Light hydrocarbon (gasoline range) parameter re®nement
of biomarker-based oil±oil correlation studies: an example
from Williston Basin
Mark Obermajer *, Kirk G. Osadetz, Martin G. Fowler, Lloyd R. Snowdon
Geological Survey of Canada, 3303-33rd Street NW, Calgary, AB T2L 2A7, CanadaReceived 28 January 2000; accepted 26 July 2000 (returned to author for revision 27 April 2000)
Abstract
We evaluated geochemical compositions of 189 crude oils produced from Paleozoic reservoirs across the Williston Basin. Emphasis is placed on compositional variations in the gasoline range (i-C5H12-n-C8H18) to verify the
biomarker-based classi®cation of oil families. The oils belong to four distinct compositional oil families Ð A, B, C and DÐ broadly con®ned to speci®c stratigraphic intervals. The unique character of each oil family, evident from theirn-alkane and biomarker signatures, is supported by distinctive gasoline range characteristics in general, and C7 (``Mango'')
parameters in particular. An invariance in the K1 parameter among oils from a single compositional group is observed for most of the oils. The K1 ratio, although relatively constant within each suite of oils, is dierent for each oil family, clearly indicating their compositional distinction. Other Mango parameters (N2, P2, P3) show a similar re¯ection of the oil families. However, while C7parameters provide excellent evidence for distinct familial association of oils from
families A, B and D, family C often overlaps with the latter two families, perhaps indicating greater genetic and source heterogeneity in the family C oils. Nevertheless, dierences in the gasoline range composition suggest that the existing biomarker-based classi®cation of oil families can be more universally applied throughout the entire Williston Basin. Moreover, because the light hydrocarbon parameters prove very useful in re®ning oil±oil correlations, routine gasoline range analysis shows good potential as a supplementary component in geochemical correlation of crude oils, especially when high levels of thermal maturity decrease the usefulness of biomarker compounds.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Light hydrocarbons; Isoheptanes; ``Mango'' parameters; Oil±oil correlation; Williston Basin
1. Introduction
Although higher molecular weight biomarkers (C20±
C40) are considered the best tools for oil±oil correlation
studies because they provide much information regard-ing an oil and its source rock (Peters and Moldowan, 1993), these compounds are unstable under thermal stress and are often absent in high maturity oils/con-densates (van Graas, 1990; ten Haven, 1996). In contrast, many lower molecular weight hydrocarbon compounds,
though more susceptible to biodegradation, typically comprise a persistent fraction of oils at high maturities. Benchmark studies (Thompson, 1983; Mango, 1990; BeMent et al., 1995; Halpern, 1995; ten Haven, 1996) have suggested that gasoline range hydrocarbons also carry useful information regarding genetic associations and alteration of oils. It has been documented that the light hydrocarbon ratios have applications for oil-oil correlation studies (Mango parameters, C7-based star
diagrams), provide an indication of the temperature of oil expulsion from its source (2,3-/2,4-dimethylpentane ratio), and re¯ect the stage of thermal decomposition of oil (paran indices). The application of these light hydrocarbon analyses is advantageous, not only because
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* Corresponding author.
they may constitute the only compositional fraction available for analysis in oils/condensates generated late during catagenesis when sterane and terpane biomarkers are below detection limit, but also because such tech-niques are relatively rapid and inexpensive. Therefore, they show excellent potential and can be extremely practical for geochemical correlations of low-density crude oil/condensate fractions providing valuable infor-mation about dierences in source kerogen, depositional paeloenvironment, genetic anities and petroleum alteration, data typically obtained through more advanced analyses of biomarkers. Moreover, it has been indicated that analyses of light hydrocarbons have application in oil-source correlation studies because the lighter (C5±C8) fraction of source rock kerogen can be
evaluated through thermal extraction (Jarvie and Walker, 1997; Odden et al., 1998).
The main objective of the present study was to exam-ine a suite of 189 oils produced from the Red River, Winnipegosis, Bakken and Madison reservoirs (Middle Ordovician Ð Lower Mississippian), from both the American and Canadian portions of the Williston Basin, with emphasis placed on the composition of the gasoline range hydrocarbons (C5±C8range). These light
hydrocarbon parameters, C7 in particular, are used to
constrain the biomarker-based classi®cation of oil families in the Williston Basin. Mango (1987, 1990) observed a unique invariance in the relative concentra-tion of methylhexanes and dimethylpentanes in oil, indicating that the ratio of [2-methylhexane+2,3-dime-thylpentane]/[3-methylhexane+2,4-dimethylpentane], the so called K1 parameter, is relatively constant and remains around unity (i.e.1.0). A high consistency of this ratio within a large set of oils (2000) was interpreted by Mango (1987) as an argument against a speci®c bio-logical precursor for isoheptanes. Instead, a chemical steady-state kinetic process, with constant rates of pro-duct formation, was proposed as a mechanism for the generation of isoheptanes. However, when discussing some other parameters derived from his C7
parent-daughter transformation scheme, Mango (1990) also indicated that distinctions between some of these para-meters likely re¯ect dierences in kerogen type and kerogen structure. Therefore, oils generated from the same source kerogen (homologous oils) should have similar ratios of isoheptanes and dimethylcyclopentanes. This concept was further tested by ten Haven (1996) who, based on a smaller (500) but global set of oils, concluded that K1 ratios should be consistent within co-genetic suites of oils. Although con®rming the remark-able invariance of isoheptanes, ten Haven (1996) docu-mented that the K1 ratio is not always around 1.0, and can vary signi®cantly between homologous series of oils. Interestingly, this variance makes the K1 ratio very useful for correlating oils because ``. . .if K1 would have been constant for oils world-wide, then there would
have been no application in correlation studies. . .'' (ten Haven, 1996, p. 962). It was stressed, however, that the light hydrocarbon parameters should be used in con-junction with other, more conventional geochemical data. More recently, Wilhelms et al. (1999) indicated that kinetic fractionation model proposed by Mango (1990) was inconsistent with compound speci®c isotopic composition of C7 hydrocarbons. These authors,
how-ever, also pointed to a common precursor for most of the C7compounds.
Following these studies, C7 parameters have been
applied successfully in the Williston Basin for grouping oils (Jarvie and Walker, 1997; Obermajer et al., 1998). In the present paper, a number of standard gasoline range hydrocarbon parameters are used not only to examine if they are universally applicable but also to validate and re®ne the existing biomarker-based classi-®cation of oil families in the Williston Basin (Osadetz et al., 1992), and to investigate if this classi®cation is applicable throughout the entire Williston Basin. Re-examination of this classi®cation based on our new analyses will allow a much better understanding of the petroleum systems in the Williston Basin, providing a framework for appraising the future hydrocarbon potential of this basin.
2. Paleozoic oils in Williston basin Ð an overview
The Williston Basin, situated on the western Canadian Shield within the interior platform structural province (Fig. 1), is a sub-circular epicratonic, preservational basin ®lled with sedimentary rocks of predominantly marine origin. These sedimentary sequences range in age from Cambrian to Tertiary reaching a maximum thick-ness of5 km near the center of the basin (Williston, North Dakota). The basin is a proli®c petroleum pro-vince with numerous occurrences of oil documented throughout the Phanerozoic succession. Petroleum occurs in structural, stratigraphic and combined struc-tural±stratigraphic traps that are often controlled by important epeirogenic basement structures such as Cedar Creek and Nesson anticlines (Clement, 1987; Gerhard et al., 1987 LeFever et al., 1987).
Fig. 1. Map showing the location and main structural elements of the Williston Basin.
Table 1
Generalized Williston Basin oil family classi®cation schemes (modi®ed from Osadetz et al., 1994)
Williams, 1974 Zumberge, 1983;
Leeheer and Zumberge, 1987
Osadetz et al., 1992, 1994 Source rocks
Type III
(Pennsylvanian oils)
Not studied Not studied Tyler Fm. (Pennsyl.)
not studied Family E
(Bakken oils)
Exshaw/Bakken Fm. (U. Dev.-Miss.)
Type II
(Devonian, Mississippian & Mesozoic oils)
Group 2
(Mission Canyon oils)
Family B (Bakken oils)
Bakken Fm. (U.Dev.-Miss.)
Family C
(Miss. & Jurassic oils)
Lodgepole Fm. (L. Miss.)
Not studied Group 4
(Nisku oils)
Family D (Winnipegosis oils)
Winnipegosis Fm. (M.Dev.)
Group 3 (Duperow oils) Type 1
(Ordovician-Silurian oils)
Group 1 (Red River oils)
Family A (Red River oils)
Winnipeg Gr. (M. Ord.) and Bighorn Gr. (U.Ord.) Group 5
(Cambrian oil)
isotope compositions conformed to these families and, together with other data, showed that many pools were water washed or biodegraded (Bailey et al., 1973a,b; Thode, 1981; Thompson, 1983). Based on a smaller sam-ple set, Zumberge (1983) and Leenheer and Zumberge (1987) classi®ed crude oils from the American Williston Basin into ®ve oil groups/families (Table 1). Group 1, identical to Type 1, was recognized as unique to Ordo-vician petroleum systems in the American mid-continent (Longman and Palmer, 1987; Jacobson et al., 1988; Foster et al., 1989) and elsewhere (Fowler and Douglas, 1984; Foster et al., 1986; Homann et al., 1987). Group 2 oils had characteristics similar to type II, Bakken-sourced oils. No equivalent to type III was identi®ed. Instead, three other groups included high and low maturity oils from Devonian pools (groups 3 and 4, respectively) and a low maturity oil from Middle Cambrian ± Lower Ordovician Deadwood Formation (group 5).
A commonly accepted model of eastern Williston Basin petroleum systems, outlined initially by Brooks et al. (1987) and then revised and more comprehensively documented by Osadetz et al. (1992, 1994) in Canada, was followed by comparable petroleum systems descri-bed from the US portion of the Williston Basin (Price and LeFever, 1994; Osadetz et al., 1995). Using combi-nations of terpane, steroidal and normal alkane char-acteristics Osadetz et al. (1992) categorized oils from the Canadian Williston Basin into four families. Family A, commonly restricted to Upper Ordovician reservoirs, had distinctive n-alkane distributions low acyclic iso-prenoid/n-alkane ratios and corresponded to type I± group 1 (Table 1) with sources in Upper Ordovician Bighorn Group kukersites (Osadetz et al., 1992; Osadetz and Snowdon, 1995), and not Winnipeg shales as pre-viously postulated (Williams, 1974; Dow, 1974).
Oils with low tricyclic/pentacyclic terpane ratios but lacking gasoline range and n-alkane characteristics of the family A, typically occurring below the Three Forks Group, were classi®ed as family D. This family was further subdivided into two sub-families recognized by their distinctive stratigraphic occurrence andn-alkane/ acyclic isoprenoid composition (Osadetz et al., 1992). Oils from Winnipegosis pinnacle reefs, family D2, are distinguished from other oils occurring predominantly in Saskatchewan and Manitoba groups reservoirs, family D1. Family D2 oils were speci®cally inferred to have source rocks in the Brightholme Member of the Winnipegosis Formation, while D1 oils were inferred to have sources in Devonian strata like, but not exclusive to, those found at the contact between the Upper and Lower members of the Winnipegosis Formation (Osa-detz et al., 1992; Osa(Osa-detz and Snowdon, 1995). Family D was not represented in the original study (Williams, 1974), but would likely correlate with group 3, 4 and possibly group 5 oils of Leenheer and Zumberge (1987) (Table 1).
Two other families, B and C, distinguished from families A and D mainly based on terpane ratios (Osa-detz et al., 1992), are found in Bakken Formation to Mannville Group reservoirs. Family B oils occur pri-marily in the Bakken Formation, while Family C oils are found primarily in the Mississippian Madison Group and Mesozoic strata. Both families are subdivi-sions of type II of Williams (1974) and group 2 of Leenheer and Zumberge (1987). Although a Bakken source was initially inferred for all these oils, it has been proposed that only family B oils are derived from Bak-ken Formation shales and family C oils are derived from Lodgepole Formation carbonates (Osadetz et al., 1992, 1994; Price and LeFever, 1994; Osadetz and Snowdon, 1995). Families B and C were then identi®ed in American Williston Basin by Price and LeFever (1994) who con®rmed the predominance of family C oils in the Mississippian subcrop play and the common restriction of family B oils to the Bakken Formation.
More recent studies of Williston Basin petroleum systems indicated a possible existence of several petro-leum sub-systems and numerous source rock intervals within Madison Group strata (Jarvie and Inden, 1997; Jarvie and Walker, 1997). Moreover, an up-to-date assessment of the Williston Basin petroleum systems provided by Jarvie (in press) documents a dominant Madison Group system with four proven and two hypothetic sub-systems, as well as functional secondary systems, such as Bakken-Lodgepole, Bakken, Duperow and Red River petroleum systems.
There are oils from a few pools, distinguished by their stratigraphic occurrence and isotopic composition, that do not comply with the general classi®cation of the Williston Basin oils. These include a Cambrian Dead-wood Formation oil at Newporte Field (Leenheer and Zumberge, 1987; Fowler et al., 1998) and a Beaverlodge Silurian pool on the Nesson Anticline (Downey, 1996). Therefore, there is a possibility that a major, currently unrecognized petroleum system (or systems) operates in the lower Paleozoic strata across the Williston Basin. More recently, Obermajer et al. (1999) indicated that oils occurring in the Upper Devonian Birdbear Forma-tion reservoirs in Saskatchewan have a distinctive geo-chemical composition and should be separated from family D Winnipegosis oils, with which they were for-merly grouped (Osadetz et al., 1992).
3. Analytical techniques
The gasoline range hydrocarbons (i-C5H12±n-C8H18)
the gas chromatograph equipped with a 60m DB-1 fused silica column. The initial temperature was held at 30C for 10 min and then programmed to 45C at a rate of 1oC/min. The ®nal temperature was held for 25 min. The
eluting hydrocarbons were detected using a ¯ame ioni-zation detector.
An aliquot of the fraction boiling above 210C was deasphalted by adding an excess of pentane (40 volumes) and then fractionated using open column liquid chromatography. Saturated hydrocarbons were
analysed using gas chromatography (GC) and gas chromatography±mass spectrometry (GC±MS). A Var-ian 3700 FID gas chromatograph was used with a 30 m DB-1 column coated with OV-1 and helium as the mobile phase. The temperature was programmed from 50 to 280C at a rate of 4C/min and then held for 30 min at the ®nal temperature. The eluting compounds were detected and quantitatively determined using a hydrogen ¯ame ionization detector. The resulting gasoline range (GRGC) and saturate fraction chromatograms (SFGC)
were integrated using Turbochrom software. ``Mango'' parameters were calculated using the normalized per-centage peak area from GRGC's instead of the weight percentage abundance in the whole oil as originally applied by Mango (1987).
Single ion monitoring GC±MS experiments were per-formed on a VG 70SQ mass spectrometer with a HP gas chromatograph attached directly to the ion source (30 m DB-5 fused silica column used for GC separation). The temperature, initially held at 100C for 2 min, was pro-grammed at 40C/min to 18C and at 4C/min to 320C, then held for 15 min at 320C. The mass spec-trometer was operated with a 70 eV ionization voltage, 300 mA ®lament emission current and interface tem-perature of 280oC. The instrument was controlled by an
Alpha Workstation using Opussoftware. Terpane and sterane ratios were calculated usingm/z191 andm/z217 mass fragmentograms.
4. Results and discussion
Most of the analyzed oils are paranic in nature as they contain a high proportion of hydrocarbons in fraction boiling above 210C. In general, the combined amounts of hydrocarbon fractions are higher in samples collected from the pools located in the southern (US) portion of the Williston Basin, often reaching values of more than
95%. The proportion of hydrocarbons is typically lower in oils from the northern (Canadian) part of the Will-iston Basin, although in most of the family D and some of the family B oils this parameter is often greater than 90%. Lower values (<80%) observed in oils from families A and C are associated with higher aromaticity and characteristic saturates/aromatics ratios of less than 1.0. In contrast, saturates predominate over aromatics in oils containing a high proportion of hydrocarbons resulting in saturates/aromatic ratios of more than 1.0.
Data from the present study not only demonstrate that the examined oils belong to four distinct oil cate-gories corresponding to previously de®ned oil families A, B, C and D (Osadetz et al., 1992; 1994) but also indicate that the previously de®ned biomarker-based classi®cation of oil families can be more universally applied in the Williston Basin.
4.1. Evidence of familial association based on higher molecular weight (C13±C40) compounds
Representative saturate fraction gas chromatograms (SFGC) and mass fragmentograms are shown on Figs. 3,6 and 8 while selected geochemical characteristics are summarized in Table 2. The dierences in the distribu-tions of normal alkanes in C13±C30range in each of the
previously de®ned oil families are evident from their SFGCs (Fig. 3). Family A oils (majority from the
Fig. 3. Representative 210C+ saturate fraction gas chromatograms showing variations in then-alkane pro®les in the Williston Basin
Ordovician Red River/Yeoman reservoirs) are char-acterized byn-alkane pro®les centered at C13±C17, very
low concentration of acyclic isoprenoids (i.e. pristane and phytane) relative to n-alkanes (lowest pristane/n -C17H36and phytane/n-C18H38ratios Ð Fig. 4) and low
relative abundance of C20+ members, resulting in high
middle/long chain n-alkane ratios, often greater than 10.0. A strong predominance of odd to even normal alkanes within the C14±C20 range and high Carbon
Preference Index values (average CPI of 1.59) are typical in these oils (Fig. 5). The absolute Pr/Ph, Pr/nC17and
Ph/nC18ratios have to be carefully veri®ed as there may
be a bias due to very low concentrations of pristane and phytane (co-elution with other compounds during GC analysis). Regardless, these oils can be easily dier-entiated from the other families based on their overall n-alkane pro®le and CPI values. As previously noted by Osadetz et al. (1992), these characteristics are common to many Ordovician oils worldwide (Reed et al., 1986; Longman and Palmer, 1987; Fowler, 1992) indicating they originated from sources geochemically similar to those of the other Ordovician oils, with the algae
Gloeocapsomorpha prisca as a main component of the source organic matter.
The remaining groups of oils have broadern-alkane pro®les with relatively higher concentrations of C20+
members and acyclic isoprenoids. Oils from Family B are characterized by a smooth n-alkane distribution with a maximum in the C13±C17 range. Their pristane/
phytane ratios are usually greater than 1.0, similar to family A, but the concentration of acyclic isoprenoids relative to n-alkanes is much higher than in family A
oils, allowing for obvious separation (Fig. 4). Moreover, the concentration of long-chain hydrocarbons is also higher in these oils when compared with family A while the CPI values are lower (Fig. 5). Families C and D are characterized by predominance of phytane over pristane (stronger and more consistent in family C) and Pr/Ph ratios of less than 1.0 (Fig. 3, Table 2). The concentra-tion of phytane relative to C18n-alkane is also higher in
these groups as compared with families A and B. How-ever, as shown on Fig. 4, there is some overlap in Pr/
nC17and Ph/nC18ratios among oils from families C and
D. The low Pr/Ph ratios and high abundances of phy-tane indicate that the source kerogen was deposited under restricted, elevated-salinity conditions. Accord-ingly, Family C oils have been correlated to sources deposited in distal anoxic settings of a broad carbonate ramp (Lodgepole Fm.) while Family D oils have been related to sources from ``o-reef'' starved basins settings (Winnipegosis Fm.) (Osadetz et al., 1992, Osadetz and Snowdon, 1995). Both oil families have comparable concentrations of long-chain hydrocarbons and cannot be easily resolved from each other based on their mid-/ long-chain hydrocarbon ratios (Table 2). Nevertheless, the SFGCs of the Winnipegosis-sourced family D oils have a dierentn-alkane envelope, typically displaying a bimodal distribution centered at C15±C17 and C23±C25
(Fig.3). Furthermore, there is a stronger odd carbon number preference in family D oils, with an average CPI ratio of 1.25 compared with 1.09 in the family C oils (Fig. 5, Table 2). Therefore, SFGC analysis provides good evidence for the existence of four compositional oil families.
Fig. 5. Cross-plot of pristane/phytane ratio (Pr/Ph) versus Carbon Preference Index (CPI, C14-20 range, see Table 2 for ratio de®nition) for the analyzed oil samples. The outlines, showing typical ®elds incorporating most of the oils from each oil family, are intended for visual approximation only. Black symbols denote one family A and one family C sample with CPI values of 1.98 and 2.19, respectively.
The geochemical dierences within each family of oils were originally shown by the distributions of steranes and terpanes. The oils are easily categorized based on the distribution of extended hopanes and C34 or C35
prominence. The family A oils are characterized by a smooth extended hopane pro®le with a steady decrease in the concentration of C31+ homologues with increasing
carbon number, although some samples have a minor predominance of the C34member (Fig. 7). A more
dis-tinctive C34hopane prominence is readily observable in
the Winnipegosis oils (Figs. 6 and 7). In contrast, most family C oils show a C35hopane prominence, a
char-acteristic not noticeable in oils from other families of eastern Williston Basin. Moreover, these oils also have the highest relative abundance of C29 norhopane and
relatively high concentration of C23 tricyclic terpanes,
and therefore, are easily distinguishable as a separate family (Figs. 9 and 10). Higher abundance of C23
tri-cyclic terpane is also seen in Family B oils (Fig. 10), but these oils also display relatively higher concentration of C24 tetracyclic terpane compared with family C oils.
Perhaps a more apparent dierence in the terpane ®nger-prints of these two families is the lack of any homohopane
Fig. 6. Representativem/z191 mass fragmentograms of the saturate fraction showing the distribution of terpanes in the analyzed Williston Basin oils. 23-C23tricyclic terpane, Ts-18(H)-trisnorhopane, 30-hopane, G-gammacerane, 34-C34homohopane.
prominence in family B oils (Fig. 7). Similarly, C23
tri-cyclic and C24tetracyclic terpane compounds are in low
relative concentrations in oils from the other two famil-ies (A and D). There is a predominance of 17(H)-tris-norhopane (Tm) over 18(H)-tris17(H)-tris-norhopane (Ts) in most of the analyzed oils with the exception of Family B oils in which the relative concentration of these two compounds is quite variable, with a majority of samples having Ts/Tm ratios greater than 1.0.
There are some dierences in the normalized relative abundance of C27:C28:C29 regular steranes. While the
proportion of the C28 member is the lowest and
gen-erally similar in all oil families, the proportion of C27
and C29members is more variable. In the family B oils,
the C29 sterane occurs in same concentration as C27
sterane (Table 2). In contrast, the relative abundance of C29 sterane are increasingly greater in the remaining
families, reaching more than 50% in oil families D and A (average of 51% and 58%, respectively). The pro-portion of C27:C28:C29regular steranes is considered to
be a highly speci®c correlation index (Peters and Mol-dowan, 1993, pp. 182±186), therefore providing further support for classifying studied oil samples into compo-sitionaly distinct oil categories.
Moreover,m/z217 mass fragmentograms show pro-minent diasterane peaks (Fig. 8). Both C29and C27
dia-steranes are present in high concentrations relative to regular steranes especially in oil families A and B (Fig. 9). However, the B oils show a higher relative abun-dance of C21regular sterane (pregnane) and the highest
C21/C29regular sterane ratio amongst all analyzed
sam-ples making distinction of the oil family B quite appar-ent (Table 2, Fig. 10). Some of the family A oils also display higher concentrations of C21 regular sterane,
which perhaps could be related to their higher thermal maturity (slightly higher C29regular sterane
isomeriza-tion ratios) as the majority of oils from this family have C21/C29regular sterane ratio of less than 1.0.
The epimerization ratios of C29 regular steranes are
quite variable and inconclusive with respect to deter-mining maturity of oils. The C29 abb/(aaa+abb)
reg-ular sterane isomerization ratio is the highest in oil families B and C, often approaching equilibrium values. Interestingly, this trend is not parallelled by the C29
S/(S+R) isomerization ratio which is the highest in oil families A and D. These variable ratios are not always consistent with maturity data derived from the gasoline range parameters and the distribution ofn-alkanes and
acyclic isoprenoids, what indicates a possible source control on the concentration of C29regular sterane and
hence the dierent genetic character of these oils.
4.2. Compositional dierences among gasoline range (C5±C8) hydrocarbons
Representative gasoline range gas chromatograms (GRGC) are shown on Fig. 11 while selected gasoline
range characteristics of the investigated oils are listed in Table 2 and shown on Figs. 12±17. As there is com-monly a variable overlapping in light hydrocarbon parameters between oil family C and other oil families, data for family C oils are typically shown on separate cross-plots (®gures ``b''). The relative abundances of the normal heptane and isoheptane, as well as the Paran Indices (Thompson, 1983), provide initial evidence for grouping the oils into separate categories (Figs. 12 and 13). However, while the distinction of oil families A, B and D is evident based on these characteristics, Family C is not clearly distinguishable. The family A oils have the highest concentration of n-heptane relative to iso-heptane (Fig. 12a), and therefore, the highest and most variable n-heptane/isoheptane ratio (nC7/bC7 Ð see
Table 2 for ratio de®nition) ranging from 1 to 10 (aver-age of 5.19). The same parameter shows less variation in the remaining oils which have similar concentrations of these two compounds with their ratio close to 1.0. There is some overlap, but in general relatively good separa-tion between oil families B and D (Fig. 12a). In contrast, it is dicult to clearly distinguish family C as most of these samples show similar proportions of heptane and isoheptane to family B, typically plotting within its ®eld (Fig. 12b). Similarly, both PI I and PI II values are generally lowest in the family B and C oils, with some overlap with family D oil samples (Fig. 13). The high values obtained for the Family A oils (Heptane Values greater than 30.0 and Isoheptane Values often greater than 1.0) easily classify these oils into a distinct group. Such variable Paran Indices would suggests that the family A oils are generally of the highest and families B and C of the lowest maturity. Accordingly, using the criteria of Thompson (1983) the A oils could be inter-preted as supermature and the remaining groups as normal mature. Although the degree of paranicity can be useful in estimating thermal maturity such an assess-ment has to be constrained with other maturity indica-tors. High Paran Indices may also result from the retention of already generated hydrocarbons in source rocks prior to expulsion or extended presence of oil in the reservoirs (Thompson, 1983). However, as the con-centration of cycloalkanes in oil depends not only on temperature and pressure but also on kerogen structure, it is more likely that observed low concentrations of cycloalkanes have resulted from natural variations in source organic facies due to the highly aliphatic, G. prisca-derived (Ordovician) source kerogen for the family A oils. The greater variability in Paran Indices observed within the family C and D oils likely re¯ects a more complex nature of source organic facies and the eects of the restricted paleodepositional environment of their source rocks.
There is also some variability in the concentrations of low molecular weight aromatic hydrocarbons such as benzene and toluene. The concentrations of toluene Fig. 10. Biomarker cross-plot of C23 tricyclic terpane/C30
hopane ratio (23h/30h) versus C21/C29regular sterane ratio (21/ 29s) for the Williston Basin oil samples. Oil family B samples with 21/29s ratios greater than 2.35 (11 samples) are not plot-ted. The outlines, showing typical ®elds incorporating most of the oils from each oil family, are intended for visual approx-imation only.
relative to methylcyclohexane and benzene relative to 3-methylpentane, are generally the lowest in the B oils, especially in the Canadian samples, probably re¯ecting water washing and longer secondary migration pathways.
Except for a few samples, benzene and toluene are pre-sent in higher relative concentrations in the remaining oil families, with family C oils typically showing the highest amounts of these compounds.
Table 2
Selected geochemical characteristics of the Williston Basin oils analysed in the present study
Family A
no.a mean st. dev.b no. mean st.dev. no. mean st.dev. no. mean st.dev.
N-alkanes/ Pr/Phc 57 1.10 0.28 31 1.40 0.20 63 0.68 0.15 36 0.87 0.20
isoprenoids Pr/nC17d 0.06 0.02 0.58 0.25 0.47 0.30 0.42 0.22
Pn/nC18e 0.23 0.08 0.51 0.18 0.70 0.23 0.74 0.46
CPIf 1.60 0.13 1.10 0.06 1.09 0.18 1.25 0.10
mc/lcg 9.07 4.74 4.02 2.24 1.81 0.52 2.11 0.69
Sterane/terpane S/(S+R)h 49 0.51 0.03 25 0.48 0.05 39 0.46 0.02 35 0.52 0.04
biomarkers = i 0.54 0.03 0.59 0.06 0.58 0.03 0.56 0.04
dia/regj 2.48 0.93 2.55 1.11 0.41 0.26 1.52 1.04
27:28:29k 28:14:58 39:20:41 38:15:47 29:20:51
21/29sl 0.61 0.34 2.46 1.77 0.47 0.17 0.46 0.49
Ts/Tmm 0.85 0.50 1.41 0.75 0.71 0.53 0.87 0.32
29h/30hn 0.59 0.10 0.71 0.41 0.95 0.11 0.60 0.09
23h/30ho 0.04 0.05 0.67 0.30 0.40 0.15 0.07 0.05
Gasoline range hydrocarbons P1 Ip 58 1.23 0.38 27 0.77 0.31 58 1.06 0.47 33 1.39 0.56
Mango parameters PI IIq 50.97 10.10 22.60 5.18 24.40 9.70 28.12 7.33
nC7r 53.41 11.85 22.91 5.47 26.80 9.70 29.64 8.37
bC7s 11.58 4.00 21.82 2.77 23.69 9.99 23.64 3.83
K1t 1.15 0.06 0.86 0.07 0.90 0.13 1.23 0.09
P2u 3.71 1.07 8.06 1.49 7.05 1.22 7.48 1.30
P3v 0.89 0.36 1.46 0.41 1.59 0.56 2.33 0.90
N2w 1.66 0.49 7.07 1.61 4.22 1.82 3.21 0.77
a no. Ð Number of analyses used. b st.dev. Ð standard deviation. c Pristane/phytane ratio. d Pristane/n-C
17ratio. e Phytane/n-C
18ratio.
f Carbon Preference Index=1=2 C
15C17C19
27diasterane/5a(H),14a(H),17a(H) 20R- C27sterane. k Normalized relative abundance of C
27, C28and C29regular steranes based onabbisomers. l Pregnane/5a(H),14a(H),17a(H) 20R- C
p Paran Index I (Isoheptane Value- Thompson, 1983). q Paran Index II (Heptane Value- Thompson, 1983).
r n-heptane*100/of compounds eluting between 2-methylhexane and 2,2-dimethylhexane.
s (2-methylhexane+2,3-dimethylpentane+3-methylhexane)*100/ of compounds eluting between 2-methylhexane and 2,2-di-methylhexane
t K1=(2-methylhexane+2,3-dimethylpentane)/(3-methylhexane+2,4-dimethylpentane). u P2=2-methylhexane+3-methylhexane.
The separation of oils from the Williston Basin as de®ned by light hydrocarbon parameters, is further re®ned using selected C7 ``Mango'' parameters (Mango, 1987,
1990). Previous studies elsewhere have demonstrated a
possible general invariance in the K1 parameter (the ratio of [2 - methylhexane+2,3 - dimethylpentane]/[3 - methyl-hexane+2,4-dimethylpentane]). In general, this is also observed for most of the studied samples. However, Fig. 12. Relative concentrations of normal heptane (nC7) and isoheptane (bC7) (see Table 2 for ratio de®nitions). The outlines are for visual approximation only. While there is some overlapping between oil families B and D (a), family C oils typically fall within family B ®eld (b).
although these ratios appear relatively constant within each biomarker-de®ned oil family they are not around unity (i.e.1.0) as claimed by Mango (1987). Following the conclusions of ten Haven (1996) that K1 ratios would be expected to be consistent within co-genetic suites of oils, dierent values of K1 parameter deter-mined for each oil family can be interpreted as verifying the genetic uniqueness of the oil families. Fig. 14a shows a good separation between families A, B and D provid-ing an additional, strong evidence for such groupprovid-ing.
However, there is strong overlap between families C and B (Fig. 14b). This observation is compatible with indi-cations of familial anity from other light hydrocarbon parameters, such as those shown on Figs. 12 and 13. Moreover, the K1 ratios for oil families B and C are typically below 1.0 (on average 0.90 and 0.86, respectively Ð Table 2) which clearly contrasts with values greater than 1.0 determined for families A and D (on average 1.15 and 1.23, respectively). This range of K1 values corresponds to that reported by Schaefer and Fig. 14. Normalized % peak area of 3-methylhexane+2,4-dimethylpentane versus 2-methylhexane+2,3-dimethylpentane showing invariance of Mango's K1 parameter in the analyzed oil samples. The outlines are for visual approximation only. There is a good separation between oil families A, B and D (a) but some overlapping between oil families C and B (b).
Littke (1988) who observed an increase in K1 values with increasing thermal maturity within a vitrinite re¯ectance range of 0.48±0.88%. However, as these results were obtained from thermal extracts of type II kerogen source rocks, where concentrations of light hydrocarbons is often aected by diusion or early expulsion and migration, the eect of temperature on K1 ratio in oils remains to be tested.
Two other cross-plots similar to those used by ten Haven (1996) show familial anity and variation in a similar manner. The data for families A, B and D plot in separate ®elds on the P2+N2 versus P3 and the N2/P3 versus P2 cross-plots (Figs. 15 and 16). This provides further support for classifying these oils into three
separate oil families. In addition, these two correlations also indicate a similar diculty of distinguishing family C oils from the other oils. Moreover, it appears that the light hydrocarbon guidelines for distinguishing lacustrine from terrigenous/marine oils based on these parameters, as used by ten Haven (1996), are not universally applicable.
4.3. Discussion
With the exception of family C oils, the light hydro-carbon data presented here not only support a bio-marker-based classi®cation, but also provide additional evidence for familial association of families A, B and D. All three families are characterized by dierent gasoline Fig. 16. Normalized % peak area of Mango parameters N2/P3 versus P2 showing (a) good separation between oil families A, B and D, and (b) some overlapping between family C and families D and B. The outlines are for visual approximation only.
range data which commonly fall within relatively nar-row compositional ranges. In contrast, the same light hydrocarbon parameters regularly show greater varia-bility amongst family C oils. Moreover, a general simi-larity in the gasoline range composition of the family C samples to the B oils is quite intriguing considering the fact that not only the compositions of their higher molecular weight fractions but also the concentrations of sulfur in both groups are distinctly dierent (there is a much greater amount of sulfur present in family C oils). This observation is not in full agreement with the results of Jarvie and Walker (1997) who, based on C6
and C7parameters, not only concluded that
Madison-reservoired oils are distinctly dierent from Bakken-reservoired oils, but also indicated a positive correlation of both groups to sources occurring in the stratigraphic intervals of their reservoirs. Both families occur in stra-tigraphically separated compartments but upward migration of Bakken-sourced oils into Madison reser-voirs with or without mixing with Madison-sourced oils, though relatively uncommon, is known to have occur-red (Burrus et al., 1996). However, such cases are easily recognised using their intermediate or Bakken-like bio-marker compositions (i.e. oils from Hummingbird and Ceylon ®elds in Saskatchewan, oils in Madison Group pools of the Nesson Anticline and Waulsortian mound oils in Stark County, North Dakota). The possibility of
mixing between family B and C oils was recently asses-sed by Jarvie (in press) who concluded that Madison Group oils which show mixed signature could have received some contribution from Bakken sources, but more likely were generated from marly shale sources within Mission Canyon strata. On the other hand, oils with a family C biomarker signature are absent from pre-Madison reservoirs (except for Hummingbird ®eld, Osadetz et al., 1992). Furthermore, some ``Mango'' parameters (P3, N2) in C oils are also similar to those of the family D oils (Figs. 15 and 16).
One possible explanation for the greater variability within the C family oils and similarities in gasoline range fraction of those oils to pre-Madison-sourced oils is the limitation of ``Mango'' parameters and the fact that this approach simply cannot be extended to the Madison Group oils. However, the light hydrocarbon fraction (boiling below 210C) constitutes the bulk of the analyzed family B and family C oils, 70% and 65%, respectively. Moreover, the absolute concentrations of higher molecular weight biomarkers used as primary evidence of familial association are low in these oils, especially in the family B Ð Bakken-reservoired oils (M. Li, personal communication). Therefore, the gasoline range data deserve a more careful evaluation.
heterogeneity of their sources, such as multiple source rock bodies within the Madison Group interval, lateral organic facies variation or variable concentration of sulfur within the Lodgepole Formation. Studies from the American Williston Basin indicate that there are at least four proven and two hypothetic petroleum sub-systems in the Mississippian strata with numerous oil-prone source rocks in the Madison Group (Jarvie and Inden, 1997; Jarvie and Walker, 1997; Jarvie, in press). Most of the Family C oils analyzed during the present study come from the Canadian Williston Basin and no attempt has been made to subdivide them. The majority of these oils have been correlated to source rocks within the Lodgepole Formation (Osadetz et al., 1992, 1994), but most pools are in the overlying Charles and Mission Canyon formations. This indicates that cross-forma-tional migration of oil from the underlying Lodgepole Formation must have occurred. Moreover, the Lodge-pole Formation in the Canadian Williston Basin is of low thermal maturity with respect to hydrocarbon gen-eration (Osadetz and Snowdon, 1995) and it has been documented that majority of these oils must have migrated considerable lateral distances from more cen-tral parts of the American Williston Basin (Osadetz et al., 1992, 1994; Burrus et al., 1996). However, although it is possible to envisage that the primary source sig-nature have been altered during the secondary migra-tion, one could expect that such long-distance (hundreds of kilometers), cross-stratal ¯ow would result in a more homogenic composition of the oil. Modelling of oil expulsion and accumulation from the Bakken shales, which were long suspected to be the major source of oil found in the Madison Group reservoirs (Dow, 1974; Price et al., 1984), documented that most of the expelled Bakken oil remains largely dispersed within the drainage system of the Madison strata; the system that formed secondary migration fairway for the Lodgepole-sourced oil. Nevertheless, this oil appears to be immobile as the oil saturation rates are too low for a free ¯ow to occur (Burrus et al., 1996). According to Muscio et al. (1994), Bakken kerogen shows enhanced tendency to generate light hydrocarbons, which generally corroborates the high API gravity character of the oils that form com-mercial accumulations in Bakken pools. If mixing of oils from Bakken and Lodgepole sources had been greater than previously estimated, the Bakken component could be assumed to be dominantly light hydrocarbons. Therefore, the Madison Group (family C) oils, despite having unique biomarker composition, would bear a strong resemblance in their gasoline fraction to the Bakken-sourced (family B) oils.
However, there are some dierences in gasoline range composition between both groups. There is a 6-carbon ring preference in the Madison Group oils whereas the Bakken-reservoired oils are preferentially enriched in 5-carbon ring hydro5-carbons (Fig. 17). These results are
similar to those of Javie (in press) who indicated that 5-carbon ring preference may be a function of clay con-tent in the Bakken Formation. Moreover, the family C oils are enriched in aromatic hydrocarbons, toluene and benzene, which are present in relatively lower con-centration in family B oils, despite the fact that kerogen in Bakken Formation has been considered as having enhanced aromatic nature (Muscio et al., 1994; Muscio and Hors®eld, 1996). The de®ciency of both compounds in Bakken-reservoired oils could be due to water wash-ing and long secondary migration (oils from northern Canadian locations generally have a lower relative con-centration of toluene compared with those from more central parts of the basin, Fig. 17). Interestingly, most of the family C oils, despite having been subjected to long secondary migration (Osadetz et al., 1992, 1994; Burrus et al., 1996), appear to have relatively higher amounts of toluene and benzene. Jarvie (in press) indicated that the higher sulfur content in the Madison Group source kerogen might have played a role enhancing cyclization of straight-chain parans leading to aromatization and greater production of toluene, although de®ciency of free hydrogen may also result in preferential formation of toluene. It appears that the dierences in the light aromatic hydrocarbons might have been at least to some extent source-controlled. Therefore, the mixing hypoth-esis remains to be tested using other techniques such as analyses of compound-speci®c isotopes, aromatic hydrocarbons, sulfur and other lower±molecular±weight fractions of both oil families and their sources.
5. Summary
Detailed analyses of gasoline range (C5±C8) fraction
in four Paleozoic oil families in the Williston Basin (families A, B, C and D) demonstrate that certain light hydrocarbon (``Mango'') parameters are useful in re®n-ing oil-oil correlations. Familial compositional distinc-tions, originally de®ned based on the distribution of saturated hydrocarbons in the C13±C30range (odd/even
in gasoline composition between these both oil families may suggest that the contribution of light hydrocarbons from Bakken sources to Madison reservoirs could have been greater than presently estimated. Alternatively, it may indicate greater source heterogeneity and organic facies changes within the Madison strata. However, these results have to be interpreted carefully as there is no general guidelines for the universal application of C7
``Mango'' parameters. Nevertheless, the gasoline range fraction is persistent even at high thermal maturities where biomarkers are often in low concentrations or below detection limit, and the ability to correlate oils of lower and higher maturities is a useful factor in petro-leum exploration. Moreover, as light hydrocarbons make up the bulk of the oil and the economic value of oil depends on its bulk characteristics, these parameters provide important information about oil relationships, source kerogen characteristics and the mechanism of subsequent oil alteration.
Acknowledgements
S. Achal, L. Mulder and M. Northcott provided analytical and technological support. Cooperation of numerous oil companies that provided oil samples for analyses is acknowledged. Drs. M. Li and C. Jiang are thanked for their scienti®c discussions. Critical reviews of Drs. R.G. Schaefer and T. Barth are greatly appre-ciated. GSC contribution no. 2000057.
Associate EditorÐL. Schwark
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