09-6 Spatial variation of Bakken or Lodgepole oils in the Canadian Williston Basin

Spatial variation of Bakken

or Lodgepole oils in the Canadian Williston Basin Zhuoheng Chen,Kirk G.Osadetz,Chunqing Jiang, and Maowen Li

ABSTRACT

There are two opposing schools of thought that infer either the Bakken Formation or the Lodgepole Formation as the primary source rock for the Madison-reservoired oils in the Canadian Williston Basin.A recent geochemical study revealed evidence indicating the existence of significant mixing of Bakken and Lod-gepole oils in the Madison reservoirs.To investigate the geo-graphic distribution of the oil compositions,we employed a multivariate statistical method to extract source and maturity-specific geochemical signatures from a geochemical data set for spatial analysis.Oil mixing appears to be geographically depen-dent and restricted by a northeast-southwest–striking zone (Torquay-Rocanville trend)in southeast Saskatchewan.Thus, fracture or fault systems are inferred to have provided high-permeability zones allowing Bakken-derived oil to migrate upward across the Lodgepole Formation.The areas without significant fault or fracture systems favor lateral oil migration along porous beds,restricting Bakken-derived oil accumula-tion to pools in Bakken reservoirs and Lodgepole-derived oils to occur primarily in overlying reservoir beds of the Madison Group.

INTRODUCTION

The Mississippian Madison Group is the most important oil-producing interval in the Canadian Williston Basin.With1.54×109m3(54.3×109ft3)in-place oil reserves(National Energy Board,2001),the Madison Group accounts for about62%of AUTHORS

Zhuoheng Chen Geological Survey of Canada,3303-33rd Street,Northwest,Calgary Alberta,T2L2A7,Canada;

Zhuoheng.Chen@nrcan-rncan.gc.ca Zhuoheng Chen obtained his Ph.D.from the Norwegian University of Science and Technology in1993and held a position as an associate professor at China University of Petroleum(Beijing) before joining the Geological Survey of Canada in1998.His research interests include petroleum resource assessment(methods and applica-tions),petroleum systems,and basin analysis. Kirk G.Osadetz Geological Survey of Canada,3303-33rd Street,Northwest,Calgary Alberta,T2L2A7,Canada;kosadetz@nrcan.gc.ca Kirk Osadetz graduated from the University of Toronto(B.Sc.,1978;M.Sc.,1983).He manages the Earth Science Sector Gas Hydrates Fuel of the Future Program and is the head of the Laboratory Services Subdivision at the Geolo-gical Survey of Canada in Calgary.He is active regarding petroleum resource evaluation and has research interests in gas hydrates,tectonics, and thermochronology.He worked previously at Gulf Canada Resources Inc.and PetroCanada Resources Inc.in Calgary.

Chunqing Jiang Geological Survey of Can-ada,3303-33rd Street,Northwest,Calgary Al-berta,T2L2A7,Canada;cjiang@ucalgary.ca Chunqing Jiang holds a Ph.D.in organic geo-chemistry.He has more than15years of expe-rience in analytical and interpretive petroleum geochemistry related to petroleum exploration and production in China,Australia,and Canada. He is a senior lab scientist at Gushor Inc.and the Petroleum Reservoir Group of the University

of Calgary.He has worked with Humble Geo-chemical Services,the Geological Survey of Can-ada,and PetroChina.

Maowen Li Geological Survey of Canada, 3303-33rd Street,Northwest,Calgary Alberta, T2L2A7,Canada;mali@nrcan.gc.ca Maowen Li has been a research scientist with the Geological Survey of Canada since1995.Since he received his Ph.D.in organic geochemistry from the University of Melbourne in1991,he has conducted petroleum system studies in China,

Copyright?2009.The American Association of Petroleum Geologists.All rights reserved. Manuscript received September5,2008;provisional acceptance November13,2008;revised manuscript received February23,2009;final acceptance March16,2009.

DOI:10.1306/0316*******

AAPG Bulletin,v.93,no.6(June2009),pp.829–851829

the total oil reserves in the Canadian part of the Williston Basin (Figures 1,2).Two different opinions exist with respect to whether the Bakken Formation or Lodgepole Formation is the primary source rock for the Madison oils of the Canadian Williston Basin.Early geochemical studies,particularly in the United States part of the Williston Basin,suggested that the organic-rich black shales of the upper and lower Bakken mem-bers are the most important source rock in the basin (Dow,1974;Meissner,1978;Dembicki and Pirkle,1985;Gerhard et al.,1990)and are considered to be the major contributor to the oils in the Madison Group https://www.360docs.net/doc/a86762385.html,ter studies iden-tified significant petroleum source rock potential in the Lodge-pole Formation (Osadetz et al.,1992;Osadetz and Snowdon,1995;Jiang et al.,2001).In general,cross-formational migra-tion of oil from Bakken shale to the Madison Group was con-sidered insignificant,implying that the Lodgepole Formation is the principal source rock for the oils in Madison reservoirs (Lefever et al.,1991;Price and Lefever,1994;Jarvie,2001).An exception to this view was the work of Burrus et al.(1996a,b),in which the basin modeling results showed that most oil generated in Bakken shales was expelled where an overpressure still exists.Why the prolific Bakken shales have contributed so little to the known oil reserves and where the Bakken-derived oil has gone have been a mystery.Osadetz et al.(1992)in their geochemical study in Canadian Williston Basin recognized two distinctive oil families in the uppermost Devo-nian and Madison Group reservoirs.The family B oils in the sandstone reservoirs of the middle member of the Bakken For-mation are characterized by high ratios of pristane/phytane (>1.40),C 23/C 30terpane (>0.80),rearranged sterane/regular sterane (>2.40)and sterane/hopane (>1.40),and a lack of C 35-homohopane predominance.These oils correlate well with type II marine source rocks in the Bakken Formation shale members.In contrast,the family C oils in the Madison Group reservoirs also display relatively high C 23/C 30terpane ratios,but they are distinguished from Bakken-sourced oils by consistently lower pristane/phytane ratios (<1.00)and the presence of clear n-alkane even-over-odd carbon number preference and C 35-homohopane predominance.These char-acteristics make the type II marine carbonates in the Mississip-pian Lodgepole Formation the primary candidate for their ef-fective source rocks.More recent geochemical studies (Jiang et al.,2001;Li and Jiang,2001;Jiang and Li,2002a,b)con-firmed this early finding and at the same time revealed a com-positional continuum between the two oil families.The latter was considered as a strong evidence for the mixing of petro-leum fluids and for a significant contribution from the Bakken

North Sea,Southeast Asia,North America,and Central Africa.His interest is in the study of petroleum system models and petroleum ex-ploration strategies.

ACKNOWLEDGEMENTS

We thank Mark Obermajer,Geological Survey of Canada (GSC)Calgary,for providing the U.S.well coordinates for the study.We thank AAPG editor Gretchen M.Gillis,reviewers Barry J.Katz and Prasanta K.Mukhopadhyay,and GSC internal reviewers Lloyd R.Snowdon and Mark Obermajer for their suggestions and comments that improved the quality of this article significantly.

The Elected Editor thanks the following re-viewers for their work on this paper:Barry J.Katz and Prasanta “Muki ”K.Mukhopadhyay.

830Spatial Variation of Bakken or Lodgepole Oils in the Canadian Williston Basin

Figure1.Cross section of Williston Basin showing the Phanerozoic stratigraphic succession and the stratigraphic position of the ef-fective Paleozoic petroleum source rocks(from Burrus et al.,1996a;used with permission from AAPG).The inserted stratigraphic chart in the lower-left showing the stratigraphic relationships of reservoir intervals in the Madison Group,Canadian Williston Basin(based on Podruski et al.,1988).See Figure2for the location of the cross section.

Chen et al.831

shales to the oil accumulations in Madison reser-voirs in southeast Saskatchewan.

It has been long postulated that there is a pre-ferred orientation of large fracture systems in the Williston Basin related to the original left lateral shear zones (e.g.,Fromberg fault zone and Colorado-Wyoming lineament of Gerhard et al.,1990,their figures 29–34).The productive part of southeast-ern Saskatchewan is related to the Brockton-Froid or associated fault systems that may have con-ducted petroleum into the oil field area (Gerhard et al.,1990).Lefever et al.(1991)suggested that migration has not occurred as a simple radial disper-sion from the generation area in the central basin.Instead,oil migration from generation kitchens ap-pears to follow preferential routes,which are con-trolled by tectonic and depositional elements.In an attempt to understand the geographic significance of the oil mixing indicated by geochemical signa-tures and their spatial variation,and its geological implications,we reprocessed the geochemical data set of 122oil samples in Jiang and Li ’s (2002a)study and examined the spatial characteristics of specific geochemical signatures in the oils.Because this study dealt with a large number of geochemical parameters and their spatial variations,mapping and visualizing the spatial variation of more than two variables at a time using a conventional mapping method are difficult.Therefore,principal com-ponent analysis (PCA)was employed to

reduce

Figure 2.Location map showing the main geological and geophysical elements in the Williston Basin and location of the data wells.The contour lines (thin brown)are the depth to base of Carboniferous (meters).NACPCA (pink shaded)=North American Central Plains conductivity anomaly (modified from Burrus et al.,1996b).

832

Spatial Variation of Bakken or Lodgepole Oils in the Canadian Williston Basin

the variable dimension and extract information associated directly with source rock character and oil maturity from the original data set.The analysis of the spatial variation of these geochemical signa-tures from PCA in a regional geological framework was then performed.

METHODS AND DATA

For a geochemical data set of m samples and n vari-ables,although each of the variables carries some specific information with respect to the genetic character of the source rock and source rock ma-turity,many of the variables are mutually corre-lated and contain redundant information.To study the spatial characteristics of the desired geochem-ical signatures,extracting relevant information from the data and combining all relevant informa-tion into a new variable are necessary,thus the de-sired geochemical signatures can be displayed ac-cording to their geographic locations.This can be accomplished by applying PCA.

Principal component analysis is a technique that transforms a set of original variables into a new set of mutually independent variables(Mardia et al.,1989).In general,PCA looks for a few linear combinations,which can be used to extract relevant information from a data matrix.After the transfor-mation,the first principal component(PC1),a lin-ear combination of the original data set,explains the largest amount of variance in the data set, and the second principal component(PC2),an-other linear combination of the data,describes the next largest variation remaining in the data set.As the first few principal components account for a large part of the variance in the original data, we can use these first few PCs to represent the data without losing significant information,thus reduc-ing the data dimension.Because the newly derived variables are independent of each other,each new variable contains independent information about the data.A specific principal component may then be identified to represent the relevant geochemical signature.From PCA,the projection of each sam-ple onto the axis defined by principal components is termed“score”,whereas the coefficient for each variable in the linear combination is called“load-ing”of the variable.The PC loading is commonly used to study the relationship between principal components and original variables,whereas the score is used to examine the characteristics of the samples.Principal component analysis has been widely used in geological interpretation of geo-chemical data(e.g.,Snowdon and Osadetz,1988), discrimination between source rock types(e.g., Odden and Kvalheim,2000),characterization of petroleum compositional families(e.g.,Pasadakis et al.,2004),and geochemical anomaly identifica-tion(Pereia et al.,2003).Chen et al.(2000)used PCA to integrate geological information controlling petroleum accumulation.In this study,we treated each geochemical parameter(molecular markers, compounds,and their ratios)as a random variable and use PCA as an exploratory tool for specific in-formation extraction.The sample scores(PC score) from PCA were employed to represent a summary of the extracted information for spatial data analy-sis.The mathematical details of PCA and its appli-cations to geological data analysis are available from Mardia et al.(1989)and Davis(1986).

Jiang and Li’s(2002a)data set was used in this study.The oil samples are grouped by the hosting reservoirs and geographic locations:(1)oils in the Bakken reservoirs in Canada,(2)oils in the Bakken reservoirs in the United States,(3)oil in reservoirs of the Madison Group located in Canada,and(4)oils reservoired in the Madison Group in the United States.Two super-Mississippian oil samples are also included in this study.Details of the oil samples and other associated information are listed in Table1. The well data in the Canadian part of the basin used here are from the Geological Survey of Canada (GSC)in-house well database.A detailed discussion of the PCA results based on aliphatic biomarker data was presented elsewhere(Pasadakis et al., 2004),and this article focuses on observations made from the gasoline range and aromatic hydrocarbons. GEOLOGICAL SETTING

The Williston Basin is a cratonic basin with its center in North Dakota(Kent and Christopher,1994)

Chen et al.833

Table1.Oil Sample Details?

https://www.360docs.net/doc/a86762385.html,b.No.Field Name Depth(m)Reservoir Location Easting Northing Canadian Bakken

1515Rocanville654.3Bakken02-11-16-31W1604849.1148610.9 2553Hummingbird2226Bakken05-26-02-19W2395185.117118.3 3554Roncott1823-1825Bakken09-34-05-25W2337824.148464.8 41279Daly879.5Bakken A13-21-10-29W1619881.794466.9 51355Torquay2001-2004.1Bakken7A-29-03-11W2467182.926888.6 61384Lake Alma N/A Bakken05-29-01-17W2409426.47347.9 71443Ceylon N/A Bakken10-15-06-19W2393998.353350.0 82121Rocanville N/A Bakken04-02-16-31W1604058.0146982.5 92122Rocanville N/A Bakken16-26-15-31W1605244.7144947.0 U.S.Bakken

101014Stoneview N/A Bakken NW31161N094W505102.8?80746.8 111166Temple N/A Bakken(?)NENW25159N096W N/A N/A 121391Antelope3154.5–3172Sanish sand8N/2E/2-1-152N N/A N/A 131392Antelope4205.5Sanish sand NWNE32153N094W N/A N/A 141393Bicentennial3381.45–4006.60Bakken(horizontal)NWNW26146N104W432406.8?173493.7 151394Charlson3076.04–3097.99Bakken-Three Forks SESE03153N095W508859.4?99811.7 161395Bicentennial3413.76–3874.26Bakken(Sanish sand)NWSE19145N103W435530.5?181598.1 171396Antelope3232.71–3235.45Sanish sand SENE33152N094W521223.9?117607.4 181397Pierre Creek3421.68–4367.78Bakken(horizontal)SESE07146N102W445300.9?168793.4 191398Elkhorn Ranch3249.17–4008.12Bakken(horizontal)SESE25143N102W458338.9?202553.2 201399Buckhorn3268.07–3902.05Bakken(horizontal)NENE17144N102W452040.2?189669.1 211400Bicentennial3312.57–3774.95Bakken(horizontal)NENE19144N103W440833.9?191167.7 221401Pierre Creek3345.79–3808.17Bakken(horizontal)NWSW01146N103W443711.6?167166.7 231402Squaw Gap North3303.73–4029.76Bakken(horizontal)SWSE21146N104W429221.5?171844.5 241403Elkhorn Ranch3165.65–3869.74Bakken(horizontal)SESW26143N102W456740.0?202541.7 251404Antelope3246.12–3249.17Bakken(Sanish sand)NWSE34152N094W542037.1?117493.1 Canadian Madison

30539Alida West1136–1334.2Alida06-06-06-33W1579135.649686.1 31573Handsworth1180.5–1182.5Alida09-21-10-09W2488149.394059.8 32585Handsworth1178.4Alida A9-36-10-08W2502390.797316.6 33516Nottingham1134.6Alida04-14-05-33W1585069.542765.4 34610Queensdale1198.7–1201Alida13-24-06-02W2558169.255385.5 35507Queensdale1185–1185.75Alida09-29-06-01W2560938.456199.7 36524Rosebank1086–1094Alida04-14-05-32W1594563.742765.4 37583Rosebank3516–3529Alida13-25-04-32W1596146.137473.2 38565Storthoaks3412–3422Alida-Tilston09-17-05-31W1600497.643579.6 40509Cantal South1143–1145Frobisher08-12-05-34W1578344.441544.1 41530Carnduff1255–1257Frobisher09-15-03-34W1575179.724038.9 42607Creelman1197–1207.3Frobisher11-07-10-09W2484193.490803.0 43521Elmore1211–1214Frobisher09-04-01-31W1602080.01241.4 44602Glen Ewen1334.7–1336.2Frobisher03-32-02-01W2561729.518339.6 45534Hastings1235.65–1236.57Frobisher14-27-03-34W1574388.527702.8 46541Hastings1144.5–1145.5Frobisher06-24-04-33W1587047.535030.6 47497Huntoon1349–1350.5Frobisher13-33-06-09W2486962.558642.3 834Spatial Variation of Bakken or Lodgepole Oils in the Canadian Williston Basin

Table1.Continued

https://www.360docs.net/doc/a86762385.html,b.No.Field Name Depth(m)Reservoir Location Easting Northing 48494Innes West1335.4Frobisher13-31-07-11W2464809.468412.6 49593Midale Central1395–1398Frobisher03-05-07-10W2476281.659049.4 50587Pinto1684.3–1686.2Frobisher16-14-01-05W2529291.04905.3 51502Steelman1444–1446Frobisher15-20-04-05W2524148.335844.8 52493Weyburn1350.5–1368.9Frobisher05-18-07-12W2455315.162713.2 53508Weyburn1368–1373.3Frobisher14-04-07-13W2449381.360270.7 54520Workman1207–1218Frobisher03-03-02-32W1593377.010197.6 55492Huntoon1343–1344Frobisher03-04-07-09W2487358.159049.4 56555Clairlaw1231.5Frobisher-Alida05-29-07-05W2523357.165970.0 57533Star Valley1161.5Frobisher-Alida13-11-09-06W2518610.080218.5 58510Arcola1179.5–1185Frobisher-Alida11-21-08-03W2544323.574519.1 59518Cantal South1120–1123Frobisher-Alida16-25-05-34W1578344.447243.5 60548Fletwode1160–1161Frobisher-Alida14-07-11-03W2541158.7100980.4 61531Queensdale1165–1168Frobisher-Alida11-22-06-01W2565685.554164.2 62491Willmar1230–1232.6Frobisher-Alida02-02-06-03W2547883.849279.0 63503Innes1320.75–1330.4Frobisher15-28-07-10W2478259.564341.6 681386Weyburn N/A Madison10-30-05-13W2446612.146836.4 691387Weyburn N/A Madison07-02-06-12W2462435.848871.9 701388Weyburn N/A Madison08-20-06-13W2448590.154571.3 711389Weyburn N/A Madison15-30-06-14W2437117.957013.9 721390Weyburn N/A Madison14-06-07-13W2446216.560270.7 74604Alameda1351.5–1357Midale13-09-04-03W2543927.932588.0 75490Alameda(Oxbow)1279.2–1291.7Midale13-14-03-02W2556586.824446.0 76500Benson1335.3Midale05-26-06-08W2499621.556199.7 77575Bryant1412–1413Midale07-30-05-07W2503577.446429.3 78527Florence1265.2–1276.2Midale01-23-02-34W1576762.115082.8 79506Glen Ewen1282.6–1289Midale05-21-03-01W2562916.325260.2 80498Huntoon(Wellore)1345–1350Midale07-32-06-09W2486171.457828.1 81574Lougheed1382–1387Midale12-30-05-14W2436326.746836.4 82586Macoun1522.8–1526.1Midale04-26-07-31W1604058.065562.9 83562Macoun West1562–1564Midale05-18-04-09W2484589.033402.2 841465Midale Central N/A Midale05-02-06-11W2471138.849686.1 851466Midale Central N/A Midale15-02-06-11W2471930.050500.3 861468Midale Central N/A Midale05-10-06-10W2479050.751314.5 871470Midale Central N/A Midale01-23-06-10W2481819.854164.2 881472Midale Central N/A Midale15-31-06-10W2475094.858642.3 89582Minard1337–1339Midale01-16-06-07W2507137.852535.8 90578Pinto1606–1608.1Midale02-02-02-05W2528895.410197.6 91514Ralph1306.7–1314.3Midale01-32-07-13W2448590.167191.3 92499Steelman1394.1Midale13-27-04-05W2526521.837473.2 93563Tagawa1429.5–1432.5Midale A13-24-06-16W2425250.155385.5 94501Weyburn1367–1388Midale09-08-07-12W2458084.361491.9 951464Weyburn N/A Midale14-31-05-12W2455710.748871.9 961467Weyburn N/A Midale14-08-06-12W2457293.152128.7 971469Weyburn N/A Midale10-10-06-13W2451359.251721.6 981471Weyburn N/A Midale12-26-06-14W2442656.256606.8 991473Weyburn N/A Midale12-33-06-13W2448985.758235.2

Chen et al.835

(Figure2).Two major sedimentary successions constitute the major fill of the basin.The Paleo-zoic succession is mostly carbonate and evaporite, whereas the Mesozoic and Cenozoic succession is mostly siliciclastic(Gerhard et al.,1990)(Figure1). The Mississippian in the Canadian Williston Basin consists of the Lodgepole,Mission Canyon,Charles, and Kibbey formations and the uppermost part of the Bakken Formation.The stratigraphic relation-ships of the Madison Group in Canadian Williston Basin are shown in Figure1.The Madison Group directly underlies the middle Jurassic unconformity

Table1.Continued

https://www.360docs.net/doc/a86762385.html,b.No.Field Name Depth(m)Reservoir Location Easting Northing 100528Workman1177–1181Midale15-18-07-31W1598519.763527.4 101535Weyburn1451–1458Midale-Frobisher B12-28-05-13W2448985.746836.4 103711Waskada921Mission Canyon16-13-01-26W1654298.24905.3 104714Waskada928–932Mission Canyon05-03-02-26W1649946.710604.7 105577Flat Lake1975Ratcliffe06-05-01-16W2419316.2834.3 106558Hoffer1940Ratcliffe05-30-01-15W2426832.56940.8 107568Hoffer1943–1946Ratcliffe09-27-01-15W2432766.47755.0 108556Hummingbird1890Ratcliffe11-26-02-19W2395580.717525.4 109559Lake Alma1984Ratcliffe04-29-01-17W2409426.46940.8 110557Neptune1752Ratcliffe04-06-04-16W2417338.329738.3 111540Oungre1823.3Ratcliffe03-29-02-14W2438304.716711.2 112584Edenvale1130–1132.3Tilston10-12-06-33W1587443.151721.6 113532Hazelwood1135–1140Tilston16-22-11-04W2537202.8104237.2 114529Moose Mountain1198–1200Tilston08-09-10-2W2554608.990395.9 115495South Parkman1090.8Tilston-Souris Valley16-09-08-33W1583091.571669.4 116526Parkman1030.5–1038.5Tilston-Souris Valley01-15-09-33W1584673.981846.8 117543Parkman1149.9–1151.5Tilston-Souris Valley10-12-10-02W2558960.490803.0 118596Moose Valley1183.5Tilston12-14-12-06W2518610.0111972.1 119511Kenosee1196.5Tilston12-26-10-03W2547092.695688.2 120721Daly725.7Lodgepole A10-13-10-28W1634914.292431.4 121722Virden620.3Lodgepole B09-26-10-26W1652715.895688.2 122725Virden607.5Lodgepole A04-27-11-26W1649946.7104644.3 U.S.Madison

641010Beaverlodge N/A Madison NW31-156N-095W504020.3?78865.4 671015Stoneview N/A Madison NW11-160N-095W506048.7?33678.9 661016Kuroki N/A Madison NW12-163N-081W638196.2?2862.5 651020Gleenburn N/A Madison NE36-159N-082W632808.2?48225.3 391442N/A N/A Charles SE32-30N-50E333452.1?74349.9 1021705Provost N/A Mission Canyon NWNW2323N56E426830.8?112357.5 731715N/A Madison T152N-R14W N/A N/A 281873State74N/A Lodgepole N/A N/A N/A 291874A-83N/A Lodgepole N/A N/A N/A Canadian Super-Mississippian

26512Red Jacket664.5Jurassic10-04-14-32W1591399.0128256.0 27523Wapella668.43–672.08Wapella sand13-34-14-01W2564498.7136805.1?Comp.=computation index number;Lab.No.=Lab sample number.

836Spatial Variation of Bakken or Lodgepole Oils in the Canadian Williston Basin

and overlies the Bakken Formation.The Bakken Formation,consisting of three members,the lower and upper black shale members and the middle sandstone-siltstone member,unconformably over-lies the Upper Devonian Big Valley or Torquay Formation.The upper and lower members are transgressive sequences,and the middle member is deposited in a low-stand setting(Smith and Bustin, 2000).

Two salt intervals,one in the Mississippian Charles Formation and another in the Middle Devo-nian Prairie Formation,are particularly important for the Mississippian petroleum systems(Figure1). The Prairie salt has threefold importance.First,the salt forms an effective regional migration barrier against any vertical migration from underlying source rocks into Mississippian reservoirs.Second, dissolution of halite has formed numerous struc-tures that comprise petroleum traps(e.g.,Hum-mingbird field in Saskatchewan).Third,most salt dissolution is controlled by an underlying struc-ture,specifically by faults that cut the Precambrian basement(Gerhard et al.,1990).Salt dissolution in the Prairie Formation caused collapse in the overlying Mississippian strata and generated frac-ture networks promoting cross-formational oil mi-gration.The Charles salt forms the top seal pre-venting oil from migrating from the Mississippian system into the overlying rocks.

Several potential source rocks in the Williston Basin exist(Osadetz and Snowdon,1995)but only the uppermost Devonian–lower Mississip-pian Bakken Formation and the overlying lower Mississippian Lodgepole Formation are significant contributors to oils accumulated in the Bakken and Madison Group reservoirs.Other potential oil source rocks in the Middle and Lower Devo-nian succession are unlikely to be the source rocks because the widespread salt in the Prairie Formation prevents oil generated by those source rocks from accumulating in the overlying reser-voirs(Osadetz and Snowdon,1995;Burrus et al., 1996a;Jiang and Li,2002a).The upper and lower black shales of the Bakken Formation are the rich-est and most widespread source rocks in the basin (Dow,1974;Dembicki and Pirkle,1985;Osadetz and Snowdon,1995).Organic carbon content and shale thickness are greatest near the Bakken depo-center east of the Nesson anticline(maximum Bak-ken shale thickness,20m[66ft],Dembicki and Pirkle,1985,their figure3).In the Canadian Will-iston Basin,the total organic carbon(TOC)con-tent of the Bakken shales can be more than30%. The average TOC value for the lower shale is 11.77%and17.63%for the upper shale.The aver-age Lodgepole TOC value is5.49%(Osadetz and Snowdon,1995).However,in the Canadian Will-iston Basin,because of shallow burial depth,neither the Bakken black shales nor the organic-rich Lodge-pole carbonate is sufficiently thermally mature to be responsible for a large volume of oil generation (Osadetz and Snowdon,1995).The oils in the Mississippian reservoirs of the Canadian Williston Basin have migrated from major oil-generation centers in the south of the basin.

The reservoir rocks in the Madison Group are shelf carbonates,and the seal is provided either by overlying evaporites or the Mesozoic erosional surface(Kent,1984;Podruski et al.,1988,their figure54).The salt in the Charles Formation forms a natural barrier for oil migrating through the Mis-sissippian system.Vertical migration through frac-ture systems was stopped by Charles salts(Kent, 1984).Most oil pools in the Madison Group of southeastern Saskatchewan occur in unconformity-related traps.The sub-Mesozoic erosional surface combined with the primary facies changes provides the traps(Kent,1984;Podruski et al.,1988).Other trap types include stratigraphic and structural traps. The common types of structural trap are basement folds,salt dissolution,and impact-related struc-tures(Kent and Christopher,1994).

A crustal geophysical anomaly lying along longitude103°W is present,which is coincident with the North American Central Plains conduc-tivity anomaly(NACPCA)(Figure2).The ther-mal maturity of source rocks may be strongly af-fected by elevated heat flows associated with this feature(Osadetz and Snowdon,1995).Enhanced maturation resulted in oil windows occurring at much shallower depths(Osadetz and Snowdon, 1995;Jarvie,2001).As shown by the data from Sta-siuk(1994),the measured vitrinite reflectance (R o)values of the Bakken Formation shales in the

Chen et al.837

Canadian Williston Basin range from0.34to 0.68%.When the vitrinite log%R o is plotted against the burial depth for the Bakken Formation, data from samples with depth greater than1400m (4593ft)(or>0.50%R o)form several different clusters,with each data subset indicating a differ-ent maturation gradient.Although the Bakken Formation shales at the vicinity of the Nesson trend reach0.7%R o at the current burial depth of about2000m(6562ft),200–400m(656–1312 ft)of additional burial is needed for stratigraphic equivalent shales to reach the same maturity level at structural locations to the west,north,and east of the Nesson trend(Li et al.,1998).Several fault or fracture systems and trends in this region are observed,which are believed to be related to base-ment tectonic elements(Gerhard et al.,1990). RESULTS AND DISCUSSION

The results of geochemical analysis in Table2form a data matrix for PCA in this study.This data matrix has46parameters and122oil samples. The PC1,accounting for37%of the total variance (inset of Figure3),is the largest variance ex-plained by a single new variable in the data matrix and contains information useful for distinguishing the Bakken and Lodgepole oils.The PC2accounts for21%of the total variance and is the second largest variance explained in the data matrix.As we dis-cussed in the previous sections,the PCs are mu-tually independent and linear combinations of the original geochemical attributes.The PC loadings indicate the relative contributions of the original geochemical attributes to the new PC variables. To reveal the genetic relationship between the new variables and the original geochemical attri-butes,Figure4a and b are bar plots of PC1and PC2loadings,respectively,showing the relative contributions of the original46geochemical attri-butes to PC1and PC2scores.The larger the abso-lute loading(the longer the bar in Figure4),the greater the contribution to the new linear combi-nation.The bars around the zero axes contribute little to the new variables.For example,the attri-butes coded4,9,38,39,40,41,44,45,and46(see Table2for the corresponding geochemical at-tributes)are good indicators for Bakken oils and1, 2,3,5,6,7,11,13,19,20,21,31,and42are good indicators for Lodgepole oils,which dominate the new variable PC1.

Source Type Indicator

A PC score plot shows the characteristics of obser-vations in terms of the linear combination of the original attributes.On a bar plot of PC1score (Figure3),the first25oils are from Bakken reser-voirs(9in the Canadian and16in the United States parts of the Williston Basin).For the next four oil samples,either the Bakken Formation made a ma-jor contribution(Red Jacket and Wapella fields in Canada,Lab.512and523in Table1)or the Bak-ken is clearly the source rock(Lodgepole Mound [Lodgepole M.]oils from State74and A-83, Lab.1873and1874in the United States,Table1). All these29oils have PC1scores smaller than?2, although the PC1scores span a wide range.The re-maining samples are from Madison reservoirs.Al-though a variable PC1score,most of the Madison-reservoired oils have PC1scores greater than?2. Within an oil family,the variation in the PC1 scores could reflect the compositional changes caused by oil maturity,migration,and even facies difference.Early studies confirmed the existence of both Bakken oil and Lodgepole oil families with distinctive geochemical characteristics and found a compositional continuum between these two oil families(Jiang and Li,2002a).The first29oils have clear geochemical characteristics of Bakken-derived oils and belong to the Bakken oil family (see PC1loading of Figure4;Table2for the cor-responding geochemical features).Jiang and Li’s (2002a)study indicated that typical Lodgepole oils have a low value of the ratio of C18trimethyl-arylisoprenoids to C18alkylbenzene(C18:TMAI/ AB<1.0)and a relatively high ratio of dibenzo-thiophene to tetramethylnaphthalenes(DBT/ TeMN>1.0).The Madison-reservoired oils exhib-it a compositional continuum between the Bakken-and Lodgepole-derived end-member oils.How-ever,the classification using PC1alone results in two oil families without distinguishing the mixed

838Spatial Variation of Bakken or Lodgepole Oils in the Canadian Williston Basin

Table2.List of Geochemical Attributes used for this Study

Variable Parameter Explanation

1DPM/MBPs Ratio of diphenylmethane to the sum of methylbiphenyls

2BCH/AB Ratio of benzylcyclohexane to alkylbenzene

3BCH/MBPs Ratio of benzylcyclohexane to methylbiphenyls

4C18:TMAI/AB Ratio of C18trimethylarylisoprenoids to C18alkylbenzene

5DBT/TeMNs Ratio of dibenzothiophene to tetramethylnaphthalenes

6DBT/Ph Ratio of dibenzothiophene to phenanthrene

7AB(%)%of C18(alkylbenzene)

8MAB(%)%of C18(methylalkylbenzene)

9TMAI(%)%of C18(trimethylarylisoprenoids)

10Bhop/Ph Ratio of benzohopanes to phenanthrene

11DBT ppm of dibenzothiophene in oil

12TeMNs ppm of tetramethylnaphthalenes in oil

13DPM ppm of diphenylmethane in oil

14MBPs ppm of methylbiphenyls in oil

15C18_TMAI ppm of C18trimethylarylisoprenoids in oil

16C18_AB ppm of C18alkylbenzene in oil

17Bhops ppm of benzohopanes in oil

18TAS ppm of triaromatic steroids in oil

194_MDBT ppm of4-methyldibenzothiophenes in oil

20(3pls2)_MDBT ppm of(3-+2-)methyldibenzothiophenes in oil

211_MDBT ppm of1-methyldibenzothiophenes in oil

22Phenanthrene ppm of phenanthrene in oil

233_MP ppm of3-methylphenanthrene in oil

242_MP ppm of2-methylphenanthrene in oil

259_MP ppm of3-,2-,9-,and1-methylphenanthrene in oil

261_MP ppm of1-methylphenanthrene in oil

27TASC26(S)ppm of C26(S)triaromatic steroids in oil

28TASC28(S)ppm of C28(S)triaromatic steroids in oil

29TAS(C20plsC21)ppm of C20+C21triaromatic steroids in oil

30TAS(C26plsC27plsC28)ppm of(C26+C27+C28)triaromatic steroids in oil

31nc7%of C7n-alkane

32Bc7%of C7branched alkanes

33n_Hexane/Methylcyclopentane Ratio of n-hexane to methylcyclopentane

34n_Heptane/Methylcyclohexane Ratio of n-heptane to methylcyclohexane

35K1Mango’s K1parameter

36%(3_MHpls2,4_DMP)%of(3-methylhexane+2,4-dimethylpentane)

37%(2_MHpls2,3_DMP)%of(2-methylhexane+2,3-dimethylpentane)

381t2c3TMCYC5/MCYC6Ratio of1-trans-2-cis-3-trimethylcyclopentane over methylcyclohexane

391t2DMCYC5/CYC6Ratio of1-trans-2-dimethylpentane over cyclohexane

401t3DMCYC5/CYC6Ratio of1-trans-3-diemthylcyclopentane over cyclohexane

411c3DMCYC5/CYC61-cis-3-dimethylcyclopentane over cyclohexane

42TolueneplsMCYC6%toluene plus methylcyclohexane in the sum of C7alkanes

43n_Heptane%of C7n-alkane in the sum of C7alkanes

44C7(branchedplscyclo)%of iso-and branched C7alkanes in the sum of C7alkanes

451367-TeMN/TeMNs Ratio of1,3,6,7-tetramethylnaphthalene to the sum of tetramethylnaphthalenes 464-MDBT/(1-MDBTplus4-MDBT)methyldibenzothiophene ratio4-/(4-+1-)

Chen et al.839

oils because the PC1score varies with oil thermal maturity.

Figure 5is a crossplot of PC1and PC2scores,on which oils in the study area can be divided into three clusters.In cluster A,the oils are the Bakken sourced and reservoired.The oils in cluster C show C18:TMAI/AB <1.0and DBT/TeMN >1.0,typi-cal characteristics of the Lodgepole oil family.The Bakken oils appear to follow a linear trend (A),in which oils located near the oil kitchen are in the lower right of the cluster.The oils in Canada are in the upper left of the trend.The Lodgepole-derived oils also follow a linear trend (cluster C),in which locally derived low-maturity oils are located in the upper right (marked C I )and the remainder are oils migrated from the south with higher thermal ma-turity.The mixed Bakken-and Lodgepole-derived

oils (in cluster B)are in between clusters A and C.The oils from Red Jacket,Wapella,Hummingbird,and Ceylon fields are Bakken mixed with Lodge-pole oils from geochemical analysis (Jiang and Li,2002a)and are in a subcluster B I .Maturity Indicator

In general,the Bakken oils in the U.S.Williston Ba-sin (computation 10to 27)show high thermal ma-turity.In a geochemical study conducted by Price and Lefever (1994),15U.S.Bakken oils were ana-lyzed for maturity using the Rock-Eval hydrogen index of the associated shale as maturity index (Price and Lefever,1994,their table 3).The re-sults suggested that the Bakken-reservoired oils around the Antelope field on the

southeastern

Figure 3.Bar diagram for the first principal component (PC1)scores derived from parameters in Table 2.The first 29samples are oils from the Bakken source rock.The others are Lodgepole oils or oils primarily from the Lodgepole source rock.The diagram (lower right)within the figure shows the variances of the data explained by each principal component.See the sample index number in Table 1.

840

Spatial Variation of Bakken or Lodgepole Oils in the Canadian Williston Basin

Figure4.Bar plots of

the first principal compo-nent(PC1)loading(a) and the second principal component(PC2)loading (b),showing the relative contributions of original geochemical attributes to the PC loading(linear combination of original geochemical attributes). See Table2for the attri-bute index number. Chen et al.841

flank of the Nesson anticline (Figure 2)have the highest maturity,whereas oils in the Squaw Gap and Bicentennial fields (Figure 2)have the lowest maturity in the United States part,which is consis-tent with our PC2scores.A large negative PC2score indicates high maturation,and a large positive score suggests low maturation.Samples from Ante-pole field (12,13,17,and 25in Figure 5)with low-est PC2scores suggest the highest maturation level,and samples from Squaw Gap and Bicentennial fields (23and 16in Figure 5)with moderate PC2scores indicate moderate maturity in this basin but the lowest in the United States part.In the Cana-dian part,samples from Rocanvile field (1,8,and 9

in Figure 5)have the highest PC2scores,indicating the lowest maturity in the Williston Basin.

The study using the ratio of 1,3,6,7-tetramethyl-naphthalene to the sum of tetramethylnaphtha-lenes (1,3,6,7-TeMN/TeMNs)and the methyldi-benzothiophene ratio 4-/(4-+1-)(4-MDBT/(1-+4-MDBT))as thermal maturity indicators (Jiang and Li,2002a)confirms Price and Lefever ’s conclu-sion and at the same time suggests that the maturity level of Bakken oils in the Canadian part of the basin is generally low.The highest oil maturity was found in the Torquay field with maturity level equivalent to the oils from the Bicentennial field.The oils with the lowest thermal maturity are found in the

Red

Figure 5.Crossplot of the first (PC1)versus the second principal component (PC2)scores showing the Bakken-reservoired oils (squares),known Lodgepole oils (C18:TMAI/AB <1.0and DBT/TeMN >1.0)(triangles),and undetermined oils (dots).The high-maturity oils in the United States are in the lower right of cluster A;low-maturity oils in the Canadian side are in the upper left of cluster A.The Bakken oils mixed with Lodgepole contribution are in subcluster B I .Cluster C is interpreted as the distribution range of Lodgepole-derived oils;cluster B:distribution range of the mixed Bakken and Lodgepole oils and C I distribution range of low-maturity Lodgepole oils.The sample index number is in Table 1.The boundaries of clusters are defined by three equations beside the boundaries.

842

Spatial Variation of Bakken or Lodgepole Oils in the Canadian Williston Basin

Jacket and Rocanville fields (Figure 2)in the north-east of the study area.

As comparing the relative thermal maturity ranks of the Bakken oils derived from geochemical analysis to the principal component scores,we found that the PC2score indicates a similar rank-ing order.We,therefore,infer that the PC2score contains information on oil thermal maturity and can be used to indicate relative oil maturity.A comparison of the PC2score and 1,3,6,7-TeMN/TeMNs is illustrated in Figure 6.Based on these thermal maturity indicators,the oils in the Madison Group show a great variation in maturity,with the highest values being comparable to those with the highest maturity of the Canadian Bakken oils.The Madison oils with the highest maturity are lo-cated in the southeastern part of Saskatchewan along the border of the United States and show a

general decreasing trend in maturity from the center to the basin margin.

Spatial Characteristics of the Geochemical Signatures

To study the spatial characteristics of specific geo-chemical signatures in the oils,the PC1and PC2are employed to represent the geochemical signa-tures of oils in relation to the source rocks.Figure 7illustrates the spatial characteristics of the PC1score.From this map,several interesting features can be observed.First,the discovered Bakken and Madison oils show certain spatial alignments,which are coincident or associated with regional tectonic lineaments (Figure 7).Second,the oils in the south-east corner of the Canadian Williston Basin (cluster ML in Figure 7)are oils in cluster C in Figure 5

,

Figure 6.Crossplot of the second principal component (PC2)scores and 1367TeMN/TeMNs (ratio of 1,3,6,7-tetramethylpaphthalene to the sum of tetramethylnaphthalenes)of oils listed in Table 1.See the text for discussions.Oil thermal maturity increases with an increase in 1367TeMN/TeMNs and a decrease in PC2.

Chen et al.

843

Figure7.Spatial variation of oil mixing based on source type indicator PC1.Bakken-reservoired oils are labeled B;and L,J,and W indicate oils in Lodgepole,Jurassic,and Cretaceous reservoirs.The circles without labels represent oils in Madison reservoirs.On the map,the Bakken-derived oils are represented by open circles,and the Lodgepole-sourced oils are indicated by solid black circles.The solid gray circles represent oils with mixed sources,and the relative contributions from the Bakken and Lodgepole are indicated by their gray level,which are calculated by the three linear equations in Figure5.The circle size represents the relative contributions from the Bakken and Lodgepole sources.The larger the circle,the higher the oil thermal maturity.Oil in the Bakken reservoir(B A=Antepole;B C= Charles;B S=Squaw Gap North;B P=Pierre Creek;B B1=Buckhorn;B B2=Bicentennial;B E=Elkhorn Ranch;B LA=Lake Alma;B D=Daly;

B R=Roncott;B H=Hummingbird;B C2=Ceylon;B RV=Rocanville;B T=Torqunay);oil in the Lodgepole reservoir(L D=Daly;L V=Virden); oil in the Jurassic reservoir(J=Red Jacket field);oil in the Cretaceous reservoir(W=Wappela field);unlabeled are oils in Madison Group.TRT=the Torqunay-Rocanville trend(Lefever et al.,1991);BFL=Brockton-Froid lineament;MFS=Mondak Fracture System (Gerhard et al.,1990);ML=Lodgepole oil family in the Madison Group.The coordinates on this map are based on a UTM project with a central meridian of103°W,a base latitude of49°N,NAD83.

844Spatial Variation of Bakken or Lodgepole Oils in the Canadian Williston Basin

reservoired in the Madison Group with geochem-ical characteristics similar to the Lodgepole oil fam-ily.This suggests that these oils have a similar source,primarily from the Lodgepole.Third,the oils in the Torquay-Rocanville trend (Lefever et al.,1991)have variable gray level,indicative of the mixing of Bakken and Lodgepole sourced oils.A lighter gray level indicates a higher contribution from Bakken source rocks.Fourth,oils with PC1score values significantly greater than that of the end-member Lodgepole oil in the central-eastern part of the Torquay-Rocanville trend are

observed.

Figure 8.Spatial variation of oil maturity based on maturity-specific indicator PC2.B:oil in the Bakken reservoir,L:oil in the Lodgepole reservoir,J:oil in the Jurassic reservoir (Red Jacket field),W:oil in the Wapella reservoir (Wapella field);unlabeled are oils in Madison Group.Bakken oils in the oil generation center have the highest maturity.Madison oils with local Lodgepole source contribution show the lowest maturity.The smaller and darker the circle is,the less mature the oil is.The coordinates are in UTM projection.B A =Antepole;B C =Charles;B P =Pierre Creek;B B1=Buckhorn;B B2=Bicentennial;B E =Elkhorn Ranch;B LA =Lake Alma;B D =Daly;B R =Roncott;B H =Hummingbird;B C2=Ceylon;B RV =Rocanville;B T =Torqunay;L D =Daly;L V =Virden.

Chen et al.

845

The PC2score indicates that the maturity level of these oils is the lowest among the samples (Figure 8).A detailed study in the Weyburn and Midale central fields revealed that these oils are generally sulfur rich,with low API gravities (Figures 9,10),geo-chemical characteristics of Lodgepole-sourced oils (Jiang and Li,2002b).As the locations of these oils coincide with the North American Central Plains conductivity anomaly (Majorowicz et al.,1986),these oils may represent oils that were generated from local Lodgepole source rocks with relatively short lateral migration distances.Finally,oil mixing appears to be structurally,instead of stratigraph-ically,controlled.Figure 11shows the locations of oil production wells in relation to their hosting reservoirs.The Mississippian strata have a general strike of northwest –southeast,but oil mixing ap-pears to be in the northeast –southwest Torquay-Rocanville trend (Figure 7),suggesting that oil mixing occurs where faults and fractures connect the Bakken and Lodgepole source rocks.

The spatial characteristics of oil maturity are represented by the PC2.The Bakken oils show a high maturity,particularly in Antepole and Charl-son fields (Lab.1404,1396,1394)around the Nesson anticline near the Bakken generation

center

Figure 9.Diagram showing the oil gravity (API)in the Weyburn and Midale central fields,Saskatchewan,Canada.The radius of the circle is proportional to API gravity.The larger the circle is,the lighter the oil is.The map location is in Figure 8.The coordinates are in UTM projection.

846

Spatial Variation of Bakken or Lodgepole Oils in the Canadian Williston Basin

(Figure 12).The Bakken oils on the Canadian side of the basin have moderate to low maturity.In con-trast,the Lodgepole oils have a variable maturity.Outside of the Torquay-Rocanville trend,Madison oils have a moderate maturity,decreasing system-atically to the north.In the Torquay-Rocanville trend,most Madison Group oils show a low ther-mal maturity.

Implication to Regional Oil Migration The geochemical characteristics of Madison-reservoired oils show a strong geographic depen-dency in the Canadian part of the Williston Basin.The oils within the Torquay-Rocanville trend have variable,source-specific signatures and low thermal maturities.The compositional continuum of these oils in between the Bakken and Lodgepole end members suggests mixing from both sources.In contrast,the Madison-reservoired oils outside of the trend (ML in Figure7)have source-specific geochemical signatures similar to the Lodgepole-derived oil,suggesting that the Lodgepole Forma-tion is the principal source rock in that region.

The occurrence of mixed oils in Madison res-ervoirs requires that the Bakken-derived oil mi-grates across the Lodgepole Formation.The fact that oil mixing appears to be primarily

restricted

Figure 10.Diagram showing the relative abundance of sulfur content in oils from the Weyburn and Midale central fields,Saskatch-ewan,Canada.The radius of the circle is proportional to the sulfur content.The larger the circle,the more sulfur content in the oil.The map location is in Figure 8.The coordinates are in UTM projection.

Chen et al.

847

to the Torquay-Rocanville trend indicates that cross-formational oil migration is geographically re-stricted and that regional oil migration may follow preferential routes.We postulated that cross-formational oil migration occurs because of the pre-sence of tectonically controlled fracture or fault systems in the Torquay-Rocanville trend in the Ca-nadian part of the basin.

Natural fractures are observed in the Weyburn oil field (Wegelin,1987).Fractures caused by Prairie salt solution were reported in the Humming-bird field and other locations (Gerhard et al.,1990).A northeast –southwest production trend in the Weyburn field suggests a preferential northeast –southwest fluid-flow orientation (Wegelin,1987),parallel to that of the Torquay-Rocanville trend.This suggests that natural fractures with a domi-nant northeast –southwest orientation are present in the Torquay-Rocanville trend.Presumably,this fracture system represents an old zone of basement weakness and reactivation during subsequent tec-tonic events.Vertical migration through the frac-ture systems was retarded by Charles salts.On reaching continuous porosity zones below the Charles evaporates,mixed-source oil migrated up-dip until trapped by porosity pinch-outs within the Madison reservoirs or at the Madison-Jurassic un-conformity.Bakken and Lodgepole oils in super-Madison reservoirs are present only beyond the depositional edge of the Charles salts (e.g.,Red Jacket and Wapella fields,Lab.512and 523,Table 1).A small group of oils (seven samples in cluster C I ,Figures 5,7)in the Torquay-Rocanville trend shows abnormal geochemical composition (high sulfur content and high PC1scores)and both very low thermal maturity and API gravity.The occurrence of these oils is geographically co-incident with the NACPCA.We infer that a less mature Lodgepole source rock may exist in west-ern parts of the Williston Basin with a connection to the Torquay-Rocanville trend.Although other Devonian source rocks (e.g.,in the Winnipegosis Formation)are also oil prone and thermally ma-ture,no geochemical evidence indicates that such source rocks have contributed significantly to the discovered Madison Group oils.

Clearly,the general geographic distribution of compositional characteristics of oil field areas ap-pears to indicate different migration routes.Early geochemical studies (Dembicki and Pirkle,

1985,

Figure 11.Map illustrating the locations of the Canadian oil wells in Table 1and their associated production formation or beds.B =Bakken Formation;L =Lodgepole Formation and Souris beds;T =Tilston beds;F =Frobisher and Alida beds;M =Midale beds;and R =Ratcliffe beds.The solid lines represent the formation-bed boundaries under super Carboniferous unconformity.The coordinates are in UTM projection.See Figure 2for the location of the map.

848

Spatial Variation of Bakken or Lodgepole Oils in the Canadian Williston Basin