The influence of source depositional conditions on the hydrocarbon and nitrogen compounds in petroleum from central Montana, USA

Organic Geochemistry Organic Geochemistry 38 (2007) 935–956 www.elsevier.com/locate/orggeochem

The influence of source depositional conditions on the hydrocarbon and nitrogen compounds in petroleum from central Montana, USA Barry Bennett a

a,*

, Samuel D. Olsen

b

Department of Geology and Geophysics, 2500 University Drive N.W., Calgary, Alta., Canada T2N 1N4 b Rogaland Research, Post Box 8046 4068, Stavanger, Norway Received 19 July 2006; received in revised form 4 January 2007; accepted 6 January 2007 Available online 26 January 2007

Abstract The hydrocarbon and pyrrolic nitrogen compound compositions of 21 crude oils from reservoirs in the Heath–Tyler– Amsden and Swift-Morrison formations of central Montana have been investigated by combined gas chromatography– mass spectrometry. The biomarker and aromatic hydrocarbon data enabled recognition of three composition types within the Heath–Tyler–Amsden oils. The Swift-Morrison oils displayed different hydrocarbon characteristics and are classified as a separate oil group. A number of geochemical parameters, e.g. dibenzothiophene/phenanthrene, C20-triaromatic steroid hydrocarbon/ (C20 + C28 triaromatic steroid hydrocarbons) indicate strong variations in petroleum composition from north to south across the region. Oils from the central Montana uplift with high relative C27-diasteranes/C29 aaa steranes and Ts/ (Ts + Tm) suggests increasing clay contribution to the source, while abundant gammacerane indicates the development of hypersaline conditions with restricted circulation. Oils from the Bull Mountain Basin, represented by the Amsden oils, show relatively low C27-diasteranes/C29 aaa steranes and Ts/(Ts + Tm) ratios indicating increasing carbonate contribution to the source, while the decrease of a gammacerane contribution suggests decreasing salinity levels, which may explain the dominance of tricyclic terpane and hence their precursors to the source organic matter. While the changes recorded by the biomarker and aromatic hydrocarbon compositions show strong regional variations, the nitrogen compounds do not respond so strongly. However, weak trends coinciding with source carbonate content were apparent in the following ratios: 1,8-dimethylcarbazole/(1,8-dimethylcarbazole + 1-ethylcarbazole) and carbazole/sum of C2-carbazoles. The second parameter may be responding to variations in salinity since it co-varies with the behaviour exhibited by the tricyclic terpanes. The source information that can be extracted from biomarkers and aromatic hydrocarbons suggests that primary migration and compositional fractionation during expulsion has exerted little influence on this fraction of petroleum. Meanwhile, primary migration and compositional fractionation during expulsion appear to effect the carbazole and benzocarbazole distributions thereby limiting their application as indicators for source rock facies composition.  2007 Elsevier Ltd. All rights reserved.

*

Corresponding author. Tel.: +1 403 2103916; fax: +1 403 2840074. E-mail address: [email protected] (B. Bennett).

0146-6380/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2007.01.004

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B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

1. Introduction Despite the common occurrence of organic nitrogen compounds in crude oils, surprisingly little is known about the influence of source rock depositional environments upon the fate and preservation of these compounds. In a study by Li et al. (1995) several crude oils from marine, freshwater and brackish-brine lacustrine to swampy lake environments revealed no characteristic pyrrolic nitrogen compound distributions that could be assigned to any given depositional conditions. In crude oils of the Rainbow-Shekilie-Zamma subbasins of NW Alberta (Canada), the saturated and aromatic hydrocarbon distributions responded to variations in thermal maturity and depositional conditions, whereas pyrrolic nitrogen compounds did not respond to the same extent (Li et al., 1999). In the Permian derived Phosphoria oils of Wyoming, molecular parameters based on the saturated hydrocarbons (e.g. Pr/Ph and Ts/ (Ts + Tm)) responded to source organic input, depositional environment and thermal maturity, whereas the pyrrolic nitrogen compounds did not appear to respond to these geological factors (Silliman et al., 2002). On the other hand, there are situations where it is clear that nitrogen compounds respond to changes in depositional environment. Clegg et al. (1997) compared the pyrrolic nitrogen compound distributions in bitumens from two carbonates of the Keg River Formation (Elk Point Group, Middle Devonian, Western Canada) representing two different source facies. A predominance of C4–C5-carbazoles was observed in sources deposited under normal marine conditions with photic zone anoxia. Meanwhile, the prominence of carbazole, 1-methylcarbazole and a high benzo[c]carbazole concentration were more typical of deposition under regressive higher salinity conditions. In crude oils from the Gulf of Suez, a positive correlation between the benzocarbazole [a]/([a] + [c]) ratio and Pr/Ph and Ts/ (Ts + Tm) parameters suggested that facies and depositional environment of the relevant source rocks influenced the benzocarbazole distributions in this setting (Bakr and Wilkes, 2002). The main petroleum system of central Montana has a well defined source rock unit, the Heath Formation of Mississippian age; overlain by a sandstone reservoir rock, the Tyler Formation of Pennsylvanian age (Cole and Drozd, 1994). The Heath Formation is characterised by lateral and

vertical facies variations (Derkey et al., 1985; Shepard, 1993; Aram, 1993). Derkey et al. (1985) suggested depositional environments consisting of small lakes, bogs and lagoons associated with coal producing swamps. Rinaldi (1988) suggested the Heath Formation was deposited under conditions of a mesosaline lagoon. Cole and Drozd (1994) proposed a restricted marine environment based on the analysis of three oils and four rock extracts. Shepard (1993) evaluated the biostratigraphy and lithostratigraphy of the Heath source rock and concluded rapid facies variations on a local rather than regional scale was attributed to the interplay of restricted marine and brackish faunas. There are few reports available describing the geochemistry of petroleum samples from central Montana oilfields. Work by Williams (1974), Swetland et al. (1978), Rinaldi (1988) and Cole and Drozd (1994) suggested that the Heath Formation represented a good source rock candidate for the Tyler oil. Aram (1993) and Obermajer et al. (2002) described three main oil types belonging to the Tyler–Amsden, Cat Creek and Mason Lake-Crooked Creek family of oils, while Obermajer et al. (2002) subdivided the Tyler–Amsden oils based on homohopane and regular sterane profiles. In this study, utilising several molecular parameters based on biomarker and aromatic hydrocarbons, we aim to delineate the compositional types and source facies relationships amongst a suite of central Montana oils. The behaviour of nitrogen compounds are then compared with the biomarker and aromatic hydrocarbon parameters to ascertain whether nitrogen compounds also inherit compositional variations that could be attributed to source rock characteristics. The Heath and Tyler accumulations are recognised for their short petroleum migration distances due to the close proximity of the Tyler reservoir and Heath source rock (Aram, 1993), although Amsden petroleum in the Bull Mountain Basin area requires migration distances up to 16–24 km (Luebking et al., 2001). In recognising the short secondary migration distances, particularly with the Heath and Tyler accumulations, compositional fractionation due to secondary petroleum migration are likely to be limited, thus changes found amongst the pyrrolic nitrogen compounds may reveal compositional features that are characteristic of the source rock.

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

2. Samples and methods 2.1. Samples General sample information for the 22 oils investigated during this study is given in Table 1. The geographical locations of the central Montana oils used during this study are shown in Fig. 1, while the stratigraphic relationship of the Heath–Tyler– Amsden formations is shown in Fig. 2. Recently, Obermajer et al. (2002) analysed sixteen oils from the central Montana region, one oil sample from Winnet Junction (well Mang #4–8) is duplicated during this investigation. Oil samples from the Sumatra and Melstone fields (Tyler Formation) have been analysed by Obermajer et al. (2002), but for this study the oils were sampled from different wells. In addition, Obermajer et al. (2002) investigated oil samples from Mason Lake (1st Cat Creek), Cat Creek (Ellis and Cat Creek) and Big Wall (Tyler) fields, in this study the oils were sampled from different reservoir formations (see Table 1). 2.2. Recovery of hydrocarbons and pyrrolic nitrogen compounds by solid phase extraction Solid phase extraction (SPE) methods were used for recovering the saturated hydrocarbons, aromatic hydrocarbons and pyrrolic nitrogen compound fractions from the crude oil samples (see Bennett et al., 2002). Approximately 100 mg of crude oil was loaded onto a C18 non-endcapped (NEC) SPE cartridge (Jones Chromatography, UK) and allowed to adsorb. Internal standards, squalane, 1,1-binaphthyl and carbazole-d8 were also added to the C18 NEC SPE. Following sample and standard mixture application to the C18 NEC SPE, the hydrocarbon fraction was eluted in n-hexane (5 ml). The pyrrolic nitrogen compound containing fraction was recovered in dichloromethane (CH2Cl2, 5 ml). N-Phenylcarbazole was added prior to GC–MS analysis to assess the recovery of carbazole-d8. The separation of the hydrocarbon fraction into saturated and aromatic hydrocarbons was carried out using the silver nitrate–silica gel SPE method described in Bennett and Larter (2000). 2.3. Gas chromatography–mass spectrometry (GC– MS) The saturated hydrocarbons, aromatic hydrocarbons and pyrrolic nitrogen compounds were ana-

937

lysed by combined gas chromatography–mass spectrometry (GC–MS) in selected ion monitoring (SIM) mode and in full scan mode (mass range, 50–550 amu). Mass spectral characterisation of compounds in the various fractions was carried out using GC–MS on a Hewlett Packard 5890 GC (using splitless injection) interfaced to a HP 5970B quadrupole mass selective detector (electron energy 70 eV, source temperature 250 C). The saturated hydrocarbons, aromatic hydrocarbons and pyrrolic nitrogen compounds were analysed on a fused silica capillary column (HP-5; 95%/5%, methyl/phenyl silicone; dimensions, 30 m · 0.32 mm i.d. · 0.25 lm film thickness (HewlettPackard)). The GC oven temperature programs were as follows: saturated hydrocarbons and aromatic hydrocarbons: 40 C held for 2 min then programmed at 4 C min 1 to 300 C and held at the final temperature for 20 min. The pyrrolic nitrogen compounds: 40 C held for 2 min then programmed at 10 C min 1 to 150 C and then 4 C min 1 to 300 C and held at final temperature for 20 min. Compound identification was based on relative retention times, comparison of mass spectra with published mass spectra and in some cases by cochromatography with authentic standards. 2.4. Quantification of hydrocarbons and pyrrolic nitrogen compounds Individual standard stock solutions were prepared in n-hexane (1,1-binaphthyl and squalane), hexane:toluene, 9:1, vol:vol, (carbazole-d8) and CH2Cl2 (N-phenylcarbazole). Peak area integration during GC–MS analysis was performed by using the Hewlett-Packard RTE integrator. The relative response factors (RRF) between internal standards and related compounds were assumed to be one e.g. carbazole-d8 (internal standard) versus carbazole. 3. Results and discussion 3.1. The hydrocarbon composition of central Montana oils A selection of gas chromatograms showing the variation in saturated hydrocarbon distributions found in the Heath–Tyler–Amsden (abbreviated to H–T–A) oils are shown in Fig. 3. The saturated hydrocarbons of the H–T–A oils are characterised by n-alkane presence from C9 through to C35, with

Reservoir formation

Well name

Group

Ace High (1) Gumbo Ridge (2) Beanblossom (3) Stud Horse (4) Winnet Junction (5) Sumatra (6) Rattler Butte (7) Hibbard (8) Tippy Buttes (9) Devil’s Pocket (10) Little Wall (11) Sheepherder (12) Melstone (13) Sumatra East (14) Mason Lake (15) Wolf Springs (16) Hawk Creek (17) Delphia (18) Big Wall (19) Cat Creek (20) Cat Creek (20)

Tyler Tyler Tyler Tyler B Tyler Tyler Tyler Amsden Tyler Heath Tyler B Upper Tyler Tyler Amsden Amsden Amsden Amsden Amsden Amsden Swift-Morrison Swift-Morrison

Coffee#27-1 Galt#2 Beanblossom A 1-19 N/A Mang 4-8A Grebe #215B Raymond#2-25 Kesterson 1 Hougen #33-21 #1-14 N/A Anderson-DeJaegher 12-3 #11-23 NP-E NCT-1 Van Arsdale 41-X-3 Horton 1 #7-8 Robinson Goffena #1 Texas#1B Northern Pacific Harlan 2 East Dome Unit 11

1a 1a 1a 1a 1a 1a 1b 1b 1b 1b 1b 1b 1b 1b 1c 1c 1c 1c 1c 2 2

Whole oil C13 isotope 30.6 30.9 30.7 29.8 31.9 30.8 30.2 30.5 29.9 30.4 30.0 29.7 30.2 30.6 29.5 29.3 29.2 29.0 29.4 30.2 30.8

Pr/Ph

Pr/nC17

Ph/nC18

20S/(20R + 20S)

22S/(22R + 22S)

0.91 0.93 0.86 0.87 0.84 0.98 0.83 0.98 0.91 0.81 0.87 0.77 0.95 1.00 0.73 0.66 0.71 0.68 0.65 0.90 1.02

1.20 1.57 1.68 1.56 0.87 1.04 1.27 0.84 1.18 0.49 1.23 1.19 1.11 1.05 0.70 0.52 0.52 0.48 0.47 0.62 0.63

1.58 2.23 2.34 2.10 1.17 1.14 1.74 0.89 1.46 0.61 1.58 1.74 1.33 1.07 1.02 0.81 0.77 0.74 0.76 1.09 0.95

0.58 0.56 0.54 0.55 0.52 0.56 0.51 0.61 0.57 0.60 0.56 0.52 0.53 0.57 0.53 0.50 0.51 0.50 0.56 0.54 0.43

0.57 0.57 0.56 0.61 0.57 0.59 0.55 0.55 0.56 0.59 0.58 0.60 0.58 0.58 0.53 0.59 0.00 0.49 0.60 0.60 0.67

Field

Group

Ts/ (Ts + Tm)

(C21 + C22)/ (C27 + C28 + C29)

(C21 aaa + abb)/ (C29 aaa20R)

(C27 baS + R)/ (C27 abbS + R)

C29H/ C30H

C23T/ C30H

ETR

MPI-1

Rc

C20/ (C20 + C28)

4-MeDBT/ 1-MeDBT

DBT/P

Ace High Gumbo Ridge Beanblossom Stud Horse Winnet Junction Sumatra Rattler Butte Hibbard Tippy Buttes Devil’s Pocket Little Wall Sheepherder Melstone Sumatra East

1a 1a 1a 1a 1a 1a 1b 1b 1b 1b 1b 1b 1b 1b

0.43 0.44 0.45 0.52 0.34 0.51 0.58 0.59 0.61 0.60 0.46 0.58 0.57 0.60

0.12 0.11 0.14 0.15 0.08 0.15 0.13 0.31 0.22 0.20 0.13 0.15 0.20 0.23

0.68 0.40 0.48 0.75 0.21 0.83 0.97 2.14 1.36 1.41 0.91 1.07 1.56 1.80

0.39 0.41 0.49 0.46 0.30 0.44 0.49 0.47 0.56 0.47 0.38 0.50 0.49 0.53

0.50 0.56 0.46 0.47 0.54 0.44 0.41 0.47 0.41 0.47 0.47 0.43 0.41 0.48

0.35 0.29 0.44 0.44 0.10 0.33 0.48 1.01 0.68 0.60 0.39 0.41 0.49 0.73

1.78 1.40 1.79 2.08 0.51 1.62 2.43 2.37 2.53 1.74 1.61 2.09 2.38 2.57

0.51 0.66 0.63 0.57 0.48 0.55 0.60 0.53 0.56 0.61 0.54 0.54 0.54 0.54

0.71 0.79 0.78 0.74 0.69 0.73 0.76 0.72 0.74 0.76 0.72 0.72 0.72 0.72

0.14 0.06 0.06 0.11 0.06 0.18 0.12 0.33 0.28 0.25 0.14 0.13 0.26 0.24

1.56 1.02 0.99 1.13 1.18 2.36 1.81 2.21 1.91 1.71 1.37 1.80 1.84 1.80

0.31 0.37 0.15 0.22 0.37 0.26 0.17 0.93 0.51 0.57 0.50 0.42 0.47 0.62

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

Field (number refer Fig. 1)

938

Table 1 General sample information and geochemical data for the central Montana oils

Key: N/A = not available; Pr/Ph = Pristane/Phytane; 20S/(20S + 20R) = C29 aaa 20S/(C29 aaa20S + C29 aaa20R)steranes; 22S/(22R + 22S) = C31 17a homohopanes 22S/ (22R + 22S); Ts/(Ts + Tm) = C27 18a-22,29,30-trisnorhopane/C27 18a-22,29,30-trisnorhopane + C27 17a-22,29,30-trisnorhopane; C21 + C22/C27 + C28 + C29 = C21 + C22 abb pregnanes/C27 + C28 + C29 abb 20R + 20S steranes; C21 aaa + abb/C29 aaa20R = C21 aaa + abb pregnanes/C29 aaa20R sterane; C27 baS + R/C27 abbS + R = C27 ba20S + 20R diasteranes/C27 abb20S + 20R steranes; C29H/C30H = C29 17a norhopane/ C30 17a hopane, C23T/C30H = C23 tricyclic terpane/C30 17a hopane; ETR = C28 + C29 tricyclic terpanes/ C28 + C29 tricyclic terpanes + C27 18a-22,29,30-trisnorhopane; MPI-1 = 3-methylphenanthrene + 2-methylphenanthrene/1.5 · phenanthrene + 9-methylphenanthrene + 1-methylphenanthrene;Rc = (0.6 · MPI-1) + 0.4; C20/(C20 + C28) = C20 triaromatic steroid/ (C20 triaromatic steroid + C28 20S + 20R triaromatic steroids); 4-MeDBT/1-MeDBT = 4-methyldibenzothiophene/1-methyldibenzothiophene; DBT/P = ; dibenzothiophene/phenanthrene.

Mason Lake Wolf Springs Hawk Creek Delphia Big Wall Cat Creek Cat Creek

1c 1c 1c 1c 1c 2 2

0.40 0.38 0.48 0.41 0.21 0.30 0.29

0.36 0.34 0.43 0.41 0.30 0.54 0.50

1.93 2.05 2.77 2.23 1.49 2.08 1.75

0.16 0.17 0.10 0.16 0.17 0.28 0.14

1.16 1.07 1.75 1.95 1.10 0.82 0.96

4.84 5.29 65.85 22.13 1.85 2.73 2.69

2.82 3.27 6.98 5.33 1.44 0.82 1.18

0.53 0.31 0.71 0.59 0.33 0.41 0.48

0.72 0.58 0.83 0.75 0.60 0.65 0.69

0.60 0.76 0.61 1.00 0.17 0.14 0.16

2.49 2.07 5.90 3.31 1.56 1.91 2.03

1.84 2.70 1.59 2.36 2.64 0.70 0.72

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

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appreciable contributions from the isoprenoid alkanes including pristane and phytane. Typically, the pristane/phytane (Pr/Ph) ratios are less than unity, with the lowest Pr/Ph ratios found in the Amsden oils (Table 1). A number of the Tyler–Amsden oils appear to show evidence for incipient biodegradation with the highest level of biodegradation indicated by the relatively high values from the Pr/ n-C17 and Ph/n-C18 parameters recorded in an oil from the Beanblossom field (Table 1). The presence of a well developed n-alkane profile indicates the oil has suffered only light biodegradation, approaching level 1 according to the Peters and Moldowan (1993) scale of biodegradation. Obermajer et al. (2002) used biomarker evidence to sub-divide the Tyler–Amsden oils into two groups; those deriving from a more carbonate source (group 1a) versus more clastic derived oils (group 1b). In this study, using biological marker features, three sub-groups within the H–T–A oils are recognised (1a, 1b and 1c; see Tables 1 and 2). Group 1a oils (Tyler reservoirs) are characterised by high relative gammacerane content, enhanced C34 17a hopane contribution compared to C35 17a hopane and low C23 tricyclic terpane versus C30 17a hopane (C23T/C30H, Table 1; Fig. 4). Additional features attributed to Group 1a oils include; 1:1 relative abundance of C27 diasteranes to C27 steranes and low relative abundance of short chain steranes (e.g. C21 aaa pregnane) compared to regular steranes (Fig. 4). Classification of oils into group 1b, by comparison to group 1a oils, are typified by a reduced contribution from gammacerane, no enhancement of C34 17a hopane compared to C35 17a hopane, enhanced tricyclic terpanes compared to hopanes (e.g. Fig. 4; Table 1), low relative abundance of short chain steranes compared to regular steranes and increased relative abundance of diasteranes compared to regular steranes (Fig. 4). Group 1b type oils are found in reservoirs from the Tyler, Amsden and Heath formations. Group 1c oils (Amsden reservoirs) are characterised by their strong tricyclic terpane distributions and unusually low contribution from the C29 to C35 17a hopanes (e.g. Hawk Creek, Fig. 4; Table 2). In addition, group 1c oils display strong relative abundance of short chain steranes compared to regular steranes and low relative abundance of diasteranes compared to regular steranes (Fig. 4). All group 1c oils are produced from reservoirs within the Amsden Formation and tend to be

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B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

112º

108º

110º ALBERTA

CANADA USA

49º

.

Great Falls

Little Snowy Mts

Big Snowy Mts

47º

45º

106º

SASKATCHEWAN

IDAHO

WYOMING

109º

107º

108º

PETROLEUM FERGUS

10

12

7

11

6

19 13

0

1

ROSEBUD

8 Sumatra Fault

9

14

18 15

Cat Creek Fault

2

CMU 5

4 3 GARFIELD

20

BMB

17

Willow Creek Fault

30 km

16

TREASURE

MUSSELSHELL YELLOWSTONE 109º

108º

107º

Fig. 1. Map showing approximate locations of oil fields (number refers to Table 1) and relevant structural elements. Key: BMB = Bull Mountain Basin, CMU = Central Montana Uplift.

located in the southern region of central Montana. However, it is also recognised that some Amsden Formation reservoired oils (Sumatra East, Hibbard and Big Wall) contain abundant tricyclic terpanes as well as appreciable quantities of extended hopanes. The hydrocarbon composition of these

oils appears to be intermediate of group 1b and group 1c oils. Based on an increased relative abundance of diasteranes to normal steranes, Sumatra East and Hibbard oils (see Table 1) appear closely related to Group 1b (Tyler oils) oils, whereas a low relative abundance of diasteranes to normal

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

SYSTEM

GROUP

JURASSIC

ELLIS

UNIT MORRISON SWIFT RIERDON PIPER

TRIASSIC PERMIAN PENNSYLVANIAN

AMSDEN

TENSLEEP ALASKA BENCH TYLER

UPPER MISSISSIPPIAM

BIG SNOWY

HEATH OTTER KIBBEY

Fig. 2. Generalised stratigraphy of the Mississippian to Jurassic strata in central Montana (modified from Obermajer et al., 2002).

steranes in Big Wall oil suggests a composition similar to the group 1c oils (Table 1). The gas chromatogram representing the saturated hydrocarbons from the East Dome Unit 11 oil of the Cat Creek field (Swift-Morrison Formation) is shown in Fig. 3. The chromatogram is dominated by a distribution of n-alkanes ranging from n-C10 through a maximum at n-C14 and tailing off towards n-C30 (Fig. 3). The Cat Creek oils display abundant isoprenoid alkanes, whose distributions are similar to those typically found in the Heath– Tyler–Amsden oils (c.f. Mason Lake, Fig. 3), which may suggest there could be a genetic relationship among these oils. Interestingly, some of the biomarker features are similar to those found for the group 1c oils, such as the relatively abundant tricyclic terpanes to extended hopanes and short chain steranes to regular steranes (see Fig. 4). The SwiftMorrison oils are referred to as group 2 oils which were previously assigned as group 2 oils by Obermajer et al. (2002). 3.2. Application of hydrocarbon molecular parameters to thermal maturity The molecular maturity of parameters based on the isomerisation at C-20 in the C29 aaa steranes (20S/(20R + 20S)) and at C-22 in the C31 17a hopanes (22S/(22R + 22S)) have achieved equilibrium end-point of 50–60% (Ensminger et al., 1977). The methylphenanthrene index (MPI; Radke et al., 1983) suggests a wide maturity variation rang-

941

ing from 0.31 to 0.71 (Table 1). The conversion of MPI to vitrinite reflectance (Rc) of the oils was calculated using the following relationship (0.6 MPI) + 0.4 proposed by Radke (1988). The Rc values range from 0.58 to 0.83 in group 1c oils, but typically most values for the H–T–A oils lie within a narrower maturity range approximating the oil window (0.7–0.8 Rc, Table 1; Fig. 5). Bennett et al. (2002) showed strong correlations between vitrinite reflectance equivalent (VRE) and the following molecular parameters: Ts/(Ts + Tm), 4-methyldibenzothiophene/1-methyldibenzothiophene and the tri-aromatic steroid side-chain cracking parameter; C20/(C20 + C28). Employing the same parameters, we illustrate the difficulty in establishing maturity relationships for the central Montana oils, through the lack of correlations shown by Ts/ (Ts + Tm) (Fig. 5a), C20/(C20 + C28) (Fig. 5b), 4methyldibenzothiophene/1-methyldibenzothiophene (Fig. 5c) versus the calculated vitrinite reflectance (Rc). Obermajer et al. (2002) using biomarker and aromatic hydrocarbon compositions also encountered difficulties in establishing maturity relationships for the central Montana oils. However, they found that the volatile hydrocarbons were less susceptible to variations in source rock composition and could be utilised to define maturity relationships amongst the central Montana oils. One of the strong compositional features recognised in the central Montana oils is attributed to the abundance of C23 tricyclic terpane compared to the extended hopanes (C23T/C30H ratio in Table 1) shown by the Amsden and Swift-Morrison oils (see Fig. 4). The ratio of C23 tricyclic terpane to C30 17a hopane (C23)/(C23 + C30) has been applied as a maturity indicator, reaching a value of 1.0 under high maturities during burial maturation (Peters and Moldowan, 1993) and contact metamorphism (Farrimond et al., 1999). Thermal alteration of crude oil under laboratory conditions also favours the tricyclic terpanes relative to pentacyclic terpanes (Aquino Neto et al., 1983). A similar feature is also recognised amongst the sterane distributions where the short chain sterane e.g. C21 aaa pregnane is the predominant compound in the m/z 217 fragmentogram (e.g. Fig. 4). At elevated temperatures under pyrolysis conditions the relative abundance of short chain steranes increases as a consequence of destruction of the regular C27 aaa cholestane (Abbott et al., 1995). In addition, a quantitative study of the biomarker abundance in a siltstone subjected to varying levels of thermal

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B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

Fig. 3. Representative gas chromatograms showing the distributions of saturated hydrocarbons encountered in central Montana oils. Key: i = isoprenoid alkanes, Pr = Pristane, Ph = Phytane.

stress during contact metamorphism also showed that elevated temperatures led to the destruction of steranes (Bishop and Abbott, 1993) and extended hopanes (Farrimond et al., 1996). Therefore, it might be considered that an increase in relative abundance of tricyclic terpanes and short chain steranes compared to their respective hopanes and ster-

anes could be an indicator of high source maturity. Fig. 6a shows the relationship of short chain steranes/C29 steranes versus C23 tricyclic terpane/C30 17a hopane for the H–T–A and Swift-Morrison oils. The Amsden and Swift-Morrison oils plot diagonally to the right of the Tyler (and Heath oil) oils depicting a relative higher maturity interpretation.

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956 Table 2 Hydrocarbon concentration (lg g

1

943

oil) data for the central Montana oils

Field

Group

C30H

Gammacerane

C34H

SumC29

C23TT

C21 + C22-preg

C27 ba 20S

DBT

Phenanthrene

Ace High Gumbo Ridge Beanblossom Stud Horse Winnet Junction Sumatra Rattler Butte Hibbard Tippy Buttes Devil’s Pocket Little Wall Sheepherder Melstone Sumatra East Mason Lake Wolf Springs Hawk Creek Delphia Big Wall Cat Creek Cat Creek

1a 1a 1a 1a 1a 1a 1b 1b 1b 1b 1b 1b 1b 1b 1c 1c 1c 1c 1c 2 2

817.3 1300.8 862.0 895.8 2945.8 896.8 550.7 319.3 591.4 691.1 930.1 656.9 673.3 542.1 190.3 188.3 18.9 50.6 440.8 62.5 57.3

625.7 1228.0 703.4 654.3 1833.6 661.2 332.1 178.8 417.3 302.7 552.1 335.4 389.4 302.9 170.0 111.3 146.1 118.4 154.9 11.3 13.7

535.0 1377.5 590.6 486.0 2075.9 536.6 175.2 97.9 234.9 179.8 400.7 185.3 223.3 162.1 50.1 46.0 0.0 0.0 106.6 0.0 0.0

524.4 1513.1 677.4 588.2 3274.5 716.2 300.1 192.8 345.7 411.2 609.7 434.0 327.8 284.3 359.8 311.5 379.0 369.5 472.3 58.0 60.9

288.5 376.6 380.7 398.0 297.6 299.3 264.4 321.6 403.3 414.7 361.4 268.3 328.9 395.1 920.0 995.5 1245.5 1120.4 815.0 170.5 154.0

124.1 192.9 118.4 126.5 331.2 161.2 77.6 113.8 144.7 158.1 146.6 108.4 140.2 130.6 319.1 250.5 444.1 392.1 252.9 51.3 48.9

135.4 141.0 177.8 188.5 275.6 141.8 146.7 78.5 165.8 179.2 163.6 180.4 149.4 133.6 51.9 48.1 64.9 54.4 54.3 7.2 7.6

80.1 68.6 11.9 29.4 130.5 153.1 75.6 270.3 257.5 137.0 221.4 127.6 274.5 221.1 388.6 601.6 381.7 573.5 574.7 46.0 36.8

261.6 187.1 78.0 136.7 354.3 597.4 456.8 289.7 508.2 242.2 438.9 306.6 585.8 355.9 211.7 222.7 240.8 243.0 218.1 20.8 25.5

Key: C30H = C30 17a hopane; C34H = C34 17a S + R hopanes; SumC29 = C29 aaa + abb 20S + 20R; C23TT = C23 tricyclic terpane; C21 + C22-preg = C21 + C22 aaa pregnanes; C27 ba 20S = C27 ba 20S diasterane; DBT = dibenzothiophene.

However, when quantitative data is considered, the maturity interpretation for both oil types appears inconsistent. The Swift-Morrison oils show the lowest concentrations of short chain steranes and regular steranes (Fig. 6b) as well as tricyclic terpanes and hopanes (Fig. 6c), which would fit a higher maturity interpretation, where most of the cyclic biomarkers have been destroyed or result from the dilution through generation of other components (Wilhelms and Larter, 2005). Meanwhile, the concentrations of C23 tricyclic terpane and short chain steranes in the Amsden petroleum are the highest of H–T–A oils, suggesting that biomarker destruction or dilution due to the generation of other components is not a significant process. In addition, the regular sterane concentrations of group 1c oils are similar to group 1a and 1b oils, implying that evidence for sterane degradation typical of high maturity oil is not consistent for the Amsden oils. ten Haven et al. (1985) identified C21 aaa pregnane and homopregnane as the most abundant compounds in gypsum core samples from a Messinian evaporitic basin in the northern Apennines, and proposed their presence as an indicator of hypersaline environment. Gypsum bearing zones have also been identified in the Heath Formation and thus such environmental conditions could have been

responsible for the presence of short chain steranes in the group 1c oils. It is clear that the determination of maturity for the H–T–A oils is difficult to appreciate employing classical molecular maturity parameters. Interestingly, biomarker ratios cannot be used to distinguish the Amsden oils from the Swift-Morrison oils and therefore both groups of oils would be erroneously grouped together and classified as high maturity oils! Quantitative data, on the other hand serve to demonstrate that the composition of the Swift-Morrison oils are dominated by a component ascribed to a high maturity source origin, whereas the molecular features of the Amsden oils are likely due to other geological factors such as source rock facies. Herein the application of the classical molecular parameters to describe the maturity of the Heath derived oils requires that the influence of source depositional environmental conditions be considered in order to understand the overall contribution to the molecular variations encountered amongst the hydrocarbon data. 3.3. Delineating source characteristics A number of molecular parameters that have been proposed as maturity parameters are also

944

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956 C30H

m/z 191

C23T

Ace High Tyler Group 1a

m/z 217

G

Tm

C29S C27D

C21P C34H

C30H

C27D

Sheepherder Tyler Group 1b C29S

Tm C23T

Intensity

C21P

C23T

Hawk Creek Amsden Group 1c

C21P

C29S

Extended tricyclic terpanes

C23T

C27D

Cat Creek Swift-Morrison Group 2

C21P

C29S C30H

C27D

Tm

Retention time Fig. 4. Representative m/z 191 and 217 mass fragmentograms of the saturated hydrocarbon fractions isolated from central Montana oils. Key: C23T, C23 tricyclic terpane; Tm, C27 17a(H)-22,29,30-trisnorhopane; C30H, C30 17a hopane; G, gammacerane; C34H, C34 17a 22S homohopane. C21P, C21 aaa pregnane; C27D, C27 ba 20S diasterane; C29S, C29 abb 20S + 20R steranes.

sensitive to source deposition characteristics (e.g. Ts/(Ts + Tm), Seifert and Moldowan, 1978). An increase in the Ts/(Ts + Tm) ratio, although typically attributed to increasing source maturity, may also increase with increasing shale content of the source (McKirdy et al., 1983), conversely low Ts/

(Ts + Tm) values may be due to carbonates (Waples and Machihara, 1990). The clay content of the source is usually indicated by the relative abundance of diasteranes compared to the regular steranes due to acid catalysed rearrangement associated with the presence of clay minerals during diagenesis (Rubin-

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956 3.0

C21 / C29 steranes

0.8

0.6

Ts / (Ts+Tm)

945

0.4

2.0

1.0

0.2 0.0 0.1

1.0 10.0 C23 tricyclic terpane / C30 Hopane

100.0

0.0 0.6

0.7

Rc

0.8

0.9

1.0

Winnet Junction

1.2

C20 / (C20+C28)

4500

C27:C28:C29 αββ 20S+20R

0.5

0.8

3000 Gumbo Ridge 1500

0

0.4

0

100

200 300 400 C21+C22 – ααα+αββ pregnanes

500

3500

0.0 0.5

0.6

0.7

0.8

0.9

C3017 α Hopane

Rc 8.0

4-MeDBT / 1-MeDBT

3000

1.0

6.0

2500 2000 1500 1000 500 0

4.0

2.0

0.0 0.5

0.6

0.7

0.8

0.9

1.0

Rc Fig. 5. Cross plots of calculated vitrinite reflectance (Rc) versus (a) Ts/(Ts + Tm), (b) C20/(C20 + C28) and (c) 4-MeDBT/1MeDBT in oils from central Montana. Key: squares = Group 1a; circles = Group 1b; open triangles = Group 1c; X = Heath oil; shaded triangles = Swift-Morrison.

0

500 1000 C23-tricyclic terpane

1500

Fig. 6. Cross plots showing the variation in (a) C23 tricyclic terpane/C30 17a hopane versus C21 aaa + abb pregnanes/C29 aaa 20R sterane and variation in concentration (lg g 1 oil) of (b) sum of C21+C22 aaa + abb short chain steranes versus sum of C27 + C28 + C29 abb steranes (data obtained from the m/z 218 mass fragmentogram) and (c) C23 tricyclic terpane versus C30 ab hopane in oils from central Montana. Key: see legend in Fig. 5.

stein et al., 1975; Peakman and Maxwell, 1988). The source composition of the central Montana oils, in terms of clastic versus carbonate content, is delineated in the cross plot of C27 ba (20R + 20S)

946

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

diasteranes/C27 abb (20R + 20S) steranes versus C29 17a norhopane/C30 17a hopane (Fig. 7a). Interestingly, the plot of C29 17a norhopane/C30 17a 0.6

C27βαD / C27αββS

clastic 0.4

carbonate

0.2

0.0 0.0

0.5

1.0

1.5

2.0

2.5

C29H/C30H 0.8

Ts / (Ts+Tm)

0.6

0.4

0.2

0 0.0

0.5

1.0

1.5

2.0

2.5

C29H/C30H

c

5

DBT / P

4

1

3

2

2 1

3

5

4

0 0

1

2

3

4

5

Pr / Ph Fig. 7. Cross plots of C29 17a norhopane/C30 17a hopane versus (a) Ts/(Ts + Tm), (b) C27 ba 20S + 20R/C27 abb 20S + 20R and (c) pristane/phytane versus dibenzothiophene/phenanthrene ratios for central Montana oils. Field boundaries from Hughes et al. (1995): 1. Marine carbonate; 2, Marine carbonate and marine marl; 3, Lacustrine hypersaline; 4, marine shale and other lacustrine; 5, fluvio-deltaic shale and coal. Key: see legend in Fig. 5.

hopane versus Ts/(Ts + Tm) shown in Fig. 7b displays a similar distribution pattern to Fig. 7a supporting that Ts/(Ts + Tm) appears to be responding to the relative clay–carbonate content of the source. A cross plot of dibenzothiophene/phenanthrene (DBT/P) versus pristane/phytane (Pr/Ph) ratios have been proposed as an indicator for establishing the depositional environment and lithology of petroleum source rocks (Hughes et al., 1995). The DBT/P versus Pr/Ph when applied to the H–T–A oils (Fig. 7c) serve to distinguish two main types of depositional setting; the carbonate dominated source for the Amsden oils and the clastic dominated sourced oils in the Heath and Tyler reservoirs, supporting the recognition of two main facies types to the source of the central Montana petroleum. The presence of gammacerane is a common feature of the Heath oil and Tyler oils and is particularly enhanced in group 1a oils. Gammacerane is thought to originate from phototrophic bacteria, which are generally abundant in saline lake environments (ten Haven et al., 1989; Peters and Moldowan, 1993). The relative abundance of C34 17a hopane compared to C35 17a hopane has also been applied as an indicator of hypersaline source conditions (Moldowan et al., 1985; Clark and Philp, 1989). Dahl et al. (1993) suggested the concentration of gammacerane versus C34 17a hopanes may reveal the nature of source conditions in terms of restricted circulation and changes in salinity, while the presence of gammacerane and a predominance of C34 17a hopanes amongst the extended hopanes, suggests an origin from anoxic to slightly sub-oxic hypersaline source rock (Moldowan et al., 1992). Fig. 8a shows a strong linear relationship between gammacerane and C34 17a hopane for the H–T–A oils suggesting that conditions responsible for controlling their variation may be related. The high concentrations of gammacerane and C34 17a hopane in Gumbo Ridge and Winnet Junction oils suggests strongly developed hypersaline conditions to the source of these oils. Conversely, the low gammacerane and C34 17a hopane content found in group 1c Amsden oils (Fig. 8a), perhaps correspond to lower salinity conditions during the accumulation of the source of Amsden oils. The Amsden oils typically contain a high relative abundance of short chain steranes compared to regular steranes and tricyclic terpanes compared to extended hopanes; thus in the absence of strong

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956 2500

C3417 α hopane

2000 1500 1000 500 0

0

500

1000

1500

2000

Gammacerane 1400

C23Tricyclic terpane

1200 1000 800 600

Hypersaline?

400 200 0

0

500

1000 Gammacerane

1500

2000

Fig. 8. Cross plots showing the variations in concentrations of gammacerane versus (a) C34 17a (22S + 22R) hopanes and (b) C23 tricyclic terpane in oils from central Montana. Key: see legend in Fig. 5.

maturity effects and a decrease in gammacerane content, this feature may be indicative of lower salinity conditions during source accumulation responsible for Amsden petroleum. The molecular ratio based on the C23 tricyclic terpane versus C30 17a hopane has been employed as a maturity parameter, although Dahl et al. (1993) suggested the parameter may be inversely related to anoxia and/or salinity. Zhusheng et al. (1988) have attributed strongly dominant tricyclic terpanes versus extended hopanes in Oil JW-24 oil from the Kelamayi Basin China due to migration fractionation. However, oil-source correlation suggested the oil was sourced from Permian aged organic rich calcareous shale formed in a fresh water lacustrine environment; therefore abundant tricyclic terpanes could represent an unusual assemblage of organisms accumulating under unique depositional conditions. For example, Kruge et al. (1990a,b) found enhanced tricyclic terpane contributions as a common feature of freshwater lacustrine environ-

947

ments. However, de Grande et al. (1993) reported prominent tricyclic terpanes in saline lacustrine and marine carbonate environments, indicating that the precursor lived in moderate salinity conditions. The presence of tricyclic terpanes has been associated with the occurrence of the alga Tasmanites (Aquino Neto et al., 1986; de Grande et al., 1993; Simoneit et al., 1993: Revill et al., 1994) though an inherent source/biomarker relationship between terpenoids and Tasmanites may not always exist (Dutta et al., 2006). In terms of source depositional conditions Dahl et al. (1993) suggest precursors of the hopanes predominate during the time of greatest anoxia and or salinity, whereas precursors of tricyclic terpane are in greater relative abundance under more oxic/less saline conditions. Fig. 8b considers the relationship between gammacerane and C23 tricyclic terpane. The Amsden oils typically contain abundant C23 tricyclic terpane as a consequence of reduced concentrations of gammacerane. Considering that gammacerane is used as an indicator of deposition under hypersaline conditions, the contributors of C23 tricyclic terpane to the organic matter appear to be more prevalent under lower salinity conditions to the precursor of gammacerane possibly indicating decreasing salinity conditions for the source of Amsden petroleum. The biomarker and aromatic hydrocarbon composition of the H–T–A petroleum are presented according to their geographical positions to highlight the north-south variation in petroleum composition across the central Montana region covered during this study. Aram (1993) showed how different sub-classes within the Tyler–Amsden formed groups elongated east-west centering between the Sumatra and Willow Creek faults. The location maps based on the classical molecular parameters C20/(C20 + C28), DBT/P and the C28 plus C29 tricyclic terpanes/Ts-ETR (Ramo´n et al., 2001) show well developed regional trends (Fig. 9). Although classically employed as a maturity parameter, the C20/(C20 + C28) (Fig. 9a) appears to be responding to facies variations, considering its similar response across the region to those parameters that respond to changes in source depositional conditions such as the DBT/P (Fig. 9b) and C28 plus C29 tricyclic terpanes/Ts-ETR (Fig. 9c). The carbon isotopes also fall within the regional trends where the heaviest values are found in the Amsden oils (Table 1). The abundance of individual compounds also depict the north-south variations with DBT (Fig. 10a)

948

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

Stud Horse 0.11

a

Devil's Pocket X 0.25

Gumbo Ridge 0.06 Sheepherder Winnet Junction 0.13 0.06 Little Wall 0.14 Melstone Big Wall 0.26 0.17 0.28 Tippy Buttes Delphia 1.0 Mason Lake 0.6 0.61 Hawk Creek

0.06 Beanblossom

0.12 Rattler Butte 0.14 Ace High 0.18Sumatra 0.33 Hibbard 0.24 Sumatra East

0.76 Wolf Springs

Stud Horse 0.22

b

Gumbo Ridge 0.37

Devil's Pocket X 0.57

Sheepherder Winnet Junction 0.42 Little Wall 0.37 0.50 Big Wall Melstone 0.51 2.64 0.47 Tippy Buttes Mason Lake 1.84

0.17 Rattler Butte 0.31 Ace High 0.26 Sumatra 0.93 Hibbard 0.62 Sumatra East

Delphia 2.36 1.59 Hawk Creek 2.70 Wolf Springs

Stud Horse 2.08

c

Devil's Pocket X 1.74

0.15 Beanblossom

Gumbo Ridge 1.40 Sheepherder Winnet Junction 2.09 0.51 Little Wall 1.61 Big Wall Melstone 2.53 1.44 2.38 Tippy Buttes Mason Lake 2.82

1.79 Beanblossom

2.43 Rattler Butte 1.78 Ace High 1.62 Sumatra 2.37 Hibbard

Delphia 5.33

2.57 Sumatra East

6.98 Hawk HawkCreek Creek 3.27 Wolf Springs

Fig. 9. Reservoir location maps displaying regional variation encountered in the (a) C20/(C20 + C28)-triaromatic steroids, (b) DBT/ Phenanthrene and (c) C28 + C29 tricyclic terpanes/Ts in the H–T–A oils from central Montana. Key: see legend in Fig. 5.

responding to increasing sulphur content, C23 tricyclic terpane (Fig. 10b) suggesting a response due to changing source depositional conditions (e.g. salin-

ity) and the abundance of C27 diasteranes (Fig. 10c) representing source relative clastic/carbonate content.

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

949

Stud Horse 29.4

a

11.9 Beanblossom 75.6 Rattler Butte

Gumbo Ridge 68.6

Devil's Pocket X 137.0

Winnet Junction Sheepherder 130.5 127.6 Little Wall 221.4 Big Wall Melstone 257.5 574.7 274.5 Tippy

80.1 Ace High 153.1 Sumatra 270.3 Hibbard

Buttes Mason Lake 388.6

221.1 Sumatra East

Delphia 573.5 381.7 Hawk Creek 601.6 Wolf Springs

Stud Horse 398.0 Gumbo Ridge 376.6 380.7

b

Beanblossom

Devil's Pocket X 414.7

364.4 Rattler Butte Sheepherder Winnet Junction 268.3 297.6 Little Wall 288.5 Ace High 361.4 299.3 Sumatra Big Wall Melstone 815.0 321.6 328.9 403.3 Hibbard Tippy Buttes 395.1 Delphia 1120.4 Mason Lake Sumatra East 920.0 1245.5 Hawk Creek 995.5 Wolf Springs

Stud Horse 188.5

c

Devil's Pocket X 179.2

380.7 Beanblossom 264.4 Rattler Butte

Gumbo Ridge 376.6

Sheepherder Winnet Junction Little Wall 180.4 275.6 135.4 Ace High 163.6 133.6 Sumatra Big Wall Melstone 54.3 78.5 165.8 149.4 Hibbard Tippy Buttes 133.6 Delphia 54.4 Sumatra East Mason Lake 64.9 51.9 Hawk Creek 48.1 Wolf Springs

Fig. 10. Reservoir location maps displaying regional variation in concentrations (lg g 1 oil) of (a) dibenzothiophene, (b) C23 tricyclic terpane and (c) C27 ba 20S diasteranes in the H–T–A oils from central Montana. Key: see legend in Fig. 5.

The application of hydrocarbon data expressed according to their geographical locations show clearly defined regional trends across central Mon-

tana. Interestingly, the tectonic development of the central Montana region was thought to have influenced the conditions of deposition during the

950

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

accumulation of the Heath source rock, thus accounting for the differences observed in the molecular signatures of the Tyler and Amsden oils

Table 3 Nitrogen compound data (concentration lg g

1

(Aram, 1993). The Willow Creek fault defines the boundary between two tectonic regions; the central Montana aulacogen (Shepard, 1993) where the

oil) for the central Montana oils

Field

Group

Carbazole

1-MC

3-MC

2-MC

4-MC

1,8DMC

1ethylC

1,3-DMC

1,6DMC

1,7DMC

Ace High Gumbo Ridge Beanblossom Stud Horse Winnet Junction Sumatra Rattler Butte Hibbard Tippy Buttes Devil’s Pocket Little Wall Sheepherder Melstone Sumatra East Mason Lake Wolf Springs Hawk Creek Delphia Big Wall

1a 1a

2.69 2.09

2.46 3.74

0.70 1.42

0.92 1.75

1.97 2.84

1.66 2.95

1.57 0.75

1.52 2.03

1.08 1.80

1.53 3.13

3.31 7.23

1a 1a 1a

0.67 0.61 0.71

0.63 1.21 1.02

0.32 0.47 0.27

0.34 0.55 0.36

0.54 0.78 0.87

0.46 0.42 0.99

0.07 1.13 0.37

0.63 1.01 0.45

1.33 0.53 0.69

0.75 0.55 0.64

1.12 1.67 2.36

1a 1b 1b 1b 1b

4.04 4.77 0.26 2.20 0.82

6.70 7.08 0.33 5.84 1.40

2.13 3.46 0.08 1.74 0.83

3.12 3.30 0.09 1.67 1.00

4.42 3.41 0.29 3.58 0.92

5.72 3.84 1.01 6.28 1.50

1.16 0.99 0.14 1.28 0.60

3.90 3.49 0.93 5.18 1.59

3.17 3.72 0.70 4.80 1.68

6.00 4.00 1.08 5.95 1.66

12.99 8.94 1.97 13.94 3.48

1b 1b 1b 1b

3.55 0.96 4.58 0.54

4.37 2.06 6.15 1.56

1.59 1.32 2.19 0.54

1.79 0.87 2.66 0.64

3.19 1.18 3.86 1.21

3.08 3.18 5.16 2.69

1.25 0.58 1.10 0.57

1.89 3.26 3.12 2.50

1.61 3.60 3.62 1.67

2.72 3.02 3.97 2.69

8.27 5.73 11.32 5.58

1c 1c 1c 1c 1c

1.17 0.19 1.06 0.81 0.26

2.53 0.27 4.17 2.82 0.34

0.99 0.19 1.02 0.62 0.13

0.92 0.14 1.20 0.70 0.11

1.03 0.27 2.15 1.66 0.10

2.69 0.77 7.31 7.00 0.00

0.60 0.07 0.35 0.68 0.00

2.29 0.46 5.55 4.32 0.43

2.30 0.34 3.50 3.04 0.35

2.21 0.40 5.64 4.86 0.29

4.67 1.16 12.70 10.07 0.96

Field

Group

2,6-DMC

2,7DMC

1,2DMC

2,4DMC

2,5DMC

2,3DMC

3,4DMC

B[a]C

B[c]C

[a]/ ([a] + [c])

C0/ C2

1,8-/(1,8+ 1-ethyl-)

Ace High Gumbo Ridge Beanblossom Stud Horse Winnet Junction Sumatra Rattler Butte Hibbard Tippy Buttes Devil’s Pocket Little Wall Sheepherder Melstone Sumatra East Mason Lake Wolf Springs Hawk Creek Delphia Big Wall

1a 1a

0.31 0.91

0.70 1.76

0.29 1.15

0.69 1.58

0.84 1.44

0.30 0.60

0.46 0.99

1.08 1.25

0.51 1.06

0.68 0.54

0.25 0.12

0.51 0.80

1a 1a 1a

0.30 0.15 0.10

0.19 0.38 0.62

0.16 0.00 0.00

0.16 0.20 0.37

0.14 0.29 0.29

0.15 0.12 0.13

0.27 0.38 0.67

0.18 0.44 0.67

0.24 0.34 0.73

0.43 0.56 0.48

0.15 0.11 0.13

0.86 0.27 0.73

1a 1b 1b 1b 1b

1.31 1.43 0.24 1.59 0.54

2.64 2.16 0.59 2.38 0.72

1.35 1.42 0.00 1.96 0.46

2.97 1.64 0.33 2.70 0.53

2.12 1.55 0.31 2.74 0.56

0.83 0.72 0.15 0.95 0.78

1.62 1.20 0.33 2.06 0.42

2.79 1.64 0.26 3.15 0.64

1.84 1.40 0.28 1.65 0.64

0.60 0.54 0.48 0.66 0.50

0.12 0.19 0.05 0.06 0.08

0.83 0.80 0.88 0.83 0.71

1b 1b 1b 1b

0.61 1.28 0.93 0.80

2.41 2.17 2.30 1.10

0.00 0.00 1.26 0.31

2.00 0.96 2.60 0.82

1.60 0.88 2.24 0.81

1.04 0.63 0.92 0.42

1.16 1.00 1.87 0.66

2.58 1.14 2.74 0.85

1.98 0.88 2.01 0.74

0.57 0.56 0.58 0.54

0.19 0.05 0.16 0.03

0.71 0.84 0.82 0.83

1c 1c 1c 1c 1c

0.82 0.15 1.14 0.86 0.00

1.77 0.15 3.70 1.91 0.00

0.00 0.17 0.00 1.21 0.00

0.86 0.06 2.72 1.71 0.00

0.75 0.12 1.92 1.44 0.00

0.39 0.13 0.29 0.42 0.00

0.60 0.12 0.92 1.17 0.00

0.70 0.00 1.04 1.62 0.27

0.74 0.00 0.41 0.55 0.53

0.49 0.00 0.72 0.75 0.34

0.08 0.06 0.03 0.03 0.13

0.82 0.92 0.95 0.91 0.00

Key: 1-MC = 1-methylcarbazole; 1,8-DMC = 1,8-dimethylcarbazole; B[a]C = benzo[a]carbazole.

1,4 + 1,5 DMC + 4- + 3-ethyl-

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

951

according to hydrocarbon composition coinciding with tectonic boundaries suggest a marine carbonate component to the source, south of the central Montana Uplift (group 1c oils); while a hypersaline deepwater basin with restricted circulation lay to the north of the Willow Creek fault where the source of group 1a and 1b oils accumulated. A number of geochemical parameters and quantitative data expressed as region based location maps respond in a similar manner across central

Tyler accumulations are found, and the tectonically stable Bull Mountain Basin (Fig. 1) where the Amsden oils are found. Luebking et al. (2001) suggested that the outlines of the tectonic regions coincided with the geographical distributions of the two subfamilies of the Heath sourced petroleum. Aram (1993) recognised different subclasses of the Tyler– Amsden Formation oils arranged as elongated west-east groupings between the Sumatra and Willow Creek faults. The areas that have been grouped

Rattler Butte Tyler 1-MC Group 1b

1,4+4-Et 1,8-DMC 1,6

C

1,2

32 4 2,5 1-Et

Gumbo Ridge Tyler Group Ia 1

1,8-DMC 1,7

Intensity

4

2,4

C 3

2

3,4

1-Et

2,3

1,8-DMC Hawk Creek Amsden Group 1c

1,5+3-Et

1,3

1-MC 2,7

4 C

32

1-Et

2,6

Retention time Fig. 11. Summed (m/z 167, 181, 195) mass chromatograms representing C0–C2-carbazoles in selected central Montana oils.

952

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

Stud Horse 0.11 Gumbo Ridge 0.12 0.15 Beanblossom 0.19 Rattler Butte

Devil's Pocket X 0.08

Sheepherder Winnet Junction 0.05 0.13 Little Wall 0.25 Ace High 0.19 0.12 Sumatra Melstone Big Wall 0.05 0.06 0.16 0.13 Hibbard Tippy Buttes 0.03 Delphia 0.03 Sumatra East Mason Lake 0.08 0.03 Hawk Creek 0.06 Wolf Springs

Stud Horse 0.27 0.86 Beanblossom 0.80 Rattler Butte

Gumbo Ridge 0.80

Devil's Pocket X 0.71

Sheepherder Winnet Junction 0.84 0.73 Little Wall 0.71 Melstone Big Wall 0.82 0.83 0.00 Tippy Buttes Mason Lake 0.82

0.51 Ace High 0.83 Sumatra 0.88 Hibbard 0.83 Sumatra East

Delphia 0.91 0.95 Hawk Creek 0.92 Wolf Springs

Stud Horse 0.56 Gumbo Ridge 0.54

Devil's Pocket X 0.50

Sheepherder Winnet Junction 0.56 0.48 Little Wall 0.57 Big Wall Melstone 0.34 0.58 0.66 Tippy Buttes Mason Lake 0.49

0.86 Beanblossom

0.54 Rattler Butte 0.68 Ace High 0.60 Sumatra 0.48 Hibbard 0.54 Sumatra East

Delphia 0.75 0.72 Hawk Creek 0.00 Wolf Springs

Fig. 12. Reservoir location maps displaying regional variations in nitrogen compound ratios (a) carbazole/summed C2-carbazoles, (b) 1,8dimethylcarbazole/(1,8-dimethylcarbazole + 1-ethylcarbazole) and (c) benzocarbazole [a]/([a] + [c]) in the H–T–A oils from central Montana. Key: see legend in Fig. 5.

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

Amsden oils. The benzocarbazole [a]/([a] + [c]) (Fig. 12c) reveals no geographical correspondence emerging where the highest and lowest [a]/ ([a] + [c]) values are found in Amsden petroleum. The behaviour of the benzocarbazole [a]/([a] + [c]) parameter is considered in terms of maturity, Rc

0.8

0.6

[a] / [a]+[c]

Montana. The main variants appear to be parameters that are based on source facies indicators and in turn indicate the highly varied source depositional conditions that are responsible for the strong variation encountered in the petroleum composition of the central Montana oils. The upper part of the Heath Formation contains organic-rich mudstones and carbonates which have been described as good to excellent sources (Cole and Drozd, 1994). The compositional features of a number of Amsden oils (e.g. Hawk Creek) are attributable to an origin from a carbonate component of the Heath source, while the Tyler (e.g. Ace High) accumulations derive from an increasing clastic (mudstone) component. Amsden oils such as Sumatra East, Hibbard and Big Wall represent intermediate compositions probably deriving from mudstone and carbonate component of the Heath source.

0.4

0.2

0.0

3.4. Nitrogen compound characteristics of central Montana petroleum

0.5

0.7

Rc

0.9

1.1

0.8

[a] / [a]+[c]

0.6

0.4

0.2

0.0 0

0.2

0.4

0.6

0.8

Ts / (Ts+Tm) 0.8

0.6

[a] / [a]+[c]

The C0–C2-carbazole and benzocarbazole concentrations and commonly applied molecular parameters are listed in Table 3. The relative carbazole versus alkylated carbazoles apparently decreases (Fig. 11) from Rattler Butte (Group 1b) to Gumbo Ridge (Group 1a) to Hawk Creek (Group 1c). The variation in relative carbazole to the sum of C2-carbazoles throughout central Montana shows a general decrease toward the south (Fig. 12a). Clegg et al. (1997) showed that the predominance of carbazole in rocks from the Upper Keg River Member which accumulated under regressive higher salinity conditions. Considering that if the response of tricyclic terpanes and gammacerane are due to changes in salinity, south of the Willow Creek fault the highest tricyclic terpanes and lowest gammacerane concentrations were found suggesting an area of relative lower salinity, which would support the interpretation based on the relative carbazole content. However, the changes encountered in the molecular parameters based on nitrogen compounds appear more complex than those generated using biomarker and aromatic hydrocarbon data suggesting that nitrogen compound variation is likely reflecting a combination of processes in addition to facies. A similar pattern emerges in the behaviour of the 1,8-dimethylcarbazole/(1,8-dimethylcarbazole + 1-ethylcarbazole) parameter (Fig. 12b), with increased contribution from the 1,8-dimethylcarbazole to the south in the

953

0.4

0.2

0.0 0.6

0.8

1.0

1.2

Pr / Ph Fig. 13. Cross plots showing the variation in (a) calculated vitrinite reflectance (Rc), (b) Ts/(Ts + Tm) and (c) Pr/Ph versus the benzocarbazole [a]/([a] + [c]) parameter in the H–T–A oils from central Montana. Key: see legend in Fig. 5.

954

B. Bennett, S.D. Olsen / Organic Geochemistry 38 (2007) 935–956

gen compounds, particularly in clastic systems, occur during petroleum expulsion from the source which is likely to reduce the compositional features that were characteristic of the source rock (e.g. maturity, facies).

1.2

1,8-/ 1,8-+1-Et

1 0.8 0.6

4. Conclusions

0.4 0.2 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Ts / (Ts+Tm)

1,8-/ 1,8-+1-Et

1.2

0.8

0.4

0 0.6

0.8 Pr /Ph

1.0

Fig. 14. Cross plots displaying (a) Ts/(Ts + Tm) versus 1,8dimethylcarbazole/(1,8-dimethylcarbazole + 1-ethylcarbazole) and (b) Pr/Ph versus 1,8-dimethylcarbazole/(1,8-dimethylcarbazole + 1-ethylcarbazole) in the H–T–A oils from central Montana. Key: see legend in Fig. 5.

(Fig. 13a); source, Ts/(Ts + Tm) (Fig. 13b); and palaeoenvironmental conditions, Pr/Ph (Fig. 13c). There appear to be no straightforward correlations between the molecular parameters and the benzo– carbazole data, which was also found by Silliman et al. (2002), whereas Bakr and Wilkes (2002) showed strong correlations in relation to facies indicators. Relationships based on the parameter 1,8-/ (1,8- + 1-Et) versus either Ts/(Ts + Tm) (Fig. 14a) and Pr/Ph (Fig. 14b) show no trends that would suggest the parameter is dependent exclusively on source depositional conditions. Nitrogen compounds show weakly developed correlations e.g. a shift toward carbazole predominance (Fig. 12a), when compared with the behaviour of biomarkers and aromatic hydrocarbons, suggesting that nitrogen compounds are influenced by other geochemical properties. Bennett et al. (2002) showed strong fractionation effects on nitro-

The saturated and aromatic hydrocarbons of central Montana petroleum show a high degree of compositional variation. Three petroleum types were recognised, within the H–T–A oils, in addition to the clearly different Swift-Morrison oils. However, based simply on molecular maturity parameters, Swift-Morrison and Amsden petroleum would appear to be related, however, using a quantitative biomarker approach these oils are clearly resolved as separate oil types. A number of classical molecular parameters commonly employed as indicators of source maturity of petroleum are not easily applied to determine the maturity of H–T–A petroleum due mainly to the inherent variation related to the many source rock components that are known to make up the Heath Formation. The source variations and their influence upon molecular parameters are strongly recorded amongst the hydrocarbon components in H–T–A petroleum. The hydrocarbon defined compositions allow recognition of three main oil sub-types in the H–T–A petroleum system. The increased carbonate content associated with the Amsden petroleum coincides with the stable platform of the Bull Mountain Basin region south of the Willow Creek fault. The Tyler oils to the north of the Willow Creek fault are typically derived from a more clay containing source, while the abundant gammacerane suggests hypersaline conditions with restricted circulation. Lower relative abundance of gammacerane combined with a strong contribution due to tricyclic terpanes may suggest a decrease in salinity from north to south across central Montana. The nitrogen compounds show weakly developed north-south trends in composition suggesting that their behaviour is susceptible to other geological factors (e.g. source retention), whereas the saturated and aromatic hydrocarbons show strong regional trends dominated by source depositional environmental conditions. This could be attributed to compositional fractionation during primary migration and source expulsion that has removed inherited source composition characteristics from the pyrrolic nitrogen compounds, whereas these

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processes do not appear to affect the information derived from the saturated and aromatic hydrocarbon composition. Acknowledgements The samples were provided by Dan Boatwright and Rich Aram of Phillips (now ConocoPhillips) and Gary Isaksen of ExxonMobil as part of a Ph.D. (Sam Olsen) investigation into the application of ICP-MS for the analysis of metals in sedimentary organic matter. Paul Donohoe is thanked for GC–MS analysis. Prof. Steve Larter, Dr. Paul Farrimond, Dr. Martin Jones and Bernie Bowler are thanked for useful discussions. We are grateful to Dr. Mark Obermajer and Dr. Maowen Li for constructive reviews of this manuscript. Associate Editor—M. Li References Abbott, G.D., Bennett, B., Petch, G.S., 1995. The thermal degradation of 5a(H)-cholestane during closed system pyrolysis. Geochimica et Cosmochimica Acta 59, 2259–2264. Aquino Neto, F.R., Trendel, J.M., Restle, A., Connan, J., Albrecht, P.A., 1983. Occurrence and formation of tricyclic and tetracyclic terpanes in sediments and petroleums. In: Bjorøy, M. et al. (Eds.), Advances in Organic Geochemistry 1981. John Wiley and Sons, pp. 659–677. Aquino Neto, F.R., Cardoso, J.N., Rodrigues, R., Trindade, L.A.F., 1986. Evolution of tricyclic alkanes in the Espirito Santo Basin, Brazil. Geochimica et Cosmochimica Acta 50, 2069–2072. Aram, R.B., 1993. Source rock study of central Montana. In: Hunter, L.D. (Ed.), Energy and Mineral Resources of Central Montana. Montana Geological Society, Billings, pp. 179–193. Bakr, M.M.Y., Wilkes, H., 2002. The influence of facies and depositional environment on the occurrence and distribution of carbazoles and benzocarbazoles in crude oils: a case study from the Gulf of Suez, Egypt. Organic Geochemistry 33, 561– 580. Bennett, B., Larter, S.R., 2000. Quantitative separation of aliphatic and aromatic hydrocarbons using silver ion-silica solid-phase extraction. Analytical Chemistry 72, 1039–1044. Bennett, B., Chen, M., Brincat, D., Gelin, F.J.P., Larter, S.R., 2002. Fractionation of benzocarbazoles between source rocks and petroleum. Organic Geochemistry 33, 545–559. Bishop, A.N., Abbott, G.D., 1993. The interrelationship of biological marker maturity parameters and molecular yields during contact metamorphism. Geochimica et Cosmochimica Acta 57, 3661–3668. Clark, J.P., Philp, R.P., 1989. Geochemical characterisation of evaporite and carbonate depositional-environments and correlation of associated crude oils in the Black Creek Basin, Alberta. Bulletin of Canadian Petroleum Geology 37, 401– 416.

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