil samples from Pennsylvanian to Cretaceous reservoirs were analyzed geochemically in the present study. Sample locations are shown on Fig. 1 with additional information given in Table 1. Gasoline range hydrocarbons (iC5–nC8) were analyzed on a HP5890 Gas Chromatograph connected to an OI Analytical Sample Concentrator. A 60 m DB-1 fused silica column (0.32 mm internal diameter) was used. The temperature, initially held at 30  C for 10 min, was programmed from 30 to 45  C at a rate of 1  C/min, and then held for 25 minutes. Eluting hydrocarbons

Table 1 List of oil samples analyzed in the present study Group

Lab.no

Location

Well name

Field

Reservoir

1a

2259 2261 2262 2263 1498

T13NR33E T11N R25E T13N R28E T13N R28E T11NR32E

#2 Clesson Mang #4–8 #1 Bethel Cat Creek #1 Continental-1 Harris

Breed Creek Winnet Junction Rattlesnake Butte Rattlesnake Butte Sumatra

Tyler Tyler Amsden Cat Creek Tyler

1b

2264 2260 1483 1486

T10NR27E T10NR29E T11N R31E T11NR32E

BN #11–5 Haugen 34–14 True Oil RP Oliver-4 State Well McAlister Fuel Co#5

Big Wall Melstone Ivanhoe Dome Sumatra

Tyler Tyler Tyler Tyler

2

2258 1494 2265 2266 1455

T6NR32E T10NR27E T15NR30E T15NR29E T8NR21N

Amma Botts Battery Texas-1B N.Pacific Carrell Oil Unit 1 Tank Battery Klinefelter #1

South Wolf Springs Big Wall Cat Creek Cat Creek Women’s Pocket

Amsden Tyler Ellis Cat Creek Ellis

3

2267 2437

T4N R25E T8NR24E

Not available Not available

Crooked Creek Mason Lake

Dakota 1st Cat Creek

M. Obermajer et al. / Organic Geochemistry 33 (2002) 1343–1359

1347

Fig. 3. Representative 210  C+ saturate fraction gas chromatograms showing variations in the n-alkane profiles in the analyzed oils. Pr, pristane; Ph, phytane; 15, 20 and 25, C15, C20 and C25 normal alkanes.

were detected and quantitatively determined using a flame ionization detector. The oils were distilled and the fraction boiling above 210  C was deasphalted and then fractionated using open column liquid chromatography. Saturated hydrocarbons were then analyzed using gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). A Varian 3700 FID gas chromatograph equipped with a 30 m DB-1 column coated with OV-1 (dimethylpolysiloxane, 0.25 mm internal diameter and helium as the mobile phase), temperature programmed from 50 to 280  C at a rate of 4  C/min and then held for 30 min at the final temperature, was used for all analyses. The resulting gasoline range (GRGC) and saturate fraction gas chromatograms (SFGC) were integrated using Turbochrom software. Saturate fraction GC–MS analyses were performed on a VG 70SQ mass spectrometer coupled with an HP gas chromatograph employing a 30 m DB-5 fused silica column (methylpolysiloxane, 0.32 mm internal diameter) for GC separation. MS operating conditions were 70 eV ionization voltage, 300 mA filament emission current and interface temperature of 280  C. The GC oven initial temperature was held at 100  C for 2 min and then increased at 40  C/min to 180  C and at 4  C/min from 180 to 320  C, then held for 15 min at

320  C. The instrument was controlled by an Alpha Workstation using Opus software. Terpane and sterane ratios were calculated using m/z 191, m/z 217 and m/z 218 mass fragmentograms.

4. Geochemistry of oils 4.1. Composition of higher molecular weight (C13–C40) fraction Representative saturate fraction gas chromatograms (SFGC) and mass fragmentograms are shown on Figs. 3 and 4 while selected geochemical parameters are listed in Table 2 and shown on Figs. 5–7. The analyzed oils belong to three categories corresponding broadly to groups previously reported from this area (Aram, 1993). Although the compositional groups identified during the present study do not show any particular correlation with the stratigraphic horizons of their reservoirs, oils from Pennsylvanian Tyler–Amsden interval share some geochemical similarities while the compositions of oils from Mesozoic reservoirs are more variable. The first compositional group (Group 1) consists of all but two oils from the Pennsylvanian reservoirs and a

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Fig. 4. Representative m/z 191 and 218 mass fragmentograms of the saturate fraction of the analyzed oils. C23, C23 tricyclic terpane; C30, hopane; G, gammacerane; C35–17a(H); 21b(H) 20S+20R- C35 homohopane; A, C27 diahopane; B, 17a(H)-C30 diahopane; !, C31–C34 diahopanes; C27, C28 and C29, C27 and C28, C29 regular steranes.

M. Obermajer et al. / Organic Geochemistry 33 (2002) 1343–1359

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Fig. 5. Biomarker cross-plot showing C35 and C34 homohopane prominence in the investigated oil samples. The outlines, showing typical fields incorporating most of the oils from each oil family, are intended for visual approximation only. 33h, 17a(H), 21b(H) 20S+20R-C33 homohopane; 34h, 17a(H), 21b(H) 20S+20R-C34 homohopane, 35h, 17a(H), 21b(H) 20S+20R-C35 homohopane.

Fig. 6. Biomarker cross-plot of C29 norhopane/C30 hopane ratio (29h/30h) versus Ts/Tm ratio for the analyzed oil samples from Central Montana. The outlines, showing typical fields incorporating most of the oils from each oil family, are intended for visual approximation only (see Table 2 for ratio definitions).

single Cat Creek oil from the Rattlesnake Butte Field. The SFGCs of these oils show relatively broad n-alkane profiles and abundant acyclic isoprenoids (including C20+ members), with Pr/nC17 and Ph/nC18 ratios typically greater than 1.0. With one exception, pristane/ phytane ratios are less than 1.0, suggesting anoxic depositional conditions for their source rocks (Didyk et al., 1978; Peters and Moldowan, 1993). Most oils in this group show minor odd/even carbon number preference. Despite overall similarity of the SFGCs, there are some significant differences in the biomarker compositions amongst Group 1 oils, that allow this group to be subdivided into two compositional categories, 1a and 1b types. The Group 1a oils display high concentrations of C31–C35 extended hopanes with prominent C34 homohopane and abundant gammacerane (Fig. 4), compounds highly specific for hypersaline environments (Moldowan et al., 1985; Clark and Philp, 1989). The concentration of C23 tricyclic terpane is low relative to hopane. The relative proportion of regular steranes shows a consistent increase with increasing carbon number (C27
The oils from the other compositional subgroup, Group 1b, also have relatively high concentrations of homohopanes, similar to the 1a type, but there is no C34 homohopane prominence (Fig. 5). The relative concentrations of C23 tricyclic terpane and trisnorhopanes are also similar. There are also significant differences in the sterane distributions between these two subgroups. The Group 1b oils contain higher amounts of diasteranes as well as C21 and C30 regular steranes compared with the 1a type oils, and display somewhat higher C29 regular sterane isomerization ratios. But perhaps more importantly, their proportion of C27:C28:C29 regular steranes is quite dissimilar (Fig. 7), with a V-shape profile on the m/z 218 mass fragmentograms displaying a predominant C27 member (Fig. 4). Such a V-shaped C27:C28:C29 regular sterane distribution is very characteristic for another compositional group (Group 2) which includes oils from Jurassic reservoirs (Woman’s Pocket and Cat Creek fields) and two Pennsylvanian oils. The two Cat Creek oils have elevated saturated/aromatic HC ratios and are very light, characteristics which typically indicate high thermal maturity. This is not fully confirmed by the gasoline range composition and SFGCs which, despite showing a smooth n-alkane profile with very low amounts of C20+ members, have high concentrations of acyclic isoprenoids and Pr/Ph ratios of less than 1.0. Two other oils from this group have much broader n-alkane profiles and minor even to odd predominance within the C18–C22 range.

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Fig. 7. Ternary diagram showing normalized relative abundance of C27, C28 and C29 regular steranes based on abb isomers in oils from Central Montana.

Table 2 Selected geochemical ratios in the investigated oil samples Group Lab.no Pr/Ph Pr/17 Ph/18 S/(S+R) b/(a+b) dia/reg 21/29

27:28:29 Ts/Tm 29/hop gam/hop 23/hop 34/33 35/34

1a

2259 2261 2262 2263 1498

0.93 0.80 0.76 0.75 1.06

1.66 0.84 2.14 1.91 1.23

2.01 1.24 2.77 2.80 1.40

0.46 0.44 0.45 0.44 0.49

0.50 0.39 0.43 0.41 0.55

0.99 0.38 0.48 0.48 1.42

0.36 0.21 0.24 0.21 0.83

23:29:48 23:35:42 24:33:43 24:33:43 25:32:43

0.73 0.51 0.74 0.77 0.88

0.50 0.62 0.48 0.47 0.37

0.42 0.48 0.64 0.63 0.35

0.30 0.11 0.26 0.22 0.28

1.70 1.78 1.67 1.70 1.30

0.44 0.76 0.53 0.53 0.74

1b

2264 2260 1483 1486

0.82 0.95 0.96 0.86

1.17 1.01 1.44 1.65

1.60 1.21 1.66 2.33

0.48 0.51 0.48 0.45

0.56 0.60 0.53 0.41

1.63 2.26 1.89 0.93

0.60 1.07 0.89 0.37

40:23:37 42:20:38 45:20:35 49:20:31

0.83 1.39 1.40 1.02

0.41 0.36 0.40 0.43

0.27 0.19 0.24 0.14

0.27 0.45 0.42 0.31

0.93 0.70 0.91 0.52

0.86 0.93 0.94 0.90

2

2258 1494 2265 2266 1455

0.77 0.74 0.88 0.93 0.63

0.53 0.53 0.61 1.00 1.06

0.71 0.87 1.01 1.69 1.49

0.48 0.47 0.38 0.36 0.47

0.60 0.59 0.49 0.50 0.60

0.46 0.41 0.28 0.38 0.33

1.32 2.95 1.64 1.89 2.25

38:23:39 41:24:35 42:24:34 41:23:36 44:23:33

0.55 0.29 0.22 0.24 0.29

0.93 0.91 0.83 0.84 0.99

0.20 0.14 0.11 0.11 0.12

2.35 2.24 2.53 2.58 2.22

0.71 0.62 0.55 0.56 0.46

1.05 1.20 1.14 1.03 0.89

3

2267 2437

2.93 3.05

0.08 0.09

0.03 0.03

41:23:36 2.10 42:20:38 1.80

0.45 0.37

0.23 0.10

0.52 0.60

0.33 0.38

n/d n/d

n/d n/d

n/d n/d

n/d n/d

n/d n/d

Pr/ph, pristane/phytane ratio; Pr/17, pristane/n-C17 ratio; Ph/18, phytane/n-C18 ratio; S/(S+R), 5a(H),14a(H),17a(H) 20S/ (20S+20R)- C29 sterane; b/(a+b), 5a(H),14b(H),17b(H)/5a(H),14a(H),17a(H)+5a(H),14b(H),17b(H) (20S+20R)- C29 sterane; dia/ reg, 10a(H),13b(H),17a(H) 20S- C27 diasterane/5a(H),14a(H),17a(H) 20R- C27 sterane; 21/29, pregnane/5a(H),14a(H),17a(H) 20RC29 sterane; 27:28:29, normalized relative abundance of C27, C28 and C29 regular steranes based on abb isomers; Ts/Tm, 18a(H)trisnorhopane/17a(H)-trisnorhopane; 29/hop, 17a(H)-norhopane/17a(H),21b(H)-hopane; gam/hop, gammacerane/17a(H),21b(H)hopane; 23/hop, C23 tricyclic terpane/17a(H),21b(H)-hopane; 34/33, 17a(H), 21b(H) 20S+20R- C34/C33 homohopane; 35/34, 17a(H), 21b(H) 20S+20R- C35/C34 homohopane; n/d, not determined.

M. Obermajer et al. / Organic Geochemistry 33 (2002) 1343–1359

The compositional uniqueness of this group of oils is readily visible in the distribution of terpanes and steranes. The m/z 191 mass fragmentograms show extremely high concentrations of C23 tricyclic terpane and minor C35 homohopane prominence. The relative amounts of 18a(H)- trisnorhopanes (Ts) and gammacerane in these oils are the lowest amongst all the studied oil samples from the Central Montana trough. Moreover, the differences in sterane distributions, shown by their C27:C28:C29 regular sterane profile, are substantiated by the highest concentration of C21 regular sterane and lowest diasterane/regular sterane ratios. Two oils from Lower Cretaceous reservoirs from the Mason Lake–Crooked Creek area are included into another compositional category—Group 3. The oils are distinctly different from any other oil in the examined set as they are very light (high saturated/aromatic hydrocarbon ratios) and have unique, ‘‘waxy’’ SFGCs. Both samples have broad n-alkane envelopes centered at C20 and extremely low concentrations of acyclic isoprenoids (lowest Pr/nC17 and Ph/nC18 ratios). Pristane/ phytane ratios of these oils are high (Table 1), but as both compounds occur in very low concentrations it is difficult to estimate how reliable these ratios are. Both oils have homohopane profiles characterized by decreasing concentrations of C31–C35 homologues with increasing carbon number. But more importantly, their

1351

terpane signatures are dominated by 17(a)H-C 30 diahopane and ?C27 diahopane (Fig. 4—peaks B and A, respectively). Moreover, their m/z 191 fragmentograms display a distinct pseudohomologous series of peaks in C31–C34 range that are most likely 17(a)H-diahopane homologues. These oils also contain the highest relative amounts of norneohopane (C29Ts) amongst the analyzed samples, while their distribution of C27:C28:C29 regular steranes is dominated by the C27 member, similar to the other two compositional oil groups. 4.2. Composition of gasoline range (C5-C8) fraction Representative gasoline range gas chromatograms (GRGC) are shown on Fig. 8 while selected C5–C8 parameters are listed in Table 3. In general, the compositional variability in the composition of higher molecular weight (C13–C40) fraction is also recognizable in the light hydrocarbon fraction but subgrouping of Group 1 is less obvious. The oils from that group notably show the lowest degree of paraffinicity. Although isoheptane and heptane values (PI 1 and PI 2, respectively; Thompson, 1983) are somewhat higher in the Type 1b oils, these values are much lower than those obtained for the remaining two groups (Fig. 9). The highest paraffin indices, especially PI 1, were observed in the Group 2 oils. Similarly, these oils display the highest

Fig. 8. Representative gasoline range gas chromatograms (i-C5H12–n-C8H18) of the analyzed central Montana oils. C6, n-hexane; C7, n-heptane; C8, n-octane, 2M6, 2-methylhexane; 3M6, 3-methylhexane; 1t3, 1,trans-3-dimethylcyclopentane; B, benzene; T, toluene.

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to methylcyclohexane (R ratio) and n-hexane to methylcyclopentane (Q ratio) vary in a similar manner in the studied oils. Both ratios are low in groups 1 and 3 (typically less than 1.0 for Group 1) and higher in Group 2 oils (more than 1.0). There are some differences in the concentrations of the light aromatic hydrocarbons such as benzene and toluene. Although Group 1 oils are relatively enriched in these two compounds, it appears that presence of these compounds is controlled to a greater extent by the stratigraphy of the reservoirs. In general, oils found in Mesozoic reservoirs have the lowest relative amounts of benzene and toluene, and consequently the highest water washing parameters such as S, T and U ratios (S=3-methylpentane/benzene; T=methylcyclohexane/ toluene; U=cyclohexane/benzene; Table 3). The compositional distinctiveness within the gasoline range fraction is also recognized using selected ‘‘Mango’’ parameters (Mango, 1987, 1990). The K1 ratio ([2 - methylhexane+2,3 - dimethylpentane])/[3 - methylhexane+2,4-dimethylpentane]), documented as an effective parameter in determining a familial affinity of oil, is somewhat similar in both Group 1 and Group 2 oil samples, typically falling into the 0.81–0.86 range. However, the normalized relative abundance of methylhexanes and dimethylpentanes is much higher in the Group 2 oils (Fig. 11). In addition, Group 2 oils have

Fig. 9. Cross-plot of Paraffin Indices (Thompson, 1983; see Table 3 for ratio definitions) in the analyzed oil samples. The outlines, showing good separation between oil families, are for visual approximation only.

relative concentrations of n-heptane and isoheptane, but it is noteworthy that compared with the distribution of paraffin indices, there is a different trend in the distributions of these parameters in the three groups of oils (Fig. 10). Furthermore, the concentration of n-heptane Table 3 Selected light hydrocarbon ratios in the investigated oil samples Group

Lab.no

PI 1

PI 2

nC7

bC7

Q

R

S

T

U

K1

P2

P3

N2

1a

2259 2261 2262 2263 1498

0.36 0.54 0.43 0.37 0.74

13.39 14.09 14.86 3.49 21.36

13.09 14.50 14.99 3.36 22.63

17.01 21.90 20.38 22.88 21.45

0.66 0.84 0.73 0.61 1.26

0.62 0.55 0.72 0.15 0.90

3.32 2.68 43.54 145.73 n/d

3.89 3.99 16.56 41.27 4.21

2.95 3.07 37.79 66.23 n/d

0.78 0.94 0.85 0.86 0.86

5.64 8.79 6.18 6.01 6.94

1.70 3.41 1.98 3.21 1.38

8.10 8.06 7.13 8.27 5.48

1b

2264 2260 1483 1486

0.64 0.58 0.77 0.59

19.03 15.19 20.76 17.54

19.84 16.52 21.15 17.43

21.07 19.06 23.56 20.13

1.29 1.06 1.48 0.92

0.81 0.57 0.97 0.75

1.35 0.94 2.90 6.07

2.76 3.07 16.89 8.38

1.55 1.51 2.56 5.73

0.81 0.84 0.82 0.71

6.18 5.56 8.36 7.04

1.70 1.65 1.57 0.91

5.10 5.45 6.24 6.84

2

2258 1494 2265 2266 1455

3.92 4.11 2.62 2.57 2.75

34.38 43.97 27.78 22.07 31.83

37.57 44.79 28.17 22.04 32.44

33.44 35.24 39.50 43.53 35.49

5.67 6.79 5.37 3.69 4.03

1.94 4.14 1.61 1.25 1.76

0.86 5.84 298.33 879.16 31.78

2.04 21.16 139.12 248.11 26.42

1.27 2.29 125.58 310.28 25.02

0.86 0.77 0.83 0.81 0.77

9.31 12.29 13.88 13.78 11.26

2.41 1.57 3.71 3.86 1.83

1.25 1.27 2.66 2.73 1.75

3

2267 2437

1.36 1.26

27.11 28.52

31.29 33.39

10.53 10.62

2.77 2.68

0.64 0.72

6.81 2.64

30.89 8.72

38.54 11.51

1.07 1.09

4.24 4.05

1.06 0.90

2.05 2.17

PI 1, (Isoheptane value)=(2-methylhexane+3-methylhexane)/sum of 1c3-, 1t3-, 1t2- dimethylcyclopentanes; PII 2, (Heptane value)=(n-heptane100)/sum of compounds eluting between cyclohexane and methylcyclohexane; nC7, weight percent normal C7 compounds; bC7, weight percent branched C7 compounds; Q, normal hexane/methylcyclopentane ratio; R, normal heptane/methylcyclohexane ratio; S, 3-methylpentane/benzene ratio; T, methylcyclohexane/toluene ratio; U, cyclohexane/benzene ratio; K1=(2methylhexane+2,3-dimethylpentane)/(3-methylhexane+2,4-dimethylpentane); P2=2-methylhexane+3-methylhexane; P3=dimethyldimethylpentane (2,2- +2,4- +3,3- +2,3-); N2=dimethylcyclopentane (1,1- +1,c3- +1,t3-); n/d- not determined.

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5. Discussion

Fig. 10. Cross-plot of the relative concentrations of normal heptane and isoheptane (see Table 3 for ratio definitions). The outlines, showing good separation between oil families, are for visual approximation only.

Fig. 11. Cross-plot of the normalized % peak area of 3methylhexane+2,4-dimethylpentane versus 2-methylhexane+ 2,3-dimethylpentane showing a distribution of K1 parameter in the analyzed oil samples. The outlines are for visual approximation only.

much lower relative concentrations of dimethylcyclopentanes resulting in lower N2 ratio (N2=1,1- +1,c3+1,t3-dimethylcyclopentane). In contrast, the two Cretaceous oils included in Group 3 have a very different K1 ratio (more than 1.0) and the lowest normalized concentrations of methylhexanes and dimethylpentanes of all the analyzed oils. The N2 parameter obtained for these oils is also low reaching values around 2.0, similar to that of the other oils produced from Mesozoic reservoirs but included in Group 2.

The observed compositional variations within the gasoline range fraction and biomarker signatures of the analyzed oil samples clearly indicate presence of at least three distinctive groups of oils. These results agree with conclusions of Aram (1993) who indicated three oil families occurring in the Central Montana trough. Following previous studies (Kranzler, 1966; Rinaldi, 1988), Aram (1993) concluded that the Limestone Member of the Upper Mississippian Heath Formation was the primary source rock for oils occurring in the Tyler–Amsden strata. The unique biomarker compositions of the majority of oils found in Mesozoic reservoirs led Aram (1993) to assume that compositional differences between oils occurring in Pennsylvanian and Mesozoic strata are genetically controlled, suggesting contribution from sources other than the Heath Formation. The results of the present study show that a biomarker-based classification of central Montana crude oils is well supported by the light hydrocarbon parameters. The oils from the Tyler–Amsden pools (Group 1) are easily distinguished from other oils and grouped into a separate broad category based on the gasoline range data. These oils not only have the lowest Paraffin Indices and relative concentrations of normal heptane, but are readily distinguishable from the other compositional groups by using selected ‘‘Mango’’ parameters. However, the subgrouping of this family, evident from terpane and sterane biomarkers, is not shown in the composition of the C5–C8 fraction. Composition of all Group 1 oils generally indicates anoxic hypersaline depositional environment (Pr/Ph ratios of less than 1.0, relatively abundant gammacerane). The 1a oils show characteristics, such as predominance of C34 homohopane, suggesting carbonate source rock lithology (Moldowan et al., 1985; Clark and Philp, 1989). In contrast, the 1b type oils have compositional traits (higher relative concentrations of C23 tricyclic terpane, 18a(H)- trisnorhopane, diasteranes and lower amounts of C29 norhopane) pointing to a more clastic source rock environment. Moreover, the very different normalized relative abundances of the C27:C28:C29 regular steranes, considered to be a highly specific correlation index (Peters and Moldowan, 1993, pp. 182–186), clearly suggest different source rocks for these oils. It appears that the Limestone Member was not the only active source rock within the Heath Formation and that this unit contained multiple source rock intervals. According to Cole and Drozd (1994), the carbonates of the Heath Formation occasionally show excellent petroleum potential, but the best source rocks occur within calcareous shales. It is interesting that the apparent source rock lithological heterogeneity, shown by different C13–C40 biomarker signatures of the 1a and 1b oils,

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is not clearly reflected in the light hydrocarbon compositions of these oils. There are several factors that could produce the homogeneity in the gasoline range composition observed in the Group 1 oils. Two common mechanisms are a similar source rock composition or a homogenization due to a long secondary migration. There is evidence that suggests that these oils were not subjected to an extensive secondary migration pathway. Firstly, it has been demonstrated that the Group 1 oils occur where Tyler–Amsden reservoir sandstones are in direct contact with the sourcing Heath Formation (Kranzler, 1966; Rinaldi, 1988). This indicates a rather short secondary migration. Secondly, we infer that the concentrations of light aromatic hydrocarbons in these oils also support a short secondary migration pathway. Where Group 1 oils occur in Pennsylvanian reservoirs they typically have relatively high concentrations of benzene and toluene, compounds often preferentially lost during migration and water washing (Palmer, 1993). Where Group 1 oils have migrated into overlying Mesozoic reservoirs (i.e. Rattlesnake Butte field), these light aromatic hydrocarbons are relatively depleted. This indicates that the concentrations of both benzene and toluene in Group 1 oils are readily affected during a cross-formational migration. Several other factors, including source, biodegradation and thermal alteration can modify benzene and toluene concentrations. Aromatic hydrocarbons are more resistant to microbial biodegradation than n-alkanes and cycloalkanes. Although Group 1 oils indeed display evidence of some early biodegradation, oils found in the Mesozoic reservoirs that have low relative concentrations of benzene and toluene show the most advanced depletion of middle-chain n-alkanes (C14–C19) and the highest isoprenoids/ n-alkanes ratios. It is the opposite of what would be expected if biodegradation affected these aromatic hydrocarbons. Moreover, constant ‘‘Mango’’ parameters and lack of other indicators for significant microbial alteration suggest biodegradation is not a major control. Biological marker thermal maturity indicators suggest only minor maturity differences among Group 1 samples (Table 2). As migration can be discounted for oils occurring in Tyler–Amsden reservoirs due to source/reservoir stratigraphic relationship, high concentrations of benzene and toluene in these oils suggest that source rock composition is the primary control on the abundance of these compounds and the generally homogeneous gasoline range composition. It is an interesting contrast that where the Group 1a– 1b biomarker differences imply that there are significant variations in source lithology and depositional environment, the homogeneous Group 1 gasoline range composition does not reflect a similar variation. It is possible that the influences of physical depositional environment and host lithology contribute differently to the composition of

the resulting oils. Jarvie (2001) has suggested that water column salinity and oxygenation affect the cyclization of straight-chain n-alkanes. Accordingly, aromatization should be enhanced in hypersaline, highly anoxic environments. Peters and Moldowan (1991) indicated that source can strongly influence the distribution of the homologous series of hopanes. Osadetz et al. (1992, 1994) documented a close association of the C34 hopane prominence in Williston Basin Paleozoic oils and source rocks with hypersaline carbonates and sulphate evaporites. Therefore, we infer that the generally highly anoxic hypersaline paleoenvironments of deposition and source rock host lithology controlled the relative prominence of C34 hopane and sterane distributions in Group 1 oils but did not result in a significant compositional variations within the gasoline range fraction of these oils. There are also other arguments indicating that the composition of light hydrocarbons in the Group 1 oils result from a single generation-entrapment episode. Burial and thermal history models (Cole and Drozd, 1994) show that the sources within the Heath Formation became fully mature only in Late Cretaceous–Early Tertiary during a foreland burial that resulted in generation and expulsion of oil now found in Tyler–Amsden reservoirs. The compositions of the oils are themselves consistent with this model of generation and entrapment. Subsequent uplift and erosion has led to lower temperatures and precludes a second phase of hydrocarbon generation. Hence, it is unlikely that a secondary oil charge into Pennsylvanian reservoirs occurred. Such an event could have homogenized the light hydrocarbon fraction in the Group 1 oils. However, if the homogenization of the gasoline range composition had resulted from a subsequent episode of oil introduction into the pools, then it is unlikely that the biomarker differences that distinguish Group 1a and 1b oils could have persisted while some, for example maturity dependant biomarkers, were homogenized. Therefore, we conclude that the homogeneity of Group 1 oil gasoline range composition is inherent due to the composition of source kerogen. The compositions of two Pennsylvanian reservoired oils (South Wolf Springs and Big Wall fields) are similar to that of the Group 2 oils. These oils appear to be more mature than the other Tyler–Amsden oils and have gasoline range and biomarker characteristics more typical for Group 2 oils. If there is no genetic relationship between oils from Pennsylvanian and Mesozoic strata, i.e. if the compositional differences between these groups are primarily source-controlled, these oils must have re-migrated into the Pennsylvanian reservoirs. Interestingly, re-migration of oil after structural inversion was proposed as a mechanism to explain how multiple reservoirs in the Cat Creek field became filled (Nelson, 1993). In addition, the occurrence of oil with characteristics typical for the Tyler–Amsden oil family

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in the Lower Cretaceous reservoir in the Rattlesnake Butte field indicates that the Mississippian–Pennsylvanian petroleum system in central Montana is not fully closed and that secondary migration from Pennsylvanian sources into Mesozoic traps has occurred. Group 2 and 3 oils, occurring predominantly in Jurassic–Cretaceous reservoirs, have different geochemical characteristics compared with the Group 1 oils. The unique biomarker signatures displayed by both groups correlate with distinct compositions within the gasoline range fractions. These oils are relatively light (API gravities of 46–52 ; Aram, 1993) and are characterized by the highest Paraffin Indices and concentrations of n-heptane and isoheptane amongst all analyzed samples from central Montana. Their high degree of paraffinicity may suggest that they are more thermally mature than the Group 1 oils, which is in agreement with their overall high gravity character, but may also indicate a highly aliphatic source of kerogen. However, the Crooked Creek type oils have very high ratios of saturated/aromatic hydrocarbons as well as overall low concentration of biomarkers, indicating that they are indeed amongst the most mature oils analyzed during this study. Other gasoline range characteristics, such as ‘‘Mango’’ parameters, are also variable. While the K1 parameter of the Group 2 oils somewhat is similar to that of the Tyler– Amsden oils, the same ratio is dissimilar for the Group 3 oils (Table 3, Fig. 11). Moreover, the concentrations of methylhexanes (P2 parameter, Figs. 12 and 13) are markedly different in both groups. As the light hydrocarbon content is generally high in these oils (French, 1985) differences in gasoline range compositions are considered important. Although the Cat Creek field was the first significant oil discovery in Montana (Fanshawe, 1985; Ames, 1993), the origin of the Cat Creek oils has remained enigmatic for over 80 years. Many of the biomarker characteristics of the Group 2 oils, such as low Pr/Ph ratios (< 1.0), diasterane/regular steranes and Ts/Tm ratios, relatively high concentration of C21 regular steranes (pregnanes) and C29 norhopanes, indicate anoxic conditions and a carbonate/evaporite source rock lithology. An unidentified rock, possibly of Cretaceous age, was postulated as a source for the oils from the Cat Creek field (French, 1985; Ames, 1993). Based on socalled ‘‘Correlation Index’’—a value derived from an average boiling point and the specific gravity of oil at 60  F—French (1985) considered the Cat Creek oils to be similar to Lower Cretaceous oils from the Cut Bank field located in north-western Montana, indicating that they might be related. Although the bulk character of such correlation would make it rather doubtful, it is worth noticing many similarities in the biomarker signatures of the Group 2 oils to Lower Cretaceous oils occurring in Northern Montana and Southern Alberta, in particular to oil Family M (Stevenson, 1998; Manzano-Kareah,

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Fig. 12. Cross-plot of the N2/P3 versus P2 gasoline range parameters showing good separation between oil families (see Table 3 for ratio definitions). The outlines are for visual approximation only.

2001). These include low Pr/Ph ratios, high relative concentrations of C21 regular steranes, C29 norhopanes, C23 tricyclic terpanes and C35 hopane predominance. The M oils, however, display much higher diasterane/ regular steranes and Ts/Tm ratios. These two parameters are similar to another previously defined group of oils, the so called Family C oils occurring in the Mississippian and Jurassic reservoirs in the adjacent Williston Basin (Osadetz et al., 1992, 1994). The Family C oils, apart from also having similar biomarker signatures to those of the Group 2 oils (Pr/Ph ratios of less than 1.0, low concentrations of diasteranes, abundant C29 norhopane, C35 hopane predominance), have comparable Mango parameters (Obermajer et al., 2000). However, the Family C oils have significantly lower relative concentrations of the C23 tricyclic terpane and C21 regular sterane, perhaps due to their lower thermal maturity compared with the Cat Creek type oils. Nevertheless, both Family M and Family C oils are believed to have been generated in source rocks of Mississippian age (Stevenson, 1998; Manzano-Kareah, 2001; Osadetz et al., 1992, 1994). Although several Jurassic–Cretaceous units in central Montana have been reported to have good quality source potential (Aram, 1993), overall similarities between Group 2 oils and oils from the two other families (M and C) may indicate that the Group 2 oils are primarily derived from the Upper Paleozoic strata equivalent to the sources of the M and C oil families. Also, the high maturity of these oils suggests that their sources were more mature than the Heath Formation, what may indicate the occurrence of yet unrecognized source rocks within the Madison Group. However, some contribution from a Mesozoic source rock cannot be ruled out. Aram (1993) speculated that Cat Creek oils might represent a composite

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mixture of oils that migrated from multiple sources. Given the fact that oil in this field has been produced from eight reservoirs ranging in age from Pennsylvanian to Cretaceous (Ames, 1993) and that fault reactivation events were quite common, it is quite likely that vertical, cross-formational migration of oil occurred in this area. According to Nelson (1993), the Cat Creek fault was the major migration fairway. This area attained its greatest burial before the formation of the present traps and some of the early oil was lost through fault migration. The oil currently found in this area would have accumulated as a result of a subsequent phase of secondary migration, comprising high gravity, mature oil. Hydrocarbons from two migrational phases could have been derived from different sources. If the oil from the preceding phase remained dispersed in the migration fairways, it is possible that the mixing occurred producing unique compositions of oils. The composition of migrating oils could also have changed if they came into contact with low maturity bitumens present within migration pathways. The two oils classified as Group 3 have a very unique geochemical composition possibly indicating a highly aliphatic source rock. According to Aram (1993), oils

from the deeper Amsden reservoirs in the Mason Lake field have compositions typical of other Pennsylvanian oils from the study area. The different light hydrocarbon distributions of the oils included within this group is conspicuous, including paraffin indices and especially the K1 parameter ( >1.0). Furthermore, the Crooked Creek–Mason Lake oils have a distinctive n-alkane profile with high concentrations of higher molecular weight homologues. High Pr/Ph ratios (around 3.0) most likely indicate an oxidized depositional environment and derivation from a source rock containing partially terrestrial organic matter. It is noteworthy that these oils not only contain relatively high concentrations of 17(a)H-C30 diahopane but also what appears to be a distinct pseudohomologues series of C31–C34 diahopanes. Such high relative concentrations of diahopanes are at least partly controlled by the high oil maturity. It has been documented that these compounds have greater thermal stability compared with regular hopanes (Moldowan et al., 1991; Farrimond et al., 1998) and their relative concentrations rise in high maturity oils and source rocks. However, it has been also shown that the presence of diahopanes is strongly influenced by the depositional environment. The origin of diahopanes is

Fig. 13. Ternary plot of C7 hydrocarbons showing 5-carbon and 6-carbon ring preference. Tol+MCYC6, sum of toluene and methylcyclohexane; 2MC6+3MC6, sum of 2-methyl- and 3-methylhexane; DMC5+DMCYC5, sum of dimethylpentanes and dimethylcyclopentanes (2,2-dmp+2,4-dmp+3,3-dmp+2,3-dmp+1,1-dmcp+1,cis-3-dmcp+1,trans-3-dmcp+1,trans-2-dmcp).

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not fully explained, yet these compounds are typically associated with a clay-rich source rock deposited in suboxic to oxic environments (Peters and Moldowan, 1993). As diahopanes are commonly found in sources rich in terrestrial input, they have been also regarded as a terrigenous marker (Philp and Gilbert, 1986; Killops and Howell, 1991; Peters and Moldowan, 1993). Telnaes et al. (1992) disputed such a specific association based on the fact that diahopanes are known to occur in oils and organic matter found in strata as old as Proterozoic (Summons et al., 1988a, b). Instead, Telnaes et al. (1992) suggested that the formation of diahopane may be associated with a bacterial component of the source kerogen and the salinity of environment. Diahopanes have also been reported from lacustrinederived oils from different sedimentary basins (Killops and Howell, 1991; Telnaes et al., 1992). It is worth noting here that the Group 3 oils have a very ‘‘waxy’’ character and are somewhat similar to a Cambrian oil from the Newporte field in the adjacent Williston Basin thought to be derived from Botryococcus-rich kerogen (Fowler et al., 1998). However, the Newporte oil has a higher concentration of isoprenoids, a Pr/Ph ratio < 1.0 and, in contrast to a V-shape distribution of regular steranes in Group 3 oils (C27 > C29 > C28), a regular sterane distribution with C29 sterane as the dominant homologue distribution (C29 > C28 > C27). Although some contribution from a lacustrine source cannot be ruled out, it is more likely that the Crooked Creek– Mason Lake type oils are primarily derived from clastic sources containing terrestrial organic component deposited in oxic-suboxic environment. Such interpretation is also supported by the higher amounts of norneohopane (C29Ts) observed in these oils. The Lower Cretaceous oil bearing Cat Creek strata in the Mason Lake area unconformably overlay continental and lacustrine deposits of the Upper Jurassic Morrison Formation. According to Peterson (1985), the uppermost Morrison Formation contains relatively widespread lacustrine dark shales and coaly deposits. Aram (1993) reported TOC values of up to 3.5% for some intervals within the Ellis–Morrison strata of central Montana that also contained good quality oil-prone kerogen. Therefore these organic-rich intervals within the Morrison Formation would appear as likely candidates for sources of the Group 3 oils. However, the Jurassic strata in central Montana are only marginally mature (0.54–0.64% Ro) which argues against the high thermal maturity of the oils. It is interesting, however, that both fields are located south of the Central Montana trough and the size of the oil accumulations in the two fields varies widely, with the Crooked Creek Field yielding a little over 23 thousand barrels of oil and the Mason Field producing more than a million barrels. It could be hypothesized that the generation of these oils was controlled by localized thermal anomalies, perhaps related to anomalous

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heat or fluid flow associated with Tertiary volcanism. Otherwise, these oils must have migrated from more mature sources. The oils found in Pennsylvanian and Lower Cretaceous strata in the Mason Lake area show different genetic affinity, suggesting that the systems are not connected. But even if the migration of oil from sources deeper than Pennsylvanian into Lower Cretaceous reservoirs is considered, it must have occurred before the Pennsylvanian reservoirs were filled, i.e. prior to Laramide orogeny when this area attained maximum burial. Therefore, migration from sources located outside the area of study is more likely. Low concentrations of benzene in these oils could then be interpreted as evidence of some water washing associated with long distance migration.

6. Summary The biomarker signatures of crude oils occurring in the Central Montana trough indicate the presence of three main compositional groups. Group 1 comprises oils from the Pennsylvanian Tyler–Amsden reservoirs and can be subdivided into at least two categories, using homohopane and regular sterane profiles. Oils from Jurassic–Lower Cretaceous reservoirs have distinctly different biomarker compositions and are included in two separate groups corresponding to the previously indicated Cat Creek and Mason Lake–Crooked Creek oil families. More importantly, however, the biomarkerbased divisions of oils from central Montana is also readily recognized by variable compositions within the gasoline range hydrocarbons. These detailed geochemical analyses indicate that the compositional differences observed among central Montana oils are genetic in nature, primarily controlled by the nature of source kerogen and paleodepositional conditions of their source rocks. The secondary alteration of oil caused by migration, biodegradation and/or reservoir conditions appear less important. The light hydrocarbon parameters prove very effective in delineating compositional differences and genetic affinities of oils from Central Montana trough. These results should be useful in assisting with future efforts of exploration and identification of source rock horizons in central Montana.

Acknowledgements Sneh Achal and Laura Mulder, GSC-Calgary, provided analytical and technological support. Cooperation of oil companies that provided oil samples for analyses is acknowledged. Critical reviews by Drs. J. Dahl and A.Y. Huc are greatly appreciated. GSC contribution no. 2001130.

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